Redshift and blueshift of the Ar-H vibrational stretching frequency in complexes of FArH and acetylene

Two planar hydrogen-bonded complexes of FArH and acetylene were found to be stable using second-order M?ller-Plesset perturbation theory (MP2) with 6-311++G(2d,2p) basis sets. The more stable complex involves bonding between the F atom of FArH and a H atom of acetylene, while the other isomer is a T-shaped complex with the H atom of FArH bonded to the center of the CC bond of acetylene; the zero-point energy corrected dissociation energies are 29 and 27 kJ mol(-1), respectively. Interestingly, the Ar-H harmonic vibrational stretching frequency is blueshifted in the more stable isomer and redshifted in the less stable form. The electron density rearrangement of FArH on complexation was investigated and used to explain these unusual findings.