Influence of the chemical composition and comonomer distribution on the dipole moments of 3,3-dimethyloxetane/tetrahydrofuran copolymers

Experimental values of the dipole moment ratio 〈μ₂〉/nm² and its temperature coefficient d ln 〈μ₂〉²/dT, where 〈μ₂〉 is the mean-square dipole moment of a chain with n skeletal bonds and m² is the mean square of the skeletal bond dipole moments, are reported for well-characterized random copolymers of 3,3-dimethyloxetane and tetrahydrofuran. The results are interpreted in terms of the rotational isomeric state theory in a manner consistent with that developed for the parent homopolymers. The theory gives a good account of the experimental results corresponding to copolymers in which the mole fraction of tetrahydrofuran lies in the range 0.11–0.89. It is found that whenever the copolymerization obeys Bernouillan statistics, the dipole moments are quite insensitive to the comonomer distribution. The theoretical analysis suggests, however, that the value of the dipole moment ratio of alternating copolymers of 3,3-dimethyloxetane and tetrehydrofuran should be near that of the parent homopolymer of lower polarity.     

Polarity and Polarisability of 3,3-Dimethyl-1-phosphabutyne

Abstract

The progress attained in the field of onecoordinated phosphorus compounds was summerised recently in review (Usp. Chim., 1985, v. 54, p. 418). Formerly we have been analysed (Izv.Akad.Nauk USSR, Ser.chim., 1984, p. 415) the polarity of P°C triple bond in phosphaalkynes on the grounds of literary date on dipole moments defined by microwave spectroscopy. In this work the dipole moment of 3,3-dimethyl-1-phosphabutyne (I) was determined in cyclohexan^e solution: μ_exp.=1,24±0,05D, α=1,250, γ=0,078, P_o=31,876 cm³. Analysis of all known up to date experimental date on polarity of phosphaalkynes brings us to the conclusion about small polarity of P°C triple bond and slight sensitivity of this value (0,7±0,2D towards carbon atom) to vareing of substituent at C_sp-atom. We can only note slight tendency of the increasing m(P≡C) in the phosphaalkynes RC?P (R=H, CH₃, F, CH=CH₂, CN, ^tBu) with the growth of -I - effect of substituent R.     

Theoretical study of solvent effects on the structures and isomerization of silylenoid H<Subscript>2</Subscript>SiLiF

The structures and isomerization of silylenoid H₂SiLiF are investigated by density functional theory (DFT) at B3LYP/6-311+G(d,p) level. The solvent effects are modeled using the self-consistent reaction field (SCRF) method with Tomasi’s polarized continuum model (PCM). Five representative solvents, i.e. benzene, ether, THF, acetone, and DMSO, are chosen for this work. Special attention is paid to THF solvent. The results indicate that the polarity of solvents has played an important role on the structures and relative stabilities of different isomers. Contrary to the three-membered ring structure 1, the relative stability of the “classical” tetrahedron structure 4 increases with increasing dielectric constants (ε) of solvents. The σ-complex isomer 3 is most unstable structure. Although the relative energies are reduced with increasing dielectric constants (ε) of solvents, the p-complex structure 2 still has the lowest energy. The effects of solvents on the dipole moments and charge distributions are also discussed.     

Biophysical chemistry of macrocycles for drug delivery: a theoretical study

Ab initio (RHF/STO-3G) quantum chemical calculations using the self-consistent reaction field (SCRF) model were carried out to analyze the effect of solvent polarity on the relative Gibbs free energies, the dipole moments, and the structural stability of peptide macrocycles based on unsubstituted cyclo[Gly₆] and its trisubstituted derivatives containing Me, NH₂, or OH groups at the Cα atom. The macrocycles studied are stable in water at both room temperature and at body fever temperature, which is important for the design of a stable nanovehicle for drug delivery in water.     

Laser control of potential energy surface crossing in ion-molecule reactions application to Na+ + CH4

Laser ion-molecule reaction interaction through dipole moments leads to potential energy sur-face crossings. We will show here by using gauge representation (electric field gauge) that we can induce laser promoted surface crossing in the region (2 a.u.) where the dipole moment changes sign. We illustrate such effects for the NaH + CH₃⁺ ↔ Na⁺ + CH₄ reaction which takes the form of inverted Morse potential (without a barrier) using ab initio methods for calculating the reaction path and the permanent dipole moment of this ion-molecule reaction.     

Electric dipole polarity of diatomic molecules

The restricted SCF (single-configuration SCF) and MCSCF (multiconfiguration SCF) calculations are performed to compute the ground-state electric dipole moments of four pairs of diatomic molecules—(1) CO and BF; (2) SiO and AlF; (3) CS and BCl; and (4) SiS and AlCl—at a number of internuclear distances on both sides of the equilibrium position. Near Hartree–Fock accuracy is obtained in the SCF calculations. All eight molecules have a range of internuclear distance in which electric dipole moments are of the polarity of A^?B⁺. The shapes of computed electric dipole moment functions are discussed in the language of the molecular orbital method and in relationship to electronegativities of atoms. The present study gives us deeper understanding of electron transfer inside molecules and consequently of the apparent contradiction between electronegativity and the dipole polarity of some molecules. © John Wiley & Sons, Inc.     

A Valence Bond Study of the Low‐Lying States of the NF Molecule

The electronic structures of the three lowest‐lying states of NF are investigated by means of modern valence bond (VB) methods such as the VB self‐consistent field (VBSCF), breathing orbital VB (BOVB), and VB configuration interaction (VBCI) methods. The wave functions for the three states are expressed in terms of 9–12 VB structures, which can be further condensed into three or four classical Lewis structures, whose weights are quantitatively estimated. Despite the compactness of the wave functions, the BOVB and VBCI methods reproduce the spectroscopic properties and dipole moments of the three states well, in good agreement with previous computational studies and experimental values. By analogy to the isoelectronic O₂ molecule, the ground state ³Σ^? possesses both a σ bond and 3‐electron π bonds. However, here the polar σ bond contributes the most to the overall bonding. It is augmented by a fractional (19 %) contribution of three‐electron π bonding that arises from π charge transfer from fluorine to nitrogen. In the singlet ¹Δ and ¹Σ⁺ excited states the π‐bonding component is classically covalent, and it contributes 28 % and 37 % to the overall bonding picture for the two states, respectively. The resonance energies are calculated and reveal that π bonding contributes at least 24, 35 and 42 kcal mol^?1 to the total bonding energies of the ³Σ^?, ¹Δ and ¹Σ⁺ states, respectively. Some unusual properties of the NF molecule, like the equilibrium distance shortening and bonding energy increasing upon excitation, the counterintuitive values of the dipole moments and the reversal of the dipole moments as the bond is stretched, are interpreted in the light of the simple valence bond picture. The overall polarity of the molecule is very small in the ground state, and is opposite to the relative electronegativity of N vs F in the singlet excited states. The values of the dipole moments in the three states are quantitatively accounted for by the calculated weights of the VB structures.     

Theoretical Infrared Radiative Lifetimes for CO a3∏

INDO self-consistent-field method was employed to calculate the potential energy and dipole moment functions for the excited a³∏ state of CO. Vibrationally averaged dipole moments and infrared radiative lifetimes were then obtained from the dipole moment function and vibrational wave functions generated by solving numerically the Schrödinger equation for nuclear motions. The calculated dipole moment is 1.468 (expt'I 1.375 D) for ν=0, and decreases with increasing ν, as found experimentally. Calculated infrared radiative lifetimes, with experimental results in parentheses, are 13.5 (17.3, 19.0±5.9), 7.3 (7.8, 13.1±2.9), and 5.0 (4.7, 5.6±1.0) msec, respectively, for ν=1, 2, and 3. The polarity of calculated dipole moment is C⁺O^?, differing from that for the ground X¹∑⁺ state. The origin of this difference is found to be due to the delocalization of the 5a orbital in the a³∏ state.     

Single‐Molecule Spin Switch Based on Voltage‐Triggered Distortion of the Coordination Sphere

Here, we report on a new single‐molecule‐switching concept based on the coordination‐sphere‐dependent spin state of Fe^II species. The perpendicular arrangement of two terpyridine (tpy) ligands within heteroleptic complexes is distorted by the applied electric field. Whereas one ligand fixes the complex in the junction, the second one exhibits an intrinsic dipole moment which senses the E field and causes the distortion of the Fe^II coordination sphere triggering the alteration of its spin state. A series of complexes with different dipole moments have been synthesized and their transport features were investigated via mechanically controlled break‐junctions. Statistical analyses support the hypothesized switching mechanism with increasing numbers of junctions displaying voltage‐dependent bistabilities upon increasing the Fe^II complexes’ intrinsic dipole moments. A constant threshold value of the E field required for switching corroborates the mechanism.     

Fine Tuning of Heme Reactivity: Hydrogen-Bonding and Dipole Interactions Affecting Ligand Binding to Hemoproteins

Heme reactivity in hemoproteins is governed by the microenvironment near the ligand binding site. In order to quantify polarity effects on heme ligand binding, the kinetics of O₂ and CO binding have been measured for a series of synthetic heme models equipped with a range of groups of varying dipole moments positioned near the heme coordination site. For hemes with polar aprotic groups, both O₂ on (k′) and off rates (k) are found to be dependent on the dipole moment. For model systems containing protic groups, the O₂ off rate is substantially reduced due to hydrogen bonding with the coordinated O₂. The hydrogen-bonding stabilization is estimated to be 0.7 and 1.6 kcal/mol for an alcohol and a secondary amide, respectively. CO binding displays little correlation with a polarity effect; instead it seems to depend upon the size and position of the polar group.     

Conformational analysis—XXI : H NMR Conformational Study of Alkyl-Substituted 2-Oxo-1,3,2-Dioxathianes

2-Oxo-1,3,2-dioxathiane and all methyl- and several alkyl-substituted 2-oxo-1,3,2-dioxathianes were prepared for a ¹H NMR conformational study. The conformational energy of the axial SO group in CCl₄, - ΔG^θ_SO = 14.8±0.3kJ mol^?1, was determined by chemical equilibration of the epimeric cis-4,6-dimethyl derivatives and it was found to decrease with the increasing solvent polarity. The conformational equilibria of alkyl-substituted derivatives were solved and the proportions of the conformers estimated using ¹H NMR chemical shifts, vicinal coupling constants and in three cases also dipole moments. The configurational interactions in the C₄C₅C₆ moiety are close to the corresponding values of 1,3-dioxanes.     

Photoinitiation. V. Effect of medium polarity on the acrylonitrile polymerization photoinitiated by naphthalene

The heterogeneous polymerization of acrylonitrile photoinitiated by naphthalene is influenced by the polarity of the reaction medium. The rate of initiation increases with the increasing dielectric strength of the reaction medium. A similar trend is observable for Stern–Volmer constants of naphthalene fluorescence quenching by acrylonitrile. The ratio k_p/k_t^1/2 of the rate constant for propagation and termination reactions is not influenced by a change in the polarity of the reaction medium. The effect of viscosity on the value of k_p/k_t^1/2 known for polymerization in a homogeneous medium was not observed in the reaction systems studied.     

Dipole moments,transition moments,oscillator strengths,radiative lifetimes,and overtone intensities for CH and CH+ as computed by quasi-degenerate many-body perturbation theory

The correlated, size-consistent, ab initio effective valence-shell dipole operator (μ^v) method is used to calculate dipole moments and transition dipole moments of the CH molecule and transition dipole moments of the CH⁺ ion as a function of internuclear distance. The dipole and transition dipole moments computed here compare well with those of other accurate ab initio methods. The transition dipole moments are then used to calculate oscillator strengths and radiative lifetimes for the A → X and B → A transitions of the CH⁺ ion and the A → X transition of the CH molecule. Comparisons are made with the best available theoretical and experimental lifetimes. Finally, the CH ground-state dipole moment function is used to evaluate overtone intensities and to examine simple models of the CH overtone intensities in polyatomic molecules.     

Tafel Slopes in Relation to the Replacement of Adsorbed Water by Organic Molecules

Abstract

In the lecture presented by Prof. Reddy the role of water in determining the potential dependence of electrosorption of neutral organic molecules has been discussed. The process one considers is that of the replacement of a number of water molecules n by each organic molecule adsorbed. The dipole moments of the adsorbed water molecules interact with the electrical field in the double layer and hence cause a potential (or charge) dependent adsorption of the neutral organic molecule, even if the latter possesses no permanent dipole moment. The theory of this phenomenon has been worked out by Bockris, Devanathan and Muller. Corrections for lateral interactions between the adsorbed water molecules and for the permanent dipole moment of the electrosorbed neutral organic molecule have been made.^1,2     

The effect of molecular polarity on nem/catic phase stability in 12-vertex carboranes

Weakly polar–polar isosteric pairs of 12-vertex p-carborane [closo-1,12-C₂B₁₀H₁₂] (1[12]) and monocarbaborate [closo-1-CB₁₁H₁₂]^? (2[12]) nematic liquid crystals, in which the difference in the calculated molecular dipole moment is 11.3 D, were synthesised, and the effect of the dipole moment on nematic phase stability was investigated. The trend observed for the 12-vertex series ([12]) was identical to that of the previously investigated 10-vertex series ([10]) containing [closo-1,10-C₂B₈H₁₀] (1[10]) and [closo-1-CB₉H₁₀]^? (2[10]): the uniform increase in the molecular dipole moment in the pairs of mesogens does not correspond to a uniform change in the clearing temperature, T_NI. This demonstrates the role of a remote substituent in modulating the intermolecular dipole–dipole interactions. The magnitude of such interactions was calculated (using density functional theory methods) for a pair of polar (2[12]d–2[12]d) and an analogous pair of weakly polar (1[12]d–1[12]d) molecules. All results for the 12-vertex series ([12]) were analysed relative to the 10-vertex analogues ([10]).     

Excited state dipole moments and geometries of the bases of nucleic acids

CNDO/s-CI and VE-PPP methods have been employed to calculate the dipole moments of the bases of nucleic acids in the ground and excited states. A component analysis in terms of μ_hyb^(σ), μ_ch and μ_π has been done using the CNDO/s-CI method and these results have been compared with those obtained by the CNDO/2 and IEHT methods. It is observed that while the CNDO/2 and CNDO/s-CI methods give almost the same total dipole moments, component-wise their predictions are very different.Dipole moments of the molecules have also been studied for the lowest excited singlet and triplet π^* ← π states. It is observed that the conventional method of calculating dipole moments using changes of only the net charges in the excited state does not give correct results for uracil and thymine, for which experimental results are available. Considering deformed non-planar excited state geometries for these molecules, the observed excited state dipole moments have been explained. A method has been suggested to include the effects of non-planarity while calculating the properties of a complex molecule in a π^* ← π excited state. For adenine, guanine and cytosine, the excited state dipole moments are found to be smaller than the ground state values.     

Polarity of [sgrave]3,λ5-Phosphoranes

Abstract

Tricoordinated pentavalent phosphorus compounds- [sgrave]³,λ⁵-phosphoranes-present a new field in unusually (low) coordinated phosphorus chemistry. Our current interest in these compounds is stimulated by the possibility of actual determination of unknown phosphorus bond polarities, using electrical methods in subsequent investigations of the spatial and electronic structure of [sgrave]³,λ⁵ -phosphoranes. We have studied the series of bis(imino)phosphoranes by the method of dipole moments R²N=P (R¹)=NR³ (I-VI) R¹ R² R³,^μexp.^{(D): (I) N}(^SiMe ₃)₂, ^SiMe ₃, SiMe₃, 2.16; (11) t-Bu, t-Bu, 2.36; (111) N(SiMe₃)₂, SiMe₃, t-Bu, 2.26; (IV) 2,4,6-Me₃C₆H₂, t-Bu, 2,4,6-t-Bu₃C₆H₂, 2.44; (V) t-Bu(Me₃Si)N, t-Bu, t-Bu, 2.74; (VI) c-2,2,6,6-Me4C5H6N, SiMe₃, SiMe₃, 2.82 and defined P=N bond polarity (3.14D). Dipole monents (I-VI) are described by the given values, the group moments R-P and R-N were previously found from dicoordinated phosphorus compounds. The tendency of increasing ^μexp, with the growth of n,Π-donor abilities of ^μexP,₁ substituent R in row (I-III)-(V)-(VI) is possibly caused by the increase of the conjugative effect contribution in stabilization of the 4-electron 3-centre Π-system N=P=N.     

Conformational equilibria in halocyclohexanones,an NMR and solvation study

The conformational equilibrium in 2-chloro cyclohexanone is measured in thirteen solvents from the 220 MHz¹H NMR spectrum using the C₂-H couplings and chemical shifts and the cis and trans 4-t-butyl-2-chlorocyclohexanones as reference compounds. ΔG_ea varies from 1.04 $\text{kcal}\text{mole}$ in n-pentane to ?0.58 $\text{kcal}\text{mole}$ in DMSO. The large concentration dependence of the NMR parameters in non-polar solvents noted previously is confirmed.Generalised reaction-field theory is used to calculate this solvent dependence, using a refined model of the geometry and dipole moments of the conformers.The cyclohexanone ring is considerably flatter than that of cyclohexane and this has an appreciable effect on the resultant dipole moments. Use of this geometry and CO and C-Cl bond moments which reproduce the observed dipole moments of the t-butyl compounds together with the generalised reaction field theory gives calculated solvation energies in good agreement with the observed data and hence allows the prediction of the vapour state energy difference.The model is applied to a variety of halo-ketones and gives both a reasonable explanation of the observed solvent dependencies and also the vapour state energy differences.The vapour state conformer energies are compared with the corresponding values for the halocyclohexanes and illustrate the large polar and steric effects due to the introduction of the CO group.     

Molecular electric properties in electronic excited states: multipole moments and polarizabilities of H2O in the lowest1 B 1 and3 B 1 excited states

Summary The dipole and quadrupole moments and the dipole polarizability tensor components are calculated for the¹ B ₁ and³ B ₁ excited states of the water molecule by using the complete active space (CAS) SCF method and an extended basis set of atomic natural orbitals. The dipole moment in the lowest¹ B ₁ (0.640 a.u.) and³ B ₁ (0.416 a.u.) states is found to be antiparallel to that in the ground electronic state of H₂O. The shape of the quadrupole moment ellipsoid is significantly modified by the electronic excitation to both states investigated in this paper. All components of the excited state dipole polarizability tensor increase by about an order of magnitude compared to their values in the ground electronic state. The present results are used to discuss some aspects of intermolecular interactions involving molecules in their excited electronic states.     

Inductive effect of substituents on the dipole moment of α-substituted vinyl methyl ether

Dipole moments of several α-substituted vinyl methyl ethers R(OMe)C:CH₂; R = Me, Et, i-Pr, t-Bu, cyclopropyl, vinyl, Ph) have been determined by the Halverstadt-Kumler method in benzene solution at 293 K. The square of the total dipole moment μ_r was found to be a linear function of the Taft's inductive constant σ_r^*: μ_r²/D²=(0.619±0.033)+(1.092±0.10) σ_r^*. The inductive contribution of the substituent R on the total dipole moment may be expressed by the equation μ_j/D = ?0.52 σ^* + 0.25. This is in good agreement with the corresponding equation for the dipole moments of alkyl-substituted ethenes: μ_i/D = ?0.58 σ^* + 0.28 (based on dipole moments obtained by PCILO calculations).