[Show abstract][Hide abstract] ABSTRACT: Temperature dependence on the structural fluctuations of Zundel cation, H5O2 (+), and its isotopomers, D5O2 (+) and T5O2 (+), have been studied using path integral molecular dynamics simulations in which nuclear quantum effect is fully taken into account. It has been found that the fluctuations of hydrogen-oxygen and oxygen-oxygen distances, which are relevant to the hydrogen bonded structure, grow drastically as the temperature increases within the range of investigation between 100 K and 900 K. The fluctuation with respect to the position of non-bonded hydrogen also increases substantially as the temperature increases. The temperature dependence on the fluctuation is greater for D5O2 (+) or T5O2 (+) than that of H5O2 (+), since the zero-point effect of the former is less than the latter.
The Journal of Chemical Physics 05/2013; 138(18):184307. · 3.12 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ab initio path integral molecular dynamics simulation was performed to understand the nuclear quantum effect on the hydrogen bond of hydrogen malonate anion. Static calculation predicted the proton transfer barrier as 0.12 kcal/mol. Conventional ab initio molecular dynamics simulation at 300 K found proton distribution with a double peak on the proton transfer coordinate. Inclusion of thermal effect alone elongates the hydrogen bond length, which increases the barrier height. Inclusion of nuclear quantum effect washes out this barrier, and distributes a single broad peak in the center. H/D isotope effect on the proton transfer is also discussed.
Chemical Physics Letters 05/2013; 571:23–27. · 2.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The geometric isotope effects on the structures of hydrated chloride ionic hydrogen bonded clusters are explored by carrying out path integral molecular dynamics simulations. First, an outer shell coordinate is selected to display the rearrangement of single and multi hydration shell cluster structures. Next, to show the competition of intramolecular and intermolecular nuclear quantum effects, the intramolecular OH∗ stretching and intermolecular ion–water wagging motions are studied for single and multi shell structures, respectively. The results indicate that the intermolecular nuclear quantum effects stabilize the ionic hydrogen bonds in single shell structures, while they are destabilized through the competition with intramolecular nuclear quantum effects in multi shell structures. In addition, the correlations between ion–water stretching motion and other cluster vibrational coordinates are discussed. The results indicate that the intermolecular nuclear quantum effects on the cluster structures are strongly related to the cooperation of the water–water hydrogen bond interactions.
Chemical Physics 01/2013; 426:38–47. · 1.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have first performed ‘gold-standard’ CCSD(T) calculations of the chromium dimer cation using a cc-pVQZ basis set in order to elucidate its electronic structures, especially for identification of its ground-state term. Our CCSD(T)/cc-pVQZ calculations have predicted that it is the highest-spin state, in contrast to a previous speculation of the lowest-spin state by density functional theory with semi-local functionals. The ground-state binding energy is found to be 1.49 eV, which reasonably agrees with the corresponding experimental value of 1.30 ± 0.06 eV. We have also provided several spectroscopic constants for references in future theoretical and experimental works.
[Show abstract][Hide abstract] ABSTRACT: We have theoretically analyzed the nuclear quantum effect on the nuclear magnetic shieldings for the intramolecular hydrogen-bonded systems of σ-hydroxy acyl aromatic species using the gauge-including atomic orbital technique combined with our multi-component density functional theory. The effect of H/D quantum nature for geometry and nuclear magnetic shielding changes are analyzed. Our study clearly demonstrated that the geometrical changes of hydrogen-bonds induced by H/D isotope effect (called geometrical isotope effect: GIE) is the dominant factor of deuterium isotope effect on 13C chemical shift.
[Show abstract][Hide abstract] ABSTRACT: a b s t r a c t Path-integral molecular dynamics simulations have been performed for porphycene and its isotopic vari-ants in order to understand the effect of isotopic substitution of inner protons on the double proton trans-fer mechanism. We have used an on-the-fly direct dynamics technique at the semiempirical PM6 level combined with specific reaction parameterization. Our quantum simulations show that double proton transfer of the unsubstituted porphycene at T = 300 K mainly occurs via a so-called concerted mechanism through the D 2h second-order saddle point. In addition, we found that both isotopic substitution and tem-perature significantly affect the double proton transfer mechanism. For example, the contribution of the stepwise mechanism increases with a temperature increase. We have also carried out hypothetical sim-ulations with the porphycene configurations being completely planar. It has been found that out-of-plane vibrational motions significantly decrease the contribution of the concerted proton transfer mechanism.
Chemical Physics 01/2012; 394(1). · 1.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Molecular dynamics simulations of hexameric molecular capsules constructed from gear-shaped amphiphile molecules with the aid of solvophobic effect and van der Waals force were performed with general AMBER force field to elucidate the thermodynamical properties of the capsules. The optimized structure of the capsule 1 6 consisting of methylated gear-shaped molecules (1) is a nearly cubic structure, whereas the distorted structure is found for the capsule 2 6 consisting of ones lacking three methyl groups, (2) so as to maximize the contact surface. Disassembly temperature for the demethylated capsule 2 6 , $210 K, is much lower than that for the methylated one 1 6 , $340 K, which is consistent with the corresponding experimental result that 1 6 is stable, whereas 2 exists as a monomer in aqueous methanol. V C 2012 Wiley Periodicals, Inc.
International Journal of Quantum Chemistry 01/2012; · 1.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Characteristic features of the positron-binding structure of some carbonyl and aldehyde species such as formaldehyde, acetaldehyde, acetone and propionaldehyde are discussed with the configuration interaction scheme of multi-component molecular orbital (MC_MO) calculations. This method can take the electron–positron correlation contribution into account through single electronic–single positronic excitation configurations. Our vertical positron affinity (PA) values of acetaldehyde and acetone with electronic 6-31 + +G(2df,2pd) and positronic [15s15p3d2f1g] basis sets are as 52 and 92 meV, which can be compared to the recent experimental values of 90 and 173 meV by Danielson et al (2010 Phys. Rev. Lett.104 233201). For formaldehyde we have also found that the PA values are enhanced by including the local vibrational contribution from the vertical PA value of 15 meV to 17, 21 and 25 meV after averaging over the zeroth, first and second vibrational states, respectively, due to the anharmonicity of the potential.
New Journal of Physics 01/2012; 14(3). · 4.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We theoretically investigated the phase transition temperature (Tc) and geometrical changes of mixed K3H1-xDx(SO4)2 and mixed (H1-xDx)2SQ crystals by using the first-principle multi-component molecular orbital method, which can take account of the quantum effect of light nucleus. The O…O distances in K3H1-xDx(SO4)2 and (H1-xDx)2SQ crystals gradually elongate and the electronic charge density around the light nucleus becomes greater, as the deuterium concentration increases. We have also predicted the Tc values of TKHS and T2SQ, with the hydrogen substituted by the tritium, as about 190 and 680 K, respectively.
[Show abstract][Hide abstract] ABSTRACT: We applied the multi-component density functional theory (MC_DFT) combined with the gauge – including atomic orbital (GIAO) and continuous set of gauge transformation (CSGT) techniques to some small molecules (H2, HD, H2O, CH4, etc.) for analyzing the nuclear quantum effect on molecular magnetic properties, such as nuclear magnetic shielding constant and molecular magnetic susceptibility. Our method clearly shows that the absolute isotropic magnetic shielding constant depends on the spatial distribution of quantum nuclei, that is, the deuteron-isotopomers have the weaker deshielding effect than the proton-isotopomers. We also found the magnetic isotope effect with respect to the molecular magnetic susceptibility, that is, the magnetic susceptibility of the deuteron-isotopomers is smaller than that of the proton-isotopomers.
Computational and Theoretical Chemistry. 11/2011; 975(s 1–3).
[Show abstract][Hide abstract] ABSTRACT: Ab initio path integral molecular dynamics based on MP2/cc-pVTZ method has been carried out for a series of small hydrogen cluster cations, H3+,H5+,H7+ and H9+. Our dissociation enthalpy for the reaction H3+(H2)n→H3+(H2)n−1+H2 is found to be 5.4, 2.1, and 3.2kcal/mol, respectively, which are slightly smaller than the corresponding experimental values. It is found that energies of quantum zero-point vibration and anharmonic vibration of the respective cations are important factors to estimate the value of dissociation energy.
Computational and Theoretical Chemistry. 11/2011; 975(s 1–3).
[Show abstract][Hide abstract] ABSTRACT: To evaluate the adsorption sites of hydrogen atom on buckybowl-like molecule (C36H12), which is a model fragment structure of zeolite-templated carbon (ZTC), we have performed path integral molecular dynamics (PIMD) simulation including thermal and nuclear quantum fluctuations under the semi-empirical PM3 potential. Here we have picked up ten carbons as the adsorption sites of additional hydrogen atom (H*), which are labeled as α-, β1-, β2-, γ-, and δ-carbon from edge to bottom carbon for inside and outside of C36H12, respectively. In the static PM3 calculation and conventional MD simulation the ten stable adsorption sites of H* are obtained both inside and outside of C36H12. In PIMD simulation, on the other hand, the nine stable adsorption sites are obtained, except for δ-carbon for inside of C36H12. This result is due to the fact that the thermal effect and zero point vibration of δ-carbon and H* stretching motion make adsorbed hydrogen atom go over potential barrier from δ- and β1-carbon for inside of C36H12 more readily. The thermal and nuclear quantum effects are important to evaluate the hydrogen adsorption site on carbon materials.
Computational and Theoretical Chemistry. 11/2011; 975(s 1–3).
[Show abstract][Hide abstract] ABSTRACT: We have carried out path-integral molecular dynamics simulations for hydrated sulfuric acid clusters to understand acid-dissociation and hydrogen-bonded structural rearrangement processes in these clusters from a quantum mechanical viewpoint. The simulations were performed using the PM6 semiempirical electronic structure level whose parameters were modified on the basis of the specific reaction parameters strategy so that relative energies of optimized structures, as well as water binding energies reproduce ab initio and density-functional theory calculations. We have found that the acid dissociation processes, first and second deprotonation, effectively occur in a hydrated cluster with a specific cluster size. The mechanisms of the proton-transfer processes were analyzed in detail and it was found that the distance between O in sulfuric acid and O in the proton-accepting water is playing an important role. We also found that the water coordination number of the poton-accepting water is important in the proton-transfer processes.
The Journal of Physical Chemistry A 09/2011; 115(42):11486-94. · 2.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Quantum Monte Carlo methods are used to investigate the binding of a positron to the alkali-metal hydrides, XH (X = Na and K). We obtain positron affinities for the NaH and KH molecules of 1.422(10) eV and 2.051(39) eV, respectively. These are considerably larger than the previous results of 1.035 eV and 1.273 eV obtained from multireference single- and double-excitation configuration interaction calculations. Together with our previous results for [LiH;e(+)] [Y. Kita et al., J. Chem. Phys. 131, 134310 (2009)], our study confirms the strong correlation between the positron affinity and dipole moment of alkali-metal hydrides.
The Journal of Chemical Physics 08/2011; 135(5):054108. · 3.12 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To elucidate the mechanism of the exceptional behavior of lysine for the ionization (protonation) yields in matrix-assisted laser desorption/ionization (MALDI) observed by Nishikaze and Takayama [Rapid Commun. Mass Spectrom. 2006, 20, 376], the temperature dependences of proton affinity (PA) and gas phase basicity for 20 amino acids are theoretically analyzed with correlated ab initio molecular orbital method under ideal gas condition. We have found that two different conformations, the linear structure with elongation of the side chain and the folded one having intramolecular hydrogen bonding, play important roles for the exceptional behavior of lysine. At low temperatures of around 298 K, the most stable conformation of the protonated lysine is the folded structure due to the formation of intramolecular hydrogen bonding. Meanwhile, at high temperatures, the Gibbs free energy of linear structure of protonated lysine becomes lower than that of the folded one because of the increment of vibrational entropic contribution. To explicitly take account of the contribution of the free energies, we have proposed the effective PA values thermally averaged using the ratio of Boltzmann distributions for two conformations. Since the effective PA value for lysine drastically decreases as the temperature increases above 1000 K, the linear correlation is clearly obtained between our effective PA values at high temperature and the ion yields in MALDI.
Biological Mass Spectrometry 04/2011; 46(4):376-82. · 3.41 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Characteristic features of the positron binding structure of some nitrile (-CN functional group) species such as acetonitrile, cyanoacetylene, acrylonitrile, and propionitrile are discussed with the configuration interaction scheme of multi-component molecular orbital calculations. This method can take the electron-positron correlation contribution into account through single electronic-single positronic excitation configurations. Our PA value of acetonitrile with the electronic 6-31++G(2df,2pd) and positronic [15s15p3d2f1g] basis set is calculated as 4.96 mhartree, which agrees to within 25% with the recent experimental value of 6.6 mhartree by Danielson et al. [Phys. Rev. Lett., 2010, 104, 233201]. Our PA values of acrylonitrile and propionitrile (5.70 and 6.04 mhartree) are the largest among these species, which is consistent with the relatively large dipole moments of the latter two systems.
Physical Chemistry Chemical Physics 02/2011; 13(7):2701-5. · 3.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
The Journal of Chemical Physics 01/2011; 134(3):031101. · 3.12 Impact Factor