J . Phys. Chem. 1989,93,1251-1261 studies are in progress for determining the M , function for TFMSA aqueous solutions by using equilibria of different indicators. Preliminary studies using Raman spectroscopy of CF3S03H-HN03-H20 mixtures with H N o 3 as indicatorSoshow for ' a titration curve related instance that in the range 80-100 wt % (50) Sampoli, M.; Marziano, N. C., unpublished results.
7257
to protonation-dehydration equilibrium of nitric acid8 is obtained. H N 0 3 + CF3S03H
H 2 0 + NO2+ + CF3S03-
The curve is similar to that observed in H2SO4-HNO3-H20 mixtures35 and the half-ionization occurs at about the Same mole fraction of the solvent, as expected from the previous discussion. Registry No. TFMSA, 1493-13-6.
Vibrational Pressure Tuning Spectroscopy of the Polymethylene Chain. 1. Various n-Alkanes from C8H,8 to C38H71t W. W. Ley and H. G . Drickamer* Materials Research Laboratory and School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801 (Received: February 27, 1989; In Final Form: May 9, 1989)
In this paper, we present the effect of pressure on the vibrational spectra of a series of 11 crystalline paraffin hydrocarbons from n-C8H18to n-C36H74plus n-CsDl8 and a few branched-chain compounds. The focus is on the increase in intensity of the CHI wagging and C-C stretching vibrations with pressure. The sum of the intensities increases in a manner that depends primarily on chain length. The distribution between the two vibrations depends on the crystal structure. The results imply that the primary event is an increase in the wagging vibration intensity due to increased intermolecular coupling. This intensity is redistributed to the C-C stretching vibration via some lattice mode (phonon-assistedcoupling or phonon-assisted resonance). This resonance is most efficient for the orthorhombic structure with 4 molecules/unit cell and least efficient for the triclinic structure with 1 moleculelunit cell. For n-C~oH62and n-C36H74, which have a monoclinic structure with 2 molecules/unit cell, the behavior is intermediate.
Introduction The properties and behavior of the n-paraffins are of great interest to physical and biological scientists alike since they serve as models and analogues for carbon-based molecules from polyethylene to phospholipids. The solid-state vibrational spectra of polymethylene chain molecules, both the n-alkanes and polyethylene, are well-known, and the theory of their normal modes of vibration has been worked out. N o attempt will be made here to describe this large body of work; instead, the reader is referred to the appropriate references.'-8 Further, and rather elegant, refinements to this body of work have been made by Strauss et al.,g-'l who describe the use of the CD2 probe and the use of high-temperature spectra to better understand these vibrations. There have been previous high-pressure investigations of the vibrational spectra of n-C16H3412 and other polymethylene chain molecules,"'6 but the effects observed and discussed in this paper were not as fully exploited due to the limitations in both the pressure cells and spectrometers available at the time. Experimental Procedure All spectra were recorded by a Nicolet Model I199 FTIR fitted with a 4X Perkin-Elmer beam condenser to focus the IR radiation on the sample. The samples were held in a diamond anvil cell with type-I1 diamonds and confined by an Inconel gasket;17 pressure was measured by the ruby fluorescence method.17 All samples were loaded neat and run for 300-1000 scans, depending on the strength, and hence S / N ratio, of the interferogram. Two samples, n-CZ3H4and n-C24H50,were run surrounded with mineral oil as a pressurizing fluid to verify the hydrostaticity of the neat hydrocarbon pressure runs. The n-alkane molecules CnHZh2( n = 8, 10, 12-16, 23, 24, 30, and 36) were studied as well as the deuterated molecule n-C8D18 and a 50 mol % mixture of n-C8H18 and n-C8DI8. The branched-chain hydrocarbons 2,2,4,4,6,8,8-heptamethylnonane, lThis work was supported in part by the Materials Science Division, Department of Energy, under Contract DE-AC02-76ER01198.
\
-500 1300 1100
Wavenum bers
Wavenum bers
Figure 1. IR spectra of n-CsHls. The CH2 wag is indicated by the heavy arrow; the C-C stretch is indicated by the thinner slanted arrow.
2,6,10,14-tetramethylpentadecane,and 4-methylheptane were studied also. (1) Painter, P. C.; Coleman, M. M.; Koenig, J. L. The Theory of Vibrational Spectroscopy and its Application to Polymeric Materials; Wiley-Interscience: New York, 1982. (2) Snyder, R. G. J . Mol. Spectrosc. 1960, 4 , 411-434. (3) Snyder, R. G. J . Mol. Spectrosc. 1961, 7, 116-144. (4) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85-1 16.
0 1989 American Chemical Society
7258 The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
U
Wovenumbers
Figure 2. IR spectra of n-C15H32.The C H 2 wag is indicated by the heavy arrow; the C C stretch is indicated by the thinner slanted arrow.
Ley and Drickamer TABLE I: Intensity of the CH, Wag vs Pressure pressure/kbar pn/ n kbar 10 30 50 70 90 C8D18 2o 1.13 1.35 1.75 2.40 8 30 1.00 1.04 1.29 1.58 10 40 0.82 1.19 1.56 1.93 12 30 1.00 1.10 1.40 1.83 13 45 1.03 1.15 1.27 14 20 1.29 1.68 2.06 2.45 15 20 1.08 1.16 1.24 1.32 16 15 0.88 1.53 2.17 2.81 3.46 23 50 1.00 1.44 1.93 24 15 0.56 2.10 3.78 5.41 6.96 30 30 1.00 1.63 2.27 2.90 36 20 0.72 1.28 1.83 2.39 2.94
110 3.19 2.00 2.31 2.34 1.38 2.83 1.40 4.10 2.37 8.59 3.53 3.50
130 4.04 2.50 2.68 2.97 1.49 3.22 1.48 4.74 2.89 10.07 4.16 4.06
TABLE II: Intensitv of the C C Stretch vs Pressure n , Dressure/kbar rnl n kbar 10 30 50 70 90 8 20 1.10 1.24 1.35 1.43 10 20 1.05 1.19 1.32 1.55 12 20 0.96 1.06 1.14 1.22 1.50 13 20 0.82 1.18 1.53 1.88 2.24 14 40 0.92 1.08 1.25 1.42 15 20 0.81 1.19 1.56 1.93 2.30 16 20 0.83 1.17 1.50 1.83 2.16 23 20 0.70 1.30 1.87 2.49 3.43 24 20 0.73 1.27 1.82 2.37 2.92 1.44 2.12 3.25 30 20 0.83 1.15 36 20 0.53 1.47 2.41 3.34 4.28
110 1.49 1.89 1.97 2.59 1.59 2.67 2.50 4.45 3.47 4.66 5.22
130 1.50 2.23 2.39 2.95 1.76 3.04 2.83 5.83 4.02 5.98 6.16
TABLE 111: Sum of the CHt Wag and C-C Stretch Intensities vs Pressure pressure/kbar Pnl n kbar 10 30 50 70 90 110 8 20 0.94 1.06 1.19 1.31 1.44 1.57 10 30 1.00 1.37 1.74 2.10 2.47 12 20 0.98 1.02 1.06 1.18 1.48 1.88 13 40 0.84 1.16 1.48 1.79 2.11 14 20 0.89 1.11 1.34 1.57 1.80 2.03 I5 30 1.00 1.40 1.80 2.21 2.61 16 20 0.77 1.23 1.69 2.16 2.62 3.08 23 30 1.00 1.76 2.52 3.29 4.05 24 20 0.65 1.35 2.05 2.75 3.45 4.15 30 30 1.00 1.90 2.80 3.71 4.61 36 20 0.56 1.44 2.33 3.22 4.11 5.00
130 1.69 2.83 2.37 2.43 2.26 3.01 3.55 4.81 4.85 5.51 5.89
attributed to changes in cell geometry. All spectra taken were found to be reversible upon release o the pressure, and all pressure runs were reproducible.
1300 1100
500 1300 1100
Wovenum bers
Wovenum bers
500
Figure 3. IR spectra of n-CI6H3,. The C H 2 wag is indicated by the heavy arrow: the C C stretch is indicated by the thinner slanted arrow.
In a diamond anvil cell, the amount of material in the light path is independent of pressure so that no intensity changes can be
Results Spectra are shown for the normal alkanes CnHZn+2 for n = 8, 15, 16, 23, and 24 in Figures 1-5, respectively. Spectra are also shown for the deuterated molecule n-C&8 in Figure 6 . In our discussion, we concentrate on three overall effects observed as a function of pressure: the methylene wagging mode, indicated by a heavy arrow, increases in intensity with pressure; the carboncarbon stretch, indicated with a light slanted arrow, also increases in intensity with pressure; and the separation in energy between these two peaks diminishes with increasing pressure. In the notation of Snyder and Scha~htschneider,~ the CH2 wag referred to in this paper is W, and the C-C stretch is R2. Other peaks in the spectrum exhibited blue shifts as has been frequently observed for many infrared peaks, and in some cases, small splittings were observed. There was, however, no consistent (10) Maroncelli, M.; Strauss, H. L.; Snyder, R. G. J . Chem. Phys. 1985,
(5) Snyder, R.G.;Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 117-168. (6)Snyder, R.G.;Schachtschneider, J. H. Spectrochim. Acta 1965, 21, 169- 1 95. (7) Sherwood, P. M. A. Vibrational Spectroscopy of Solids; Cambridge University Press: London, 1972. (E) Turrell, G. Infrared and Raman Spectra of Crystals; Academic Press: New York, 1972. (9) Maroncelli, M.; Strauss, H . L.; Snyder, R. G. J . Phys. Chem. 1985, 89, 4390-4395.
82. - - ,281 - - 1-2824.
(1 1) Snyder, R.G.;Maroncelli, M.; Strauss, H . L.; Hallmark, V. M. J . Phys. Chem. 1986, 90,5623-5630. (12) Wong, P. T.T.; Chagwedera, T. E.; Mantsch, H. H. J . Chem. Phys. 1987, 87,4487-4497.
(13) Wu, C.; Nicol, M. J . Chem. Phys. 1973, 58, 5150-5162. (14) Wu, C.; Nicol, M. Chem. Phys. Lett. 1973, 18, 83-86. (15) Wu, C.;Nicol, M. Chem. Phys. Lett. 1974, 24, 395-398. (16) Wu, C.;Jura, G. Spectrochim. Acta 1974, 30A, 797-812. (17) Sherman, W. F.;Stadtmuller, A. A. Experimental Techniques in High Pressure Research; Wiley: New York, 1987.
v
Vibrational Spectroscopy of Various n-Alkanes
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7259
0 kbar
80 kbar
m C
0
c ._ 4-
41 kbar
f E
108 k bo r
8
128 kbar
1500 1300 1100
Wavenum bers
,500 1300 1100
Wavenum bers
Figure 4. IR spectra of n-C2,Ha. The CH2 wag is indicated by the heavy arrow; the C C stretch is indicated by the thinner slanted arrow.
v
1200 1000 800
1200 1000 800
Wavenum bers
Wavenum bers
Figure 6. IR spectra of n-C8D18.The CH2wag is indicated by the heavy
arrow. .
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Pressure (kbar) Figure 7. Intensity of the CH2 wag vs pressure for the following: n-
C24HS0,squares; n-C3oH62, diamonds; n-C23H4s, circles.
500 1300 1100
Wovenumbers
1500 1500
1100
Wavenum bers
Figure 5. IR spectra of n-C2?HSo.The CH2 wag is indicated by the heavy arrow; the C-C stretch is indicated by the thinner slanted arrow.
change in intensity for any of the other peaks in the spectrum with pressure. For the deuterated molecule C8D18,the CD2 wagging mode is 330 cm-' lower in energy than the CH2 wag in CsH18. The intensity of the CD2 wag also increases with pressure, which confirms the identity and behavior of the wagging mode in C8H18 and the other paraffin species. Unfortunately, the C-C stretch peak is obscured by the CD2 and CD, vibrations. All of the intensity changes and peak shifts are shown quantitatively in Tables 1-111 and in Figures 7-9. The intensity data was obtained by integrating the area under the peak, or multiplet of peaks as in the case of C8Di8 and C8H18,and normalizing to
unity at the first reliably measurable pressure, P,. The standard used for peak intensity was the CH2 bending mode at 1470 cm-' that showed no significant intensity change with pressure. The shift data were taken as the simple position of the transmittance minimum of the peak or the intensity-weighted minima of any multiplet. The n-alkane molecules studied have one of three different crystal structures. The molecules with an even number of carbon atoms, n, less than or equal to 26 (Le., n = 8, 10, 12, 14, 16, and 24) crystallize in a triclinic lattice with 1 molecule/unit cell. The odd-numbered species (n = 13, 15, and 23) have an orthorhombic lattice with 4 molecules/unit cell. The even-numbered species, n = 30 and 36, have a monoclinic lattice with 2 molecules/unit cell. Table I11 represents the intensity growth for the CHI wag and C-C stretch combined. Inspection of the last column, 130 kbar, shows that this combined intensity growth increases primarily as a function of chain length and is relatively independent of crystal structure. Inspection of the last column of Table I shows that the increase in the CHI wag intensity is much greater for the even-numbered triclinic hydrocarbons than for their odd-numbered orthorhombic
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
7260
ra,
lZ30 1220
i
triclinic species with 1 molecule/unit cell, this intensity is distributed more strongly to the CH, wagging mode. In the oddnumbered orthorhombic species with 4 molecules/unit cell, the intensity is distributed more strongly to the C-C stretching mode. The even-numbered monoclinic species with 2 molecules/unit cell show an intermediate behavior. The shift data for the two peaks are shown in Figure 8a,b. The energy of the wag vibration decreases with increasing molecular chain length; the curves for successive values of n are increasingly closer to one another and asymptotically approach the value for the wag in polyethylene, 1176 cm-' at 1 atm. The energy of the C-C stretch increases with increasing chain length and asymptotically approaches its Raman-active value in polyethylene, 1132 cm-l at 1 atm. As is true for most vibrations, the C-C stretch shows a net shift to higher energy, a blue shift, as higher pressure causes an overall increase in the lattice and bond forces. For the larger chains, the CH, wag, however, shows an initial shift to lower energy, a red shift, which reverses at about 50 kbar and begins to shift blue. We have not yet fully developed an explanation for this behavior. The difference in energy between the wagging and stretching modes is plotted in Figure 9. The difference in energy becomes smaller with increasing chain length and with increasing pressure. For n-CsHl8, the two peaks approach each other only modestly, but as the chain length increases to n = 23 and above, the two peaks approach each other at a rate of about 0.4 cm-'/kbar. This continues up to about 60 kbar where the trend begins to gradually level off to a value that becomes smaller with increasing n.
CH, Wag oo
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Ley and Drickamer
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40
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80
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120
140
P r e s s u r e (kbar) Figure 8. (a) Shift of the CH2wag vs pressure in the n-paraffinsC,,HW2. n is indicated beside each curve. (b) Shift of the C C stretch vs pressure in the n-paraffins C,,H2"+2. n is indicated beside each curve. 0"
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P r e s s u r e (kbar) Figure 9. Difference in energy between the CH2 wag and C-C stretch vs pressure. n is indicated beside each curve.
counterparts with the monoclinic hydrocarbons, n = 30 and 36, falling in between. Table I1 shows that for the C-C stretch this situation is reversed, and again, the monoclinic hydrocarbons show an intermediate behavior. In summary, the combined intensity growth for both peaks is a function of molecular chain length only. in the even-numbered
'
Discussion The above observations seem to indicate a 2-fold coupling process that involves intermolecular coupling, which is responsible for the combined intensity increase, and a resonance between the CH2 wag and C-C stretching vibrational coordinates, which is responsible for the relative distribution of intensity between these two peaks. We propose that the intermolecular coupling is responsible for the increase in the CH2 wag intensity and that the resonant process between the wag and C-C stretch vibrational coordinates causes the C-C stretch to borrow intensity from the wag. The resonance and intensity borrowing between the CH, wag and the C-C stretch probably occurs via a lattice mode, also referred to as a phonon-assisted process. In Figure 9, one can see that the minimum difference, i.e., separation, in energy between the CH2 wag and the C-C stretch is about 21 cm-l for C36H74 at 150 kbar. Typically, this is too great an energy difference for Fermi resonance to occur. The combined intensity of the wag and stretch gives an indication of the total amount of intensity contributed via the CH2 wag; this is primarily a function of the molecular chain length and independent of the crystal structure as shown by Table 111. That the C-C stretch borrows intensity from the wag is seen in Tables I and 11. The odd-numbered orthorhombic species with 4 molecules/unit cell exhibit the strongest resonance between the wag and stretch vibrational coordinates; thus, the C-C stretch grows in more strongly and the wag less strongly vs that of the even-numbered triclinic species with 1 molecule/unit cell. In the latter case, the resonance between the two vibrational coordinates is weaker and the wag grows more as compared to the C-C stretch. In the even-numbered monoclinic species with 2 molecules/unit cell, the resonance between vibrational coordinates exhibits be~ ~intermediate ~ ' havior to the other two species. This coupling/ resonance scheme is summarized in Table IV. That the C-C stretch borrows intensity from the CH, wag, and not vice versa, is demonstrated in the following paper in this issue where this same problem is examined in various polyethylenes. At sufficiently long CHI chain lengths, increasing pressure shifts the C-C stretch and CH, wag close enough in energy for the resonance between their vibrational coordinates to become so strong that the intensity of the C-C stretch can be observed to overwhelm the CH, wag completely. One would expect the first-order intermolecular coupling process to occur in a direction perpendicular to the molecular chain axis
Vibrational Spectroscopy of Various n-Alkanes
TABLE IV: Summary of Intensity Results and Couplings in n-Paraffins
lattice type molec UC overall intens inc/coupling CH2 wag intens inc C-C str intens inc wag-str normal coord coupling
n even 5 2 6 even 1 2 6 odd triclinic monoclinic orthorhombic 1 2 4 approx the same for all n-paraffins, function of CH2 chain length only strong moderate weak weak moderate strong moderate strong weak
since that is the direction in which the CHI moieties on neighboring molecules are exposed to one another. This is demonstrated by the data of Table 111. This combined increase in intensity ultimately originates from the CHI wag and thus indicates the degree to which intermolecular coupling occurs. As mentioned above, this coupling does not appear to be a function of the crystal structure and increases with increasing chain length only. The longer the molecular chain, the less the intermolecular coupling process is sterically hindered by the end methyl groups. The peak shift behavior supports the 2-fold coupling argument as well. Figure 9 shows that the inherent tendency of the CH2 wag and C-C stretch peaks is to shift closer together. In other words, the C-C stretch shifts to higher energy more rapidly than the CH2 wag at lower pressures; evidently, the repulsive forces associated with the C-C stretch increase more rapidly with compression than those for the CHI wag. This trend diminishes with pressure and eventually levels off at about 110-120 kbar. It is well-known that for any resonant process involving two states, i and j , solution of the second-order secular equation
where Hij = (iim), yields two energy levels that are spread apart relative to their original position.I8 This tendency to spread the two spectroscopic peaks apart is counteractive to their inherent tendency to shift toward one another and produces the leveling off effect seen in Figure 9 when the two effects begin to balance one another. A series of experiments were carried out to verify the above arguments. As mentioned previously, the deuterated molecule CsDls was studied and exhibited intensity growth for the CD2 wag. This first of all helped identify the wagging mode in the nparaffins. It also demonstrates that the first-order effect of intermolecular interaction is to increase the CH2 wag intensity. The CD2 wag is shifted to significantly lower energy, about 887 cm-I at 1 atm, while the C-C stretch is relatively unshifted. Therefore, in the deuterated molecule, it is unlikely that these two vibrational coordinates resonantly couple the same was as they do in C8Hl8. The only remaining effect that may be responsible for the increase in the CD2 wag intensity is the intermolecular coupling. A 50 mol % mixture of C8H18 and c8D18 was also studied. If the primary mechanism of interaction is intermolecular coupling, then one would expect that two adjacent, unlike molecules would not be able to couple due to the large difference in the vibrational frequencies of their wagging modes. In a 50 mol 7’% mixture, the number of like nearest neighbors will be reduced by half and the capability for overall cooperativity between unit cells in a crys( 1 8) Wilson, E. B.; Decius, J. C.; Cross,P . C . Molecular Vibrations: the Theory of Infrared and Raman VibrationalSpectra; Dover: New York, 1955;
pp 197-198.
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7261 talline domain will be destroyed. One would expect to see the increase in wag intensity but to a greatly diminished extent due to the lower number of possible nearest-neighbor interactions. This is verified by experiment; both wagging vibrations grew in intensity but to a much lesser degree, approximately a factor of 4, than in either pure sample. As mentioned above, the branched hydrocarbon molecules 2,2,4,4,6,8,8-heptamethylnonane, 2,6,10,14-tetramethylpentadecane, and 4-methylheptane were studied. These molecules form a glassy amorphous solid state. If the intensity growth of the wag is caused by some local distortion in the molecule itself, as opposed to the intermolecular coupling of molecules in the crystal lattice, then one might expect to see this growth occur in these amorphous materials. No such changes were observed, however; it is therefore unlikely that any localized molecular distortion is responsible for the observed behavior. An alternative interpretation of the total intensity increase for the CH2 wag is the possibility of intensity borrowing from a strong peak. The nearest such peak that occurs consistently in each of the spectra is the C H 2 bend at about 1470 cm-I, more than 280 cm-I away from either the wag or the C-C stretch. As the carbon chain length increases from n = 8 to 36, the energy of the wag and C-C stretch each changes by only about 35 cm-’, which should not be enough to significantly change any borrowing from such a distant peak. Therefore, one would expect to see little if any change in the total intensity increase as a function of chain length. The last column of Table I11 shows that this is not the case. The increase in total intensity is very much a function of chain length, which is consistent with the concept of intermolecular coupling. And, as mentioned above, the primary event of intermolecular coupling via the CHI wag is verified by the experiment with a 50 mol % mixture of C8Hl8 and CSDl8.
Summary The effect of pressure u p m the intensity and position of the CH2 wag and C-C stretch vibrations in linear hydrocarbon molecules has been interpreted in terms of two major phenomena: intermolecular coupling and phonon-assisted resonance between C H 2 wag and C-C stretch vibrational coordinates. The intermolecular coupling arises from the compression of adjacent molecules and is a direct result of pressure. The resonance is apparently assisted by the decrease in energy separation of the two modes, which in turn arises from the inherent shift behavior of the various n-alkane molecules; as such, it is a “second generation” effect of pressure. The primary effect of pressure is to increase the intensity of the CH2 wagging mode via intermolecular coupling. At sufficiently high pressures, and particularly for the larger chain molecules, the second-order effect of phonon-assisted resonance becomes an increasingly dominant factor in the behavior of the two vibrational modes studied. This is further demonstrated in a subsequent paper19 dealing with various polyethylenes. Therein the phonon-assisted resonance effect is shown to be the dominating factor as the length of the polymethylene chain is increased. Note Added in Proof. Throughout this and the following paper in this issue, we use the molecular descriptions “wag” and “stretch” for the peaks discussed. This is entirely appropriate at moderate pressures where intermolecular interactions are relatively weak. At higher compressions, the site symmetry C, (for n odd) or Ci (for n even) will be of increasing importance and the peaks will increasingly represent the combinations of molecular vibrations allowed under the different site symmetries. (!9) Ley, W. W.; Drickamer, H. G. J . Phys. Chem., following paper in this issue.
