Rotational Isomerism in O-Methylphenol: Matrix-Isolation Infrared Study and MNDO Calculations

Rotational isomerism in o-methylphenol was studied by matrix-isolation infrared spectroscopy and MNDO calculations. Monomer molecules were isolated both in the argon matrix and in the nitrogen matrix near 10°K. The resolution of one weak band from the main hydroxyl absorption in the stretching and torsional regions evidently indicates the existence of two stable rotational isomers. Results of MNDO semi-empirical molecular orbital calculations with full geometry optimization indicate that only two stable conformations can exist with the trans (stag) conformation being more stable than the cis (stag) conformation by 3.32 KJ/mole. The present experimental data are interpreted with the aid of theoretical MNDO calculations. The agreement between experiment and theoretical calculations is excellent. However, it was found that the CNDO/2 calculations would give misleading predictions on the relative stabilities of rotational isomers in the present case.