Photodissociation Dynamics of Small Aromatic Molecules Studied by Multimass Ion Imaging

Abstract

The photodissociation dynamics of various aromatic molecules, studied using multimass ion imaging techniques, is reviewed. The experimental data reveals new isomerization and dissociation mechanisms. Our investigation of benzene, pyridine, and pyrimidine finds that H-atom elimination thresholds remain the same for the three molecules. We also notice that ring-opening dissociation thresholds decrease rapidly with the increase of the number of nitrogen atoms in the aromatic ring. Hydrogen atom elimination is the sole dissociation channel for benzene at 193 nm. Along with H-atom elimination, we observe five distinct ring-opening dissociation channels for pyridine at 193 nm. No dissociation channels were observed for benzene and pyridine at 248 nm. Ring-opening dissociation channels are the major channels for pyrimidine, which dissociates at 193 nm and also at 248 nm. A six-membered to seven-membered ring isomerization was observed for photodissociation processes involving toluene, m-xylene, aniline, 4-methylpyridine, alpha-fluorotoluene, and 4-fluorotoluene, indicating a general isomerization mechanism for all such aromatic molecules. What is significant, is that during the isomerization, atoms (i.e., carbon, nitrogen, fluorine, and hydrogen) belonging to respective alkyl or amino groups are involved in an exchange with atoms within the aromatic ring. This type of isomerization is not observed in other aromatic isomerization mechanisms. For small tyrosine chromophores, such as phenol, 4-methylphenol, and 4-ethylphenol, H-atom elimination from a repulsive excited state plays a key role. However, dissociation is quenched in large chromophores like 4-(2-aminoethyl)-phenol. Our work demonstrates the capability and high sensitivity of multimass ion imaging techniques in the study of aromatic compounds.

Photoisomerization mechanism of 4-methylpyridine explored by electronic structure calculations and nonadiabatic dynamics simulations

“…Meanwhile, a conical intersection between S 2 and S 0 states was characterized by the state-averaged complete active space self-consistent field (CASSCF) calculations, which was suggested to be responsible for the ultrafast deactivation of the pyridine molecule. 17,20 A lot of distinct channels were observed by measuring photofragment translational energy distributions and dissociation rates. 13 The observed infrared bands were assigned to an isomer that has a C-C bridging bond (Dewar isomers) with the aid of density functional theory (DFT) calculation.…”

“…11 The FT-IR spectroscopy has been used to study photoisomerization processes of pyridine, 2-methyl-pyridine, and 4-methylpyridine in a low-temperature argon matrix. 20 The first type of distributions were of typical characteristics of nonadiabatic dissociation with no exit barrier, yielding two radicals as products. But no other isomer bands were observed in the FT-IR spectroscopy.…”

“…13 The observed infrared bands were assigned to an isomer that has a C-C bridging bond (Dewar isomers) with the aid of density functional theory (DFT) calculation. 20 Cao and co-workers 15 have performed a systematic theoretical study on photoisomerization processes of 2,3dimethylpyridine and 3,5-dimethylpyridine. Zewail and co-workers reported the direct determination of the molecular structure during the nonradiative transition in aromatic pyridine excited at ∼266 nm (∼37 594 cm −1 ).…”

Vacuum Ultraviolet Single-Photon Postionization of Amino Acids

“…Compared with other nonlinear optical generation schemes, which have complicated experimental configurations, ninth harmonic generation from a Nd:YAG laser provides a convenient, low-cost, and compact VUV source for SP ionization in a mass spectrometer. Several SP ionization methods using the ninth harmonic of a Nd:YAG laser coupled with TOF-MS have been demonstrated [52][53][54][55][56].…”

Formação de moléculas orgânicas em ambientes interestelares

“…Essas propriedades, contudo, podem ser derivadas de outros trabalhos (Lin et al (2005), Ni et al (2007) Assim como no caso do pirrol e do benzeno, a baixa abundância da piridinaé justificada por haver apenas uma reação de formação para algumas reações de destruição deste composto. Além disso, como nos casos anteriores deve-se levar em consideração a limitação do modelo em usar apenas reações na fase gasosa, não considerando a fase de grãos, que constituiriam o sítio de formação privilegiada para uma molécula tão complexa como esta.…”