Water versus acetonitrile coordination to uranyl. Density functional study of cooperative polarization effects in solution

Optimizations at the BLYP and B3LYP levels are reported for mixed uranyl-water/acetonitrile complexes [UO(2)(H(2)O)(5-n)(MeCN)(n)](2+) (n = 0-5), in both the gas phase and a polarizable continuum modeling acetonitrile. Car-Parrinello molecular dynamics (CPMD) simulations have been performed for these complexes in the gas phase, and for selected species (n = 0, 1, 3, 5) in a periodic box of liquid acetonitrile. According to structural and energetic data, uranyl has a higher affinity for acetonitrile than for water in the gas phase, in keeping with the higher dipole moment and polarizability of acetonitrile. In acetonitrile solution, however, water is the better ligand because of specific solvation effects. Analysis of the dipole moment of the coordinated water molecule in [UO(2)(H(2)O)(MeCN)(4)](2+) reveals that the interaction with the second-shell solvent molecules (through fairly strong and persistent O-H···N hydrogen bonds) causes a significant increase of this dipole moment (by more than 1 D). This cooperative polarization of water reinforces the uranyl-water bond as well as the water solvation via strengthened (UO(2))OH(2)···NCMe hydrogen bonds. Such cooperativity is essentially absent in the acetonitrile ligands that make much weaker (UO(2))NCMe···NCMe hydrogen bonds. Beyond the uranyl case, this study points to the importance of cooperative polarization effects to enhance the M(n+) ion affinity for water in condensed phases involving M(n+)-OH(2)···A fragments, where A is a H-bond proton acceptor and M(n+) is a hard cation.     

Excess protons in water-acetone mixtures

Using molecular dynamics experiments, we analyze equilibrium and dynamical characteristics related to the solvation of excess protons in water-acetone mixtures. Our approach is based on the implementation of an extended valence-bond Hamiltonian, which incorporates translocation of the excess charge between neighboring water molecules. Different mixtures have been analyzed, starting from the pure water case down to solutions with a water molar fraction x(w) = 0.25. In all cases, we have verified that the structure of the first solvation shell of the H(3)O(+) moiety remains practically unchanged, compared to the one observed in pure water. This shell is composed by three water molecules acting as hydrogen bond acceptors, with no evidence of hydrogen bond donor-like connectivity. Moreover, the increment in the acetone concentration leads to a gradual stabilization of Eigen-like [H(3)O[middle dot](H(2)O)(3)](+) configurations, in detriment of Zundel-like [H[middle dot](H(2)O)(2)](+) ones. Rates of proton transfer and proton diffusion coefficients have been recorded at various water-acetone relative concentrations. In both cases, we have found a transition region, in the vicinity of x(w) ～ 0.8, where the concentration dependences of the two magnitudes change at a quantitative level. A crude estimate shows that, at this tagged concentration, the volumes "occupied" by the two solvents become comparable. The origins of this transition separating water-rich from acetone-rich realms is rationalized in terms of modifications operated in the nearby, second solvation shell, which in the latter solutions, normally includes at least, one acetone molecule. Our results would suggest that one possible mechanism controlling the proton transfer in acetone-rich solutions is the exchange of one of these tagged acetone molecules, by nearby water ones. This exchange would give rise to Zundel-like structures, exhibiting a symmetric, first solvation shell composed exclusively by water molecules, and would facilitate the transfer between neighboring water molecules along the resonant complex.     

Molecular structure of the solvated proton in isolated salts. Short,strong, low barrier (SSLB) H-bonds

Large, inert, weakly basic carborane anions of the icosahedral type CHB(11)R(5)X(6)(-) (R = H, Me; X = Cl, Br) allow ready isolation and structural characterization of discrete salts of the solvated proton, [H(solvent)(x)][CHB(11)R(5)X(6)], (solvent = common O-atom donor). These oxonium ion Br?nsted acids are convenient reagents for the tuned delivery of protons to organic solvents with a specified number of donor solvent molecules and with acidities leveled to those of the chosen donor solvent. They have greater thermal stability than the popular [H(OEt(2))(2)][BAr(F)] acids based on fluorinated tetraphenylborate counterions because carborane anions can sustain much higher levels of acidity. When organic O-atom donors such as diethyl ether, tetrahydrofuran, benzophenone, and nitrobenzene are involved, the coordination number of the proton (x) in [H(solvent)(x)()](+) is two. A mixed species involving the [H(H(2)O)(diethyl ether)](+) ion has also been isolated. These solid-state structures provide expectations for the predominant molecular structures of solvated protons in solution and take into account that water is an inevitable impurity in organic solvents. The O.O distances are all short, lying within the range from 2.35 to 2.48 A. They are consistent with strong, linear O.H.O hydrogen bonding. Density functional theory calculations indicate that all H(solvent)(2)(+) cations have low barriers to movement of the proton within an interval along the O.H.O trajectory, i.e., they are examples of so-called SSLB H-bonds (short, strong, low-barrier). Unusually broadened IR bands, diagnostic of SSLB H-bonds, are observed in these H(solvent)(2)(+) cations.     

Hydrated alizarin complexes: hydrogen bonding and proton transfer

We investigated the hydrogen bonding structures and proton transfer for the hydration complexes of alizarin (Az) produced in a supersonic jet using fluorescence excitation (FE), dispersed laser induced fluorescence (LIF), visible-visible hole burning (HB), and fluorescence detected infrared (FDIR) spectroscopy. The FDIR spectrum of bare Az with two O-H groups exhibits two vibrational bands at 3092 and 3579 cm(-1), which, respectively, correspond to the stretching vibration of O1-H1 that forms a strong intramolecular hydrogen bond with the C9=O9 carbonyl group and the stretching vibration of O2-H2 that is weakly hydrogen-bonded to O1-H1. For the 1:1 hydration complex Az(H(2)O)(1), we identified three conformers. In the most stable conformer, the water molecule forms hydrogen bonds with the O1-H1 and O2-H2 groups of Az as a proton donor and proton acceptor, respectively. In the other conformers, the water binds to the C10=O10 group in two nearly isoenergetic configurations. In contrast to the sharp vibronic peaks in the FE spectra of Az and Az(H(2)O)(1), only broad, structureless absorption was observed for Az(H(2)O)(n) (n≥ 2), indicating a facile decay process, possibly due to proton transfer in the electronic excited state. The FDIR spectrum with the wavelength of the probe laser fixed at the broad band exhibited a broad vibrational band near the O2-H2 stretching vibration frequency of the most stable conformer of Az(H(2)O)(1). With the help of theoretical calculations, we suggest that the broad vibrational band may represent the occurrence of proton transfer by tunnelling in the electronic ground state of Az(H(2)O)(n) (n≥ 2) upon excitation of the O2-H2 vibration.     

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.     

The vibrational predissociation spectra of the H5O2 +RGn(RG = Ar,Ne) clusters: correlation of the solvent perturbations in the free OH and shared proton transitions of the Zundel ion

Predissociation spectra of the H(5)O(2) (+)RG(n)(RG = Ar,Ne) cluster ions are reported in energy regions corresponding to both the OH stretching (3350-3850 cm(-1)) and shared proton (850-1950 cm(-1)) vibrations. The two free OH stretching bands displayed by the Ne complex are quite close to the band origins identified earlier in bare H(5)O(2) (+) [L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, 7319 (1989)], indicating that the symmetrical H(5)O(2) (+) "Zundel" ion remains largely intact in H(5)O(2) (+)Ne. The low-energy spectrum of the Ne complex is simpler than that observed previously for H(5)O(2) (+)Ar, and is dominated by two sharp transitions at 928 and 1047 cm(-1), with a weaker feature at 1763 cm(-1). The H(5)O(2) (+)Ar(n),n = 1-5 spectra generally exhibit complex band structures reflecting solvent-induced symmetry breaking of the Zundel core ion. The extent of solvent perturbation is evaluated with electronic structure calculations, which predict that the rare gas atoms should attach to the spectator OH groups of H(5)O(2) (+) rather than to the shared proton. In the asymmetric complexes, the shared proton resides closer to the more heavily solvated water molecule, leading to redshifts in the rare gas atom-solvated OH stretches and to blueshifts in the shared proton vibrations. The experimental spectra are compared with recent full-dimensional vibrational calculations (diffusion Monte Carlo and multimode/vibrational configuration interaction) on H(5)O(2) (+). These results are consistent with assignment of the strong low-energy bands in the H(5)O(2) (+)Ne spectrum to the vibration of the shared proton mostly along the O-O axis, with the 1763 cm(-1) band traced primarily to the out-of-phase, intramolecular bending vibrations of the two water molecules.     

Charge penetration and the origin of large O-H vibrational red-shifts in hydrated-electron clusters, (H2O)n-

The origin of O-H vibrational red-shifts observed experimentally in (H2O)n(-) clusters is analyzed using electronic structure calculations, including natural bond orbital analysis. The red-shifts are shown to arise from significant charge transfer and strong donor-acceptor stabilization between the unpaired electron and O-H sigma* orbitals on a nearby water molecule in a double hydrogen-bond-acceptor ("AA") configuration. The extent of e(-) --> sigma* charge transfer is comparable to the n --> sigma* charge transfer in the most strongly hydrogen-bonded X(-)(H2O) complexes (e.g., X = F, O, OH), even though the latter systems exhibit much larger vibrational red-shifts. In X(-)(H2O), the proton affinity of X(-) induces a low-energy XH...(-)OH diabatic state that becomes accessible in v = 1 of the shared-proton stretch, leading to substantial anharmonicity in this mode. In contrast, the H + (-)OH(H2O)(n-1) diabat of (H2O)n(-) is not energetically accessible; thus, the O-H stretching modes of the AA water are reasonably harmonic, and their red-shifts are less dramatic. Only a small amount of charge penetrates beyond the AA water molecule, even upon vibrational excitation of these AA modes. Implications for modeling of the aqueous electron are discussed.     

First steps towards dissolution of NaSO4- by water

NaSO(4)(-)(H(2)O)(n) (n = 0-4) clusters have been generated in the gas phase as model systems to simulate the first dissolution steps of sulfate salts in water; photoelectron spectroscopy and theoretical calculations indicate that the first three water molecules strongly interact with both Na(+) and SO(4)(2-), forming a three-water solvation ring to start to pry apart the Na(+)SO(4)(2-) contact ion pair.     

Stepwise synthesis of [Ru(trpy)(L)(X)](n+) (trpy = 2,2':6',2' '-terpyridine; L = 2,2'-dipyridylamine; X = Cl-, H2O, NO2-, NO+, O2-). Crystal structure, spectral, electron-transfer, and photophysical aspects

Ruthenium-terpyridine complexes incorporating a 2,2'-dipyridylamine ancillary ligand [Ru(II)(trpy)(L)(X)](ClO(4))(n) [trpy = 2,2':6',2' '-terpyridine; L = 2,2'-dipyridylamine; and X = Cl(-), n = 1 (1); X = H(2)O, n = 2 (2); X = NO(2)(-), n = 1 (3); X = NO(+), n = 3 (4)] were synthesized in a stepwise manner starting from Ru(III)(trpy)(Cl)(3). The single-crystal X-ray structures of all of the four members (1-4) were determined. The Ru(III)/Ru(II) couple of 1 and 3 appeared at 0.64 and 0.88 V versus the saturated calomel electrode in acetonitrile. The aqua complex 2 exhibited a metal-based couple at 0.48 V in water, and the potential increased linearly with the decrease in pH. The electron-proton content of the redox process over the pH range of 6.8-1.0 was calculated to be a 2e(-)/1H(+) process. However, the chemical oxidation of 2 by an aq Ce(IV) solution in 1 N H(2)SO(4) led to the direct formation of corresponding oxo species [Ru(IV)(trpy)(L)(O)](2+) via the concerted 2e(-)/2H(+) oxidation process. The two successive reductions of the coordinated nitrosyl function of 4 appeared at +0.34 and -0.34 V corresponding to Ru(II)-NO(+) --> Ru(II)-NO* and Ru(II)-NO* --> Ru(II)-NO(-), respectively. The one-electron-reduced Ru(II)-NO* species exhibited a free-radical electron paramagnetic resonance signal at g = 1.990 with nitrogen hyperfine structures at 77 K. The NO stretching frequency of 4 (1945 cm(-1)) was shifted to 1830 cm(-1) in the case of [Ru(II)(trpy)(L)(NO*)](2+). In aqueous solution, the nitrosyl complex 4 slowly transformed to the nitro derivative 3 with the pseudo-first-order rate constant of k(298)/s(-1) = 1.7 x 10(-4). The chloro complex 1 exhibited a dual luminescence at 650 and 715 nm with excited-state lifetimes of 6 and 1 micros, respectively.     

Kinetics of proton transfer from cationic carbon acids in water and aqueous DMSO. Effect of activating groups and solvent on intrinsic rate constants

[reaction: see text] Acidity constants and rates of reversible deprotonation of acetonyltriphenylphosphonium ion (1H+), phenacyltriphenylphosphonium ion (2H+), N-methyl-4-phenacylpyridinium ion (3H+), and N-methyl-4-(phenylsulfonylmethyl)pyridinium ion (4H+) by amines in water, 50% DMSO-50% water (v/v), and 90% DMSO-10% water (v/v) have been determined. From the respective Br?nsted plots, log k(o) values for the intrinsic rate constants of the various proton transfers were obtained. Solvent transfer activity coefficients of the carbon acids and their respective conjugate bases were also determined which helped in understanding how the pKa values and intrinsic rate constants depend on the solvent. Some of the main conclusions are as follows: (1) The pK(a) values of 1H+, 2H+, and 3H+ are significantly higher than that of 4H+ because of a stronger resonance stabilization of the corresponding conjugate bases 1, 2 and 3, respectively. (2) The electronic effects of the PPh3+ and the N-methyl-4-pyridylium group are similar but the mix between inductive and resonance effect is different. (3) All four acids become more acidic upon addition of DMSO to the solvent. In all cases, the main factor is the stronger solvation of H3O+ in DMSO; for 1H+, 2H+, and 3H+ but not 4H+ this factor is significantly attenuated by stronger solvation of the carbon acid in DMSO. (4) The intrinsic rate constants for proton transfer are relatively high for all four carbon acids and show little solvent dependence; this contrasts with nitroalkanes which have much lower intrinsic rate constants and show a strong solvent dependence. These results can be understood by a detailed analysis of the interplay between inductive, resonance, and solvation effects.     

Survey on the phase transitions and their effect on the ion-exchange and on the proton-conduction properties of a flexible and robust Zr phosphonate coordination polymer

The flexible zirconium tetraphosphonate coordination polymer with formula Zr(O(3)PCH(2))(2)N-C(6)H(10)-N(O(3)CH(2)P)(2)X(2-x)H(2+x)·nH(2)O (X = H, Li, Na, K, 0 < x < 1, 4 < n < 7.5) (1) possesses an open framework structure with 1D cavities decorated with polar and acids P═O and P-OH groups. 1 has been fully protonated by adding HCl and then subjected to several acid-base ion-exchange reactions with alkaline metals hydroxides. 1 is a very robust coordination polymer because it can be regenerated in H- form using strong acid solutions and ri-exchanged several times without hydrolysis and loss of crystallinity. The flexibility of 1 has been also studied by means of TDXD (temperature dependent X-ray diffraction) evidencing remarkable phase transformations that lead to a different disposition of the water molecules. These transformations also influence the accessibility of the cations on the P-OH groups placed inside the channels and thus the ion-exchange properties. The dependence of the proton conductivity properties on these phase transitions has been also investigated and discussed.     

用密度泛函方法研究质子化苯基丙酮-水团簇

The structure and growth trend of the protonated acetophenone-water clusters have been investigated using the DFT-B3LYP method combined with the standard 6-31+G(d,p) basis set. In order to obtain more accurate single-point energy the B3LYP/6-311++G(3df,2p) method was adapted. The results show that the formation of H+C8H8O-H2O is a barrierless reaction process and the equilibrium distance between the proton and the O atom in C8H8O molecule is 1.015 A. For H+C8H8O-(H2O)n(n=1,2,3) clusters, the proton lies between the acetophenone molecule C8H8O and the water molecule H2O. The distance between the proton and the O atom of the C8H8O molecule increased from n=1 to n=3; C8H8O-H+-H2O can be regarded as an solvation shell. For H+C8H8O (H2O)n (n=4,5,6,7,8) clusters, the proton lies between the two H2O molecules forming a H5O2+ structure, C8H8O-H5O2+ is an important structure, which the other H2O molecules will attack from different sides.     

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.     

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.     

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.     

Ultrafast proton transfer to solvent: molecularity and intermediates from solvation- and diffusion-controlled regimes

Photoinduced proton transfer (PT) from cations 6-hydroxyquinolinium (6HQc) and 6-hydroxy-1-methylquinolinium (6MQc) to water and alcohols, and solvation of the zwitterionic conjugate base 1-methylquinolinium-6-olate (6MQz) were studied with stationary and transient absorption spectroscopy and by quantum chemical calculations. Transient emission spectra from 6MQz in acetonitrile and protic solvents shift dynamically to the red without changing their shape and intensity. The shift matches the solvation correlation function C(t) either measured with known solvatochromic probes coumarin 343 and coumarin 153 or derived from infrared/dielectric-loss data on neat solvents. This indicates that 6MQz monitors the solvation dynamics and that no intramolecular electron transfer occurs on a subpicosecond or longer time scale. The PT dynamics S(t) from 6HQc and 6MQc closely follows C(t), being initially 2-3 times slower. This allows for the conclusion that PT is controlled by solvation, with a barrier of 2 kJ/mol. In water, a pre-condition of this ultrafast reaction seems to be hydrogen-bonding between the negatively charged oxygen and two water molecules, resulting in a complex 6HQc:H2O:H2O. The complex is stable due to a high (47 kJ/mol) bonding energy between 6HQc and a water molecule. In acetonitrile, the reaction equilibrium is strongly shifted to the cation. There an intermediate PT state was detected, which may be ascribed to the cationic form 6HQc:H2O due to residual water impurities. In water-acetonitrile mixtures, the ultrafast solvent-controlled PT is followed by a diffusion-controlled reaction; the measured rate kD approximately 1010 s-1 M-1 is characteristic for simple bimolecular diffusion. The dependence of the short-time PT signal on water concentration can be fitted with a Poisson distribution of water molecules around the cation. Altogether, the short-time and long-time behaviors provide strong evidence that diffusion of only one water molecule is sufficient to detach the proton. Subsequent solvent stabilization of the products completes the PT reaction.     

Compatibility between methanol and water in the three-dimensional cage formation of large-sized protonated methanol-water mixed clusters

Infrared spectra of large-sized protonated methanol-water mixed clusters, H(+)(MeOH)(m)(H(2)O)(n) (m=1-4, n=4-22), were measured in the OH stretch region. The free OH stretch bands of the water moiety converged to a single peak due to the three-coordinated sites at the sizes of m+n=21, which is the magic number of the protonated water cluster. This is a spectroscopic signature for the formation of the three-dimensional cage structure in the mixed cluster, and it demonstrates the compatibility of a small number of methanol molecules with water in the hydrogen-bonded cage formation. Density functional theory calculations were carried out to examine the relative stability and structures of selected isomers of the mixed clusters. The calculation results supported the microscopic compatibility of methanol and water in the hydrogen-bonded cage development. The authors also found that in the magic number clusters, the surface protonated sites are energetically favored over their internal counterparts and the excess proton prefers to take the form of H(3)O(+) despite the fact that the proton affinity of methanol is greater than that of water.     

A computational study of interactions between acetic acid and water molecules

Density functional theory calculations were performed for the title reactions to elucidate the difference between the strong cyclic hydrogen bond of (Me-COOH)(2) and the electrolytic dissociation, MeCOOH <==> Me-COO(-) + H(+), as a weak acid. The association of water clusters with acetic acid dimers strengthens the cyclic hydrogen bond. A nucleophilic attack of the carboxylic carbon by a water cluster leads to a first zwitterionic intermediate, MeCOO(-) + H(3)O(+) + (HO)(3)C-Me. The intermediate is unstable and is isomerized to a neutral interacting system, MeCOOH...(HO)(3)C-Me + H(2)O. The ethanetriol, (HO)(3)-CMe is transformed to an acetic acid monomer. The monomer may be dissociated to give a second zwitterionic intermediate with reasonable proton-relay patterns and energy changes. In proton relay reaction channels, H in MeCOOH is not an acidic proton but is always a hydroxy proton.     

A bond-order analysis of the mechanism for hydrated proton mobility in liquid water

Bond-order analysis is introduced to facilitate the study of cooperative many-molecule effects on proton mobility in liquid water, as simulated using the multistate empirical valence-bond methodology. We calculate the temperature dependence for proton mobility and the total effective bond orders in the first two solvation shells surrounding the H(5)O(2) (+) proton-transferring complex. We find that proton-hopping between adjacent water molecules proceeds via this intermediate, but couples to hydrogen-bond dynamics in larger water clusters than previously anticipated. A two-color classification of these hydrogen bonds leads to an extended mechanism for proton mobility.