Hydrogen bonding in the mixed HF/HCl dimer: Is it better to give or receive?

The ClH⋯FH and FH⋯ClH configurations of the mixed HF/HCl dimer (where the donor⋯acceptor notation indicates the directionality of the hydrogen bond) as well as the transition state connecting the two configurations have been optimized using MP2 and CCSD(T) with correlation consistent basis sets as large as aug-cc-pV(5 + d)Z. Harmonic vibrational frequencies confirmed that both configurations correspond to minima and that the transition state has exactly one imaginary frequency. In addition, anharmonic vibrational frequencies computed with second-order vibrational perturbation theory (VPT2) are within 6 cm^-1 of the available experimental values and deviate by no more than 4 cm^-1 for the complexation induced HF frequency shifts. The CCSD(T) electronic energies obtained with the largest basis set indicate that the barrier height is 0.40 kcal mol^-1 and the FH⋯ClH configuration lies 0.19 kcal mol^-1 below the ClH⋯FH configuration. While only modestly attenuating the barrier height, the inclusion of either the harmonic or anharmonic zero-point vibrational energy effectively makes both minima isoenergetic, with the ClH⋯FH configuration being lower by only 0.03 kcal mol^-1 . © 2018 Wiley Periodicals, Inc.