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Vibrational Study of Alkyl Isocyanates in Solution

Volume 46, Number 6 (June 1992) Page 972-980

Nyquist, Richard A.; Luoma, Davin A.; Putzig, Curt L.

The vasym.NCO frequencies for alkyl isocyanates occur at higher frequency in CHCl3 solution than in CCl4 solution. With the exception of tert-butyl isocyanate, the vasym.NCO mode increases in frequency as the mole % CHCl3/CCl4 increases. The vasym.NCO mode for tert-butyl analog increases in frequency up to a certain mole % CHCl3/CCl4 and then vasym.NCO decreases in frequency. The vasym.NCO mode for n-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 % CHCl3/CCl4 as determined by study of the vasym.NCO frequencies. In general, the vasym.NCO and the vsym.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 vasym.NCO, and the methyl analog exhibits three significant bands in this region of the spectrum. These IR bands are the result of vasym.NCO in Fermi resonance with combination tones, and the unperturbed frequencies have been calculated with the use of the recorded data.