Microwave Spectrum and Barrier to Internal Rotation of 4-Chlorophenol

Abstract

Abstract Microwave spectra of 4-chlorophenol were observed in the frequency ranges 8–18 and 26.5–40 GHz. Rotational constants were obtained for the ground vibrational states of three isotopic species, 35ClC6H4OH, 37ClC6H4OH, and 35ClC6H4OD, and for the first excited state of C–Cl out-of-plane bending mode for 35ClC6H4OH. All the b-type Q-branch transition lines of the OH species split into doublets, but those of the OD species do not. The separation of doublets is 158.99±0.16 MHz for the ground state of 35ClC6H4OH. An analysis of the doublets on the basis of the pseudo-symmetric internal rotor model gives the barrier height to the internal rotation of hydroxyl group as V2=13.98±0.37 kJ mol−1 for the 35Cl species. The V2 value obtained is compared with the theoretical V2 value calculated for 4-X-phenols (X=H, F, Cl) by means of ab initio MO with the 4-31G basis set.