Influence of electric field on the hydrogen bond network of water

Understanding the inherent response of water to an external electric (E)-field is useful towards decoupling the role of E-field and surface in several practically encountered situations, such as that near an ion, near a charged surface, or within a biological nanopore. While this problem has been studied in some detail through simulations in the past, it has not been very amenable for theoretical analysis owing to the complexities presented by the hydrogen (H) bond interactions in water. It is also difficult to perform experiments with water in externally imposed, high E-fields owing to dielectric breakdown problems; it is hence all the more important that theoretical progress in this area complements the progress achieved through simulations. In an attempt to fill this lacuna, we develop a theory based on relatively simple concepts of reaction equilibria and Boltzmann distribution. The results are discussed in three parts: one pertaining to a comparison of the key features of the theory vis a vis published simulation/experimental results; second pertaining to insights into the H-bond stoichiometry and molecular orientations at different field strengths and temperatures; and the third relating to a surprising but explainable finding that H-bonds can stabilize molecules whose dipoles are oriented perpendicular to the direction of field (in addition to the E-field and H-bonds both stabilizing molecules with dipoles aligned in the direction of the field).     

Theoretical analyses of the morphological development of the hydrogen bond network in protonated methanol clusters

Extensive density functional theory (DFT) calculations are carried out on various structural isomers of protonated methanol clusters, H(+)(MeOH)n (n = 2-9), to analyze the morphological development of the hydrogen bond network in the clusters with an increase of the cluster size. Coexistence of multiple structural isomers is demonstrated by the nearly degenerated energies. Moreover, significant temperature dependence of the preferential isomer structure is shown by the calculated Gibbs free energies. The previously reported infrared spectra of H(+)(MeOH)n (J. Phys. Chem. A 2005, 109, 138) are revisited on the basis of the spectral simulations of the isomers by DFT calculations.     

Influence of intramolecular hydrogen bond strength on OH-stretching overtones

Vapor-phase OH-stretching overtone spectra of 1,3-propanediol and 1,4-butanediol were recorded and compared to the spectra of ethylene glycol to investigate the effect of increased intramolecular hydrogen bond strength on OH-stretching overtone transitions. The spectra were recorded with laser photoacoustic spectroscopy in the second and third OH-stretching overtone regions. The room-temperature spectra of each molecule are dominated by two conformers that show intramolecular hydrogen bonding. Anharmonic oscillator local-mode calculations of the OH-stretching transitions have been performed to aid assignment of the different conformers in the spectra and to illustrate the effect of the intramolecular hydrogen bonding. The hydrogen bond strength increases in the order ethylene glycol, 1,3-propanediol, and 1,4-butanediol. The overtone transitions of the hydrogen-bonded hydroxyl groups are more difficult to observe with increasing intramolecular hydrogen bond strength. We suggest that the bandwidth of these transitions increases with increasing hydrogen bond strength and with increasing overtone and furthermore that these changes are in part responsible for the lack of observed overtone spectra for complexes.     

Influence of an electric field on the buoyancy-driven instabilities

The influence of dc electric fields (EFs) on the development of buoyancy-driven instabilities of reaction fronts is investigated experimentally in a modified Hele-Shaw cell for the arsenous acid-iodate system. Assessment of effects of external EFs is made both visually and through dispersion curves. It is shown that density fingering, observed on ascending fronts, is suppressed by the EF if the front propagates towards the positive electrode and is enhanced when the front propagates towards the negative electrode. The stabilizing (destabilizing) effects include slower (faster) development of fingers and the decrease (increase) in their numbers. The descending front, stable under no EF conditions, remains stable when an EF is applied with the positive electrode facing the approaching front. When the descending front faces the negative electrode, the tiny fingerlike structure develops after quite a long time.     

Influence of the electric field on BN conical structures

We apply first‐principles calculations to investigate the effect of the electric field on boron nitride conical structures. The studies involve nanocones with different disclination angles. We applied fields of 0.3 V/Å and 0.6 V/Å parallel to the cone axis. It is shown that a small field does not affect the stability of such structures; however, for a larger field, a decrease of 0.1 eV/atom for all structures is observed. We also find modification in the energy gap due to the intensity of the electric field. The bandgap decreased proportionally to the intensity of the electric field, indicating that these results have consequences in the field emission properties of these structures. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

External electric field will control the selectivity of enzymatic-like bond activations

Controlling the selectivity of a chemical reaction is a Holy Grail in chemistry. This paper reports theoretical results of unprecedented effects induced by moderately strong electric fields on the selectivity of two competing nonpolar bond activation processes, C-H hydroxylation vs C=C epoxidation, promoted by an active species that is common to heme-enzymes and to metallo-organic catalysts. The molecular system by itself shows no selectivity whatsoever. However, the presence of an electric field induces absolute selectivity that can be controlled at will. Thus, the choice of the orientation and direction of the field vis-à-vis the molecular axes drives the reaction in the direction of complete C-H hydroxylation or complete C=C epoxidation.     

Spectral characterization of hydrogen bond network dynamics in water

Some computational aspects of the characterization of the complex hydrogen bond network dynamics using power spectral analysis are discussed. In the case of hydrogen-bonded liquids, the tagged molecule potential energy is shown to be a useful quantity for capturing the behavior of the networked liquid on different lengths and time scales. The computation of the tagged potential energy for rigid-body effective pair potentials, such as the TIP5P-E and SPC-E models, is discussed. The more structured nature of the TIP5P-E potential, compared to the SPC/E potential, shows up as differences in the high-frequency librational band of the power spectra of the tagged molecule potential energies. The static distributions of the tagged molecule potential energies are also more structured in the case of TIP5P-E, rather than SPC/E, water. The overall behavior of the key power spectral features remains the same in both the models. The possibility of detailed characterization of the power spectrum, and therefore of the underlying dynamics, using a model-based parametric fitting procedure for the power spectra is also discussed. We show that a parametric fitting can allow one to test alternative models of the dynamics underlying the liquid state dynamics.     

A molecular orbital study of the effect of an external electric field on methanol

We report the results of INDO-MO calculations to investigate the influence of an external electric field on methanol. We predict that during a field-ionisation experiment, ionisation will occur from a quite different molecular geometry than the field-free olie.     

Topological building blocks of hydrogen bond network in water

Basic three-dimensional units of the network, called fragments, are introduced to characterize the hydrogen bond (HB) network structure of water. Topological differences among normal liquid water, water at low temperature, and water under high pressure are elucidated by their fragment statistics. Water at low temperature has almost defect-free network and is filled with stable fragments with small distortion. It is found that there exists a certain way on how fragments mutually aggregate. Well-formed aggregates heterogeneously constitute very stable network structures. HB network rearrangements occur scarcely inside these aggregated domains but take place in their surface areas. The heterogeneity of HB structure and rearrangement in water is thus explained in terms of the fragment structure and its rearrangements. The fragment analysis thus elucidates the intermediate-range order in water HB network.     

Effects of alkali metal halide salts on the hydrogen bond network of liquid water

Measurements of the oxygen K-edge X-ray absorption spectrum (XAS) of aqueous sodium halide solutions demonstrate that ions significantly perturb the electronic structure of adjacent water molecules. The addition of halide salts to water engenders an increase in the preedge intensity and a decrease in the postedge intensity of the XAS, analogous to those observed when increasing the temperature of pure water. The main-edge feature exhibits unique behavior and becomes more intense when salt is added. Density functional theory calculations of the XAS indicate that the observed red shift of the water transitions as a function of salt concentration arises from a strong, direct perturbation of the unoccupied molecular orbitals on water by anions, and does not require significant distortion of the hydrogen bond network beyond the first solvation shell. This contrasts the temperature-dependent spectral variations, which result primarily from intensity changes of specific transitions due to geometric rearrangement of the hydrogen bond network.     

Restructuring the crystalline cellulose hydrogen bond network enhances its depolymerization rate

Conversion of lignocellulose to biofuels is partly inefficient due to the deleterious impact of cellulose crystallinity on enzymatic saccharification. We demonstrate how the synergistic activity of cellulases was enhanced by altering the hydrogen bond network within crystalline cellulose fibrils. We provide a molecular-scale explanation of these phenomena through molecular dynamics (MD) simulations and enzymatic assays. Ammonia transformed the naturally occurring crystalline allomorph I(β) to III(I), which led to a decrease in the number of cellulose intrasheet hydrogen bonds and an increase in the number of intersheet hydrogen bonds. This rearrangement of the hydrogen bond network within cellulose III(I), which increased the number of solvent-exposed glucan chain hydrogen bonds with water by ~50%, was accompanied by enhanced saccharification rates by up to 5-fold (closest to amorphous cellulose) and 60-70% lower maximum surface-bound cellulase capacity. The enhancement in apparent cellulase activity was attributed to the "amorphous-like" nature of the cellulose III(I) fibril surface that facilitated easier glucan chain extraction. Unrestricted substrate accessibility to active-site clefts of certain endocellulase families further accelerated deconstruction of cellulose III(I). Structural and dynamical features of cellulose III(I), revealed by MD simulations, gave additional insights into the role of cellulose crystal structure on fibril surface hydration that influences interfacial enzyme binding. Subtle alterations within the cellulose hydrogen bond network provide an attractive way to enhance its deconstruction and offer unique insight into the nature of cellulose recalcitrance. This approach can lead to unconventional pathways for development of novel pretreatments and engineered cellulases for cost-effective biofuels production.     

Many-body calculation of the electric field gradient in the hydrogen molecule

The electric field gradient in the hydrogen molecule has been calculated by diagrammatic many-body perturbation theory (MBPT ) in Gaussian basis sets. The procedure through third order in electron correlation gives a value for the field gradient of 0.34041 a.u., which is 0.8% greater than the accurate value. The result is discussed in terms of the completeness of the basis sets and the convergence of the perturbation expansion.     

Theoretical investigation of the hydrogen bond interactions of methanol and dimethylamine with hydrazone and its derivatives

The interactions between hydrogen bond donors (dimethylamine (DMA) and methanol (MeOH)) and acceptors (formaldehyde dimethylhydrazone, acetaldehyde N,N-dimethylhydrazone and N-nitrosodimethylamine) were theoretically investigated by DFT. The hydrogen bonding interactions were found on several bonding sites of the acceptors. The important properties of structure, binding energy, enthalpy of formation, Gibbs free energy of formation and equilibrium constant were investigated. Compared to the monomer, the DMA complexes show a small red shift of the NH-stretching vibrational transition but a significantly intensity enhancement. On the other hand, the MeOH complexes have a large red shift but a relatively small intensity enhancement of the OH-stretching transition. Atoms-in-molecules analysis revealed that several types of hydrogen bond interaction were present in the complexes. Since natural bond orbital analysis overestimated the effect of charge transfer, the more reliable localized molecular orbital energy decomposition analysis was performed and it shows that the major contribution to the total interaction energy is the electrostatic interaction. All these parameters suggest that the hydrogen bond donor strength of MeOH is substantially greater than DMA.

     

Density functional theory study of the hydrogen bond interaction between lactones, lactams, and methanol

The structure and relative stability of methanol complexes with various cyclic ketones, lactones, lactams, and N-methyl lactams from three- to seven-membered rings have been investigated using the density functional theory method. The geometries, harmonic frequencies, and energies were calculated at the B3LYP/6-311+G(d,p) level. Three stable structures, cis-a, cis-b, and trans, with respect to the ring oxygen (nitrogen) atom, were found to be local minima of the potential energy surface. For lactones and N-methyl lactams, the most stable structure is trans; it is stabilized, as in cyclic ketones, through the conventional hydrogen bond (HB) interaction between the basic carbonyl oxygen and the acidic methanolic hydrogen and an unconventional HB interaction between the methanolic oxygen and the CH hydrogen, in the alpha position of the carbonyl group. For unsubstituted lactams, the cis-a structure, stabilized through a HB interaction between the NH group and the methanol oxygen in addition to the conventional HB interaction, is the most stable. The topological properties of the electron density ratify the existence of conventional (N,O-H. . .O) and unconventional (C-H. . .O) hydrogen bonding. A good correlation was found between the HB distances and the electron density at the HB critical point. The unsubstituted lactams yield more stable complexes with methanol than N-methyl lactams, lactones, and cyclic ketones. In the most stable complexes, both components behave simultaneously as a HB donor and as a HB acceptor.     

Deuteron magnetic resonance and hydrogen bond network of ammonium trihydrogen selenite

The DMR spectra of single-crystal ND₄D₃(SeO₃)₂ have been studied. The principal values and the direction cosines of the field-gradient tensor of deuterons located on three nonequivalent O · · · O hydrogen bonds have been determined. The lengths of hydrogen bonds have been calculated from $\text{eQq}\text{h}$ values; the deuterons have been located on hydrogen bonds. The comparison with the DMR data of isomorphous compound RbD₃(SeO₃)₂ was made; the influence of NH · · · O hydrogen bonds on the structural parameters O · · · O hydrogen bonds is discussed.     

Influence of external electric field on the rate of hydrogen isotope exchange between hydroxyl-containing molecules in CCl4

Rates of the fast proton-deuteron exchange between CH₃OD and C₆H₅OH (1), and CH₃OD and CH₃COOH (2) in dilute CCl₄ solutions have been measured by a kinetic IR spectroscopy technique. Under an imposed external electric field (10 V/m), the exchange rate in system (2) increases, which is not observed in (1). The effect is interpreted assuming that, in the second case, a cyclic ion pair with a high dipole moment is formed as a reaction intermediate.

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- CH₃OD C₆H₅OH (1), CH₃OD CH₃COOH (2) CCl₄. (IO⁷ B/) (2) , (1). (2) , .

     

The hydrogen atom in a cubic field of electric dipoles

The full rotational symmetry of the hydrogen atom is lowered to that of the double group of O_h. This is done by a cubic field of electric dipoles. Symmetry-adapted spin orbitals have been calculated and with them the energies of the lowest states as functions of the distance from the dipoles to the atom, the dipole moment, and the size of the atoms forming the dipoles. The Hamiltonian used is obtained by starting with the Dirac equations and then making some simplifications and approximations.     

Organic nanostructures on hydrogen-terminated silicon report on electric field modulation of dangling bond charge state

We pursue dynamic charge and occupancy modulation of silicon dangling bond sites on H-Si(100)-2 × 1 with a biased scanning tunneling microscope tip and demonstrate that the reactivity and mechanism of product formation of cyclobutylmethylketone (CBMK) on the surface at the active sites may be thus spatially regulated. Reactivity is observed to be dependent on the polarity between tip and surface while the area over which reactivity modulation is established scales according to the dopant concentration in the sample. We account for these observations with examination of the competition kinetics applicable to the CBMK/H-Si reaction and determine how said kinetics are affected by the charge state of DB sites associated with reaction initiation and propagation. Our experiments demonstrate a new paradigm in lithographic control of a self-assembly process on H-Si and reveal a variant to the well-known radical mediated chain reaction chemistry applicable to the H-Si surface where self-assembly is initiated with dative bond formation between the molecule and a DB site.