The frequency-dependent gas-phase infrared multiple photon dissociation (IRMPD) spectrum for the protonbound dimer of water is reported. The present spectrum is shown to be only in fair agreement with a spectrum reported in an earlier communication but is in agreement with spectra predicted by theoretical means. Two different possible assignments of the observed infrared bands are provided. The first is based on the harmonic oscillator approximation from density functional theory calculations, and a second is based on a quantum four-dimensional model calculation of anharmonic frequencies and intensities. Both calculated spectra agree fairly well, but the density functional calculation assignments are in better agreement. This is expected despite the anharmonic nature of the asymmetric stretch due to the flat potential energy surface associated with this mode.The proton-bound dimer of water ( Figure 1) and larger protonated water clusters are of considerable importance both because of the interest in proton mobility from a biochemical point of view as well as the interest in the strong hydrogen bonding that takes place in these clusters. A normal hydrogen bond is on the order of 10-20 kJ mol -1 , whereas the protonbound water dimer is bound by some 130 kJ mol -1 . 1 Many theoretical studies of protonated water clusters have been conducted in the past. 2-7 Experimentally, the vibration-rotation spectrum has been observed for H 5 O 2 + and H 9 O 4 + in the OH stretching region (3550-3850 cm -1 ) using infrared multiplephoton dissociation spectroscopy (IRMPD). 8 Much more recently, the gas-phase vibrational spectrum of the proton-bound water dimer was measured by observing IRMPD in an ion trap using the free-electron laser for infrared experiments (FELIX) in The Netherlands. 9 The H 5 O 2 + ions were formed with an electrospray source and transferred to a linear radio frequency hexapole ion trap at 100 K where the ions were exposed to high intensity and tunable infrared light from the FEL and the dissociation products were monitored.In the present work, we present an infrared spectrum for the proton-bound water dimer also determined by observing IRMPD using a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer (MICRA, 10 a mobile ion cyclotron resonance analyzer), coupled to the FEL in Orsay, France (CLIO). The FTICR is based on the use of a permanent magnet, which is an assembly composed of two Halbach 11 cylinders producing a nominal magnetic field of 1.25 T. The ICR cell presents an open geometry derived from a cubic cell (20 × 20 × 20 mm 3 ) where the excitation electrodes have been replaced by two tunnels made of four interconnected electrodes. Gases are introduced through the combination of a leak valve and followed by a pulsed three-way valve that directs the gas flow either to the mass spectrometer or to the pump of the gas inlet system.The FEL at CLIO (Centre Laser Infrarouge d'Orsay), 12 a European facility in France, provides highly intense IR radiation from 3-120 µm. This FEL consists of a...

In this article, the new and exciting techniques of infrared consequence spectroscopy (sometimes called action spectroscopy) of gaseous ions are reviewed. These techniques include vibrational predissociation spectroscopy and infrared multiple photon dissociation spectroscopy and they typically complement one another in the systems studied and the information gained. In recent years infrared consequence spectroscopy has provided long-awaited direct evidence into the structures of gaseous ions from organometallic species to strong ionic hydrogen bonded structures to large biomolecules. Much is being learned with respect to the structures of ions without their stabilizing solvent which can be used to better understand the effect of solvent on their structures. This review mainly covers the topics with which the author has been directly involved in research: structures of proton-bound dimers, protonated amino acids and DNA bases, amino acid and DNA bases bound to metal ions and, more recently, solvated ionic complexes. It is hoped that this review reveals the impact that infrared consequence spectroscopy has had on the field of gaseous ion chemistry.

Methyl formate, HCOOCH 3 , is a well-known interstellar molecule prominent in the spectra of hot molecular cores. The current view of its formation is that it occurs in the gas phase from precursor methanol, which is synthesized on the surfaces of grain mantles during a previous colder era and evaporates while temperatures increase during the process of high-mass star formation. The specific reaction sequence thought to form methyl formate, the ion-molecule reaction between protonated methanol and formaldehyde followed by dissociative recombination of the protonated ion [HCO(H)OCH 3 ] + , has not been studied in detail in the laboratory. We present here the results of both a quantum chemical study of the ion-molecule reaction between [CH 3 OH 2 ] + and H 2 CO as well as new experimental work on the system. In addition, we report theoretical and experimental studies for a variety of other possible gas-phase reactions leading to ion precursors of methyl formate. The studied chemical processes leading to methyl formate are included in a chemical model of hot cores. Our results show that none of these gas-phase processes produces enough methyl formate to explain its observed abundance.

Benzylium versus tropylium ion yields from the fragmentation of ethylbenzene cations at various excitation energies are studied by forming excited ethylbenzene cations by charge transfer from a series of chargetransfer agents and by identifying the benzylium ion by its secondary reaction with neutral ethylbenzene. At lower excitation energies, the tropylium ion yield decreases with increasing energy from values near 16% (at an energy of 230 kJ mol -1 ) to 5% (at an energy of 500 kJ mol -1 ). At higher excitation energies, the tropylium ion yield increases again, which is attributed to secondary isomerization of the vibrationally highly excited benzylium ion arising from the primary fragmentation. It is suggested that this isomerization competes with radiative cooling of the excited benzylium ion. The experimental observations are rationalized in the framework of statistical unimolecular rate theory and electronic structure calculations.

The interaction of lithium ions with two pyrimidine nucleobases, thymine and uracil, as well as the solvation of various complexes by one and two water molecules, has been studied in the gas phase. IRMPD spectra are reported for each of B-Li(+)-(H(2)O)(n) (n = 1-2) and B(2)-Li-(H(2)O)(m) (m = 0-1) for B = thymine, uracil over the 2500-4000 cm(-1) region. Calculations were performed using the B3LYP density functional in conjunction with the 6-31+G(d,p) basis set to model the vibrational spectra as well as MP2/6-311++G(2d,p) theory to model the thermochemistry of potential structures. Experimental and theoretical results were used in combination to determine structures of each complex, which are reported here. The lithium cation in all complexes was found to bond to the O4 oxygen in both thymine and uracil, and the first two water molecules of solvation were found to bond to Li(+). The experimental spectra obtained for BLi(+)(H(2)O)(n) (n = 1-2) and B(2)Li(+) for thymine and uracil clearly resemble one another, suggesting similar structural features in terms of bonding between the base and Li(+), as well as for solvation. This was confirmed through theoretical work. The addition of water to the lithium ion-bound DNA base dimers has been shown to induce a significant change in structure of the dimer to a hydrogen-bonded system similar to base pairing in the Watson-Crick model of DNA.

The infrared multiphoton dissociation (IRMPD) spectra of three homogenous proton-bound dimers are presented and the major features are assigned based on comparisons with the neutral alcohol and with density functional theory calculations. As well, the IRMPD spectra of protonated propanol and the propanol/water proton-bound dimer (or singly hydrated protonated propanol) are presented and analysed. Two primary IRMPD photoproducts were observed for each of the alcohol proton bound dimers and were found to vary with the frequency of the radiation impinging upon the ions. For example, when the proton-bound dimer absorbs weakly a larger amount of S(N)2 product, protonated ether and water, are observed. When the proton-bound dimer absorbs more strongly, an increase in the simple dissociation product, protonated alcohol and neutral alcohol, is observed. With the aid of RRKM calculations this frequency dependence of the branching ratio is explained by assuming that photon absorption is faster than dissociation for these species and that only a few photons extra are necessary to make the higher-energy dissociation channel (simple cleavage) competitive with the lower energy (S(N)2) reaction channel.

The proton- and the sodium ion-bound glycine homodimers are studied by a combination of infrared multiple photon dissociation (IRMPD) spectroscopy in the N-H and O-H stretching region and electronic structure calculations. For the proton-bound glycine dimer, in the region above 3100 cm (-1), the present spectrum agrees well with one recorded previously. The present work also reveals a weak, broad absorption spanning the region from 2650 to 3300 cm (-1). This feature is assigned to the strongly hydrogen-bonded and anharmonic N-H and O-H stretching modes. As well, the shared proton stretch is observed at 2440 cm (-1). The IRMPD spectra for the proton-bound glycine dimer confirms that the lowest energy structure is an ion-dipole complex between N-protonated glycine and the carboxyl group of the second glycine. This spectrum also helps to eliminate the existence of any of the higher-energy structures considered. The IRMPD spectrum for the sodium ion-bound dimer is a much simpler spectrum consisting of three bands assigned to the O-H stretch and the asymmetric and symmetric NH 2 stretching modes. The positions of these bands are very similar to those observed for the proton-bound glycine dimer. Numerous structures were considered and the experimental spectrum agrees with the B3LYP/6-31+G(d,p) predicted spectrum for the lowest energy structure, two bidentate glycine molecules bound to Na (+). Though some of the structures cannot be completely ruled out by comparing the experimental and theoretical spectra, they are energetically disfavored by at least 20 kJ mol (-1).

Infrared multiphoton dissociation spectra of three homogeneous and two heterogeneous proton-bound dimers were recorded in the gas phase. Comparison of the experimental infrared spectra recorded in the fingerprint region of the proton-bound dimers with spectra predicted by electronic structure calculations shows that all modes which are observed contain motion of the proton oscillating between the two monomers. The O-H-O asymmetric stretch for the homogeneous dimers is shown to occur at around 800 cm-1. As expected, the O-H-O asymmetric stretching modes for the heterogeneous proton-bound dimers are observed to shift to significantly higher energy with respect to those for the homogeneous proton-bound dimers due to the asymmetry of the O-H-O moeity. This shift is shown to be predictable from the difference in proton affinities between the two monomers. Density functional predictions of the infrared spectra based on the harmonic oscillator model are demonstrated to predict the observed spectra of the homogeneous proton-bound dimers with reasonable accuracy. Calculations of the structure and infrared spectrum of protonated diglyme at the B3LYP/6-31+G** level and basis also agree well with an infrared spectrum recorded previously. For both heterogeneous proton-bound dimers, however, the predicted spectra are blue-shifted with respect to experiment.