2-Methoxyphenyl isocyanate and 2-Methoxyphenyl isothiocyanate: Conformers, vibration structure and multiplet Fermi resonance

Abstract

IR and Raman spectral measurements in the region 3500–400/50cm−1 have been made for the liquid samples of 2-Methoxyphenyl isocyanate and 2-Methoxyphenyl isothiocyanate. A complete assignment of the measured bands has been proposed as aided by conformational and vibration analyses at B3LYP/6-311++G** level of calculations. Three conformers for 2-Methoxyphenyl isocyanate and two for 2-Methoxyphenyl isothiocyanate have been determined. The tilt of the isocyanate (NCO) and isothiocyanate (NCS) moieties with respect to phenyl ring are in broad agreement with their parents. Stretching mode frequencies of methyl group (–OCH3) in 2-Methoxyphenyl isocyanate have been lowered in the 2900–2800cm−1; deformation asymmetric modes are IR strong and symmetric one Raman strong. In 2-Methoxyphenyl isothiocyanate, a similar pattern is true for stretching modes but deformation asymmetric modes are IR strong and symmetric mode has not been observed. Multiplet absorption band system near 2200cm−1 in 2-Methoxyphenyl isocyanate has been interpreted to be caused by Fermi resonance. A similar pattern in absorption near 2100cm−1 in 2-Methoxyphenyl isothiocyanate but more complex Raman band pattern has also been explained through Fermi resonance from heuristic stand-point. Many Raman modes in 1300–1100cm−1 are intensified apparently owing to isothiocyanate than isocyanate moiety. Phenyl ring breathing mode is shifted to 1040cm−1 as strong Raman; the symmetric stretching mode of O–CH3 near 1023cm−1 as strong absorption.