Torsional barriers and nonlinear optical properties of 2-, 3-, 4-phenylpyridine molecules

The torsional barriers and nonlinear optical properties for all phenylpyridine molecules were calculated by using Hartree-Fock (HF) theory and Becke three-parameter functional (B3LYP) hybrid approaches within the density functional theory framework with the 6-31++G(d, p) basis set, and via the GAUSSIAN 98W. The torsional barrier computations show that dihedral angle between the two rings increases with the number of H-H vicinal interactions and torsional barriers with dihedral angles for 3-, 4-phenylpyridines are too similar for both HF and B3LYP level calculations. Also, HOMO-LUMO energy gaps, polarizabilities, anisotropy of polarizabilities, and static hyperpolarizabilities are calculated as a function of dihedral angle between benzene and pyridine rings. The study reveals that the phenylpyridines show very low nonlinear optical properties. The calculated torsional barrier, equilibrium dihedral angle and molecular dipole moment results for these molecules were compared with available experimental and other results determining from different computational methods.     

The magnetic two-centre problem: Stability and critical bonding

Stability properties of the homonuclear diatomic one-electron system are investigated by finite-elements methods. Assuming the Born-Oppenheimer approximation and parallel magnetic fields, the computed critical charges for stability and metastability reveal a surprisingly simple dependence on the magnetic field strength B.     

Extended Measurement of the v2 (1- ←0+) Band of H3O+ by Mid-Infrared Diode Laser Spectroscopy

Twenty-five new R-branch lines of the v2（1^-← 0^＋） band of H3O^＋ are measured using diode laser velocity modulation spectroscopy between 1070 and 1230 cm^-1. The H3O^＋ ions are produced in a high voltage ac discharge with water diluted in helium. The observed lines together with all the previously published measurements are fit to the standard vibration-rotational Hamiltonian of an oblate symmetric top, yielding a set of improved molecular constants. All the sextic centrifugal distortion constants for both 0^＋ and 1^- states are determined precisely. The observed R（13, 0） transition is shifted about -0.129 cm^-1 from its calculated value, indicating that a near degeneracy exists between the （13, 0）^＋ and （13, 3）^- ground-state rotation-inversion levels.     

Electronic structure and driving forces in α-cyclodextrin:butylparaben inclusion complexes

We investigate the geometry and electronic structure for complexes of α-cyclodextrin with butylparaben using DFT and Hartree-Fock calculations. The effect of solvent is explicitly taken into account. A Morokuma-Kiatura analysis of the bond energy is performed. We emphasize the role of the water, by pointing out the changes in the solvent's electronic structure for different docking geometries.     

Dissociation Energies of Diatomic Molecules

Molecular dissociation energies of 10 electronic states of alkali molecules of KH, ^7LiD, ^7 LiH, ^6LiH, NaK, NaLi and NaRb are studied using the highest three accurate vibrational energies of each electronic state, and an improved parameter-free analytical formula which is obtained starting from the LeRoy Bernstein vibrational energy expression near the dissociation limit. The results show that as long as the highest three vibrational energies are accurate, the current analytical formula will give accurate theoretical dissociation energies De^theory, which are in excellent agreement with the experimental dissociation energies De^expt.     

Quantum ergodicity and energy flow in molecules

We review a theory for coupled many-nonlinear oscillator systems that describes quantum ergodicity and energy flow in molecules. The theory exploits the isomorphism between quantum energy flow in Fock space, that is, vibrational state space, and single-particle quantum transport in disordered solid-state systems. The quantum ergodicity transition in molecules is thereby analogous to the Anderson transition in disordered solids. The theory reviewed here, local random matrix theory (LRMT), describes the nature of the quantum ergodicity transition, statistical properties of vibrational eigenstates, and quantum energy flow through the vibrational states of molecules. Predictions of LRMT have been observed in computational studies of coupled nonlinear oscillator systems, which are summarized here. We also review applications of LRMT to molecular spectroscopy and chemical reaction rate theory, including adoption of LRMT in theories that predict rates of conformational change of molecules taking place at energies corresponding to those below and above the quantum ergodicity transition. A number of specific examples are reviewed, including the application of LRMT to predict (1) dilution factors of IR spectra of organic molecules, (2) rates of conformational change in chemical and photochemical reactions, (3) conformational dynamics of biological molecules in molecular beams, (4) rates of hydrogen bond breaking and rearrangement in clusters of biological molecules and water, and (5) excited state proton transfer reactions in proteins.     

High-resolution infrared study on DCCI: harmonic force field of monoiodoacetylene

A high-resolution infrared spectrum of DCCI has been measured in the regions of the bands of 2ν₁, ν₁+ν₂, ν₂+ν₃, ν₂+ν₄, ν₂+ν₅, and ν₄+ν₅. Using these bands together with the spectra measured previously, the rovibrational states of 2ν₁, ν₁+ν₂, ν₁+ν₃, ν₁+ν₄, ν₁+ν₅, ν₁+2ν₅, ν₂+ν₃, ν₂+ν₄, ν₂+ν₅, ν₂+2ν₅, ν₃+ν₄, ν₄+ν₅, and ν₄+2ν₅ have been analyzed. Various l-type and Fermi resonances have been taken into account in the analysis. In addition, the weak Coriolis resonance between the levels υ₃=υ₄=1 and υ₄=2 has been considered. Accurate values for the molecular constants and the appropriate resonance parameters have been obtained. The unperturbed normal vibrational frequencies associated to the equilibrium state have been calculated for both isotopic species HCCI and DCCI. Then the harmonic force constants for monoiodoacetylene have been determined.     

The ground state rotational spectrum of SO2F2

The analysis of the ground state rotational spectrum of SO₂F₂ [K. Sarka, J. Demaison, L. Margulès, I. Merke, N. Heineking, H. Bürger, H. Ruland, J. Mol. Spectrosc. 200 (2000) 55] has been performed with the Watson’s Hamiltonian up to sextic terms but shows some limits due to the A and S reductions. Since SO₂F₂ is a quasi-spherical top, it can also be regarded as derived from an hypothetical XY₄ molecule. Thus we have developed a new tensorial formalism in the O(3)⊃T_d⊃C_2v group chain (M. Rotger, V. Boudon, M. Loëte, J. Mol. Spectrosc. 216 (2002) 297]. We test it on the ground state of this molecule using the same experimental data (10 GHz-1 THz region, J up to 99). Both fits are comparable even if the formalisms are slightly different. This paper intends to establish a link between the classical approach and the tensorial formalism. In particular, our tensorial parameters at a given order of the development are related to the usual ones. Programs for spectrum simulation and fit using these methods are named C_2vTDS. They are freely available at the URL:http://www.u-bourgogne.fr/LPUB/c2vTDS.html.     

Single-molecule force spectroscopy: Practical limitations beyond Bell’s model

Single-molecule force spectroscopy experiments, and a number of other physical systems, are governed by thermally activated transitions out of a metastable state under the action of a steadily increasing external force. The main observable in such experiments is the distribution of the forces, at which the escape events occur. The challenge in interpreting the experimental data is to relate them to the microscopic system properties. We work out a maximum likelihood approach and show that it is the optimal method to tackle this problem. When fitting actual experimental data it is unavoidable to assume some functional form for the force-dependent escape rate. Focusing on escape processes over a single activation barrier, we consider a quite general and common such functional form and demonstrate by means of data from a realistic computer experiment that the maximum number of fit parameters that can be determined reliably is three. They are related to the force-free escape rate and the position and height of the activation barrier. Furthermore, the results for the first two of these fit parameters show little dependence on the assumption about the manner in which the barrier decreases with the applied force, while the last one, the barrier height in the absence of force, depends strongly on this assumption.     

Sub-collision hyperfine structure of nonlinear-optical resonance with field scanning

Some experimental evidences for methane are produced that the simple transition from frequency scanning of nonlinear-optical resonances to magnetic one may be accompanied with transition from sub-Doppler collisionally broadened structure to sub-collision hyperfine one. It is conditioned by nonlinearity of splitting of hyperfine sublevel for molecules in the adiabatically varied magnetic field and, respectively, breaking the analogy of magnetic and frequency scannings. The exact calculation of the resonance structure is considered for molecules with only one spin subsystem. The approximately spin-additive calculation of the structure is given for sufficiently fast rotating molecules with greater number of spin subsystems. Within the same approximation an example of hyperfine doubling in the magnetic and electric spectra of nonlinear-optical resonance is considered for fluoromethane.     

Nonlinear Optical Properties of Novel C60 Derivatives under Picosecond Laser Excitation

We investigate the third-order nonlinear optical properties of six novel fullerene derivatives under picosecond laser excitation by Z-scan technique. The experimental results reveal that all the derivatives have very large nonlinear absorption coefficient under 532 nm pulses excitation and great third-order nonlinear refraction index under 1064 nm pulses excitation. The molecular second hyperpolarizabilities are obtained from the experimental results.     

The CΠu state of Na2 revisited: A comprehensive study by polarization labeling spectroscopy technique

This paper contains the analysis of the C(2)¹Π_u state of Na₂ molecule based on a large data set (3030 lines) obtained from a polarization labeling spectroscopy experiment on the transition. The energies of rovibrational levels in the C state, spanning a wide range of v (0 ? v ? 43) and J (12 ? J ? 100) quantum numbers, are fitted to a Dunham polynomial expansion and also to a numerical potential curve constructed by the inverted perturbation approach (IPA) method.     

High resolution microwave spectroscopy of the isomeric pair vinylcyanoacetylene and cyanovinylacetylene

The carbon chain molecules vinylcyanoacetylene and cyanovinylacetylene have been investigated between 8 and 41 GHz by Fourier transform microwave spectroscopy of a supersonic molecular beam. Owing to the high spectral resolution of the present technique, significantly more accurate rotational and centrifugal distortion constants have been derived for both molecules. In addition, the dipole moments have been calculated at the B3LYP/cc-pVTZ level of theory. Because these carbon chains are similar in structure and composition to known astronomical molecules and because of their high polarity, both species are good candidates for radioastronomical detection.     

Analysis of （1, 2） （1, 3） and （2, 3） Bands in the c^3Ⅱu- b^3Ⅱg System of P2

We investigate the high resolution absorption spectroscopy of P2 radical, generated in ac glow discharge of PC13 buffered with helium, using optical heterodyne magnetic rotation enhanced concentration modulation spectroscopy in the visible region. The （1, 2）, （1, 3） and （2, 3） bands of c^3Ⅱu- b^3Ⅱg in the range 16620-17860cm^-1 are observed and their 3II2 3II2 subbands are rotationally analysed. A set of effective molecular constants for the Ω= 2 component of the states involved are determined.     

Z-scan study of optical nonlinearities in two fullerene derivatives

The third-order nonlinear optical properties of two fullerene derivatives under picosecond laser excitation have been investigated by Z-scan technique. The experimental results reveal that all the derivatives have very large nonlinear absorption coefficient under 532 nm pulses excitation and great third-order nonlinear refraction index under 1064 nm pulses excitation. The molecular second hyperpolarizabilities have been obtained from the experimental results.     

Cavity ring-down spectroscopy of H2O in the range 16 570-17 125 cm

Following previous investigations on H₂¹⁶O and H₂¹⁸O by cavity ring-down spectroscopy, this method has now been applied to investigate the energy region of the 5ν polyad in the absorption spectrum of H₂¹⁷O. In the range 16 570-17 125 cm⁻¹, the highest energy range investigated for the H₂¹⁷O isotopologue so far, 516 lines are attributed to H₂¹⁷O and assigned from a newly generated line list.     

Theoretical Study of Photoabsorption Spectra near Si 2p Edges of Silanes： to Determine Orientations of Adsorbed Silanes

In the frame work of quantum defect theory, photoabsorption spectra near Si 219 edges of silane have been studied. When silanes are adsorbed on a physical surface and excited by polarized x-ray photons, relative intensities of the spectra will be different from that of free molecules. Such features can be used to determine orientations of adsorbed silanes based on selection rules.     

Doublet and quartet states of Li 2 −

Electronic states of the molecular lithium anion are investigated by configuration-interaction calculations. Comparison with the analogously computed potential energy curves for the lowest singlet and triplet states of the neutral Li₂ shows that in addition to the well-known stable ground state X there also exist metastable excited states of Li ₂ ^- . Within the quartet sector, two candidates for such long-lived states are identified and their spectroscopic properties studied. Received 23 March 1999     

Ab initio investigation of hydrogenation of (BN)12

Ab initio molecular orbital theory was used to examine the hydrogenation of a B₁₂N₁₂ molecule. The 1,2 addition of the 4,6 bond is an energetically favorable adsorption site in one-hydrogen-molecule adsorption. We found that the averaged bind energy of hydrogen molecule is maximized in B₁₂N₁₂H₁₂. The largest energy gaps of B₁₂N₁₂H₁₂ and B₁₂N₁₂H₂₄ suggest they have special stability. Moreover, calculation of the Gibbs free energy of the B₁₂N₁₂ + 12H₂ → B₁₂N₁₂H₂₄ reaction showed that this reaction becomes endothermic above 320 K.     

Connection between the 2-body energy of the Kaxiras-Pandey and the Biswas-Hamann potentials

Relationship among interatomic potential functions can be useful in shedding insight on the extent of similarity, and in obtaining a potential function from parameters of another potential function. The 2-body portion of the Biswas-Hamann (BH) and the Kaxiras-Pandey (KP) potential functions are related by equating both functions, as well as their corresponding derivatives up to the third order at the equilibrium bond length. Validity of the parametric relationship is verified by plotting the loose form of the 2-body BH potential in terms of KP parameters and comparing it with the KP potential function. The parametric relationships developed herein are then compared with those that concern other potential functions, with particular emphasis on the scaling factors.