Infrared Photodissociation Spectroscopy of Protonated Formic Acid and Acetic Acid Clusters

Infrared photodissociation spectra of protonated formic acid clusters, H+·(HCOOH)n (n = 2−5), are measured in the 3000−3700 cm-1 region. Density functional theory calculation is applied to H+·(HCOOH)n (n = 2−5). Geometry optimization of H+·(HCOOH)n (n = 2−5) indicates that stable forms of these clusters are open-chain structures with free OH groups at both ends. In the infrared photodissociation spectra of the n = 2 and n = 3 species, there are two sharp bands in the range of 3400−3700 cm-1. The lower- and higher-frequency bands of them are attributed to the free OH stretching vibrations of the peripheral COOH groups in the E and Z conformations, respectively. The intensity of the higher-frequency band, relative to that of the lower-frequency band, decreases from n = 2 to n = 3. The n = 4 and n = 5 ions exhibit only one sharp band in the same region. This band is assigned to the free OH stretching vibrations of the end COOH groups in the E conformation; the n = 4 and n = 5 ions have the peripheral COOH gr...