Re-crystallization of MNA under a strong dc electric field

Crystals of 2-methyl-4-nitroaniline were grown from solution while subjected to a dc electric field of 9.8×10⁴ V m⁻¹. This resulted in an alteration of the unit cell lattice parameters while the space group was modified from Cc to one of its subgroups Pc. The modified crystals display an increased second harmonic generation efficiency accompanied by a decrease in the dielectric permittivity as well as changes in the internal and external vibrational modes.     

Water vapor nucleation on a crystal surface in a strong electric field

Water vapor nucleation at 260 K in a transverse electric field has been simulated by the Monte Carlo method under conditions corresponding to an internal wall of a spatially extended microcrack in a silver iodide crystal. The bicanonical statistical ensemble method has been employed to calculate, at the molecular level, the free energy of addition and the work of formation of dense phase nuclei in fields with different strengths. In a moderate field, the film mechanism of nucleation characterized by intense distortions on the film surface remains preserved. A domain structure of a film layer in contact with the surface exhibits a high stability with respect to an external field and remains preserved until the film is completely destroyed. In a strong electric field, the nucleation mechanism is fundamentally changed; i.e., the film is destroyed to yield threadlike structures. Therewith, the area of the contact with the surface drastically decreases. The orientation of nanothreads along the electric field lines overcomes a low free energy barrier. The point of equilibrium of nanothreads with vapor depends on the presence of hydrogen bonds, while their stability is determined by longer-range dipole-dipole interactions. The observed form of existence of the condensate as polarized nanothreads seems to be analogous to the superpolarized state previously revealed for water microdroplets, the transition to which has the character of the first-order phase transition.     

Implications of comparative spectral doublets observed for neon-isolated and gaseous tropolone(OH) and tropolone(OD)

Spectral doublet separations reported for gas phase and neon matrix-isolated samples of tropolone(OH) and tropolone(OD) are found to support recent work suggesting the possibility that tropolone has a slightly nonplanar geometry in the S1 (A 1B2) (pi*-pi) electronic state. Tautomerizations of gaseous tropolones in the S0 and S1 states are governed by equal double-minimum potential energy functions (PEFs), but interactions in the neon matrix environment transform the tautomerization PEFs of the slightly nonplanar S1 tropolones into unequal double-minimum PEFs. The spectral doublets reported for the zero-point S1-S0 transitions imply energy minima for the nonplanar S1 state in a neon matrix are offset by about 7 cm-1, and tunneling splittings in the symmetric double minimum PEFs of the gaseous molecules are damped about 2 cm-1 by the matrix environment. This means gas phase tunneling splittings smaller than 2 cm-1 are fully quenched in the neon matrix, and gas phase tunneling splittings near 20 cm-1 are damped by only 10%.     

Calculation of the electric hypershielding at the nuclei of molecules in a strong magnetic field

The third-rank electric hypershielding at the nuclei of 14 small molecules has been evaluated at the Hartree-Fock level of accuracy, by a pointwise procedure for the geometrical derivatives of magnetic susceptibilities and by a straightforward use of its definition within the Rayleigh-Schrodinger perturbation theory. The connection between these two quantities is provided by the Hellmann-Feynman theorem. The magnetically induced hypershielding at the nuclei accounts for distortion of molecular geometry caused by strong magnetic fields and for related changes of magnetic susceptibility. In homonuclear diatomics H(2), N(2), and F(2), a field along the bond direction squeezes the electron cloud toward the center, determining shorter but stronger bond. It is shown that constraints for rotational and translational invariances and hypervirial theorems provide a natural criterion for Hartree-Fock quality of computed nuclear electric hypershielding.     

Decay of atomic systems in a strong electric field. Negative ion and hydrogen atom

An exact transcendental equation was obtained determining the energy levels of a particle bound by a short-range force and placed in uniform electric field. At high intensities there is a considerable difference between the values of decay probabilities calculated by previous theory and by this one. The results obtained were used to determine the electron binding energy and the polarizability of negative ions by employing experimental data on ion decay during a flight through the field region. Concrete calculations were carried out for the C^? ion.The calculation of the resonance energy E_r and the decay probability 2Γ of the quasistationary state n = S, n₁ = 3, n₂ = 0, and m = 1 of the hydrogen atom in a uniform electric field was performed by means of the direct integration of the time-independent Schrödinger equation for field intensities ranging from 0.8 × 10⁶V/cm to 1.7 × 10⁶V/cm. The calculated values fo E_r perfectly agree with those of the Rayleigh—Schrödinger perturbation energies calculated up to the fourth power in the field strength and differ substantially from the resonance energies obtained earlier by Hirschfelder and Curtiss. Our results concerning the rate of spontaneous ionization also differ substantially from the results of Hirschfelder and Curtiss and agree rather well with the results of Bailey, Hiskes and Riviere who used the general WKB method of Rice and Good.     

Anisotropic rotational diffusion and transient nonlinear responses of rigid macromolecules in a strong external electric field

Nonlinear transient responses of polar polarizable particles (macromolecules) diluted in a nonpolar solvent to a sudden change in magnitude of a strong external dc field are evaluated using the anisotropic noninertial rotational diffusion model. The relaxation functions and relaxation times appropriate to the transient dynamic Kerr effect and nonlinear dielectric relaxation are calculated by solving the infinite hierarchy of differential-recurrence equations for statistical moments (ensemble averages of the Wigner D functions). The calculations involve matrix continued fractions, which ultimately yield the exact solution of the infinite hierarchy of differential-recurrence relations for the first- and second-order transient responses.     

Capture and recombination of charge carriers in a strong electric field in quasi-one-dimensional crystals with scattering centers

The asymmetric random walk method is used to find the time dependence of the density of free charge carriers in a chain containing traps and neutral barriers, assuming a strong electric field. Fluctuations in the density of defects are taken into account exactly. The kinetics of the density decay has two stages: fast exponential decay at small times and slow decay (-[ln(t) + const]t^-, where a l is a combination of the parameters of the system) at long times. It is also shown that in the presence of barrier defects the asymptotic forms of the decrease in the number of charge carriers due to capture by traps and due to germinal recombination are the same.Institute of Theoretical Physics, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 2, pp. 129–133, March–April, 1991. Original article submitted May 21, 1990.     

Quantum-mechanical investigation of the properties of the molecules and ions of acetals in a strong electric field

Theoretical and Experimental Chemistry -     

Investigating electric field induced molecular distortions in polypropylene using Raman spectroscopy

Polymeric electric insulators are an integral part of many electronic circuits and systems. Changes induced by an electric field can affect various mechanisms; including electrical polarisation and electromechanical properties. Changes in the dielectric material can be tracked using spectroscopic methods. This study has shown that analysing polypropylene under electric field stress using Raman spectroscopy in combination with principal component analysis allows small changes in the non-crystalline phase to be identified. We have observed that for polypropylene, vibrational motion and changes in conformation occur mostly within the tie molecules connecting the overall cluster network. Amorphous molecular chains in the spherulites were also found to orient and form into a smectic mesophase. These electromechanical changes at the micro- and macromolecular level were found to be generally reversible once the stress is removed. However, with increased aging, these changes may lead to adverse structural changes and thus, in the future, this information may be used to inform faults and defect detection within polymeric dielectric materials.     

Birefringence and dielectric relaxation decay transients and anisotropic rotational diffusion of macromolecules in a strong external electric field

The decay transient response of polar and polarizable rigid bodies (macromolecules) diluted in a nonpolar solvent after a sudden switch-off of a strong external dc field is evaluated in the context of the anisotropic noninertial rotational diffusion model. On solving the differential-recurrence equations for the statistical moments (expectation values of Wigner's D functions), the decay transients of the birefringence and dielectric relaxation are obtained. The solutions (valid for arbitrary strengths of an external electric field) are given in a closed form suitable for comparison with experiment and Brownian dynamics simulations.     

The buildup of the electric field during positron annihilation lifetime spectroscopy measurement

Positron annihilation lifetime spectra were measured on pure polystyrene (PS) and PS samples whose film surfaces were coated with gold, graphite (GR‐PS), or M_oS₂. The results showed the longest lifetime remained constant with measured time in all experimental samples, whereas the corresponding intensity decreased with time at different rates. The experimental phenomena were associated with the buildup of an electric field inside the polymer during extended positron annihilation lifetime spectroscopy (PALS) measurement. The decrease in the rate was attributed to the presence of conductive film causing the neutrality between positive charges and negative charges, thereby reducing the buildup of the electric field. Additionally, we also performed PALS measurement on GR‐PS under different experimental conditions, such as the conductive film being grounded or not grounded or the presence of an external electric field. These results further indicated that the buildup of the electric field was responsible for the decrease in the intensity with time. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 332–336, 2001     

Internal electric field in cytochrome C explored by visible electronic circular dichroism spectroscopy

Electronic circular dichroism (ECD) is a valuable tool to explore the secondary and tertiary structure of proteins. With respect to heme proteins, the corresponding visible ECD spectra, which probe the chirality of the heme environment, have been used to explore functionally relevant structural changes in the heme vicinity. While the physical basis of the obtained ECD signal has been analyzed by Woody and co-workers in terms of multiple electronic coupling mechanism between the electronic transitions of the heme chromophore and of the protein (Hsu, M.C.; Woody, R.W. J. Am. Chem. Soc. 1971, 93, 3515), a theory for a detailed quantitative analysis of ECD profiles has only recently been developed (Schweitzer-Stenner, R.; Gorden, J. P.; Hagarman, A. J. Chem. Phys. 2007, 127, 135103). In the present study this theory is applied to analyze the visible ECD-spectra of both oxidation states of three cytochromes c from horse, cow and yeast. The results reveal that both B- and Q-bands are subject to band splitting, which is caused by a combination of electronic and vibronic perturbations. The B-band splittings are substantially larger than the corresponding Q-band splittings in both oxidation states. For the B-bands, the electronic contribution to the band splitting can be assigned to the internal electric field in the heme pocket, whereas the corresponding Q-band splitting is likely to reflect its gradient (Manas, E. S.; Vanderkooi, J. M.; Sharp, K. A. J. Phys. Chem. B 1999, 103, 6344). We found that the electronic and vibronic splitting is substantially larger in the oxidized than in the reduced state. Moreover, these states exhibit different signs of electronic splitting. These findings suggest that the oxidation process increases the internal electric field and changes its orientation with respect to the molecular coordinate system associated with the N-Fe-N lines of the heme group. For the reduced state, we used our data to calculate electric field strengths between 27 and 31 MV/cm for the investigated cytochrome c species. The field of the oxidized state is more difficult to estimate, owing to the lack of information about its orientation in the heme plane. Based on band splitting and the wavenumber of the band position we estimated a field-strength of ca. 40 MV/cm for oxidized horse heart cytochrome c. The thus derived difference between the field strengths of the oxidized and reduced state would contribute at least -55 kJ/mol to the enthalpic stabilization of the oxidized state. Our data indicate that the corresponding stabilization energy of yeast cytochrome c is smaller.     

Melt electrospinning in a parallel electric field

Electrospinning is an efficient and direct method of fabricating nanofibers. Fibers are frequently unstable in the electrospinning process, and the uneven distribution of the electric field is an important factor leading to instability. Experimental and finite element simulation studies are conducted on the process of melt electrospinning in a parallel electric field. Two parallel metal disks are used to successfully generate a uniform electric field. Electric field intensity on the edges of the metal disk is always stronger than the field at the center of the disk or at the spinneret bottom. The diameters, distances, and relative areas of these disks significantly affect the distribution of the electric field. Thus, the parallel electric field effectively reduces jet buckling in melt electrospinning. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 946–952     

Decomposition of hexafluoropropylene oxide in a strong IR field

Multiphoton dissociation (MPD) of hexafluoropropylene oxide (HFPO) under focused CO₂-laser radiation was studied. A high-energy channel of HFPO decomposition was revealed. The dependences of the MPD yield and the ratio between various channels of HFPO decomposition on the laser radiation frequency in the range from 967.7 to 1090.0 cm^–1, the incident radiation energy, and the initial pressure of HFPO were obtained. A considerable contribution of thermal decomposition of HFPO occurring outside of the irradiated zone to the total yield of its decomposition was established. An explanation for the pressure dependence of the yield of HFPO decomposition was suggested.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2174–2180, November, 1995.The authors are grateful to the Russian Foundation for Basic Research for financial support of this work (Project No. 94-03-09059).