Characterization of competing halogen-bonding and hydrogen-bonding motifs in the acetonitrile/hydrogen iodide dimer

Abstract

Three different C3v configurations of the CH3CN/HI dimer have been characterized using MP2 and CCSD(T) methods with large correlation consistent basis sets. All three stationary points are minima. The hydrogen bonded CH3CN⋯HI configuration is the global minimum (GM) with an electronic dissociation energy exceeding 4 kcal mol−1 near the CCSD(T) complete basis set (CBS) limit. A strongly bound halogen-bonded form of the dimer, CH3CN⋯IH, lies within 0.9 kcal mol−1 of the GM according to CCSD(T) electronic energies extrapolated to the CBS limit. The energetic separation between the two minima decreases by approximately 0.1 kcal mol−1 at all levels of theory when the harmonic zero-point vibrational energies are included. A third minimum was identified in which iodine interacts with the hydrogens of the methyl group (HI⋯CH3CN). This minimum is significantly higher in energy than the GM, almost within 1 kcal mol−1 of the dissociation limit. The GM has been previously identified via matrix isolation infrared spectroscopy. The energetics, vibrational frequencies, and infrared intensities computed here corroborate the tentative assignment of a second feature in the HI stretching region of the experimental infrared spectrum to the halogen-bonded configuration.