A Frequency Based Theory of Catalysts ··\1uch of chemistry invol\es energy. In the right place. right time. and right amount. energ) dfr.es the,.----- P reactions. separations and other functtons of the chemical uni\erse. In the \.\-rong place. \Hong time. or ""ron Q \ ~ .imount. energ) can ruin a batch. create um~anted b:, -products. or otheN ise interfere"" ith the desired outc me. ,;. Thus. -:hemical research is intimate!:, imol\ed ""1th the control of the t)-pe. amount. Jnd timing of energ) :mrodw..:t10n into a s:,stem:· \1icro,,.,ave-Enhanced Che:nistr:,: Eds. H\1 Kingston & SJ Has\ltell.l 997. 08: 2 • 0 1999 ~ ° 1t Background h-1 ~,§ rRAo~~~ Chemical reactions are driven by energy. The energy comes in many different fonns - chemical, thermal. mechanical, acoustic, and electromag- netic. Various features of each type of energy are thought to contribute in different ways to the driving of chemical reactions. Irrespective of the type of energy involved, chemical reactions are undeniably and inextricably intertwined with the transfer and combination of energv. An understanding ~* ~ ::i9f energy is, therefore, vital to an understanding of chemical reactions. ,,, Chemical energy drives a reaction by catalyzing it. Catalysts are J,;:!:hemicals that speed the rate of a reaction, but are neither used nor consumed !:;:!n the reaction. Many reactions are said to be catalyzed. That is, the addition ;:;pf a small amount of a chemical substance (the catalyst) can greatly speed the i::xate of the reaction. How did this definition evolve? •• In 1770, only 15 elements were known. After Boyle's recognition that ,JJ:hemistry was properly the study of matter', catalytic action was noted for /,,the first time. In 1835, Berzelius proposed a theory of "catalysis". He :::;believed that since the catalyst was unchanged by the reaction, a catalytic :'.J"force'' must be involved. By the tum of the century the kinetic theory of chemistry had been developed. In 1911, Ostwald proposed the modem theory of catalysis, which has stood unchallenged for nearly one hundred years. This theory abandons Berzelius' catalytic force, and states there are five essential steps in catalysis: 1. Diffusion of the reactant to the catalyst site; 2. Bond formation between reactant and catalyst; 3. Reaction of the catalyst-reactant complex; 4. Bond rupture at the catalytic site to produce product; 5. Diffusion of product a\vay from the catalyst. Thus. catalysts are thought to speed reactions by bonding with chemicals. CJtJ!) sis ,mJ ChemicJI processes: EJs. Pc..m.:e R & PJtterson \\.R. Wik; & Sons. ICJ8 l. Page -1- Thennal energy is used to drive chemical reactions by applying heat and increasing the temperature. The addition of heat increases the kinetic (motion) energy of the chemical reactants. A reactant with more kinetic energy moves faster and farther, and is more likely to take part in a chemical reaction. Mechanical energy likewise, by stirring and moving the chemicals, increases their kinetic energy and thus their reactivity. The addition of mechanical energy often increases temperature, by increasing kinetic energy. Acoustic energy is applied to chemical reactions as orderly mechanical waves. Because it is mechanical, acoustic energy increases kinetic energy of chemical reactants, and can also elevate temperature. Electromagnetic (EM) energy consists of waves of electric and magnetic fields. EM energy increases kinetic energy and heat in reaction systems. It also energizes electronic orbitals in some reactions. Both acoustic and electromagnetic energy consist of waves. The number of waves in a period of time can be counted. Waves are often drawn, ;:~s in Figure 1.. below. Usually, time is placed on the horizontal X-axis. The i:;·ertical Y-axis shows the strength or intensity of the wave. This is also ;'1~alled the amplitude. A weak wave will be of weak intensity and will have ::~ow amplitude (Figure 2.a., below.) A strong wave will have high amplitude ;;~~Figure 2.b.) Amplitude j Time - Figure 1. - Wave representation (acoustic or electromagnetic) Pao< lfJPJ/;J-,,q'-y'JP - .fJF,,__,, t,J PJ 1L}1!11 i . ..\mplitude i Amplitude Time -+ ( a) Time -+ Figure 2. - Waves of different amplitudes but the same frequency. (a) Low intensity \vave with low amplitude. (b) High intensity wave with high amplitude. Traditionally, the number of waves per second is counted, to give the ::frequency. Frequency = Number of waves/ time = Waves/second= Hz. Another name for "waves per second", is "hertz" (abbreviated as Hz) 1:so named for Heinrich Hertz who did much of the early research in-defining ;;~lectromagnetic waves. Frequency is drawn on wave diagrams by showing :Si~ifferent numbers of waves in a period of time (Figure 3. (a) below.) It is /]Hso drawn by placing frequency itself, rather than time, on the X-axis i::~Figure 3. (b).). Amplitude T • • 0.5 1 Time (seconds) (a) 0 l 2 34 Frequency (b) Figure 3. Frequenc~ diagrams. (a) Time on the X-axis. 2 and 3 Hz frequency waves. (b) Frequcncv on the X-axis. 2 and 3 Hz \\a\es. - Page-,;;?-/q:.-)7';? 0fJl1/t1/rr Energy waves and frequency have some interesting properties, and combine in interesting \vays. The manner in which wave energy transfers and combines, depends largely on the frequency. Say, for instance, that we combine nvo waves of energy, each of the same amplitude, but one at a frequency of 400 Hz (waves per second) and the other at I00 Hz. The waves will combine and add their frequencies, to produce a new frequency of 500 Hz. the sum frequency. They will also subtrac_t \vhen they combine, to produce a difference frequency of 300 Hz. All wave energies add and subtract this wav, and it is called heterodvning. The results of heterodvning ., .I "-' • '-' are probably familiar to most people as the harmonies in music. There is a mathematical, as well as musical basis, to the harmonics produced by heterodyning. Consider a continuous progression of heterodyned frequencies. Using our example above, beginning with 400 Hz and 100 Hz. we get 500 Hz and 300 Hz. If we heterodyne (add and subtract) these new frequencies, we get 800 (500 + 300) and 200 (500 - 300). 1;[:iHeterodyning 800 and 200, gives us 1,000 and 600 (see Figure 4. below). ',"" Soon a mathematical pattern begins to emerge. Both the addition and :,,;:the subtraction columns contain alternating series of numbers that double ::nwith each set of heterodynes. In the additive column, 400 Hz, 800 Hz, and ,"••; 1,600 Hz, alternates with 500 Hz, 1000 Hz, and 2000 Hz. The same sort of :;·/doubling phenomenon occurs in the subtraction column. FIGURE 4. - Heterodyne Progression Initial Frequencies (Hz) 400 And 100 400 + 100 = 500 and 400 - 100 = 300 500 + 300 = 800 and 500 - 300 = 200 800 + 200 = 1000 and 800 - 200 = 600 1000 + 600 = 1600 and 1000 - 600 = 400 Sum ' ~2000 +----------+--+---+--+--+---+- :::, O" ~ I.L.1000 - - - - - - - 0 Heterodyne generation Figure 5.• Visual pattern created by heterod)ned series from Figure 4. Mathematics has a name for this - it is called a fractal. A fractal is defined as a mathematical function which produces a series of self-similar patterns or numbers. Fractal patterns have spurred a great deal of interest because they are found everywhere around us in nature. They can be found in the patterning of large expanses of coastline, all the way down to microorganisms. They are found in the behavior of organized insects and in the behavior of fluids. The visual patterns produced by fractals are very characteristic (see Figure 6.~) :Enc: clopi;:dia of Ph: sics: Eds. Li;:mer RG & Trigg GL. \'CH Pub!. Inc.. 1991. Page . 5 . ~ ;{) -/qt:-~ ~ ('J{J (1/c'f/71' A heterodv-ne is a mathematical function. governed '- bv " mathematical equations, just like a fractal. A heterodyne also produces self~similar patterns of numbers, like a fractal. If graphed, a heterodyne series produces the same familiar visual shape and fonn which is so characteristic of fractals. Com- pare the heterodyne series in Figure 5. with the fractal series in Figure 6. Figure 6. - Fractal diagram Heterodynes are fractals - the conclusion is inescapable. Heterodynes id and fractals are both mathematical functions which produce a series of self- similar patterns or numbers. All wave energies combine in heterodyne patterns. Thus, all wave energies actually combine in fractal patterns. Once you understand that the fundamental process of combining energies is itself a fractal process, it becomes easier to understand why so many creatures and systems in nature also exhibit fractal patterns. The fractal processes and patterns of nature are built in at a fundamentally basic level. As the scorpion said to the frog, "It's in my nature."3 Well, you say, nature is made up of more than just energy. There is quite a bit of matter out there also. What about matter? Good question. After all, if energy can transfer and combine by heterodyning, what about :The scorpion asked the frog to give him a ride across a stream. He promised not to sting the frog. Just as the frog carried the scorpion to the other side of the stream. the scorpion stung him ;.in~\\a:,. When the frog :.isked \\h:,. the scorpion replied...It's in m: nature:· Page - 6 ~ / ,;J-#-f9" • tJ "-' fl/4 lr1 matter? Chemical reactions occur between atoms and molecules, not just -- between little bits of enern:v. The answer is, matter interacts bv heterodvning "' ,, ....... also. All matter. whether large or small, has \vhat is called a natural oscillatory frequency. The natural oscillatory frequency (NOF) of an object, be it atom or elephant, is the frequency at which the object prefers to vibrate, once set in motion. The ~OF of an object is related to many factors such as size, dimensions. and composition. Basically, the smaller an object is. the smaller the distance it has to cover when it wriggles back and forth. The smaller the distance. the faster it can wriggle back and forth, and the higher its NOF. Consider a wire composed of metal atoms. The wire has a natural oscillatory frequency. The individual metal atoms also have unique oscillatory frequencies. The NOF of the atoms and the NOF of the wire heterodyne by adding and subtracting, just the way energy heterodynes, NOFatom + NOF-.1,m: = Added Frequencyatom-\\Ire, and NOFatom - NOF\\ire = Subtracted Frequencyatom-\\lre' If you stimulate an atom on the wire with the Subtracted Frequencyatom- \1.ire, ( this experiment was actually done using a laser to generate the subtracted frequency 4 ) it will heterodyne (add) with the N O F .,..ire to produce NOFatom• the natural oscillatory frequency of the atom, Subtracted Frequencyatom-.. ire + NOF\\ire= NOFatom· Matter heterodynes with matter the same way that wave energy does. This means that matter combines in fractal processes also. It now becomes doubly easy to understand why so many creatures and systems in nature exhibit fractal processes and patterns. Matter, as well as energy, combines by the mathematical equations of heterodynes, to produce harmonics and fractal patterns. That is why we see fractals everywhere we turn. There is a third leg to this stool. We already know that energy heterodvnes with energv. and matter heterodvnes with matter. Can matter ,., 1,,_.., ., heterodyne with energy? The ans\ver is yes. Remember our metal wire and atoms? The Subtracted Frequencyatom-1,1,ire in the experiment \Vas provided by a laser. using electromagnetic wave energy at a frequency equal to the Subtracted Frequencyatom-1,1,ire· The matter in the wire via its' natural oscillatory frequency, heterodyned with the laser's wave energy frequency. to produce the frequency of an individual atom of matter. So, energy and matter do heterodyne with each other. This of course leads to the interesting curiosity ofjust how matter and energy interact. It helps in understanding chemical reactions, to understand energy and matter interactions. When energy encounters matter, one of three things generally happens. The energy either bounces off the matter (reflected energy), passes through the matter (transmitted energy), or combines with the matter (heterodyning). The crucial factor which detennines which of these three things will happen is - you guessed it - the frequency of the energy ,,,,,compared to the frequency of the matter. If the frequencies do not match, the ::;,;energy will either be reflected, or will pass on through as transmitted energy. :;~If the frequencies match either exactly or as harmonics (heterodynes), then ;:~:the energy combines with the matter. ,.... Think of matter as a glass of water. When energy heterodynes and !:~~combines with the matter, it is just like adding another spoonful of matching :;"''water to the glass of water. It all mixes and combines together until you 1::1:cannot tell what was in the glass and what was from the spoon. Add a spoonful of oil to the glass of water however, and things will be quite different. The oil and water don't match, so they will not mix. The oil will float to the top of the glass, like reflected energy. So when the frequencies of matter and energy match, they will ·combine by heterodyning. When the frequencies do not match, they will not combine, and the energy will either be reflected or transmitted. There is another term often used for matching of frequencies. That term is resonance. A lot is written about resonance, much of it erroneous. It all boils down to one simp.le concept. Resonance means the frequencies match. If the frequency of energy and the frequency of matter match, they are in resonance and the energy combines with the matter. Resonance, or frequency matching, is merely an aspect of heterodyning that enables the coherent transfer and combination of energy with matter. So in our example above with the wire and atoms, we could have created resonance with the atom. and transferred energy to it directly. by stimulating it with a laser exactly matching its NOF. We would be 53/' Page -8~ J;f!JJ3•/,;2-/if- 0 rs 1~,r/2; energizing it with its· resonant frequency. Or, as the clever scientists did \Vho perfonned the experiment, we could take advantage of the heterodyning that naturally occurs between frequencies of energy and matter. We can produce the resonant frequency of the atom (NOFatom) indirectly, as an additive (or subtractive) heterodyned frequency, between the resonant frequency of the wire (NOFv.m:) and the frequency of the laser. Either direct resonance, or indirect resonance through heterodyned frequency matching produces resonance and combining of the matter and energy. Another indirect form of resonance not used by the scientists in that experiment is quite familiar to music lovers in the form of harmonics. A harmonic is a frequency that is an integer multiple of the resonant (NOF) frequency. For instance, we can play the note "A" at 440 Hz. If we double that frequency to 880 Hz. we will play "'A'' an octave higher. This first octave is called the first harmonic. Doubling our note or frequency again, ".Jrom 880 Hz to l 760 Hz (four times the frequency of the original note) gives ::i1s another "A", two octaves above the original note. This is called the third :\~armonic. Every time you double the frequency you get another octave, so i:'.~hese are the even integer multiples of the resonant frequency. '" In between the first and third harmonic is the second harmonic, which ;.,::is three times the original note. This is not an octave like the first and third 0 ;, harmonics. It is an octave and a fifth, equal to the second "E" above our ;:;:roriginal "A". All of the odd integer multiples are fifths, rather than octaves. i::,-;Because harmonics are simply multiples of the fundamental natural !:,;=oscillatory frequency, they stimulate the NOF or resonant frequency :::':indirectly. Play the high "A" at 880 Hz on a piano, and the string for middle ,,,,"A" at 440 Hz will also begin to vibrate. Matter and energy in chemical reactions respond to harmonics of resonant frequencies much the way musical instruments do. We can stimulate the resonant frequency of the atom (NOFatom) indirectly, using its' harmonics. This is because the harmonic frequency heterodynes with the resonant frequency ofthe atom itself(NOFatom). Say, for instance our laser is tuned to 800 Thz and our atom resonates at 400 Thz. Heterodyning the two frequencies gives us: 800 Thz - 400 Thz = 400 Thz. The 800 Thz (the atom's first harmonic), heterodynes with the atom· s own resonant frequency, to produce the atom's own resonant frequency. Thus, the Pm!e - -9-~ f.£lJJJ) / 4 ) - / c f - f / ' r rJfJ fl/["( fr, first harmonic indirectly resonates with the atom's NOF, and stimulates the atom's resonant frequency as a first generation heterodyne. - Of course, the two frequencies will also heterodyne in the other direction, producing: 800 Thz .... 400 Thz = 1,200 Thz. The 1,200 Thz frequency is not the resonant frequency of the atom. Part of the laser's energy will heterodyne to produce the resonant frequency of the atom. The other part of the laser's energy heterodynes to a different frequency, that does not itself stimulate the resonant frequency of the atom. That is why the stimulation of an object by a harmonic frequency of particular strength or amplitude, is always less than the stimulation by its' own exact resonant (NOF) frequency at the same strength. "'"' The preceding discussion highlights the fact that the same fundamental ::;;iprocess ofheterodyning governs all the interactions of matter, energy, and :\,~their combinations. It was this very similarity in process that prompted /:~~Albert Einstein to write several important papers in 1905 modeling atomic it,events using statistical methods of chemical dynamics. His papers provided :::1~the springboard for the later development of relativity and quantum theories. ::· One of these papers described interactions of energy with matter in his :;:~explanation of the photoelectric effect5. He postulated that electromagnetic energy (ie. a beam of light) was made up of small packets of energy, called photons. Based on the photon concepts in his landmark paper, textbooks now teach that the energy E of a photon is related to its' frequency f, by the equation, E = hf, in which h is a universal constant, called Planck's constant. When a packet or photon of electromagnetic (EM) energy strikes an atom, it may transfer its energy to the atom if their frequencies match. Then the photon ceases to exist as an individual packet of energy. It blends and merges with the electron cloud of the atom, just like a spoonful of water blending into a cup of water. 'Einstein A: On a Heuristic Point of View Concerning the Production and Transfonnation of Light: Annalen da Physzk. l9 0 5 ~ _ /Cf-45 Pa2e-l / c;J / 7 - ;vf5 flJ.rf,7 Einstein believed that quantum mechanical descriptions were not the whole story, and that they might someday be replaced by a more complete theory. Over his objections, his energy quanta theories were used to develop theories in which the entire concept of the quantum state was inherently statistical. He is often quoted as cautioning. "God does not play dice with the universe." There may yet be good reason to heed that admonition. - The original intent of his work \\as misunderstood bv ~ manv. . Take for example the modern definition of a photon. Current science teaches that the energy of a photon is equal to Plank's constant multiplied by the frequency. Thus. the higher the frequency, the higher the energy. Textbooks are full of examples and explanations of scienti fie phenomena based on ''high frequency photons'' having high energy, as opposed to "low frequency photons" with lower energy by comparison. But Planck's constant and photon frequency are both arbitrarily defined in units of Joule·seconds and seconds· 1 , respectively \Vhen multiplied, the ::~:econds cancel out and a unit of energy is obtained. The unit of energy, i;J,jowever, is merely the amount of energy in one second's worth of EM !,waves. If one calculates the amount of energy in one wave of EM energy, it ;;~ms out that all EM energy has the exact same amount of energy per wave, ::~o matter what the frequency (Appendix B). High frequency EM energy has ~:the same amount of energy in one wave, as low frequency EM energy has. •• The "high energy photons" simply fit more waves into a second than ;Jjhe low energy photons. Naturally, if all waves have the same amount of Cinergy, and you double the number of waves, you will double the energy. :;2[he whole concept of photons with different amounts of energy is an artifact 1"Which detracts from an understanding of the processes around_ us. So what is going on around us in the process of chemical reactions? Ostwalds's theories on catalysts and bond formation were based on the kinetic theories of chemistry from the turn of the century. We know quite a bit more than Ostwald did when he proposed his theory. We now know that chemical reactions are interactions of matter, and that matter interacts '"'ith other matter through heterodyning of frequencie?. We know that energy can just as easily interact with matter through this same process of heterodyning. With the advent of spectroscopy (more on that later) we also learned that matter produces electromagnetic energy at the same frequencies at which it vibrates. Energy and matter move about and recombine with other energy or matter. depending on their frequencies. In many respects. both philosophically and mathematically, both matter and energy are frequency. This leads to the inescapable conclusion that since chemical reactions are recombinations of matter driven by energy, they are in effect~ driven just as much by frequency. Well, this all sounds well and good, but if we analyze a reaction, is this what is really going on? Actually, it is. And a good reaction to look at is the formation of water from hydrogen and oxygen gases, catalyzed by platinum. Platinum has been known for some time to be a good hydrogen catalyst, although the reason for this has not been well understood. Pt This reaction is proposed to be a chain reaction, dependent on the generation and stabilization of the hydrogen and hydroxy radicals. The j 1proposed reaction chain is: -+-+--+-..-+--+-+_. ½H.. 1 H i 1 i H + 0 2 + H2 i 1 T H..O + OH· T 1 T OH·+ H2 T 1 Creation and stabilization of the hydrogen and hydroxy radicals are thought to be crucial to this reaction chain. Under nonnal circumstances, - h.v, drog......en and oxvgen -''-" gas can be mixed together '-" in a container indefinitelv., . and thev., do not form water. \\'henever the occasional h.vdro-g-en molecule splits apart. the hydrogen atoms do not have enough energy to go out and Pag~-1~~§-/~~ - ~' AJ/lJ 17,{' 300" C Benzene + 2H2 Pd catalyst 2. Cyclohexene < 300' C -------- --- ..... Benzene + 2Cyclohexane Pd catalyst The same catalyst with the same reactant. produces quite different products - molecular hydrogen or cyclohexane - depending on the reaction temperature. If catalyst activity is merely a matter of the frequencies it Page -3rflll3 '9-/q,L'frsl1/,f?r produces. then how can this be? If a physical catalyst actually catalyzes a reaction by virtue of the electromagnetic spectral pattern it emits. then why \vould temperature affect catalyst activity? The answer to this question again comes from advances in the field of spectroscopy. \.-1any factors are known to affect the direction and intensity with which a physical catalyst guides a reaction. Temperature is but one of these factors. Other factors include pressure. volume. surface area. solvents and support materials. contaminants. catalyst size and shape. reaction vessel size and shape, electric fields. magnetic fields. and acoustic fields. These factors all have one thing in common. They change the spectral frequency patterns of atoms and molecules. Some of these changes are so well studied that entire branches of spectroscopy have been designed around them. The next several sections will discuss each factor and its' influence on catalyst mechanisms of action, and chemical reactions in general. Temperature Let's start with temperature. At very low temperatures, the spectral pattern of an atom or molecule has clean crisp peaks (Fig. 15.a.). As the temperature increases, the peaks begin to broaden, producing a bell curve type of spectral pattern (Fig. 15.b.). At even higher temperatures, the bell curve broadens even more, to include more and more frequencies on either side of the primary frequency (Fig. 15.c). This phenomenon is called "broadening". These spectral curves are very much like the resonance curves we discussed in the previous section. Spectroscopists use resonance curve terminology to describe spectral frequency curves for atoms and molecules (Fig. 16). The frequency at the top of the curve- f0 - is called the resonance frequency. There is a frequency (f2) above the resonance frequency and another (f1) below it, at which the energy or intensity is 50% of that for the resonant frequency f0• When we discussed resonance curves we used the ratio of: to calculate the resonance quality. Q, for a resonance curve. Spectroscopists use the frequencies a little bit differently. The quantity (t~ - f1) is a measure Page -35,,c-/~~1f,(,, .-\mpl•nude .A.- Frequenc;, ➔ a) Spectral pattern at lov, temperature. Amplitude: \ I / j \j V Frequency ➔ b) Spectral pattern at moderate temperature. . ... Amplitude..- J Frequency ➔ c) Spectral pattern at high temperature. Figure 15. Spectral patterns at different temperatures. the effects of temperature on spectral emissions. a) Spectral pattern at lov,· temperatures \\ith crisp narrow peaks. b)Spectral p::mern at moderate temperature \\ ith broader peaks. cl Spectral pattern .lt high temperature \\ith mw.:h broadc-ning of pc-aks. lJfJ/Ja_,/<72--t3/- Page - 3 ~ • . ,J(J '1/tt(rf of how wide or narrow the spectral frequency curve is. Spectroscopists call this quantity (t~ - f1) the ··line width". A spectrum with narrow curves has a small line width, while one with \\:ide curves has a lar.g....e line width. Amplitude: Line \\idth I f, Frequency ➔ Figure 16. Spectral curve showing line width (f: - f1). Temperature affects the line width of spectral curves. Line width can affect catalyst performance and chemical reactions. At low temperatures, the spectral curves of chemical species will be separate and distinct, with little transfer of resonant energy (Fig. 17.a). As the line widths of the chemical species get broader and broader, their spectral curves may start to overlap with matching frequencies (Fig. 17.b.). When the frequencies match at higher temperatures, resonant transfer of energy takes place and the reaction can proceed in a different direction than it otherwise would have at a lower temperature. Besides affecting the line width of the spectral curves, temperature also can change the resonant frequency and the amplitude of the curves. For some chemical species the resonant frequency will shift as temperature changes. This can be seen in the infrared absorption spectra and blackbody radiation graphs in Figure 18. Atoms and molecules do not all shift their resonant frequencies by the same amount or in the same direction, at the same temperatures. This can affect catalyst performance. If a catalyst resonant frequency shifts more with increased temperature than the resonant frequency of its· target chemical. then resonance may be created where none pre\'iously existed. (Fig. 18.c.} Amplitude: j _jl L II ______,,, Frequency ➔ a) Separate and distinct spectral curves at lo\\ tetnperature. Amplitude : Frequency ➔ b) Overlapping spectral curves at higher temperature. allowing resonant energy transfer. Figure 17. Effects of temperature on spectral curve line widths and resonant transfer ofenergy. I::? 10 I 6 ),4 a) Lowering of resonant frequency (increasing wavelength) as temperature increases in right hand set of curves. Figure 18. Effects of temperature on resonant frequency. a. Influence of temperature on the resolution of infrared absorption spectra''' Page -3 •'/~tc(--£7~ /\Jf f (1/r'(f, f I 11 20-•1 I I ,;;- I00 ~j-1 ' 8 i I c I I :i 80- -, I \ I "' 6-.) -1 0 5 20 >... ll 10 4 (cm) 18.b. Spectral radiant emittance of a blackbody: (a) 800 K. (b) 1200 K. (c) 1600 K. (d) 1600 K {Wien). (e) 1600 K (Rayleigh). (t) 6000 K. (g) 10.000 K. The decrease of resonance wavelength. and hence increase of resonance frequency. is sho-v.n by the dashed curve. 11 A Figure 18.c. At low temperature the catalyst (C) does not resonate with target chemical species (A). At high temperature. the catalyst's resonant frequency shifts and resonance exists between C and A. The amplitude or intensity of a spectral line may be affected by temperature also9. For instance, linear and symmetric rotor molecules will increase intensity as the temperature is lowered. Conversely, rotational or vibrational spectra may decrease intensity as the temperature is lowered. These changes of spectral intensity can affect catalyst performance. Consider the example where a low intensity spectral curve of a catalyst is resonant with a chemical target"s frequency. Only small amounts of energy can be transferred from the catalyst to the target chemical. Say that as the temperature increases, the amplitude of the catalyst's curve increases also. In . -- this case the catalvst can transfer much larg;er amounts of energy to the ~ chemical target. If the chemical target is the intermediate chemical species for an alternative reaction route. the type and ratio of end products may be affected. Look at our cyclohexene, palladium reaction again. At temperatures beloir 300 C the products are benzene and hydrogen gas. When the temperature is above 300 C the products are benzene and cyclohexane. Temperature is affecting the palladium and/or the rest of the reaction system (including reactants, intermediates, and products) is such a way that an alternative reaction pathv.,·ay leading to the formation of cyclohexane is favored above 300 C. This could be a result of increased line width, altered resonance frequencies, or changes in spectral curve intensities for any of the chemical species in the reaction system. It is important to consider the spectral catalyst frequencies under the reaction conditions we want to duplicate. Say for instance that at temperatures above 300 C. the reaction system is unaffected but the palladium has an increased line width. lower resonant frequency, and increased intensity. Also say for instance that the wider line width and lower resonant frequency interact with an intermediate important for the formation of cyclohexane. If we wanted to spectrally catalyze the formation of cylcohexane at room temperature, we would need to use the wider, lower spectral catalyst frequency for palladium above 300° C. Thus it can be important to understand the reaction system dynamics in designing and determining a spectral catalyst. Otherwise, one is reduced to using random, trial and error or feedback-type analyses which; although they \Vill eventually identify the spectral catalyst frequencies, will be very timeconsuming. The trial and error techniques for determining spectral catalysts also have the added drawback, that having once identified a frequency, one is left with no idea of what it means 12. If one wishes to modify the reaction, another trial and error analysis becomes necessary rather than a simple, quick calculation. Pressure Pressure and temperature are direct!) related to each other. Everyone . probably remembers the ideal gas law from high school chemistry: PV = nRT, where Pis pressure.Vis volume. n is the number of moles of gas. R is the gas constant, and Tis the absolute temperature (Celsius plus 273 - ). All things being equal. if temperature increases. so does pressure. Given this similarity between the effects of pressure and temperature in chemical systems. one might wonder if they have similar effects on spectral emission patterns. The answer is yes. Increased pressure causes broadening and other changes in spectral curves. just as increased temperature does (Fig. 19). 1ox10·• , - - - - - - - - - , - - - - - . . . - - - - , - - - - - - , PRESSURES: o p=C8JmmHt; • p:OVmmHt; Cv2pZ ~ CURVES: ( v·vo)Z T(2~4 p)Z ;:- 6x10-• 1------,....--.....-----.tt-t-+--+-1t----+------1 '§ 2c· o 4,110-• 1------+---+-__,,...,-'--t--'rt----+------1 a. § 0 ~ 2x10-• 1-----,.51---=+--.:;.<---+--+-l'--"11,1-'s,1,1 1057.9 MHz ••. _ _...,.__ _ 2pl/,I Lyma, a at 1215.67 A n•l 'sin.· .-·· _£a.6 GHz 2s,,2 Figure 34. Fine structure of then= I and n =2 levels of the hydrogen atom. 19 «w cm' !-?;n ,~p_, Z26.S cm•-1_ _ 43.5 ~ 2 16..4 ,_,.·1 30 wt•~-1 0 cm C Po o em-'-0 -3P2 ° 0 Cm_-1 .__...2..pl/2 F Figure 35. The multiplet splittings for the lowest energ:: !'evels for Carbon. Oxygen. and Fluorine. -1-3.5 cnf = 1.3 THz. 16.-1- cm··= 490 GHz. 226.5 cm·1 = 6.77 THz. 158.5 cm· 1 =4.74 THz. 404 cm 1.:.1 THz. Page -63 Now that you know atoms have fine structure frequencies, which are split apart by differences or intervals (heterodynes) called ·fine splitting fre- quencies, you are probably beginning to wonder about molecules as well. Molecules also have fine structure frequencies. The origin and derivation for molecular fine structure and splitting is a little different from that for atoms, but the graphical and practical results are quite similar. In atoms, the fine structure frequencies are said to result from the interaction of the spinning electron \Vith its· own magnetic field. Basicallv, this means the electron cloud ~ ~ of a single atomic sphere, rotating and interacting with its' own magnetic field, produces the atomic fine structure frequencies. In the literature, you will find this referred to as ·"spin-orbit coupling·•. For molecules, the fine structure frequencies are the actual rotational frequencies of the electronic or vibrational frequencies. So the fine structure frequencies for atoms and molecules both result from rotation. In the case of atoms, it is the atom spinning and rotating around itself, much the way the earth rotates around its axis. In the case of molecules, it is the molecule spinning and rotating head- over-heels. like a candy bar tumbling through the air. Take a look at Figure 36. This shows the infrared absorption spectrum of the SF6 vibration band near 28.3 THz (10.6 µm wavelength, wavenumber Figure 36. The vibration band ofSF6 at wavelength IO µm. 1 948 cm·1) of the SF6 molecule.2 The molecule is highly symmetrical and rotates like a top. The spectral tracing was obtained with a high resolution grating spectrometer. There is a broad band between 941 and 952 cm·' (28. l and 28.5 THz) with three sharp spectral curves at 946, 947, and 948 cm·1 (28.3, 28.32. and ~8.34 THz). Now what if we take a narrow slice of the broad band and look at it in more detail? Figure 37.a. shows a narrow slice being taken from between 949 and 950 cm· 1, which is blown up to show more detail below and in 37.b. A tunable semiconductor diode laser was used to obtain the detail. There are many more spectral curves which appear when we look at the spectrum in finer detail. These curves are called the fine structure frequencies for this molecule. 3850 MHz .. ......... ·• ·• ·•,....... ~ ....... co. ... -,.... a) Slice from vibration band of SF6. Ill MIi '"29' 3850 MHZ b) Fine structure frequencies sho\rn in detail. Figure 37....\ narrow slice from the vibration band of SF~· blown up to sho,\ more detail. with the tine structure frequencies. Page -65- /;7-/t:r-7? µIf I ! 1/.rfn -- . The total enern:v of an atom or molecule is the sum of its' electronic vibrational. and rotational energies. The simple Planck equation discussed in the Background section: E = hf. can be rewritten as follows: where Eis the total energy, Ee the electronic energy, Ev the vibrational energy, and Er the rotational energy. Diagrammatically, this looks like Figure 37. for molecules. The electronic energy,£"', involves a change in the orbit of one of the electrons in the molecule. 10 It is designated by the orbital number n = O, 1, J V n ➔ 3 2 I 3 4 3 ;: : 2 4 3 2 4 __ 3 __ 2_ _ -----3 4 __ 3_ _ 2 __ -----2 Figure 38. Diagram of rL'tational (J ). \ ibrativnal (\"). and ele-:tr0nic (n Jener~ le\ ds for a molecule. 2, 3, etc. The vibrational energy, E._., is produced by a change in the vibration rate benveen two atoms within the molecule 1°, and is designated by a vibrational number v = l, 2, 3, etc. Lastly, the rotational energy, Er, is the energy of rotation caused by the molecule rotating around its' center of masst0. The rotational energy is designated by the quantum number J = l, 2, 3, etc .. as detennined from angular momentum equations. Thus. \.vhen we look at the vibrational frequencies of SF6 in more detail, we see the fine structure molecular frequencies. These fine structure frequencies are actually produced by the molecular rotations, J, as a subset of each vibrational frequency. Just as the rotational levels J are evenly separated in Figure 38. they are also evenly separated when plotted as frequencies. This may by easier to understand by looking at some other frequency diagrams. Figure 39 shows the rotational spectrum for hydrogen chloride21 . In Fig. 39.a., the separate waves that look like teeth on a comb, are the individual rotational frequencies. The whole big wave (the whole comb) that goes from 20 to 500 cm· 1 is the entire vibrational frequency. At low resolution, this would look like a single frequency peaking at 20 cm •1 (598 GHz) (Fig. 39.b.). This is very similar to the way atomic frequencies such as 456 THz look like just one frequency at low resolution, but tum out to be several different frequencies at higher magnification. 500 380 260 140 20 a) .:: 1 lnfra-rt!d Spt!Ctroscop:, and \.folecular Structure. \.I Da\leS. El· • r Publ. Co. I963. /c7-/ct--?P7 NPS rz/t{,;7 500 380 260 140 20 bl Figure 39. The rotational spectrum for hydrogen chloride shO\\."ing fine structure detail. a) Pure rotational absorption spectrum of gaseous hydrogen chloride recorded "'1ith an interferometer. b) The same spectrum at low resolution. In Figure 40, the rotational spectrum (fine structure) of hydrogen cyanide is shown.20 Note again the regular spacing of the rotational levels. (This spectral tracing is oriented the opposite of what we are used to. It uses transmission rather than emission on the horizontal Y axis, so our familiar intensity notation increases as you go down on the Y axis, rather than up.) ;,C 25 30 , I v\ I .2C • 20 .... .- ..... -. .,,,,,.. -....... ....._ Figure -+0. The rotational spectrum for h:, drogen c: anide. ··r is the rotational level. Page -68 1cJ~1(-ff/ 0/f l!/tc,f.,f Finally. for a veritable symphony of rotational frequencies, take a look at Fig-ure 41, showing- a vibrational band for FCCF22. All of the fine sa\\1ooth spikes are the fine structural frequencies, corresponding to the rotational frequencies. Note the regular spacing of the rotational frequencies. Also note the undulating pattern of the rotational frequency intensity, as well as the alternating pattern of the rotational frequency intensities. 27!JO cm l 27201 ---- I ------ 7' ] •Cl ,.. Ptel I i ~- I I I I ,I '41 ctO l11J I I C 91 I a10 ~•:!) i 11 rfl oS d 2702 cm 1 I ;! I ii i: ,.,_ ' 1 j 'I ' ' ..~~49, .; •l,J D 270) I ~3 I 0 11,r, Figure 41." 1 The v, - v5 band of FCCF. ,·,. is vibrational level one. and v5 is vibrational level 5. The vibrational level frequencies heterodyne (add and subtract) just like all frequencies. The additive heterodyne of,.,. and v.- are depicted in the spectral band showing the frequency band at ··.-1 •• ri·: - ,·_.). B = i·. - ~v;. -- \'ibrat10n-R0tat1ona! Spectros.:op~ and \ 1olecular D~ n:imics. Ed Papousek.. World Sci.. I QQ7 G,_,/c(-9f (UI 1 r1.1tr4r Consider the actual rotational frequencies (fine structure frequencies) for the ground state of carbon monoxide listed in Figure 42. Figure 42. Rotational Frequencies and Derived Rotational Constant for CO in the Ground State23 J Transition Frequency (\-fHZ) Frequencv iOF of the wire heterodyned by adding and subtracting, the way all frequencies heterodyne, NOF:iwm - NOFv.ire = Subtracted Frequency.itom-v.ire- \Vhen they stimulated an atom on the wire with the Subtracted Frequencyatoml\ire• it heterodyned (added) with the NOF\,ire to produce NOFatom, the natural oscillatory frequency of the atom, Subtracted Frequencyawm-v.ire + NOF1,ire = NOFatom· The rotational frequencies of molecules can be manipulated the same way. The first rotational level has a natural oscillatory frequency (NOF) of 89,740 !v1HZ. The second rotational level has a NOF of 179,470 ~1HZ. NOFrotational 1➔2 - NOFrotat1onalo-+I = Subtracted Frequency rotational2-t· 179,470 MHZ - 89,740 MHZ - 89,730 MHZ. The NOFs of the rotational frequencies heterodyne by adding and subtracting, the way all frequencies heterodyne. The two rotational frequencies heterodyne to produce a subtracted frequency. This subtracted frequency happens to be exactly twice as big as the derived rotational constant "B" listed in nuclear physics and spectroscopy manuals. When you stimulate the first rotational frequency in the molecule with the Subtracted Frequency rotational 2.1, it will heterodyne (add) \Vith the NOFrotational o-+t (first rotational frequency) to produce NOFrotauonal I➔:?, the natural oscillatory frequency of the molecule's second rotational level. Subtracted Frequency rotat10r.:ii 2-1 + NOFrotat1onai 0-tl = NOFrot:monal I ➔:? 89,730 MHZ + 89.740 MHZ = 179,470 ~1HZ Since the rotational frequencies are evenly spaced harmonics. the subtracted frequency will also add with the second level NOF to produce the third level \;OF. It will add with the third level NOF to produce the fourth level NOF . .-\nd so on and so on. By using one single microwave frequency you will stimulate all the rotational levels in a vibratory band. Now if you excite all the rotational levels for a vibrational frequency, you will excite the vibrational frequency also. And if you excite all the vibrational levels for an electronic level, you will excite the electronic level as well. Thus, one can drive up into the highest levels of the electronic and vibrational structure of the molecule using a single microwave frequency. That is why the spectroscopists are wrong in stating that the use of microwaves restricts you to the ground state of the molecule. If you are trying to resonate directly with an upper vibrational or electronic level you cannot use a microwave frequency. If, however~ you imitate a mechanism of action from the catalysts - namely resonating with target species indirectly through heterodynes, you can use a microwave frequency to energize an upper level vibrational or electronic state. \Vith an understanding of the simple processes of heterodyning it becomes readily apparent why microwave frequencies do not really limit one to the ground state levels of molecules. Catalysts use this trick, of stimulating target species indirectly by zapping them with heterodyned frequencies (harmonics). Catalysts also stimulate the target species by direct resonance with the fundamental frequency of interest. Here the rotational frequencies give us a bonus. Take a look at the first frequency in Figure 42. The first rotational frequency for CO is 115 GHz. The heterodyned difference between rotational frequencies is also 115 GHz. The first rotational frequency and the heterodyned difference between frequencies are identical. All of the upper level rotational frequencies are harmonics of the first frequency. This relationship is not as apparent when one deals only with the rotational constant "B", but a frequency based analysis makes it easier to see. Take a look at the first level rotational frequencies for LiF as well. It is nearly identical to the heterod) ned difference bet\veen it and the second level rotational frequency. The rotational frequencies are sequential harmonics of the first rotational frequency. Nov~ if you stimulate a molecule with a frequency equal to 2B. the heterodyned harmonic difference between rotational frequencies, you will kill two birds with one stone. You will resonate with all the upper rotational frequencies indirectly through heterodynes, and you will also resonate directly with the first rotational frequency. There are a whole host of constants used in spectroscopy that relate in some \vay or another to the frequencies of atoms and molecules, just as the rotational constant --B'' relates to the harmonic spacing of rotational fine structure molecular frequencies. The alpha (a) rotation-vibration constant is a good example of this. The alpha rotation-vibration frequency constant is related to slight changes in the frequencies for the same rotational level. when the vibrational level changes. Take a look at the frequencies for the same rotational levels, but different vibrational levels of LiF in Figure 44. This has been reformatted in Figure 45. The frequencies are almost the same, but vary by a few percent as one moves between Yibrational levels. The rotational transition 0 ➔ l has frequency 89,740.46 ~1HZ at vibrational level 0. At vibrational level 1, the 0 ➔ l transition is 88,319.18 :\1HZ, and at vibrational level 2 the 0 ➔ 1 transition is 86,921.20 MHZ. These slight differences between the same J rotational level for different vibrational levels are related to harmonics of the alpha frequency constant. Figure 45. Rotational and Vibrational Frequencies for LiF Rotational Transition O➔ l 1 ➔2 2➔3 3 ➔4 4➔5 5➔6 89,740.46 179,470.35 269,179.18 358,856.19 448,491.07 538,072.65 88,319.18 176,627.91 264,915.79 353,172.23 441,386.83 86,921.20 173,832.04 260,722.24 347,581.39 171,082.27 256,597.84 342,082.66 Vibrational 0 I 2 3 Level k]-~/cf-9/f /IJ(J I t/1 ft1 Consider the rotational and vibrational states for OCS shown in Figure -1-6.~ This figure shows the same rotational level (J = 1 ➔ ·2) for different vibrational states in the OCS molecule. For the ground vibrational (000) level. J = l ➔ 2 transition; and the excited state ( l 00) J = 1 ➔ 2 transition, the difference between the two frequencies is equal to 4 X alpha1 (4a 1). In another excited state. the frequency difference bet\veen the ground vibrational (000) level. J = l ➔ 2 transition. and the center of the two /-type doublets 4 X alpha2 (4a2). In a higher excited vibrational state, the frequency difference between (000) and (0t)0) is 8 X alpha2( 8a2). Thus it can be seen that the rotation-vibration constants "a" are actually harmonic molecular frequencies. Stimulating a molecule with an a frequency or a harmonic of it will either directly resonate \Vith or indirectly heterodyne harmonically with various rotational-vibrational frequencies for the molecule. 13001 10011 12001 W!Ol 11001 ln:01 lOOOI 101:01 102'101 (03l0) 121101 111~1 (12°ol IOl~I 103101 IOI-at Figure 46". Rotational transition J = l ➔ 2 for the triatomic molecule OCS. The vibrational state is given by vibrational quantum numbers in brackets (v I v: v 3 ), V: having a superscript i/i. In case I = l. a subscript l is applied to the lower-frequency component of the /-type doublet. and 2 to the higher-frequency component. The two lines at (01 10) and (01 10) are an /-type doublet. s~parated by q1.. Another interesting constant is the /-type doubling constant. This can also be seen in Figure 45. Just as the atomic frequencies are sometime split into doublets or multiplets, the rotational frequencies are also sometimes split into doublets. The difference between them is call the /-type doubling constant. These constants are usually smaller (lower frequency) than the a constants. For OCS the a constants are 20.56 and 10.56 \1HZ, while the/- type doubling constant is 6.3 ~-tHZ. These frequencies are all in the radiow~n·e portion of the electro-magnetic spectrum. 1%!?) /cJ-/cf--9'~ Page - 7 ~ lJfJ rYr,[r, \Veil after that lengthy (and hopefully not too boring) discourse on fine structure frequencies of atoms and molecules, this seems like a good time to come up for a breath of fresh air, and discuss just what all this has to do with spectral catalysts and chemical reactions. \Ve shall begin by considering the case of atomic fine structure frequencies and splitting frequencies. From there we \Vill move on to applications of molecular fine structure frequencies. By way of review, the electronic frequencies seen in the atomic spectra are not neat crisp individual curves. If we look at them in sufficient detail, we see that each curve is made up of several other curves spaced so closely together that they just look like one curve or line on a conventional spectrum. The 456 THz frequency for hydrogen is actually made up of 7 different but closely spaced frequencies. These are the fine structure frequencies. The amount that the closely spaced frequencies are split apart is called the fine splitting frequency. Energy is transferred by two fundamental frequency mechanisms. If the frequencies are the same, then energy transfers by direct resonance. Energy can also transfer indirectly by heterodyning, ie. the frequencies match after having been added or subtracted with another frequency. The direct or ;, indirect resonant frequencies do not have to match exactly. If they are merely close, a lot of energy will still transfer. In our reaction combining hydrogen and oxygen to form water, we learned that energizing the reaction intermediates of atomic hydrogen and the hydroxy radical are crucial to sustaining the reaction. The catalyst platinum energizes both reaction intermediates by directly and indirectly resonating with them. Platinum also energizes the intermediates at multiple energy levels, creating the conditions for energy amplification. Let's say we want to copy platinum's mechanism of action by making use of atomic fine structure frequencies. If we directly resonate with the fine structure frequencies we will simply resonate with the same frequencies we discussed earlier with only slight variations between the frequencies (456.676 and 456.686 THz for example). This is no different from what we did earlier and has no advantages to what we already know. If we indirectly resonate vvith the fine structure frequencies, howe\·er. it is another story altogether. How do we indirectly resonate with the atomic tine structure frequencies? By using the fine splittirnz frequencies which are -~ lcJ~/¢:.-7f/ Page - 7 7 ~ (Vf!J fl/.,fr, simply the differences or heterodynes betv.een the fine structure frequencies. Look at the hydrogen 456 THz fine structure and fine splitting frequencies for hydrogen (figures 30, 32, and 34). 456.686 THz - 456.676 THz = 0.0102 THz = 10.2 GHz If we irradiate hydrogen atoms \.vith 10.2 GHz electromagnetic energy (micro\vaves), \Ve \vill energize the 456 THz electronic spectrum frequency by - resonatirnr with it indirectlv. The l 0.2 GHz will add to 456.676 THz to ~ produce the resonant frequency of 456.686 THz. The 10.2 GHz will also subtract from the 456.686 THz to produce the resonant fr.equency of 456.676 THz. Thus zapping a hydrogen atom with l 0.2 GHz will energize its' overall 456 THz frequency. If \Ve want to use a combination of mimicked catalyst mechanisms, we can try the following: 1) resonate with the hydrogen atom frequencies indirectly through heterodynes (fine splitting frequencies); and 2) resonate with the hydrogen atom at multiple frequencies. We would do this by using a combination of microwave frequencies either simultaneously, in sequence, or in chirps or bursts. For instance we could use the individual microwave frequencies of 10.87 GHz, 10.2 GHz, 3.23 GHz, 1.38 GHz, and 1.06 GHz in a sequence. Ifwe wanted to use radiowave frequencies, we could use 239 and 448 MHZ in sequence. There are many fine splitting frequencies for hydrogen not listed here, so depending on the frequency range of our equipment, we can Tailor the chosen frequencies to the capabilities of our equipment. Another way to deliver multiple electromagnetic energy frequencies would be to use a lower frequency as a carrier wave for a higher frequency. This can be done for instance by producing 10.2 GHz EM energy in short bursts, with the bursts coming at a rate of 239 MHZ. This can also be done by continuously delivering 10.2 GHZ EM energy and by varying the amplitude at a rate of 239 MHZ. Thus by mimicking the mechanism of action of catalysts and by making use of the atomic fine structure and splitting frequencies we can energize upper levels of atoms using microwave and radiowave frequencies. By knov,;ingly energizing particular atoms, we catalyze and guide the reactions to desired end products. Depending on the circumstances, the option to use lo\\"er frequencies may have many advantages. Lower frequencies have much better penetrance into large reaction spaces and volumes, and may be better -7[JJ,r Page /42 7 0 ~ -/