Dipole moment derivatives and infrared intensities. II. Polar tensors in methyl halide molecules

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Abstract
Atomic polar tensors have been determined for H, C, and X atoms of the methyl halide molecules (CH3X), using infrared intensity data reported by Russell, Needham, and Overend. The signs for the ∂p/∂Qi’s have been chosen by considering Coriolis coupling constants, agreement between intensity parameter values for CH3X and CD3X, and by considering the values of the atomic polar tensors calculated by CNDO methods for CH3F and CH3Cl. The resulting atomic polar tensors show systematic trends for the CH3X molecules as X change from F to I. The possibility that atomic polar tensors can be transferred from one molecule to another (with some allowance for expected trends) cannot be ignored. Atomic effective charges for each atom have been calculated, and ξH is found to be nearly constant for the four molecules. Additional properties of the atomic polar tensor are examined, including the anisotropy. A ''modified bond moment hypothesis'' appears to be reasonably successful in modeling the infrared intensities of the methyl halides.