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 groups only in the E conformation. We observe infrared photodissociation spectra of H+.(HCOOH)(n) (n = 6, 7) in the 3500-3600 cm(-1) region. The absence of any free OH band for n = 7 shows that both ends of chain structures of n = 7 are terminated by cyclic dimers. Infrared photodissociation spectra of H+.(CH3COOH)(n) (n = 2-5) are also examined in the 3000-3700 cm(-1) region. Resemblance of the spectra of H+.(CH3COOH)(n) to those of H+.(HCOOH)(n) suggests that the acetic acid clusters have structures that are similar to those of the formic acid clusters; the intermolecular network is formed only by the COOH groups.