2852
J Phys Chem. 1990, 94, 2852-2857
Identification of Conformational Isomers of Methyl-Substituted Cyclohexanone and Tetrahydropyran Frozen in a Molecular Beam Timothy J. Cornish and Tomas Baer* Chemistry Department, University of North Carolina, Chapel Hill, North Carolina 27599-3290 (Received: September 15, 1989)
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Examination of the n 3s Rydberg transition in several methyl-substituted cyclohexanones and tetrahydropyrans (THPs) cooled in a free-jet expansion reveals the presence of two distinct molecular species. These have been assigned as pairs of conformational isomers that freeze out in the molecular beam at their room temperature populations as a result of the substantial inversion barrier of the saturated six-membered ring system. Included in this study are 3-methylcyclohexanone, 4methylcyclohexanone, 3-methyltetrahydropyran, 4-methyltetrahydropyran, and cis-3,4-dimethyltetrahydropyran.The approximate equilibrium concentration for the conformer pairs has been determined from the ratio of peak intensities of the electronic transition origins. The derived AGO’S for vapor-phase ring inversion are compared to similar AGO’S for the liquid phase.
Introduction
A previous study of the n
-
3 s transition of saturated cyclic ketones cooled in a free-jet expansion demonstrates that a considerable amount of stereochemical information is contained in the Rydberg spectrum.’ These U V absorptions are generated by using resonance-enhanced multiphoton ionization (2 1 REMPI) with the two-photon absorption to the 3s state serving as the real intermediate level. It was found, for example, that the n 3s transition energies for a number of configurational isomers of methyl-substituted cyclohexanones differed by several hundred wavenumbers. Furthermore, the relative shifts of the transition origins were found to be additive with respect to substitutent position and orientation (axial vs equatorial) on the ring, thereby enabling structure identification. This is similar to analyses of the N M R chemical shift for a variety of compounds that exist as stereoisomers. Recently, a similar pattern of shifts was noted in a series of saturated cyclic ethers, i.e., substituted tetrahydropyrans.2 I n view of the substantial differences in the transition origin energies of configurational isomers, conformational isomers should also have distinguishable optical spectra since the time involved for electronic excitation is many orders of magnitude shorter than that for inversion of the conformational isomers. I t should thus be possible to use the Rydberg spectra to identify inversion isomers and to measure their respective vapor-phase-equilibrium ratios. The measurement of solution-phase equilibrium constants associated with isomerization reactions by N M R methods is a well-established t e c h n i q ~ e . ~At room temperature the interconversion is often sufficiently rapid, however, that the two structures cannot be individually observed by this method. To circumvent this problem, the reaction rate can be slowed by cooling the solution so that the N M R peaks de-coalesce. Two difficulties associated with this method are, first, as the sample is cooled the equilibrium constant shifts markedly in favor of the more stable isomer so that the peak associated with the least stable structure often becomes prohibitively small to measure. Second, in some cases, the solution may freeze, or the solute may crystallize, before the equilibration rate is sufficiently slow for the N M R method to be applied. Thus, an alternate technique for obtaining such data with a wider range of temperatures is highly desirable. In addition, equilibrium measurements for vapor-phase samples would eliminate perturbations due to solvent interactions as well as provide thermodynamic information that can be directly related
+
-
( I ) Cornish, T. J.; Baer. T.J . Am. Chem. Soc. 1988, 110, 3099. (2) Cornish, T. J.; Baer. T. Manuscript in preparation. (3) E.g.: Eliel, E. L.; Hargrave, K . D.; Pietrusiewicz, K . M.; Manoharan, M. J . Am. Chem. SOC.1982, 104, 3635.
0022-3654/90/2094-2852$02.50/0
to values determined by computational methods. The equatorial-axial isomerization rates are related to the activation energies for interconversion of the two structures, which have also been measured by the above-mentioned variable-temperature N M R methods. In the case of cyclohexanone and tetrahydropyran, the activation energies have been reported to be 4.04 and 10.3 kcal/m01,~respectively. Several recent studies have focused on multiple conformations of samples in a molecular b e a ~ n . ~Examples .~ include loosely bound van der Waals complexes,6 as well as substituted aromatic compounds7 in which the rotations about u bonds of ring substituents can be frozen. The van der Waals complexes were identified by their high-resolution IR spectra, while the aromatic rotamers were investigated by fluorescence techniques7a4 or by resonance i~nization.~~-g I n the two most recent studies power saturation techniques have been used to distinguish the origin peaks7dor the whole spectra7gof specific structures in the presence of other conformations. The approach of Lipert and C o l s ~ n , ~ s in which a probe laser is set to a particular transition while the saturation laser is scanned, seems particularly promising because of its versatility. In this paper, we report the n 3s Rydberg spectra of several methyl-substituted cyclic ethers and cyclic ketones that exist as mixtures of conformations. This is accomplished by using a 2 + I resonance-enhanced multiphoton ionization (REMPI) technique. The conformation energies can then be determined from the relative spectral intensities of the n 3s transitions. Since conformational energies of saturated six-membered ring systems are fairly well established, it is possible to compare the optical spectroscopic data with previously reported results.
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Experimental Approach
The REMPI apparatus has been described in detail earlier.8 Briefly, the output from a Lumonics dye laser pumped by a Lumonics excimer laser was focused with a 25-cm focal length lens at the center of the photoionization region where it intersected (4) Anet, F. A. 1 . ; Chmurny, G . N.; Krane, J . J . Am. Chem. SOC.1973, 9.5,4424. ( 5 ) Lambert, J. B.; Mixan, C. E.; Johnson, D. H. J . A m . Chem. SOC.1973, 95. 4634. (6) (a) Dayton, D. C.; Miller, R. E. Chem. Phys. Lerr. 1988, 143, 580. (b) Jucks, K. W.; Miller, R. E. J . Chem. Phys. 1988, 88, 2196. (7) (a) Song, K.; Hayes, J. M. J . Mol. Specrrosc. 1989, 134, 82. (b) Philips, 1 . A.; Levy, D. H. J . Chem. Phys. 1988,89,86. (c) Oikawa, A,; Abe, H.;Mikami, N . ; Ita, M. J . Phys. Chem. 1984, 88, 5180. (d) Martinez 111, S. J.; Alfano, J. C.; Levy, D. H. J . Mol. Spectrosc. 1989, 137, 420. (e) Breen, P. J.; Warren, J . A.; Berstein, E. R.; Seeman, J. I . J . Chem. Phys. 1987, 87, 1917. (f) Dunn, T. M.; Tembreull, R.; Lubman, D. M. Chem. Phys. Lerr. 1985, 121,453. (9) Lipert, R. J.; Colson, S. D. J . Phys. Chem. 1989, 93, 3894. ( 8 ) Cornish. T. J.: Baer. T. J . Am. Chem. Soc. 1987, 109. 6915.
C2 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 2853
Methyl-Substituted Cyclohexanone and T H P Isomers
TABLE I: Transition Origin Energies (cm-I), Q Values, Equilibrium Constants, and AGO (kcal/mol) Values for Methyl Isomers of THP and Cyclohexanone n
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3s origin 5 1 579 50982 52 384 51 277 51 319 50 696 50 862 50 297
3-methyl-THP (eq) 3-methyl-THP (ax) 4-methyl-THP (eq) 4-methyl-THP (ax) cis-3,4-dimethyl-THP (4-eq) cis-3,4-dimethyl-THP (4-ax) 3-methylcyclohexanone (eq) 3-methylcyclohexanone (ax) 4-methylcyclohexanone (eq) 4-methylcyclohexanone (ax)
calc
lit.
K,(303 K) 11.9
AG0303
AG0=170
1.49
1.44’
49.5
2.34
1.95’
0.08 0.30
4.6
0.92
0.76’
I .Ob
14.9
1.62
1.30‘
1.25 I .5 1.68
62
2.52
I .9d
n 0.18 0.14 0.09
0.10
50710 50 463
‘Reference 3. bThis value was incorrectly reported as 1.5 in ref I . ‘Reference 12a (inferred from energy of epimerization in cis- and trans3,5-dimethylcyclohexanone). Reference 12b (inferred from conformational energy of methylcyclohexane in the vapor phase).
I I/ 1
376
1
378
1
1
380
+
1
1
582
1
1
384
1
1
1
386
1
388
1
1
390
-I 1
1
392
1
1
394
1
396
380
582
w w (MI)
Figure 1. The 2 1 resonance-enhanced multiphoton ionization (REMPI) spectra of methyl-substituted tetrahydropyrans. The main spectra were collected with circularly polarized light, while the inserts were collected with linearly polarized laser light.
a seeded and skimmed molecular sample beam at right angles. The laser power was held at a constant intensity over the entire wavelength scan with a feedback loop coupled to a Scientech calorimeter. The pump laser intensity was adjusted with a motor-driven, mechanical flag inserted into the path of the excimer beam. In this manner, the dye laser output was held constant at 400 pJ/pulse. The molecular beam was generated with a Laser Technics pulsed valve with backing pressures (Ar) ranging between 70 and 600 Torr. The former conditions were used for the “hot” sample spectra, while the high backing pressures were used for the “cold” sample spectra. Typical pressures in the nozzle and experimental chambers during the experiment were and Torr. Laser polarization was accomplished in three stages. A calcite prism was used to produce pure plane-polarized light, followed by a half-wave plate consisting of two Fresnel rhombs used t o rotate the plane of polarization. Finally, a single rhomb was used to alternate between circular and linear polarization. The electrons (or ions) were extracted with a strong electric field (1 500 V/cm) and collected with a set of microchannel plates. The microchannel plate output was sent to a gated integrator followed by digital conversion and processing. The spectra were collected by scanning the laser while collecting the total electron signal. Ion time-of-flight mass spectra were also obtained at
384
+
586
388
-w
390
392
394
396
398
400
(d
Figure 2. The 2 1 R E M P I spectra of two methyl-substituted cyclohexanones. All spectra were collected with circularly polarized light.
selected laser wavelengths in order to verify the purity of the samples. The samples of 3-methylcyclohexanone, 3-methyltetrahydropyran (3-methyl-THP), and 4-methylycyclohexanone were purchased from Aldrich. cis-3,4-Dimethyltetrahydropyran (cis-3,4-dimethyl-THP) was prepared as described in ref 9. Samples of 4-methyltetrahydropyran (4-methyl-THP) were prepared by ring closure of 3methylpentane- 1J-diol as described in ref 10. Authenticity of the compounds was confirmed by I3CNMR measurements.“ Each sample was purified by preparative gas chromatography using an SF-96 column.
-
Results The n 3s spectra of 3-methyl-, 4-methyl-, and cis-3,4-dimethyl-THP are shown in Figure 1. Transition origin energies, D values, and relative intensities of the origin peaks for these structures are listed in Table I. Each spectrum exhibits a strong (9) Scott, L. T.; Naples, J. 0. Synthesis 1973, 209. (10) Robinson, P. L.; Barry, C. N.; Kelly, J. W.; Evans, S. A. J . Am. Chem. SOC.1985, 107, 5210. ( I 1 ) Eliel, E. L.; Manoharan, M.; Pietrusiewicz, K. M.; Hargrave, K. D. Org. Magn. Reson. 1983, 21, 94. (12) (a) Allinger, N . L.; Hirsch, J. A.; Miller, M. A.; Tyminski, 1. J . J . Am. Chem. SOC.1969, 91, 337. (b) Prosen, E. J.; Johnson, W.H.; Rossini, F. D. J . Res. Nail. Bur. Stand. 1947, 39, 173.
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The Journal of Physical Chemistry, Vol. 94, No. 7 , I990 I/
Cornish and Baer
cc1 1
1
383
385
387
389
39 1
393
395
Wavelength (nm)
Figure 4. The effect of laser polarization on the spectra of the two conformers of cis-3,4-dimethyl-THP. 381
383
385
387
389
-
39 1
Wavelength (nm)
Figure 3. Two REMPI spectra of 4-methylcyclohexanone under the low backing pressure of 70 Torr (labeled hot), and the high backing pressure of 600 Torr (labeled cold). The extra peaks in the upper spectrum are the hot transitions of the equatorial conformer.
origin band and a number of excited-state vibrational peaks which are attributable to the major conformer. Additionally, to the red of these prominent bands is a much weaker spectrum that is assigned to the minor conformer. The longer scans were collected by use of circular laser polarization in order to show relative concentrations of the two species present in the sample (discussed later) while the magnified inserts were produced by use of linear polarization because of higher signal-to-noise ratios. Figure 2 is a plot of the n 3s transitions of 3- and 4-methylcyclohexanones. The strongest portions of these spectra have been reported previously' and were attributed to the dominant conformations, but the weaker, red-shifted peaks were not discussed. In Figure 2, the complete spectrum, as well as the inserts, were collected under circular polarization conditions. A number of tests were conducted to determine whether the small peaks are, in fact, due to the less stable conformers. First, it was observed that the minor peaks in similar cold spectra of unsubstituted cyclohexanone and T H P are not present, indicating that the methyl substituent is necessary for their observation. In addition, time-of-flight mass spectra were collected while the laser was tuned to the resonance energies of the bands assigned as the transition origins. In each case, the REMPI mass spectra of the two suspected conformers were essentially identical. The possibility that the weak red-shifted peaks were the result of hot bands was tested by varying the temperature of the molecular beam. In Figure 3, for example, a warm spectrum of 4-methyl-THP exhibits several very low frequency sequence bands clustered around the transition origin peak. In addition, a few hot fundamental vibrations are observed within a range of about 500 cm-' to the red of the origin. This is similar to those peaks identified in the hot spectrum of the unsubstituted T H P ring.13 Unlike the T H P spectrum, however, in which all bands to the red of the origin disappear at higher backing pressures, the relative intensities of the peaks assigned as alternate conformations do not vary as a function of the molecular beam temperature. From this evidence, and that discussed above, it is clear the red-shifted bands in the cold spectra plotted in Figures 1 and 2 are due to neither sample impurities nor hot bands. They are therefore attributable either to different electronic transitions of a single structure or to excitations of alternate conformations. The former possibility has been ruled out since the energy separations of the strong vs weak bands are significantly smaller than one would expect for different electronic excitations. In THP, for example,
-
-
Robini4 reports that the n 3s and the next higher n 3p transitions are separated by about 2500 cm-'. We conclude, therefore, that these overlapping spectra are due to unique conformational isomers. Transition Energy Shifrs. The sign and magnitude of shifts in transition energies between pairs of conformers are useful in assigning the spectra to specific stereoisomers.' In the present examples, the axial conformations in 3- and 4-methyl isomers of T H P (Figure 1) as well as 3- and 4-methylyclohexanone (Figure 2 ) are all found to be red shifted from those of the equatorial conformers. This is consistent with the 3s energy shifts observed in the spectra of several cis- and trans-dimethyl isomers of cyclohexanone.' For example, it was reported that the spectrum of trans-3,5-dimethylcyclohexanonewas observed at -491 cm-' relative to the cis spectrum (the negative sign indicating a red shift). This variation in the transition energy must be related to the reorientation of the methyl group at the C3 position from an equatorial to an axial position. The shift observed for axial 3methylcyclohexanone relative to the equatorial conformer is -565 cm-', which is within 74 cm-' of the expected shift. In the cyclic ether ring system* the spectroscopic energy difference between the configurational isomers cis- and trans-3,5dimethyl-THP is -43 I cm-' while the energy difference of the 3-methyl conformational isomers in Figure I is -598 cm-I. In general, the predictability of the shifts in the methyl isomers of cyclic ketones is higher than those of the cyclic ether methyl isomers.2 In each case, however, the consistency of the spectral shifts between configurational and conformational isomers is sufficiently high to support the proposed assignments. The 4-methyl conformers can be analyzed in a similar fashion. Two configurational isomers of 3,4,5-trimethyltetrahydropyran (3,4,5-trimethyl-THP) were examined in order to fix the 4-methyl group in either the axial or equatorial position. A spectroscopic shift of -1 195 cm-' was observed between the eq-eq-eq and the eq-ax-eq configurations.2 The observed shift of -1 107 cm-' between the axial and equatorial conformers in 4-methyl-THP (Figure 1) is again consistent with that predicted from multisubstituted isomers. No similar data are available for the 4-methyl isomer of the cyclic ketone system. Laser Polarization Eflects. The intensitiesof the spectra plotted in Figures 1 and 2 show a strong dependence on laser polarization, thereby complicating the relative population analysis. Not only does the intensity of each spectrum vary with laser polarization, but in some cases the spectra of two conformers show a different intensity dependence. The most extreme example of this is found in cis-3,4-dimethyl-THP. The two-photon Rydberg absorption spectra under circular and linear polarizations are shown in Figure 4. The intensity of the minor peak at 394.6 nm is significantly increased relative to that of the major peak as the laser polarization
~
(13) Cornish, T. J.; Baer, T.; Pedersen, L. 6064
G.J . Phys. Chem. 1989, 93,
(14) Robin, M . B. Higher Excited States of Polyafomie Molecules; Academic Press: New York, 1975; Vol I, p 260.
Methyl-Substituted Cyclohexanone and T H P Isomers is changed from linear to circular. (In fact, both major and minor peaks are attenuated, but the latter less so.) This effect is quantified by each conformer’s R value, the ratio of the two-photon signal strength under the two polarization conditions.15 In two-photon transitions, the peak height ratio (R) under circular and linear polarizations is equal to 3/2 for all rotational branches except the Q branch. The Q value for the Q branch depends on the symmetry of the optical transition as well as the symmetry of the molecule.’ It is therefore primarily the intensity changes of the Q branch under different polarization conditions that determine the variations in the R values. If the peaks in the electronic spectrum exhibit a strong Q branch, as in the case of THP,” then the intensities of these bands will be dramatically reduced under circular polarization conditions and R will be small. If, on the other hand, predominant Q branches are not present in the rotational fine structure of the vibrational peaks, as in the case of cyclohexanone, then the bands will be enhanced under circular polarization conditions and the value of R will be 3/2. Variations in backbone conformations due to methyl substitution cause R to decrease from the maximum value of 1.5 in the cyclohexanone isomers and increase from the minimum value of 0.08 in the T H P isomers. It is important to determine which laser polarization should be utilized in measuring the conformer ratio. As discussed by Lin et al.Isb in two-photon transitions, the intensities of the 0, P, R, and S branches always are enhanced by 312 under excitation with circularly polarized light. Similarly, the Q branch is enhanced by 312 if the transition is between two states of different symmetry. On the other hand, R will be less than 312 for Q branches in transitions between states of the same symmetry. Therefore, the areas of the peaks collected under circular polarization should exhibit the appropriate intensities for measuring the concentration. This was tested by collecting spectra of mixtures with known sample quantities. In one case, the spectrum of a 1:l mixture of cyc1ohexanone:THP was collected under circular and linear polarization. These were chosen because the polarization ratios for cyclohexanone and THP, R = 1.5 and 0.08, respectively, represent the two extremes in polarizaton effects observed in the compounds investigated. The peak area ratios of the transition origins were approximately 40:l under linear and 2:1 under circular laser polarization. It is possible that this 2:l ratio, which still does not accurately reflect the actual composition, may be a result of residual elliptical polarization in the circularly polarized laser beam. An expanded spectrum of the origin peak in T H P showed that some Q-branch character remained in its rotational fine structure. If this sharp central branch is neglected, the ratio of the remaining areas reduces to 1 : l . We were not successful in determining the purity of the circularly polarized light emanating from the Fresnel rhomb quarter-wave plate. Therefore, in light of this uncertainty in quantitation, the present technique appears to be limited to systems in which the Q values for the two conformers are similar. With the exception of cis-3,4-dimethyl-THP, this is the case for the conformational pairs considered here. Determination of Equilibrium Constants. It is evident that the sample experiences substantial rotational and vibrational cooling in the free-jet expansion, causing the peaks to sharpen and the hot bands to disappear at high backing pressure, yet the relative populations of axial and equatorial conformers remain essentially constant over a range of temperatures (backing pressures). For instance, the intensity ratio of the transition origins in equatorial and axial 3-methyl-THP is about 12/1 under both warm and cold beam conditions. While this evidence alone does not prove that the relative proportions seen in the cold beam spectrum represent ambient temperature populations, it is consistent with results obtained in other molecular beam studies16 (15) (a) Monson, P. R.; McClain, W. M. J . Chem. fhys. 1970,53, 29. (b) Lin, S. H.;Fujimura, Y.;Neusser, H. J.; Schlag, E. W. Multiphoton Spectroscopy of Molecules; Academic Press: Orlando, FL, 1984; p 116. (16) (a) Klots, T. D.; Ruoff, R. S.; Emilsson, T.; Gutowsky, H. S. Abstracts, Forty-Fourth Symposium on Molecular Spectroscopy, The Ohio State University. June 1989. (b) Felder, P.; Gunthard, Hs.H. Chem. Phys. 1982, 71, 9.
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 2855
e Frequency (cm-1)
The 3s Rydberg state vibrational spectra of the axial and equatorial conformers of 3-methylcyclohexanone and 3-methyl-THP. In each spectrum, the origin band serves as the zero energy reference. Figure 5.
in which the extent of conformational relaxation was examined in compounds capable of isomerizing via simple free rotation about a bond axis. According to Klots et a1.,16athe equilibria for interconverting structures with barrier heights of less than about 500 cm-’ (1.4 kcal/mol) relax toward the more stable isomer during supersonic expansion. In contrast, structures with interconversion barriers greater than this value tend to freeze out in the initial stage of expansion with virtually no change in ambient temperature populations of conformational isomers. In T H P and cyclohexanone, the reported inversion barriers are 10.3 and 4.0 kcal/mol, respectively, so the freezing out of conformational isomers at ambient temperature proportions is expected. Therefore the temperature of the valve nozzle was used for the calculations of free energy differences between the conformational isomers. In order to calculate AGO values using the electronic excitation data, it must be established that the relative intensities of the overlapping spectra reflect true ratios of conformer populations. In principle, the concentrations are proportional to the integrated intensity of the entire spectrum. However, since the spectra of the axial conformers are pure only over the low-energy region and overlap the spectra of the equatorial isomers at higher energies, this approach is not possible. Hence the peak ratios of the origin bands were used as an approximate measure of the isomer concentration. The method is valid as long as the Franck-Condon factors for the n 3s transition for both conformers, and therefore the excited-state vibrational intensities, are approximately equal. It was possible to test this by comparing the low-frequency region of several conformer pairs. This region reflects the backbone ring vibrations which are the modes that are most sensitive to structural changes upon electronic excitation. In Figure 5 , the 0-500-cm-l vibrational region for each conformational isomer of 3-methylT H P and 3-methylcyclohexanone has been plotted with the transition origin serving as the zero energy reference. This comparison reveals that the excited ring vibrations in conformational pairs are similar both in number and overall intensity. Thus, the postulate that the Franck-Condon factors are comparable for conformational pairs appears to be a good one, at least among these few examples. A less satisfactory situation exists in the case of 4-methyl-THP
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The Journal of Physical Chemistry, Vol. 94, No. 7, 1990
(Figure I ) . Here, two of the low-energy ring vibrations in the equatorial conformer are totally absent in the spectrum of the axial conformer. However, in this case the energy separation of the two spectra is so large that the integrated intensity of the spectrum from 0 to IO00 cm-I can be used. This caused the Kq to increase from 49.5 to 57, a difference that is probably within the errors imposed by other factors. The difference in the derived AGO’S (see Discussion) is only 0.1 kcal/mol. An interesting feature of the 3s spectra in Figure 5 is that the low-frequency modes in the axial conformer are of lower frequencies relative to the equatorial conformer. A similar effect has been reported for chlorocyclohexane in the ground electronic state.” According to Hofner et al.,l* this observation is expected because vibrations of the axial conformation can release ring strain energy. A final problem in relating peak intensities to conformer populations is the possibility that alternate conformations have different absorption cross sections. This can only be determined if the relative concentrations of the specific components in the molecular beam are known beforehand, which is, of course, the unknown quantity that is being measured. Until further evidence can be collected to determine otherwise, the assumption that each species in a pair of conformational isomers will exhibit similar absorptivities seems reasonable. Table I lists the K values that are the ratio of peak areas determined under circJar polarization. Also listed are the AGO’S at 303 K calculated from the relation AGO = -RT In Kq. For the T H P conformers, solution-phase AGO values were obtained by NMR methods at temperatures ranging from 160 to 180 K. A low barrier of inversion in the cyclic ketones prevents the direct measurement of their conformational energies by NMR. Consequently, the literature value for 3-methylcyclohexanone is inferred from studies of the trans to cis epimerization of 3,5-dimethylcyclohexanone, while that reported for the 4-methyl isomer is simply the energy associated with the ring inversion of methylcyclohexane. Agreement between the spectroscopic and literature values of AGO is fairly good.
Discussion There are two important differences between the equilibrium constants determined by N M R and by the present laser method. First, the temperatures of the two experiments are different. Second, the NMR data refer to solution-phase equilibria, whereas the molecular beam measurements reflect gas-phase equilibria. A value for AGO at the temperature of the experiment can be extracted from the equilibrium constant, Kq Temperature effects on AGO are determined by AHo and ASo,quantities that can be determined experimentally only by measuring the equilibrium constant as a function of temperature. Very few studies have provided information about ASo of the axial-equatorial equilibria. However, the indications are that ASo is small. Reisselg and Hofner et al.’* measured ASo and AHo for the ax eq equilibration by variable temperature NMR for chloro-, bromc-, cyanc-, and methoxycyclohexane and found the ASovalues to be between 0.1 and 0.6 cal/(mol.K). Thus the difference in AGO between 160 and 300 K would be only between 0.01 and 0.08 kcal/mol. Somewhat larger and negative (-1 cal/(mol.K)) values have been reported for the equilibration of cis- and trans-dimethylcyclohexanes.20 However, these latter entropy changes are dominated by entropies of mixing and symmetry effects, both of which are absent in simple conformational equilibria. The instrumental setup in its present configuration does not permit the measurement of the equilibrium constant over a wide range of temperatures, but this should be possible in the near future by using an alternate
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(17) (a) Klaeboe, P.; Lothe, J. J.; Lunde, K. Acta Chim. Scand. 1956, 10, 1465. (b) Klaebcc, P. Acta Chim. Scand. 1969, 23, 2641. (18) Hofner, D.; Lesko,S. A.; Binsch,G. Org. Magn. Reson. 1978,II, 179. ( I 9) Reisse, J . In Conformational Analysis, Scope and Present Limitations; Chiurdoglu, G . , Ed.; Academic Press: New York, 1971; pp 219-228. (20) Pedley, J. B.; Naylor, R. D.; Kirby, S. P. Thermochemical Dora of Organic Compounds, 2nd ed.; Chapman and Hall: London, 1986.
Cornish and Baer pulsed nozzle. Another approach would be to determine ASofrom molecular orbital ab initio techniques by calculating the vibrational frequencies of the axial and equatorial isomers. However, such calculations are extremely time consuming for molecules of this size. The second difference between the present conditions and those of the NMR experiment is the solvent. Riddellzi reports that the axial-equatorial equilibria are markedly dependent on the nature of the solvent. Even in molecules where intramolecular dipoledipole interactions and specific solvent effects are small or absent, gas-phase enthalpy differences between axial and equatorial conformations differ from those in the pure liquid phase because of the difference in the heat of vaporization of axial and equatorial isomers. A comparison of the gas- and liquid-phase heats of reaction for the equatorial, equatorial equatorial, axial dimethylcyclohexanes reveals that the gas-phase AHo exceeds that of the liquid phase by about 0.2 kcal/mol.20 This difference is, in fact, quite close to the differences in the gas- and solution-phase A G O values reported in Table I. All of the REMPI results in Table I are derived from a direct determination of relative peak areas. On the other hand several of the N M R results were determined indirectly. For example, the AGO for 4-methyl-THP was found by applying the counterpoise m e t h ~ d . ~ ~ This - * ~ approach was necessary because the concentration of the minor component is too small to measure by NMR. The counterpoise method is best illustrated with the first three entries in Table I. The AGO’S for the first two molecules refer to the free energy differences between equatorial and axial orientations of the monomethyl conformations. In contrast, AGO for the third entry refers to a combination of motions in which one substituent changes from axial to equatorial, while the other substituent changes in the opposite direction. The net result for this case is that AGO is considerably smaller than the other two since the energies of the two changes offset each other. If we assume that the energy changes associated with the two methyl flips are independent, then AGO for cis-3,4-dimethyl-THP can be calculated from the difference between the AGO values for the 3- and 4-methyl-THP structures. Thus, if any two of the A G O ’ S are known, the third can be calculated from their sum or difference. As pointed out by Eliel et aI.,j this particular example is one in which the assumption of no interaction between the two groups is not particularly good because of possible differences in vicinal interactions for the two conformers. However, other schemes were utilized and the data from at least two independent determinations gave similar values of 1.95 kcal/mol for 4methyl-THP.
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Conclusions Studies of the n 3s spectra of several methyl-substituted cyclic ethers and cyclic ketones have shown that a mixture of conformational isomers are frozen out in a free-jet expansion. The data suggest that relative proportions of conformer pairs in the molecular beam are equal to their ambient temperature populations. In addition, equilibrium constants determined from peak ratios lead to calculated values for AGO that are in fair agreement with those determined from NMR measurements. Several values calculated directly from the molecular beam data cannot be determined directly by NMR because the proportion of the minor conformation is too small to measure at low temperatures. The vapor-phase measurements of Kq tend to produce AGO values that are somewhat higher than those from the N M R method. This difference may be due to solvent effects which have been shown to lower AGO for equatorialaxial equilibria in the solution relative to the gas-phase values. Further work is in progress on variable-temperature equilibrium measurements and ab initio calcu(21) Riddell, F. The Conformational Analysis of Heterocyclic Compounds; Academic Press: London, 1980 p 69. (22) Eliel, E. L.; Kandasamy, D. J . Org. Chem. 1976, 41, 3899. (23) Eliel, E. L.; Della, E. W.; Williams, T. H. Tetrahedron Lett. 1963, 831. (24) Booth, H.; Everett, J. R . J . Chem. Soc., Perkin Trans. 2 1980, 255.
J . Phys. Chem. 1990, 94, 2851-2865
lations of vibrational frequencies. These should help in providing more precise answers to some of the questions raised in this study. Acknowledgment. We are grateful to Prof. Ernest Eliel for providing several of the compounds used in this study, critical
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review of the manuscript, and many enlightening discussions about the data analysis. In addition, we would like to thank GLAXO Inc. and the National Science Foundatin for financial support of this work. We are also grateful to Dr. William Murray for his efforts in the synthesis of several compounds.
TlmeResolved Absorption Changes of Thin CS2 Samples under Shock Compression: Electronic and Chemical Implications C.S . Yoo* and Y. M. Gupta Department of Physics, Washington State University, Pullman, Washington 991 64-281 4 (Received: September 18. 1989)
Electronic and chemical changes in CS2shocked to 12 GPa have been examined by using time-resolved absorption measurements. Experiments were carried out on thin samples ( = l pm) to provide good resolution of the absorption spectrum between 280 and 500 nm in pure CS2and to separate pressure, temperature, and time effects. In addition to the V-band existing at ambient conditions, a new band is observed on the red side of the V-band at high pressures. Unlike the V-band, the peak position and intensity of the new band vary substantially with pressure and temperature. The new band shows a "hot" band character and is conjectured to be the T-band resulting from a transition to the 'A, state. The edge shifts of the absorption band, at a given pressure, decrease with decreasing temperature. The time-dependent increase in absorbance and the magnitude of the edge shifts cannot be reconciled with temperature changes and are believed to represent changes in the electronic structure due to diffusive molecular motions. Spectral changes are irreversible above 11-12 GPa due to a shock-induced chemical reaction. The absorption changes from this and previous work suggest that an associative type of chemical reaction occurs in shocked CS2.
I. Introduction The electronic structure of molecules can be altered significantly by increasing pressure and temperature. These electronic changes can either directly or indirectly initiate chemical reactions, particularly in unsaturated CS2 is a good example of a molecule that, under compression, shows large spectral changes and undergoes irreversible chemical reactions. The spectral and chemical changes in CS2 have been studied previously under both and shock c o m p r e ~ s i o n . ~Shock ~ wave studies on CS2 have led to the following developments: measurement of a break in the CS, Hugoniotq6development of an equation of state for CS2,' experiments to establish a chemical reaction in CS2and to measure the kinetic parameters of this reaction,8 and application of time-resolved spectroscopy to examine spectral change^.^ Changes in the transmission spectrum of shocked CS,, in the near-UV and visible region, revealed substantial changes including a shift of the red edge of the 320-nm band toward the red at a rate of approximately 20 nm/GPa.Io Further studiesIi-l3 on the transmission spectrum of shocked CS2 by Duvall and his coworkers have led to a quantification of this shift as a function of pressure and temperature, to the finding that the edge shift is irreversible at pressures greater than 9 GPa, and to a recognition of the difference between the shift observed in shock compressed CS2 and that observed in statically compressed CS2. Recently, we have reported on spectral changes of shocked CS2/hexane mixtures under step wave 10ading.I~ It was evident that absorption band changes with pressure include intensity enhancement, asymmetric broadening of the band, and the red shift of the band, all of which were dependent on the CS2 concentration. A molecular model, based on the staggered-parallel orientation of CS,,was suggested to explain the experimental results in the mixtures. This molecular orientation should involve a diffusive molecular rotation on nanosecond time scale; however, evidence of such motion has not been obtained. Part of the *Author to whom correspondence should be addressed. Present address: Physics Department, Division H, P.O. Box 808, Lawrence Livermore National Laboratory, Livermore, CA 94550.
0022-3654f 90 f 2094-2851$02.50 f 0
difficulty is that, in step wave loading of thick (100-200 pm) samples, it is difficult to separate pressure, time, and temperature effects. The CS2/hexane mixture studyi4 also revealed a new feature, on the red side of the absorption band centered a t 320 nm, for some of the mixtures. This leads to the possibility that CS2 absorption band characteristics at high pressures are different from these at ambient conditions; examination of the band edge alone, as was done in previous studies,"I would not resolve this issue. The work reported here is a continuation of earlier efforts to understand spectral and chemical changes in shocked CS2. Unlike previous studies, we examined the full absorption band of pure CS2,between 280 and 500 nm, using very thin (approximately 1 pm) samples. The present work had three main objectives: (i) to resolve the full absorption band of pure CS2 to achieve an improved understanding of the pressure and temperature effects on the electronic transitions; (ii) to determine temporal changes ( I ) Yakushev, V. V.; Nabatov, S. S.; Yakushev, 0. B. h k 1 . Akad. Nauk USSR 1973, 214, 879. (2) Duvall, G. E. 'Electronic Spectra of Various Liquids under Shock Compressions";ONR Annual Report, No. NOOO14-77C-0232, 1985. (3) Bridgman, P. W. Proc. Am. Acad. Arts Sci. 1942, 74, 399. (4) Whalley, E. Can. J . Chem. 1960, 38, 2105. (5) Agnew, S. F.; Mischke, R. E.; Swanson, B. I. J . Phys. Chem. 1988,92, 4201. (6) Dick, R. D. J . Chem. Phys. 1970, 52,6021. (7) Sheffield, S. A.; Duvall, G. E. J . Chem. Phys. 1983,79,1981. See also: Sheffield, S. A. Shock-Induced Reaction in Carbon Disulfide. Ph.D. Thesis, Washington State University, 1978. (8) Sheffield, S. A. J . Chem. Phys. 1984, 81, 3048. (9) Duvall, G. E.; Ogilvie, K. M.; Wilson, R.; Bellamy, P. M.; Wei, P. S. P. Nature 1982, 296, 846. (10) Ogilvie, K. M.; Duvall, G. E. J . Chem. Phys. 1983, 78, 1077. ( I 1) Duvall, G. E.; Granholm, R. H.; Bellamy, P. M.; Hegland, J. E. Effect of Temperature on the UV-Visible Spectrum of Dynamically Compressed CS2. In Shock Waves in Condensed Matter; Gupta, Y . M., Ed.; Plenum: New York, 1986; p 213. (12) Duvall, G. E. "Optical Spectroscopy of Dynamically Compressed Liquids"; ONR Final Report, No. NOOO14-77C-0232, 1986. (13) Yoo, C. S.; Furrer, J. J.; Duvall, G. E.; Agnew, S. F.; Swanson, B. 1. J . Phys. Chem. 1987, 91. 657. (14) Yoo, C. S.; Duvall, G. E.; Furrer, J. J.; Granholm, R. J . Phys. Chem. 1989, 93, 3012.
0 1990 American Chemical Society
