Unimolecular processes in CH2OH below the dissociation barrier: O–H stretch overtone excitation and dissociation

Abstract

The OH-stretch overtone spectroscopy and dynamics of the hydroxymethyl radical, CH(2)OH, are reported in the region of the second and third overtones, which is above the thermochemical threshold to dissociation to H+CH(2)O (D(0)=9600 cm(-1)). The second overtone spectrum at 10 484 cm(-1) is obtained by double resonance IR-UV resonance enhanced multiphoton ionization (REMPI) spectroscopy via the 3p(z) electronic state. It is rotationally resolved with a linewidth of 0.4 cm(-1) and displays properties of local-mode vibration. No dissociation products are observed. The third overtone spectra of CH(2)OH and CD(2)OH are observed at approximately 13 600 cm(-1) by monitoring H-atom photofragments while scanning the excitation laser frequency. No double resonance REMPI spectrum is detected, and no D fragments are produced. The spectra of both isotope analogs can be simulated with a linewidth of 1.3 cm(-1), indicating dissociation via tunneling. By treating the tunneling as one dimensional and using the calculated imaginary frequency, the barrier to dissociation is estimated at about 15 200 cm(-1), in good agreement with theoretical estimations. The Birge-Sponer plot is linear for OH-stretch vibrations 1nu(1)-4nu(1), demonstrating behavior of a one-dimensional Morse oscillator. The anharmonicity parameter derived from the plot is similar to the values obtained for other small OH containing molecules. Isomerization to methoxy does not contribute to the predissociation signal and the mechanism appears to be direct O-H fission via tunneling. CH(2)OH presents a unique example in which the reaction coordinate is excited directly and leads to predissociation via tunneling while preserving the local-mode character of the stretch vibration.

Ab Initio Chemical Kinetics for the CH3 + O(3P) Reaction and Related Isomerization–Decomposition of CH3O and CH2OH Radicals

“…Recently, Dames and Golden 49 computed the barrier of 40.0 kcal/mol at the RCCSD(T)/CBS level. In 2006, Wei et al 73 published their spectroscopic experimental result for CH 2 OH → CH 2 O + H. The decomposition barrier they obtained is 43.5 kcal/mol, which is greater than our CCSD(T) value by 4.0 kcal/mol, although their heat of reaction, 27.5 kcal/mol, is almost the same as ours predicted at the CCSD(T) level. It should be mentioned that earlier kinetic measurements 41 gave much smaller barriers, below 30 kcal/mol, for the decomposition process attributable in part to the pressure effect.…”

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

“…The production of H atoms upon excitation near the estimated 4ν 1 ← 0 transition was observed with the action spectrum showing a partially resolved rotational structure. In similar studies of CD 2 OH, 13 only H fragments were observed, indicating that isomerization to CHD 2 O was at best a minor channel, despite the fact that the available calculations 3,15 all placed the barrier for isomerization to methoxy lower than the barrier for OH bond breaking. As described previously, 10, 15 the minimum energy structure of CH 2 OH is non-planar and non-symmetric (C 1 ) with unequal CH bond lengths.…”

“…Gas-phase spectroscopic studies of vibrational levels such as the symmetric and antisymmetric CH stretch fundamental vibrations of CH 2 OH, as well as excitation of 1-4 quanta of the OH stretch (ν 1 mode) were reported previously. [11][12][13] Perhaps the most intriguing result of the previous studies was the observation of overtone-induced vibrational predissociation of CH 2 OH upon excitation of the 4ν 1 vibrational level. 13 In these studies, the excited OH bond coincided with the bond that was broken.…”

“…[11][12][13] Perhaps the most intriguing result of the previous studies was the observation of overtone-induced vibrational predissociation of CH 2 OH upon excitation of the 4ν 1 vibrational level. 13 In these studies, the excited OH bond coincided with the bond that was broken. In other cases of overtone-initiated a) Author to whom correspondence should be addressed.…”

The equilibrium OH bond length

“…The fundamental OH stretch was measured at 3650 cm À1 in Ar-matrix and at 3637 cm À1 in N 2 -matrix. The overtone spectra were also measured up to v OH ¼ 4 [58] giving! ¼ 3857.7(29) cm À1 and!x ¼ 91.4(10) cm À1 , leading to is (OH) ¼ 3674.9 cm À1 .…”