Infrared Intensities of Crystalline HCN and DCN

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Abstract
The absolute infrared intensities of crystalline HGN and DCN were measured at the temperature of liquid N2. In comparison with the gaseous values, both stretching fundamentals (v1, v3) exhibited a striking increase in the integrated absorption, while the bending (v2) was almost unchanged. From the observed intensity change, the change in the dipole-moment derivatives with respect to the molecular internal coordinates was calculated to be ∂ΔM⁄∂rC=N=1.43 D/Å and ∂ΔM⁄∂rC−H=2.01 D/Å, those values were chosen from two alternative possibilities as the more probable set. The significance of these values, however, is somewhat peculiar to the optically-active crystalline vibrations, so they were then resolved into derivatives with respect to purely intramolecular and intermolecular coordinates: [∂ΔM⁄∂rC−H]=0.58 D/Å and [∂ΔM⁄∂rH···N]=−1.43 D/Å. The general intensity features were reasonably interpreted by considering the contribution of a charge-transfer structure to the H–N hydrogen bonding.