An experimental and theoretical study of the electronic spectrum of the HBCl free radical

Abstract

Following our previous discovery of the spectra of the HBX (X = F, Cl, and Br) free radicals [S.-G. He, F. X. Sunahori, and D. J. Clouthier, J. Am. Chem. Soc. 127, 10814 (2005)], the Ã(2)A(″)Π-X̃(2)A(') band systems of the HBCl and DBCl free radicals have been studied in detail. The radicals have been prepared in a pulsed electric discharge jet using a precursor mixture of BCl3 and H2 or D2 in high pressure argon. Laser-induced fluorescence (LIF) and single vibronic level emission spectra have been recorded to map out the ground and excited state vibrational energy levels. The band system involves a linear-bent transition between the two Renner-Teller components of what would be a (2)Π electronic state at linearity. We have used high level ab initio theory to calculate the ground and excited state potential energy surfaces and have determined the vibronic energy levels variationally. The theory results were used to assign the LIF spectra which involve transitions from the ground state zero-point level to high vibrational levels of the excited state. The correspondence between theory and experiment, including the transition frequencies, upper state band symmetries, and H, B, and Cl isotope shifts, was used to validate the assignments.

An experimental and theoretical study of the Ã2A″Π–X̃2A′ band system of the jet-cooled HBBr/DBBr free radical

“…In the variational Renner-Teller calculations (see below), the fully ab initio PESs yielded vibronicà 2 A ′′ state levels, which were ∼100 cm −1 too high for the main bands of the spectra. Similar deviations were observed also for HBF 14 and HBCl, 16 which lead to errors of ∼2% in the corresponding barrier to linearity in the lower electronic state. For these two radicals, these approximately constant shifts did not constitute an obstacle to the assignment of the experimental bands, thanks to the comparatively simpler structure of the observed transitions.…”

“…The degeneracy of the two surfaces at linearity was imposed by using 52 linear geometries in the fitting of the ground state coefficients. This procedure is identical to that adopted for HBCl 16 and it is able to recover surfaces which are nearly degenerate for linear geometries, with differences of the same order of magnitude of the RMSD's of the fittings. From the fitted surfaces, the barrier to linearity of the ground state was calculated to be 5607 cm −1 , slightly lower than the 6073 cm −1 value found for HBCl, 16 but much lower than the 10 084 cm −1 value found for HBF.…”

“…Since the 0 0 0 band occurs at very low energies in the near-infrared and the observed bands terminate on quite high bending levels in the excited state, the detailed assignment of the vibrational structure in the LIF spectra is challenging. In a previous work on HBF/DBF 14,15 and HBCl/DBCl, 16 we have used a combined theoretical and experimental approach to tackle the problem. In each case, we first used a high level ab initio theory to calculate a suitable grid of points on the ground and excited state potential energy surfaces.…”

A stimulated emission study of the ground state bending levels of BH2 through the barrier to linearity and ab initio calculations of near-spectroscopic accuracy

“…To circumvent these problems, we used the LIF synchronous scanning (sync-scan) technique described previously. 10 In this method, the fluorescence is dispersed by a scanning monochromator that is fixed on a prominent emission band of the isotopologue(s) of interest. The excitation laser and the monochromator are scanned synchronously under computer control so that the resulting spectrum exhibits only those transitions that emit down to the chosen level, focusing on the spectrum of a subset of the molecular isotopologues and minimizing impurity emission.…”

“…We also studied the spectrum of 11 BD 2 , detecting bands in the bending progression from 2 14 0 to 2 23 0 and some stretch-bend combinations. The corresponding 10 B isotopologues were also studied. Many of the bands exhibited spin splittings, especially at low N values.…”