The Infrared Intensities of Stretching Fundamentals in Gaseous and Crystalline Cyanoacetylene

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Abstract
The absolute intensities of the stretching fundamentals of cyanoacetylene and cyanoacetylene-d were measured in gaseous and crystalline (77 K) states. As had been expected from the previous result for hydrogen cyanide, a striking intensity increase upon crystallization was observed not only for νCH(CD) also for the other fundamentals. The observed intensities were reduced to dipole-moment derivative with respect to the internal coordinates, ∂μ⁄∂r. Then, by taking advantage of the strictly linear geometry of hydrogen-bonded chains in the crystals, it was demonstrated that the derivative with respect to the hydrogen-bonded distance, [∂μ⁄∂rN···H], was primarily responsible for the remarkable change in ∂μ⁄∂r; this was in accordance with the previous conclusion for hydrogen cyanide. A discrepancy from this view which appeared for ∂μ⁄∂r3 was interpreted as being mainly caused by the vibronic behavior of π-electrons.

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