Bond Moments Research Articles

Configuration and Conformation of N,N'-Diarylamidines: A Dipole Moment Study

Dipole moments of N,N'-diphenylformamidine and its para-halogen derivatives were measured in benzene solution. Since the prototropic rearrangement is degenerate, the dipole moments can be simply interpreted in terms of the E configuration on C=N and a conformational equilibrium on C-N. The two forms, Eap and Esp are populated approximately in the 1 : 1 ratio: the former is the conformation found in the crystal. The dipole moments anticipated for individual conformations were calculated by vectoraddition of bond moments: the previously determined correction for conjugation was found unnecessary.

Calculation and measurement of the dipole moment of small proteins: Use of protein data base

The dipole moments of small protein molecules were determined experimentally in order to validate the calculated dipole moments by previous investigators. We found that the agreements are satisfactory for some proteins. There are, however, many proteins for which the agreement is less than satisfactory. In order to find the cause of the disagreement, the dipole moments of these proteins were recalculated using the Brookhaven Protein Data Bank. We calculated the dipole moment due to fixed surface charges and the bond moments of all the carbonyl groups in main chain and side chains. The calculation consists of the mean moments and their mean square fluctuations. In addition, we investigated the effect of electrostatic interactions between charged sites for several proteins. These results show that incorporation of the interactions does not affect substantially the calculated dipole moments. The rms fluctuation of the dipole moment is found to be small but not negligible. In conclusion, recalculated dipole moments are in good agreement with the observed values.

2,4-Substituted-3-aza-1-thiabutadienes: a conformational study by dipolmetry

Dipole moments of fifteen 2,4-substituted4- N,N-dimethylamino-3-aza-1-thia(oxa)butadienes were measured in carbon tetrachloride solution. Calculations of molecular moments for five compounds in the sp (CS and NC on the same side) and ap conformations were performed according to the bond moment model. A comparison of calculated values with the experimental ones shows that the sp conformation is favoured in the solutions studied, in accordance with the reactivity observed for 4+2 cycloadditions.

CO2 laser Stark spectroscopy of the ν4 band of SiHF3: The C rotational constant and vibrationally induced dipole moment

The ν4 (SiF3 degenerate stretch) fundamental band of SiHF3 has been investigated by the CO2 laser Stark spectroscopy. The vibrationally induced dipole moment was found to give remarkable perturbations of the type (Δl,ΔK)=(2,−1) in the ν4 vibrational state under intense electric field. Several perturbation-induced rovibrational transitions obeying the Δ(K−l)=±3 selection rules were observed in the Stark spectrum. The C0 rotational constant, which is otherwise hard to obtain for a symmetric top molecule, was determined to be 4134.73(71) MHz. The vibrationally induced dipole moment μr in the ν4 state was derived as ±0.034 38(17) D, the sign being experimentally indeterminable. The band origin, the Coriolis constant Cζ4, and the vibration–rotation constant αC4 are 999.581 076(30) cm−1, 2445.59(71) MHz, and 9.243(22) MHz, respectively. The dipole moment is 1.359 20(22) D in the ground state, and increases by 0.026 73(10) D on excitation of the ν4 vibration. The uncertainties given in the parentheses correspond to three standard deviations. The present results are compared with our previous results of a similar analysis for the deuterated species. The dipole moment in the ground vibrational state decreases by 0.002 01(28) D on deuteration. The vibrationally induced dipole moments μr and the vibrational changes δμ4 of the dipole moment for both isotropic species are discussed theoretically. We introduce the coefficient of the mean square amplitude dependence for the bond moment, which is useful in the discussion.

Molecular geometries and heats of formation of C60 and C70 as computed by MM2‐87

AbstractMM2‐87 calculations have been performed on C60 (buckminsterfullerence; footballene) and C70 with full energy minimization. The steric energies for C60 and C70 were computed to be 179.9 and 177.9 kcal/mol, respectively. The heats of formation for C60 is found to be more stable than C60. The two bond lengths for C60 were computed to be 1.393 and 1.444 Å. Eight different bond lengths were found for C70 ranging from 1.386 to 1.452 Å. Bond angles, dihedral angles, and moments of inertia are also reported for the compounds.

DIPOLE MOMENTS OF SOME PHENYL AND CHLOROPHENYL PHOSPHORUS-HALOGEN COMPOUNDS

Abstract The electric dipole moments of o-ClC6H4PCl2 1, C6H5PF4 2, o-ClC6H4PF4 3, C6H5P(O)F2 4, C6H5P(O)HF 5. C6H5P(O)H(OH) 6, and o-ClC6H4P(O)Cl2 7, were determined in dilute benzene solution at 25°C. The phosphoryl bond moment PO in compounds 4, 5, and 7 was estimated using simple bond-additivity calculations. The results obtained were 4.55D, 3.64D, and 5.52D for compounds 4, 5, and 7, respectively. Dipole moments for compounds 2 and 3 were also calculated theoretically using INDO MO techniques. The calculated values differed from the experimental values by about 12%.

Mesomeric dipole moments. Part 12

The dipole moments of para-substituted azoxybenzenes (1, a-e) and phenylazoxycyanides (2, a-e) were analyzed in terms of bond moments and additional contributions expressing electron delocalization. The results are compatible with a significant participation of the polar mesomeric formula B (25% or 34% for1 and2, respectively), the N-O bond acquiring considerable double-bond character. Comparison of known x-ray structures reveals that this bond is significantly shortened, whereas the N=N bond is extended only very slightly. Hence we can consider the possibility that the N1 atom allocates somewhat more than eight valence electrons, similarly to nitro compounds. In all respects the properties of the azoxy group are between those of the nitro and imino-N-oxide groups.

Dipole moments and electron distribution of acyl chlorides and acyl bromides

AbstractDipole moments of para‐substituted benzoyl chlorides and bromides, partly measured in benzene and partly recalculated from elderly literature, were analysed in terms of bond moments and additional contributions expressing the conjugation. Although the gross moments are similar, their decomposition yields different pictures for acyl chlorides and bromides. Only in the latter can significant electron transfer from the halogen be observed. On the other hand, conjugation with the benzene nucleus is strong in both aromatic acyl chlorides and bromides, but stronger in chlorides. There is agreement with the evidence from some other physical quantities but not from all.

Molecular mechanics and molecular shape: Part VII. Structural factors in the estimation of solvation energies

Pathways are proposed to facilitate the estimation of parameters that enter the computation of solvation energies: the cavity radius and the set of bond moments. The radius can be estimated in ways similar to the estimation of molecular van der Waals radii, except that the atomic radii are different. Bond moments can be obtained by Del Re's procedure of charge distribution, since it works through the intermediacy of bond charges. If the procedure is modified to include “pseudobonds”, bond moments can be estimated for molecules in an aggregate. These modes of estimation were tried in the context of Abraham's solvation energies. Three test-cases, which could not be tackled by previous versions of the method, are described.

Infrared intensities of liquids. 5. Optical and dielectric constants, integrated intensities, and dipole moment derivatives of water and water-d2 at 22.degree.C

The authors have recorded multiple attenuated total reflection spectra of liquid H{sub 2}O and D{sub 2}O, using the Spectra-Tech CIRCLE cell, and calculated from them the infrared optical and dielectric constants and molar conductivities from 9000 to 1250 cm{sup {minus}1} for H{sub 2}O and from 8500 to 700 cm{sup {minus}1} for D{sub 2}O. Our results agree well with the literature for H{sub 2}O, while our results for D{sub 2}O are the most extensive reported to date. We have calculated the dipole moment derivatives with respect to stretching and bending internal coordinates from the areas under the bands in our molar conductivity spectra. For lack of information, we have used the assumptions of the simple bond-moment model, a diagonal force field, and neglect of stretch-bend interaction. We found {partial derivative}{mu}/{partial derivative}r for the stretching vibrations to be 3.02 D/{angstrom} {plus minus} 1% and 3.04 D/{angstrom} {plus minus} 0.5% for H{sub 2}O and D{sub 2}O, respectively, and {partial derivative}{mu}/{partial derivative}{theta} for the bending vibration to be 0.73 D {plus minus} 3% for H{sub 2}O and 0.63 D {plus minus} 5% for D{sub 2}O. The two values for the stretching vibrations are indistinguishable, but this is not true for the bend. The disagreementmore » for the bending vibration is probably due, at least in part, to our simulation of the absorption by three distinct bands of mixed Gauss-Lorentzian character, in order to try to separate the bending mode from the background absorption. It is probable that no such separation exists precisely. The bond moments for H{sub 2}O and D{sub 2}O agree with those calculated by the same approximations form literature data for HDO to about the extent allowed by the approximations.« less

Ferroelectric liquid crystals with large spontaneous polarization and high speed display devices

Abstract Ferroelectric liquid crystals with large spontaneous polarization exceeding 10-7 C/cm2 prepared by taking the following design factors into consideration are discussed; (a) separation between the chiral carbon and the dipole moment, (b) bulky molecular group connected to the chiral carbon, (c) introduction of large bond moments, (d) orientation of many bond moments around chiral carbon, (e) dipole moment between the core and chiral carbon, (f) the dipole moment at the core, and (g) the effect of hydrogen bonds. Anomalous dielectric behavior in these new ferroelectric liquid crystals and high speed response times of less than several tens of μs in electro-optic cells have also been discussed.

Magnitude and direction of the spontaneous polarization of ferroelectric liquid crystals with several bond moments

Abstract The magnitude and direction of the spontaneous polarization in most ferroelectric liquid crystals have been confirmed to be determined by the location and magnitude of the bond moment around the chiral carbon and the core. In compounds with several bond moments their relative orientation is very important for obtaining a large spontaneous polarization. Compounds with benzene rings in the core substituted with OH also have a large spontaneous polarization, perhaps due to the formation of hydrogen bonds. Reversal of the direction of the spontaneous polarization with temperature has been found for EFPPOPB. This anomalous behaviour has been explained tentatively in terms of a conformation change due to the existence of a flexible -CH2- unit between the chiral carbon and the dipole moment.

Electrostatic bond dipole moments from dimethyl ether, methanol, methane and water

Bond dipole moments are derived for the O—H, C—H, and C—O bonds from ab initio calculated and experimentally determined electrostatic multipoles of dimethyl ether, methanol, methane and water, using an electrostatic model with point dipoles placed at the midpoints of the bonds between atoms and directed along the bonds. Multipole moments up to and including L= 6 moments for each of the molecules are used. It has been found that the model leads only to a fair description of the molecular potential. Owing to the large correlation between the carbon–oxygen bond moments and the oxygen–lone pair moments the latter could be omitted from the model without affecting the quality of the fits.

Study of dipole moments of phenylsulphonyl isothiocyanates

The dipole moments of 4-substituted phenylsulphonyl isothiocyanates have been measured in benzene at 20 °C. The steric arrangement of SO2NCS group is discussed on the basis of comparison of the dipole moment values measured with those calculated by vectorial addition of bond moments for the conformers formed by rotation around the S-N bond.

MMPEP: Development and evaluation of peptide parameters for Allinger's MMP2(85) programme, including calculations on crambin and insulin

Allinger's MMP2(85) program has been converted to an IBM environment, and the dimensions expanded to a current maximum of 999 atoms. Substantial additional expansion will be possible. An all-atom set of parameters, which permit Allinger's comprehensive force field to be applied to the molecular mechanics treatment of peptides, has been determined. These parameters, termed MMPEP, contain 21 atom types: 5 for carbon, 6 for hydrogen, 5 for nitrogen, 4 for oxygen, and 1 for sulfur, and are based on crystallographic heavy atom bond lengths and bond angles, vibrational and microwave spectra, and ab initio calculations. To minimize the conformational energy of a peptide from an initial starting geometry, all internally stored parameters are released, and replaced by PEPCON, a 360-line external file containing the MMPEP parameters.The ability of the MMPEP parameterization of MM85 to reproduce experimental crystal structures has been tested on several peptides and polypeptides, and the use of a dielectric constant ε = 78.5 D leads to the following results: Ala-Ala-Gly (rms = 0.261); Gly-Gly-Val (rms = 0.349); glutathione (rms = 0.417); crambin (327 heavy atoms; rms = 0.310 for all heavy atoms); insulin (389 heavy atoms; rms = 0.646 for all heavy atoms); the origins of deviations can be interpreted. No problems have been encountered in the application of the Newton–Raphson minimization procedure to such large molecules as crambin and insulin, even though all possible nonbonded interactions have been retained. On the IBM 3081 computer, real time minimization of trip)eptides requires 1–2 min, crambin requires 250 min, and insulin 200 min. Since hydrogen bonding in Allinger's force field is a natural result of electrostatic and van der Waals interactions, in MMPEP hydrogen bonding is taken into account through the large number of hydrogen atom types and their different bond moments and van der Waals radii.

Effect of Molecular Structure of Core on Ferroelectricity in Ferroelectric Liquid Crystals

By using various couples of ferroelectric liquid crystals with molecular structures which were only different in the core center in each couple, the effect of the bond moment C=O at the center of core on ferroelectricity could be studied. The bond moment C=O at the center of the core was found to enhance the spontaneous polarization except for one case.

Microwave spectrum and electric dipole moment of trifluoroethylene oxide

The rotational spectrum of the normal isotopic species of trifluoroethylene oxide, CHFCF 2O , was fit to a centrifugal distortion model giving rotational constants (MHz) of A = 6669.565(2), B = 3126.567(1), and C = 2652.440(1). The electric dipole moment (debyes) was determined from Stark effect measurements to be μ a = 0.402(1), μ b = 1.046(38), and μ c = 1.349(9). The experimental rotational constants are consistent with a proposed molecular structure derived from an additivity principle based upon ab initio calculations and experimental structures of related fluorinated oxiranes. Electric dipole components of trifluoroethylene oxide calculated from the proposed molecular structure and previously determined CO, CF, and HC bond moments are in reasonable agreement with the experimental values.

Dependence of spontaneous polarization on orientation and position of several bond moments near chiral parts in ferroelectric liquid crystals

Abstract New series of ferroelectric liquid crystals with several bond moments near the chiral part have been prepared. The position and the orientation of those bond moments are found to strongly influence the magnitude of the spontaneous polarization in these ferroelectric liquid crystals, and the spontaneous polarization as high as 240 nC/cm2 has been obtained.

On the additivity of bond dipole moment changes arising from bond distortions

The concept of bond moments and derivatives in the context of vibrational transition moments has been examined by calculations on methylene fluoride and ethene at the 3–21 G* level and N, N-dimethylformamide at the 3–21 G level. It is shown that transition moments on stretching and bending lie up to 15° off the bond axis and perpendiculars. Nearest atom non-bonded interactions appear to be responsible for these deviations in that as bonds stretch the moments swing in the direction expected for a reduced spatial interaction. It is proposed that as more data is accumulated it may be possible to establish the magnitude of these interaction moments in any defined system and hence to improve considerably predictive capabilities for large systems without the expensive procedure of ab initio calculations.

On the CH bond dipole moment in alkanes

The Thomas–Reiche–Kuhn sum rule, within the acceleration gauge for the transition dipole moment, is used to partition the total number of electrons in a molecule and to define atomic populations, which can be related to corresponding experimental estimates of atomic polar tensors from IR intensities. As a bond dipole moment can only be defined for diagonal atomic polar tensors, it is shown that the assumption of a C–H bond moment, transferable from molecule to molecule in the alkane series, is physically unreliable. From experimental IR intensities of methane we infer that, for the equilibrium geometry, μC–H =0.339 D, directed C−H+. Accordingly, it is argued that the theoretical bond dipole moments, estimated for the C–H bond in methane via localization procedures of SCF wave functions, are questionable, as they predict the opposite polarity. Finally, a resolution of the electric dipole moment into atomic contributions is suggested.