Molecular and Supramolecular Structures ofN-Phenyl Formamide and its Hydrated Clusters

Abstract

The amide, N-phenyl formamide (formanilide), and its water clusters have been studied in a jet expansion using laser-induced fluorescence excitation and mass-selected, resonant two-photon ionization (R2PI) techniques. The isomer with a trans configuration of the amide group (defined in Figure 1) is identified through analysis of the partially resolved contour of its S1 ← S0 band origin. Ion-dip “hole-burn” spectra of the nonplanar cis isomer contain either symmetric or antisymmetric components of low-frequency progressions, providing evidence of a double-minimum ground state potential. Excited-state vibrations at 76 and 152 cm-1, which are strongly Franck−Condon active, show evidence of Duschinski mixing of the ground-state modes including Cring−N torsion. Water clusters have been observed for trans-formanilide only: two distinct 1:1 hydrates, two 1:2 hydrates, and a complex with at least four bound water molecules. (The cis isomer is also expected to form extremely stable complexes with water, but none have been detected experimentally in the present study.) The observed clusters are assigned using spectroscopic data, including band contours, to structural alternatives computed ab initio at the HF/6-31G* level. The 1:1 hydrates are assigned to a cluster in which water binds at the NH site and one in which water binds at the HCO site. In the 1:2 clusters, the addition of a further water molecule to each of the 1:1 clusters results in cyclic hydrogen-bonded structures, with the water dimer bridging between proton donor and proton acceptor sites of the host. The interactions are HCO···HOH and OCH···OH2 in one case and NH···OH2 and πring···HOH in the other. At the MP2/6-31G*//HF/6-31G* level, these structures are ca. 10 kJ mol-1 more stable than the nearest competitor, in part because of cooperative effects. The R2PI spectrum of the NH bound 1:2 cluster “C” is very similar to that of the indole(H2O)2 complex assigned by Zwier and co-workers.19 Its origin is red-shifted 482 cm-1 from trans-formanilide, and the electronic transition excites long intermingled vibrational progressions with frequencies of 29, 38, and 51 cm-1.