Proton transfer in ammonia cluster cations: molecular dynamics in a self consistent field

A theoretical study of the energetics and intramolecular dynamics for ammonia cluster cations (NH₃) ₂ ⁺ and (NH₃) ₃ ⁺ is presented. The proton transfer mechanism after a vertical ionisation is followed in detail. Structural and energetic calculations are performed near the Hartree-Fock self consistent field (HF-SCF) limit; all open shell calculations are spin restricted (RHF). The calculations reconcile experimental results from thermochemical and photoionisation measurements: discrepancies in the energetics are shown to be due to the probing of different regions of the potential energy surface (PES). For the dynamics calculations, it is impractical to determine a large region of the multidimensional PES and then integrate Newton's equations. The calculations therefore incorporate a quantum mechanical determination of the electronic energy of the system after each time increment while nuclear degrees of freedom are handled classically. In this way, the classical reaction path across the Born-Oppenheimer surface is obtained.     

Theoretical investigation into the nature of the second vertical ionic state of the water dimer

Ab initio molecular orbital calculations have been carried out to resolve questions concerning the location of the second ionization in the water dimer. The results indicate that it is from the proton acceptor molecule of the water dimer.     

Ionization of carbon acids in liquid ammonia

The acidities of various carbon acids in liquid ammonia (LNH(3)) at room temperature were determined by NMR and rates of D-exchange. There is a reasonable linear correlation of the pK(a)s in LNH(3) with those in water and DMSO of slope 0.7 and 0.8, respectively. Carbon acids with an aqueous pK(a) of less than 12 are fully ionized in liquid ammonia. Nucleophilic substitution of benzyl chloride by carbanions in liquid ammonia generates a Br?nsted β(nuc) = 0.38.     

Ionization dynamics of a water dimer: specific reaction selectivity

Ionization dynamics of a water dimer have been investigated by means of a direct ab initio molecular dynamics (MD) method. Two electronic state potential energy surfaces of (H(2)O)(2)(+) (ground and first excited states, (2)A' and (2)A') were examined as cationic states of (H(2)O)(2)(+). Three intermediate complexes were found as product channels. One is a proton transfer channel where a proton of H(2)O(+) is transferred into the H(2)O and then a complex composed of H(3)O(+)(OH) was formed. The second is a face-to-face complex channel denoted by (H(2)O-OH(2))(+) where the oxygen-oxygen atoms directly bind each other. Both water molecules are equivalent to each other. The third one is a dynamical complex where H(2)O(+) and H(2)O interact weakly and vibrate largely with a large intermolecular amplitude motion. The dynamics calculations showed that in the ionization to the (2)A' state, a proton transfer complex H(3)O(+)(OH) is only formed as a long-lived complex. On the other hand, in the ionization to the (2)A' state, two complexes, the face-to-face and dynamical complexes, were found as product channels. The proton of H(2)O(+) was transferred to H(2)O within 25-50 fs at the (2)A' state, meaning that the proton transfer on the ground state is a very fast process. On the other hand, the decay process on the first excited state is a slow process due to the molecular rotation. The mechanism of the ionization dynamics of (H(2)O)(2) was discussed on the basis of theoretical results.     

In search of excited-state proton transfer in the lumichrome dimer in the solid state: theoretical and experimental approach

Quantum chemical density functional theory (DFT) calculations and spectral data were employed to investigate the possibility of the excited-state double proton transfer (ESDPT) in lumichrome crystals. The calculations in a lumichrome dimer predict a transfer of a proton in the first excited state, leading to a cation-anion pair. The presently reported X-ray structure of 1,3-dimethyllumichrome and its complex solid-state luminescence indicate that also in this molecule intermolecular hydrogen bonds might be involved in the photophysics. The long-wavelength emission in lumichrome crystals peaked at 530 nm is attributed to excited-state proton transfer, whereas a wider emission band in methylated lumichrome derivatives peaked at 560 nm is attributed to ions formed upon photoexcitation of the crystals.     

Proton and electron transfer in the excited state quenching of phenosafranine by aliphatic amines

The quenching of the excited singlet and triplet states of phenosafranine by aliphatic amines was investigated in acetonitrile and methanol. The rate constants for the quenching of the excited singlet state depend on the one-electron redox potential of the amine suggesting a charge transfer process. However, for the triplet state, quenching dependence on the redox potential either is opposite to the expectation or there is not dependence at all. Moreover, in MeOH the first-order rate constant for the decay of the triplet state, k(obs) presents a downward curvature as a function of the amine concentration. This behavior was interpreted in terms of the reversible formation of an intermediate excited complex, and from a kinetic analysis the equilibrium constant K(exc) could be extracted. The log K(exc) shows a linear relationship with the pKb of the amine. On the other hand, for the triplet state quenching in acetonitrile k(obs) varies linearly with the amine concentration. Nevertheless, the quenching rate constants correlate satisfactorily with pKb and not with the redox potential. The results were interpreted in terms of a proton transfer quenching, reversible in the case of MeOH and irreversible in MeCN. This was further confirmed by the transient absorption spectra obtained by laser flash photolysis. The transient absorption immediately after the triplet state quenching could be assigned to the unprotonated form of the dye. At later times the spectrum matches the semireduced form of the dye. The overall process corresponds to a one-electron reduction of the dye mediated by the deprotonated triplet state.     

Excited state two photon absorption of a charge transfer radical dimer in the near infrared

Nonlinear transmission measurements of a solution of radical dimers of tetramethyl-tetrathiafulvalene, (TMTTF+)2, recorded with 9 ns laser pulses at 1064 nm are reported and interpreted on the basis of a multiphoton absorption process. One finds that the process can be interpreted with a sequence of three photon absorption, the first being a one photon absorption related to the intermolecular charge transfer process characteristic of the dimers and the second a two photon absorption from the excited state created with the first process. A model calculation allows one to obtain the value of the two photon absorption cross section which is found to be several orders of magnitude larger than those usually found for two photon absorbing systems excited from the ground state. These results show the importance of an excited-state population for obtaining large nonlinear optical responses.     

Excited state proton transfer in 3-methyl-7-azaindole dimer. Symmetry control

The concerted double proton transfer undergone by the C(2)(h) dimer of 7-azaindole upon electronic excitation has also been reported to occur in 3-methyl-7-azaindole monocrystals and in dimers of this compound under free-jet conditions. However, the results obtained in this work for the 3-methyl-7-azaindole dimer formed in a 10(-4) M solution of the compound in 2-methylbutane suggest that the dimer produces no fluorescent signal consistent with a double proton transfer in the liquid phase or in a matrix. In this paper, the spectroscopic behavior of the doubly hydrogen bonded dimer of 3-methyl-7-azaindole is shown to provide a prominent example of molecular symmetry control over the spectroscopy of a substance. This interpretation opens up a new, interesting research avenue for exploring the ability of molecular symmetry to switch between proton-transfer mechanisms. It should be noted that symmetry changes in the 3-methyl-7-azaindole dimer are caused by an out-of-phase internal rotation of the two methyl groups.     

Proton collisions with the water dimer at keV energies

Proton collisions with the water dimer are studied using a nonadiabatic, direct, time‐dependent approach called electron nuclear dynamics (END). Fragmentation of the water dimer in collisions with protons at energies of 5.0, 1.0 keV and 200 eV is the primary aim of this initial study of water clusters using END. We report on the initial fragmentation dynamic, that is, for times less than 200 fs. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Proton dosimetry in the radiation chemistry of ionic salts

Layers of thickness less than the range are used in conjunction with the known flux density. The energy loss may be calculated or may be measured from the yield per particle for specimens of several thicknesses.     

Ionization potentials of large sodium doped ammonia clusters

In a continuous neat supersonic expansion ammonia clusters are generated and doped with sodium atoms in a pickup cell. Thus clusters of the form Na(NH(3))(n) are produced that are photoionized by a tunable dye laser system. The ions are mass analyzed in a reflectron time-of-flight mass spectrometer, and the wavelength dependent ion signals serve for the determination of the ionization potentials (IP) of the different clusters in the size range 10< or =n< or =1500. Aside from a plateau for 10< or =n< or =17 and smaller steps at n=24, 35, and 59 on the average a continuous decrease of the IP with cluster size is observed. The IPs in this size range are linear with (n+1)(-13) and extrapolate to IP(n=infinity)=1.66+/-0.01 eV. The slope is consistent with a dielectric continuum model of the solvated electron and the dielectric constant of the solid. The extrapolated IPs are compared with results obtained for negative ammonia cluster ions and metallic solutions in liquid ammonia. Differences are explained by the presence of counterions and their various distances from the solvated electron.     

Proton antenna effect of the gamma-cyclodextrin outer surface, measured by excited state proton transfer

The reversible proton dissociation and geminate recombination of the common photoacid, 8-hydroxypyrene-1,3,6-trisulfonate (pyranine), either in dilute aqueous solution or when forming a complex with gamma-cyclodextrin (gamma-CD), has been studied by time-resolved fluorescence spectroscopy and supplemented by molecular modeling and dynamics simulations. We find that the dissociation rate of the proton from the excited molecule was decreased to about approximately 50% of its value in water, while the rate of recombination was doubled. These observations were evaluated by molecular modeling of the reactants at atomic resolution. The combination of the two methodologies indicates that the pyranine in the complex can assume more than one level of interaction with the solvent. The polysugar torus surrounding the pyranine perturbs the hydrogen bond in the dye's immediate vicinity and deforms the electrostatic potential inside the Coulomb cage, causing major deviations from a simple spheric symmetry. These observations can account for the special kinetic features measured for the complex. We suggest that this system can be used as a basic model for evaluating the mechanism of proton transfer in non-homogeneous systems, such as the surface of proteins or biomembranes.     

Structure of the ammonia dimer studied by density functional theory

Self-consistent Kohn–Sham density functional calculations have been carried out to study the structure of the ammonia dimer. The local-density approximation yields unusually large binding energy and short internitrogen distance compared with the experimental and more accurate theoretical data. The results from the Becke–Perdew gradient-corrected functionals are generally in good agreement with those at the SCF MP 2 level when the geometry is fully optimized with various large basis sets. With our best estimation, the staggered quasi-linear structure (C_s) is 0.6 kcal/mol lower in energy than the symmetric cyclic one (C_2h). The hydrogen-bonded N—H bond in the staggered quasi-linear structure is found to be 0.008 Å longer than the N—H bond in ammonia. In our calculations, we could not find the minima on the energy surface corresponding to the two asymmetric cyclic structures suggested by microwave spectra and coupled pair functional calculations. © 1994 John Wiley & Sons, Inc.     

Dissociation energy and vibrational predissociation dynamics of the ammonia dimer

Experiments using infrared excitation of either the intramolecular symmetric N-H stretch (ν(NH,S)) or the intramolecular antisymmetric N-H stretch (ν(NH,A)) of the ammonia dimer ((NH(3))(2)) in combination with velocity-map ion imaging provide new information on the dissociation energy of the dimer and on the energy disposal in its dissociation. Ion imaging using resonance enhanced multiphoton ionization to probe individual rovibrational states of one of the ammonia monomer fragments provides recoil speed distributions. Analyzing these distributions for different product states gives a dissociation energy of D(0) = 660 ± 20 cm(-1) for the dimer. Fitting the distributions shows that rotations are excited up to their energetic limit and determines the correlation of the fragment vibrations. The fragments NH(3)(ν(2) = 3(+)) and NH(3)(ν(2) = 2(+)) have a vibrational ground-state partner NH(3)(ν = 0), but NH(3)(ν(2) = 1(+)) appears in partnership with another fragment in ν(2) = 1. This propensity is consistent with the idea of minimizing the momentum gap between the initial and final states by depositing a substantial fraction of the available energy into internal excitation.     

Proton transfer events in GFP

Proton transfer is one of the most important elementary processes in biology. Green fluorescent protein (GFP) serves as an important model system to elucidate the mechanistic details of this reaction, because in GFP proton transfer can be induced by light absorption. Illumination initiates proton transfer through a 'proton-wire', formed by the chromophore (the proton donor), water molecule W22, Ser205 and Glu222 (the acceptor), on a picosecond time scale. To obtain a more refined view of this process, we have used a combined approach of time resolved mid-infrared spectroscopy and visible pump-dump-probe spectroscopy to resolve with atomic resolution how and how fast protons move through this wire. Our results indicate that absorption of light by GFP induces in 3 ps (10 ps in D(2)O) a shift of the equilibrium positions of all protons in the H-bonded network, leading to a partial protonation of Glu222 and to a so-called low barrier hydrogen bond (LBHB) for the chromophore's proton, giving rise to dual emission at 475 and 508 nm. This state is followed by a repositioning of the protons on the wire in 10 ps (80 ps in D(2)O), ultimately forming the fully deprotonated chromophore and protonated Glu222.     

Excited state hydrogen transfer in fluorophenol.ammonia clusters studied by two-color REMPI spectroscopy

Two-color (1 + 1') REMPI mass spectra of o-, m- and p-fluorophenol.ammonia (1 ration) clusters were measured with a long delay time between excitation and ionization lasers. The appearance of NH(4)(NH(3))(n-1)(+) with 100 ns delay after exciting the S(1) state is a strong indication of generation of long-lived species via S(1). In analogy with the phenol.ammonia clusters, we conclude that an excited state hydrogen transfer reaction occurs in o-, m- and p-fluorophenol.ammonia clusters. The S(1)-S(0) transition of o-, m- and p-fluorophenol.ammonia (1 : 1) clusters were measured by the (1 + 1') REMPI spectra, while larger (1 ration) cluster (n = 2-4) were observed by monitoring the long-lived NH(4)(NH(3))(n-1) clusters action spectra. The vibronic structures of m- and p-fluorophenol.ammonia clusters are assigned based on vibrational calculations in S(0). The o-fluorophenol.ammonia (1 : 1) cluster shows an anharmonic progression that is analyzed by a one-dimensional internal rotational motion of the ammonia molecule. The interaction between the ammonia molecule and the fluorine atom, and its change upon electronic excitation are suggested. The broad action spectra observed for the o-fluorophenol.ammonia (1 : n) cluster (n>== 2) suggest the excited state hydrogen transfer is faster than in m- and p-fluorophenol.ammonia clusters. The different reaction rates between o-, m- and p-fluorophenol.ammonia clusters are found from comparison between the REMPI and action spectra.     

Ionization potentials and structure of simple ionic compounds

Conclusions From the analysis we have presented here, we can state that the mutual influence of ions in simple diatomic and triatomic molecules essentially comes down to polarization effects similar to the Stark shift of levels. An expression for the Stark shift, obtained within the framework of perturbation theory, is valid only for weak fields. However, experiments indicate that phenomena similar to the Stark shift of levels occur for rather strong intramolecular fields as well.Translated from Zhurnal Strukturnoi Khimii, Vol. 26, No. 5, pp. 27–34, September–October, 1985.     

Solubilities of ammonia in basic imidazolium ionic liquids

Solubilities of ammonia in four conventional imidazolium ionic liquids: [C_nmim][BF₄] (n = 2, 4, 6, 8) have been measured. Isothermally fixed temperatures are 293.15, 303.15, 313.15, 323.15 and 333.15 K; the pressure is from 0 to 1.0 MPa. High solubilities of ammonia are found, and it is also found that the solubilities of ammonia increase when the length of cations’ alkyl increases (the ILs have the same anion), that is: [C₈mim]⁺ > [C₆mim]⁺ > [C₄mim]⁺ > [C₂mim]⁺. The solubility data have been correlated by the Krichevisky–Kasarnovsky equation, and then Henry's constants and partial molar volumes of NH₃ at infinite dilution are obtained. The thermodynamic properties such as solution enthalpy (Δ_solH), solution Gibbs free energy (Δ_solG), solution entropy (Δ_solS), and solution heat capacity (Δ_solC_p) of these systems are obtained.