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Structure and Dynamics of the Methane-Propane van der Waals Complex
journal contribution
posted on 2019-06-12, 19:42 authored by Karen I. Peterson, D. P. Pullman, Wei Lin, Andrea J. Minei, Eric A. Arsenault, Stewart E. NovickMicrowave transitions in the region
7–26 GHz were measured
for the methane–propane van der Waals complex. The nearly free
internal rotation of methane within the complex gives rise to three
states that do not relax even in a 5 K supersonic expansion. Eighteen
lines have been assigned to the lowest state and are well fitted to
a semirigid rotor model, with rotational constants A = 7553.8229 (24) MHz, B = 2483.9200 (8) MHz, and C = 2041.8692 (5) MHz, and six distortion constants. The
structure has the methane positioned above the plane defined by the
propane carbon atoms with a center-of-mass van der Waals bond distance
of 3.98 Å. This is significantly larger than the equilibrium
value of 3.71 Å found with ab initio calculations
done at the CCSD(T)-F12a/aug-cc-pVTZ level of theory. Further calculations
encompassing a large range of angular orientations of the methane
subunit indicate that angular motion produces a large zero-point contribution
to the energy, which not only lowers the effective barrier to internal
rotation of the methane but also increases the radial distance between
subunits. Therefore, although in the lowest energy structure the methane
can get close to the propane by interdigitating the hydrogens atoms,
the zero-point energy effectively flattens out the potential so that
the hydrogens become less restricting.
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zero-point energyequilibrium valuehydrogens atomsMethane-Propane van der Waals Complex Microwave transitionsmethane subunitzero-point contributionpropane carbon atomsenergy structuresemirigid rotor modelMHzCCSDdistortion constants3.98 Å.center-of-mass van der Waals bond distanceab initio calculations5 K3.71 Å
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