Theoretical Dipole Moment Research Articles

Torsional isomerization of biologically active bicyclic molecules

AbstractRotational isomerization of bipyridines C5H4NC5H4N was studied by CNDO/2, PPP/CI, and CNDO/CI methods. It is shown that CNDO/2 overestimates the angle of rotation ϕ between the pyridine rings ca. two times. The angle ϕ was determined for 2,2′‐bipyridine by means of correlation of the theoretical (CNDO/2) and experimental dipole moment. It was also found from the correlation between the theoretical and experimental UV spectra. It is shown that there is an explicit dependence of the results upon the distance between heteroatoms (PPP/CI). It has been found that the CNDO/CI method correctly predicts the value of the rotational angles and their sequence in bipyridines.

Ab initio theory of the polarizability and polarizability derivatives in H 2S

Ab initio finite-perturbation self-consistent-field (SCF) and configuration interaction (CI) calculations utilizing an extensive basis of cartesian gaussian orbitals are reported for the electric dipole polarizability tensor, α, and the polarizability derivative tensor, α′, of H 2S. Particular attention was devoted to the selection of an appropriate molecular orbital basis and configuration expansion for determining electron correlation contributions to the polarizability. The common practice of truncating configuration expansions based on a perturbation theory correction to the energy is shown to be inadequate for this property. Contrary to the predictions of such an approach, we conclude that electron correlation has little effect on α and α′ in H 2S. Our calculation yielded a nearly isotropic polarizability tensor, whose trace, α , is ca. 5% below the experimental result. The magnitude of the derivative ( α ′) along the symmetric stretch is also in good agreementwith experiment. It is positive and roughly an order of magnitude larger than the components along the other two modes. The derivative anisotropy parameter is found to be positive for all three normal modes. Theoretical dipole moment derivatives have also been evaluated and are compared to those determined from the infrared spectrum. Recent data from Rayleigh and Raman scattering experiments on H 2O are discussed in light of our results for H 2S.

Dipole moments. Structure and conformation of asymmetrically 4,5‐substituted 1‐(α‐aroyloxyarylideneamino)‐1,2,3‐triazoles

AbstractThe dipole moments of several asymmetrically 4,5‐substituted 1‐(α‐aroyloxyarilideneamino)‐1,2,3‐triazoles, produced by oxidation of bis‐aroylhydrazones of α‐dicarbonyl compounds, were measured in benzene solution. The structure and the probable conformation of each of these compounds was determined by comparing their experimental and theoretical dipole moments. It has been found that when one of the substituents R1 or R2 is an aryl group, it is located at the 4‐position of the triazole ring. Considering the conformation of the ester‐imino function, it has been found to be the same as that of symmetrically substituted derivatives.

Theoretical study of the X1Σ+, A1Π, C1Σ− and E1Σ+ states fo the SiO molecule

The self-consistent-field plus configuration-interaction method has been used to compute potential energy curves and certain one-electron properties for the X1Σ+, C1Σ−, A1Π, and E1Σ+ states of SiO. This study employed a basis consisting of 51 Slater-type orbitals which is an expanded version of the one reported by McLean and Yoshimine. The computed ground-state dissociation energy (De) of 8.1 eV is in excellent agreement with the experimental value of 8.26±0.13 eV. The theoretical ground-state electric dipole moment function is in good agreement with the experimental curve constructed from the microwave data for the v=0–3 vibrational levels. Einstein A coefficients for vibration–rotation transitions computed from existing theoretical and experimental data are in good agreement. The E1Σ+ state is shown to dissociate adiabatically to ground-state atoms over a potential barrier with a maximum near 5 bohr. Calculated transition probabilities and radiative lifetimes for the A1Π–X1Σ+ and E1Σ+–X1Σ+ band systems agree well with recent laboratory experiments. Absorption cross sections as a function of wavelength have been computed and used to determine the opacity of SiO boundary layers that will form on the surface of probe vehicles entering the Jovian atmosphere at high speeds. These calculations demonstrate that the brilliant shock layer emission will be significantly absorbed by the SiO A1Π–X1Σ+ and SiO E1Σ+–X1Σ+ band systems in the boundary layer in the spectral region between 170 and 230 nm.

Theoretical dipole moment derivatives and force constants for HCN

Valence configuration interaction wavefunctions are used to calculate dipole moment derivatives and force constants for linear hydrogen cyanide. The magnitude of the dipole moment is found to decrease when the C–N bond is stretched from its equilibrium value. The implications for hydrogen bonding in crystalline HCN are discussed. Self-consistent field calculations are used to confirm that the SCF dipole derivative with respect to the CN bond length has the wrong sign and to explore the sensitivity of the dipole derivatives to the choice of atomic basis set.

Theoretical study of molecular dipole moment functions. II. The d 3Δ and a′ 3Σ+ states of CO

Approximate dipole moment functions for the d 3Δ and a′ 3Σ+ states of CO have been calculated using configuration interaction wavefunctions including internal and semi-internal electronic correlation effects. The best theoretical dipole moments for individual vibrational levels are in good agreement with experimental data. The theoretical dipole moment functions are of higher accuracy than those derivable from existing experimental data.

On the sign anomaly in the experimental and theoretical dipole moment derivative of HCN

On the sign anomaly in the experimental and theoretical dipole moment derivative of HCN

The influence of intracollisional interference on the dipole spectrum of HD

The influence of intracollisional interference on the dipole spectrum of pure HD is investigated within the framework of an adiabatic separation of the relative translational motion and non-mixing of rotational levels. The effect leads to a density dependence of the integrated intensities for the sharp spectral lines. It is found that the large discrepancy between the theoretical and experimental pure rotational dipole moment matrix elements for HD reported in the literature can be interpreted as destructive interference between the allowed and the collision-induced dipoles. Similar calculations for the fundamental band are in excellent agreement with the experimental results for the R1(1) line, but agree less well for the R1(0) line. We interpret this, as well as the asymmetrical line shape, as evidence of rotational level mixing due to anisotropic intermolecular forces.

Theoretical intensity parameters for the vibration–rotation bands of ClO

Intensity parameters for the vibration-rotation bands of ClO were calculated by evaluating matrix elements of the vibrational eigenfunctions using the theoretical dipole moment curve. These vibrational eigenfunctions are based on realistic Klein-Dunham potentials deduced from experimental values (Coxon et al, 1976) for certain vibrational energy levels. The data can be applied to the interpretation of remote or in situ experiments to deduce stratospheric ClO concentrations from vibration-rotation absorption spectra.

Étude structurale de spiropyrannes benzothiazoliniques par détermination expérimentale et théorique des moments dipolaires

For the conformational study of a series of benzothiazolinic spiropyrans, experimental and theoretical (E.H.T. method) dipole moments have been checked. The energetic data of the semi-empirical calculations for the eight possible spiropyrannic conformations (four limit conformations for each enantiomer) and the comparison between experimental and calculated dipole moments fit quite well, and allow an interesting approach to the conformation of these molecules, in spite of their complexity.

Theoretical study of molecular dipole moment functions. I. The X 1Σ+ state of CO

Approximate dipole moment functions for the X 1Σ+ state of CO have been calculated using several configuration interaction wavefunctions. The best theoretical results obtained for the dipole derivatives at Re and vibrational transition matrix elements are found to be in quantitative agreement with available experimental data. The theoretical dipole moment functions, in contrast to empirical functions, possess correct long range behavior. Through a detailed comparison of different approximate wavefunctions, useful information has been obtained on the relative importance of different types of correlation effects on the accurate determination of molecular dipole moment functions.

Steric effects in 5-membered rings—VIII : A conformational analytical study of cis- and trans-1,3-dichlorocyclopentane

A study of the title systems was performed using theoretical calculations, dipole moments (experimental and calculated) and NMR spectroscopy. The results were discussed in the light of literature data on related systems. It was concluded that a multitude of conformations occur in a potential well for the cis-isomer and that for the trans-one, an even flatter and more variegated distribution in a double minimum is indicated.

Studies in the chemistry of erythrina alkaloid derivatives—III : Two isomers of 2,3,4,4a,5,6,7,7a,8,9,10,11-dodecahydro-2,10-dioxo-1 H-benzo[d]naphthalene

Diketone 2,3,4,4a,5,6,7,7a,8,9,10,11-dodecahydro-2,10-dioxo-1 H-benzo[d]-naphthalene prepared by annulation of N-(1-cyclohexenyl)pyrrolidine with vinyl methyl ketone, was found to consist of two stereoisomers. Theoretical dipole moments of these were calculated, and their structures were shown by moment dipole measurements to be cis A/B, cis A/C and trans A/B, cis A/C, respectively. The isomer trans A/B, cis A/B was converted into trans A/B, cis A/C dodecahydro-1 H-benzo[d]naphthalene by the Wolff-Kishner reduction.

Barrier to rotation and conformation of the -NR 2 group in cytosine and its derivatives. Part II. Experimental and theoretical dipole moments of methylated cytosines

The dipole moments of several cytosine, methylaminocytosine and dimethylaminocytosine derivatives with and without an ortho methyl group were determined experimentally in dioxane and benzene. Calculations of total energies and dipole moments were performed by the CNDO/2 and INDO methods for sp 2 and sp 3 hybridization of exocyclic nitrogen for different values of rotational angle ϕ C-N. Comparison of the experimental dipole moments with those calculated for the energy minima suggests that the conformation of the dimethylamino group is not planar and differs from that found in cytosine. 1,5,7-Trimethylcytosine, with the dipole moment of 7 Debye units, was considered to be the model compound which closely reproduces the dipole moment of cytosine.

Studies of polarized ethylenes.: PART VII. experimental and theoretical dipole moments and electronic structures by CNDO/2 calculations

The dipole moments of a number of representative “push-pull” ethylenes have been calculated by the CNDO/2 method, and a comparison is made with experimental dipole moments, which are in general surprisingly well reproduced. An analysis of the calculated charge distributions shows that the π-electron displacements are in agreement with qualitative expectations, but no back-polarization of the σ-electrons can be observed. Instead, the distribution of the σ-electrons seems to be governed by atom electronegativities. The experimental dipole moments of some more or less flexible analogues are discussed in relation to conformations and expected charge distributions.

Study of the ground state potential curve and dipole moment of OH by the method of optimized valence configurations

Accurate theoretical potential and dipole moment curves are presented for the X2Πi state of the hydroxyl radical. The theoretically determined dissociation energy is 4.53 eV as compared to the experimental value of 4.63 eV. The computed dipole moment at the experimental equilibrium internuclear separation is 1.674 D, which is in excellent agreement with the most reliable experimental value of 1.66±0.01 D. A detailed, general prescription for constructing optimized valence configuration wavefunctions for diatomic hydrides is presented with OH as a specific example.

Electronic structure and bonding of allyllithium

The results of CNDO/2 calculations have been used to determine the equilibrium geometry for allyllithium which is predicted to be a non-planar “bridge” structure. The lowest energy configuration (LEC) is described in molecular orbital terms which stress the importance of the lithium's position in permitting it to utilize all its valence orbitals in bonding with the allyl moiety. Charge densities, the theoretical dipole moment, and variations of configurations are discussed in relation to the spectroscopic measurements of allyllithium. Utilizing the geometry of the allyllithium monomer (LEC), a proposed dimer configuration is reported which is in feasible agreement with the experimental aggregation properties. The solvent interactions were considered by utilizing two molecules of ammonia as the electron donor with allyllithium. The results of the calculations predict that the solvation of the lithium by ammonia increases the allyllithium fragment separation with concomitant changes in the electron densities and bonding populations.

High Field Stark Effects in CH and NH

A LoSurdo discharge was used to apply electric fields of up to 290 kV/cm to a mixture of cyanogen and hydrogen, and to ammonia, permitting the observation of Stark effects in the optical spectra of the CH and NH molecules, respectively. These experiments yielded the following values of the molecular electric dipole moment in the ground vibrational states: μ(CH, A2Δ) = 0.887 ± 0.045 D, and μ(NH, X3Σ) = 1.389 ± 0.075 D. A table of experimental and theoretical dipole moments of first-row hydrides is included.

Comparison between Theoretical and Experimental Electric Dipole Moments of Selected N,N-Dimethylaniline Derivatives

Electric dipole moments of nine N, N-dimethylaniline derivatives were determined experimentally in benzene solution according to literature methods. The theoretical dipole moments of these compounds were calculated from molecular orbital approximations according to the Del Re σ-electron treatment, the Hückel π-electron method, the Pariser-Parr-Pople treatment, the complete neglect of differential overlap method, and selected combinations of these methods. Parameters not previously reported were determined for the nitro group by reproduction of experimental dipole moments for use in the Del Re calculations. Comparisons between the experimental and theoretical moments indicate that the combination of the Del Re σ-electron and Hückel π-electron methods gave the best reproduction of the experimental dipole moment values while being considerably less expensive in terms of computer size and time, as well as requiring less time in preparing the data than more sophisticated methods examined.

Conformation of non‐aromatic ring compounds ‐ 84 Dipole moments and 100 MHz Fourier‐transform Proton Magnetic Resonance spectra of some substituted 3‐phenylglutaric anhydrides; a conformational analysis

AbstractThe dipole moments and PMR spectra of a series of 3‐(p‐X‐phenyl)glutaric anhydrides [X: H (2), F (3), Cl (4), Br (5), NO2 (6)] and of 3‐(p‐chlorophenyl)‐3‐methylglutaric anhydride (7) were measured and interpreted in terms of conformational properties of the anhydride ring and its side‐chain. The dipole moments of 2‐7 (measured in benzene) were 4.73, 3.82, 3.66, 3.70, 2.53 and 4.59 Debyes, respectively. Theoretical dipole moments for the individual conformers were calculated from atomic coordinates of model structures and standard point charges. The equilibrium compositions derived from vicinal proton coupling constants are compared with those obtained from the dipole moments. A satisfactory agreement is noted. The p‐X‐C6H4 group in 2‐6 is shown to prefer the equatorial Epl conformation (Ph//βH) over the axial Apr conformation (Ph // βH) to the extent of 0.6‐1.2 kcal/mol. In compound 7 the equatorial Ph group probably takes up the Epr conformation.