Study of structural and dynamic properties of liquid phenyltrimethoxysilane

Herein, we present a combined experimental and computational study of liquid phenyltrimethoxysilane. A femtosecond time-resolved optical Kerr effect experiment has been performed to study the rotational diffusion of the molecule. A new all-atoms molecular model of the compound, based on the OPLS force field, has been developed to reproduce the rotational diffusion time constant and other physical and dynamic properties available in the literature. The density obtained from the simulations is 1074 ± 4 kg m(-3), which is within 1% of the experimental value of 1062 kg m(-3). The viscosity from the simulations is 1.6 ± 0.1 mPa s while the experimental value is 2.1 mPa s. The average bulk dipole moment of 1.8 ± 0.5 Debye obtained from the simulation matches the experimental value of 1.77 Debye. The average relative dielectric constant from the simulations is 3.86 ± 0.04, which is within 13% of the experimental value (4.4). The rotational diffusion time of the dipole moment obtained from the simulations is 20.39 ± 0.06 ps, which is in excellent agreement with the experimental value of 20 ± 1 ps obtained from our measurements. The new model has also been used to calculate structural and dynamic properties of the molecule not yet determined experimentally.     

Comparison of the experimental, semi-experimental and ab initio equilibrium structures of acetylene: influence of relativisitic effects and of the diagonal Born-Oppenheimer corrections

The equilibrium structure of acetylene (also named ethyne) has been reinvestigated to resolve the small discrepancies noted between different determinations. The size of the system as well as the large amount of available experimental data provides the quite unique opportunity to check the magnitude and relevance of various contributions to equilibrium structure as well as to verify the accuracy of experimental results. With respect to pure theoretical investigation, quantum-chemical calculations at the coupled-cluster level have been employed together with extrapolation to the basis set limit, consideration of higher excitations in the cluster operator, inclusion of core correlation effects as well as relativistic and diagonal Born-Oppenheimer corrections. In particular, it is found that the extrapolation to the complete basis set limit, the inclusion of higher excitations in the electronic-correlation treatment and the relativistic corrections are of the same order of magnitude. It also appears that a basis set as large as a core-valence quintuple-zeta set is required for accurately accounting for the inner-shell correlation contribution. From a pure experimental point of view, the equilibrium structure has been determined using very accurate rotational constants recently obtained by a "global analysis" (that is to say that all non-negligible interactions are explicitly included in the Hamiltonian matrix) of rovibrational spectra. Finally, a semi-experimental equilibrium structure (where the equilibrium rotational constants are obtained from the experimental ground state rotational constants and computed rovibrational corrections) has been obtained from the available experimental ground-state rotational constants for ten isotopic species corrected for computed vibrational corrections. Such a determination led to the revision of the ground-state rotational constants of two isotopologues, thus showing that structural determination is a good method to identify errors in experimental rotational constants. The three structures are found in a very good agreement, and our recommended values are r(CC) = 120.2958(7) pm and r(CH) = 106.164(1) pm.     

Analytical solution of vibrational–rotational energy transfer in the dissociation and relaxation of N2, Br2, and CO at high temperature

An approximate analytical solution of relaxation in a low-pressure system with exponential transition probabilities is given for vibrational–rotational energy transfer in the dissociation of diatomics. The main assumption is that the rotational degrees of freedom are in thermal equilibrium at all times, and that the barrier to dissociation in the vibrational–rotational plane is linear and asymmetric. The theory is applied to high-temperature dissociations of N₂, Br₂, and CO in excess argon, with satisfactory agreement with available experimental data.     

The chiral molecule CHClFI: first determination of its molecular parameters by Fourier transform microwave and millimeter-wave spectroscopies supplemented by ab initio calculations

The rotational spectrum of chlorofluoroiodomethane (CHClFI) has been investigated. Because its rotational spectrum is extremely crowded, extensive ab initio calculations were first performed in order to predict the molecular parameters. The low J transitions were measured using a pulsed-molecular-beam Fourier transform spectrometer, and the millimeter-wave spectrum was measured to determine accurate centrifugal distortion constants. Because of the high resolution of the experimental techniques, the analysis yielded accurate rotational constants, centrifugal distortion corrections, and the complete quadrupole coupling tensors for the iodine and chlorine nuclei, as well as the contribution of iodine to the spin-rotation interaction. These molecular parameters were determined for the two isotopologs CH35ClFI and CH37ClFI. They reproduce the observed transitions within the experimental accuracy. Moreover, the ab initio calculations have provided a precise equilibrium molecular structure. Furthermore, the ab initio molecular parameters are found in good agreement with the corresponding experimental values.     

Rotational contour analysis of the electronic origin band in the emission spectrum of the orthoxylyl radical at 4683 Å

The rotational contour of the 4683 Å emission band of the o-xylyl radical was studied at high resolution. Calculations of the rotational contour of this hybrid band were made in the rigid rotor approximation for various sets of values of the excited state rotational constants and directions of the transition moment μ. Matching of computed and experimental rotational features showed that μ is oriented at +37° or ?37° with respect to the b inertial axis. The nature of the excited states of o-xylyl and the methyl-to-ring interaction are discussed with respect to these two possible assignments     

Simultaneous determination of fundamental parameters for ferroelectric liquid crystals

An analysis of an experimental technique which allows for a simultaneous determination of several parameters with special interest for ferroelectric liquid crystals is presented. These parameters are the spontaneous polarization, the rotational viscosity and the switching time. In addition, the susceptibility associated with the soft mode free of contributions related to the helicoidal structure can also be obtained. The experimental results for these parameters for the ferroelectric liquid crystal HDOBACEEC are reported. A comparison between the switching time values deduced from the rotational viscosity and those obtained by optical measurements is performed.     

Intermolecular electrostatic interactions and Brownian tumbling in protein solutions

It is often implicitly assumed that the long-range intermolecular electrostatic interactions in homogeneous protein solutions either are negligible for affecting protein Brownian tumbling or cause its deceleration without changing the shape of rotational auto-correlation function. This review presents a wide set of experimental data (NMR relaxation, dielectric spectroscopy and Brownian dynamics simulations) demonstrating that the interprotein electrostatic steering leads to a complication of the rotational correlation function. The key point of this effect is the rotational anisotropy caused by the interaction of the electric dipole moment of a protein with the external electric field produced by charges of neighboring proteins. Taking this effect into account in some cases might be of critical importance for the correct interpretation of various experimental data.     

A classical trajectory study of the photodissociation of T1 acetaldehyde: the transition from impulsive to statistical dynamics

Previous experimental and theoretical studies of the radical dissociation channel of T(1) acetaldehyde show conflicting behavior in the HCO and CH(3) product distributions. To resolve these conflicts, a full-dimensional potential-energy surface for the dissociation of CH(3)CHO into HCO and CH(3) fragments over the barrier on the T(1) surface is developed based on RO-CCSD(T)/cc-pVTZ(DZ) ab initio calculations. 20,000 classical trajectories are calculated on this surface at each of five initial excess energies, spanning the excitation energies used in previous experimental studies, and translational, vibrational, and rotational distributions of the radical products are determined. For excess energies near the dissociation threshold, both the HCO and CH(3) products are vibrationally cold; there is a small amount of HCO rotational excitation and little CH(3) rotational excitation, and the reaction energy is partitioned dominantly (>90% at threshold) into relative translational motion. Close to threshold the HCO and CH(3) rotational distributions are symmetrically shaped, resembling a Gaussian function, in agreement with observed experimental HCO rotational distributions. As the excess energy increases the calculated HCO and CH(3) rotational distributions are observed to change from a Gaussian shape at threshold to one more resembling a Boltzmann distribution, a behavior also seen by various experimental groups. Thus the distribution of energy in these rotational degrees of freedom is observed to change from nonstatistical to apparently statistical, as excess energy increases. As the energy above threshold increases all the internal and external degrees of freedom are observed to gain population at a similar rate, broadly consistent with equipartitioning of the available energy at the transition state. These observations generally support the practice of separating the reaction dynamics into two reservoirs: an impulsive reservoir, fed by the exit channel dynamics, and a statistical reservoir, supported by the random distribution of excess energy above the barrier. The HCO rotation, however, is favored by approximately a factor of 3 over the statistical prediction. Thus, at sufficiently high excess energies, although the HCO rotational distribution may be considered statistical, the partitioning of energy into HCO rotation is not.     

星际尘埃HC_(3-x)NC_x(x=0～3)与HC_(5-x)NC_x(x=0～5)分子的DFT研究

运用密度泛函理论（DFT）中的B3LYP方法,计算了星际尘埃HC_3N,HC_5N分子 及它们的同分异构体的分子结构、相对稳定性、偶极矩、转动常数等,对于HC_(3- x)NC_x(x = 0～3),HC_3N分子比其它的异构体要稳定,将计算所得的HC_3N, HC_2NC的转动常数B_0与实际的观测值B_0相比较,发现两者能较好地吻合。比较 HC_(5-x)NC_x(x = 0～5)的各同分异构体的总能量,发现HC_5N的相对稳定性最高 ,但它的偶极矩相对较低。将HC_5N的转动常数B_0的理论计算值与实际观测值进行 比较,其相对标准误差仅0.6%。     

Brownian Motion of Lipid Molecules

Lateral and transmembrane diffusions in lipid molecules are discussed in terms of translational and rotational motions of a long cylinder. A procedure is suggested to deduce the dimensions (length and radius) of lipid molecules using experimental diffusion or viscosity data. It is shown that the hydrodynamic equation for transmembrane diffusion predicts a slower motion than for lateral diffusion, in qualitative agreement with experimental results. Further, the consequences of coupling transmembrane motion with rotational motion are discussed.     

Beam broadening of polar molecules and clusters in deflection experiments

A beam of rotating dipolar particles (molecules or clusters) will broaden when passed through an electric or magnetic field gradient region. This broadening, which is a common experimental observable, can be expressed in terms of the variance of the distribution of the resulting polarization orientation (the direction cosine). Here, the broadening for symmetric-top and linear rotors is discussed. These two types of rotors have qualitatively different low-field orientation distribution functions, but behave similarly in a strong field. While analytical expressions for the polarization variance can be derived from first-order perturbation theory, for experimental guidance it is important to identify the applicability and limitations of these expressions, and the general dependence of the broadening on the experimental parameters. For this purpose, the analytical results are compared with the full diagonalization of the rotational Stark-effect matrices. Conveniently for experimental estimations, it is found that for symmetric tops, the dependence of the broadening parameter on the rotational constant, the axial ratio, and the field strength remains similar to the analytical expression even outside of the perturbative regime. Also, it is observed that the shape envelope, the centroid, and the width of the orientation distribution function for a symmetric top are quite insensitive to the value of its rotational constant (except at low rotational temperatures).     

A two-state stochastic model for the dynamics of constrained water in reversed micelles

Translational diffusion leads the tagged water molecule to explore regions of the reversed micelle with a varying degree of space hindrance, thereby modulating the rotational relaxation process. A two-state modulation is shown to result in satisfactory agreement with the NMR experimental data on the rotational correlation time as a function of the micelle radius.     

Path integral Monte Carlo approach for weakly bound van der Waals complexes with rotations: algorithm and benchmark calculations

A path integral Monte Carlo technique suitable for the treatment of doped helium clusters with inclusion of the rotational degrees of freedom of the dopant is introduced. The extrapolation of the results to the limit of infinite Trotter number is discussed in detail. Benchmark calculations for small weakly bound (4)He(N)--OCS clusters are presented. The Monte Carlo results are compared with those of basis set calculations for the He--OCS dimer. A technique to analyze the orientational imaginary time correlation function is suggested. It allows one to obtain information regarding the effective rotational constant for a doped helium cluster based on a model for the rotational Hamiltonian. The renormalization of the effective rotational constant for (4)He(N)--OCS clusters derived from the orientational imaginary time correlation function is in good agreement with experimental results.     

Density functional theoretical (DFT) study for the prediction of spectroscopic parameters of ClCCCN

DFT(B3LYP, B3PW91) calculations in conjunction with three different basis sets have been utilized to investigate the variations in the bond lengths, dipole moment, rotational constants, IR frequencies, IR intensities and rotational invariants of ClCCCN. The nuclear quadrupole constants of chlorine ((35)Cl, (37)Cl) and nitrogen ((14)N) of ClCCCN have been calculated on the experimental r(s) structure as well as on the B3PW91/6-311++g(d,p) optimized geometry and were found to be within the scale length of the experimental uncertainty. The slope and intercept obtained from the regression analysis between the B3LYP/6-311++g(d,p) level calculated and experimental B(o) values of ClCCCN were used to calculate reasonable values of rotational constants of all the rare isotopic species of ClCCCN having standard deviation +/-0.048 MHz. All the spectroscopic parameters obtained from DFT calculations show satisfactory agreement with the available experimental data.     

The microwave and infrared spectroscopy of benzaldehyde: conflict between theory and experimental deductions

Recently, it has been proposed that ab initio calculations cannot accurately treat molecules comprised of a benzene ring with a pi-conjugated substituent, for example, benzaldehyde. Theoretical predictions of the benzaldehyde barrier to internal rotation are typically a factor of 2 too high in comparison to the experimental values of 4.67 (infared) and 4.90 (microwave) kcal mol(-1). However, both experiments use Pitzer's 1946 model to compute the reduced moment of inertia and employ the experimentally observed torsional frequency to deduce benzaldehyde's rotational barrier. When Pitzer's model is applied to a system with a nonconjugated functional group, such as phenol, the model and theoretical values are in close agreement. Therefore, we conclude the model may not account for conjugation between the substituent and the pi-system of benzene. The experimental values of the benzaldehyde rotational barrier are therefore misleading. The true rotational barrier lies closer to the theoretically extrapolated limit of 7.7 kcal mol(-1), based on coupled cluster theory.     

Effect of Rotational Couplings on Vibrational Spectrum Line Shape of the Bending Mode in Low-Density Supercritical Water: Density and Hydrogen Isotopes Dependencies

The effect of rotations on the line shape of the bending vibrational spectrum for supercritical water was analyzed using classical molecular dynamics simulation for the flexible point-charge SPC/Fw model. The experimental infrared spectrum of the bending mode at the low densities of 0.01–0.04 g·cm^?3 and at 400 °C was essentially reproduced without any other assumptions. The spectrum line shape at low densities consists of two broad rotational bands due to the rotational couplings, as in the case of the O–H stretch mode. This is due to the time-scale separation breakdown but is not due to the presence of any definite clusters. The rotational couplings become more significant at higher temperatures. The separations between the bending band center and the rotational broad side-bands are found to be linearly correlated with the inverse of the total moment of inertia of the water isotopic species, which is clear molecular-level evidence for the rotational couplings.     

Theoretical Study on Quantum Dynamics for Ar-HF Inelastic Collision

The integral cross sections and rate constants of pure rotational and ro-vibrational energy transfer processes for the Ar-HF system are thoroughly studied by using the timeindependent close coupling method based on our newly constructed potential energy surface. Compared to previous theoretical results, pure rotational transitions in this work achieve better agreement with the experimental data. For ro-vibrational energy transfer, it is found that quasi-resonant transitions dominate the cross sections in all cases. Furthermore, the vibrational-resolved rate constant of transition v=1→v=0 increases very quickly with the temperature from 100 K to 1500 K and is also in good agreement with the available experimental results.     

Investigation of the equality constraint effect on the reduction of the rotational ambiguity in three-component system using a novel grid search method

The obtained results by soft modeling multivariate curve resolution methods often are not unique and are questionable because of rotational ambiguity. It means a range of feasible solutions equally fit experimental data and fulfill the constraints. Regarding to chemometric literature, a survey of useful constraints for the reduction of the rotational ambiguity is a big challenge for chemometrician. It is worth to study the effects of applying constraints on the reduction of rotational ambiguity, since it can help us to choose the useful constraints in order to impose in multivariate curve resolution methods for analyzing data sets. In this work, we have investigated the effect of equality constraint on decreasing of the rotational ambiguity. For calculation of all feasible solutions corresponding with known spectrum, a novel systematic grid search method based on Species-based Particle Swarm Optimization is proposed in a three-component system.     

Theoretical and experimental studies of CH3X-Y2 rotational line shapes for atmospheric spectra modelling: application to room-temperature CH3Cl-O2

The case of symmetric tops CH(3)X (X = Br, Cl, F, …) perturbed by non-polar diatoms Y(2) (Y = N(2), O(2), …) is analysed from the viewpoint of theoretical collisional broadening of their rotational lines observed in atmospheric spectra. A semi-classical approach involving an exponential representation of the scattering operator and exact trajectories governed by the isotropic potential is presented. For the first time the active molecule is strictly treated as a symmetric top and the atom-atom interactions are included in the intermolecular potential model. It is shown for the CH(3)Cl-O(2) system that these interactions contribute significantly to the line width for all values of the rotational quantum numbers J and K. Additional testing of modifications required in the semi-classical formalism for a correct application of the cumulant expansion is performed and it is shown that the use of the cumulant average on the rotational states of the perturbing molecule leads to entirely negligible effects for the not very strongly interacting CH(3)Cl-O(2) system. In order to check the theoretical predictions and to extend the scarce experimental data available in the literature to higher values of the rotational quantum numbers, new measurements of room-temperature O(2)-broadened CH(3)Cl rotational lines are carried out by a photomixing continuous-wave terahertz spectrometer. The experimental line widths extracted with a Voigt profile model demonstrate an excellent agreement with theoretical results up to very high J-values (J = 31, 37, 40, 45, 50).