Dynamics of Methyl Radical Formation Following 266 nm Dissociative Photoionization of Xylenes and Mesitylene

Abstract

The 266 nm dissociative photoionization of three xylene isomers and mesitylene leading to the formation of methyl radical was examined. The total translational energy distribution profiles [P(E T )] for the methyl radical were almost identical for all of the three isomers of xylene and mesitylene, while a substantial difference was observed for the corresponding P(E T ) profile of the co-fragment produced by loss of one methyl group in m-xylene. This observation is attributed to the formation of the methyl radical from alternate channels induced by the probe. The P(E T ) profiles were rationalized based on the dissociation of {sp 2 }C−C{sp 3 } bond in the cationic state, wherein the {sp 2 }C−C{sp 3 } bond dissociation energy is substantially lower relative to the neutral ground state. The dissociation in the cationic state follows a resonant three-photon absorption process, resulting in a maximum translational energy of about 1.6−1.8 eV for the photofragments in the center-of-mass frame. Fitting of the P(E T ) profiles to empirical function reveals that the dynamics of {sp 2 }C−C{sp 3 } bond dissociation is insensitive to the position of substitution but marginally dependent on the number of methyl groups.

Modulating the Roaming Dynamics for the NO Release in ortho-Nitrobenzenes

“…The total translational energy distributions P ( E T ) for the NO release were broad (see Figure ) and were fitted to the bimodal empirical function − P ( E T ) = C · [ ( E T ) a 1 · ( E T max − E T ) b 1 + ( E T ) a 2 · ( E T max − E T ) b 2 ] where a 1 , a 2 , b 1 , and b 2 are adjustable parameters; C is a normalization constant; and E T max is the maximum total translational energy available for the fragments in the center-of-mass frame after dissociation. Figure illustrates that the fitting method clearly distinguishes the slow component (red dotted curve) and fast component (blue dotted curve), and the linear combination of the two functions gives a very good fit (green solid curve) to the experimental data (black solid curve).…”

Dissociative Photoionization of Dimethylpyridines and Trimethylpyridine at 266 nm: Dynamics of Methyl Radical Release

“…The remarkable agreement between the calculated {sp 2 }C–C{sp 3 } bond dissociation energies and the experimental data indicates that the dissociation of the methylpyridines occurs from the cationic ground state through a [1 + 1 + 1] three-photon dissociation process, which is favored due to the lower dissociation energy in comparison to the neutral ground state. Importantly, the observed dissociation dynamics of the {sp 2 }C–C{sp 3 } bond in methylpyridines are similar to methylbenzenes (xylenes) and suggest that the nitrogen atom substitution in the ring has only a marginal effect on the dissociation dynamics of the system. However, a favored dissociation of the {sp 2 }C–C{sp 3 } bond adjacent to the nitrogen is observed in the case of methylpyridines, unlike methylbenzenes.…”

“…30−32 In the case of xylenes and mesitylene, 266 nm excitation leads to dissociative photoionization resulting in {sp 2 }C−C{sp 3 } bond fission and formation of a methyl radical via a resonant three-photon absorption process, which is attributed to the substantial lowering of the {sp 2 }C−C{sp 3 } bond dissociation energy in the cation ground state relative to the neutral ground state. 33 In light of these observations, photodissociation studies on methylpyridines following 266 nm excitation were carried out on N-heterocyclic analogues of xylene and mesitylene using the velocity map ion imaging (VMI) technique. Comparison of the dissociative photoionization process in xylene and mesitylene with the corresponding pyridine analogs provides insights into the role of electronic Unlike methylbenzenes, the methyl groups present in methylpyridines exhibit variations based on their proximity to the nitrogen atom, which are denoted as x and y labels for methyl groups located closer and farther from pyridine nitrogen, respectively, with the exception of symmetric 2,6-dimethylpyridine.…”

“…Nevertheless, the alkyl-substituted derivatives of both the systems, namely, methylbenzenes and methylpyridines, undergo extensive isomerization following excitation to the excited singlet electronic states, resulting in various positional isomers due to the scrambling of the alkyl substituent. ,− Subsequent to internal conversion to the ground electronic state, the {sp 2 }­C–C­{sp 3 } and C–H bond fission are the major photodissociation channels in these molecules. − Furthermore, isomerization from being a six-membered to seven-membered via a C–C bond insertion mechanism has also been reported. ,, Interestingly, the energy barrier associated with the insertion-isomerization process (about 4.60 eV) ,, is comparable to the {sp 2 }­C–C­{sp 3 } bond dissociation threshold (around 4.50 eV), thereby resulting in a compelling interplay of competing pathways. Furthermore, the presence of an aromatic chromophore enhances the likelihood of (resonant) multiphoton absorption, allowing the molecules to access both the ground and excited states of the cation. − In the case of xylenes and mesitylene, 266 nm excitation leads to dissociative photoionization resulting in {sp 2 }­C–C­{sp 3 } bond fission and formation of a methyl radical via a resonant three-photon absorption process, which is attributed to the substantial lowering of the {sp 2 }­C–C­{sp 3 } bond dissociation energy in the cation ground state relative to the neutral ground state . In light of these observations, photodissociation studies on methylpyridines following 266 nm excitation were carried out on N-heterocyclic analogues of xylene and mesitylene using the velocity map ion imaging (VMI) technique.…”

Dynamics of Methyl Radical Formation Following 266 nm Dissociative Photoionization of Dimethyl- and Trimethyl- Pyridines

“…[30][31][32] In the case of xylenes and mesitylene 266 nm excitation leads to dissociative photoionization resulting in {sp 2 }C−C{sp 3 } bond fission and formation of methyl radical via a resonant three-photon absorption process, which is attributed to the substantial lowering of the {sp 2 }C−C{sp 3 } bond dissociation energy in the cation ground state relative to the neutral ground state. 33 In the light of these observations, photodissociation studies on methylpyridines following 266 nm excitation were carried out on N-heterocyclic analogues of xylene and mesitylene, using the Velocity Map Ion Imaging (VMI) technique. Comparison of the dissociative photoionization process in xylene and mesitylene with the corresponding pyridine analogues provides insights to role of electronic effects due to heteroatom substitution on the {sp 2 }C−C{sp 3 } bond dissociation.…”