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Optica Publishing Group
  • Applied Spectroscopy
  • Vol. 46,
  • Issue 6,
  • pp. 972-980
  • (1992)

Vibrational Study of Alkyl Isocyanates in Solution

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Abstract

The <i>v</i><sub>asym.</sub>NCO frequencies for alkyl isocyanates occur at higher frequency in CHCl<sub>3</sub> solution than in CCl<sub>4</sub> solution. With the exception of tert-butyl isocyanate, the <i>v</i><sub>asym.</sub>NCO mode increases in frequency as the mole % CHCl<sub>3</sub>/CCl<sub>4</sub> increases. The <i>v</i><sub>asym.</sub>NCO mode for tert-butyl analog increases in frequency up to a certain mole % CHCl<sub>3</sub>/CCl<sub>4</sub> and then <i>v</i><sub>asym.</sub>NCO decreases in frequency. The <i>v</i><sub>asym.</sub>NCO mode for <i>n</i>-butyl isocyanate occurs at an exceptionally high frequency for the alkyl isocyanates studied, and this result is explained in terms of the formation of a pseudo-six-membered intermolecular hydrogen-bonded ring. Inductive and steric factors also influence the type and form of solvent/solute complexes formed vs. mole % CHCl<sub>3</sub>/CCl<sub>4</sub> as determined by study of the <i>v</i><sub>asym.</sub>NCO frequencies. In general, the <i>v</i><sub>asym.</sub>NCO and the <i>v</i><sub>sym.</sub>NCO modes for alkyl isocyanates decrease in frequency as the number of hydrogen atoms on the α-carbon atom of the alkyl group decreases from 3, to 2, to 1, to 0. Some of the alkyl isocyanates exhibit two significant IR bands in the region expected for <i>v</i><sub>asym.</sub>NCO, and the methyl analog exhibits three significant bands in this region of the spectrum. These IR bands are the result of <i>v</i><sub>asym.</sub>NCO in Fermi resonance with combination tones, and the unperturbed frequencies have been calculated with the use of the recorded data.

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