The induced magnetic field in cyclic molecules

The response of a molecule to an applied external magnetic field can be evaluated by a graphical representation of the induced magnetic field. We have applied this technique to four representative, cyclic organic molecules, that is, to aromatic (C(6)H(6), D(6h)), anti-aromatic (C(4)H(4), D(2h)) and non-aromatic (C(4)H(8), D(4h), and C(6)H(12), D(3d)) molecules. The results show that molecules that contain a pi system possess a long-range magnetic response, while the induced magnetic field is short-range for molecules without pi systems. The induced magnetic field of aromatic molecules shields the external field. In contrast, the anti-aromatic molecules increase the applied field inside the ring. Aromatic, anti-aromatic, and non-aromatic molecules can be characterized by the appearance of the magnetic response. We also show that the magnetic response is directly connected to nucleus-independent chemical shifts (NICS).     

Montmorillonite alignment induced by magnetic field: evidence based on the diffusion anisotropy of water molecules

Diffusion coefficients of water in Na-montmorillonite (Na-mon) suspensions have been determined by pulsed-field gradient spin-echo (PGSE) NMR spectroscopy for three directions (x, y, and z), where x and y mean the directions perpendicular to the static magnetic field, and z the direction parallel to it. Diffusion anisotropy was observed in the suspensions with Na-mon weight fractions of 0.63, 1.82, and 3.32%; i.e., the diffusivity of water in the z direction is faster than that in the x or y direction. The largest diffusion anisotropy of water was observed at the Na-mon fraction of 3.32%. However, diffusion anisotropy disappeared in the suspensions with Na-mon fraction more than 5.02%. The fast diffusivity in the z direction was slightly enhanced in a stronger static magnetic field (14.1 T). These results indicate that the platelike Na-mon particles are aligned with their platelike faces parallel to the static magnetic field of NMR. We also measured diffusion coefficients of water for the z direction in the temperature range from 24 to 85 degrees C. The plot of diffusion coefficients of water against reciprocal temperature showed a refraction point at 65 degrees C. This phenomenon explicitly means that the alignment is gradually relaxed at higher temperatures.     

Sigma,pi, and delta wavefunction forms for the hydrogen molecule

Using variational Monte Carlo methods, we examine simple, explicitly‐correlated trial wavefunction forms for the X¹Σ, B¹Σ, a³Σ, b³Σ, I¹Π_g, C¹Π_u, i³Π_g, c³Π_u, J¹Δ_g, and j³Δ_g states of the hydrogen molecule. The energies produced by our best wavefunctions are slightly above the best values in the literature. When we combine our trial wavefunction forms with the generalized Feynman‐Kac path integral method, our results are in excellent agreement with the best nonrelativistic values for these systems except for the I¹Π_g state. Our best energy for this state, ?0.65951554(6), is lower by several microhartrees than that obtained by Wolniewicz [J Mol Spectrosc 1995, 169, 329]. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Magnetic properties in three electrons under Rashba spin-orbit interaction and magnetic field

In this work, we study three-electron magnetic susceptibility in quantum dots under Rashba spin-orbit interaction (SOI) and magnetic field by an analytical methodology. The Hamiltonian of the system is separated to center of mass and relative terms using the Jacobi transformations and the hyperspherical coordinates. By solving Schrodinger equation, energy levels and thereby the susceptibility are calculated using canonical ensemble. At zero temperature, the magnetization reduces with increasing magnetic field with and without Rashba SOI in three-electron-quantum dot without electron-electron (e-e) interaction. Also, SOI slightly changes the magnetization for three-electron-quantum dot without e-e interaction. At nonzero temperature, the magnetization shows a paramagnetic peak when the magnetic field increases. This peak position changes under the SOI. In the presence of e-e interaction, the susceptibility enhances with raising magnetic field and it shows a maximum. The susceptibility at low magnetic field is negative and then it becomes positive. The susceptibility with e-e interaction and without SOI is always diamagnetic and its magnitude reduces with enhancing magnetic field. The susceptibility shows a transition between diamagnetic and paramagnetic with e-e interaction and SOI.     

Study of the kinetic energy densities of electrons as applied to quantum dots in a magnetic field

There are three expressions for the kinetic energy density t( r ) expressed in terms of its quantal source, the single-particle density matrix: t _A( r ) , the integrand of the kinetic energy expectation value; t _B( r ) , the trace of the kinetic energy tensor; t _C( r ) , a virial form in terms of the ‘classical’ kinetic field. These kinetic energy densities are studied by application to ‘artificial atoms‘ or quantum dots in a magnetic field in a ground and excited singlet state. A comparison with the densities for natural atoms and molecules in their ground state is made. The near nucleus structure of these densities for natural atoms is explained. We suggest that in theoretical frameworks which employ the kinetic energy density such as molecular fragmentation, density functional theory, and information-entropic theories, one use all three expressions on application to quantum dots, and the virial expression for natural atoms and molecules. New physics could thereby be gleaned.     

Canonical orbital contributions to the magnetic fields induced by global and local diatropic and paratropic ring currents

The induced magnetic field (IMF) of naphthalene, biphenyl, biphenylene, benzocyclobutadiene, and pentalene is dissected to contributions from the total π system, canonical π‐molecular orbitals (CMO), and HOMO→π* excitations, to evaluate and interpret relative global and local diatropicity and paratropicity. Maps of the IMF of the total π system reveal its relative strength and topology that corresponds to global and local diatropic and paratropic ring currents. The total π magnetic response is determined by this of canonical HOMOs and particularly by paratropic contributions of rotational excitations from HOMOs to unoccupied π * orbitals. Low energy excitations and similar nodal structure of HOMO and π * induce strong paratropic fields that dominate on antiaromatic rings. High energy excitations and different nodal structures lead to weak paratropic contributions of canonical HOMOs, which are overwhelmed by diatropic response of lower energy canonical orbitals in aromatic rings. CMO‐IMF analysis is found in agreement with ring current analysis. © 2017 Wiley Periodicals, Inc.     

On the calculation of induced electric and magnetic moments of atoms and molecules

We present a many-electron method for calculating first and second order perturbed wavefunctions due to external electric and magnetic fields, which identifies the important correlation effects for the response function a priori and calculates them variationally. It is accurate, economical and applicable to ground and excited states with the same ease. Thus, it presents a useful alternative to the well-known coupled Hartree-Fock methods. As an application of this method, we calculate the static electric dipole polarizability of the Be ground state. We find α_d = 5.49 ų in agreement with recent extensive calculations.     

Effect of electric field switching on the electrophoretic mobility of single-stranded DNA molecules in polyacrylamide gels

We have examined the effects of pulsed electric fields on the separation of single-stranded DNA molecules in polyacrylamide sequencing gels. Using different electric field pulsing regimens, the mobilities of single-stranded DNA molecules can be retarded or increased as compared to conventional electrophoresis. These results indicated that pulsed field techniques can be applied to gel electrophoresis of small single-stranded DNA molecules.     

On the possibility of aligning paramagnetic molecules or ions in a magnetic field

Strong magnetic fields can hybridize low rotational states of paramagnetic molecules or molecular ions whose electronic angular momentum is coupled to the molecular axis. The hybridization creates pendular states in which the molecular axis is confined to librate over a limited angular range about the field direction. In this way substantial spatial alignment associated with large Zeeman shifts can be attained for many ground-state radicals or ions and electronically excited states of diatomic or linear molecules. The magnetic hybridization is analogous to that recently demonstrated for polar molecules in electric fields. The magnetic version can only provide ensemble alignment rather than orientation, but offers complementary chemical scope by virtue of its applicability to nonpolar molecules and ions.     

pi bonding in second and third row molecules: testing the strength of Linus's blanket

The flexibility of valence bond (VB) theory provides a new method of calculating pi-bond energies in the double-bonded species H(m)A=BH(n), where A, B = C, N, O, Si, P, S. This new method circumvents the problems usually associated with obtaining pi-bond strengths by targeting only the pi bond, while all other factors remain constant. In this manner, a clean separation between sigma- and pi effects can be achieved which highlights some expected trends in bond strength upon moving from left to right and up and down the Periodic Table. Intra-row pi bonds conform to the classic statement by Pauling [L. Pauling, The Natiure of the Chemical Bond, Cornell University Press, Ithaca, 1960, 3rd edition] regarding the relationship of heteronuclear bond strengths to their homonuclear constituents whereas inter-row pi bonds do not. This variance with Pauling's statement is shown to be due to the constraining effect of the underlying sigma bonds which prevents optimal p(pi)-p(pi) overlap. While Pauling's statement was based on the assumption that the resonance energy (RE) would be large for heteronuclear and small for homonuclear bonds, we have found large REs for all bonds studied herein; this leads to the conclusion that REs are dependent not only on the electronegativity difference but also the electronegativity sum of the constituent atoms. This situation where the bond is neither covalent nor ionic but originates in the covalent-ionic mixing has been termed charge shift (CS) bonding [S. Shaik, P. Maitre, G. Sini, P. C. Hiberty, J. Am. Chem. Soc. 1992, 114, 7861]. We have shown that CS bonding extends beyond single sigma bonds in first row molecules, thus supporting the idea that CS-bonding is a ubiquitous bonding form.     

Switching individual molecules by light and electrons: From isomerisation to chirality flip

Molecular electronics offers a promising way for constructing nano-electronic devices in future with faster performance and smaller dimensions. For this aim, electronic switches are essential as basic components for storage and logical operations. The main requirements for a molecular switch are reversibility and bistability. This necessitates the existence of at least two different thermally stable forms of a molecule that may be changed repeatedly from one state to the other one through an external stimulus. The transition should then be connected to a measurable change in molecular properties. The development of such molecular switches on the single molecule level is a major challenge on the path towards incorporating molecules as building units into nanoelectronic circuits. Since isomers may differ significantly in physical and chemical properties, isomerisation opens a way for a molecular switch.In this article, an overview is provided over those isomerisation reactions of single molecules adsorbed on surfaces that are investigated with a scanning tunnelling microscope and that have a potential as a molecular switch in future molecular electronics. These are mainly, but not exclusively, constitutional, configurational, and geometric isomerisation reactions. The external stimulus is either light or the possible interaction with the tip of a scanning tunnelling microscope, i.e. electrons, electric field, or mechanical force. Some reactions are similar to those observed for the molecule in the liquid phase, but some are observed or even possible only on a surface. The detailed investigation of the isomerisation yield dependence on several parameters gives insight into the underlying processes of the reaction.     

Two-dimensional simulations of CuPc-PCTDA solar cells: the importance of mobility and molecular pi stacking

An existing two-dimensional microkinetical model for the photovoltaic effect of molecular-based solar cells has been extended to include electron-hole pair recombination between donor and acceptor sites. Simulations of the short circuit current for simple two-dimensional model heterojunction structures composed of copper phthalocyanine (CuPc) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) are presented. The short circuit current was investigated as a function of the thickness of the photoactive layer for different choices of mobility for CuPc and PTCDA. The hole mobility of CuPc and/or the electron mobility of PTCDA limits the photovoltaic performance if chosen below a certain threshold determined by the net electron-hole generation rate at the CuPc-PTCDA interface. Also, the mobilities should be of the same order of magnitude. The effect of changing the interplanar separation alpha between the pi stacking molecules was investigated, and it was found that increasing alpha from 0.33 to 0.6 nm increases the short circuit current up to 5 orders of magnitude. This was rationalized in terms of the charge separation energetics of geminate electron-hole pairs and its dependence on alpha. As mobilities decrease with increasing alpha and thus opposes this effect, an optimum for alpha approximately 0.66 nm was found for the CuPc-PTCDA heterojunction model structures. The simulations are interpreted in a simple kinetic picture of an electron-hole pair generation step at the CuPc-PTCDA interface and subsequent transport in the CuPc and PTCDA domains. It is argued that an optimal device configuration involves an amorphous region at the CuPc-PTCDA interface and a gradual increase of the molecular order as the electrodes are approached in the respective CuPc and PTCDA transport regions.     

Observed adiabatic corrections to the born-oppenheimer approximation for diatomic molecules with ten valence electrons

Using millimeter-wave (mw) spectroscopy pure rotational transitions were measured with very high precision in several vibrational states for many compounds of the group III/VII and IV/VI diatomic molecules. The spectra were fitted to the usual Dunham expansion adopting the normal mass relations for the Y_lk except for Y₀₁ in order to combine all data of different isotopes for the same compound. For Y₀₁ the atomic mass relation given by Watson is used which introduces phenomenological parameters Δ₀₁^A, Δ₀₁^B for molecule AB taking the adiabatic and nonadiabatic corrections to the Born-Oppenheimer approximation into account. All observed spectra are well described by such a procedure. From these calculations the correction parameters Δ₀₁^A, Δ₀₁^B were obtained with an accuracy of ≈ 10% or better. Using known values of the rotational gJ factor and of the electric dipole moment the nonadiabatic part was calculated and with this result the adiabatic part was evaluated from Δ₀₁ for each atom. The adiabatic correction does not change very much for one specific atom by varying the chemical counterpart and in general it is less than 30% of the total correction for this class of molecules. The only exceptions are InI and the Tl and Pb compounds for which the adiabatic corrections are obtained ten to hundred times larger than those of the other compounds. No explanation is known for this behavior in the published literature.     

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.     

Parameters for the calculation of the ring current and atomic magnetic anisotropy contributions to magnetic shielding constants: Nucleic acid bases and intercalating agents

The values of the non-zero elements of the diamagnetic and paramagnetic parts of the atomic magnetic susceptibility tensors are reported for nucleic acid bases and for the conjugated parts of some intercalating agents. These quantities are necessary for the computation of the contribution of the local magnetic anisotropy to the intermolecular shielding which is produced by this type of molecule. The intensities of the ring currents as well as the radii of the conjugated rings are reported for these compounds so that the contribution of the magnetic anisotropy of these molecules to the chemical shift of any nucleus located at any point in space may be computed in an NMR study.     

Effect of magnetic field on diamagnetic susceptibility of two interacting electrons in a quantum dot

We have estimated the energy levels of the low‐lying states as a function of magnetic field when two electrons are introduced in a quantum dot (QD). Oscillator strength of interacting electrons for different magnetic field strengths has been calculated. There is no appreciable change in oscillator strength for stronger confinements for all the magnetic field strengths. We present the shift of diamagnetic susceptibility of a hydrogenic donor impurity in GaAs/Ga_1?xAl_xAs QD systems for the ground and low lying excited states. The effect of magnetic field on diamagnetic susceptibilities is estimated by two different methods and it has been found that values obtained from both the methods resemble each other. The diamagnetic shift is in good agreement with the other investigators. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Decay of trapped electrons by tunnelling to scavenger molecules in low-temperature glasses

The decay of trapped electrons by tunnelling to scavenger molecules is investigated theoretically. An exact expression has been derived which gives an exponential decrease of the trapped electron yield with the scavenger concentration at any given time.     

Accounting for translational invariance in analyzing contributions of perturbation theory to force-constant tensors of molecules

It has been shown that there are an infinite number of modes of representation of force constants in the form of a sum of the contributions of first-order and second-order perturbation theory, depending on the selection of the admixture of translation of the nuclear skeleton as a whole to the displacement of the nuclei. In particular, any force constant can be reprsented in the form of a specially constructed second-order relaxation term (or first-order with zero contribution); a representation has been found in which the first-order contributions for the displacements of all nuclei in the molecule are identical; rules of the sums for the relaxation terms have been derived, relating the changes in the Gel-Mann-Feynman forces on the nuclei in the molecule with displacements of various nuclei. For the electronic ground state of the molecule, the optimal representation of the force constants has been found, in which the minimum in the first-order contribution is combined with the minimum in the absolute magnitude of the second-order relaxation contribution. It has been proven that in the case of an unperturbed electronic state, the first-order contribution to any diagonal force constant is positive.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 3, pp. 280–287, May–June, 1985.