Electric Dipole Moments Of Molecules Research Articles

The Role of Computational Chemistry in the Experimental Determination of the Dipole Moment of Molecules in Solution.

The methods for the experimental determination of electric dipole moment of molecules in solution from measurements of dielectric permittivity and refractive index are traditionally based on the classical Onsager model. In this model the molecular solute is approximated as a simple polarizable point dipole inside a spherical or ellipsoidal cavity of a dielectric medium representing the solvent. However, the inadequacies of the model resulting from the assumption of a simple shape of the cavity, for the evaluation of the cavity field effect, and from the uncertainty of the polarizability of the molecular solute influences the results and hampers the comparison with the electric dipole moments computed from quantum chemical solvation models. In this article we propose a new method for the experimental determination of the electric dipole moment in solution in which information from the Polarizable Continuum Model calculations are used in place of the Onsager model. The new method overcomes the limitations of this latter model regarding both the cavity field effect and the polarizability of the molecular solutes, and thus allows a coherent comparison between experimental and computed dipole moments of solvated molecules. © 2019 Wiley Periodicals, Inc.

High SNR single measurements of trace gas phase spectra at THz frequencies

Fast detection and identification of trace gases in ambient conditions demand a high signal-to-ratio (SNR) and superior resolution from a single measurement. We performed time-domain terahertz (THz) spectroscopy on gas (or vapor) phase samples of carbon monoxide, methanol, water, and acetonitrile at concentrations less than 10 ppm and demonstrated 50–60 dB SNR on a single measurement. Using data measured at different sample concentrations and terahertz probe beam powers, we investigated the interplay between the SNR, the probe beam power, the sample concentration, and the electric dipole moment of molecules, for which we extended an absorbance theory to THz frequencies. When comparing our data with references and a theoretical model, we found some discrepancy in certain spectral line intensities, suggesting that certain rotational resonance quantum state may have higher populations or transition probabilities than our model predicts at atmospheric conditions. We could achieve the above results largely due to our high power THz source capable of generating up to 3 mW, an order of magnitude greater than those available commercially.

Permanent electric dipole moment of strontium monofluoride as a test of the accuracy of a relativistic coupled-cluster method

The permanent electric dipole moment of the X 2 {\Sigma}+ electronic ground state of the strontium monofluoride molecule is calculated using a relativistic coupled cluster method. Our result is compared with those of other calculations and that of experiment. Individual contributions arising from different physical effects are presented. The result obtained suggests that the relativistic coupled cluster method used in the present work is capable of yielding accurate results for the permanent electric dipole moments of molecules for which relativistic effects cannot be ignored.

Carriers Spectra of Functionalized Semiconducting Nanowire and Conformational Transition in Molecules

The tuning of the spectrum of semiconducting nanowires as a result of the functionalization by a layer of molecules with a conformational transition is investigated. The situation where the electric charge carrier induces the conformational transition with a change of the orientation of the intrinsic electric dipole moments of molecules is expected. The spectrum of a carrier and the parameters of the arising quantum well are determined by the derived self-consistent system of transcendent equations. The system includes the Schrödinger equation for a charge carrier, nonlinear equations for the intrinsic electric-dipole moments, the material equations de4scribing the interaction of an extra carrier in the nanowire and molecular electric dipoles. In a semiconductor nanowire, the hole and electron spectra are symmetric. It is shown that the layer of adsorbed molecules breaks this symmetry when themolecular dipoles create the conditions for a localization of carriers of only one kind, which depends on the charge sign and the orientation of dipoles. The functionalized nanowires can be used as a semiconductor rectifier.

New all-optical method for measuring molecular permanent dipole moment difference using two-photon absorption spectroscopy

Stark effect, in combination with spectral hole burning and single-molecule spectroscopy, has been a fruitful technique to study permanent electric dipole moment of molecules in condensed phase. However, because measuring Stark shifts relies on external fields and narrow line- or hole-widths, the applicability of this method at ambient conditions required by most biological systems has remained limited. Here we demonstrate a new all-optical method for measuring the molecular dipole moment difference between ground and excited states using two-photon absorption (2PA) spectroscopy. We show that the value and orientation of the static dipole moment difference can be determined from the corresponding absolute 2PA cross-section. We use this new method to determine for the first time the strength of local electric field E loc =0.1–1.0×10 8 V/cm inside beta-barrel of Fruit series of red fluorescent proteins. Because our method does not rely on external field and is applicable in liquid solutions, it is well suited for the study of biological systems.

Photophysical Properties of Ricin

The molar absorption coefficient of ricin in phosphate-buffered saline (PBS) at 279 nm was measured as (93,900+/-3300) L mol(-1) cm(-1). The concentration of ricin was determined using amino acid analysis. The absorption spectrum of ricin was interpreted in terms of 69% contribution from absorption by tryptophan residues and 31% contribution from absorption by tyrosine residues. The total dipole strength of the ricin band at 280 nm was determined to be (147+/-8) D2 and was consistent with the combined dipole strengths of 10 tryptophan ([11.7+/-1.0] D2) and 23 tyrosine ([1.4+/-0.2] D2) residues. The structure of ricin was used to determine the coupling of the tryptophan residues in ricin. The maximum interaction energy was found to be 424 cm(-1)/epsilon while the average interaction between any two pairs of tryptophan residues was approximately 18 cm(-1)/epsilon. In this study, epsilon is the dielectric constant inside the protein. The fluorescence from ricin, excited at 280 nm, was dominated by fluorescence from tryptophan residues suggesting the presence of energy transfer from tyrosine to tryptophan residues. The absorbance and fluorescence of ricin increased slightly when ricin was denatured in a high concentration of guanidine. Irreversible thermal unfolding of ricin occurred between 65 degrees C and 70 degrees C. (D=3.3364*10(-30) Cm, not SI unit, convenient unit for the magnitude of the electric dipole moment of molecules.).

Suppression of inelastic collisions of polarΣ1state molecules in an electrostatic field

Collisions of polar $^{1}\ensuremath{\Sigma}$ state molecules at ultralow energies are considered, within a model that accounts for long-range dipole-dipole interactions, plus rotation of the molecules. We predict a substantial suppression of dipole-driven inelastic collisions at high values of the applied electric field, namely, field values of several times ${B}_{0}∕\ensuremath{\mu}$. Here ${B}_{0}$ is the rotational constant, and $\ensuremath{\mu}$ is the electric dipole moment of molecules. The sudden large drop in the inelastic cross section is attributed to the onset of degeneracy between molecular rotational levels, which dramatically alters the scattering Hamiltonian. This capability could, in principle, be used to stabilize ultracold gases against collisional losses.

C-axis resistivity of graphite intercalation compounds

The c-axis resistivity of stage-2, 3, 4, 5, 6, 7, and 9 graphite intercalation compounds (GICs) has been measured in the temperature range between 4.2 and 300 K with and without an external magnetic field along the c-axis. In these compounds the c-axis conduction is dominated by the in-plane conduction because of the highly anisotropic resistivity. The temperature dependence of strongly depends on the stage number. The stage-2 and 3 GICs show a metallic behaviour: increasing with increasing temperature. The logarithmic behaviour in is observed in a limited temperature range for stages 5 and 6, and negative longitudinal magnetoresistance is observed for stages 3 to 7, indicating that the two-dimensional weak localization effect may occur mainly in the interior graphite layers with a small charge transfer. The c-axis resistivity of stage-3 to 9 GICs shows a unusual thermal hysteresis in the temperature range between 180 and 240 K. It has two local minima at critical temperatures - 210 K) and (= 223 - 231 K) depending on the stage number. The carriers in the bounding graphite layers with a large charge transfer are scattered by enhanced fluctuations due to electric dipole moments of molecules. There may occur a two-dimensional- (2D-) like dipole ordered phase below and a 3D-like dipole ordered phase below .

Photoinduced charge transfer as revealed by ground and excited state dipole moments

Abstract

Electric dipole moment of molecules of ellipsoidal shape determined from dielectric measurements

An equation was derived for calculating dipole moments from dielectric measurements in dilute solutions of dipole molecules in nonpolar solvents. For this purpose the Onsager formula, as generalized by Scholte, was applied for molecules of ellipsoidal shape. The expression obtained was verified using experimental data from earlier studies and also from literature data. In each case good agreement was found between values of dipole moments calculated from dielectric measurements and those measured for molecules in the vapor phase.