Angular-radial coupling in the tunneling motion of (HCCH)2

Suni, Ian I.; Klemperer, William
January 1993
Journal of Chemical Physics;1/15/1993, Vol. 98 Issue 2, p988
Academic Journal
The torsion–rotation Hamiltonian is developed for the problem of two rods with a C2v equilibrium structure tunneling through a C2h saddle point, one rod rotating clockwise and the other counterclockwise with the two rods constrained to lie in a plane. The Hamiltonian is developed both for the case where the rod center-of-mass spacing is constant and the case where this spacing is an elliptical function of the torsional angle. The latter case is formally similar to Bunker’s semirigid bender. Both forms of the torsion–rotation Hamiltonian are then solved for (HCCH)2 and (DCCD)2 and spectral fits obtained. The results show that perturbation by an excited torsional state explains some spectral anomalies observed previously. The only spectral fit that includes all observed microwave and radiofrequency transitions and gives physically reasonable results is the model that allows for dependence of the intermolecular spacing on torsional angle. The best fit for (HCCH)2 yields an equilibrium C2v intermolecular distance of 4.465(2) Å and an intermolecular distance of 4.014(10) Å at the C2h saddle point. For (DCCD)2 intermolecular spacings of 4.450(2) and 3.991(15) Å are obtained. The radial shrinkage between the two configurations agree quite well for (HCCH)2 and (DCCD)2, 0.45(1) and 0.46(1) Å, respectively.


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