Photodissociation Dynamics of Phosphorus Trichloride (PCl3) at 235 nm Using Resonance Enhanced Multiphoton Ionization (REMPI) with Time-of-Flight (TOF) Mass Spectrometry

The influence of substituents in governing the strength of the P-X bonds of substituted halophosphines R1R2P-X (X = F and Cl)

In this study, the gas-phase homolytic P-F and P-Cl bond dissociation energies (BDEs) of a set of thirty fluorophosphine (R1R2P-F) and thirty chlorophosphine-type (R1R2P-Cl) molecules have been obtained using the high-level W2 thermochemical protocol. For the R1R2P-F species, the P-F BDEs (at 298 K) differ by up to 117.0 kJ mol-1, with (H3Si)2P-F having the lowest BDE (439.5 kJ mol-1) and F2P-F having the largest BDE (556.5 kJ mol-1). In the case of the chlorophosphine-type molecules, the difference in BDEs is considerably smaller (i.e., 72.6 kJ mol-1), with (NC)2P-Cl having the lowest P-Cl BDE (299.8 kJ mol-1) and (HO)2P-Cl having the largest (372.4 kJ mol-1). We have further analyzed the effect of substituents in governing the P-F and P-Cl BDEs by considering the effect of substituents in the parent halogenated precursors (using molecule stabilization enthalpies) and the effect of substituents in the product radicals (using radical stabilization enthalpies). Finally, we have also assessed the performance of a wide range of DFT methods for their ability to compute the gas-phase P-F and P-Cl BDEs contained in this dataset. We find that, overall, the double hybrid functional DSD-PBEB95 offers the best performance for both bond types, with mean absolute deviations of just 2.1 (P-F BDEs) and 2.2 (P-Cl BDEs) kJ mol-1.

Ultraviolet photodissociation dynamics of PCl3 at 235 nm: three-dimensional ion imaging and theoretical analysis

The photodissociation dynamics of PCl3 at 235 nm has been studied by monitoring ground state Cl(2P3/2) and spin-orbitally excited Cl(2P1/2) atoms by resonance enhanced multiphoton ionization (REMPI). Also, the PCln+ (n = 0, 1, 2) photoions were observed non-resonantly. The speed distributions and speed-dependent anisotropy parameters β for all these particles have been determined by three-dimensional photofragment ion imaging. The β parameters are close to zero for all these particles. The relative yield of excited Cl(2P1/2) atoms for photodissociation of PCl3 is 0.49 ± 0.03. The speed distributions of Cl(2P3/2) and Cl(2P1/2) atoms are bimodal, mainly resulting from the sequential photodissociation PCl3 → PCl2 → PCl. Near-resonant two-photon ionisation followed by near-resonance one-photon photodissociation is proposed for the production of PCln+ photoions. Using Condon's reflection principle, we constructed the vibrational mode dependent absorption spectrum by accounting for the vibrational motion for all six normal coordinates. As a result, the absorption of 235 nm radiation by PCl3 occurs due to zero vibrational motion rather far from the equilibrium geometry. In particular, movement along the Q2 normal coordinate results in an efficient photoinduced transition into the excited state à of D3h symmetry due to a parallel transition.

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

RATIONALE

Halogenated thiophenes are an important class of compounds mostly used in the synthesis of various materials, showing unusual electronic and optical properties. The Thiophene Ring Fragmentation (TRF) process is widely used in synthetic chemistry. In this study, the fragmentation pattern of the molecular cation of halogenated thiophene, namely, 2-chlorothiophene, was monitored to establish its dissociation mechanism.

METHODS

The molecular cation of 2-chlorothiophene was prepared using multiphoton excitation using a laser at 235 nm. Various product ions upon fragmentation of the molecular ion were mass analyzed using time-of-flight mass spectrometry. Laser power dependence studies were also conducted for various product ions to arrive at the dissociation mechanism. Theoretical calculations were carried out to estimate the reaction enthalpies for various reactions and compared with the experimental data available in the literature.

RESULTS

The most abundant product ion was observed as the HCS⁺ radical cation followed by the C₃ H₃ ⁺ ion and the H₂ CCCCS⁺ radical cation. Other product ions such as SCCl⁺ , ClHCCS⁺ radical cations were also observed to a lesser extent in the fragmentation pattern of the parent molecular ion. Various dissociation channels were identified and supported with ab initio calculation. It has been inferred that the TRF process is usually initiated by the H/Cl atom transfer process. The appearance energies of the various fragment ions were also estimated theoretically and compared with literature values.

CONCLUSIONS

In conclusion, the fragmentation pattern of the molecular cation of 2-chlorothiophene was studied and the formation mechanisms of various product ions have been assigned. The appearance energies of the various fragment ions were also calculated. Finally, it is inferred that a TRF process is initiated by the H/Cl atom migration and subsequent ring opening either by C-C or C-S bond cleavage leading to the various isomers and their subsequent fragmentation. The ionization energies were accurately predicted for various species using ab initio calculation.

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

RATIONALE

1,2,5-Thiadiazoles are an important class of compounds mostly used in synthetic chemistry, and as herbicides, insecticides, drugs, organic conductors, etc. Recently, they have been used as a source for the generation and study of nitrile N-sulfides, RCNS, and its isomers. In this study, we monitor the fragmentation pattern of ionic halogenated 1,2,5-thiadiazoles, namely, 3,4-dichloro-1,2,5-thiadiazole, which generates the nitrile sulfides, to establish its various dissociation mechanisms.

METHODS

The molecular cation of 3,4-dichloro-1,2,5-thiadiazole was prepared using multiphoton excitation using a laser at 235 nm. Various product ions upon fragmentation of the molecular ion were mass analyzed using time-of-flight mass spectrometry. Laser power dependence studies were conducted for various product ions to arrive at the dissociation mechanism. Theoretical calculations were performed for the estimation of the ΔH values for various reactions to support the experimental data.

RESULTS

The most abundant product ion was observed to be the NS⁺ radical cation followed by the S⁺ ion and the SCl⁺ radical cation. The other product ions such as the CNS⁺ radical cation and the ClCNS⁺ and ClCN⁺ cations were also observed to a lesser extent in the fragmentation pattern of the parent molecular ion. Various dissociation channels were identified and supported with ab initio calculations.

CONCLUSIONS

In conclusion, we have studied the fragmentation pattern of the molecular cation of 3,4-dichloro-1,2,5-thiadiazole and the formation mechanisms of various product ions have been assigned. It has been also observed that most of the product ions are nitrile N-sulfides. Finally, it is inferred that there are two primary paths for the fragmentation of the parent molecular cation, namely, (1) Cl atom migration and subsequent ring opening by N-S bond cleavage and (2) direct ring opening by N-S bond cleavage. The ionization energies were accurately predicted for various species using ab initio calculations. Copyright © 2016 John Wiley & Sons, Ltd.

Near-threshold photodissociation dynamics of CHCl3

Energy- and angle-resolved photofragment distributions for ground-state Cl ((2)P3/2) and spin-orbit excited Cl* ((2)P1/2) have been recorded using the velocity map imaging technique after photodissociation of chloroform at wavelengths of 193 and ∼235 nm. Translational energy distributions are rather broad and peak between 0.6 and 1.0 eV. The spin-orbit branching ratios [Cl*]/[Cl] are 1 and 0.3 at 193 and 235 nm, respectively, indicating the involvement of two or more excited state surfaces. Considering the anisotropy parameters and branching ratios collectively, we conclude that the reaction at 193 nm takes place predominantly on the (1)Q1 surface, while the (3)Q1 surface gains importance at lower dissociation energies around 235 nm.