Infrared and microwave investigations of interconversion tunneling in the acetylene dimer

Abstract

A sub-Doppler infrared spectrum of (HCCH)2 has been obtained in the region of the acetylene C–H stretching fundamental using an optothermal molecular-beam color-center laser spectrometer. Microwave spectra were obtained for the ground vibrational state using a pulsed-nozzle Fourier transform microwave spectrometer. In the infrared spectrum, both a parallel and perpendicular band are observed with the parallel band being previously assigned to a T-shaped C2v complex by Prichard, Nandi, and Muenter and the perpendicular band to a C2h complex by Bryant, Eggers, and Watts. The parallel band exhibits three Ka=0 and three asymmetry-doubled Ka=1 series. The transitions show a clear intensity alternation with Kc with two of the Ka=0 series missing every other line. In addition, the perpendicular band has the same ground-state combination differences as the parallel band. To explain these apparent anomalies in the spectrum, we invoke a model consisting of a T-shaped complex with interconversion tunneling between four isoenergetic hydrogen-bonded minima. In this picture, the parallel and perpendicular bands arise from excitation of the acetylene units parallel and perpendicular to the hydrogen bond. The observation of rotation–inversion transitions in the microwave spectrum, in addition to the pure rotation transitions of Prichard, Nandi, and Muenter, verifies the model. The measured microwave splittings yield a tunneling frequency of 2.2 GHz which is consistent with a ∼33 cm−1 barrier separating the four minima.

High resolution overtone spectroscopy of the acetylene van der Waals dimer, (12C2H2)2

“…2 This statement is confirmed by more detailed comparisons between observed and simulated structures, as already appearing in Fig. 5-7.…”

“…1 An extensive experimental and ab initio literature was since devoted to (C 2 H 2 ) 2 , summarized in ref. [2][3][4]. A crucial step in the investigation of this species was the work by Fraser et al 2 These authors demonstrated that 1CH vibrational excitation could occur in either monomeric unit.…”

“…[2][3][4]. A crucial step in the investigation of this species was the work by Fraser et al 2 These authors demonstrated that 1CH vibrational excitation could occur in either monomeric unit. Tunneling effects were modeled to explain detailed spectral patterns and their analysis unified the various existing microwave (MW) and IR high resolution data around a single, T-shaped geometry involving two linear units.…”

“…2,7 These authors demonstrated that the relevant permutation-inversion group is G 16 . Their symmetry labels will be used here (see character table of G 16 in Table 3 More K sub-bands (up to K a = 4 and K a 0 0 = 3) and higher J-values (up to J 0 0 = 16) are observed in 2CH compared to 1CH, thanks to higher rotational temperature.…”

“…2. A rigid rotor program (PGOPHER) 18 was used to handle each series of tunneling symmetry levels as a separate set, initially constraining the GS constants to the values given by Fraser et al 2 This rigid rotor approach was reasonably performed for assigning R and P branches within the 0-1 sub-band and, later within other sub-bands. It even helped extending assignments from one sub-band to another.…”