Determination of the proton tunneling splitting of the vinyl radical in the ground state by millimeter-wave spectroscopy combined with supersonic jet expansion and ultraviolet photolysis

Abstract

The vinyl radical in the ground vibronic state produced in a supersonic jet expansion by 193 nm excimer laser photolysis of vinyl bromide was investigated by millimeter-wave spectroscopy. Due to the proton tunneling, the ground state is split into two components, of which the lower and higher ones are denoted as 0+ and 0-, respectively. Eight pure rotational transitions with Ka = 0 and 1 obeying a-type selection rules were observed for each of the 0+ and 0- states in the frequency region of 60-250 GHz. Tunneling-rotation transitions connecting the lower (0+) and upper (0-) components of the tunneling doublet, obeying b-type selection rules, were also observed in the frequency region of 190-310 GHz, including three R- and six Q-branch transitions. The observed frequencies of the pure rotational and tunneling-rotation transitions were analyzed by using an effective Hamiltonian in which the coupling between the 0+ and 0- states was taken into account. A set of precise molecular constants was obtained. Among others, the proton tunneling splitting in the ground state was determined to be DeltaE0 = 16,272(2) MHz. The potential barrier height was estimated to be 1580 cm(-1) from the proton tunneling splitting, by an analysis using a detailed one-dimensional model. The spin-rotation and hyperfine interaction constants were also determined for the 0+ and 0- states together with the off-diagonal interaction constants connecting the 0+ and 0- states, epsilonab + epsilonba for the spin-rotation interaction and Tab for the hyperfine interaction of the alpha (CH) proton. The hyperfine interaction constants, due to the alpha proton and the beta (CH2) protons, are consistent with those derived from electron spin resonance studies.

Nonadiabatic Chemical Dynamics: Comprehension and Control of Dynamics, and Manifestation of Molecular Functions

“…The final results are Á 0 ðHÞ ¼ 0:14 cm À1 and Á 0 ðHÞ ¼ 0:53 cm À1 by the MP2/6-31G(d,p) and the CCSD(T)/aug-cc-pVTZ method, respectively. The converged result, Á 0 ðHÞ ¼ 0:53 cm À1 , is in good agreement with the experimental value, Á 0 ðHÞ ¼ 0:54 cm À1 [103]. This guarantees the accuracy of the present semiclassical theory at least for the ground vibrational state.…”

“…14). Very recently, Tanaka et al [103] investigated this radical by millimeter-wave spectroscopy and reported a set of precise molecular constants together with the tunneling splitting. This fiveatom molecule is a good example so that we can accomplish the full-scale high level quantum chemical calculations and confirm the accuracy of our semiclassical theory of tunneling splitting.…”

Laboratory detection of the rotational-tunnelling spectrum of the hydroxymethyl radical, CH2OH

“…The Hamiltonian used in the analysis is that for an asymmetric top radical in a doublet state including the energy separation between the tunnelling levels, that is to say, no interaction terms connecting the two tunnelling substates were needed to account for the observed spectrum: H = H rot + H sr + H ns + H 01 . Details on these terms can be found in Tanaka et al (2004). Briefly, H rot and H sr stand for the Watson A reduction in the I r representation of the rotational and of the electron-spin rotational coupling tensors, respectively, both including the centrifugal distortion effects.…”

Ortho-para mixing interaction in the vinyl radical detected by millimeter-wave spectroscopy

“…Although we could not determine the ␦a F value of H 2 CCH in the previous study, 12 it should be similar to that in H 2 CCD. However, the mixing of para and ortho levels would be much smaller ͑about 0.00043%͒ than in deuterated species, because of large energy differences corresponding to ⌬E 0 = 16 272 MHz.…”

“…The energy separation between 0 + and 0 − is denoted by ⌬E 0 . Previously, 12 we observed millimeter-wave transitions connecting the 0 + and 0 − states of H 2 CCH to determine the tunneling splitting ⌬E 0 as 16 271.8429͑59͒ MHz. For this tunneling molecule, the molecular symmetry group C 2v ͑M͒ is appropriate, and the ground electronic state belongs to the 2 B 2 species of this group.…”