β Scribed by P.R. Bunker
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This theoretical paper is concerned with the proton rearrangement energy levels and spectrum of the CH / 5 molecular ion, and it is based on the ab initio results of P. R. Schreiner, S-J. Kim, H. F. Schaefer, and P. v. R. Schleyer [J. Chem. Phys. 99, 3716-3720 (1993)]. The ab initio work predicts that the molecule should be considered as an H 2 molecule bound with a dissociation energy of about 15 000 cm 01 at the apex of a pyramidal CH / 3 group. At equilibrium the H 2 axis is nearly perpendicular to the C 3 axis of the CH / 3 group, eclipsing a CH bond. The internal rotation of the H 2 about the C 3 axis has a barrier height of 30 cm 01 . There is also an internal ''flip'' motion through a C 2Β£ structure, with a barrier of 300 cm 01 , that exchanges a CH / 3 and an H 2 proton in the molecule and that makes all 120 symmetrically equivalent minima accessible. Using the ab initio equilibrium structure and torsional barrier, the rotation-torsion energy levels and spectrum are calculated. The tilt of the C 3 axis of the CH / 3 group away from the internal rotation axis of the H 2 has a very significant effect on the energy levels. The tunneling resulting from the flip motion will produce splittings in the rotation-torsion energy levels and a spectrum that will have characteristic relative intensities because of the nuclear spin statistical weights. These weights are calculated using the complete nuclear permutation inversion group G 240 Γ S 5 1 {E, E*}, all elements of which are feasible. There are many levels with zero nuclear spin statistical weight, and this will make the spectrum simpler than would otherwise be the case.
π SIMILAR VOLUMES
The structures ofcthylidcne and propylidene in their lowest singlet states :ue considered using INDO lnd MIND 112 calculations. Using a gadicnt optimization method, it is found that R!INDO/Z predicts no encrg minimu.n f-or singlet ethylidenc, while INDO predicts non-&ssic4 bridged structures for the
Diiicrences between SiHS and CH; are more significant than the similarities. The proton affinity of Sib exceeds that of CH4 by ~25 kcal/mol. but the heat of hydrogenation of SiH; is smaller than that of CH: by nearly the mrne amount. Like CH:, the Cs structures of SiH: are preferred, but SiH: is bes