Kimichi Suzuki

Temperature and isotope effects on water cluster ions with path integral molecular dynamics based on the fourth order Trotter expansion

Path integral molecular dynamics simulation based on the fourth order Trotter expansion has been performed to elucidate the geometrical isotope effect of water dimer anions, H(3)O(2)(-), D(3)O(2)(-), and T(3)O(2)(-), at different temperatures from 50 to 600 K. At low temperatures below 200 K the hydrogen-bonded hydrogen nucleus is near the center of two oxygen atoms with mostly O...X...O geometry (where X = H, D, or T), while at high temperatures above 400 K, hydrogen becomes more delocalized, showing the coexistence between O...X-O and O-X...O. The OO distance tends to be shorter as the isotopomer is heavier at low temperatures, while this ordering becomes opposite at high temperatures. It is concluded that the coupling between the OO stretching mode and proton transfer modes is a key to understand such a temperature dependence of a hydrogen-bonded structure.

Efficient ab initio path integral hybrid Monte Carlo based on the fourth-order Trotter expansion: Application to fluoride ion-water cluster

We propose an efficient path integral hybrid Monte Carlo (PIHMC) method based on fourth-order Trotter expansion. Here, the second-order effective force is employed to generate short trial trajectories to avoid computationally expensive Hessian matrix, while the final acceptance is judged based on fourth-order effective potential. The computational performance of our PIHMC scheme is compared with that of conventional PIHMC and PIMD methods based on second- and fourth-order Trotter expansions. Our method is applied to on-the-fly ab initio PIHMC calculation of fluoride ion-water complexes, F(-)(H(2)O) and F(-)(D(2)O), at ambient temperature, particularly focusing on the geometrical isotope effect.

The chemical shift of deprotonated water dimer: Ab initio path integral simulation

The (1)H NMR chemical shift in deprotonated water dimer H(3)O(2)(-) has been studied by ab initio path integral simulation. The simulation predicts that the isotropic shielding of hydrogen-bonded proton increases as a function of temperature by about 0.003 ppm/K. This change is about an order of magnitude larger than that of the nonhydrogen-bonded proton. It is concluded that this is caused by the significant difference in the quantum distribution of proton at high and low temperatures in the low barrier hydrogen bond.

Ab initio path integral simulations for the fluoride ion-water clusters: competitive nuclear quantum effect between F(-)-water and water-water hydrogen bonds.

Small hydrated fluoride ion complexes, F(-)(H2O)n (n = 1-3), have been studied by ab initio hybrid Monte Carlo (HMC) and ab initio path integral hybrid Monte Carlo (PIHMC) simulations. Because of the quantum effect, our simulation shows that the average hydrogen-bonded F(-)···HO distance in the quantum F(-)(H2O) is shorter than that in the classical one, while the relation inverts at the three water molecular F(-)(H2O)3 cluster. In the case of F(-)(H2O)3, we have found that the nuclear quantum effect enhances the formation of hydrogen bonds between two water molecules. In F(-)(H2O)2 and F(-)(H2O)3, the nuclear quantum effect on two different kinds of hydrogen bonds, F(-)-water and water-water hydrogen bonds, competes against each other. In F(-)(H2O)3, thus, the nuclear quantum effect on the water-water hydrogen bond leads to the elongation of hydrogen-bonded F(-)···HO distance, which we suggest this as the possible origin of the structural inversion from F(-)(H2O) to F(-)(H2O)3.

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.

Multicomponent Molecular Orbital–Climbing Image–Nudged Elastic Band Method to Analyze Chemical Reactions Including Nuclear Quantum Effect

To analyze the H/D isotope effects on hydrogen transfer reactions in XHCHCHCHY↔XCHCHCHYH (X, Y=O, NH, or CH2 ) including the nuclear quantum effect of proton and deuteron, we propose a multicomponent molecular orbital-climbing image-nudged elastic band (MC_MO-CI-NEB) method. We obtain not only transition state structures but also minimum-energy paths (MEPs) on the MC_MO effective potential energy surface by using MC_MO-CI-NEB method. We find that nuclear quantum effect affects not only stationary-point geometries but also MEPs and electronic structures in the reactions. We clearly demonstrate the importance of including nuclear quantum effects for H/D isotope effect on rate constants (kH /kD ).

Temperature dependence on the structure of Zundel cation and its isotopomers

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.

Unusual H/D isotope effect in isomerization and keto–enol tautomerism reactions of pyruvic acid: nuclear quantum effect restricts some rotational isomerization reactions

Isomerization and keto–enol tautomerism reactions of the pyruvic acid molecule have been investigated using the multicomponent B3LYP (MC_B3LYP) methods, which can take account of the nuclear quantum effect (NQE) of a light nucleus, such as a proton and a deuteron. While the conventional harmonic zero point vibrational energy (ZPVE) correction makes the activation energies of all the reactions in this system lower, a contrasting behavior is found in our MC_B3LYP results for several rotational reactions. In such cases, the H/D isotope effect on the activation energy is also completely opposite between harmonic ZPVE-corrected B3LYP and MC_B3LYP calculations. In our MC_B3LYP calculation, the activation energies of several C–C or O–H rotational reactions of H species are slightly higher than those of D species, since the NQE of a hydrogen-bonded proton strengthens the hydrogen-bonded interaction more than that of a deuteron, and, thus, the rotational motion of H species is restricted. Such an “unusual” H/D isotope effect on the activation energies can be observed only in the MC_B3LYP results. Our MC_B3LYP calculations clearly demonstrate that direct inclusion of NQE is indispensable to analyze H/D isotope effects on activation energies of not only hydrogen transfer reactions but also C–C and O–H rotational reactions in the isomerization and keto–enol tautomerism of pyruvic acid molecule.

Semiempirical investigations on the stabilization energies and ionic hydrogen-bonded structures of F−(H2O)n and Cl−(H2O)n (n = 1–4) clusters

Several semiempirical methods were utilized to analyze the structures and stabilities of X−(H2O)n (X = F, Cl; n = 1–4) clusters with respect to the number of water molecules through their comparison with ab initio molecular orbital calculations. Our results show that the recently developed PM6-DH+ semiempirical method can provide reasonable binding energies of hydrated fluoride and chloride ion clusters, which are consistent with the corresponding experimental results. For the optimized geometries of X = F, however, the semiempirical methods show that the global minima are close to HF(OH)−(H2O)n−1 structures, which are different from the ab initio calculations. Meanwhile, the topological characteristics for the global minima of X = Cl obtained by semiempirical methods have the same symmetries with ab initio calculations. All calculation levels agree on the trend of decreasing ion-water interaction with the increasing number of water molecules. We also found a new structure of Cl−(H2O)4 with a second hydration shell as a complement of previous studies. Those are very important data for our near-future study of on-the-fly semiempirical molecular dynamics (MD) or path integral MD simulation.PACS36.40.Mr, 31.15.bu, 31.15.A-

Ab Initio Path Integral Molecular Dynamics Simulations of F2H− and F2H3+

The quantum nature of the strong hydrogen bonds for the F2H−and F2H3 +ions and their deuterated isotopomers at the room temperature has been studied using ab initio path integral molecular dynamics (PIMD) simulations. It is found that, for both of these ions, the hydrogen-bonded H/D atoms largely fluctuate around the central position of two F atoms. The average FH/FF distances of F2H−and F2H3 +are longer than the average FD/FF distances of F2D−and F2D3 +due to the primary/secondary isotope effects, which stem from the difference of the quantum nature of H and D nuclei. These results are compared with the family of Zundel-type ions, O2H3 −, N2H5 −, O2H5 +, and N2H7 +, which have been studied previously with the same ab initio PIMD approach. A comparison is also made with the previous experimental and ab initio vibrational configuration interaction results of F2H−.