A study of the structure and dynamics of the hydronium ion by high resolution infrared laser spectroscopy. III. The ν3 band of D3O+ J. Chem. Phys. 92, 3257 (1990); 10.1063/1.457884 Infrared spectroscopy of carboions. V. Classical vs nonclassical structure of protonated acetylene C2H+ 3 J. Chem. Phys. 91, 5139 (1989); 10.1063/1.457612 Infrared spectroscopy of carboions. IV. The A 2Π u -X 2Σ+ g electronic transition of C− 2 J. Chem. Phys. 89, 129 (1988); 10.1063/1.455731Infrared spectra of carboions. II. ν3 band of acetylene ion C2H+ 2(2Π u )

The infrared spectra of the band of the 2Π–2Π asymmetric hydrogen stretching vibration in the three isotopic acetylene ions C2H2+ (ν3), 13C2H2+ (ν3), and DCCH+ (ν1) have been observed and analyzed. The high resolution infrared spectra were recorded using a difference-frequency laser spectrometer as the tunable coherent infrared source probing an ac glow discharge. Velocity modulation, noise subtraction, and unidirectional multipassing of the infrared beam through the discharge cell provided high sensitivity. C2H2+ was produced in a gas mixture of H2, He, and either CH4 or C2H2, with a total pressure of ≊7 Torr in multiple-inlet–outlet air-, water-, and liquid-nitrogen-cooled discharge tubes; C2H2 freezing precluded its use in liquid-N2-cooled discharges. Complicated by a strong perturbation whose maximum occurred at N′=15 for F1 and N′=14 for F2, the assignment of the spectrum of normal C2H2+ was made possible by (1) fortuitous discharge conditions which provided unambiguous discrimination of C2H2+ lines from among concurrent CH3+ and C2H3+ lines, and (2) fitting the ground state combination differences. Sufficiently high N transitions were observed where Λ doubling was evident. The average bond lengths rz(CH)=1.077 (5) Å and rz(CC)=1.257 (8) Å were calculated from the spectroscopic constants determined from nonlinear least-squares fitting. Vibration–rotation interactions, the Renner–Teller interaction of perturbing states, plasma chemistry, and the relevance of the work in astrophysics are discussed.

The (2ν2,l=2←ν2), (2ν2,l=0←ν2), and (ν1+ν2←ν1) hot bands of H+3 were observed. The vibrationally hot ions were produced in a liquid nitrogen cooled 6 kHz ac discharge using gas mixtures of H2 and He. The spectra were detected in direct absorption using a newly extended tunable difference frequency spectrometer using both LiNbO3 and LiIO3 crystals as nonlinear optical elements. The range of this spectrometer is now ∼5300–∼1900 cm−1. The positions of the rovibrational transitions compare extremely well with the theoretical predictions of Miller and Tennyson. A vibrational temperature study of the discharge indicates a significant population inversion between the ν1 and ν2 levels.

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.

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