Photodissociation Dynamics of Halogenated Thiophenes at 235 nm: A Resonance Enhanced Multiphoton Ionization-Time-of-Flight (REMPI-TOF) Study

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 116; no. 44; pp. 10656 - 10667
• Main Authors: Kawade, Monali, Saha, Ankur, Upadhyaya, Hari P, Kumar, Awadhesh, Naik, Prakash D, Bajaj, P.N
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 08-11-2012
• Subjects: Anisotropy; Bromine; Channels; Dynamic tests; Energy distribution; Halogens - chemistry; Lasers; Mathematical models; Photochemical Processes; Photodissociation; Quantum Theory; Spectrometry, Mass, Electrospray Ionization; Thiophenes - chemistry; Time Factors
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp3068602

The photodissociation dynamics of halogen-substituted thiophenes, namely, 2-chlorothiophene and 2-bromo-5-chlorothiophene, has been studied in a supersonic molecular beam around 235 nm, using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique, by detecting the nascent state of the primary halogen atoms. A single laser has been used for excitation of halothiophenes, as well as for the REMPI detection of photoproducts, namely, chlorine and bromine atoms, in their spin–orbit states X(2P3/2) and X*(2P1/2). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin–orbit branching ratio, for chlorine and bromine atom elimination channels. State-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment anisotropies, βι. The TOF profiles for Cl, Cl*, Br, and Br* are found to be independent of laser polarization; i.e., the β is well characterized by a value of ∼0.0, within the experimental uncertainties. For 2-chlorothiophene, we have observed two components for the Cl and only one component for the Cl* atom elimination channel in the translational energy distributions. The average translational energies for the fast and the slow components of the Cl channel are 3.0 ± 1.0 and 1.0 ± 0.5 kcal/mol, respectively. For Cl*, the average translational energy is 3.5 ± 1.0 kcal/mol. For 2-bromo-5-chlorothiophene, we have observed only one component for Cl, Cl*, Br, and Br* in the translational energy distributions. The average translational energies for the Cl and Cl* channels are 3.5 ± 1.0 and 5.0 ± 1.0 kcal/mol, respectively, whereas the average translational energies for the Br and Br* channels are 2.0 ± 1.0 and 3.5 ± 1.0 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. The ΔH f 298 value for 2-chlorothiophene has been estimated theoretically to be 23.5 kcal/mol.