Infrared Study of Alkyl Isothiocyanates in Carbon Tetrachloride and/or Chloroform Solutions
Abstract:The v asym. NCS frequencies for alkyl isothiocyanates occur at higher frequency in CHCI3 or CDCl3 solution than in CCl4 solution. The v asym. NCS mode increases in frequency as the mole % CHCl3/CCl4 increases. The v asym. NCS mode for butyl isothiocyanate occurs at an exceptionally high frequency, and this result is explained in terms of the formation of a pseudo-six-membered intramolecularly hydrogen-bonded ring. The v asym. NCS mode for alkyl isothiocyanates are corrected for Fermi resonance (FR), with the exception of the propyl analog. The propyl analog appears to have three other modes in FR with v asym. NCS, and an equation has not yet been developed to correct for FR in this case. The unperturbed v asym. NCS frequencies for alkyl isothiocyanates decrease in the alkyl order: methyl, butyl, ethyl, and tert-butyl. The decrease in frequency of v asym. NCS in the order methyl, ethyl, and tert-butyl isothiocyanate is attributed to weakening of the N=C=S bonds due to σ electron donation of each alkyl group. In the case of the butyl analog, the σ electron donation is partially compensated for by the postulated existence of a pseudo-six-membered hydrogen-bonded ring.
Document Type: Research Article
Affiliations: Analytical Sciences and Laboratory, 1897F Building, Dow Chemical Company, Midland, Michigan 48667
Publication date: June 1, 1993
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