Hydrogen bonding and induced dipole moments in water: predictions from the Gaussian charge polarizable model and Car-Parrinello molecular dynamics

We compare a new classical water model, which features Gaussian charges and polarizability (GCPM) with ab initio Car-Parrinello molecular dynamics (CPMD) simulations. We compare the total dipole moment, the total dipole moment distribution, and degree of hydrogen bonding at ambient to supercritical conditions. We also compared the total dipole moment calculated from both the electron density (partitioning the electron density among molecules based on a zero electron flux condition), and from the center of localized Wannier function centers (WFCs). Compared to CPMD, we found that GCPM overpredicts the dipole moment derived by partitioning the electron density and underpredicts that obtained from the WFCs, but exhibits similar trends and distribution of values. We also found that GCPM predicted similar degrees of hydrogen bonding compared to CPMD and has a similar structure.     

The unexpected and large enhancement of the dipole moment in the 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione (DMACB) molecule upon crystallization: a new role of the intermolecular CH...O interactions.

The molecular dipole moment of the 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione (DMACB) molecule and its enhancement in the crystal was evaluated by periodic RHF ab initio computations. A discrete boundary partitioning of the electronic density that allows an unambiguous partitioning of the molecular space in the condensed phase was adopted. The resulting molecular dipole in the crystal compares favorably with the experimental value obtained by a multipolar analysis of single-crystal X-ray diffraction data recorded at 20 K, using a fuzzy boundary partitioning of the derived pseudoatom densities. We show that a large and highly significant molecular dipole enhancement may occur upon crystallization, despite the lack of a strongly hydrogen bonded environment in the crystal. The 23 unique C-H...O interactions which are formed upon packing of the DMACB molecule induce an increase in the molecular dipole (over 75%) that is comparable to or greater than that found in systems which are characterized by the stronger O-H...O and N-H...O hydrogen bonds. The DMACB molecule constitutes an excellent system for the study of C-H...O interactions in the condensed phase, since no other kind of competing hydrogen bonds is present in its crystal. A simple and qualitative model for the matrix contribution to the DMACB molecular dipole enhancement in the crystal is proposed. The formation of several weak C-H...O bonds is found to yield a small (about 0.2 e) net flux of electronic charge flowing from the hydrogens of the methyl groups to the carbonyl oxygen atoms. Despite the limited increase of the intramolecular charge transfer upon crystallization, a large molecular dipole enhancement occurs because the centroids of the positive and negative induced charges are quite far apart. This work highlights a new and important role of the C-H...O bond, besides those already known in the literature.     

Dipole and quadrupole moments of molecules in crystals: a novel approach based on integration over Hirshfeld surfaces

Elegant expressions are derived for the computation of dipole and quadrupole moments of molecules using the electrostatic potential and electric field evaluated on an oriented molecular surface. These expressions are implemented for Hirshfeld surfaces, applied to various molecular crystals, and compared with the results from the quantum theory of atoms in molecules. The effect of intermolecular interactions is also explored by examining the differences between electrostatic moments derived from a periodic Hartree-Fock electron density and an electron density resulting from a superposition of noninteracting molecules. The enhancement of the dipole moment for hydrogen bonded molecular crystals is typically 30%-40% and shown to be largely independent of the partitioning scheme. Dipole moments calculated from Hirshfeld surfaces systematically underestimate those from zero-flux surfaces, a result attributed to the translation of the Hirshfeld surface relative to the zero-flux surfaces for these molecules. For acetylene and benzene, the differences between a crystal calculation and the sum of noninteracting molecules are small, and both partitioning schemes yield quadrupole and second moment results in close agreement.     

Ab initio calculation of the dipole moment function of hydrogen iodide

SCEP/CEPA and MC SCF potential energy and dipole moment functions for hydrogen iodide have been calculated. Spectroscopic constants and vibrational dipole matrix elements obtained from the CEPA functions are in good agreement with experimental data. In contrast to previous results for hydrogen fluoride, the MC SCF dipole moment function is less accurate than the CEPA function.     

Hirshfeld partitioning of the electron density: atomic dipoles and their relation with functional group properties

Atomic dipole moments, derived within the Hirshfeld partitioning of the molecular electron density, have been studied for compounds of the type H-X and Cl-X, for a series of functional groups X frequently encountered in organic molecules. In the case of the H-X compounds, the component of the atomic dipole moment on H along the axis connecting H with the central atom in X is found to be linearly correlated with the electronegativity of X, the hardness of X playing no significant role. In the case of the Cl-X compounds, the situation is less clear. However, evidence seems to point to the conclusion that for these compounds, also the group hardness plays an important role.     

An interpretation of the enhancement of the water dipole moment due to the presence of other water molecules

The dipole moment of the gas phase water monomer is 1.85 D. When solvated in bulk water, the dipole moment of an individual water molecule is observed to be enhanced to the much larger value of 2.9 +/- 0.6 D. To understand the origin of this dipole moment enhancement, the effective fragment potential (EFP) method is used to solvate an ab initio water molecule to predict the dipole moments for various cluster sizes. The dipole moment as a function of cluster size, nH 2O, is investigated [for n = 6-20 (even n), 26, 32, 41, and 50]. Localized charge distributions are used in conjunction with localized molecular orbitals to interpret the dipole moment enhancement. These calculations suggest that the enhancement of the dipole moment originates from the decrease of the angle between the dipole vectors of the lone pairs on oxygen as the number of hydrogen bonds to that oxygen increases. Thus, the decreased angle, and the consequent increase in water dipole moment, is most likely to occur in environments with a larger number of hydrogen bonds, such as the center of a cluster of water molecules.     

Charge distributions and multipole moments in molecules

The consideration of multipole moments is suggested as a new criterion for the validity of assignments of atomic charges in molecules. The total quadrupole and octupole moments generated by our definition of atomic charges are compared with the exact moments of the underlying wavefunction for various basis sets in selected diatomics. The analysis includes also total overlap and total dipole moment partitioning as well as 1σ MO overlap partitioning. All considerations together allow us to assess the validity of our charge definition as compared to Mulliken's and Löwdin's and the quality of the basis set.     

Properties of the X 1Σ state of BF

A perturbation theoretical approach for treating electron correlation has been used to calculate the potential energy curve and dipole moment function of BF near its equilibrium bond length. A dipole moment of 0.89 D (B^?F⁺) is predicted at R_e. When the bond is stretched by ≈0.2 Å the dipole reverses sign.     

Calculation of partition coefficient and hydrophobic moment of the secondary structure of lysozyme

A method that permits a semiquantitative estimate of the partitioning of any solute between any two media is presented. As an example, the partition coefficients and hydrophobic moment of the secondary structure of lysozyme are calculated. Program GSCAP is written as a version of Pascal's solvent-dependent conformational analysis (SCAP) program. The dipole moments calculated for the helices are trebled with respect to that for the sheet. For helices, the main contribution to the water-accessible surface area is the hydrophobic term, while the hydrophilic part dominates in the sheet. Molecular globularity and the three studied partition coefficients differentiate between helices and sheet.     

Relationship between the charge distribution and dipole moment functions of CO and the related molecules CS, SiO, and SiS

The dipole moment functions of the titled molecules are written as the sum of a charge and induced atomic dipole contribution and the distance dependence interpreted in terms of these components. These two contributions have opposite signs over a large range of internuclear distances, and when they have equal magnitudes, the dipole moment vanishes. This happens with CO near the equilibrium bond length and is responsible for its small dipole moment. The dipole moment of CS is 0.770(ea0), rather large for a diatomic in which the two atoms have essentially the same electronegativities; this is because for CS, the two components of the dipole moment have the same sign at equilibrium and reinforce one another.     

Influence of dipole moments on the medicinal activities of diverse organic compounds

In this study, synthesis, biological activity and structure-activity relationships of diverse compounds are described. In general, the relationships between dipole moment and biological activities are discussed in detail. Despite progress of interdisciplinary science, the use of dipole moment values of organic compounds to understand their potent medicinal activities in various diseases remains unexplored. In contrast, it can be seen that many compounds demonstrate a direct correlation between biological activity and dipole moment. Therefore, analyzing the dipole moment values, scientists may design more potent compounds prior to their synthesis which is tedious, costly and time-consuming.     

Dipole moments of some substituted benzaldehydes. Conformational preference of substituents ortho to the aldehyde group

The dipole moments of a number of substituted benzaldehydes are measured in benzene solution. The angle which the dipole axis of the CHO group makes with the axis of rotation of the group is determined. The observed moments of the ortho-substituted benzaldehydes are compared with the moments calculated for free rotation as well as fors-trans ands-cis orientations of the -CHO group.o-Fluorobenzaldehyde exists mostly in thes-trans conformation.o-Chloro-,o-bromo-ando-nitro-benzaldehydcs also exist in thes-trans conformation; their observed dipole moments are even lower than the values calculated fors-trans forms, indicating mutual induction of the ortho substituents. Though 2,5-dichlorobenzaldehyde is expected to have the same dipole moment as benzaldehyde, the observed moment is significantly lower due to mutual induction of the ortho substituents. 2,5-Dimethylbcnzaldehyde has, however, almost the same moment as benzaldehyde. The dipole moment ofo-methoxybcnzaldchyde is considerably higher than the values calculated for boths-cis ands-trans conformations. An explanation is given for this.o-Hydroxybenzaldehyde exists exclusively in thes-cis form due to internal H-bonding.     

甲酸,乙酸或水单一溶剂的偶极矩与盐浓度的关系

近十几年来,汽液平衡盐效应的研究一直是国内外十分活跃的课题.但是,对于多组分含盐体系,特别是多组分羧酸体系,如甲酸-乙酸-水-盐体系的汽液平衡盐效应研究报道甚少。一方面因为多元汽液平衡盐效应的测定较难;另一方面由于盐的加入,使原来的强极性、强缔合的羧酸体系变得更为复杂,给热力学关联带来了困难.尽管有的关联方法引入盐后引起偶极矩改变,且使偏心因子产生相应变化,但在实验上并未得到偶极矩随盐浓度定量变化的关系。     

Evaluation of the solid state dipole moment and pyroelectric coefficient of phosphangulene by multipolar modeling of X-ray structure factors

The electron density distribution of the molecular pyroelectric material phosphangulene has been studied by multipolar modeling of X-ray diffraction data. The "in-crystal" molecular dipole moment has been evaluated to 4.7 D corresponding to a 42% dipole moment enhancement compared with the dipole moment measured in a chloroform solution. It is substantiated that the estimated standard deviation of the dipole moment is about 0.8 D. The standard uncertainty (s.u.) of the derived dipole moment has been derived by splitting the dataset into three independent data-sets. A novel method for obtaining pyroelectric coefficients has been introduced by combining the derived dipole moment with temperature-dependent measurements of the unit cell volume. The derived pyroelectric coefficient of 3.8(7) x 10-6 Cm 2K-1 is in very good agreement with the measured pyroelectric coefficient of p = 3 +/- 1 x 10-6 Cm-2 K-1. This method for obtaining the pyroelectric coefficient uses information from the X-ray diffraction experiment alone and can be applied to much smaller crystals than traditional methods.     

13C NMR investigations of phenol–triethylamine complexes: Influence of hydrogen bond interaction on the electronic structure of the aliphatic chain

The influence of hydrogen bond formation on ¹³C chemical shifts at the α and β positions of triethylamine and tri-n-butylamine has been investigated by dipole moment measurements and CNDO/2 calculations. It has been shown that a hydrogen bridge dipole moment occurs during complexation. Moreover, the change observed in the C-α? C-β bond dipole moment is proportional to the hydrogen bridge dipole moment, but is approximately 100 times smaller. This change has been related to differences between the ¹³C chemical shifts at the α and β positions of amines.     

Ground state and excited state dipole moments of alkyl substituted para-nitroaniline derivatives

The ground state and excited state dipole moment of a series of alkyl substituted para-nitroaniline derivatives is reported. Ground state dipole moment was determined by the Debye-Guggenheim method and the excited state dipole moment was estimated using the solvatochromic method. For all molecules under investigation, the excited state dipole moment was found to be higher than the ground state dipole moment. The molecules exhibited positive solvatochromism.     

On the negative dipole moment derivatives of HRgX (Rg=He, Ar, Kr; X=F, Cl) molecules

The large dipole moment and the negative dipole moment derivatives with respect to H–Rg displacement of the neutral HRgX (Rg=He, Ar, Kr; X=F, Cl) molecules have been rationalised by a charge/charge flux/dipole flux decomposition of the charge density using the ChelpG method. This approach was also applied to the hydrogen halides HF and HCl for the sake of comparison. It was found that the dipole moment of HRgX is dominated by the large positive charge contribution while the negative dipole moment derivative of HRgX is due to the dominance of the negative charge flux contribution.     

Predicting the infrared transition intensities in the Ar-HF complex: the key role of the dipole moment surface accuracy

The method proposed earlier for the generation of the full-dimensional energy surface for van der Waals complexes [P. Jankowski, J. Chem. Phys. 121, 1655 (2004)] is used to obtain a fulldimensional dipole moment surface for the atom-diatom complex in calculations based on the coupled-cluster with single, double, and noniterative triple excitation approach and the aug-cc-pVQZ basis sets. This surface has been employed to calculate transition intensities of the infrared spectra of Ar-HF. Special attention has been paid to study the problem of relative intensities of the different bands which have not been properly predicted within the long-range models of the dipole moment [A. E. Thornley and J. M. Hutson, J. Chem. Phys. 101, 5578 (1994)]. The intensities calculated with the present dipole moment surface agree very well with the experimental data, which indicate that the short-range interactions significantly affect the dipole moment surface and the calculated intensities. To investigate the role of the accuracy of the dipole moment surface on infrared transition intensities in atom-diatom complexes, four models of increasing complexity are studied. Their performance is shown to strongly depend on the region of the interaction energy surface probed by the initial and final states of the individual transitions.     

Calculation of Dipole Moment of Fractal Asphaltene Cluster

Computer simulation has been used to investigate the cluster formation of the asphaltenes. Asphaltene nanoaggregates, forming a cluster, possessed permanent dipole moments. The influence of various factors, namely, temperature of the medium, size and dipole moment of the nanoaggregates, on the dipole moment of the asphaltene cluster was studied. The orientation interactions between the nanoaggregates were demonstrated to rise with increasing specific dipole moment of the nanoaggregates. It is has been found that the dipole moment of the asphaltene cluster varies according to power law in relation to the number of the nanoaggregates, forming the cluster.