Microwave Spectra of Some Isotopically Substituted Phenols

The rotational spectra of C₆H₅¹⁸OH, C₆D₅OH, and C₆D₅OD were successfully analyzed to obtain their respective rotational constants in the region from 20 000 to 31 000 Mc/sec. The splitting of the rotational transitions due to the hindered internal rotation of the hydroxyl group was noted in C₆H₅¹⁸OH and C₆D₅OH while the spectrum of the C₆D₅OD isotope was unsplit. As is the case in C₆H₅¹⁶OH, the high barrier to internal rotation produced a doublet spectrum, where the splittings were effectively independent of the J transition and correspond to 115 and 103 Mc/sec for the C₆D₅OH and C₆H₅¹⁸OH isotopes, respectively. The experimental rotational constants are: $$\begin{array}{cccc}\mathrm{Isotope}& \mathrm{A(}\mathrm{Mc}/\sec )& \mathrm{B(}\mathrm{Mc}/\sec )& \mathrm{C(}\mathrm{Mc}/\sec )\\ {\mathrm{C}}_6{\mathrm{D}}_5^{16}\mathrm{OH}& \text{4682.677±0.006}& \text{2422.815±0.004}& \text{1596.930±0.004}\\ {\mathrm{C}}_6{\mathrm{D}}_5^{16}\mathrm{OD}& \text{4654.378±0.005}& \text{2342.102±0.004}& \text{1558.383±0.004}\\ {\mathrm{C}}_6{\mathrm{H}}_5^{18}\mathrm{OH}& \text{5650.076±0.008}& \text{2487.321±0.005}& \text{1727.232±0.005.}\end{array}$$ It was found that the structure of the C–O–H fragment could not be uniquely determined from the rotational constants. Two structures were found which were essentially the same except for C–O–H configuration relative to the internal rotation axis. Both structures fit the experimental spectra equally well and have standard deviations of 3.2 Mc/sec. The parameters for Structures (I) and (II) are, respectively: C–C, 1.3956±0.0002 and 1.3954±0.0002 Å; C–H, 1.081±0.001 and 1.082±0.001 Å; O–H, 0.985±0.01 and 0.944±0.009 Å; C–O, 1.370±0.001 and 1.379±0.001 Å; COH, 111.87±0.35 and 105.04±0.35 deg; α (off angle from rotation axis), 6.34±0.60 and −11.17±0.50 deg. In Structure (I) the oxygen and hydrogen are on the same side of the internal rotation axis while in Structure (II) they straddle it. Structure (II) is believed to be the more reasonable. Neither C–O–H structure could correlate the splitting data to one value of the barrier height, but good agreement was obtained if the moment of inertia of the hydroxyl group was based on CH₃OH. The barrier height obtained was 1150 cm⁻¹.