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

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 center dot center dot center dot 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 center dot center dot center dot 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 center dot center dot center dot 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.

Automated summary

Three different C3v configurations of the CH3CN/HI dimer have been characterized. The hydrogen bonded CH3CN center dot center dot center dot HI configuration is the global minimum (GM) and a strongly bound halogen-bonded form of the dimer, CH3CN center dot center dot center dot IH, is close in energy to the GM. A third minimum, HI center dot center dot center dot CH3CN, is significantly higher in energy. The computational results support the experimental assignment of a second feature in the HI stretching region of the infrared spectrum to the halogen-bonded configuration.