Vibrational signatures of hydrogen bonding in the protonated ammonia clusters NH4(+)(NH3)(1-4).

The gas phase vibrational spectroscopy of the protonated ammonia dimer N(2)H(7)(+), a prototypical system for strong hydrogen bonding, is studied in the spectral region from 330 to 1650 cm(-1) by combining infrared multiple photon dissociation and multidimensional quantum mechanical simulations. The fundamental transition of the antisymmetric proton stretching vibration is observed at 374 cm(-1) and assigned on the basis of a six-dimensional model Hamiltonian, which predicts this transition at 471 cm(-1). Photodissociation spectra of the larger protonated ammonia clusters NH(4)(+)(NH(3))(n) with n=2-4 are also reported for the range from 1050 to 1575 cm(-1). The main absorption features can be assigned within the harmonic approximation, supporting earlier evidence that hydrogen bonding in these clusters is considerably weaker than for n=1.

Isomorphous substitution in bimetallic oxide clusters.

The geometric and electronic structure of bimetallic oxide clusters is studied as a function of their composition with gas phase vibrational spectroscopy. Infrared multiple photon dissociation spectra of titanium-vanadium oxide cluster anions are measured in the 500 to 1200 wave number range and assigned on the basis of harmonic frequencies calculated using density functional theory. Singly substituted (V(2)O(5))(n-1)(VTiO(5))(-) (n=2-4) cluster anions are shown to form polyhedral caged structures similar to those predicted for their isoelectronic counterparts, the neutral (V(2)O(5))(n) clusters. Upon systematic exchange of V by Ti atoms in V(4-n)Ti(n)O(-)(10) (n=1-4), the structure does not change. The stress induced by the isomorphous substitution results in an increased number of unpaired electrons (n-1) for the Ti-rich systems, leading to a quartet ground state for Ti(4)O(-)(10).

Experimental and theoretical study of the infrared spectra of BrHI- and BrDI-.

Gas phase vibrational spectra of BrHI- and BrDI- have been measured from 6 to 17 microm (590-1666 cm(-1)) using tunable infrared radiation from the free electron laser for infrared experiments in order to characterize the strong hydrogen bond in these species. BrHI-.Ar and BrDI-.Ar complexes were produced and mass selected, and the depletion of their signal due to vibrational predissociation was monitored as a function of photon energy. Additionally, BrHI- and BrDI- were dissociated into HBr (DBr) and I- via resonant infrared multiphoton dissociation. The spectra show numerous transitions, which had not been observed by previous matrix studies. New ab initio calculations of the potential-energy surface and the dipole moment are presented and are used in variational ro-vibrational calculations to assign the spectral features. These calculations highlight the importance of basis set in the simulation of heavy atoms such as iodine. Further, they demonstrate extensive mode mixing between the bend and the H-atom stretch modes in BrHI- and BrDI- due to Fermi resonances. These interactions result in major deviations from simple harmonic estimates of the vibrational energies. As a result of this new analysis, previous matrix-isolation spectra assignments are reevaluated.