Infrared spectroscopy of H-bonded bridges stretched across the cis-amide group: I. Water bridges

The water-containing clusters of oxindole (OI) and 3,4-dihydro-2(1H)-quinol inone (DQ) have been studied in the hydride stretch region of the infrared by the technique of resonant ion-dip infrared spectroscopy (RIDIRS). Both O I and DQ are constrained cis-amides with adjacent N-H and C=O groups betwee n which water can form H-bonded bridges. The hydride stretch fundamentals o f OI-W-n with n = 1-3 and DQ-W-n with n = 1, 2 without exception divide up into free OH stretch fundamentals near 3700 cm(-1) and a set of H-bonded br idge fundamentals in the 3200-3450 cm(-1) region. The bridge fundamentals s how a distribution of intensities that reflects strong coupling among the X H oscillators in the bridge. When more than one water is involved in the br idge, the bridge fundamentals are unusually broad, with widths of 50-80 cm( -1) full width at half-maximum. Minimum-energy structures, binding energies , vibrational frequencies, and infrared intensities have been calculated by density functional theory with a Becke3LYP functional and a 6-31+G* basis set. The calculated infrared spectra match experiment well, confirming the bridge structures for the clusters. The form of the calculated normal modes provides insight into the nature of the bridge fundamentals.