Conformational structures of 3-phenyl-1-propionic acid, its p-hydroxy derivative, and its hydrated clusters

We present fluorescence excitation and one- and two-color resonant two-photon ionization spectra of 3-phenyl-1-propionic acid (PPA), its complexes with water, PPA(H2O)(n), where n = 1-3, and its p-hydroxy derivative, 3-(4-hydroxyphenyl)-1-propionic acid (HPPA), at the vibronic and partially resolved rovibronic level. Different conformations of the alkyl side chain in the bare molecule produce two spectral bands in the 0(0)(0) region for PPA and three bands for HPPA. Analysis of the rovibronic band contours of these features is consistent with two conformational structures, referred to as anti and gauche. The anti structure is characterized by an extended side chain and a predominantly b-type band contour. The gauche structure is characterized by a side chain folded back toward the phenyl ring and a hybrid band contour. Analysis of this contour shows that there is a significant rotation of the transition moment within the molecular frame. Rovibronic band contour analysis of the 1:1 water complex of PPA yields a structure consistent with the acid acting as a proton donor. The rotational band contour of the 1:2 water complex is consistent with a cyclic structure in which each water is involved in two hydrogen bonds.