Structural and Infrared Spectroscopic Study on Solvation of Acetylene by Protonated Water Molecules

The effect of solvation on the conformation of acetylene has been studied by adding one water molecule at a time. Quantum chemical calculations of the H+(C2H2) (H2O)(n) (n=1-5) clusters indicate that the H2O molecules prefer to form the OH center dot center dot center dot pi interaction rather than the CH center dot center dot center dot O interaction. This solvation motif is different from that of neutral (C2H2)(H2O)Th (n=1-4) clusters, in which the H2O molecules prefer to form the CH center dot center dot center dot O and OH center dot center dot center dot C H bonds. For the H+(C2H2)(H2O)(n) cationic clusters, the first solvation shell consists of one ring structure with two OH center dot center dot center dot pi H-bonds and three water molecules, which is completed at n=4. Simulated infrared spectra reveal that vibrational frequencies of OH center dot center dot center dot H-bonded O H stretching afford a sensitive probe for exploring the solvation of acetylene by protonated water molecules. Infrared spectra of the H+(C2H2)(H2O)Th (n=1-5) clusters could be readily measured by the infrared photodissociation technique and thus provide useful information for the understanding of solvation processes