Structural and Thermochemical Properties of Hydroxymethyl (CH2OH) Radicals and Cations Derived from Observations of B̃ 2A‘(3p) ← X̃ 2A‘‘ Electronic Spectra and from ab Initio Calculations

Abstract

B? ²A‘(3p) ← X? ²A‘‘ spectra of the isotopically substituted hydroxymethyl radicals (CH₂OH, CH₂OD, CD₂OH, CD₂OD) 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 ν₈ torsion modes and ν₉ CH₂-wag mode are strongly coupled and governed by nonharmonic potential energy functions; for example, for ¹²CH₂¹⁶OH(X? ²A‘‘) we obtain 2ν₈ = 846 ± 6 cm^-1(1σ), 1ν₉ = 234 ± 5 cm^-1, and 2ν₉ = 615 ± 6 cm^-1. Using MP2/6-311G(2df,2p) ab initio calculations, we constructed the two-dimensional potential energy surfaces that govern the ν₈ torsion modes and ν₉ CH₂-wag in the X? ²A‘‘ radical and the X? ¹A‘ cation core of the B? ²A‘(3p) Rydberg state. Energy levels calculated with these potential energy surfaces account for the REMPI bands originating from the ν₈ hindered rotor and the ν₉ CH₂-wag modes. The experimental and ab initio results lead to improved heat capacities and entropies (S°_298.15(CH₂OH) = 244.170 ± 0.018 J (mol K)^-1). Ab initio CBS-QCI/APNO calculations predict that Δ_fH°_298.15(CH₂OH) = ?18.4 ± 1.3 kJ mol^-1. Re-evaluation of photoionization data yields IP_a(CH₂OH) = 7.562 ± 0.004 eV. Re-evaluated photoionization appearance data, kinetic equilibrium data, and shock tube data indicate that Δ_fH°₀(CH₂OH⁺) = 718.1 ± 1.8 kJ mol^-1, Δ_fH°_298.15(CH₂OH⁺) = 716.4 ± 1.8 kJ mol^-1, Δ_fH°₀(CH₂OH) = ?11.5 ± 1.3 kJ mol^-1, and Δ_fH°_298.15(CH₂OH) = ?17.8 ± 1.3 kJ mol^-1. We report the proton affinity, PA₀(CH₂O) = 705.2 ± 1.9 kJ mol^-1. Thermochemical tables based upon these values are presented for CH₂OH and CH₂OH⁺.