Dipole Moment Components Research Articles

Theoretical study of molecular ordering in a nematic liquid crystal

Analysis of molecular ordering in a nematic liquid crystal, namely Terepthal-bis-4- n-butylaniline (TBMA) has been carried out using computer simulation techniques. Net charge and dipole moment components located at various atomic centres of the molecule have been computed by CNDO/2 method. Modified second-order perturbation theory valid at intermediate range has been used to evaluate the intermolecular interactions between a pair of molecules during stacking, in-plane and terminal interactions. Probability of occurrence of a configuration has been estimated using Maxwell–Boltzmann formula. Results have been discussed in light of other theoretical and experimental observations with an aim to explain molecular ordering in nematogens.

Ordering of p–n-alkoxybenzoic acids at phase transition temperatures: a comparative computational analysis

A comparative analysis of molecular ordering of nematogenic p-n-alkoxybenzoic acids has been carried out with respect to translatory and orientational motions for the acids with seven (7OBAC), eight (8OBAC), nine (9OBAC) and 10 (10OBAC) carbon atoms in the alkyl chain. The CNDO/2 method has been used to compute the net atomic charge and dipole moment components at each atomic center. Modified Rayleigh-Schrodinger perturbation theory with multicentered-multipole expansion method has been used to evaluate long-range intermolecular interactions while a '6-exp' potential function has been assumed for short-range interactions. The total interaction-energy values obtained by these computations were used to calculate the probability of each configuration at the phase-transition temperature using the Maxwell-Boltzmann formula. The flexibility of various configurations has been studied in terms of variation of probability due to small departures from the most probable configuration. A comparative picture of molecular parameters like total energy, binding energy and total dipole moment has been given. An attempt has been made to explain the nematogenicity of these acids in terms of their relative order with the molecular parameter introduced in this paper.

Can we estimate total magnetization directions from aeromagnetic data using Helbig’s integrals?

Abstract An algorithm that implements Helbig’s (1963) integrals for estimating the vector components (m x, m y, m z) of the magnetic dipole moment from the first order moments of the vector magnetic field components (ΔX, ΔY, ΔZ) is tested on real and synthetic data. After a grid of total field aeromagnetic data is converted to vector component grids using Fourier filtering, Helbig’s infinite integrals are evaluated as finite integrals in small moving windows using a quadrature algorithm based on the 2-D trapezoidal rule. Prior to integration, best-fit planar surfaces must be removed from the component data within the data windows in order to make the results independent of the coordinate system origin. Two different approaches are described for interpreting the results of the integration. In the “direct” method, results from pairs of different window sizes are compared to identify grid nodes where the angular difference between solutions is small. These solutions provide valid estimates of total magnetization directions for compact sources such as spheres or dipoles, but not for horizontally elongated or 2-D sources. In the “indirect” method, which is more forgiving of source geometry, results of the quadrature analysis are scanned for solutions that are parallel to a specified total magnetization direction.

Adsorption of 2-mercaptobenzimidazole on a Au(1 1 1) electrode

The description of the monolayer formed at Au(1 1 1) by 2-mercaptobenzimidazole (MBI) under potential control has been based on electrochemical data (charge measurements) and spectroscopic information from the subtractively normalized interfacial Fourier transform infrared spectroscopy method (SNIFTIRS). From the quantitative analysis of the SNIFTIR spectra, a surface coverage Γ/ Γ max was extracted for each sample potential. The evolution of the coverage with potential was in full agreement with the charge density curve. The shift of the pzc in the presence of MBI indicates that the adsorbed molecules have a nonzero component of the permanent dipole moment in the direction perpendicular to the electrode surface. Thanks to the high quality of the spectra, it was possible to determine the orientation of MBI molecules at the surface in the monolayer and submonolayer range. The angle between the C 2-axis of the molecule and the direction normal to the surface is close to 64 ± 4° and its small change (<15°) with potential indicates that the orientation of the molecules is chiefly controlled by the chemical interaction between the sulphur atom and the gold surface.

The Millimeter‐ and Submillimeter‐Wave Spectrum of Cyanoformamide

The organic species cyanoformamide (NCCONH2) is the cyano (CN) derivative of formamide (NH2CHO), a known interstellar molecule with a role in the synthesis of nucleic acid precursors under prebiotic conditions. Until quite recently, the rotational spectrum of cyanoformamide had not been studied. Based partially on the results of a newly published microwave study and on the results of our ab initio calculations, we have measured and assigned 2691 transitions of the millimeter- and submillimeter-wave spectrum of this molecule through 360 GHz with the fast-scan submillimeter spectroscopic technique (FASSST). These transitions have been added to the previously measured 135 microwave transitions to produce a global data set of 2826 transitions that was analyzed to within experimental accuracy with a standard asymmetric-top fitting procedure. With the determined spectroscopic constants and dipole moment components, we are able to predict the frequencies and intensities of many additional lines through 400 GHz. Cyanoformamide can now be searched for over a wide frequency range in likely interstellar sources such as hot molecular cores.

Rotational Spectrum of the Dimethyl Ether−Acetylene Complex: Evidence for an Effective C2v Geometry

The rotational spectra for five isotopomers of the 1:1 weakly bound complex formed between dimethyl ether (DME) and acetylene (HCCH) have been measured by Fourier transform microwave spectroscopy. The experimental rotational constants, planar moments, and dipole moment components are consistent with a floppy complex possessing an effective C2v structure in which the hydrogen atom of acetylene is hydrogen bonded to the oxygen atom of dimethyl ether with an intermolecular H...O separation of 2.08(3) A. Experimental rotational constants for the normal isotopic species are A = 10382.5(17) MHz, B = 1535.7187(18) MHz, and C = 1328.3990(17) MHz and the dipole moment components are mua= mutotal = 1.91(10) D. Ab initio calculations at the MP2/6-311++G(2d,2p) level indicate that the energy barrier for motion of the HCCH subunit between the lone pairs of the DME, via a C2v intermediate structure, is very low (approximately 0.29 kJ mol(-1)). Inclusion of basis set superposition error and zero point energy corrections to the energies of four stationary points located on the potential energy surface shows that the relative stabilities are particularly sensitive to these corrections. The ab initio optimizations give rotational constants for the C2v structure of A = 10066 MHz, B = 1496 MHz, and C = 1324 MHz, and a dipole moment of mua= mu(total) = 2.12 D, in reasonable agreement with the experimentally determined values. The structural parameters and energetics of the DME-HCCH complex will be discussed and compared to similar complexes such as H2O-HCCH.

Molecular dynamics simulations of tubulin structure and calculations of electrostatic properties of microtubules

We present the results of molecular dynamics computations based on the atomic resolution structure of tubulin. Values of net charge, charge distribution and dipole moment components are obtained for the tubulin heterodimer. Physical consequences of these results are discussed for microtubules in terms of the effects on test charges, test dipoles, and neighboring microtubules.

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.

Study of relationship of atomic orbital hybridization with bonding using hybridization displacement charge: optimal hybridization principle

Hybridization displacement charges (HDC) were computed for several symmetric molecules employing the ab initio HF/6-31G** method. HDC calculations provide several point charges near as well as far from the atoms in molecules. The HDC method preserves the contribution of each atom to the component of molecular dipole moment arising due to atomic orbital hybridization. Variation of total HDC with bond length showed a maximum or a shoulder near the optimized bond length in each case. The linear correlation coefficient between the optimized bond lengths and the positions of maxima or shoulders was found to be 0.81. According to the t-test, this correlation coefficient is significant at a level less than 0.5% showing that the correlation is meaningful. Thus it is concluded that bonding in molecules follows a principle that can be termed as Optimal Hybridization Principle (OHP). This principle, like the Maximum Overlap Principle (MOP), can be used to explain bonding in molecules in a qualitative manner. The HDC components arising due to mixing of different s and p types of orbitals usually vary with bond length quite differently. Thus bonding is usually controlled by hybridization of some specific atomic orbitals in molecules which generally include those belonging to the next higher shell than the valence shell of the atom under consideration.

Infrared overtone spectroscopy and unimolecular decay dynamics of peroxynitrous acid

Peroxynitrous acid (HOONO) is generated in a pulsed supersonic expansion through recombination of photolytically generated OH and NO(2) radicals. A rotationally resolved infrared action spectrum of HOONO is obtained in the OH overtone region at 6971.351(4) cm(-1) (origin), providing definitive spectroscopic identification of the trans-perp (tp) conformer of HOONO. Analysis of the rotational band structure yields rotational constants for the near prolate asymmetric top, the ratio of the a-type to c-type components of the transition dipole moment for the hybrid band, and a homogeneous linewidth arising from intramolecular vibrational energy redistribution and/or dissociation. The quantum state distribution of the OH (nu=0,J(OH)) products from dissociation is well characterized by a microcanonical statistical distribution constrained only by the energy available to products, 1304+/-38 cm(-1). This yields a 5667+/-38 cm(-1) [16.2(1) kcal mol(-1)] binding energy for tp-HOONO. An equivalent available energy and corresponding binding energy are obtained from the highest observed OH product state. Complementary high level ab initio calculations are carried out in conjunction with second-order vibrational perturbation theory to predict the spectroscopic observables associated with the OH overtone transition of tp-HOONO including its vibrational frequency, rotational constants, and transition dipole moment. The same approach is used to compute frequencies and intensities of multiple quantum transitions that aid in the assignment of weaker features observed in the OH overtone region, in particular, a combination band of tp-HOONO involving the HOON torsional mode.

Microwave spectrum, r0 structure, barriers to internal rotation and ab initio calculations of gauche-1,1-difluoropropane

The microwave spectra of four isotopomers, CH 3CH 2 12CHF 2, CH 3CH 2 13CHF 2, CD 3CH 2 12CHF 2 and CD 3CD 2 12CHF 2, of gauche-1,1-difluoropropane, in the ground vibrational state were recorded and assigned. From the measured frequencies for the R and Q branches of the a-, b- and c-type transitions, accurate values for the rotational constants were obtained along with values for Δ J , Δ JK , δ J and Δ K . The dipole moment components were evaluated from the measurements of the Stark effects of the ground state rotational transitions and the determined values are: | μ a|=2.044(3), | μ b|=0.851(5), | μ c|=0.711(35) and | μ tot|=2.326(3) D. Ab initio molecular orbital calculations have been carried out with valence electron correlation by the perturbation method to second-order with various basis sets up to MP2(full)/6-311+G(d,p) from which structural parameters, barriers to internal rotation, and distortion constants have been obtained. The C–H distance of 1.095±0.002 Å for the CHF 2 moiety has been determined from the isolated stretching frequency from the CD 3CD 2 12CHF 2 molecule. The r 0 parameters have been obtained by combining the ab initio predicted values with the microwave rotational constants and these parameters for the heavy atoms are: rC 1–F 5=1.370(5), rC 1–F 6=1.372(5), rC 1–C 2=1.505(3), rC 2–C 3=1.522(3) Å, ∠C 1C 2C 3=112.2(5), ∠C 2C 1F 5=109.8(5), ∠C 2C 1F 6=110.9(5)°. The adjusted r 0 parameters have also been obtained for 2,2-difluoropropane, CH 3CF 2CH 3. The results are compared to those of some similar molecules.

The microwave spectrum of cyanoformamide

The spectrum of cyanoformamide, NCCONH 2, has been measured between 18 and 40 GHz using a Hewlett–Packard spectrometer with Stark modulation. The molecule is somewhat unstable and could most conveniently be measured in a flow system. The quadrupole hyperfine structure due to the two nitrogen nuclei has been analyzed for the ground state, and quadrupole coupling constants, rotational constants, and centrifugal distortion constants have been determined for the ground state. A rough determination of the components of the electric dipole moment was possible from the Stark shifts of suitable transitions.

Rotational spectrum, structure and modeling of the OCS–CS2van der Waals dimer

The rotational spectrum of a 1 : 1 weakly bound complex between OCS and CS2 has been measured by Fourier-transform microwave spectroscopy, giving ground state rotational constants of A = 2369.6942(9) MHz, B = 994.4467(6) MHz and C = 700.5137(3) MHz for the normal isotopic species. The experimental dipole moment components are μa = 0.2893(4) D and μb = 0.6364(27) D, with μtotal = 0.6991(25) D. The rotational constants and dipole moment components are consistent with a structure of Cs symmetry, in which the CS2 and OCS monomers are aligned almost parallel to one another, with a center of mass separation of 3.8017(2) A. This structure is in good agreement with the lowest energy geometry obtained from an ab initio calculation at the MP2/6-311++G(2d,2p) level which predicts rotational constants of A = 2322 MHz, B = 1036 MHz and C = 716 MHz and dipole moment components of μa = 0.32 D and μb = 0.69 D. Semi-empirical modeling using the ORIENT program gives similarly good agreement, although the predicted rotational constants and dipole moment are a little further from the experimental results (A = 2458 MHz, B = 1027 MHz and C = 725 MHz and μa = 0.34 D and μb = 0.68 D).

Structure, dipole moment and large amplitude motions of 1-benzofuran

The rotational spectrum of 1-benzofuran has been investigated by three different rotational spectroscopy techniques: (i) millimeterwave absorption free jet spectroscopy, useful for a fast assignment of the spectrum; (ii) molecular beam Fourier transform microwave spectroscopy, sensitive to detect less abundant isotopic species in natural abundance; (iii) waveguide conventional microwave spectroscopy, useful for the study of intramolecular dynamics, through the rotational spectra of the vibrational satellites of low energy modes. Besides the rotational spectrum of the ground state of the normal species, the spectra of 9 singly substituted 13C and 18O isotopomers in natural abundance, and of 6 vibrational satellites, have been measured. Precise structural parameters for the molecule, as well as information on the potential energy surface of the low energy vibrations, have been obtained. The dipole moment components have been determined to be micro(a)= 0.216 (2) and micro(b)= 0.720 (3) D, respectively.

Electric dipole polarizability and hyperpolarizability of NCCN, NCCP, and PCCP

Components of the static electric dipole polarizabilty (α), and second hyperpolarizability (γ); tensors have been determined by accurate ab initio calculations for three molecules containing two conjugated carbon - nitrogen and/or carbon - phosphorus triple bonds. For the NCCP molecule the dipole moment (μ) and non-vanishing components of the first hyperpolarizability (β) tensor have also been calculated. Electron correlation effects have been taken into account by the second-order Many Body Perturbation Theory (MBPT(2)), and coupled cluster (CCSD and CCSD(T)) calculations. The basis set developed by Sadlej (Pol) and designed to be used in electric properties calculations together with its extension (HyPol) have been applied and compared to the results obtained with the standard aug-cc-PVTZ sets of Dunning.

Curvature contributions to the static electrical properties of push-pull molecules

Abstract Calculations of the curvature contribution to the diagonals components of the static dipole moment (μ), polarizability (α), first (β) and second (γ) hyperpolarizability of push-pull molecules are presented. This contribution was obtained from the analytical evaluation of electrical properties method using the harmonic zero-point energy. The valence-bond charge-transfer model was employed to obtain the field-dependent force constant and their derivates with respect to electric field. Our results show a relationship between the curvature and electronic contributions. We have also found that the curvature contribution is important in a numerical estimation of β and γ.

Millimeter and submillimeter wave spectroscopic investigations into the rotation-tunneling spectrum of gGg′ ethylene glycol, HOCH 2CH 2OH

The rotation-tunneling spectrum of the second most stable gGg ′ conformer of ethylene glycol (1,2-ethanediol) in its ground vibrational state has been studied in selected regions between 77 and 579 GHz. Compared to the study of the more stable aGg ′ conformer, a much larger frequency range was studied, resulting in a much extended frequency list covering similar quantum numbers, J⩽55 and K a ⩽19. While the input data were reproduced within experimental uncertainties up to moderately high values of J and K a larger residuals remain at higher quantum numbers. The severe mixing of the states caused by the Coriolis interaction between the two tunneling substates is suggested to provide a considerable part of the explanation. In addition, a Coriolis interaction of the gGg ′ ground vibrational state with an excited state of the aGg ′ conformer may also contribute. Relative intensities of closely spaced lines have been investigated to determine the signs of the Coriolis constants between the two tunneling substates relative to the dipole moment components and to estimate the magnitudes of the dipole moment components and the energy difference between the gGg ′ and the aGg ′ conformers. Results of ab initio calculations on the total dipole moment and the vibrational spectrum were needed for these estimates. The current analysis is limited to transitions with quantum numbers J⩽40 and K a ⩽6 plus those having J⩽22 and K a ⩽17 which could be reproduced within experimental uncertainties. The results are aimed at aiding radioastronomers to search for gGg ′ ethylene glycol in comets and in interstellar space.

Dynamics of Nematogenic Molecules with Transverse Component of Dipole Moment

Dynamics of Nematogenic Molecules with Transverse Component of Dipole Moment

Vibronic transition moments and line intensities for H 2O +

Ab initio calculations of the electronic dipole moment components for the A ̃ 2A 1 and X ̃ 2B 1 electronic states and the electronic transition moment for the A ̃ 2A 1 – X ̃ 2B 1 transition of H 2O + have been carried out. Parameterized analytical functions have been fitted through the computed ab initio data points, and the resulting dipole moment and transition moment surfaces have been used, along with potential energy surfaces derived from the ab initio results of Brommer et al. [J. Chem. Phys. 98 (1993) 5222], to simulate H 2O + spectra and to generate an extensive set of vibronic transition moments for the A ̃ – X ̃ and X ̃ – X ̃ band systems of H 2O +. The work is made with the dual purpose of facilitating further assignments of high-resolution spectra [J. Mol. Spectrosc. 219 (2003) 258] and of allowing cometary spectra of H 2O + to be simulated [Ap. J. 574 (2002) L183].

Phase-dependent decoherence of optical transitions inPr3+:LaF3in the presence of a driving field

The decoherence times of orthogonally phased components of the optical transition dipole moment in a two-level system have been observed to differ by an order of magnitude. This phase anisotropy is observed in coherent transient experiments where an optical driving field is present during extended periods of decoherence. The decoherence time of the component of the dipole moment in phase with the driving field is extended compared to T, obtained from two-pulse photon echoes, in analogy with the spin locking technique of NMR. This is the first phase-dependent investigation of optical decoherence in the presence of a driving field.