Ab Initio Path Integral Molecular Dynamics Study of the Nuclear Quantum Effect on Out-of-Plane Ring Deformation of Hydrogen Maleate Anion

Low-Barrier Hydrogen Bond in Fujikurin A-D: A Computational Study

The compounds Fujikurin A, B, and D, recently isolated from Fusarium fujikuroi, possess intramolecular low-barrier hydrogen bonds (LBHBs), which are hydrogen bonds with a very low-energy barrier for proton transfer. The isolated compounds have a hydrogen-bonded proton that appears to rapidly switch between two equilibrium states via a transition state (TS). To understand the characteristics of these intramolecular LBHBs in detail, we performed path integral molecular dynamics (PIMD) simulations, which can consider nuclear quantum effects (NQEs) under a finite temperature. The PIMD simulations predicted that the NQE completely washed out the energy barrier for the proton transfer reaction. Consequently, a single-well shape emerged in the results, along with the effective free-energy potential surface for the hydrogen-bonded proton distribution. Thus, we conclude that the hydrogen-bonded proton in Fujikurin does not in fact transfer between two equilibrium structures but widely delocalizes around the global minimum structure involving the TS region.

Theoretical Studies on the Oligomerization of Silicate Species in Basic Solution

The possible reaction pathways of silicate species to linear- and ring-structure oligomers up to silicate hexamers in the basic medium have been studied using the density functional theory. The calculations were performed at the ωB97XD/6-31+G(d,p) level, and the integral equation formalism polarizable continuum model was employed to simulate the solvent effects, and it was found that they are appropriate in exploring the reaction mechanism of silicate species condensation. There are two steps in the anionic silicate condensation reactions: the SiO-Si bond formation step and the dehydration step. Moreover, the latter is the rate-limiting step for most of the reaction pathways except for the cyclization reaction of the linear pentamer to the 5-ring. The short linear oligomers would be likely formed from the reaction between monomers and oligomers, while the longer ones are easily formed through the reactions between short oligomers. The 4-ring and branched 5-ring oligomers are found to be formed very favorable both in kinetic and thermodynamic and could have great influences on the initial stage of zeolite synthesis. The intramolecular and intermolecular hydrogen bond effect of silicate species is an important factor affecting the reaction mechanism.

Theoretical vibrational spectra of OH(-)(H2O)2: the effect of quantum distribution and vibrational coupling

We performed ab initio path integral molecular dynamics simulations for the hydroxide-water cluster, OH(-)(H2O)2, at 50 K, 100 K, and 150 K to investigate its flexible structure. From our simulations, we found that nuclear quantum effects enhance hydroxide hydrogen atom inversion and the conformational change between isomers occurs by simultaneous rotation of the free hydrogen atom. We propose the importance of including the transition state conformer with C2 symmetry, for the description of this system at temperatures realized in predissociation experiments. Temperature dependence of relative populations of each conformer along with multidimensional vibrational calculations were used to simulate the vibrational spectra and compare with the experimental spectra of Johnson and coworkers. We assign the doublet peaks seen in the experiment at 2500 to 3000 cm(-1), as the mixture of the ionic hydrogen bonded OH stretching overtone, ionic hydrogen bonded OH bending overtone, and the combination band of the ionic hydrogen bonded OH stretch and bend, which are modulated by the van der Waals OO vibrations. We concluded that for OH(-)(H2O)2, the vibrational couplings between the ionic hydrogen bonded motion and floppy modes contribute to the broadening of peaks observed in the 2500 to 3000 cm(-1) region.

Intermolecular OHN hydrogen bond with a proton moving in 3-methylpyridinium 2,6-dichloro-4-nitrophenolate

Temperature-dependent changes in the strong OHN hydrogen bond in 3-methylpyridinium 2,6-dichloro-4-nitrophenolate are used to discuss the proton transfer mechanism.