High resolution spectrum of the HCl dimer

Abstract

Rotational structure in the spectrum of the H–Cl stretching bands of the HCl dimer has been fully resolved using a tunable difference-frequency laser. The spectrum of a natural isotopic sample was recorded under thermal equilibrium conditions near the condensation point (T∼130 K) of HCl at low pressures (3–4 Torr) and long path lengths (64–80 m). Rotational assignments have been made for K″=0, 1, and 2 subbands of a perpendicular-type band centered near 2880 cm−1 and a parallel-type band near 2857 cm−1 sharing a common vibrational ground state. For the ground state of the H 35Cl–H 35Cl dimer, the K″=0 rotational constant B̄=1944.29(7) MHz and the K″=1 asymmetry splitting B−C=19.5(3) MHz yield a zero-point center-of-mass separation of the two monomer constituents of 3.797(1) Å. Another difference band for the 35Cl isotope has also been observed as well as a similar set of bands for the mixed H 35Cl–H 37Cl species. The spectra suggest an extremely rapid tunneling motion between equivalent forms of nearly orthogonally oriented monomer units.

Structures of the van der Waals Isomers of Halosulfuric Acids: Microwave Spectra of HX–SO3(X= F, Cl, Br)

“…For a rigid bent structure, however, they do not. Nevertheless, in the limit of infinite separation of the HX and SO 3 moieties, K is given by k SO 3 ϩ m HX , where k SO 3 is the K quantum number for free SO 3 and m HX is the projection of the torsional angular momentum of the HX onto the pseudosymmetry axis of the complex. Thus, with k SO 3 rigorously restricted to 3n (n ϭ an integer), the K Ϯ3n states of the complex must correlate with rotationally excited HX.…”

“…The observation of symmetric rotor spectra for each of the three HX-SO 3 complexes indicates that the vibrationally averaged orientation of the HX subunits lies along the C 3 axis of the SO 3 . Preliminary analysis of the rotational constants gives intermolecular S-X bond lengths which are near the sum of the sulfur and halogen van der Waals radii, and the force constants obtained from centrifugal distortion are of a magnitude characteristic of van der Waals bonds (0.109, 0.092, and 0.094 mdyn/Å for the HF, HCl, and HBr adducts, respectively).…”

“…Since the hydrogen halides react with SO 3 , the complexes of interest were generated in a "coinjection" nozzle (16) in which pure HF, HCl, or HBr was flowed through a small (0.007Љ ID, 0.013Љ OD) needle into the early stages of a pulsed expansion of Ar and SO 3 . The Ar/SO 3 expansion was prepared by flowing argon at a backing pressure of 1.7 atm over a solid sample of SO 3 cooled to 0°C.…”

“…Systems such as HCl/ AlCl 3 , HBr/AlBr 3 , and HF/BF 3 , for example, comprise superacid media which are widely used as Friedel-Crafts catalysts (1). Similarly, the reaction of SO 3 with HF or HCl is used to produce superacids, and the fluoro or chlorosulfuric acids which result are often employed as catalysts and reagents in reactions such as alkylation, acylation, polymerization, sulfonation, and isomerization processes (1).…”

“…Indeed, only a relatively few binary HX complexes involving a "basic halogen" have been investigated by high-resolution techniques. Examples include (HF) 2 (2), (HCl) 2 (3), HF-HCl and HCl-HF (4), HF-ClF and HF-Cl 2 (5), HBr-DBr (6), HI-HF (7), HF-BF 3 (8), and HCl-BF 3 (9), in which one of the halogen atoms is directly involved in the intermolecular bond.…”