Molecular Electric Moment Research Articles

Molecular electric moments calculated by using natural orbital functional theory.

The molecular electric dipole, quadrupole, and octupole moments of a selected set of 21 spin-compensated molecules are determined employing the extended version of the Piris natural orbital functional 6 (PNOF6), using the triple-ζ Gaussian basis set with polarization functions developed by Sadlej, at the experimental geometries. The performance of the PNOF6 is established by carrying out a statistical analysis of the mean absolute errors with respect to the experiment. The calculated PNOF6 electric moments agree satisfactorily with the corresponding experimental data and are in good agreement with the values obtained by accurate ab initio methods, namely, the coupled-cluster single and doubles and multi-reference single and double excitation configuration interaction methods.

Crystal structure and dielectric property of supramolecular macrocyclic [(NDPA)·(18-crown-6)]2+·(DMA)+·3ClO4− assemblies

Abstract One novel organic–inorganic hybrid supramolecular assemblies [(NDPA)·(18-crown-6)]2+·(DMA)+·3ClO4− (1), has been successfully constructed through the prominent strategies of crystal engineering (NDPA = N,N-dimethyl-1,4-phenylenediamine, DMA = dimethylamine), and characterized by IR, powder XRD and single crystal X-ray diffraction. In the structure, the supramolecular organic cations and inorganic ClO4− anions are arranged alternately and linked by N O⋯H hydrogen bonds. It is worthy to note that the ClO4− are linked to form one-dimensional inorganic chain through strong N H⋯O hydrogen bonds along b-axis. There is no distinct dielectric anomaly in the temperature-dependent and frequency-dependent dielectric constant curves, suggesting that no phase transition exists within the measured temperature range (120–420 K). The relative displacement of cations and anions, the turned polarization of molecular electric moment and macrocyclic molecule rotator are the main factors to determine the trend of dielectric constant.

Benchmarking quantum chemical methods for the calculation of molecular dipole moments and polarizabilities.

The calculation of molecular electric moments, polarizabilities, and electrostatic potentials is a widespread application of quantum chemistry. Although a range of wave function and density functional theory (DFT) methods have been applied in these calculations, combined with a variety of basis sets, there has not been a comprehensive evaluation of how accurate these methods are. To benchmark the accuracy of these methods, the dipole moments and polarizabilities of a set of 46 molecules were calculated using a broad set of quantum chemical methods and basis sets. Wave function methods Hartree-Fock (HF), second-order Møller-Plesset (MP2), and coupled cluster-singles and doubles (CCSD) were evaluated, along with the PBE, TPSS, TPSSh, PBE0, B3LYP, M06, and B2PLYP DFT functionals. The cc-pVDZ, cc-pVTZ, aug-cc-pVDZ, aug-cc-pVTZ, and Sadlej cc-pVTZ basis sets were tested. The aug-cc-pVDZ, Sadlej cc-pVTZ, and aug-cc-pVTZ basis sets all yield results with comparable accuracy, with the aug-cc-pVTZ calculations being the most accurate. CCSD, MP2, or hybrid DFT methods using the aug-cc-pVTZ basis set are all able to predict dipole moments with RMSD errors in the 0.12-0.13 D range and polarizabilities with RMSD errors in the 0.30-0.38 Å(3) range. Calculations using Hartree-Fock theory systematically overestimated dipole moments and underestimate polarizabilities. The pure DFT functionals included in this study (PBE and TPSS) slightly underestimate dipole moments and overestimate polarizability. Polarization anisotropy and implications for charge fitting are discussed.

Systematic Theoretical Study of Non-nuclear Electron Density Maxima in Some Diatomic Molecules

First, exploratory calculations were performed to investigate the presence of non-nuclear maxima (NNMs) in ground-state electron densities of homonuclear diatomic molecules from hydrogen up to calcium at their equilibrium geometries. In a second stage, only for the cases in which these features were previously detected, a rigorous analysis was carried out by several combinations of theoretical methods and basis sets in order to ensure that they are not only calculation artifacts. Our best results support that Li2, B2, C2, and P2 are molecules that possess true NNMs. A NNM was found in values obtained from the largest basis sets for Na2, but it disappeared at the experimental geometry because optimized bond lengths are significantly inaccurate for this case (deviations of 0.10 Å). Two of these maxima are also observed in Si2 with CCSD and large basis sets, but they are no longer detected as core-valence correlation or multiconfigurational wave functions are taken into account. Therefore, the NNMs in Si2 can be considered unphysical features due to an incomplete treatment of electron correlation. Finally, we show that a NNM is encountered in LiNa, representing the first discovery of such electron density maxima in a heteronuclear diatomic system at its equilibrium geometry, to our knowledge. Some results for LiNa, found in variations in internuclear distances, suggest that molecular electric moments, such as dipole and quadrupole, are sensitive to the presence of NNMs.

Physicochemical Properties of Zwitterionicl- anddl-Alanine Crystals from Their Experimental and Theoretical Charge Densities

The total experimental electron density distributions rho(r) of zwitterionic L- and DL-alanine crystals, as derived from extensive sets of X-ray diffracted intensities collected at 23 and 19 K, are compared to gain an insight into the different physical properties of the two related chiral compounds in the solid state and to explore the extent of the rho(r) transferability. Relevant parameters that characterize the two crystal forms are obtained, showing differences and similarities in terms of (i) geometric descriptors, (ii) topological indexes, (iii) molecular electrostatic potential Phi(r) distributions, (iv) atomic volumes and charges, (v) molecular electric moments, and (vi) electrostatic interaction energies. To assess the relative stability of the racemate with respect to the pure enantiomer, the crystal lattice energies, as obtained through DFT fully periodic calculations, are also discussed and compared with the experimental sublimation enthalpies after correction for the proton-transfer energies. In-crystal group charges, evaluated with the quantum theory of atoms in molecules, are found to be transferable between the racemic and the pure enantiomer, at variance with group volumes. Similarly, molecular first and third moments are not strictly transferable and indicate that for the zwitterionic alanine molecule the molecular charge distribution in the DL-crystal is more polarized in the c direction by about 10%. By contrast, quantitative agreement is observed for second and fourth moments. Significant differences arise from (1) the crystal packing of the dipole vectors, which are aligned in an antiparallel fashion in the L-crystal, to be compared with a parallel alignment in the racemate, due the polar space group Pna21 of the latter, (2) the strongly attractive electrostatic energy of a homochiral pair in the L-crystal, which is opposed to the corresponding heterochiral pair in the DL-crystal form. The difference between these Ees values amounts to 135-150 kJ mol(-1). Despite this, the two crystal forms are predicted as equally thermodynamically favored by the theoretical P-B3LYP estimates of the crystal lattice energies. Finally, the necessity of an upgrading of the dispersion and exchange-repulsion terms currently adopted within the experimental charge density approach to intermolecular interactions is recognized and discussed.

Progress in the Understanding of Drug–Receptor Interactions, Part 2: Experimental and Theoretical Electrostatic Moments and Interaction Energies of an Angiotensin II Receptor Antagonist (C30H30N6O3S)

A combined experimental and theoretical charge density study of an angiotensin II receptor antagonist (1) is presented focusing on electrostatic properties such as atomic charges, molecular electric moments up to the fourth rank and energies of the intermolecular interactions, to gain an insight into the physical nature of the drug-receptor interaction. Electrostatic properties were derived from both the experimental electron density (multipole refinement of X-ray data collected at T=17 K) and the ab initio wavefunction (single molecule and fully periodic calculations at the DFT level). The relevance of SO and SN intramolecular interactions on the activity of 1 is highlighted by using both the crystal and gas-phase geometries and their electrostatic nature is documented by means of QTAIM atomic charges. The derived electrostatic properties are consistent with a nearly spherical electron density distribution, characterised by an intermingling of electropositive and -negative zones rather than by a unique electrophilic region opposed to a nucleophilic area. This makes the first molecular moment scarcely significant and ill-determined, whereas the second moment is large, significant and highly reliable. A comparison between experimental and theoretical components of the third electric moment shows a few discrepancies, whereas the agreement for the fourth electric moment is excellent. The most favourable intermolecular bond is show to be an NHN hydrogen bond with an energy of about 50 kJ mol(-1). Key pharmacophoric features responsible for attractive electrostatic interactions include CHX hydrogen bonds. It is shown that methyl and methylene groups, known to be essential for the biological activity of the drug, provide a significant energetic contribution to the total binding energy. Dispersive interactions are important at the thiophene and at both the phenyl fragments. The experimental estimates of the electrostatic contribution to the intermolecular interaction energies of six molecular pairs, obtained by a new model proposed by Spackman, predict the correct relative electrostatic energies with no exceptions.

Response tensors for chiral discrimination in NMR spectroscopy

Response tensors that may be used for rationalising chiral discrimination by nuclear magnetic resonance spectroscopy are selected relying on the criterion that the induced molecular electric moment and the induced magnetic field at a nucleus are invariant to a translation of the coordinate system.

A Model for the Chemical Bond

A simple model of the chemical bond is developed on the basis of an elementary MO theory requiring solution of at most a 2 × 2 Hückel secular equation including overlap. The fundamental quantities are the atomic integrals α1 and α2 giving the energies of the isolated atoms or ions, and the bond integral b which depends on the overlap S between the interacting AOs. For homonuclear diatomics, the interaction is first order in b, and the bond is formed as long as the number of electrons in bonding MOs is greater than the number of electrons in antibonding MOs. When the number of bonding electrons equals the number of antibonding electrons Pauli repulsion occurs, provided overlap between the interacting AOs is explicitly considered. Model bond energies with |β| « 60 kcal mol–1 and |βπ| « 55 kcal mol–1 reproduce experimental atomization energy results for σ- and π-electrons of most first-row homonuclear diatomics with an average error not exceeding 10%. Bond stereochemistry is introduced in terms of bond energy maximization. Symmetry arguments are used to reduce consideration of CH4 to a set of 2 × 2 secular equations, whose best energy corresponds to the tetrahedral arrangement of four straight CH bonds, explaining in a natural way the formation of four equivalent sp3 hybrids on carbon. Next, electron distribution in a two-electron bond is considered, and atomic, overlap, gross atomic, and formal charges are introduced to explain the origin of molecular electric moments.

Molecular electric moments and electric field gradients from X-ray diffraction data: model studies

Model X-ray data sets, with and without the inclusion of experimental thermal motion parameters, have been computed via Fourier transformation of ab initio molecular electron densities for 12 different molecular crystals. These datasets were then analysed with three different multipole models of varying sophistication and, from the multipole functions, molecular dipole and second moments, as well as electric field gradients (EFG's), at each nuclear site were computed and compared with results obtained from the original ab initio wavefunctions. The results provide valuable insight into the reliability of these properties, extracted in the same way from experimental X-ray data. Not all molecular systems display identical trends, but a general pattern is discernible. Specifically, dipole moments are typically underestimated by a small but significant amount (~ 10–15%), the trace of the second moment tensor is well determined but overestimated by a few per cent and electric field gradients at protons are confirmed to be well within reach of a careful charge density analysis of X-ray diffraction data.

Molecular electric moments from x-ray diffraction data

Molecular electric moments from x-ray diffraction data

Piezoelectric properties of electron-irradiated poly (vinylidene fluoride)

Some of the piezoelectric constants of PVDF are strictly stress dependent. This behaviour is assumed to originate in a contribution of the interphase region between crystalline and amorphous phases to the piezoelectric effect in this material. This assumption is supported by the changes in piezoelectricity after irradiation with fast electrons. Such treatment causes crosslinking predominantly in the amorphous regions and, consequently, significant changes in the mobility of molecular electric moments. The piezoelectric constants become smaller upon irradiation, this being caused by a reduced permanent polarization, by increased mechanical stiffness, and by the reduced reorientability of the molecular electric moments in the interphase. One of the nonlinear piezoelectric coefficients changes its sign at a certain irradiation dose.

The use of “ab initio” net charges to improve Fraga's atom-atom pair potential for molecular association

The assignment of some parameters in an atom-atom pair potential proposed by Fraga is checked through calculations of some azine stacked dimers. So, the use of Mulliken net charges and interpolated polarizabilities instead of the standard Fraga's procedure reveals to be a reliable method to maintain the generality and simplicity of Fraga's potential while yielding improved results. Further improvement is gained if the net charges are scaled taking as a reference some molecular electric moments.

Finite-field many-body perturbation theory. VII. A complete fourth-order MBPT study of multipole moments of the CO molecule

A systematic study of multipole moments of CO molecules is carried out by using the funite-field many-body pertubartion theory. The dipole and quadrupole moment functions are the calculated by the complete fourth-order MBPT schemes and the corresponding correlation corrections are analysed. The basis set dependence of the calculated properties is also investigated. The most accurate fourth-order MBPT results calculated with large, partly optimized basis sets are in fair aggreement with the available experimental data. Owing to the systematic character of the MBPT approach to correlation effects in properties some estimates of the accuracy of the calculated quantities can be obtained. They indicate the limitations of the accuracy of calculation is performed nowadays for molecular electric moments. Estimates of octupole and hexadecapole moments of CO are obtained from HF SCF and SD Cl calculations and their reliability is discussed.

Electric Induction in Molecules and the Polarity of the C—H Bond

FOR non-mesomeric molecules the molecular electric moment may be formally divided into the vector sums of the group moments and of the induced moments. Calculation of these latter has been attempted by Smallwood and Herzfeld1, Smyth2 and Groves and Sugden3; but the number of examples of any one molecular type considered has been too small to substantiate the methods. The calculations of the last-named authors, based on a method suggested by Frank4, appear to be the most valid.

On the surface potentials of unimolecular films of long chain fatty acids. Part III.–The potentials of solid, liquid condensed and double layer films

It was shown in Parts I and II that the vertical component of the mean molecular electric moment, μ¯, calculated with the aid of the Helmholtz equation 4 πn μ¯ = ∆V was dependent on the state of the film. Th experiments of Lyons and Rideal ( loc. cit. ) indicated that the state of a film could be altered not only by means of the usual variables, the two dimensional pressure F, and the temperature T, but also by the extent of the adhesion of the polar group to the substrate, conveniently altered in the case of acids by a change in the p H of the solution. That the change in this adhesion could not be attributed to the ionisation of the acid alone was shown by the fact that great changes in the stability of the film could be made over ranges of p H well outside the range of dissociation; further for identical hydrogen ion activities the effects of different anions possessing various capillary activities were widely diverse, <error>e. g</error>., the surface film on a citrate buffer behaves as if it were relatively more acid, and that on a borate buffer as if it were relatively more alkaline than the bulk phase.

On the surface potentials of unimolecular films of long chain fatty acids. Part II.–The evaluation of the resolved electric moments

In Part I a method was described for the determination of the change in interphase potential at an air liquid interface caused by the presence of a unimolecular film of myristic acid and the values for this change (∆V) were plotted as a function of the molecular area. On application of the Helmholtz equation ∆V = 4 πn μ we obtain a mean value for the vertical component of the molecular electric moment of a molecule of myristic acid when present in a film in the expanded state of μ = 2.29. 10 -19 e. s. u.