Electric properties of molecules confined by the spherical harmonic potential

The harmonic oscillator potential is very often used in quantum chemical studies of electric properties to model the effect of spatial confinement. In the vast majority of works, the harmonic potential of cylindrical symmetry was applied. Thus far, its spherical counterpart was used mainly to describe properties of spatially restricted atomic systems. Therefore, our main goal was to study the molecular electric properties in the presence of the spherically symmetric harmonic oscillator potential and to characterize the impact of the relative position of the considered molecules and spherical confinement on these properties. Moreover, we analyzed how the topology of confining environment affects the dipole moment and (hyper)polarizability, by comparing the results obtained in the spherical and cylindrical harmonic potential. Based on the conducted research, it was found that the position of the molecules relative to the spherical confinement strongly influences their electric properties. The observed trends of changes in the electric properties, caused by increasing the confinement strength, vary significantly. Moreover, it was shown that in the vast majority of cases, significant differences in the values of electric properties, obtained in the cylindrical and spherical confinement of a given strength, occur.     

Electrochromism and Solvatochromism

The position and the intensity of electronic bands are influenced by an electric field. Pronounced changes in the position of absorption bands are mainly due to the dipole moment of the molecule in the ground state and the change in the dipole moment during the excitation process, and pronounced changes in intensity are due to the field dependence of the transition moment, which can be described by the transition polarizability. The effect of an external electric field on the optical absorption (electrochromism) of suitable molecules can be used to determine the dipole moment in the ground state, the change in dipole moment during the excitation process, the direction of the transition moment of the electronic band, and certain components of the transition polarizability tensor. These data largely determine the strong solvatochromism (solvent-dependence of the position and intensity of electronic bands), which is observed in particular with molecules having large dipole moments. Smaller contributions to solvatochromism result from dispersion interactions, which predominate in the case of nonpolar molecules. The models developed have been experimentally checked and verified by a combination of electro-optical absorption measurements (influence of an external electric field on absorption) and investigation of the solvent-dependence of the electronic bands.     

Effect of External Electric Field on H‐Bonding and π‐Stacking Interactions in Guanine Aggregates

The structure and electronic properties of guanine oligomers and π stacks of guanine quartets (G‐quartets) with circulene are investigated under an external field through first‐principles calculations. An electric field induces nonplanarity in the guanine aggregates and also leads to an increase in the H‐bond distances. The calculations reveal that the binding energy of the circulenes with G‐quartets increases on application of an electric field along the stacking direction. The HOMO–LUMO gap decreases substantially under the influence of an external field. The contribution of a simple dipole–dipole interaction to the stability of the stacked system is also analyzed. The electric field along the perpendicular axis increases the dipole moments of the guanine dimer, trimer, and quartet. Such an increase in the dipole moment facilitates stacking with circulenes. The stability of G‐quartet–circulene π stacks depends on the phase of the dipole moment (in‐phase or out‐of‐phase) induced by an external electric field. The stability of stacks of bowl‐shaped circulenes with G‐quartets depends on the direction of the applied field.     

The temperature dependence of the d-electron dipole strengths of cobalt(II) tetrahalides

The dipole strengths of the ⁴A₂ → ⁴T₁(F), ⁴T₁(P), d-electron transitions of the cobalt(II) tetrahalides are found to decrease with an increase in temperature, in agreement with a dynamic ligand-polarisation model for the absorption mechanism. The reduction in dipole strength is accounted for by a decrease in the coulombic coupling between the quadrupole moment of the d-electron transition of the metal ion and the transient electric dipole moment induced in the ligands, due to the anharmonic increase in the mean bond length and the progressively larger mean-square amplitudes of the bending modes of the complex ion in its ground electronic state as the temperature is raised.     

MECHANISM OF ORIENTATION AND LINEAR DICHROISM OF PHOTOSYNTHETIC PARTICLES IN ELECTRIC FIELDS: CHLOROPLASTS AND CHLOROPHYLL-PROTEIN COMPLEXES

Abstract— The mechanisms of orientation in pulsed and alternating electric fields of thylakoids (derived from the sonication of spinach chloroplasts) and of light-harvesting chlorophyll a/b-protein complexes (CPII) were investigated by utilizing linear dichroism techniques. Comparisons of the linear dichroism spectra of thylakoids and CPII particles suggest that the latter are oriented with their directions of largest electronic polarizabilities (and thus probably their largest dimensions) within the thylakoid membrane planes. At low electric field strengths (< 12 V cm^?1), and at low frequencies of alternating electric fields (< 0.25 Hz), thylakoid membranes tend to align with their normals parallel to the direction of the applied electric field; the mechanism of orientation involves a permanent dipole moment of the thylakoids which is oriented perpendicular to the planes of the membranes. However, at high field strengths and high frequencies of the applied alternating electric fields, the thylakoids tend to orient with their planes parallel to the applied field, thus exhibiting an inversion of the sign of the linear dichroism as the electric field strength is increased. At the higher frequencies and at higher field strengths, the orientation mechanisms of the thylakoids involve induced dipole moments related to anisotropies in the electronic polarizabilities. The polarizability is higher within the plane than along a normal to the plane, thus accounting for the inversion of the dichroism as the electric field strength is increased. The CPII particles align with their largest dimension parallel to the applied field at all field strength, indicating that the induced dipole moment dominates the orientation mechanisms in pulsed electric fields. The magnitude of the absolute linear dichroism of CPII suspensions increases with increasing dilution, indicating that aggregates of lower symmetry are formed at higher concentrations of the CPII complexes.     

Dipole moments of HDO in highly excited vibrational states measured by Stark induced photofragment quantum beat spectroscopy

We report here a measurement of electric dipole moments in highly vibrationally excited HDO molecules. We use photofragment yield detected quantum beat spectroscopy to determine electric field induced splittings of the J=1 rotational levels of HDO excited with 4, 5, and 8 quanta of vibration in the OH stretching mode. The splittings allow us to deduce mua and mub, the projections of dipole moment onto the molecular rotation inertial axes. We compare the measured HDO dipole moment components with the results of quantitative calculations based on Morse oscillator wave functions and an ab initio dipole moment surface. The vibrational dependence of the dipole moment components reflect both structural and electronic changes in HDO upon vibrational excitation; principally the vibrational dependence of the O-H bond length and bond angle, and the resulting change in orientation of the principal inertial coordinate system. The dipole moment data also provide a sensitive test of theoretical dipole moment and potential energy surfaces, particularly for molecular configurations far from equilibrium.     

The predicted spectrum of the hypermetallic molecule MgOMg

The present study of MgOMg is a continuation of our theoretical work on Group 2 M(2)O hypermetallic oxides. Previous ab initio calculations have shown that MgOMg has a linear (1)Σ(g)+ ground electronic state and a very low lying first excited triplet electronic state that is also linear; the triplet state has (3)Σ(u)+ symmetry. No gas phase spectrum of this molecule has been assigned, and here we simulate the infrared absorption spectrum for both states. We calculate the three-dimensional potential energy surface, and the electric dipole moment surfaces, of each of the two states using a multireference configuration interaction (MRCISD) approach based on full-valence complete active space self-consistent field (FV-CASSCF) wavefunctions with a cc-pCVQZ basis set. A variational MORBID calculation using our potential energy and dipole moment surfaces is performed to determine rovibrational term values and to simulate the infrared absorption spectrum of the two states. We also calculate the dipole polarizability of both states at their equilibrium geometry in order to assist in the interpretation of future beam deflection experiments. Finally, in order to assist in the analysis of the electronic spectrum, we calculate the vertical excitation energies, and electric dipole transition matrix elements, for six excited singlet states and five excited triplet states using the state-average full valence CASSCF-MRCISD/aug-cc-pCVQZ procedure.     

Permanent electric dipole moments of four tryptamine conformers in the gas phase: a new diagnostic of structure and dynamics

Rotationally resolved electronic spectroscopy in the gas phase, in the absence and presence of an applied electric field, has been used to determine the charge distribution of a cross section of the energy landscape of tryptamine (TRA). We report the magnitude and direction of the permanent electric dipole moments of the four TRA conformers GPyout, GPyup, GPhup, and Antiup in their S0 and S1 electronic states. Each dipole moment is unique, providing a powerful new tool for the conformational analysis of biomolecules in the gas phase. A comparison of the results for the different conformers of TRA reveals that the position and orientation of the ethylamine side chain play a major role in determining both the permanent and induced electric dipole moments of the different species in both electronic states.     

Application of potential constants: Charge transfer and electric dipole moment change in the formation of heteronuclear diatomic molecules—IV

The harmonic and anharmonic potential (force) constants of heteronuclear diatomic molecules, which are usually available from normal coordinate analyses, are applied to problems of determining the number of electrons transferred (charge transfer) and electric dipole moment functions of such molecules. The approach developed here is mainly based on Slater's orbital expansion method, that is, in a non-spin-polarized calculation atomic energies in a molecule are expanded with respect to the occupation number of electrons of atomic orbitals. To confirm the accuracy and the reliability of the approach, we have calculated the number of electrons transferred and electric dipole moments for alkali halides and other heteronuclear diatomic molecules. Specially, detailed analyses of electric dipole moment functions have been carried out on hydrogen fluoride (HF) and hydrogen oxide (OH) for which reliable experimental dipole moment functions are presently known over a wide range of internuclear distances. It is concluded from these analyses that the present approach is simple and useful in evaluating the charge transfer and the dipole moment change in the formation of heteronuclear diatomic molecules.     

强外电场作用下二甲基硅酮的分子结构和激发态

The ground states of dimethyl siloxane under different intense electric fields ranging from - 0. 04 to 0. 04 a. u. are optimized using density functional theory DFT / B3P86 at 6-311 ++ G（d,p）level. The excitation energies and oscillator strengths under the same intense applied electric fields are calculated employing the revised hybrid CIS-DFT method. The result shows that the electronic state,molecular geometry,total energy,dipole moment and excitation energy are strongly dependent on the field strength and behave asymmetry to the direction of the applied electric field. As the electric field changes from - 0. 04 to 0. 04 a. u. ,the bond length of Si-O increases whereas the bond length of Si-C decreases because of the charge transfer induced by the applied electric field. The dipole moment of the ground state decreases linearly with the applied field strength. However,the dipole moment of molecule changes from positive to negative as the inverse electric field increase to - 0. 03 a. u. Further increase of the inverse electric field results in an increase of the total energy of the molecule. The dependence of the calculated excitation energies on the applied electric field strength is fitting well to the relationship proposed by Grozema. The excitation energies of the first five excited states of dimethyl siloxane decrease as the applied electric filed increases because the energy gap between the HOMO and LUMO become close with the field,which shows that the molecule is easy to be excited under electric field and hence can be easily dissociated.     

Finite-field method calculations. III. Dipole moment gradients,polarisability gradients and field-induced shifts in bond lengths,vibrational levels,

In this paper, the finite-field method is applied in calculating polarisabilities and, by extension, polarisability gradients. The method is also used to examine the shifts in a number of properties induced by electric fields of given strength. Model calculations at the SCF and Cl levels are reported for LiH. Estimates of the Raman scattering intensity and depolarisation ratio, obtained from the calculated polarisability gradient components, are presented. The dipole moment gradient, which is related to the infrared absorption intensity, is also reported. The properties for which field-dependent shifts are given comprise bond length, vibrational levels, spectroscopic constants, harmonic force constant, first anharmonic constant and dipole moment gradient.     

Influence of cation binding on the electro-optically determined electric moments of purple membranes

Analysis of the elect ro-optically determined permanent dipole moment and electric polarizability of purple membrane fragments reveals the complex nature of the membrane electric moments.The problem to distinguish between the contribution of the membrane structural charges (charged groups of the polypeptide chain and polar lipid headarouos), bound cations and the electric double layer structure deserves particular attention not only because of its importance for electro-optics but also in respect to the relation of the membrane surface electric properties to the membarans transport function.The removal of divalent cations (Ca²⁺ and Mg²⁺) bound to purple membrane in the native state induces a cat ion-free species or purple membrane (deionized - blue membrane) with drastically changed spectroscopic properties and function. The preseent paper summarizes our study on the electric moments of blue membrane and their changes during the blue to purple transition. We intended to provide an insight into the possible regulation of this reversible transition (purple-to-blue and blue-to-purple) through changes of the asymmetric charge distribution and the importance of the asymmetric interfacial charge distribution for the proton transfer in purple membranes.The changes in the electric moments (permanent and induced dipole moments) of purple membrane fragments upon di- and trivalent cations binding to cation-depleted purple membranes were studied by electric light scattering (rotational electrokinetics) in d.c. and a.c. electric fields, and by electric pulses with reversing polarity, the results show a recovery of the membrane charge asymmetry (permanent dipole moment) though not of the induced dipole moment.     

Boundary effects on electrophoresis of a colloidal cylinder with a nonuniform zeta potential distribution

The electrophoretic motion of a long dielectric circular cylinder with a general angular distribution of its surface potential under a transversely imposed electric field in the vicinity of a large plane wall parallel to its axis is analyzed. The thickness of the electric double layers adjacent to the solid surfaces is assumed to be much smaller than the particle radius and the gap width between the surfaces, but the applied electric field can be either perpendicular or parallel to the plane wall. The presence of the confining wall causes three basic effects on the particle velocity: (1) the local electric field on the particle surface is enhanced or reduced by the wall; (2) the wall increases viscous retardation of the moving particle; (3) an electroosmotic flow of the suspending fluid may exist due to the interaction between the charged wall and the tangentially imposed electric field. Through the use of cylindrical bipolar coordinates, the Laplace and Stokes equations are solved analytically for the two-dimensional electric potential and velocity fields, respectively, in the fluid phase, and explicit formulas for the quasisteady electrophoretic and angular velocities of the cylindrical particle are obtained. To apply these formulas, one has only to calculate the multipole moments of the zeta potential distribution at the particle surface. It is found that the existence of a plane wall near a nonuniformly charged particle can cause its translation or rotation which does not occur in an unbounded fluid with the same applied electric field.     

First Principles Calculations of Electric Field Effect on the (6,0) Zigzag Single-Walled Silicon Carbide Nanotube for use in Nano-Electronic Circuits

Structural, electronic, and electrical responses of the H-capped (6,0) zigzag single-walled silicon carbide nanotube (SiCNT) was studied under the parallel and transverse electric fields with strengths 0–140 × 10^?4 a.u. by using density functional calculations. Analysis of the structural parameters indicates that resistance of the nanotube against the applied parallel electric field is more than resistance of the nanotube against the applied transverse electric field. The dipole moments, atomic charge variations, and total energy of the (6,0) zigzag SiCNT show increases with any increase in the applied external electric field strengths. The length, tip diameters, electronic spatial extent, and molecular volume of the nanotube do not change significantly with any increasing in the electric field strength. The energy gap of the nanotube increases with any increases in the electric field strength and its reactivity is decreased. Increase of the ionization potential, electron affinity, chemical potential, and HOMO and LOMO in the nanotube with increase of the applied external electric field strengths indicates that the properties of SiCNTs can be controlled by the proper external electric field for use in nano-electronic circuits.     

Electric parameters of photosystem II particles — electric light scattering study

Electric light scattering and microelectrophoresis were applied to investigate the electric moments (permanent dipole moment and electric polarizability and electrophoretic mobility of envelope-free chloroplasts and photosystem II (PS II particles. The effect of the removal of the extrinsic polypeptides (18, 24 and 33 kDa) on the electric moments was also studied. A significant difference was observed between the orientation behaviour of chloroplasts and PS II preparations. The data indicate that the permanent and induced dipole moments contribute to the orientation of the PS II particles, whereas chloroplasts possess induced dipole moment only.

NaCl and Tris treatments of PS II preparations influence both the transverse permanent dipole moment and the electric polarizability of PS II particles. The increase in the electrophoretic mobility of PS II particles on removal of the extrinsic proteins corresponds to an increase in the electric polarizability value, demonstrating its interfacial nature.     

Polarized basis sets and the calculation of infrared intensities from nuclear electric shielding tensors

The idea of the basis set polarization which follows from the known dependence of basis set functions on the perturbation strength is applied to the calculation of the dipole moment derivatives with respect to nuclear displacements. The differentiation of the dipole moment function is replaced by the straightforward evaluation of derivatives of the intramolecular electric field with respect to the external electric field strength. The method and its efficiency are illustrated by a series of calculations of the dipole moment derivatives for the water molecule. Already a polarized basis set of 26 CGTO's derived from the minimal CGTO basis set provides fairly reasonable results.     

Can octupolar molecules be poled by an external electric field?

Octupolar molecules are generally believed to be of potential use in developing nonlinear optical materials owing to the fact that they do not easily form molecular aggregates. This is often put against the conjectured drawback that electric fields have no poling, or ordering, effect for this class of molecules because of the lack of a permanent ground state dipole moment. In this paper, we analyze this notion in some detail and present results from molecular dynamics computer simulations of an ensemble of a prototypical octupolar molecule, the 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) molecule, dissolved in chloroform. It is found that TATB molecules indeed show rather significant dipole moments in solutions because of the dual action of the thermal motions of the atoms and the strong intermolecular interactions. Applied electric fields accordingly show significant effects on the orientations of the molecular dipole moments. We also find that TATB molecules can aggregate because of the strong hydrogen-bonding interactions between the molecules, though they lack a static permanent dipole moment. Thus, the simulation results for TATB molecules in solution present us with a totally different notion about the collective properties of octupolar molecules. Taking account of quantum chemistry results, we found that the collective molecular nonlinear optical (NLO) properties are enhanced after the onset of the electric field, showing significant anisotropic characteristics.     

外电场作用下甲基乙烯基硅酮分子结构和电子光谱

采用密度泛函B3P86方法在6-311++G(d, p)基组水平上优化得到了沿分子轴方向不同外电场(0-0.04 a.u.)作用下, 甲基乙烯基硅酮分子的基态电子状态、几何结构、电偶极矩和分子总能量. 在优化构型下利用杂化CIS-DFT方法(CIS-B3P86)研究了同样外电场条件下对甲基乙烯基硅酮的激发能和振子强度的影响. 计算结果表明, 分子几何构型与电场大小呈现强烈的依赖关系, 分子偶极矩μ随电场的增加先减小后急剧增大. 电场为零时, 分子总能量为-483.5532137 a.u., 随着电场增加, 能量升高, 在F=0.02 a.u.时达到最大值-483.5393952 a.u., 此后, 继续增大电场系统总能量则开始降低. 激发能随电场增加急剧减小, 表明在电场作用下, 分子易于激发和离解.     

Theoretical study of the adsorption and dissociation of oxygen on Pt(111) in the presence of homogeneous electric fields

The effect of homogeneous electric fields on the adsorption energies of atomic and molecular oxygen and the dissociation activation energy of molecular oxygen on Pt(111) were studied by density functional theory (DFT). Positive electric fields, corresponding to positively charged surfaces, reduce the adsorption energies of the oxygen species on Pt(111), whereas negative fields increase the adsorption energies. The magnitude of the energy change for a given field is primarily determined by the static surface dipole moment induced by adsorption. On 10-atom Pt(111) clusters, the adsorption energy of atomic oxygen decreased by ca. 0.25 eV in the presence of a 0.51 V/A (0.01 au) electric field. This energy change, however, is heavily dependent on the number of atoms in the Pt(111) cluster, as the static dipole moment decreases with cluster size. Similar calculations with periodic slab models revealed a change in energy smaller by roughly an order of magnitude relative to the 10-atom cluster results. Calculations with adsorbed molecular oxygen and its transition state for dissociation showed similar behavior. Additionally, substrate relaxation in periodic slab models lowers the static dipole moment and, therefore, the effect of electric field on binding energy. The results presented in this paper indicate that the electrostatic effect of electric fields at fuel cell cathodes may be sufficiently large to influence the oxygen reduction reaction kinetics by increasing the activation energy for dissociation.