Loss of CO from Monodeprotonated Phthalic Acid upon Photodissociation and Dissociative Electron Detachment.

The decarboxylation (CO loss) mechanism of cold monodeprotonated phthalic acid was studied in a photodissociation action spectrometer by quantifying mass-selected product anions and neutral particles as a function of the excitation energy. The analysis proceeded by interpreting the translational energy distribution of the generated uncharged products, and with the help of quantum calculations. In particular, this study reveals different fragmentation pathways in the deprotonated anion and in the radical generated upon electron photodetachment.

Influence of the N atom and its position on electron photodetachment of deprotonated indole and azaindole.

Electron photodetachment of cold deprotonated indole and azaindole anions has been studied by use of a mass-selective photofragmentation spectrometer capable of negative ion and neutral particle detection. The electron affinities of the indolyl radical and the 5-, 6- and 7-azaindolyl radicals have been measured with an uncertainty of less than 0.002 eV.

Influence of the N atom position on the excited state photodynamics of protonated azaindole.

We present a study of the photofragmentation of three protonated azaindole molecules - 7-azaindole, 6-azaindole, and 5-azaindole - consisting of fused pyrrole-pyridine bicyclic aromatic systems, in which the pyridinic (protonated) nitrogen heteroatom is located at the 7, 6, and 5 positions, respectively. Photofragmentation electronic spectra of the isolated aforementioned azaindolinium cations reveal that their photodynamics extends over timescales covering nine orders of magnitude and provide evidence about the resultant fragmentation pathways. Moreover, we show how the position of the heteroatom in the aromatic skeleton influences the excited state energetics, fragmentation pathways, and fragmentation timescales.

Carbonyl stretch of CH⋯O hydrogen-bonded methyl acetate in supercritical trifluoromethane.

Infrared spectroscopy in the gas phase was used to study the formation reaction of the CH⋯O hydrogen bonding complex involving the CH group of trifluoromethane, as a hydrogen donor, and the carbonyl group of methyl acetate, as a hydrogen acceptor, under different (T, p) conditions. The hydrogen-bonded carbonyl stretch of the molecular pair was monitored in dilute mixtures of methyl acetate in trifluoromethane at near-critical temperatures, from gas- to liquid-like densities. In the gas region, it was possible to discriminate the carbonyl signal of the hydrogen-bonded complex from that of the free ester and have access to their relative concentration.

Static and dynamic scavenging of ammoniated electrons by nitromethane.

We studied the time-resolved scavenging efficiency of nitromethane for transient electron species in liquid ammonia, at a temperature of 298 K. UV excitation of iodide ions produced fully solvated electrons, as well as transient (I, e-) and (counterion, e-) pairs, the overall concentration of which was monitored by NIR absorption with subpicosecond time resolution. After the UV pulse, the solution absorbance decays almost completely in a few hundreds of picoseconds due to geminate electron-iodine atom recombination and a competitive annihilation channel involving the scavenger.

Tautomerism and electronic spectroscopy of protonated 1- and 2-aminonaphthalene.

Experimental and theoretical investigations of the excited states of protonated 1- and 2-aminonaphthalene are presented. The electronic spectra are obtained by laser induced photofragmentation of the ions captured in a cold ion trap. Using ab initio calculations, the electronic spectra can be assigned to different tautomers which have the proton on the amino group or on the naphthalene moiety.

Counterion effects on the ultrafast dynamics of charge-transfer-to-solvent electrons.

We performed femtosecond transient absorption (TA) experiments to monitor the solvation dynamics of charge-transfer-to-solvent (CTTS) electrons originating from UV photoexcitation of ammoniated iodide in close proximity to the counterions. Solutions of KI were prepared in liquid ammonia and TA experiments were carried out at different temperatures and densities, along the liquid-gas coexistence curve of the fluid. The results complement previous femtosecond TA work by P.

Photodissociation Electronic Spectra of Cold Protonated Quinoline and Isoquinoline in the Gas Phase.

Photofragmentation electronic spectra of isolated single-isomeric N-protonated quinoline (quinolinium) and isoquinoline (isoquinolinium) ions have been measured at a temperature of ∼40 K using a mass-selective, 10 cm spectral resolution, photodissociation spectrometer. Additionally, ab initio adiabatic transition energies calculated using the RI-ADC(2) method have been employed to assist in the assignment of the spectra. Three electronic transitions having ππ* character were clearly evidenced for both protonated ions within the UV and deep-UV spectral ranges.

Photodissociation UV-Vis Spectra of Cold Protonated Azobenzene and 4-(Dimethylamino)azobenzene and Their Benzenediazonium Cation Fragment.

Gas phase photodissociation electronic spectra of protonated azobenzene (ABH(+)) and 4-(dimethylamino)azobenzene (dmaABH(+)) were measured in a cryogenically cooled ion trap at temperatures of a few tens of Kelvin. Experimental results were complemented with electronic structure calculations in the ground state at the MP2/cc-pVDZ level of theory, and in the low lying excited states using the RI-CC2 method. Calculated energies revealed that only the trans isomers of the azonium molecular ions (protonation site on the azo group) will likely exist in the trap at the temperatures achieved in the experiment.

Electric deflection of middle-size ammonia clusters containing (E(-), Na(+)) pairs.

Effective polarizabilities of Na(NH3)n (n = 8-27) clusters were measured by electric deflection as a function of the particle size. A significant field-induced shift of the beam intensity profile without the occurrence of broadening revealed that the clusters behave as liquidlike polar objects in the conditions of the experiment (cluster temperatures were estimated in the range of 110-145 K). Most of the cluster polarity is attributed to the spontaneous promotion of the alkali atom valence electron to a diffuse state stabilized by the cluster solvent field, with the consequent formation of (e(-), Na(+)) pairs.

Electronic spectra of the protonated indole chromophore in the gas phase.

The electronic spectroscopy of cold protonated indole was investigated experimentally and theoretically. Two isomers were observed by experiment: The first isomer corresponds to the lowest-energy isomer in the calculations, absorbing at ~350 nm and protonated on the C3 atom of the pyrrole ring. According to our calculations, the absorptions of the other isomers protonated on carbon atoms (C2, C4, C5, C6, and C7) are in the visible region.

Role of the charge-transfer state in the electronic absorption of protonated hydrocarbon molecules.

The vibrationally resolved electronic spectra of isolated protonated polycyclic aromatic hydrocarbons (PAHs)--naphthalene, anthracene, and tetracene--have been recorded via neutral photofragment spectroscopy. The S1←S0 transitions are all in the visible region and do not show a monotonic red shift as a function of the molecular size, as observed for the neutral analogues. Comparison with ab initio calculations indicates that this behavior is due to the nature of the excited state, which has a pronounced charge-transfer character for protonated linear PAHs with an even number of aromatic rings.

Excited state hydrogen transfer dynamics in substituted phenols and their complexes with ammonia: ππ*-πσ* energy gap propensity and ortho-substitution effect.

Lifetimes of the first electronic excited state (S(1)) of fluorine and methyl (o-, m-, and p-) substituted phenols and their complexes with one ammonia molecule have been measured for the 0(0) transition and for the intermolecular stretching σ(1) levels in complexes using picosecond pump-probe spectroscopy. Excitation energies to the S(1) (ππ*) and S(2) (πσ*) states are obtained by quantum chemical calculations at the MP2 and CC2 level using the aug-cc-pVDZ basis set for the ground-state and the S(1) optimized geometries. The observed lifetimes and the energy gaps between the ππ* and πσ* states show a good correlation, the lifetime being shorter for a smaller energy gap.

Structural transitions and dipole moment of water clusters (H2O)(n=4-100).

The properties of water clusters (H(2)O)(n) over a broad range of sizes (n=4-100) were studied by microcanonical parallel tempering Monte Carlo and replica exchange molecular dynamics simulations at temperatures between 20 and 300 K, with special emphasis in the understanding of relation between the structural transitions and dipole behavior. The effect of the water interaction potential was analyzed using six nonpolarizable models, but more extensive calculations were performed using the TIP4P-ice water model. We find that, in general, the dipole moment of the cluster increases significantly as the cluster melts, suggesting that it could be used to discriminate between the solidlike and liquidlike phases.

A fast ruthenium polypyridine cage complex photoreleases glutamate with visible or IR light in one and two photon regimes.

We introduce a new caged glutamate, based in a ruthenium bipyridyl core, that undergoes heterolytic cleavage after irradiation with visible light with wavelengths up to 532nm, yielding free glutamate in less than 50ns. Glutamate photorelease occurs also efficiently following two-photon (2P) excitation at 800nm, and has a functional cross section of 0.14GM.

Electric susceptibility of sodium-doped water clusters by beam deflection.

The electric susceptibility of neutral sodium-doped water clusters Na(H(2)O)(N), N = 6-33, was determined by beam electric deflection. The clusters behave as polarizable particles; their intensity profiles exhibit global shifts toward the high-field region without the occurrence of broadening. In the conditions of the experiment, sodium-water clusters have a "floppy" structure and hence the electric susceptibility presents both electronic and orientacional terms.

Is ammonia a better solvent than water for contact ion pairs?

The existence of a charge-transfer-to-solvent process when a KI contact ion pair (CIP) dissolved in supercritical water (SCW) is excited by UV light was confirmed by use of electronic structure calculations applied to molecular dynamics trajectories. We observed similar behavior with fluid density as that found for the KI-CIP in supercritical ammonia (SCA); nevertheless, there are some distinct features in the two supercritical solvents. First, the effect of the solvent field due to the molecules lying beyond the first solvation shell is very different in SCW compared with that observed in SCA; in SCW it actually has a destabilizing effect over the ground and excited states.

Short-range and long-range solvent effects on charge-transfer-to-solvent transitions of I- and K+I- contact ion pair dissolved in supercritical ammonia.

Vertical excitation and electron detachment energies associated with the optical absorption of iodide ions dissolved in supercritical ammonia at 420 K have been calculated in two limiting scenarios: as a solvated free I- ion and forming a K+I- contact ion pair (CIP). The evolution of the transition energies as a result of the gradual building up of the solvation structure was studied for each absorbing species as the solvent's density increased, i.e.

Development of the charge-transfer-to-solvent process with increasing solvent fluid density: the effect of ion pairing.

The study of the UV spectroscopic behaviour of alkali metal iodides dissolved in supercritical ammonia showed that two absorbing species contributed to the UV absorption of the solutions. The two species differed in the type of interaction of iodide with the cation, i.e.

Influence of ion pairing on the UV-spectral behavior of KI dissolved in supercritical NH3: from vapor phase to condensed liquid.

The UV-spectroscopic behavior of KI dissolved in supercritical ammonia enabled us to identify two species that contribute to the optical absorption depending on the fluid density rho1 and the temperature T. At low rho1 and high T, contact ion pairs (CIPs) prevail, while at high density of ammonia, solvent separated ion pairs (SSIPs) and free iodide ions dominate the optical absorption of the solute. The features of the electron excitation process depend on the state of the K+ I- species present.

Solvent triggered change of the electron excitation route of KI in supercritical NH3.

The UV-spectroscopic behavior of KI contact ion pairs (CIPs) dissolved in supercritical NH3 was studied combining classical molecular dynamics simulations with electronic structure calculations, and the results show that an abrupt change of the photoexcitation route of KI CIPs occurs at very low solvent densities. Few NH3 solvating molecules are required to hamper the well-known photoinduced intramolecular electron (e-) transfer observed in isolated ion pairs of alkali metal halides in the vapor drawing the e- to solvent cavities leading to a charge-transfer-to-solvent process.