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

Abstract

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((2)P(3/2)) and X*((2)P(1/2)). 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, β(ι). 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.

Ground‐state dissociation pathways for the molecular cation of 2‐chlorothiophene: A time‐of‐flight mass spectrometry and computational study

“…At this point, it will be useful to compare our earlier results on the photodissociation of neutral 2‐chlorothiophene upon excitation at 235 nm where we monitored the dynamics of only one channel, i.e. the Cl atom formation channel . The present study which monitors the various product ions in the fragmentation of the 2‐chlorothiophene cation gives an indication of various possible channels that might be happening in the neutral 2‐chlorothiophene as well.…”

“…the Cl atom formation channel. 36 The present study which monitors the various product ions in the fragmentation of the 2chlorothiophene cation gives an indication of various possible channels that might be happening in the neutral 2-chlorothiophene as well. Assuming that the dissociation channels in the neutral and cationic parent compound are similar, a lot of information can be gathered in this respect.…”

“…The laser‐based ionization experimental setup with a supersonic molecular beam (MB) facility equipped with a time‐of‐flight (TOF) mass spectrometer has been described in previous publications . Briefly, it consists of an expansion chamber, ionization chamber and a flight tube.…”

“…The laser-based ionization experimental setup with a supersonic molecular beam (MB) facility equipped with a time-of-flight (TOF) mass spectrometer has been described in previous publications. [35][36][37][38] Briefly, it consists of an expansion chamber, ionization chamber and a flight tube. A pulsed supersonic molecular beam was generated using a solenoid valve, with a 0.8 mm nozzle and 500 μs opening time.…”

Dissociation pathways for the molecular cation of 3,4‐dichloro‐1,2,5‐thiadiazole: A time‐of‐flight mass spectrometry and computational study

“…The laser‐based ionization experimental setup with a supersonic molecular beam (MB) facility equipped with a time‐of‐flight (TOF) mass spectrometer has been described in detail elsewhere . Briefly, it consists of an expansion chamber, ionization chamber and a flight tube.…”