O–D bond dissociation from the 3s state of deuterated hydroxymethyl radical (CH2OD)

Abstract

The photodissociation of the deuterated hydroxymethyl radical CH 2 OD is investigated on the lowest excited state, the 3s Rydberg state, in the wavelength region 365-318 nm where the D atom is the only significant product. The translational energy distribution and kinetic energy-dependent anisotropy parameter of the D channel are determined by the core-sampling time-of-flight technique at 352.5 nm. The negative recoil anisotropy parameter ␤ eff ϭϪ0.7Ϯ0.1 is consistent with the perpendicular nature of the transition from the ground state. The fraction of the available energy partitioned into the translational degree of freedom is 0.69. Ground state products CH 2 O (1 1 A 1) ϩD constitute the main photodissociation channel, and no significant H product is detected at these wavelengths. Comparison with the conical intersection calculations of Hoffman and Yarkony suggests that O-D bond rupture involves crossing from the 3s potential energy surface to a repulsive region of the ground state surface at a large O-D bond distance. Isomerization of CH 2 OD to CH 2 DO is not competitive with the fast O-D dissociation.

Interacting Rydberg and valence states in radicals and molecules: experimental and theoretical studies

“…In photodissociation of the hydroxyalkyl radicals, conical intersections with the ground state control the branching ratios and fragment internal energy distributions [22][23][24][25]121,122]. Electronic structure calculations on the prototype radical CH 2 OH show that the lowest four electronic states have a Rydberg 3s, 3p x , 3p y and 3p z character.…”

“…Take, for example, the series of substituted methyl radicals: CH 3 , CH 2 F, CH 2 Cl and CH 2 OH (to be described in more detail further) [21][22][23][24][25]. Despite their similarity, each gives rise to different excited states, optical transitions and dissociation mechanisms.…”

“…Moreover, the valence and Rydberg orbitals in CH 2 Cl are mixed in a way that depends sensitively on molecular geometry [26]. In CH 2 OH, on the other hand, multiple conical intersections couple the Rydberg states to the ground state in two simultaneous dissociative coordinates [22][23][24][25].…”

“…Using the deuterated analogue CH 2 OD, yield spectra of H and D photofragments were recorded to distinguish between O-H and C-H bond breaking pathways [22,23,121]. With 3s photolysis near the origin of the absorption (365-318 nm), only the O-D bond-breaking channel was observed, but at shorter wavelengths (5318 nm) H products were detected as well.…”

“…D photofragments were correlated energetically with the CH 2 O þ D channel, and their recoil anisotropy parameter, eff ¼ À0.7AE0.1, was close to the limiting value of À1 for the perpendicular 1 2 A 0 ð3sÞ 1 2 A 00 and 1 2 A 0 ð3p x Þ 1 2 A 00 transitions. Most of the available energy was channelled into translation, with cold formaldehyde as a co-product [22,23].…”