Absolute infrared intensities of cyclopropane-d and cyclopropane-d6: Dipole moment derivatives and polar tensors

Abstract

Integrated infrared absorption intensities for the fundamental vibrations of gas‐phase cyclopropane‐d₀ and cyclopropane‐d₆ were experimentally determined using nitrogen pressure‐broadening techniques. Although the experimental data are insufficient to resolve the relative sign combinations for the (∂P/∂Q_i) and (∂P/∂S_j) charge distribution parameters in the A₂′′ symmetry species, a definitive relative sign choice is indicated for the E′ dipole moment gradients. The normal coordinate transformations required for the dipole derivative analysis are based on the highly reliable force field derived by Duncan and Burns [J. Mol. Spectrosc. 30, 253 (1969)]. For ease both in computing the atomic polar tensor elements of cyclopropane and in comparing cyclopropane force constants and dipole derivatives to related systems, the Duncan and Burns potential function is recast in terms of a set of more convenient internal valence force constants. (∂P/∂Q_ii) dipole gradients determined by semiempirical CNDO/2 molecular orbital calculations are used to indicate a preferred sign for the dipole derivatives. Atomic polar tensors for the carbon and hydrogen atoms of cyclopropane are calculated and compared to those for methane and ethylene. The effective charge parameter for the hydrogen atom in cyclopropane is also discussed.