Infrared spectroscopy of carbo-ions. V. Classical vs nonclassical structure of protonated acetylene C2H+3

Abstract

The problem of classical vs nonclassical structure of protonated acetylene (vinyl cation) C2H+3 has been studied using high resolution infrared spectroscopy. The spectrum has been observed in the 3.2 μm region in air-cooled and water-cooled plasmas using C2H2:H2:He mixtures and in liquid nitrogen-cooled plasmas using CH4:H2:He mixtures. The difference frequency spectrometer with the velocity modulation method has been used to conduct the Doppler-limited, high sensitivity spectroscopy. The observed vibration–rotation pattern with the band origin at 3142.2 cm−1 has been identified as due to the antisymmetric CH stretching ν6 band of the C2H+3 ion with the nonclassical (bridged) structure. The observed spectral pattern was anomalous, but definitive assignments could be made for a part of the spectrum using the ground state combination differences which fit to the usual asymmetric rotor pattern. The discrimination between the classical and nonclassical structures is based on the observed spectral intensity pattern due to spin statistical weights. Agreement of vibrational band patterns and the rotational constants with ab initio values gives supporting evidence. The anomaly of the spectrum is at least partly ascribed to the small energy difference between the classical and nonclassical structures and possible rearrangement between them, the idea used by organic chemists over the years in wet chemistry. Systematic splittings with the intensity ratio of 2:1 have been noticed in some parts of the spectrum indicating that the protons tunnel between the apex and the two end equilibrium positions of the bridged structure. Using a simplified internal rotation model proposed by Hougen, the barrier height of the tunneling has been estimated. Chemical kinetics in plasmas related to C2H+3 is also discussed. We conclude that (1) the nonclassical structure is lower in energy than the classical structure, and (2) the apex proton and the two end protons exchange their positions with a measurable time scale.

Quantum astrochemical spectroscopy

“…The CcCR QFF computations actually put the antisymmetric CAH stretch for the lowest-energy, cyclic isomer within 0.1 cm 21 of the experimental frequency at 3142.2 cm 21 , the B and C rotational constants within 4.0 MHz at 34237.5 and 31371.8 MHz, and the D J value within 0.7 kHz of experiment at 37.65 MHz. [69,79,80] The CcCR QFF is still a very trustworthy tool. The cyclic form of C 3 H 2 has also been suggested as a potential interstellar species, as well, based on CcCR QFF computations, [81] but l-C 3 H 1 is present in the Horsehead nebula PDR.…”

Abundance anomaly of the13C species of CCH

“…In the ion-molecule reaction scheme, the two carbon atoms are identical. Note that C 2 H + 3 has a non classical structure with the two identical carbon atoms (Crofton et al 1989). In contrast, two carbon atoms are not equivalent for the neutralneutral reaction (2).…”

Unimolecular dynamics from kinetic energy release distributions. V. How does the efficiency of phase space sampling vary with internal energy?

“…Numerous reliable ab initio calculations 36,37 as well as a detailed spectroscopic analysis 38,39 have established that the minimum energy structure of the C 2 H 3 ϩ ion corresponds to a nonclassical bridged structure belonging to the C 2v point group, which can be described as protonated acetylene, hereafter denoted structure 1. The classical, Y-shaped, vinyl structure H 2 CCH ϩ ͑denoted 2͒ is found to lie about 0.13 eV higher in energy and is now believed to be very close to a transition state ͑denoted TS 12 ͒ in an internal rotation motion during which the three hydrogen atoms rotate as an equilateral triangle about a dumbell of two carbon atoms ͑Fig.…”