The results of spectroscopic studies of the C6H6(NH3), Na(H2O), and Na(NH3) gas phase complexes are reported. The goal of these experiments was to obtain information upon which a more quantitative understanding of the intermolecular interactions found within them could be based. These intermolecular interactions, amino-aromatic hydrogen bonding in the C6H6(NH3) complex and the primarily electrostatic bonding of Na+ to polar molecules in the Na+(H2O) and Na+(NH3) complexes, form the basis for many biologically significant processes. The C6H6(NH3) dimer was investigated with resonant two-photon ionization (R2PI) and microwave spectroscopies. Using R2PI via the 610 transition of the benzene in the complex, it was found that the NH3 is located above the benzene plane, on or near the benzene C6 axis, and that the monomers undergo free or nearly free internal rotation. Subsequent high resolution microwave absorption and emission measurements of the rotational spectrum revealed that the center of mass separation (Rcm-cm) of the monomers is 3.59 Å and that the ammonia C3 axis is tilted 58° from the benzene C6 axis. In this geometry, the ammonia protons interact with the benzene n-cloud in a hydrogen-bonded geometry similar to that observed in high-resolution x-ray crystallography of protein structures. The Na(H2O), Na(D2O), Na(NH3), and Na(ND3) complexes were studied with ZEKE-PFI (zero kinetic energy-pulsed field ionization) photoelectron spectroscopy. Spectra of all four complexes were obtained by single-photon ionization. For the Na(NH3) and Na(ND3) complexes, two-color (1+1') photoionization was used as well, with the à 2E state serving as the intermediate resonance. Improved values for the ionization energies (IEs) and intermolecular stretching vibrational frequencies of the complexes were determined, while the intermolecular bending vibrational frequencies of Na(NH3) and Na(ND3) were measured for the first time. From the rotational structure observed in the ZEKE-PFI spectra, it was found that the Na-O and Na-N bond lengths shrink by 5.2 ± 0.5% when the complexes are ionized. The single-photon ZEKE-PFI spectra show transitions only between states of the same vibrational symmetry, in accord with the selection rule for allowed electronic transitions. Some of the two-color ZEKE-PFI spectra, however, show strong transitions between states of different vibrational symmetry which are attributed to vibronic coupling in the intermediate state. All of the single-photon spectra as well as the two-color spectra of Na(NH3) showed unusually strong O-branches, presumably the result of field-induced rotational autoionization. This effect is commonly observed in ZEKE-PFI spectroscopy, particularly for species such as Na(H2O) and Na(NH3) which combine small rotational constants with strongly anisotropic ion-electron scattering potentials.
David Allen Rodham (Thu,) studied this question.