Five stable isomers of 2-chloroethanol and 2-bromoethanol have been found by the density functional theory with the hybrid density-functional B3LYP and the second-order Møller-Plesset perturbation method using 6-31++G(d,p) and 6-311++G(2d,2p) basis sets. Intramolecular hydrogen bonding plays an important role in the most stable isomers. Assignments, especially for the low ionization potential bands, in the He*(23S) Penning ionization electron spectra and He I ultraviolet photoelectron spectra were made on the basis of the characteristics of molecular orbitals, collision energy dependence of partial Penning ionization cross sections (CEDPICS), and the related calculations. The intramolecular hydrogen bonding (C-Cl⋯H-O and C-Br⋯HO) leads to the significant steric shielding effects, which further results in distinctly different slopes of CEDPICS for the low ionization potential bands.
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