High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of imidazole

Abstract

The fragmentation dynamics of imidazole molecules following excitation at 193.3 nm and at many wavelengths in the range of 210< or =lambda(phot)< or =240 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Long wavelength excitation within this range results in population of the 1 (1)A(")((1)pisigma(*)) excited state, but the 2 (1)A(')<--X (1)A(')(pi(*)<--pi) transition becomes the dominant absorption once lambda(phot)< or =220 nm. The measured energy disposals show parallels with those found in recent studies of the UV photolysis of pyrrole [Cronin et al., Phys Chem. Chem. Phys. 6, 5031 (2004)]. The total kinetic energy release (TKER) spectra display a "fast" feature, centred at TKER approximately 9200 cm(-1). The analysis of the structure evident in the fast feature reveals the selective population of specific in-plane stretching vibrational levels of the imidazolyl cofragment; these fragments are deduced to carry only modest amounts of rotational excitation. Comparison with calculated normal mode vibrational frequencies allows the assignment of the populated levels and a precise determination of the N-H bond strength in imidazole: D(0)=33,240+/-40 cm(-1). The observed energy disposal can be rationalized using Franck-Condon arguments, assuming that the potential energy surface (PES) for the 1 (1)A(")((1)pisigma(*)) state has a topology similar to that of the corresponding (1)pisigma(*) state of pyrrole. As in pyrrole, photoexcitation populates skeletal motions in the S(1) state (in-plane motions in the present case) that are only weakly coupled to the N-H dissociation coordinate and thus map through into the corresponding product vibrations. A second, "slow" feature is increasingly evident in TKER spectra recorded at shorter lambda(phot). This component, which exhibits no recoil anisotropy, is attributed to H atoms formed by the "statistical" decay of highly vibrationally excited ground state molecules. The form of the TKER spectra observed at short lambda(phot) is rationalized by assuming two possible decay routes for imidazole molecules excited to the 2 (1)A(')((1)pipi(*)) state. One involves fast 2 (1)A(')((1)pipi(*)) right arrow-wavy 1 (1)A(")((1)pisigma(*)) radiationless transfer and subsequent fragmentation on the 1 (1)A(')((1)pisigma(*)) PES, yielding fast H atoms (and imidazolyl cofragments)-reminiscent of behavior seen at longer excitation wavelengths where the 1 (1)A(")((1)pisigma(*)) PES is accessed directly. The second is assumed to involve radiationless transfer to the ground state, most probably by successive 2 (1)A(') right arrow-wavy 1 (1)A(") right arrow-wavy X (1)A(') couplings, mediated by conical intersections between the relevant PESs and the subsequent unimolecular decay of the resulting highly vibrationally excited ground state molecules yielding slow H atoms.

Ultrafast Dynamics of UV‐Excited Imidazole

“…Table 2 reports selected results for the TZVP + mod basis set together with the root mean square deviation (RMSD) between the present theoretical results and the experimental results of Devine et al [39] using the fitted values reported in reference [27] In contrast to the results obtained for pyrrole, [23] the B3LYP functional is in better agreement with experimental values than the PBE0 functional. The functional with the largest fraction of Hartree-Fock (HF) exchange contribution (BHLYP) provides the worst results.…”

“…[26,39] For excitation at 200 nm (window B), the time constants associated with the fast and slow hydrogen atoms are 78 AE 37 and 163 AE 50 fs, respectively. [26] The fast hydrogen release was related with direct dissociation along the 1 ps * NH state, while the slow hydrogen release was associated with dissociation along the first excited state of the imidazolyl radical.…”

“…At 265.7 (4.67 eV; Figure 1 d) and 253.9 nm (4.88 eV; Figure 1 c), the one-photon energies are below the onset of the imidazole absorption spectrum, [39] and the photoelectron spectra show nonresonant two-photon ionization into the ground state of the cation. Vibrational structure is observed with an energetic spacing of 0.18 eV that is consistent with cationic inplane skeletal stretching modes observed by He(I) photoelectron spectroscopy.…”

Spectroscopic investigation (FT-IR and FT-Raman), vibrational assignments, HOMO–LUMO, NBO, MEP analysis and molecular docking study of 2-(4-hydroxyphenyl)-4,5-dimethyl-1H-imidazole 3-oxide

“…Devine et al [7] reported the high resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of imidazoles. DFT method is at present widely used for simulating IR and Raman spectrum.…”

Mass spectrometry of hydrogen bonded clusters of heterocyclic molecules: Electron ionization vs. photoionization

“…The photodynamics of the isolated nitrogen heterocycle molecules is reasonably well understood, based on extensive experimental studies [11,12,7,8] and theoretical calculations [13][14][15][16]. Briefly, the dissociation of the N-H bond via the * state was identified as a major deactivation pathway at lower wavelengths while internal conversion via * /S 0 conical intersection seems to become more important at larger photon energies.…”