Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). I. A theoretical study

Abstract

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following the excitation of high OH stretch overtones is studied by quasi-classical molecular dynamics calculations using a global potential energy surface (PES) fitted to ab initio calculations. The PES includes CH(2)OH and CH(3)O minima, dissociation products, and all relevant barriers. Its analysis shows that the transition states for OH bond fission and isomerization are both very close in energy to the excited vibrational levels reached in recent experiments and involve significant geometry changes relative to the CH(2)OH equilibrium structure. The energies of key stationary points are refined using high-level electronic structure calculations. Vibrational energies and wavefunctions are computed by coupled anharmonic vibrational calculations. They show that high OH-stretch overtones are mixed with other modes. Consequently, trajectory calculations carried out at energies about ~3000 cm(-1) above the barriers reveal that despite initial excitation of the OH stretch, the direct OH bond fission is relatively slow (10 ps) and a considerable fraction of the radicals undergoes isomerization to the methoxy radical. The computed dissociation energies are: D(0)(CH(2)OH → CH(2)O + H) = 10,188 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,167 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,787 cm(-1). All are in excellent agreement with the experimental results. For CH(2)OH, the barriers for the direct OH bond fission and isomerization are: 14,205 and 13,839 cm(-1), respectively.

Combined quantum chemical and modeling study of CO hydrogenation on water ice

“…This conclusion agrees with the experimental exclusive detection of CH 3 O (Pirim et al 2010). Remarkably, calculated ∆E /L2 values for CH 3 O and CH 2 OH formation (14.5 kJ mol −1 and 41.1 kJ mol −1 ) agree reasonably well with those obtained by Kamarchik et al (2012) at the CCSD(T)/augcc-pvtz (22.6 kJ mol −1 and 47.1 kJ mol −1 ). The ZPE corrections do not affect the general trends described above (see values in brackets of Table 1), the main effects are an increase of the ∆E and ∆E r values by about 4 kJ mol −1 and 30 kJ mol −1 at most.…”

“…Previous works (Saebo et al 1983;Geers et al 1993;Walch 1993;Dertinger et al 1995;Taketsugu et al 1996;Marcy et al 2001;Petraco et al 2002;Kamarchik et al 2012) examined the isomerization of CH 3 O into CH 2 OH as a way to reach the most stable isomer, but both experiment and theory concluded that this isomerization is unlikely because the barrier for isomerization is significantly higher than the CH 3 O dissociation barrier. Accordingly, and considering the cryogenic temperatures of the interstellar medium at which the processes occur, formation of CH 2 OH is kinetically hampered in favor of CH 3 O formation.…”

Laboratory detection of the rotational-tunnelling spectrum of the hydroxymethyl radical, CH2OH

“…Therefore the C s symmetry-constrained geometry optimization has been performed by employing the B3LYP/6-311++G(d, p) method. The Gaussian package (Frisch et al 2009) has been used, and the resulting geometry, in close agreement with that of Kamarchik et al (2012), is cast in Table 1.…”

“…At equilibrium, CH 2 OH is a non-planar species belonging to the C 1 point group with unequal CH bond lengths. However, it is well accepted (Johnson & Hudgens 1996;Kamarchik et al 2012) that this radical in its ground vibrational state effectively behaves as a planar molecule of C s symmetry with a 2 A electronic ground state. Therefore the C s symmetry-constrained geometry optimization has been performed by employing the B3LYP/6-311++G(d, p) method.…”

High-Resolution Direct-Absorption Spectroscopy of Hydroxymethyl Radical in the CH Symmetric Stretching Region

“…Hydroxymethyl radical, CH 2 OH, has been the subject of over 100 experimental and theoretical papers in the last 40 years, 1 in large measure due to the important role it plays as a reactive intermediate in combustion and environmental chemistry. 2 As one example, the combustion initiation step for oxygenated hydrocarbon fuels such as methanol is thought to be hydrogen abstraction by O 2 to produce hydroxymethyl radical, i.e., CH 3 OH + O 2 → CH 2 OH + HO 2 .…”