Vacuum ultraviolet (VUV) photoionization of small water clusters

Tunable vacuum ultraviolet (VUV) photoionization studies of water clusters are performed using 10-14 eV synchrotron radiation and analyzed by reflectron time-of-flight (TOF) mass spectrometry. Photoionization efficiency (PIE) curves for protonated water clusters (H2O)(n)H+ are measured with 50 meV energy resolution. The appearance energies of a series of protonated water clusters are determined from the photoionization threshold for clusters composed of up to 79 molecules. These appearance energies represent an upper limit of the adiabatic ionization energy of the corresponding parent neutral water cluster in the supersonic molecular beam. The experimental results show a sharp drop in the appearance energy for the small neutral water clusters (from 12.62 +/- 0.05 to 10.94 +/- 0.06 eV, for H2O and (H2O)4, respectively), followed by a gradual decrease for clusters up to (H2O)23 converging to a value of 10.6 eV (+/-0.2 eV). The dissociation energy to remove a water molecule from the corresponding neutral water cluster is derived through thermodynamic cycles utilizing the dissociation energies of protonated water clusters reported previously in the literature. The experimental results show a gradual decrease of the dissociation energy for removal of one water molecule for small neutral water clusters (3 相似文献   

Ab initio theoretical calculations of the electronic excitation energies of small water clusters

A direct ab initio molecular dynamics method has been applied to a water monomer and water clusters (H(2)O)(n) (n = 1-3) to elucidate the effects of zero-point energy (ZPE) vibration on the absorption spectra of water clusters. Static ab initio calculations without ZPE showed that the first electronic transitions of (H(2)O)(n), (1)B(1)←(1)A(1), are blue-shifted as a function of cluster size (n): 7.38 eV (n = 1), 7.58 eV (n = 2) and 8.01 eV (n = 3). The inclusion of the ZPE vibration strongly affects the excitation energies of a water dimer, and a long red-tail appears in the range of 6.42-6.90 eV due to the structural flexibility of a water dimer. The ultraviolet photodissociation of water clusters and water ice surfaces is relevant to these results.     

Benchmark structures and binding energies of small water clusters with anharmonicity corrections

For (H(2)O)(n) where n = 1-10, we used a scheme combining molecular dynamics sampling with high level ab initio calculations to locate the global and many low lying local minima for each cluster. For each isomer, we extrapolated the RI-MP2 energies to their complete basis set limit, included a CCSD(T) correction using a smaller basis set and added finite temperature corrections within the rigid-rotor-harmonic-oscillator (RRHO) model using scaled and unscaled harmonic vibrational frequencies. The vibrational scaling factors were determined specifically for water clusters by comparing harmonic frequencies with VPT2 fundamental frequencies. We find the CCSD(T) correction to the RI-MP2 binding energy to be small (<1%) but still important in determining accurate conformational energies. Anharmonic corrections are found to be non-negligble; they do not alter the energetic ordering of isomers, but they do lower the free energies of formation of the water clusters by as much as 4 kcal/mol at 298.15 K.     

Calculation of photoionization cross sections of small metal clusters

In this paper we present photoionization cross sections of the Cu and Al dimers and tetramers. The local spin density method is applied to calculate the electronic structure and the ground state potential. The cross sections are calculated using the continuum multiple scattering method, and a basic analysis of the photoionization process is given within the independent particle picture.     

Hydrogen dissociation on small aluminum clusters

Transition states and reaction paths for a hydrogen molecule dissociating on small aluminum clusters have been calculated using density functional theory. The two lowest spin states have been taken into account for all the Al(n) clusters considered, with n=2-6. The aluminum dimer, which shows a (3)Π(u) electronic ground state, has also been studied at the coupled cluster and configuration interaction level for comparison and to check the accuracy of single determinant calculations in this special case, where two degenerate configurations should be taken into account. The calculated reaction barriers give an explanation of the experimentally observed reactivity of hydrogen on Al clusters of different size [Cox et al., J. Chem. Phys. 84, 4651 (1986)] and reproduce the high observed reactivity of the Al(6) cluster. The electronic structure of the Al(n)-H(2) systems was also systematically investigated in order to determine the role played by interactions of specific molecular orbitals for different nuclear arrangements. Singlet Al(n) clusters (with n even) exhibit the lowest barriers to H(2) dissociation because their highest doubly occupied molecular orbitals allow for a more favorable interaction with the antibonding σ(u) molecular orbital of H(2).     

Zwitterion-dianion complexes and anion-anion clusters with negative dissociation energies

Clusters of oxalate and malonate dianions with glycine in its zwitterionic form were found by ab initio and density functional theory calculations. Proton transfer is impeded by an electrostatic barrier, but the resulting anion-anion pairs form complexes despite having negative dissociation energies. A variety of X-/Y- species with exothermic dissociation energies ranging from 10 to 141 kcal mol-1, but large enough barriers to be experimentally produced, are reported. These dianions may represent an unrecognized control element in nature and provide a unique opportunity to probe electrostatic effects and a wealth of novel clusters.     

Accurate determination of the dissociation energies of alkali halides

Photofragmentation spectroscopy offers a possibility to determine the dissociation energy of diatomic molecules with an accuracy of about 0.01 eV, which is half an order of magnitude better than any other applicable method. In this letter the method will be compared with other known techniques and results will be given for NaBr, KBr, NaI ad KI.     

On the contribution of vibrational anharmonicity to the binding energies of water clusters

The second-order vibrational perturbation theory method has been used together with the B3LYP and MP2 electronic structure methods to investigate the effects of anharmonicity on the vibrational zero-point energy (ZPE) contributions to the binding energies of (H2O)n, n = 2-6, clusters. For the low-lying isomers of (H2O)6, the anharmonicity correction to the binding energy is calculated to range from -248 to -355 cm(-1). It is also demonstrated that although high-order electron correlation effects are important for the individual vibrational frequencies, they are relatively unimportant for the net ZPE contributions to the binding energies of water clusters.     

Determining the dissociation threshold of ammonia trimers from action spectroscopy of small clusters

Infrared-action spectroscopy of small ammonia clusters obtained by detecting ammonia fragments from vibrational predissociation provides an estimate of the dissociation energy of the trimer. The product detection uses resonance enhanced multiphoton ionization (REMPI) of individual rovibrational states of ammonia identified by simulations using a consistent set of ground-electronic-state spectroscopic constants in the PGOPHER program. Comparison of the infrared-action spectra to a less congested spectrum measured in He droplets [M. N. Slipchenko, B. G. Sartakov, A. F. Vilesov, and S. S. Xantheas, J. Phys. Chem. A 111, 7460 (2007)] identifies the contributions from the dimer and the trimer. The relative intensities of the dimer and trimer features in the infrared-action spectra depend on the amount of energy available for breaking the hydrogen bonds in the cluster, a quantity that depends on the energy content of the detected fragment. Infrared-action spectra for ammonia fragments with large amounts of internal energy have almost no trimer component because there is not enough energy available to break two bonds in the cyclic trimer. By contrast, infrared-action spectra for fragments with low amounts of internal energy have a substantial trimer component. Analyzing the trimer contribution quantitatively shows that fragmentation of the trimer into a monomer and dimer requires an energy of 1700 to 1800 cm(-1), a range that is consistent with several theoretical estimates.     

Ionization energies and hydrogen-bond strength of the water clusters

The first vertical ionization energies of the water clusters (H₂O)_n for n = 1–8 have been evaluated from ab initio SCF MO calculations. The values obtained for n = 5 and 8 are in reasonable agreement with the experimental values of ice. The stability of the clusters is examined in terms of the hydrogen-bond strength per bond (B_h).     

On the photoionization and fragmentation of ammonia clusters using TPEPICO

Measurements of the appearance potentials of ammonia cluster ions (NH₃) _n ⁺ and (NH₃) _n ?2NH ₄ ⁺ using the threshold photoelectron photoion coincidence (TPEPICO) time-of-flight method are performed. The results agree well with other available data, however, solvation energies derived from these and from thermochemical data show significant discrepancies. The energetics for ammonia clusters are elucidated from various points of view.     

Vacuum-ultraviolet (VUV) photoionization of small methanol and methanol-water clusters

In this work, we report on the vacuum-ultraviolet (VUV) photoionization of small methanol and methanol-water clusters. Clusters of methanol with water are generated via co-expansion of the gas phase constituents in a continuous supersonic jet expansion of methanol and water seeded in Ar. The resulting clusters are investigated by single photon ionization with tunable vacuum-ultraviolet synchrotron radiation and mass analyzed using reflectron mass spectrometry. Protonated methanol clusters of the form (CH3OH)nH(+) (n = 1-12) dominate the mass spectrum below the ionization energy of the methanol monomer. With an increase in water concentration, small amounts of mixed clusters of the form (CH3OH)n(H2O)H(+) (n = 2-11) are detected. The only unprotonated species observed in this work are the methanol monomer and dimer. Appearance energies are obtained from the photoionization efficiency (PIE) curves for CH3OH(+), (CH3OH)2(+), (CH3OH)nH(+) (n = 1-9), and (CH3OH)n(H2O)H(+) (n = 2-9) as a function of photon energy. With an increase in the water content in the molecular beam, there is an enhancement of photoionization intensity for the methanol dimer and protonated methanol monomer at threshold. These results are compared and contrasted to previous experimental observations.     

Combined bond-polarization basis sets for accurate determination of dissociation energies

Summary The effect of bond functions on the basis set superposition error (BSSE) is investigated at both SCF (self consistent field) and correlated levels for a number of basis sets using the pairwise additive function counterpoise (PAFC), the site-site function counterpoise (SSFC), and the newly proposed successive reaction counterpoise method (SRCP). BSSEs using bond functions are shown to be roughly twice those without bond functions, whereas the latter may still be quite sizeable. The addition of f functions dramatically decreases the bond function BSSE. The results obtained support the empirical decision in our earlier papers to neglect BSSE altogether.     

On the advantages of hydrocarbon radical stabilization energies based on R-H bond dissociation energies

Hydrocarbon radical stabilization energy (RSE) estimates based on the differences in R-H vs CH(3)-H bond dissociation energies have inherent advantages over RSEs based on R-CH(3) vs CH(3)-CH(3), as well as R-R vs CH(3)-CH(3) comparisons, since the R-CH(3) and R-R reference systems are prone to unbalanced contaminating intramolecular interactions involving the R groups. When the effects of steric crowding, branching, protobranching, conjugation, and hyperconjugation are taken into account, R-CH(3) and R-R based RSE values are nearly identical to R-H RSEs. Corrections for electronegativity differences between H and R are not needed to achieve agreement.     

Revisiting small clusters of water molecules

The geometries and relative energies of small clusters of water molecules, (H₂O)_n with 4 ⩽ n ⩽ 8, are reported. For each value of n we have considered the conformations corresponding to the lowest-energy minimum and those in nearby relative minima. Thus we report on six tetramers, four pentamers, six hexanlers, four heptamers, and eigth octamers. The geometrical conformations have been obtained using the Metropolis Monte Carlo method as a minimization technique, where the interaction energy is computed with the MCY potential plus three- and four-body corrections previously discussed. All the reported structures for a given cluster size are found to be close in energy. For the lowest conformation the geometry was optimized with ab initio SCF computations using energy gradients. Our results are compared with previous theoretical studies. We discuss the convergence of the interaction potential for liquid water when expressed in terms of a many-body series expansion.     

Thermodynamic characteristics of small water clusters

Physicotechnical Institute of Low Temperatures, Academy of Sciences of the Ukrainian SSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 29, No. 5, pp. 89–94, September–October, 1988.     

On the determination of monomer reactivities in copolymerization

Existing methods to determine reactivity ratios in copolymerizations are critically discussed; particular emphasis is placed on the difference between exact and approximate procedures. Among the latter, the “intersection” method (Mayo and Lewis) and the “linearization” method (Fineman and Ross) are still preferred. They are compared from the point of view of projective geometry, and a “duality” is shown to exist between them.A new procedure, based on the so-called “method of grouping”, is then suggested: it allows ready evaluation of reactivity ratios with a minimum of computational difficulties. The results obtained by this procedure are, as such, good estimates for systems reported in the literature without the information necessary for a careful statistical treatment, and may best be used as the starting data for subsequent and more sophisticated elaborations.     

On ionisation induced unimolecular dissociation of sodium clusters

Fragmentation of sodium cluster ions (Na _x ⁺ ,x<42) was studied via photoionisation of neutral precursors. Expansions of metal vapor out of cylindrical and conical nozzles yielded supersonic beams with differing cluster compositions. Measurements of photoionisation efficiency curves in the 3–6 eV range for both types of expansion allow quantitative separation of direct ionisation and unimolecular dissociation contributions to specific ion signals. Data for Na ₈ ⁺ and Na ₇ ⁺ are analysed to yield lower limits on bond energies. Results obtained for larger clusters are also discussed.     

Experimental determination of the alpha and beta C--H bond dissociation energies in naphthalene

The acidities of the two different sites in naphthalene (1alpha and 1beta) and the electron affinities of the alpha- and beta-naphthyl radicals were measured using a Fourier transform mass spectrometer. Both carbon-hydrogen bond dissociation energies for naphthalene also were obtained, in this case via the application of a thermodynamic cycle. The final results are DeltaH(o)acid (1alpha) = 394.2+/-1.2 kcal mol(-1), DeltaH(o)acid (1beta) = 395.5+/-1.3 kcal mol(-1), EA(alpha) = 31.6+/-0.5 kcal mol(-1), EA(beta) = 31.6+/-0.5 kcal mol(-1), BDE(1alpha) = 112.2+/-1.3 kcal mol(-1) and BDE(1alpha) = 111.9+/-1.4 kcal mol(-1), and they are compared to benzene and phenyl radical as well as ab initio and density functional theory (B3LYP) calculations.     

Visualizing internal stabilization in weakly bound systems using atomic energies: hydrogen bonding in small water clusters

Atomic energies are used to visualize the local stabilizing and destabilizing energy changes in water clusters. Small clusters, (H(2)O)(n), from n = 2-5, at MP2/aug-cc-pVTZ geometries are evaluated using energies defined by the quantum theory of atoms in molecules (QTAIM). The atomic energies reproduce MP2 total energies to within 0.005 kcal mol(-1). Oxygen atoms are stabilized for all systems and hydrogen atoms are destabilized. The increased stability of the water clusters due to hydrogen bond cooperativity is demonstrated at an atomic level. Variations in atomic energies within the clusters are correlated to the geometry of the waters and reveal variations in the hydrogen bond strengths. The method of visualization of the energy changes applied here is especially suited for application to large biomolecules.