Structural and Thermochemical Properties of Hydroxymethyl (CH₂OH) Radicals and Cations Derived from Observations of B²A‘(3p) ← X²A‘‘ Electronic Spectra and fromab InitioCalculations

B̃ 2 A‘(3p) ← X̃ 2 A‘‘ spectra of the isotopically substituted hydroxymethyl radicals (CH 2 OH, CH 2 OD, CD 2 OH, CD 2 OD) were observed between 39 700 and 43 000 cm -1 by 2+1, 2+2, and 1+1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy. Analyses of the vibrational hot bands in these spectra show that the ν 8 torsion modes and ν 9 CH 2 -wag mode are strongly coupled and governed by nonharmonic potential energy functions; for example, for 12 CH 2 16 OH(X̃ 2 A‘‘) we obtain 2ν 8 = 846 ± 6 cm -1 (1σ), 1ν 9 = 234 ± 5 cm -1, and 2ν 9 = 615 ± 6 cm -1 . Using MP2/6-311G(2df,2p) ab initio calculations, we constructed the two-dimensional potential energy surfaces that govern the ν 8 torsion modes and ν 9 CH 2 -wag in the X̃ 2 A‘‘ radical and the X̃ 1 A‘ cation core of the B̃ 2 A‘(3p) Rydberg state. Energy levels calculated with these potential energy surfaces account for the REMPI bands originating from the ν 8 hindered rotor and the ν 9 CH 2 -wag modes. The experimental and ab initio results lead to improved heat capacities and entropies ( S ° 298.15 (CH 2 OH) = 244.170 ± 0.018 J (mol K) -1 ). Ab initio CBS-QCI/APNO calculations predict that Δ f H ° 298.15 (CH 2 OH) = −18.4 ± 1.3 kJ mol -1 . Re-evaluation of photoionization data yields IP a (CH 2 OH) = 7.562 ± 0.004 eV. Re-evaluated photoionization appearance data, kinetic equilibrium data, and shock tube data indicate that Δ f H ° 0 (CH 2 OH + ) = 718.1 ± 1.8 kJ mol -1, Δ f H ° 298.15 (CH 2 OH + ) = 716.4 ± 1.8 kJ mol -1, Δ f H ° 0 (CH 2 OH) = −11.5 ± 1.3 kJ mol -1, and Δ f H ° 298.15 (CH 2 OH) = −17.8 ± 1.3 kJ mol -1 . We report the proton affinity, PA 0 (CH 2 O) = 705.2 ± 1.9 kJ mol -1 . Thermochemical tables based upon these values are presented for CH 2 OH and CH 2 OH + .