J. Phys. Chem. 1983, 87,462%-4630
4628
Low-Resolution Microwave Spectroscopy. Conformation and Low Internal Rotation Barrier in p-Cyclopropylbenzaldehyde Nancy S. True, Department of Chemistry, The Universm of California, Davis, California 95616
Robert K. Bohn," Anthony Chleffalo, and J. Radhakrlrhnan Department of Chemistiy and Institute of Meterials Science, Universe of Connecticut, Storrs, Connecticut 06268 (Received: October 4, 1982; I n Final Form: March 18, 1983)
Three species are present in low-resolution microwave (LRMW) spectra of p-cyclopropylbenzaldehyde. B + C values are 1066.0 (2) and 1042.7 (4) MHz for the syn (S) and anti (A) conformers having the methine C-H bond coplanar with the benzene ring and 1055.5 (1)for states freely rotating (FR) above the phenyl-cyclopropyl internal rotation barrier. The relative intensity ratio S:A:FR is 1:1:20, indicating a very low internal rotation barrier for the cyclopropyl group.
Introduction Direct information about internal rotation barriers can be obtained from low-resolution microwave (LRMW) spectra.'-5 Virtually all the states freely rotating above an internal rotation barrier have similar rotational constants, their rotational spectra superpose, and molecules in this dense collection of states are observed as a single spectroscopic species in LRMW spectra of a variety of compounds. Recently, molecules in free-rotor states have been observed and characterized in gaseous samples of substituted alkylbenzenes,' esters,? aldehydesI3and nitrites: The intensity of free-rotor species relative to the stable conformers indicates that internal rotation barriers in these molecules are low. For isopropylbenzenes' the barrier to internal rotation about the phenyl-isopropyl bond is 250 f 140 cal/mol. The present study provides information about the internal rotation barrier about the phenyl-cyclopropyl bond in p-cyclopropylbenzaldehyde. Recent experimental and theoretical studies of conformation and internal rotation in cyclopropylbenzene have reached a variety of conclusions. On the basis of the magnitude of the first four ionization potentials in the He(1) photoelectron spectrum of cycl~propylbenzene,~ it was concluded that little conjugation exists between the aromatic and cyclopropyl rings. Electron diffraction data for cyclopropylbenzene have been interpreted in terms of two conformations, the bisected conformation (Figure 1) being the more stable.6 Molecular orbital calculations7 consistently predict that the stable conformer of cyclopropylbenzene has the bisected configuration. INDO/2 calculations yield a twofold barrier height of 2 kcal/mol. Ab initio calculations at the STO-3G level' with no geometry optimization yield a 4.3 kcal/mol barrier. Recently, the spin-spin coupling constant from the para hydrogen (1) N. S. True, M. S. Farag, R. K. Bohn, M. A. MecGregor, and J. Radhakrishnan, J. Phys. Chem., preceding paper in this issue. (2) N. S. True, C. J. Silvia, and R. K. Bohn, J.Phys. Chem., 86, 1132 (1981). (3) N. S. True, L. P. Thomas, and R. K. Bohn, J. Phys. Chem., 84, 1785 (1980). (4) N. S. True and R. K. Bohn, J. Phys. Chem., 86, 2327 (1982). (5) I. Prins, J. W. Verhoeven, T. J. De Boer, and C. Worrel, Tetrahedron, 33, 127 (1977). (6) L. V. Vilkov and W. I. Sadova, Dokl. Akad. Nauk SSSR, 162,565 (1965). (7) W. J. E. Parr and T. Schaefer, J. Am. Chem. SOC.,99,1033 (1977).
to the methine proton of the cyclopropyl moiety has been measured and interpreted in terms of a 2.0 (3) kcal/mol barrier.7 An earlier NMR study reported a 1.4 kcal/mol barrier based on temperature dependence of the chemical shifts.* A Raman study of liquid p-cyclopropylbenzene9 reported observation of the first overtone of the torsional frequency and calculated a 5.8 kcal/mol barrier. To produce useful LRMW band spectra a molecule must be polar and prolate. In order for the LRMW rotational constant, B + C, to be sensitive to conformational change, the groups attached to the torsional bond must be asymmetrical.1° Accordingly, we studied p-cyclopropylbenzaldehyde (Figure 1)rather than cyclopropylbenzene. This study determines the conformational species present and the barrier to internal rotation about the phenylcyclopropyl bond in gaseous p-cyclopropylbenzaldehyde.
Experimental Section LRMW spectra of p-cyclopropylbenzaldehydewere recorded at room temperature from 26.5 to 35 GHz on a Hewlett-Packard 8460-A microwave spectrometer using standard procedures.' The sample pressure was 30 mtorr. Additional spectra were recorded with slower scan rates and a 0.3-s time constant. Spectra were recorded with Stark fields of 3200 and 320 V/cm. A mixture of 0- and p-cyclopropylbenzaldehyde was prepared." p-Cyclopropylbenzaldehyde was isolated by preparative GLC on a 6-ft, diethylene glycol succinate on Chromosorb W column. The product produced NMR and IR spectra characteristic of p-cyclopropylbenzaldehyde.
-
Results and Discussion LRMW spectral data of p-cyclopropylbenzaldehyde appear in Table I. Three R-branch, a-type band series designated syn (S), anti (A), and free rotor (FR) are present. They have B + C values of 1066.0 (2), 1042.7 (4), and 1055.5 (1)MHz, respectively. The relative intensity ratio, S:AFR, is -1:1:20. Half-height bandwidths are -75 MHz for all three species. Two torsional angles are present in p-cyclopropylbenzaldehydeshown in Figure 1. The (8) G. L. Closs and H. B. Klinger, J.Am. Chem. Soc., 87,3265 (1965). (9) A. V. Bobrov, Opt. Spectrosc. (Engl. Transl.), 33, 471 (1972). (IO) M. S. Farag and R. K. Bohn, J. Chem. Phys., 62, 3946 (1975). (11) M. R. Harnden, R. R. Rasmussen, and E. J. Baker, J. Chem. SOC. C, 8, 2095 (1968).
0022-3654/03/20%7-462%!§0 1.50/0 0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87,
LRMW Studies of p -Cyclopropylbenzaldehyde
No. 23, 1983 4629
TABLE I: LRMW Spectral Data of p-Cyclopropylbenzaldehyde syn conformer
J+ 1 26 27 28 29 30 31 32 33 34 avB+ C
v,
MHz
27 710 28 790 29 840 30 910 31 980 33 050 34 115 35 190
anti conformer
B t C , MHz 1065.8 1066.3 1065.7 1065.9 1066.0 1066.1 1066.1 1066.4
v,
MHz
28 165 29 210 30 225 31 270 32 320 33 370 34 390 35 460
1066.0 ( 2 )
Ka
- 0.97 (1)
fwhm,b MHz relative intensity ( 2 5 "C)
75 1
B
+ C, MHz
1043.2 1043.2 1042.2 1042.3 1042.6 1042.8 1042.1 1042.9 1042.7 ( 4 ) -0.97 (1) 75 -1
a K = ( 2 B - A - C ) / ( A- C ) where A, B , and C were calculated from model structures. maximum.
n
H
L
U
bisected 'I=C?l8O0
orthogogal 'I= 90
Figure 1. Structure and possible conformations of p -cyclopropylbenzaldehyde.
barrier to internal rotation of the aldehyde group in benzaldehyde has been studied by microwave12and infrared13 spectroscopy and determined to be -5 kcal/mol and the molecule is planar. For p-cyclopropylbenzaldehyderotational constants were calculated by using geometry reported for benzaldehyde" coupled with C-C bond lengths of 1.507 8, in the cyclopropyl ring and 1.48-8, exocyclic bond length; B + C, the LRMW rotational constant, and F , the internal rotational constant, were calculated as a function of ?r by using a computer program written by H. Pickett. B + C was found to vary sinusoidally from 1065 MHz for the syn conformer to 1039 MHz for the anti conformer. F varies from 0.61 cm-l for the syn conformer to 0.68 cm-l for the anti conformer. The observed B + C values of series designated syn and anti are consistent with syn and anti conformers. The very intense series designated FR has a B + C value which is the average of the B C values of the syn and anti conformers. Two interpretations for this series can be proposed. The FR series may correspond to a stable conformer with a torsioeal angle of -9OO. Its relative intensity is then consistent with a species 2 kcal/mol lower in energy than the syn and anti forms assuming that dipole moments, asymmetry parameters, and vibrational partition functions of the three forms are equivalent. A more plausible interpretation of the FR series is that it corresponds to molecules in states freely rotating above the internal rotation barrier about the phenylcyclopropyl bond. Previous calculations' indicate that these states will have B + C values which are the average of the function E + C (7). Analogous free-rotor band series have been
+
(12) R. K. Kakar, E.A. Rinehart, C. R. Quade, and T. Kojima, J. Chem. Phys., 52, 3803 (1970). (13)F. A. Miller, W. G. Fately, and R. E. Witkowski, Spectrochim. Acta, Part A , 23, 891 (1967).
free-rotor states v,
MHz
27 440 28 500 29 550 30 610 31 665 32 725 33 770 34 830 35 885
B
+ C, MHz
1055.4 1055.6 1055.4 1055.5 1055.5 1055.6 1055.3 1055.5 1055.4 1055.5 (1)
-7520 fwhm: full width at half.
identified and characterized for derivatives of isopropylbenzene. The free-rotor population as a function of barrier height for a twofold potential function has previously been reported' and found to be almost independent of the internal rotation constant F. Using this relationship' the observed relative intensity ratio S:AFR of 1:1;20 is consistent with a barrier height of 170 cal/mol. This is an upper limit on the barrier heights since bands of the FR series probably are not completely modulated at 3200 V/cm. At 320 V/cm the relative intensity ratio is only 1:1:2. The Stark field dependence of the relative intensity ratio is consistent with our model which predicts that free-rotor bands will be composed of a dense manifold of weak lines in contrast to bands of the stable conformers which will be composed of strong lines. If the series designated FR is assigned to a stable 90° conformer, the very strong dependence of the relative intensity on Stark voltage remains a mystery. The authors suggest 120 f 90 cal/mol as the best value and uncertainty range consistent with the LRMW results. The He(1) photoelectron spectral study of cyclopropylbenzene derivatives5 concluded that, since substitution of bulky groups near the phenyl-cyclopropyl bond did not significantly alter spectra, the phenyl-cyclopropyl conformation must be orthogonal. Implicit in their analysis is the assumption that the potential energy barrier is large. However, if the rotational barrier is low, Le., the electronic energy varies little with internal rotation, then it is not necessarily true that the photoelectron spectra will be sensitive to conformation. Our result is probably also not inconsistent with the Russian electron diffraction study6 which reported a mixture of two conformers. It is very difficult by electron diffraction to distinguish a mixture of two conformers from a low torsional barrier model in cyclopropylbenzene since only about one-fifth14 of the scattering data are sensitive to the torsional angle and those data are overlapped by scattering from interatomic distances which are independent of the torsion. NMR studies using the J method have reported a higher barrier of 2.0 (3) kcal/mol for cyclopropylbenzene in CS2 solution.' Recently, gas-phase NMR studies of N,N-di(14) Total electron scattering intensity from a molecule is approximately proportional to the sum over d atom pairs of the products of their atomic numbers, Z a: x&ZiZ.. Only the scattering from the atom pairs between ortho and meta C a n d k atoms to the H and methylene C atoms of the cyclopropyl group varies with the torsional angle. Thus, about onefifth of the total electron scattering by cyclopropylbenzene is sensitive to conformation. Vilkov and Sadova modeled this contribution on a mixture of two conformers. The distinction between a mixture of two conformers and a mixture sampling all structures in between is difficult by electron diffraction.
J. Phys. Chem. 1083, 87, 4630-4637
4630
methyltrifluoroacetamide demonstrated that solvent internal Dressure effects oan sienificantlv increase activation energies for processes with large positive activation volumes such as conformer internal rotation^.'^ This effect is consistent with the apparent phase dependence of the internal rotation barrier height for cyclopropylbenzene. The observation here of a low internal rotation barrier in p-cyclopropylbenzaldehydein the gas phase does not necessarily imply a lack of conjugation but rather that the Y
(15)B. D. Ross, N. S. True, and D. L. Decker, J. Phys. Chem., 87,89 (1983).
various factors contributing to the potential barrier nearly cancel. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We are grateful to Professor E. Bright Wilson of Harvard University for allowing the use of the microwave spectrometer supported by NSF Grant No. GP37066X. Calculations were carried out a t the University of Connecticut Computer Center. Registry No. p-Cyclopropylbenzaldehyde, 20034-50-8.
Intraconfigurational Absorption Spectroscopy of OS4+ in K,SnC16 and K,OsCI, Crystals 6. A. Kozikowsklf and T. A. Kelderllng' Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60680 (Received: January 11, 1983; I n Final Form: March 28, 1983)
Absorption and excitation spectra for Os4+:KzSnCg,KzOsCh, and KzOsBr6are discussed. At low temperatures crystals of these systems undergo phase transitions to lower symmetry structures. Our data indicate that the effects of lowered symmetry are in general small and that the spectra can be interpreted in a manner paralleling our earlier studies of Os4+in cubic environments. In pure K20sC16,new transitions were located which cannot be assigned to intraconfigurational transitions of Os4+. These may be due to either impurities or interconfigurational effects.
Introduction In recent years, several papers have appeared dealing with the absorption and luminescence spectroscopy of Os4+ Making use of in octahedral hexahalide both near-infrared absorption and magnetic circular dichroism (MCD) data, we have previously reported and assigned all of the intraconfigurational d-d transitions of Ir4+,Os4+,and Re4+in cubic host crystals having both C1and Br- ligands.*12 Our approach to interpretation of these data has used ligand field theory to fit calculated energy levels to the experimental ones by a variation of the ligand field parameters. We have found that parameters can be found which reproduce the lowest energy states but fail for those states closest to the charge-transfer (CT) threshold.12 Since other host systems of lower symmetry are also available, it would be of interest to determine if their lowered symmetry would greatly affect the energies of these d-electron states. The possibility of detecting new states, particularly those of excited d-electron configurations, in a lower symmetry and/or higher concentration (i.e., pure crystal) environment is also of interest. Additional data from interconfigurational transitions are needed to determine reliable values for the spectroscopic parameters, especially lODq, for these systems. In this paper, we present an analysis of the low-temperature absorption and excitation spectra of mixed crystals of Os4+ in the K2SnC16host lattice, and of pure K20sC1, and KzOsBrs crystals. KzSnC&, K20sC1,, and K20sBr6crystals undergo phase transitions to lower symmetry space groups at cryogenic temperatures. Thus, they provide distorted octahedral environments for study of 'Present address: Procter and Gamble, Miami Valley Labs, Cincinati. OH 45247.
0022-3654/83/2087-4630$0 1.50/0
these ions. Similar studies of Re4+and Ir4+will be published ~eparate1y.I~ At room temperature, K2SnC16crystallizes in the cubic anti-fluorite s t r u ~ t u r e . ' ~The crystal undergoes a phase (1)(a) Piepho, S. B.; Dickinson, J. R.; Spencer, J. A.; Schatz, P. N. Mol. Phys. 1972,24,609. (b) Weiss, L. C.; McCarthy, P. J.; Jasinski, J. P.; Schatz, P. N Inorg. Chem. 1978,17,2689. (2)(a) Nims, J. L.; Patterson, H. H.; Khan, S. M.; Valencia, C. M. Inorg. Chem. 1973,12, 1602. (b) Khan, S. M.; Patterson, H. H.; E n g strom, H. Mol. Phys. 1978,35,1623. (c) Reinberg, A. R. Phys. Rev. E. 1971,3,41. (3)Flint, C. D.; Paulusz, A. G. Mol. Phys. 1980,41, 907. (4)(a) Wernicke, R.;Eying, G.; Schmidtke, H. H. Chem. Phys. Lett. 1978,58,267.(b) Schonherr, T.; Wernicke, R.; Schmidtke, H. H. Spectrochim. Acta, Part A 1982,38, 679. (5)Durocher, D.; Dorain, P. B. J. Chem. Phys. 1974,61,1361.Dorain, P. B.;Patterson, H. H.; Jordan, P. C. Ibid. 1968,49,3845. (6)Homborg, H.; Preetz, W.; Barka, G.; Schatzel, G. 2.Naturforsch. E 1980,35,554.Homborg, H.2.Anorg. Allg. Chem. 1980,460,27.Allan, G. C.; Al-Mobarak, R.; El-Sharkawy, G. A. M.; Warren, K. D. Inorg. Chem. 1972,11, 787. Ikeda, K. I.; Maeda, S. Ibid. 1978,17, 2698. Dickinson, JdR.; Johnson, K. E. Mol. Phys. 1970,19,19. (7)Schatz, P. N.;Shiflett, R. B.; Spencer, J. A.; McCaffery, A. J.; Piepho, S. B.; Dickinson, J. R.; Lester, T. E. Symp. Faraday SOC.1969, 3,14. Piepho, S. B.; Dickinson, J. R.; Spencer, J. A.; Schatz, P. N. Mol. Phys. 1972,24,609.Inskeep, W. H.; Schwartz, R. W.; Schatz, P. N. Ibid. 1973,25,805.Piepho, S. B.;Inskeep, W. H.; Schatz, P. N.; Preetz, W.; Homborg, H. Ibid. 1975,30,1569. (8) Jorgensen, C. K. Discuss. Faraday SOC.1958,26,175.Jorgensen, C. K. Mol. Phys. 1959,2,309.Jorgensen, C. K. Ibid. 1960,3,201.Jorgensen, C. K. Acta Chem. Scand. 1962,16, 793. Johannesen, R. B.; Candela, G. A. Inorg. Chem. 1963,2,67. (9) Keiderling, T. A,; Stephens, P. J.; Piepho, S. B.; Slater, J. L.; Schatz, P. N. Chem. Phys. 1975,1I,343. (10)Kozikowski, B. A,; Keiderling, T. A. Mol. Phys. 1980,40, 477. (11)Kozikowski, B. A.; Keiderling, T. A. Chem. Phys. 1980,53,323. (12)Kozikowski, B.A. Ph.D. Thesis, University of Illinois at Chicago Circle, Chicago, IL, 1982. (13)Kozikowski, B. A.; Yoo, R. K.; Keiderling, T. A., unpublished results. (14)Lerbscher, J. A.; Trotter, J. Acta Crystallogr., Sect. E 1976,32, 2671.
0 1983 American Chemical Society
