Microwave Spectra and Intramolecular Hydrogen Bonding in the 2-Haloethanols: Molecular Structure and Quadrupole Coupling Constants for 2-Chloroethanol and 2-Bromoethanol

Abstract

A proposed intramolecular hydrogen bond in the 2-haloethanols has been investigated with analysis of the microwave spectra of 2-chloroethanol and 2-bromoethanol. For each moledule, only a form gauche about the CC bond has been observed. For the chloro compound nine isotopic species have been studied to yield the molecular coordinates of the chlorine, oxygen, hydroxyl hydrogen, and carbon atoms, and thus a molecular structure. For bromoethanol a molecular structure has been found based upon the coordinates of the bromine and hydroxyl hydrogen atoms. Principal structural conclusions are that, of the probable rotameric forms, the lowest energy form is the one allowing close approach of hydrogen and halogen—namely, the gauche–gauche for dihedral angles (CCX) (COH) close to 60°. The O–H bond length in chloroethanol is 1.008 Å, about 5% longer than in ethanol. The H···X distance is ∼ 0.5 Å less than the sum of the atom van der Waals radii for X = Cl, Br. The O···X distance is approximately equal to the sum of the van der Waals radii. The (CCX) (CCO) dihedral angles are quite similar (63.2°, 64.2°, respectively, for X = Cl, Br vs 72.2° for X = F). The OH and CCl bonds are nearly parallel. The quadrupole coupling constants of the chlorine nucleus have been determined from analysis of fortuitous second-order effects: χzz = − 70.0 ± 2 Mc/sec, ηb = + 0.02 ± 0.03, θz = 20′ ± 20′. Approximate coupling constants have been determined for 2-bromoethanol. These quadrupole data reveal that the charge distribution in the vicinity of the halogen atom is similar to that in the corresponding ethyl halide, despite the H···X interaction. Vibrationally exited states for 2-chloroethanol have been assigned to the C–C torsion (155 cm−1), the ∠CCCl bend (310 cm−1), and the C–O torsion (350 cm−1). The relative strengths of the hydrogen–halogen and deuterium–halogen interactions have been considered. These results clearly indicate a hydrogen–halogen interaction in the 2-haloethanols and strongly suggest that this interaction is electrostatic in nature—possibly of the dipole–dipole type.