M(+)(H(2)O)(n) and M(+)(H(2)O)(n)Ar ions (M=Cu and Ag) are studied for exploring coordination and solvation structures of noble-metal ions. These species are produced in a laser-vaporization cluster source and probed with infrared (IR) photodissociation spectroscopy in the OH-stretch region using a triple quadrupole mass spectrometer. Density functional theory calculations are also carried out for analyzing the experimental IR spectra. Partially resolved rotational structure observed in the spectrum of Ag(+)(H(2)O)(1) x Ar indicates that the complex is quasilinear in an Ar-Ag(+)-O configuration with the H atoms symmetrically displaced off axis. The spectra of the Ar-tagged M(+)(H(2)O)(2) are consistent with twofold coordination with a linear O-M(+)-O arrangement for these ions, which is stabilized by the s-d hybridization in M(+). Hydrogen bonding between H(2)O molecules is absent in Ag(+)(H(2)O)(3) x Ar but detected in Cu(+)(H(2)O)(3) x Ar through characteristic changes in the position and intensity of the OH-stretch transitions. The third H(2)O attaches directly to Ag(+) in a tricoordinated form, while it occupies a hydrogen-bonding site in the second shell of the dicoordinated Cu(+). The preference of the tricoordination is attributable to the inefficient 5s-4d hybridization in Ag(+), in contrast to the extensive 4s-3d hybridization in Cu(+) which retains the dicoordination. This is most likely because the s-d energy gap of Ag(+) is much larger than that of Cu(+). The fourth H(2)O occupies the second shells of the tricoordinated Ag(+) and the dicoordinated Cu(+), as extensive hydrogen bonding is observed in M(+)(H(2)O)(4) x Ar. Interestingly, the Ag(+)(H(2)O)(4) x Ar ions adopt not only the tricoordinated form but also the dicoordinated forms, which are absent in Ag(+)(H(2)O)(3) x Ar but revived at n=4. Size dependent variations in the spectra of Cu(+)(H(2)O)(n) for n=5-7 provide evidence for the completion of the second shell at n=6, where the dicoordinated Cu(+)(H(2)O)(2) subunit is surrounded by four H(2)O molecules. The gas-phase coordination number of Cu(+) is 2 and the resulting linearly coordinated structure acts as the core of further solvation processes.

The photodissociation spectrum of (C6H6)2+ is obtained from the yields of fragment C6H6+ ion as a function of photodissociation wavelength in the 400–1400 nm region. Two bands at 440 and 580 nm are attributed to the C ← X and the B ← X local excitation (LE) bands, respectively. Both the most intense band at 920 nm and relatively weak one at 1160 nm are assigned to charge resonance (CR) bands. The red-shift of the B ← X band from that of C6H6+ and the cross sections at the CR bands much larger than those at the LE bands are consistent with a sandwich structure for (C6H6)2+. The appearance of the two CR bands is explained on the basis of displaced sandwich structures for (C6H6)2+.

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The electronic spectrum of (CloH8)2+ is obtained in the 455-1400 nm region by applying photodissociation spectroscopy to the ion produced by the laser-induced plasma technique. The spectrum shows a local excitation band at 580 nm and a charge resonance band at 1180 nm. The locations of these bands coincide with those reported for intramolecular dimer cations of dinaphthylpropanes with a partially overlapped conformation, suggesting that (ClOHg)2+ has a similar conformation in the gas phase.

The photodissociation of size-selected benzene cluster ions, (C6H6)+m+hω→(C6H6)+n+ (m−n)C6H6, has been investigated in the 410–750 nm wavelength range using tunable dye laser radiation. The measurements were performed using a tandem mass spectrometer [a linear time-of-flight (TOF)/reflectron TOF] combined with multiphoton ionization (MPI) for ion preparation. Only C6H+6 was detected as a photofragment of (C6H6)+2 and (C6H6)+3, while both C6H+6 and (C6H6)+2 fragments were observed in the case of (C6H6)+4 photodissociation. Photodissociation spectra, i.e., photofragment yield spectra as a function of wavelength, of (C6H6)+m (m=2,3) were obtained. Two local excitation bands of (C6H6)+m were seen in this region and assigned to the C(A2u)←X(E1g) and the B(E2g)←X(E1g) transitions of a C6H+6 unit in the clusters. The origin of the B←X transition of (C6H6)+2 and (C6H6)+3 was redshifted relative to that of C6H+6 by about 1400 and 2400 cm−1, respectively, while the C←X bands of (C6H6)+2 and (C6H6)+3 were seen at the same wavelengths of 440 nm. Possible structures for the cluster ions are discussed based on the spectral shifts.

Coordination and solvation structures of the Cu + (H 2 O) n ions with n = 1-4 are studied by infrared photodissociation spectroscopy and density functional theory calculations. Hydrogen bonding between H 2 O molecules is detected in Cu + (H 2 O) 3 and Cu + (H 2 O) 4 through a characteristic change in the position and intensity of OH-stretching transitions. The third and fourth waters prefer hydrogen-bonding sites in the second solvation shell rather than direct coordination to Cu +. The infrared spectroscopy verifies that the gas-phase coordination number of Cu + in Cu + (H 2 O) n is two and the resulting linearly coordinated structure acts as the core of further solvation processes.

Photodepletion spectra of (C&6)2+ and (c6H6)3+ are obtained from the depletion yields of the parent ions as functions of photodissociation wavelengths of 750-970 nm. The spectrum of (c6H6)2+ shows a broad and featureless band with a maximum at -920 nm and an (estimated) width of 4 0 0 0 cm-I. The spectrum of ( c, &) 3:shows a similar absorption to the (c&6)2+ band, suggesting that the (C6H6)3+ band is due to the charge resonance transition in the dimer ion subunit.

Infrared photodissociation spectra of [aniline-(H 2 O) n ] + (n = 1-8) are measured in the 2700-3800 cm-1 region. The spectra are interpreted with the aid of density functional theory calculations. The n = 1 ion has an N-H•••O hydrogen bond. The spectrum of the n = 2 ion demonstrates a large perturbation to both of the NH oscillators, indicating the 1-1 structure where each NH bond is bound to a water molecule. For the n = 3 ion, the calculated spectrum of the 2-1 branched structure coincides well with the observed one. For the n = 4 ion, there exist three strong bands at 2960, 3100, and 3430 cm-1 , and a very weak one at 3550 cm-1. The observed spectrum in the 3600-3800 cm-1 region is decomposed into four bands centered at 3640, 3698, 3710, and 3734 cm-1. The 2-2 branched isomer is responsible for all the features except the 3550 and 3710 cm-1 bands. These two bands are due to another isomer with a five-membered ring. An infrared band characteristic of the n = 5 ion appears at 3684 cm-1 , which is not seen in the spectra of the n = 1-4 ions. This band is indicative of a ring structure and assigned to the free OH stretching vibration of the three-coordinated (double-acceptor-single-donor