Determining the CH3SO2→CH3+SO2 barrier from methylsulfonyl chloride photodissociation at 193 nm using velocity map imaging

Abstract

These imaging experiments study the formation of the methylsulfonyl radical, CH(3)SO(2), from the photodissociation of CH(3)SO(2)Cl at 193 nm and determine the energetic barrier for the radical's subsequent dissociation to CH(3) + SO(2). We first state-selectively detect the angular and recoil velocity distributions of the Cl((2)P(3/2)) and Cl((2)P(1/2)) atoms to further refine the distribution of internal energy partitioned to the momentum-matched CH(3)SO(2) radicals. The internal energy distribution of the radicals is bimodal, indicating that CH(3)SO(2) is formed in both the ground state and low-lying excited electronic states. All electronically excited CH(3)SO(2) radicals dissociate, while those formed in the ground electronic state have an internal energy distribution which spans the dissociation barrier to CH(3) + SO(2). We detect the recoil velocities of the energetically stable methylsulfonyl radicals with 118 nm photoionization. Comparison of the total recoil translational energy distribution for all radicals to the distribution obtained from the detection of stable radicals yields an onset for dissociation at a translational energy of 70+/-2 kcal/mol. This onset allows us to derive a CH(3)SO(2) --> CH(3) + SO(2) barrier height of 14+/-2 kcal/mol; this determination relies on the S-Cl bond dissociation energy, taken here as the CCSD(T) predicted energy of 65.6 kcal/mol. With 118 nm photoionization, we also detect the velocity distribution of the CH(3) radicals produced in this experiment. Using the velocity distributions of the SO(2) products from the dissociation of CH(3)SO(2) to CH(3) + SO(2) presented in the following paper, we show that our fastest detected methyl radicals are not from these radical dissociation channels, but rather from a primary S-CH(3) bond photofission channel in CH(3)SO(2)Cl. We also present critical points on the ground state potential energy surface of CH(3)SO(2) at the //CCSD(T)/aug-cc-pV(Q + d)ZCCSD(T)/6-311++G(2df,p) level. We include harmonic zero-point vibrational corrections as well as core-valence and scalar-relativistic corrections. The CCSD(T) predicted barrier of 14.6 kcal/mol for CH(3)SO(2) --> CH(3) + SO(2) agrees well with our experimental measurement. These results allow us to predict the unimolecular dissociation kinetics of CH(3)SO(2) radicals and critique the analysis of prior time-resolved photoionization studies on this system.

Further Studies into the Photodissociation Pathways of 2-Bromo-2-nitropropane and the Dissociation Channels of the 2-Nitro-2-propyl Radical Intermediate

“…Approximately 40 ns after interaction, the fragments are ionized by 118 nm light (10.5 eV), created by tripling the third harmonic of a Nd:YAG laser through a xenon gas cell. 32 The ions pass through an electrostatic lens system containing a repeller and extractor plate held in a voltage ratio of 1.4:1. This lens assembly accelerates the fragments down a grounded TOF tube where they impact the detector.…”

Infrared absorption of CH3SO2 observed upon irradiation of a p-H2 matrix containing CH3I and SO2

“…11 Several theoretical computations predict the energy, geometry, and vibrational wavenumbers of CH 3 SO 2 and methoxysulfinyl radical (CH 3 OSO). [12][13][14][15][16][17][18] According to the most extensive computations, reactions of CH 3 with SO 2 might proceed via two paths: a barrierless channel to produce CH 3 SO 2 and a channel with a barrier of 49-58 kJ mol −1 to produce two conformers of CH 3 OSO, syn-CH 3 OSO, and anti-CH 3 OSO; anti-CH 3 OSO might transform via a barrier of ∼1 kJ mol −1 to syn-CH 3 OSO that is more stable than anti-CH 3 OSO by ∼8 kJ mol −1 , and more stable than CH 3 SO 2 by 21−37 kJ mol −1 . [16][17][18] Isomerization from CH 3 SO 2 to syn-CH 3 OSO is unlikely because of a large barrier of ∼200 kJ mol −1 .…”

Dissociation dynamics of the methylsulfonyl radical and its photolytic precursor CH3SO2Cl

“…Even though this latter conclusion was flawed, as the small basis size was not taken into account, it does illustrate the challenge in arriving at good predictions for these sulfur-containing molecules and the need for reliable experimental benchmarks. The preceding paper presents a basis set convergence analysis 7 showing that good energetics for the dissociation barrier to CH 3 +SO 2 are obtained with the CCSD͑T͒ method if one includes not only inner polarization functions ͑tight-d functions͒ in the basis functions for the sulfur atom, as suggested in seminal work by Martin,8 but also core-valence and relativistic corrections.…”