Matrix isolation study of the complexes of small ring heterocycles: Part 1. Complexes of the hydrogen halides with ethylene oxide and related species

Abstract

The matrix isolation technique has been employed for the synthesis and isolation of the hydrogen-bonded complexes of oxygen-containing heterocycles with the hydrogen halides. For the saturated heterocycles, (CH 2) n O, n = 2–4, a 1:1 complex was observed in each case, while for furan, (CH) 4O 1 both 1:1 and 1:2 complexes were observed. Infrared spectra of the 1:1 complexes of the saturated heterocycles with HF 1 HCl or HBr were dominated by intense, broad absorptions due to the hydrogen stretching motion of the complexed hydrogen halide, shifted considerably to lower energies. In addition, several vibrational modes of the perturbed base subunit in the complex were detected. The furan complexes, on the other hand, showed a much smaller shift of the hydrogen stretching motion, along with a considerably reduced bandwidth (4 cm −1 vs. approximately 50 cm −1). However, the spectra collectively suggested a stronger perturbation to the furan ring than to the skeletons of the saturated heterocycles, probably as a consequence of the rigidity of the furan ring. Within the sequence of saturated heterocycles, the degree of perturbation to the hydrogen halide, as measured by shift from the free hydrogen halide, increased with n, the number of CH 1 units in the ring. This is in accord with the proton affinities of these species, although the results for the furan complexes cannot be rationalized in this fashion.