Abstract
The <i>v</i><sub>asym</sub>. NCS frequencies for alkyl isothiocyanates occur at higher frequency in CHCI<sub>3</sub> or CDCl<sub>3</sub> solution than in CCl<sub>4</sub> solution. The <i>v</i><sub>asym</sub>. NCS mode increases in frequency as the mole % CHCl<sub>3</sub>/CCl<sub>4</sub> increases. The <i>v</i><sub>asym</sub>. 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 <i>v</i><sub>asym</sub>. 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 <i>v</i><sub>asym</sub>. NCS, and an equation has not yet been developed to correct for FR in this case. The unperturbed <i>v</i><sub>asym</sub>. NCS frequencies for alkyl isothiocyanates decrease in the alkyl order: methyl, butyl, ethyl, and tert-butyl. The decrease in frequency of <i>v</i><sub>asym</sub>. 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.
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