J. Phys. Chem. 1987, 91, 1063-1066 is absent in T C N E complexes such as those considered in this work. Prochorow and Tramerlg measured the polarized fluorescence spectra of complexes of T C N E with alkylbenzenes in low-temperature organic glass and concluded that the LE and C T transitions moments are perpendicular or very nearly perpendicular in the more stable complexes studied. Recent picosecond absorption studies of T C N E complexes with anthracene derivatives20also reveal the radical ion (D'+-A'-)character of the excited state reached upon irradiation in the charge-transfer band. The calculations presented in this work lend further support to the absence of LE/CT mixing in HMB:TCNE and HMB: TCNE:HMB. If, in these complexes, the line connecting the centers of symmetry of each molecule is perpendicular to the planes of the molecule, then the out-of-plane C T transition and in-plane LE transitions cannot share intensity. (19) Prochorow, J.; Tramer, A. J. Chem. Phys. 1967, 47, 775. (20) (a) Hilinski, E. F.; Masnovi, J. M.; Amatore, C.; Kochi, J. K.; Rentzepis, P. M. J. Am. Chem. SOC.1983, 105, 6167. (b) Hilinski, E. F.; Masnovi, J. M.; Kochi, J. K.; Rentzepis, P. M. J. Am. Chem. SOC.1984, 106, 807 1.
1063
Finally, it should be mentioned that a similar localization of the C T excitation in anthracene-trinitrobenzene crystal has been reported.21 In this work it was found that, although the donor is symmetrically associated with two acceptor molecules (according to the crystal structure), the C T excitation appears to localize on one DA pair and is accompanied by a lattice distortion. As in the HMB/TCNE system, the increase in oscillator strength over that of the 1:l complex is somewhat less than the value of two which would be expected if both acceptor molecules of the ADA triad participated in charge transfer.21a Acknowledgment. The support of the National Science Foundation through Grant CHE-8311762 is gratefully acknowledged. Registry No. (HMB)-(TCNE), 2605-01-8; (HMB),.(TCNE), 16012- 18-3; @-xylene)2.(bromoani1),96927-4 1-2. (21) (a) Hochstrasser, R. M.; Lower, S. K.; Reid, C. J . Mol. Spectrosc. 1965, 25,257. (b) Hochstrasser, R. M.; Lower, S. K.; Reid, C. J . Chem. Phys. 1964, 41, 1073.
Matrix Isolation Investigation of the Hydrogen-Bonded Complexes and Halogen-Exchange Reactions between the Hydrogen Halides and the fert-Butyl Halides Bruce S. Ault* and Candace E. Sass Department of Chemistry. University of Cincinnati, Cincinnati, Ohio 45221 (Received: August 11, 1986; I n Final Form: October 20, 1986)
The twin jet codeposition of HCl or HBr with the tert-butyl halides into argon matrices has led to the formation of weakly hydrogn bonded complexes. The hydrogen halide stretching mode shifted 100-200 cm-' to lower energy; several modes of the base (in each case including the carbon-halogen stretch) were perturbed as well. Single jet codeposition, where gas-phase equilibration of the reactants occurs prior to matrix condensation, of the same pairs of reactants led, in many cases, to different spectra than the twin jet and indicated that halogen exchange had occurred. When the halogen of the hydrogen halide was heavier than the halogen of the tert-butyl halide, gas-phase exchange took place, an observation which is in accord with bond dissociation energies and heats of formation. The perturbation to a given hydrogen halide increased as the halogen of the tert-butyl halide increased in atomic weight.
Introduction Numerous kinetic studies in the past 25 years have demonstrated that HCl and HBr serve as effective homogeneous catalysts for gas-phase isomerization and elimination reactions.'" In each case, the species undergoing isomerization has at least one basic site, and the formation of a hydrogen-bonded complex has been postulated as a reaction intermediate. The tert-butyl group is well-known for its ability to stabilize a positive charge,' suggesting that the hydrogen halide elimination of the tert-butyl halides might also be catalyzed by the hydrogen halides, through hydrogen-bond formation to the halogen atom. A similar effect has been reported for the dehydrobromination reaction of sec-butyl bromide, which is catalyzed by gaseous HC1.2 The tert-butyl halides are known to form complexes with certain strong Lewis acids, including BF3 (1) Ross, R. A.; Stimson, V. R. J . Chem. SOC.1962, 1602. (2) Stimson, V. R. Aust. J. Chem. 1971, 24, 961. (3) Maccoll, A,; Stimson, V. R. J . Chem. SOC.1960, 3087. (4) Cross, J. T. D.; Stimson, V. R. J . Chem. SOC.B 1967, 880. (5) Kairaitis, D. A.; Stimson, V. R. Aust. J. Chem. 1968, 21, 1711. (6) Maccoll, A.; Ross, R. A. J. Am. Chem. SOC.1965.87, 4997. (7) Butler, G. B.; Berlin, K. D. Fundamentals of Organic Chemistry; Press
Co.: New York, 1972; p 175. (8) Maccoll, A.; Stone, R. H . J. Chem. SOC.1961, 2756.
0022-3654/87/2091- 1063S01SO10
and GaBr3.9n10 With other Lewis acids, including the heavier boron trihalides, rapid halogen exchange has been observed." In addition, weak hydrogen bonding of the tert-butyl halides to solvent methanol has been reported, leading to a small shift (40 cm-I) in the 0-H stretching mode.I2 The matrix isolation techniqueI3-l5is ideally suited for the study of molecular interactions, of both a Lewis acid/baseI6-'* and h y d r ~ g e n - b o n d i n g ' ~nature. - ~ ~ Intermediate complexes may be (9) Perkampus, von H. H.; Baumgarten, E. Ber. Bunsen-Ges. Phys. Chem. 1964, 68, 496.
(10) Nakane, R.; Kurihara, 0.;Natsubori, A. J . Phys. Chem. 1964, 68, 2876. (1 1) Goldstein, M.; Haines, L. I. B.; Hemmings, J. A. G.J . Chem. SOC. Dalton Trans. 1972, 2260. (12) Krueger, P. J.; Mettee, H. D. Can. J. Chem. 1964, 42, 288. (13) Hallam, H., Ed. Vibrational Spectroscopy of Trapped Species; Wiley: New York, 1973. (14) Craddock, S.; Hinchliffe, A. J. Matrix Isolation; Cambridge University Press: New York, 1975. (1 5 ) Moskowits, M.; Ozin, G., Eds. Cryochemistry; Wiley: New York, 1976. (16) Ault, B. S. Znorg. Chem. 1981, 20, 2817. (17) Ault, B. S. J. Am. Chem. Soc. 1983, 105, 5742. (18) Sass, C. S.; Ault, B. S. J . Phys. Chem. 1986, 90, 1547. (19) Andrews, L. J . Mol. Strucr. 1983, 100, 281.
0 1987 American Chemical Societv
Ault and Sass
1064 The Jburnal of Physical Chemistry, Vol. 91, No. 5, I987
isolated immediately after formation, prior to any subsequent rearrangement. A n d r e w ~recently ~~ reported 1:1 complexes of HF with tert-butyl chloride and tert-butyl bromide, as part of study of the interaction of HF with alkyl halides. A distinct, but weak, interaction was noted, and the tert-butyl group did appear to stabilize hydrogen-bond formation somewhat compared to primary alkyl halides. Barnes and co-worker recently reported very briefly the formation of a complex between (CH3)3CCland HCl in argon mat rice^.^^,^^ With the continuing interest in hydrogen bonding25 and in homogeneous catalysis, a study was undertaken to characterize the intermediate molecular complexes of the tert-butyl halides with HCl and HBr.
Experimental Section The experiments in this investigation were carried out on a conventional matrix isolation apparatus, which has been described previously.26 HCl and HBr (both Matheson) were introduced into the stainless steel vacuum line from lecture bottles and purified by repeated free=thaw cycles at 77 K. tert-Butyl fluoride (Alfa) was introduced in a similar manner and found to contain substantial C 0 2 impurity, which was removed by distillation from a hexane/LN2 slush bath (-96 “C). Samples of tert-butyl chloride and tert-butyl bromide were prepared from the vapor above the purified liquid. Argon was used as the matrix gas in all experiments and was used without purification. Experiments were carried out in both single jet and twin jet modes; in the former experiments, the reagents were mixed in a single vacuum line and diluted to the desired ratio with argon. Consequently, complete mixing at room temperature occurred prior to deposition. In the twin-jet mode, each reagent was diluted in argon in a separate vacuum line and sprayed simultaneously onto the 14 K cold window. In these experiments, mixing only occurred immediately in front of the cold window, during the condensation process. Specta were recorded on one of three spectrometers, an IBM 98 Fourier transform IR, a Perkin-Elmer 983 IR, or a Beckman IR12 spectrophotometer. In all cases, the resolution was on the order of 1 cm-I, and the band positions were consistent from one instrument to the next, to within 1 cm-I.
Ar/(CH3I3CF * A r / H B r
i
2760
2680
2600 2520 2460 ENERGY ( c d )
*1300
1220
‘14C
Figure 1. Argon matrix infrared spectra of the reaction products arising from the twin jet codeposition of ?err-butyl fluoride with HCI (middle trace) and HBr (bottom trace) compared to a blank of tert-butyl fluoride (top trace), over selected spectral regions. Bands marked with an asterisk are due to parent HBr.
creased in intensity at the same rate. When HC1 and tert-butyl fluoride were deposited in a single jet experiment, a very different spectrum was obtained, with a major absorption at 3696 cm-’. None of thc product bands observed in the twin jet experiments were detected in the single jet experiment. Instead, numerous strong absorptions were noted which are easily assigned to tertbutyl chloride, by comparison to authentic spectra of this comResults pound. Finally, no absorptions of either HC1 or ?err-butyl fluoride Prior to the codeposition of the tert-butyl halides with the were observed in this single jet experiment. hydrogen halides, blank spectra were run of each reagent alone The twin jet codeposition of tert-butyl fluoride with HBr into in an argon matrix. The spectra obtained for the hydrogen halides an argon matrix gave results quite similar to those obtained from were in good agreement with previous matrix st~dies,~’-~O as were the spectra of tert-butyl bromide3’ and ~ h l o r i d e . ~ N~ o- lit~ ~ * ~ ~the HC1 experiment, namely, a strong, sharp product absorption at 2472 cm-’. In addition, new absorptions near the tert-butyl erature spectra of matrix-isolated tert-butyl fluoride were found fluoride parent modes were detected at 739, 858, and 1186 cm-l. for comparison; the spectrum obtained was in good agreement Again, a single jet experiment with this pair of reactants gave a with previous gas-phase spectra.33 quite different spectrum, with a strong absorption at 3690 cm-’, tert-Butyl Fluoride. The twin jet codeposition of tert-butyl as well as a number of strong absorptions which are readily fluoride with HCl into an argon matrix (each sample at a dilution assigned to tert-butyl bromide. N o bands of parent tert-butyl of 500/1) gave rise to a very sharp, intense absorption at 2756 fluoride were detected, and absorptions of parent HBr were very cm-‘ . weak. Figure 1 shows the infrared spectra obtained after the twin In addition, absorptions were noted near parent bands of jet deposition of tert-butyl fluoride with HCl and with HBr into tert-butyl fluoride at 739, 898, and 1185 cm-I. When the conargon matrices. centration of HCl was increased, while the concentration of In several experiments, “merged” jet deposition was employed, tert-butyl fluoride was held constant, all four product bands inwhere the two samples were prepared in separate vacuum lines (as in the twin jet experiments), but the two deposition lines were (20) Barnes, A. J. J . Mol. Struct. 1983, 100, 259. joined some 40 cm from the cold window. The goal of these (21) Ault, B. S.;Pimentel, G . C . J . Phys. Chem. 1973, 77, 1649. experiments was to enhance room-temperature mixing compared (22) Truscott, C. E.; Ault, B. S. J . Phys. Chem. 1984.88, 2323. to twin jet experiments, yet not reach equilibrium as in single jet (23) Arlinghaus, R. T.; Andrews, L. J . Phys. Chem. 1984, 88, 4032. (24) Evans, M. L. PhD. Thesis, University of Wales, 1975. deposition. In these experiments, the products observed were quite (25) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H. similar to those in the twin jet deposition experiments; only a small Freeman Co.: San Francisco, 1960. amount (95%). Whether this vibrational modes of the tert-butyl halide subunit. The single jet reaction is a homogeneous, gas-phase reaction or occurs on the reaction, on the other hand, led to a gas-phase halogen-exchange surface of the stainless steel vacuum line is not apparent. Further reaction during the equilibration process whenever the halogen kinetic studies will be required to clarify this point. of the hydrogen halide was heavier than the halogen of the The AH‘S of the exchange reactions, as calculated from either tert-butyl halide. This exchange reaction apparently occurs with bond dissociation e n e r g i e ~ or ~ ~standard s ~ ~ heats of f o r m a t i ~ n ~ ~ , ~ ’ low activation barrier; the AH for the exchange is quite small and (where available), are in agreement as to the exothermic direction exothermic as well. Kinetic studies should provide more details of reaction. However, these AH values are quite small, -2 to -5 into the nature of the reaction and the activation parameters. kcal/mol, at room temperature. Nonetheless, if entropic effects are neglected, the calculated AGO values lead to equilibrium Acknowledgment. We gratefully acknowledge support of this constants of 100 or more, in agreement with the observation above research by the National Science Foundation under Grant C H E 8400450. C.E.S. also acknowledges the Neff Foundation for fellowship support. (34) Eggers, K. W.; Cocks, A. T. Helu. Chim. Acta 1973, 56, 1516. (35) Eggers, K. W.; Cocks, A. T. Helv. Chim. Acta 1973, 56, 1537. (36) Stull, D. R.; Westrum, E.F., Jr.; Sinke, G. C. The Chemical Thermodynamics of Organic Compounds; Wiley: New York, 1969. (37) Cox, ,J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds; Academic: London, 1970.
Registry No. (CH&CBr, 507-19-7; HBr, 10035-10-6; HCI, 764701-0; (CH,)SCF, 353-61-7; (CH,),CCI, 507-20-0. (38) Jorgensen, W. L. Chem. Phys. Lett. 1978, 53, 5 2 5 .
