Quantitatively resolving mixtures of isobaric compounds using chemical ionization mass spectrometry by modulating the reactant ion composition

Acrolein (C(3)H(4)O) and 1-butene (C(4)H(8)) can both be individually detected by proton transfer chemical ionization mass spectrometry (CI-MS). However, because these compounds are isobaric, mixtures of these two compounds cannot be resolved since both compounds react with H(3)O(+) via a proton-transfer reaction to form a protonated molecule that is detected at a nominal mass-to-charge ratio of 57 (m/z 57). While both compounds react with H(3)O(+) only acrolein reacts to any significant extent with H(3)O(+)(H(2)O). Recognizing that the electrical potential applied to a drift tube reaction mass spectrometer provides a simple and effective means for varying the relative intensity of the H(3)O(+) and H(3)O(+)(H(2)O) reactant ions we have developed a method whereby we make use of this reactivity difference to resolve mixtures of these two compounds. We demonstrate a technique where the individual contributions of acrolein and 1-butene within a mixture can be quantitatively resolved by systematically changing the reagent ion from H(3)O(+) to H(3)O(+)(H(2)O) through control of the electric potential applied to the drift tube reaction region of a proton transfer reaction mass spectrometer.     

Unimolecular reactions of dihydrated alkaline earth metal dications M2+(H2O)2, M = Be, Mg, Ca, Sr, and Ba: salt-bridge mechanism in the proton-transfer reaction M2+(H2O)2 --> MOH+ + H3O

The unimolecular reactivity of M(2+)(H(2)O)(2), M = Be, Mg, Ca, Sr, and Ba, is investigated by density functional theory. Dissociation of the complex occurs either by proton transfer to form singly charged metal hydroxide, MOH(+), and protonated water, H(3)O(+), or by loss of water to form M(2+)(H(2)O) and H(2)O. Charge transfer from water to the metal forming H(2)O(+) and M(+)(H(2)O) is not favorable for any of the metal complexes. The relative energetics of these processes are dominated by the metal dication size. Formation of MOH(+) proceeds first by one water ligand moving to the second solvation shell followed by proton transfer to this second-shell water molecule and subsequent Coulomb explosion. These hydroxide formation reactions are exothermic with activation energies that are comparable to the water binding energy for the larger metals. This results in a competition between proton transfer and loss of a water molecule. The arrangement with one water ligand in the second solvation shell is a local minimum on the potential energy surface for all metals except Be. The two transition states separating this intermediate from the reactant and the products are identified. The second transition state determines the height of the activation barrier and corresponds to a M(2+)-OH(-)-H(3)O(+) "salt-bridge" structure. The computed B3LYP energy of this structure can be quantitatively reproduced by a simple ionic model in which Lewis charges are localized on individual atoms. This salt-bridge arrangement lowers the activation energy of the proton-transfer reaction by providing a loophole on the potential energy surface for the escape of H(3)O(+). Similar salt-bridge mechanisms may be involved in a number of proton-transfer reactions in small solvated metal ion complexes, as well as in other ionic reactions.     

Femtosecond dynamics of Cu(H(2)O)(2)

The ultrafast relaxation dynamics of Cu(H(2)O)(2) is investigated using femtosecond photodetachment-photoionization spectroscopy. In addition, stationary points on the Cu(H(2)O)(2) anion, neutral, and cation potential energy surfaces are characterized by ab initio electronic structure calculations. Electron photodetachment from Cu(-)(H(2)O)(2) initiates the dynamics on the ground-state potential energy surface of neutral Cu(H(2)O)(2). The resulting Cu(H(2)O)(2) complexes experience large-amplitude H(2)O reorientation and dissociation. The time evolution of the Cu(H(2)O)(2) fragmentation products is monitored by time-resolved resonant multiphoton ionization. The parent ion, Cu(+)(H(2)O)(2), is not detected above background levels. The rise to a maximum of the Cu(+) signal from Cu(-)(H(2)O)(2), and the decay of the Cu(+)(H(2)O) signal from Cu(-)(H(2)O)(2) have similar tau approximately 10 ps time dependences to the corresponding signals from Cu(-)(H(2)O), but display clear differences at very short and long times. The experimental observations can be understood in terms of the following picture. Prompt dissociation of H(2)O from nascent Cu(H(2)O)(2) gives rise to a vibrationally excited Cu(H(2)O) complex, which dissociates to Cu+H(2)O due to coupling of H(2)O internal rotation to the dissociation coordinate. This prompt dissociation removes all intra-H(2)O vibrational excitation from the intermediate Cu(H(2)O) fragment, which quenches the long time vibrational predissociation to Cu+H(2)O previously observed in analogous experiments on Cu(-)(H(2)O).     

Alternative low-energy mechanisms for isotopic exchange in gas-phase D2O-H+(H2O)n reactions.

Molecular-dynamics (MD) trajectories and high-level ab initio methods have been used to study the low-energy mechanism for D(2)O-H(+)(H(2)O)(n) reactions. At low collisional energies, MD simulations show that the collisional complexes are long-lived and undergo fast monomolecular isomerization, converting between different isomers within 50-500 ps. Such processes, primarily involving water-molecule shifts along a water chain, require the surmounting of very-low-energy barriers and present sizable non- Rice-Ramsperger-Kassel-Marcus (RRKM) effects, which are interpreted as a lack of randomization of the internal kinetic energy. Interestingly, the rate of water shifts was found to increase upon increasing the size of the cluster. Based on these findings, we propose to incorporate the following steps into the mechanism for low-energy isotopic scrambling these D(2)O-H(+)(H(2)O)(n) reactions: a) formation of the collisional complex [H(+)(H(2)O)(n)D(2)O]* in a vibro-rotational excited state; b) incorporation of the heavy-water molecule in the cluster core as HD(2)O(+) by means of isomerization involving molecular shifts; c) displacement of solvation molecules from the first shell of HD(2)O(+) inducing de-deuteration (shift of a D(+) to a neighbor water molecule); d) reorganization of the clusters and/or expulsion of one of the isotopic variants of water (H(2)O, HDO or D(2)O) from the periphery of the complex.     

Dynamics and mechanism of structural diffusion in linear hydrogen bond

Dynamics and mechanism of proton transfer in a protonated hydrogen bond (H-bond) chain were studied, using the CH(3)OH(2)(+)(CH(3)OH)(n) complexes, n = 1-4, as model systems. The present investigations used B3LYP/TZVP calculations and Born-Oppenheimer MD (BOMD) simulations at 350 K to obtain characteristic H-bond structures, energetic and IR spectra of the transferring protons in the gas phase and continuum liquid. The static and dynamic results were compared with the H(3)O(+)(H(2)O)(n) and CH(3)OH(2)(+)(H(2)O)(n) complexes, n = 1-4. It was found that the H-bond chains with n = 1 and 3 represent the most active intermediate states and the CH(3)OH(2)(+)(CH(3)OH)(n) complexes possess the lowest threshold frequency of proton transfer. The IR spectra obtained from BOMD simulations revealed that the thermal energy fluctuation and dynamics help promote proton transfer in the shared-proton structure with n = 3 by lowering the vibrational energy for the interconversion between the oscillatory shuttling and structural diffusion motions, leading to a higher population of the structural diffusion motion than in the shared-proton structure with n = 1. Additional explanation on the previously proposed mechanisms was introduced, with the emphases on the energetic of the transferring proton, the fluctuation of the number of the CH(3)OH molecules in the H-bond chain, and the quasi-dynamic equilibriums between the shared-proton structure (n = 3) and the close-contact structures (n ≥ 4). The latter prohibits proton transfer reaction in the H-bond chain from being concerted, since the rate of the structural diffusion depends upon the lifetime of the shared-proton intermediate state.     

Infrared spectroscopy of Cu+(H2O)(n) and Ag+(H2O)(n): coordination and solvation of noble-metal ions

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.     

Structural rearrangements and magic numbers in reactions between pyridine-containing water clusters and ammonia

Molecular cluster ions H(+)(H(2)O)(n), H(+)(pyridine)(H(2)O)(n), H(+)(pyridine)(2)(H(2)O)(n), and H(+)(NH(3))(pyridine)(H(2)O)(n) (n = 16-27) and their reactions with ammonia have been studied experimentally using a quadrupole-time-of-flight mass spectrometer. Abundance spectra, evaporation spectra, and reaction branching ratios display magic numbers for H(+)(NH(3))(pyridine)(H(2)O)(n) and H(+)(NH(3))(pyridine)(2)(H(2)O)(n) at n = 18, 20, and 27. The reactions between H(+)(pyridine)(m)(H(2)O)(n) and ammonia all seem to involve intracluster proton transfer to ammonia, thus giving clusters of high stability as evident from the loss of several water molecules from the reacting cluster. The pattern of the observed magic numbers suggest that H(+)(NH(3))(pyridine)(H(2)O)(n) have structures consisting of a NH(4)(+)(H(2)O)(n) core with the pyridine molecule hydrogen-bonded to the surface of the core. This is consistent with the results of high-level ab initio calculations of small protonated pyridine/ammonia/water clusters.     

Reactions that generate aromatic molecules: is aromatic stabilization less or more advanced than bond changes at the transition state? Kinetic and thermodynamic acidities of rhenium carbene complexes

A kinetic study of the reversible deprotonation of the rhenium carbene complexes 1H(+)(O), 1H(+)(S) and 2H(+)(O) by carboxylate ions, primary aliphatic and secondary alicyclic amines, water and OH(-) in 50% MeCN-50% water (v/v) at 25 degrees C is reported. These carbene complexes are of special interest because in their deprotonated form they represent derivatives of the aromatic heterocycles furan, thiophene and benzofuran. Intrinsic rate constants (k(o) for Delta G degrees = 0) determined from appropriate Br?nsted plots for these rhenium carbene complexes and for the corresponding selenophene (1H(+)(Se)) and benzothiophene (2H(+)(S)) derivatives investigated earlier follow the orders furan < selenophene < thiophene and benzofuran less, similar benzothiophene. These orders indicate that an increase in aromaticity leads to an increase in the intrinsic rate constant or a decrease in the intrinsic barrier. This is an unexpected result; it implies that, in contrast to common resonance effects, the development of aromaticity at the transition state is ahead of proton transfer, i.e., the percentage development of the aromatic stabilization energy at the transition state is higher than the percentage of proton transfer.     

Communication: A concerted mechanism between proton transfer of Zundel anion and displacement of counter cation

Ab initio path integral molecular dynamics simulation of M(+)(H(3)O(2)(-)) (M = Li, Na, and K) has been carried out to analyze how the structure and dynamics of a low-barrier hydrogen-bonded Zundel anion, H(3)O(2)(-), can be affected by the counter alkali metal cation, M(+). Our simulation predicts that the quantum proton transfer in Zundel anion can be strongly coupled to the motion of counter cation located nearby. A smaller cation can induce larger structural distortion of the Zundel anion fragment making the proton transfer barrier higher, and hence, lower the vibrational excitation energy. It is also argued that a large H∕D isotope effect is present.     

Hydration of potassiated amino acids in the gas phase

The thermochemistry of stepwise hydration of several potassiated amino acids was studied by measuring the gas-phase equilibria, AAK(+)(H(2)O)(n-1) + H(2)O = AAK(+)(H(2)O)(n) (AA = Gly, AL, Val, Met, Pro, and Phe), using a high-pressure mass spectrometer. The AAK(+) ions were obtained by electrospray and the equilibrium constants K(n-1,n) were measured in a pulsed reaction chamber at 10 mbar bath gas, N(2), containing a known partial pressure of water vapor. Determination of the equilibrium constants at different temperatures was used to obtain the DeltaH(n)(o), DeltaS(n)(o), and DeltaG(n)(o) values. The results indicate that the water binding energy in AAK(+)(H(2)O) decreases as the K(+) affinity to AA increases. This trend in binding energies is explained in terms of changes in the side-chain substituent, which delocalize the positive charge from K(+) to AA in AAK(+) complexes, varying the AAK(+)-H(2)O electrostatic interaction.     

Structural study on 2,2'-(methylimino)bis(N,N-dioctylacetamide) complex with Re(VII)O4- and Tc(VII)O4- by 1H NMR, EXAFS, and IR spectroscopy

The structures of the complex of 2,2'-(methylimino)bis(N,N-dioctylacetamide) (MIDOA) with M(VII)O(4)(-) (M = Re and Tc), which were prepared by liquid-liquid solvent extraction, were investigated by using (1)H nuclear magnetic resonance (NMR), extended X-ray absorption fine structure (EXAFS), and infrared (IR) spectroscopy. The (1)H NMR spectra of the complex of MIDOA with Re(VII)O(4)(-) prepared in the organic solution suggest the transfer of a proton from aqueous to organic solution and the formation of the H(+)MIDOA ion. The EXAFS spectra of the complexes of H(+)MIDOA with Re(VII)O(4)(-) and Tc(VII)O(4)(-) show only the M-O coordination of the aquo complexes, suggesting that the chemical state of M(VII)O(4)(-) was unchanged during the extraction process. The results from (1)H NMR and EXAFS, therefore, provide evidence of M(VII)O(4)(-)···H(+)MIDOA complex formation in the organic solution. The IR spectra of Re(VII)O(4)(-)···H(+)MIDOA and Tc(VII)O(4)(-)···H(+)MIDOA were analyzed based on the structures and the IR spectra that were calculated at the B3LYP/cc-pVDZ level. Comparison of the observed and calculated IR spectra demonstrates that an intramolecular hydrogen bond is formed in H(+)MIDOA, and the M(VII)O(4)(-) ion interacts with H(+)MIDOA through multiple C-H(n)···O hydrogen bonds.     

Reactivity of phosphate monoester monoanions in aqueous solution. 1. Quantum mechanical calculations support the existence of "anionic zwitterion" MeO(+)(H)PO(3)(2-) as a key intermediate in the dissociative hydrolysis of the methyl phosphate anion

The dissociative hydrolysis reaction of the methyl phosphate monoanion has been studied for the reactant species CH(3)OPO(3)H(-) (1) and CH(3)OPO(3)H(-) x H(2)O (1a) in the gas and aqueous phases by density functional theory (B3LYP) calculations. Nonspecific solvation effects were taken into account with the polarizable continuum model PCM either by solvating the gas-phase reaction paths or by performing geometry searches directly in the presence of the solvation correction. In agreement with previous theoretical studies, our gas-phase calculations indicate that proton transfer to the methoxy group of 1 is concerted with P-O bond cleavage. In contrast, optimizations performed with the PCM solvation model establish the existence of the tautomeric form CH(3)O(+)(H)PO(3)(2-) (2) as an intermediate, indicating that proton transfer and P-O bond cleavage become uncoupled in aqueous solution. The dissociative pathway of 1a is energetically favored over the dissociative pathway of 1 only when the added water molecule plays an active catalytic role in the prototropic rearrangement 1 <--> 2. In that case, it is found that the collapse (via P-O bond cleavage) of the hydrated zwitterionic form CH(3)O(+)(H)PO(3)(2-) x H(2)O (2a) is rate-determining. This collapse may occur by a stepwise mechanism through a very short-lived metaphosphate intermediate (PO(3)(-)), or by a concerted S(N)2-like displacement through a loose metaphosphate-like transition state. The present calculations do not allow a distinction to be made between these two alternatives, which are both in excellent agreement with experiment. The present study also reveals that PO(3)(-) reacts selectively with CH(3)OH and H(2)O nucleophiles in aqueous solution. However, the observed selectivity of metaphosphate is governed by solvation effects, not nucleophilicity (water being much more effective than methanol in capturing PO(3)(-)). This arises from a better solvation of the addition product H(2)O(+)PO(3)(2-) as compared to CH(3)O(+)(H)PO(3)(2-).     

Electron impact ionization of water-doped superfluid helium nanodroplets: observation of He(H(2)O)(n)(+) clusters

Electron impact mass spectra have been recorded for helium nanodroplets containing water clusters. In addition to identification of both H(+)(H(2)O)(n) and (H(2)O)(n)(+) ions in the gas phase, additional peaks are observed which are assigned to He(H(2)O)(n)(+) clusters for up to n=27. No clusters are detected with more than one helium atom attached. The interpretation of these findings is that quenching of (H(2)O)(n)(+) by the surrounding helium can cool the cluster to the point where not only is fragmentation to H(+)(H(2)O)(m) (where m < or = n-1) avoided, but also, in some cases, a helium atom can remain attached to the cluster ion as it escapes into the gas phase. Ab initio calculations suggest that the first step after ionization is the rapid formation of distinct H(3)O(+) and OH units within the (H(2)O)(n)(+) cluster. To explain the formation and survival of He(H(2)O)(n)(+) clusters through to detection, the H(3)O(+) is assumed to be located at the surface of the cluster with a dangling O-H bond to which a single helium atom can attach via a charge-induced dipole interaction. This study suggests that, like H(+)(H(2)O)(n) ions, the preferential location for the positive charge in large (H(2)O)(n)(+) clusters is on the surface rather than as a solvated ion in the interior of the cluster.     

Infrared photodissociation spectroscopy of H(+)(H2O)6·M(m) (M = Ne, Ar, Kr, Xe, H2, N2, and CH4): messenger-dependent balance between H3O(+) and H5O2(+) core isomers

Although messenger mediated spectroscopy is a widely-used technique to study gas phase ionic species, effects of messengers themselves are not necessarily clear. In this study, we report infrared photodissociation spectroscopy of H(+)(H(2)O)(6)·M(m) (M = Ne, Ar, Kr, Xe, H(2), N(2), and CH(4)) in the OH stretch region to investigate messenger(M)-dependent cluster structures of the H(+)(H(2)O)(6) moiety. The H(+)(H(2)O)(6), the protonated water hexamer, is the smallest system in which both the H(3)O(+) (Eigen) and H(5)O(2)(+) (Zundel) hydrated proton motifs coexist. All the spectra show narrower band widths reflecting reduced internal energy (lower vibrational temperature) in comparison with bare H(+)(H(2)O)(6). The Xe-, CH(4)-, and N(2)-mediated spectra show additional band features due to the relatively strong perturbation of the messenger. The observed band patterns in the Ar-, Kr-, Xe-, N(2)-, and CH(4)-mediated spectra are attributed mainly to the "Zundel" type isomer, which is more stable. On the other hand, the Ne- and H(2)-mediated spectra are accounted for by a mixture of the "Eigen" and "Zundel" types, like that of bare H(+)(H(2)O)(6). These results suggest that a messenger sometimes imposes unexpected isomer-selectivity even though it has been thought to be inert. Plausible origins of the isomer-selectivity are also discussed.     

Apparent stoichiometry of water in proton hydration and proton dehydration reactions in CH3CN/H2O solutions

Gradual solvation of protons by water is observed in liquids by mixing strong mineral acids with various amounts of water in acetonitrile solutions, a process which promotes rapid dissociation of the acids in these solutions. The stoichiometry of the reaction XH(+) + n(H(2)O) = X + (H(2)O)(n)H(+) was studied for strong mineral acids (negatively charged X, X = ClO(4)ˉ, Clˉ, Brˉ, Iˉ, CF(3)SO(3)ˉ) and for strong cationic acids (uncharged X, X = R*NH(2), H(2)O). We have found by direct quantitative analysis preference of n = 2 over n = 1 for both groups of proton transfer reactions at relatively low water concentrations in acetonitrile. At high water concentrations, we have found that larger water solvates must also be involved in the solvation of the proton while the spectral features already observed for n = 2, H(+)(H(2)O)(2), remain almost unchanged at large n values up to at least 10 M of water.     

Isotope exchange in reactions between D2O and size-selected ionic water clusters containing pyridine, H+ (pyridine)m(H2O)n

Pyridine containing water clusters, H(+)(pyridine)(m)(H(2)O)(n), have been studied both experimentally by a quadrupole time-of-flight mass spectrometer and by quantum chemical calculations. In the experiments, H(+)(pyridine)(m)(H(2)O)(n) with m = 1-4 and n = 0-80 are observed. For the cluster distributions observed, there are no magic numbers, neither in the abundance spectra, nor in the evaporation spectra from size selected clusters. Experiments with size-selected clusters H(+)(pyridine)(m)(H(2)O)(n), with m = 0-3, reacting with D(2)O at a center-of-mass energy of 0.1 eV were also performed. The cross-sections for H/D isotope exchange depend mainly on the number of water molecules in the cluster and not on the number of pyridine molecules. Clusters having only one pyridine molecule undergo D(2)O/H(2)O ligand exchange, while H(+)(pyridine)(m)(H(2)O)(n), with m = 2, 3, exhibit significant H/D scrambling. These results are rationalized by quantum chemical calculations (B3LYP and MP2) for H(+)(pyridine)(1)(H(2)O)(n) and H(+)(pyridine)(2)(H(2)O)(n), with n = 1-6. In clusters containing one pyridine, the water molecules form an interconnected network of hydrogen bonds associated with the pyridinium ion via a single hydrogen bond. For clusters containing two pyridines, the two pyridine molecules are completely separated by the water molecules, with each pyridine being positioned diametrically opposite within the cluster. In agreement with experimental observations, these calculations suggest a "see-saw mechanism" for pendular proton transfer between the two pyridines in H(+)(pyridine)(2)(H(2)O)(n) clusters.     

Cluster-enhanced X-O2 photochemistry (X=CH3I, C3H6, C6H12, and Xe)

The effect of a local environment on the photodissociation of molecular oxygen is investigated in the van der Waals complex X-O(2) (X=CH(3)I, C(3)H(6), C(6)H(12), and Xe). A single laser operating at wavelengths around 226 nm is used for both photodissociation of the van der Waals complex and simultaneous detection of the O((3)P(J),J=2,1,0) atom photoproduct via (2+1) resonance enhanced multiphoton ionization. The kinetic energy distribution (KED) and angular anisotropy of the product O atom recoil in this dissociation are measured using the velocity map imaging technique configured for either full ("crush") or partial ("slice") detection of the three-dimensional O((3)P(J)) atom product Newton sphere. The measured KED and angular anisotropy reveal a distinct difference in the mechanism of O atom generation from an X-O(2) complex compared to a free O(2) molecule. The authors identify two one-photon excitation pathways, the relative importance of which depends on IPx, the ionization potential of the X partner. One pathway, observed for all complexes independent of IPx, involves a direct transition to the perturbed covalent state X-O(2)(A'(3)Delta(u)) with excitation localized on the O(2) subunit. The predominantly perpendicular character of this channel relative to the laser polarization detection, together with data on the structure of the complex, allows us to confirm that X partner induced admixing of an X(+)-O(2) (-) charge transfer (CT) state is the perturbing factor resulting in the well-known enhancement of photoabsorption within the Herzberg continuum of molecular oxygen. The second excitation pathway, observed for X-O(2) complexes with X=CH(3)I and C(3)H(6), involves direct excitation into the (3)(X(+)-O(2) (-)) CT state of the complex. The subsequent photodissociation of this CT state by the same laser pulse gives rise to the superoxide anion O(2) (-), which then photodissociates, providing fast (0.69 eV) O atoms with a parallel image pattern. Products from the photodissociation of singlet oxygen O(2)(b (1)Sigma(g) (+)) are also observed when the CH(3)I-O(2) complex was irradiated. Potential energy surfaces (PES) for the ground and relevant excited states of the X-O(2) complex have been constructed for CH(3)I-O(2) using the results of CASSCF calculations for the ground and CT states of the complex as well as literature data on PES of the subunits. These model potential energy surfaces allowed us to interpret all of the observed O((3)P(J)) atom production channels.     

Electronic effect on protonated hydrogen-bonded imidazole trimer and corresponding derivatives cationized by alkali metals (Li(+), Na(+), and K(+))

The electronic effects on the protonated hydrogen-bonded imidazole trimer (Im)(3)H(+) and the derivatives cationized by alkali metals (Li(+), Na(+), and K(+)) are investigated using B3LYP method in conjunction with the 6-311+G( *) basis set. The prominent characteristics of (Im)(3)H(+) on reduction are the backflow of the transferred proton to its original fragment and the remoteness of the H atom from the attached side bare N atom. The proton transfer occurs on both reduction and oxidation for the corresponding hydrogen-bonded imidazole trimer. For the derivatives cationized by Li(+), (Im)(3)Li(+), the backflow of the transferred proton occurs on reduction. The electron detachment from respective highest occupied molecular orbital of (Im)(3)Na(+) and (Im)(3)K(+) causes the proton transferring from the fragment attached by the alkali metal cation to the middle one. The order of the adiabatic ionization potentials of (Im)(3)M(+) is (Im)(3)H(+)>(Im)(3)Li(+)>(Im)(3)Na(+)>(Im)(3)K(+); the order of (Im)(3)M indicates that (Im)(3)H is the easicst complex to be ionized. The polarity of (Im)(3)M(+) (M denotes H, Li, Na, and K) increases on both oxidation and reduction. The (Im)(3)M(+) complexes dissociate into (Im)(3) and M(+) except (Im)(3)H(+), which dissociates preferably into (Im)(3) (+) and H atom, while the neutral complexes [(Im)(3)M] dissociate into (Im)(3) and M. The stabilization energy of (Im)(3)Li(2+), (Im)(3)Na(2+), and (Im)(3)K(2+) indicate that their energies are higher as compared to those of the monomers.     

Microsolvation of Co2+ and Ni2+ by acetonitrile and water: photodissociation dynamics of M(2+)(CH3CN)n(H2O)m

The microsolvation of cobalt and nickel dications by acetonitrile and water is studied by measuring photofragment spectra at 355, 532 and 560-660 nm. Ions are produced by electrospray, thermalized in an ion trap and mass selected by time of flight. The photodissociation yield, products and their branching ratios depend on the metal, cluster size and composition. Proton transfer is only observed in water-containing clusters and is enhanced with increasing water content. Also, nickel-containing clusters are more likely to undergo charge reduction than those with cobalt. The homogeneous clusters with acetonitrile M(2+)(CH(3)CN)(n) (n = 3 and 4) dissociate by simple solvent loss; n = 2 clusters dissociate by electron transfer. Mixed acetonitrile/water clusters display more interesting dissociation dynamics. Again, larger clusters (n = 3 and 4) show simple solvent loss. Water loss is substantially favored over acetonitrile loss, which is understandable because acetonitrile is a stronger ligand due to its higher dipole moment and polarizability. Proton transfer, forming H(+)(CH(3)CN), is observed as a minor channel for M(2+)(CH(3)CN)(2)(H(2)O)(2) and M(2+)(CH(3)CN)(2)(H(2)O) but is not seen in M(2+)(CH(3)CN)(3)(H(2)O). Studies of deuterated clusters confirm that water acts as the proton donor. We previously observed proton loss as the major channel for photolysis of M(2+)(H(2)O)(4). Measurements of the photodissociation yield reveal that four-coordinate Co(2+) clusters dissociate more readily than Ni(2+) clusters whereas for the three-coordinate clusters, dissociation is more efficient for Ni(2+) clusters. For the two-coordinate clusters, dissociation is via electron transfer and the yield is low for both metals. Calculations of reaction energetics, dissociation barriers, and the positions of excited electronic states complement the experimental work. Proton transfer in photolysis of Co(2+)(CH(3)CN)(2)(H(2)O) is calculated to occur via a (CH(3)CN)Co(2+)-OH(-)-H(+)(NCCH(3)) salt-bridge transition state, reducing kinetic energy release in the dissociation.     

Tuning of the internal energy and isomer distribution in small protonated water clusters H(+)(H2O)(4-8): an application of the inert gas messenger technique

Infrared spectroscopy of gas-phase hydrated clusters provides us much information on structures and dynamics of water networks. However, interpretation of spectra is often difficult because of high internal energy (vibrational temperature) of clusters and coexistence of many isomers. Here we report an approach to vary these factors by using the inert gas (so-called "messenger")-mediated cooling technique. Protonated water clusters with a messenger (M), H(+)(H(2)O)(4-8)·M (M = Ne, Ar, (H(2))(2)), are formed in a molecular beam and probed with infrared photodissociation spectroscopy in the OH stretch region. Observed spectra are compared with each other and with bare H(+)(H(2)O)(n). They show clear messenger dependence in their bandwidths and relative band intensities, reflecting different internal energy and isomer distribution, respectively. It is shown that the internal energy follows the order H(+)(H(2)O)(n) > H(+)(H(2)O)(n)·(H(2))(2) > H(+)(H(2)O)(n)·Ar > H(+)(H(2)O)(n)·Ne, while the isomer-selectivity, which changes the isomer distribution in the bare system, follows the order H(+)(H(2)O)(n)·Ar > H(+)(H(2)O)(n)·(H(2))(2) > H(+)(H(2)O)(n)·Ne ~ (H(+)(H(2)O)(n)). Although the origin of the isomer-selectivity is unclear, comparison among spectra measured with different messengers is very powerful in spectral analyses and makes it possible to easily assign spectral features of each isomer.