Direction Of Dipole Moment Research Articles

Functional oxide films with engineered ferroelectric properties

ABSTRACTThe ferroelectric properties of anisotropically strained SrTiO3 films are analyzed by detailed measurements of the complex dielectric constant as function of temperature, frequency, bias voltage and electric field direction. The strain induces a relaxor-ferroelectric phase that persists up to room temperature. However, transition temperature and ferroelectric properties strongly depend on the orientation of the electric field and therefore on the amount of structural strain in the given electric field direction. Frequency and time dependent relaxation experiments reveal the presence and properties of polar nanoregions with randomly distributed directions of dipole moments in the film.

Linear, non-linear optical susceptibilities and the hyperpolarizability of the mixed crystals Ag0.5Pb1.75Ge(S1−xSex)4: experiment and theory

As the starting point for a comprehensive theoretical investigation of the linear and nonlinear optical susceptibilities, we have used our experimental crystallographic data for Ag0.5Pb1.75GeS3Se (Ag2Pb7Ge4S12Se4) reported. The experimental crystallographic positions were optimized by minimizing the forces acting on each atom to get meaningful theoretical predictions of the optical properties. The linear optical susceptibilities are calculated. We find that the optical band gap shows very good agreement with our measured gap. The second-order nonlinear optical (NLO) susceptibilities dispersion namely the optical second harmonic generation (SHG) is calculated and compared with our experimental measurements. The microscopic first order hyperpolarizability, β123, vector component along the principal dipole moment directions for the χ((2))(123)(ω) component was obtained theoretically and compared with our measured values at different temperatures. The dependence of the two-photon absorption (TPA) for the pump-probing at SHG of the microsecond CO2 laser was measured. In addition we explored the linear electro-optical effect in these crystals. This effect is described by the third rank polar tensors similarly to the SHG. However, for the Pockels effect besides the electronic contribution, the phonon subsystem also begins to play a principal role. As a consequence we study the dispersion of the linear electro-optical effects in the mentioned crystals.

Effect of Solvent-Induced Coil to Helix Conformational Change on the Two-Photon Absorption Spectrum of Poly(3,6-phenanthrene)

This paper investigates the effect of solvent-induced conformational changes of poly(3,6-phenanthrene) on their two-photon absorption (2PA). Such effect was studied employing the wavelength-tunable femtosecond Z-scan technique and modeled using the sum-over-essential states approach. We observed a strong reduction of the 2PA cross-section when the sample was prepared in hexane (poor solvent) in comparison to chloroform (good solvent), which is related to the conformation adopted by the polymer in each case. In chloroform it adopts a random coil conformation, as opposed to the one-handed helix conformation in hexane. Our results pointed out that the coil to helix conformation change decreases the degree of molecular planarity of the polymer π-conjugated backbone, which is primarily responsible for their optical nonlinearity, contributing to diminishing the effective transition dipole moments and, consequently, the 2PA cross-section. Moreover, by studying the nonlinear response with different light polarization, we showed that, although the solvent-induced conformational change does not alter the molecular symmetry of the polymer, it modifies considerably the direction of the transition dipole moments between the excited states.

Dispersion of the linear and nonlinear optical susceptibilities of disilver germanium sulfide from DFT calculations

The dispersion of the linear and nonlinear optical susceptibilities is calculated for disilver germanium sulfide (Ag2GeS3) using the all-electron full potential linearized augmented plane wave (FP-LAPW) method. Calculations are performed with four exchange correlations namely local density approximation (LDA), general gradient approximation (GGA), Engel–Vosko generalized gradient approximation (EVGGA), and modified Becke–Johnson potential (mBJ). Our calculations give a band gap of 0.40 eV (LDA), 0.42 eV (GGA), 1.03 eV (EVGGA), and 1.30 eV (mBJ) in comparison with our measured gap (1.98 eV). The mBJ exchange correlation gives the best agreement with experiment. We find that the calculated linear optical susceptibilities of Ag2GeS3 show considerable anisotropy which is useful for second harmonic generation and optical parametric oscillation. To analyze the spectra of the calculated χ 113 (2) (ω), χ 232 (2) (ω), χ 311 (2) (ω), χ 322 (2) (ω), and χ 333 (2) (ω), we have correlated the features of these spectra with the features of ɛ2(ω) spectra as a function of ω/2 and ω. From the calculated dominant component |χ 333 (2) (ω)|, we find that the microscopic second-order hyperpolarizability, β333, the vector components along the dipole moment direction is 41.2 × 10−30 esu at static limit and 222.9 × 10−30 esu at λ = 1064 nm.

Multidipoles model of ship's magnetic field

Ships made of ferromagnetic metals interfere with Earth's magnetic field in their surrounding. The disturbance of the magnetic field makes possible localization and even identification of the ship, which could determine a threat to the ship. The measurement of the magnetic field around the ship enables to determine its magnetic signature. The paper presents a multidipoles model of the ship magnetic field, which allows to determine parameters of the ship magnetic signature. The ship built of ferromagnetic elements disrupts Earth's magneticfield in its surrounding. This disor- der is defined as the ship's own magnetic field. This field contains constant and induced components(2). Thecharacteristicfeaturesofthe ship'sownmagneticfield are called magnetic signature(2,4). Magnetic signature allows to specify the distribution of the magnetic field around the ship. Magnetic signature can be determined using numerical modeling or the measurement method (3). The numerical modeling of the ship's magnetic field with commercial software has limited capacity, because the thickness of ship's metal sheets is very small in comparison with its size (3). That makes it necessary to use a dense grid of a huge number of finite elements. Additionally, the existence of the ship's permanent magnetism complicates the numerical modeling. The paper proposes a new computational method for measuring anddeterminingthe magneticsignatureofa ship, whichis basedon measurementof themagneticfieldin selected points in space around the ship, and then on creating a model of the multidipoles magnetic field of the ship. In the multidipoles model of magneticfield, a certain numberof magnetic dipoles distributed in space inside the ship are assumed. Subsequently, the spatial distribution, the values and directions of the magnetic dipole moments are found using the optimization method. The multidipoles model allows to reconstruct the ship's multidipoles magnetic field distribution in the ship's surrounding (2,4). 2. The own ship's magnetic field Measurements of the ship magnetic flux density are carried out on the measuring range. In practice, the magnetic field distribution is measured along a certain trajectory around the ship at one depth. Due to the passive protection of the ship, knowledge of the distribution of the magnetic field across the lower half-spaceinthevicinity oftheshipisrequired. Thereforeoneneedsaneffectivemethodfordetermining

Out-of-Plane Low-Frequency Vibrations and Nonradiative Decay in the 1ππ* State of Jet-Cooled 5-Methylcytosine

We investigate the UV vibronic spectrum and excited-state nonradiative processes of jet-cooled 5-methylcytosine (5MCyt) using two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm(–1) resolution. In contrast to cytosine, which shows only five bands above its electronic origin, the lowest electronic transition of 5MCyt exhibits about 25 low-frequency vibronic bands that extend to 0(0)(0) + 450 cm(–1), allowing to extract detailed information on the excited-state electronic and nuclear structure. Most bands are overtones and combinations of the out-of-plane vibrations ν'(1), ν'(2), and ν'(3). Their large intensities reflect butterfly-, boat-, and twist-deformations of the 5MCyt framework upon electronic excitation. From the rotational contours of the 0(0)(0), 1(0)(2), 2(0)(2), and 3(0)(2) bands, the transition is found to be polarized along the in-plane a/b axes, characteristic of a (1)ππ* transition. Approximate second-order coupled-cluster (CC2) and time-dependent B3LYP calculations both predict that 5MCyt undergoes an out-of-plane deformation in its (1)ππ* (S(2)) state but both methods overestimate the out-of-plane ν'(1), ν'(2), and ν'(3) vibrational frequencies by a factor of 3–5. The TD-B3LYP (1)ππ* transition dipole moment direction is 10%:90% a:b, in good agreement with experiment. From the Lorentzian line shape contributions needed to fit the rotational contours, a lower limit to the 5MCyt (1)ππ* state lifetime at the 0(0)(0), 1(0)(2), 2(0)(2), and 3(0)(2) bands is determined as τ ≥ 30 ps. These values are in stark contrast to the ultrafast (picosecond) lifetimes measured for jet-cooled cytosine by femtosecond pump–probe techniques. They also confirm the observation from the R2PI spectrum that 5-methylation of cytosine increases its excited-state lifetime. The higher out-of-plane overtone and combination bands disappear from the spectrum by ~460 cm(–1), signaling the onset of lifetimes τ < 0.5 ps, induced by excitation of the ν'(1), ν'(2), and ν'(3) modes. Above 460 cm(–1) the in-plane fundamentals 9(0)(1), 10(0)(1), and 17(0)(1) are weakly observed, implying that the in-plane vibrations couple weakly to the out-of-plane modes and have lifetimes τ > 3 ps up to 720 cm(–1) excess vibrational energy.

Picosecond Rotational Interconversion Adjacent to a C═O Bond Studied by Two-Dimensional Infrared Spectroscopy

Molecular conformations around the C═O group of carbonyl compounds like ketones and aldehydes play an important role in determining their reaction properties in solutions, including reaction rate, mechanism, steric structure, and chirality of products. Investigating different rotational conformers and their rapid exchange at room temperature will provide information on the rotational barrier and insights into how different rotamers may contribute to fundamental reactions in chemistry. We applied two-dimensional infrared (2D IR) spectroscopy and polarization-dependent IR transient grating technique to the study of 4,4-dimethyl-2-pentanone in CCl(4). Spectroscopic evidence showed that the internal rotation around the single carbon-carbon bond adjacent to the C═O group takes place on a picosecond time scale. DFT calculations suggested the presence of three different rotational conformations, one eclipsed and two staggered forms. Spectral simulation utilized the stochastic Liouville equation with a three-state jump model and incorporated the polarization factors that take into account the different direction of transition dipole moment in the three rotamers. The effects of the intramolecular vibrational energy redistribution process on the waiting time dependence of the 2D absorptive spectra were also included. Through comprehensive simulation of the observed spectral features, the exchange time constants between the three rotamers were determined: 5.4 ps from the eclipsed to staggered forms and 1.7 ps for the reverse direction.

Acentric Nonlinear Optical 2,4-Dihydroxyl Hydrazone Isomorphic Crystals with Large Linear, Nonlinear Optical Susceptibilities and Hyperpolarizability

A systematic ab initio study of the linear, nonlinear optical susceptibilities, and hyperpolarizability of noncentrosymmetric-monoclinic 2,4-dihydroxyl hydrazone isomorphic crystals (DHNPH) within density functional theory in the local density approximation (LDA), general gradient approximation (GGA), the Engel-Vosko generalized gradient approximation (EV-GGA) and modified Becke-Johnson potential (mBJ) has been performed. The complex dielectric susceptibility dispersion, its zero-frequency limit and the birefringence are studied. Using scissors’ corrected mBJ we find a large uniaxial dielectric anisotropy (-0.56) resulting in a significant birefringence (0.61). We also find that 2,4- DHNPH possess large second harmonic generation. The calculated second order susceptibility tensor components for the static limit |χ(111)(2)(0)| and |χ(111)(2)(ω)| at λ=1.9 μm (0.651 eV) and at λ = 1.064 μm (1.165 eV) are 53, 91, and 209 pm/V, respectively. A remarkable finding, applying the scissors’ correction has a profound effect on value, magnitude and sign of χ(ijk)(2)(ω). In additional we have calculated the microscopic hyperpolarizability, β(111), vector component along the principal dipole moment directions for the dominant component. We find that the value of β(111) equal to 47× 10(-30) esu, in good agreement with the measured value (48.2× 10(-30) esu).

Polarization angle dependence of stark absorption spectra of spirilloxanthin bound to the reconstituted LH1 complexes using LH1-subunits isolated from the purple photosynthetic bacterium Rhodospirillum rubrum.

Reconstituted LH1 complexes were prepared using the LH1 subunit-type complexes, isolated from the purple photosynthetic bacterium Rhodospirillum (Rs.) rubrum, and purified all-trans spirilloxanthin. Stark absorption spectra of spirilloxanthin bound to both the native and reconstituted LH1 complexes were compared in different polarization angles (χ) against the external electric field. From the polarization angle dependence of the Stark absorption spectra, two angles were determined in reference to the direction of transition dipole moment (m) of spirilloxanthin: one is the change in polarizability upon photoexcitation (Δα), θ(Δα) and the other is the change in static dipole moment upon photoexcitation (Δμ), θ(Δμ). Despite the symmetric molecular structure of all-trans spirilloxanthin, its Stark absorption spectra show pronounced values of Δμ. This large Δμ values essentially caused by the effect of induced dipole moment through Δα both in the cases for native and reconstituted LH1 complexes. However, slightly different values of θ(Δα) and θ(Δμ) observed for the native LH1 complex suggest that spirilloxanthin is asymmetrically distorted when bound to the native LH1 complex and gives rise to intrinsic Δμ value.

Photochemistry of Benzylallene: Ring-Closing Reactions to Form Naphthalene

Conformer-specific, vibrationally resolved electronic spectroscopy of benzylallene (4-phenyl-1,2-butadiene) is presented along with a detailed analysis of the products formed via its ultraviolet photoexcitation. Benzylallene is the minor product of the recombination of benzyl and propargyl radicals. The mass-selective resonant two-photon ionization spectrum of benzylallene was recorded under jet-cooled conditions, with its S(0)-S(1) origin at 37,483 cm(-1). UV-UV holeburning spectroscopy was used to show that only one conformer was present in the expansion. Rotational band contour analysis provided rotational constants and transition dipole moment direction consistent with a conformation in which the allene side chain is in the anti position, pointing away from the phenyl ring. The photochemistry of benzylallene was studied in a pump-probe geometry in which photoexcitation occurred by counter-propagating the expansion with a photoexcitation laser. The laser was timed to interact with the gas pulse in a short tube that extended the collisional region of the expansion. The products were cooled during expansion of the gas mixture into vacuum, before being interrogated using mass-selective resonant two-photon ionization. The UV-vis spectra of the photochemical products were compared to literature spectra for identification. Several wavelengths were chosen for photoexcitation, ranging from the S(0)-S(1) origin transition (266.79 nm) to 193 nm. Comparison of the product spectral intensities as a function of photoexcitation wavelength provides information on the wavelength dependence of the product yields. Photoexcitation at 266.79 nm yielded five products (benzyl radical, benzylallenyl radical, 1-phenyl-1,3-butadiene, 1,2-dihydronaphthalene, and naphthalene), with naphthalene and benzylallenyl radicals dominant. At 193 nm, the benzylallenyl radical signal was greatly reduced in intensity, while three additional C(10)H(8) isomeric products were observed. An extensive set of calculations of key stationary points on the ground state C(10)H(10) and C(10)H(9) potential energy surfaces were carried out at the DFT B3LYP/6-311G(d,p) level of theory. Mechanisms for formation of the observed products are proposed based on these potential energy surfaces, constrained by the results of cursory studies of the photochemistry of 1-phenyl-1,3-butadiene and 4-phenyl-1-butyne. A role for tunneling on the excited state surface in the formation of naphthalene is suggested by studies of partially deuterated benzylallene, which blocked naphthalene formation.

Molecular engineering and solvent dependence of excitation energy hopping in self-assembled porphyrin boxes

It has been demonstrated that the direction and magnitude of transition dipole moments, and hence rates in the excitation energy hopping in the self-assembled porphyrin boxes can be tuned by insertion of ethynyl groups as well as the dielectric constant of solvent.

A Theoretical Estimation for Dipole Moment Direction of Tubulin Dimer and Assessment of Microtubule Folding Possibility

Microtubules (MTs) are protein filaments forming a major part of the cytoskeleton of all eukaryotic cells. They are hollow cylinders with an inner radius of 15 nm and outer radius of 25 nm and with a length of up to the cell radius made up of tubulin dimers. There exist various theories about the formation of MTs from tubulin dimers. They mostly agree that MTs grow mainly along the plus-end with the addition of tubulin dimers one by one and in this process the configuration of the polymer is cylindrical. There are also some hypothesis that under specific conditions nucleating tubulin dimers polymerize to form a plain sheet with 13 parallel protofilaments and then the plain sheet folds into a cylinder. In this work we consider such a possibility with calculating the total dipole-dipole interaction energy between all tubulin dimers directly within the plain sheet and in the cylindrical form, MT. This calculation also help us to define a sub space for the direction of the dipole moment of a tubulin dimer in order to allow conformation of a MT.

Electric Field Reversal of Na2SO4, (NH4)2SO4, and Na2CO3 Relative to CaCl2 and NaCl at the Air/Aqueous Interface Revealed by Heterodyne Detected Phase-Sensitive Sum Frequency

Phase-sensitive sum frequency generation (PS-SFG) spectroscopy, heterodyne detected, was used to investigate the average direction of the transition dipole moment of interfacial water molecules that is intrinsically contained in the sign of the second-order nonlinear susceptibility, χ(2). The organization of water at air/aqueous inorganic salts interfaces of CaCl2, NaCl, Na2SO4, (NH4)2SO4, and Na2CO3 was inferred. We attribute our findings to the net charge separation arising from the ion distributions at the air/water interface assuming similar ion distribution widths for all systems studied. This is most evident for the aqueous ammonium sulfate solution where the electric field has a greater magnitude relative to the other salt solutions studied. The magnitude of the electric field in the interfacial region decreases in the order (NH4)2SO4 > Na2SO4 > Na2CO3 ≥ CaCl2 > NaCl; the electric field is opposite in direction for the sulfate and carbonate salts relative to the chloride salts.

Presence and Absence of Excited State Intramolecular Charge Transfer with the Six Isomers of Dicyano-N,N-dimethylaniline and Dicyano-(N-methyl-N-isopropyl)aniline

The excited state behavior of the six m,n-dicyano-N,N-dimethylanilines (mnDCDMA) and m,n-dicyano-(N-methyl-N-isopropyl)anilines (mnDCMIA) is discussed as a function of solvent polarity and temperature. The dicyano moiety in these electron donor (D)/acceptor (A) molecules has a considerably larger electron affinity than the benzonitrile subgroup in 4-(dimethylamino)benzonitrile (DMABN). Nevertheless, the fluorescence spectra of the mnDCDMAs and mnDCMIAs in n-hexane all consist of a single emission originating from the locally excited (LE) state, indicating that a reaction from LE to an intramolecular charge transfer (ICT) state does not take place. The calculated energies E(ICT), obtained by employing the reduction potential of the dicyanobenzene subgroups and the oxidation potential of the amino substituents trimethylamine (N(Me)(3)) and isopropyldimethylamine (iPrNMe(2)), are lower than E(LE). The absence of an LE → ICT reaction therefore makes clear that the D and A units in the dicyanoanilines are not electronically decoupled. In the polar solvent acetonitrile (MeCN), dual (LE + ICT) fluorescence is found with 24DCDMA and 34DCDMA, as well as with 24DCMIA, 25DCMIA, and 34DCMIA. For all other mnDCDMAs and mnDCMIAs, only LE emission is observed in MeCN. The ICT/LE fluorescence quantum yield ratio Φ'(ICT)/Φ(LE) in MeCN at 25 °C is larger for 24DCDMA (1.2) than for 34DCDMA (0.35). The replacement of methyl by isopropyl in the amino substituent leads to a considerable increase of Φ'(ICT)/Φ(LE), 8.8 for 24DCMIA and 1.4 for 34DCMIA, showing that the LE ⇄ ICT equilibrium has shifted further toward ICT. The appearance of an ICT reaction with the 2,4- and 3,4-dicyanoanilines is caused by a relatively small energy gap ΔE(S(1),S(2)) between the two lowest excited singlet states as compared with the other m,n-dicyanoanilines, in accordance with the PICT model. The observation that the ICT reaction is more efficient for 24DCMIA and 34DCMIA than for their mnDCDMA counterparts is mainly caused by the fact that iPrNMe(2) is a better electron donor than N(Me)(3): E(D/D(+)) = 0.84 against 1.05 V vs SCE. That ICT also occurs with 25DCMIA, notwithstanding its large ΔE(S(1),S(2)), is due to the substantial amino twist angle θ = 42.6°, which leads to partial electronic decoupling of the D and A subgroups. The dipole moments μ(e)(ICT) range between 18 D for 34DCMIA and 12 D for 25DCMIA, larger than the corresponding μ(e)(LE) of 16 and 11 D. The difference between μ(e)(ICT) and μ(e)(LE) is smaller than with DMABN (17 and 10 D) because of the noncollinear arrangement of the amino and cyano substituents (different dipole moment directions). The dicyanoanilines that do not undergo ICT, have LE dipole moments between 9 and 16 D. From plots of ln(Φ'(ICT)/Φ(LE)) vs 1000/T, the (rather small) ICT reaction enthalpies ΔH could be measured in MeCN: 5.4 kJ/mol (24DCDMA), 4.7 kJ/mol (24DCMIA), and 3.9 kJ/mol (34DCMIA). With the mnDCDMAs and mnDCMIAs only showing LE emission, the fluorescence decays are single exponential, whereas for those undergoing an LE → ICT reaction the LE and ICT picosecond fluorescence decays are double exponential. In MeCN at 25 °C, the decay times τ(2) have values between 1.8 ps for 24DCMIA and 4.6 ps for 34DCMIA at 25 °C. Longer times are observed at lower temperatures. Arrhenius plots of the forward and backward ICT rate constants k(a) and k(d) of 25DCMIA in tetrahydrofuran, obtained from the LE and ICT fluorescence decays, give the activation energies E(a) = 4.5 kJ/mol and E(d) = 11.9 kJ/mol, i.e., ΔH = -7.4 kJ/mol. From femtosecond transient absorption spectra of 24DCDMA and 34DCDMA at 22 °C, ICT reaction times τ(2) = 1/(k(a) + k(d)) of 1.8 and 3.1 ps are determined. By combining these results with the data for the fluorescence decays and Φ'(ICT)/Φ(LE), the values k(a) = 49 × 10(10) s(-1) (24DCDMA) and k(a) = 23 × 10(10) s(-1) (34DCDMA) are calculated. An LE and ICT excited state absorption is present even at a pump/probe delay time of 100 ps, showing that an LE ⇄ ICT equilibrium is established.

Tailoring organic heterojunction interfaces in bilayer polymer photovoltaic devices

In an ideal model, a p-n junction is formed by two stacked slabs of semiconductors. Although the construction of actual devices is generally more complex, we show that such a simple method can in fact be applied to the formation of organic heterojunctions. Two films of the organic semiconductors poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) can be connected by a simple film-transfer method without disturbing their flat surfaces. Each film can further be modified with a surface-segregated monolayer to tune the strength and direction of the surface dipole moment. Using this method, we fabricated bilayer organic photovoltaic devices with interfacial dipole moments that were selected to align the energy levels at the heterojunction. The open-circuit voltages of the P3HT/PCBM devices could be tuned over a wide range between 0.3 and 0.95 V, indicating that, even if the same combination of bulk materials is used, the interfacial properties drastically alter the performance of organic photovoltaic devices.

Imidazole Trimer-Water Complexes in Superfluid Helium Nanodroplets: Water Stretching Modes

Imidazole (IM) is an important moiety, having a proton donor and acceptor, in nucleic acid bases, i.e., adenine and guanine, and plays an important role in the intermolecular interactions of biological processes so that many theoretical and experimental studies of IM and its clusters have been investigated. Previously, we have presented and characterized the high-resolution infrared laser spectra of IM, imidazole dimer (IMD) and two isomers of IMand IMDwater complexes (IMW and IMDW) formed in helium nanodroplets. Recently, we have extended the study of imidazole clusters to imidazole trimer (IMT) reporting the vibrational spectra of IMT in the free NH spectral region and found out that the IM molecules tend to form linear structures due to their dipole-dipole interactions in helium nanodroplets. The unambiguous free NH band assignment was made by the experimental techniques, such as pick-up oven temperature dependence and vibrational transition moment angle (VTMA), and density functional theory (DFT) calculations. In the present note, higher order complexes, IMT with one water molecule (IMTWs), formed in He nanodroplets are presented. To the best of our knowledge, there is no report on the spectroscopic studies of IMTWs systems due to experimental difficulties in obtaining well resolved spectra. In this study we have used advantage of using helium nanodroplets since the very weak interactions between the target molecules and the helium give rise to almost unperturbed environments from that of the gas phase. Furthermore, the temperature of the helium nanodroplets is extremely low, 0.37 K, thus the spectral broadening is significantly reduced making the He droplets as an ideal matrix for infrared spectroscopy. Using the helium nanodroplet techniques for obtaining well resolved spectra, we were able to measure the angle between the transition and permanent dipole moments of the target molecules, called the vibrational transition moment angles (VTMAs), which turned out to be a very sensitive structural and unambiguous vibrational band assignment tool. Furthermore, VTMAs can be also easily calculated and compared with the experimental values obtained in this study. The use of VTMAs for the specific tautomer and vibrational band assignments of small biomolecules is well described in our previous reports. In this note we present the first spectroscopic investigation on the IMTW systems applying the VTMA techniques in He nanodroplets. Figure 1 shows the two lowest energy structures for IMTWs (> NH...OH2 and > N: ...H-O-H), where IMT is acting as a proton acceptor and donor from the nitrogen atom of the IM ring in the two lowest energy isomers, IMTW1 and 2, respectively. The structures were optimized at the B3LYP/6-311++G(d,p) level of theory and the calculated vectors representing the directions of the permanent electric dipole moments (solid arrows) and the vibrational transition moments (empty arrows) are superimposed onto the clusters. The magnitudes of the various moments for IMTWs are given in Table 1. It is clear from the figure that the patterns of VTMAs for the two isomers are quite different, making them a useful tool for distinguishing these

Theoretical investigation for Li 2CuSb as multifunctional materials: Electrode for high capacity rechargeable batteries and novel materials for second harmonic generation

Based on the first-principles electronic structure calculations, we predict that Li 2CuSb should be good electrode materials for high capacity rechargeable batteries and novel materials for second harmonic generation. This prediction is based on the experimental measurements of Fransson et al. [1], and as step forward to do deep investigation on these materials we addressed ourselves for performing theoretical calculation. We found that intercalation of lithium leads to phase transitions, which agrees well with the experiment, increasing the conductivity of the material, and break the symmetry along the optical axis making the material useful for second harmonic generation (SHG) applications. We should emphasize that lithiated compound show very high second order optical susceptibility. We present the total charge densities in the (1 1 0) and (1 0 0) planes for the parent and lithiated phases and it was found that the parent compound shows a considerable anisotropy between the two planes in consistence with our calculated optical properties. We found that Li 2CuSb possesses high second harmonic generation and its second order optical susceptibility of the total absolute value at zero frequency is equal to 142 pm/V. Based on the value of the second order optical susceptibility the microscopic second order hyperpolarizability, β ijk , the vector component along the dipole moment direction is about 31.01 × 10 −30 esu.

Pesticide interaction with environmentally important cations: A theoretical study of atrazine

The 2-chloro-1,3,5-triazine-4,6-diamine denoted AtraMod thereafter was used as a model of the atrazine pesticide molecule. AtraMod complexes with Na + and Ca 2+, two soil abundant cations, have been studied by means of Hartree–Fock, density functional theory (B3LYP), Møller–Plesset perturbation theory (MP2) and coupled cluster theory (CCSD(T)). The order of stability of the three types of complexes is the same whatever the computational level but depends on the cation. Finally, the complexation energy is strongly overestimated with B3LYP using a large basis set, all the more in the case of Ca 2+. The MP2 results, in good agreement with the CCSD(T) values, will be considered as the reference. A study of atrazine isomers with Na + and Ca 2+ inspired from the three types of AtraMod complex is also presented. AtraMod is a good model for atrazine with Na + as it allows to determine unambiguously the relative stability of the three families of isomers. For atrazine–Ca 2+, the situation is less obvious as two families of isomers overlap. However, the study of AtraMod–Ca 2+ enables to describe correctly the binding site in atrazine isomers. Finally, thermodynamic values (at 298.15 K and 1 atm) have been computed for AtraMod and atrazine complexes at various levels. For a better understanding of the interaction, a Reduced Variational Space Self Consistent Field (RVS SCF) energy decomposition has been performed for each complex with AtraMod and for a selection of isomers for atrazine. The electrostatic contribution is the predominant term of the interaction energy, however the preparation energy (to promote AtraMod from its isolated geometry to the one in the complex) must be considered to recover the right order in complexation energy. In none of the complexes the cation interacts along the dipole moment direction. A multipolar expansion of the electrostatic interaction between a charge (cation) and the AtraMod molecule (dipole, quadrupole and octupole…) has to be used to model the electrostatic contribution.

Role of dipole–dipole interaction on enhancing Brownian coagulation of charge-neutral nanoparticles in the free molecular regime

In contrast to van der Waals (vdW) forces, Coulombic dipolar forces may play a significant role in the coagulation of nanoparticles (NPs) but has received little or no attention. In this work, the effect of dipole-dipole interaction on the enhancement of the coagulation of two spherically shaped charge-neutral TiO(2) NPs, in the free molecular regime, is studied using classical molecular dynamics (MD) simulation. The enhancement factor is evaluated by determining the critical capture radius of two approaching NPs for different cases of initial dipole direction with respect to path (parallel∕perpendicular) and orientation with respect to each other (co-orientated∕counterorientated). As particle diameter decreases, the enhancement of coagulation is augmented as the ratio of dipole-dipole force to vdW force becomes larger. For 2-nm TiO(2) NPs at 273 K, the MD simulation predicts an average enhancement factor of about 8.59, which is much greater than the value of 3.78 when only the vdW force is considered. Nevertheless, as temperature increases, the enhancement factor due to dipole-dipole interaction drops quickly because the time-averaged dipole moment becomes small due to increased thermal fluctuations (in both magnitude and direction) of the instantaneous dipole moment.

Water dynamics in chabazite

Self-diffusion, libration and reorientation of water molecules in the zeolite chabazite are examined by molecular dynamics (MD) computer simulations at different temperatures and loadings. Comparison with experiment provides satisfactory agreement of the self-diffusivities and excellent agreement of the anisotropy factors and explains the failure of their prediction by the concept of structure-correlated diffusion anisotropy. Long-range electrostatic interactions are found to affect self-diffusion at high loadings and temperatures. Spectral densities of librational motion are similar to those in aqueous salt solutions, while reorientation is much slower and much more anisotropic. The vector in direction of the molecular dipole moment reorients only very slowly, as a consequence of the attraction by the almost immobile Ca + + -ions and by the walls of the zeolite. The other two vectors seem to reorient by jumps rather than by rotational diffusion.