Nuclear relaxation and molecular motion of 1,3,5-trifluorobenzene-d3 in liquid solutions

The ²D and ¹⁹F spin-lattice relaxation times of 1,3,5-trifluorobenzene-d₃ in 0.15 mol fraction solutions in various solvents have been measured over the temperature range 270–400 K. The relationship between the reorientational correlation times, τ_θ, and the angular momentum correlation times, τ_J, obtained from the nuclear relaxation data has been compared with the theoretical relationships predicted by the J-diffusion limit of the extended diffusion (EDJ) model and the Fokker-Planck-Langevin (FPL) model for symmetric top molecules. It was found that the rotational motion of 1,3,5-trifluorobenzene-d₃ in most solutions was better described by the FPL model with frictional anisotropy τ₆/τ_· in the range 1–2. (τ₆ and τ_· are the correlation times for the angular velocity components along the axes parallel and perpendicular to the molecular symmetry axis.) The viscosity and temperature dependence of τ_θ has been analyzed in terms of a modified Debye equation and values for the anisotropic interaction parameter, κ, in each solvent are reported. The variation in κ with solvent is attributed mainly to the variation in the dipole moment of the solvent molecules. The frictional anisotropy (τ₆/τ_·) is found to decrease as κ increases. This is interpreted in terms of the effects of molecular shape and electrostatic interactions between CF bond dipole moment and solvent dipole moments.     

On the determination of short lifetimes of vibronic states from resonance Raman excitation profiles

The relaxational width (Γ) of an excited state and the solvent-induced broadening (σ) produce a considerable and non-additive effect on the intensity distribution inside the resonance Raman excitation profiles. An approach based on the generating function method is proposed which makes it possible to separate these effects and determine the Γ and σ values. It is shown that the first moment M₁ of an excitation profile depends on Γ alone. The variance of the profile is a sum of σ² and a function of Γ alone. a theoretical dependence of M₁ (ρ) derived under very general assumptions is shown to be a sharp and monotonous function of Γ. This dependence makes it possible to determine the value of Γ from experimentally available value of M₁. The requirements are formulated that should be met by experimental data for a practical realization of the proposed approach.     

Effects of mechanical and thermal stresses on electric degradation of polyolefins and related materials

Degradation under the simultaneous effects of mechanical stress and temperature in polyolefins (PE, PP), composites on their basis (PE+PP fibre, PP+PP fibre, PP+glass fibre) and radiation low-density polyethylene (X-LDPE) used in high-voltage cables obeys the thermofluctuation theory of Zhourkov (in certain σ and τ₀ intervals) based on the theory of Arrhenius is presented in the following form: τ_σ = τ₀ exp[(U₀ − γσ)/ RT] (1) where τ is durability. τ₀ is a constant (10⁻¹²-10⁻¹³s) equal to period of vibrations of atoms around equilibrium position, U₀ is the activation energy of the mechanical destruction process (at σ = 0), γ is a structure-sensitive parameter, T is absolute temperature and R is universal gas constant. Electric degradation under the effects of electric field and temperature in the materials mentioned above obeys the equation: τ_E = τ₀ exp[(W₀ − χE)/ RT] (2) Here, τ_E, W₀ and χ are analogous to τ_σ, U₀ and γ, respectively. It is assumed that the following equation is valid under the simultaneous effects of E, σ and T: τ_σ,E = τ₀ exp[(U₀ − (γσ + χE))/ RT]. (3) electric degradation     

Photoionization of carbon disulfide: theoretical studies of outer-valence-shell partial cross sections

Studies in vertical-electronic, static-exchange approximation are reported of the outer-valence-shell (2π_g⁻¹, 2π_u⁻¹, 5σ_u⁻¹, 6σ_g⁻¹) partial-channel photoionization cross sections of CS₂. The origins of strong features present in the calculated profiles, and in corresponding experimental cross sections, are attributed to the 6σ_u(σ¹) and 7σ_g (σ¹) orbitals of Mulliken on basis of Stieltjes orbital diagnostics.     

Magnetic circular dichroism of manganese pentacarbonyl bromide

The absorption and magnetic circular dichroism (MCD) spectra of manganese pentacarbonyl bromide Mn(CO)₅Br in a solution of 2-methyl tetrahydrofuran were measured at various temperatures over the 3500–4500 Å region. Within experimental error the MCD spectra are not temperature dependent and in addition to a negative B term exhibit a positive A term, indicative for a degenerate excited state. Following the irreducible tensor method we conclude that the transition involved will be mainly e[π, Br] → a₁[σ^*, 3d₂²] and for the excited state we calculate a magnetic moment which agrees quite closely with the experimental value.     

The evaluation and characterization of voigt lineshape

Methods used to evaluate and characterize the resonance lineshapes observed in magnetic resonance experiments are presented, with particular reference to the treatment of Voigt lineshape which results from the convolution of Gaussian and Lorentzian broadening functions. Since the Voigt integral lacks an analytical solution, a simple numerical procedure is suggested to compute the Voigt function for any value of r, the ratio of the Lorentzian linewidth (σ_L) to the Gaussian linewidth (σ_G). The method is used to characterize the profiles by their incomplete moments. Using least-squares procedure , the Voigt profiles are fitted to a polynomial of sixth degree. The methods discussed here are quite general, and can be applied to the analysis of lineshapes in other branches of spectroscopy.     

Radial moment analysis of isovalent hybridization in N2

A radial moment analysis has been performed for the Hartree–Fock molecular orbitals of the nitrogen molecule. The objective of the analysis was to determine the extent of isovalent hybridization in even and odd sigma molecular orbitals. The radial moment analysis for the LC -SCF -AO fragments of the 2σ_g, 2σ_u, and 3σ_g molecular orbitals substantiates Mulliken's earlier conjecture concerning promotion into 3s atomic orbitals for the 3σ_g molecular orbital. The concept of free isovalent hybridization is discussed in terms of the atomic orbital shape defined by the extracted moments.     

Symmetry of the dissociation of hydrogen: angular dependence of the doppler profile of the excited hydrogen atom (n = 4) produced in e-H2 collisions

The angular dependence of the Doppler profile of the Balmer-β line indicates that the asymmetry parameter, b, is positive and the polarization of the electric vector, J_p, is 0.7 ± 0.1 for the formation of H¹(n=4) from H₂. Thus, H¹(n=4) is produced in a parallel transition, and the transition moment of H¹ lies along the dissociation axis. This result suggests that the intermediate states for the fast and anisotropic H¹(n=4) atoms should be of the type ¹Σ⁺_u(2pσ_u)(n/σ_g).     

Lanthanide luminescence efficiency in eight- and nine-coordinate complexes: Role of the radiative lifetime

The photophysical parameters for the sensitization of metal-centred luminescence are analyzed in two series of complexes with tridentate and hexadentate ligands having N_xO_y chelating units. In particular, the radiative lifetime τ_rad is experimentally estimated for 29 nine-coordinate Eu^III complexes and 10 eight- and nine-coordinate Yb^III complexes. The known dependence of τ_rad on refractive index is substantiated by comparing data for solid-state samples and solutions. Moreover, a clear dependence of τ_rad with the coordination environment is evidenced and in the case of Eu^III, a comparison between τ_rad and the nephelauxetic effect generated by the ligands is attempted. Altogether, this extensive analysis points to the importance of having a handle on τ_rad when designing ligands for highly luminescent lanthanide-containing molecular edifices. This, in turn, should stimulate initiating theoretical considerations to unravel a reliable relationship between τ_rad and the electronic structure of the ligands.     

Magnetochemistry of dimers with iron group ions

General theory of matrix interactions of dimers of iron group ions is considered. Equations for calculation of matrix elements of the exchange energy of dimers and equations for calculation of the magnetic moments of dimers of different d ⁿ configurations and different exchange parameters are reported. The energies of the spin states of dimers are calculated with inclusion of exchange interactions of the first J ₀ and second J ₂ orders, and energy state diagrams for dimers with different spins and exchange energies are reported. For heterospin dimers, equations for calculation of the magnetic moment ?? and magnetic susceptibility ?? are introduced, and for homospin dimers, analytical formulas of calculation are presented. The tables of magnetic moments are calculated and their diagrams as a function of J ₀ for homospin dimers with spins S = 5/2, 2, 3/2, 1, and 1/2 are given. The experimental data are interpreted for Co²⁺ dimers as an example: the experimental and theoretical dependences of ?? and ?? on temperature are considered.     

A study of the precision and accuracy of peak quantification in comprehensive two-dimensional liquid chromatography in time

Simulated chromatographic data were used to determine the precision and accuracy in the estimation of peak volumes (i.e., peak sizes) in comprehensive two-dimensional liquid chromatography in time (LC × LC). Peak volumes were determined both by summing the areas in the second dimension chromatograms and by fitting the second dimension areas to a Gaussian peak. The Gaussian method is better at predicting the peak volume than the moments method provided there are at least three second dimension injections above the limit of detection (LOD). However, when only two of the second dimension signals are substantially above baseline, the accuracy and precision of the Gaussian fit method become quite poor because the results from the fitting algorithm become indeterminate. Based on simulations in which the modulation ratio (M_R = 4¹σ/t_s) and sampling phase (?) were varied, we conclude for well-resolved peaks that the optimum precision in peak volumes in 2D separations will be obtained when the M_R is between two and five, such that there are typically four to ten second dimension peaks recorded over the eight σ width of the first dimension peak. This sampling rate is similar to that suggested by the Murphy–Schure–Foley criterion. This provides an RSD of approximately 2% for the signal-to-noise ratio used in the present simulations. The precision of the peak volume of experimental data was also assessed, and RSD values were in the range of 4–5%. We conclude that the poorer precision found in the LC × LC experimental data as compared to LC may be due to experimental imprecision in sampling the effluent from the first dimension column.     

Polarizability effect in silatranes and related compounds

The dipole moments (μ) of the molecules, the dipole moments (μ_DA) and the length (d_DA) of the Si ← N bonds, and the electrochemical oxidation potentials (E_p) of Si-substituted silatranes, the dipole moments (μ^hs) of the molecules of Si-substituted 3-homosilatranes as well as the enthalpies of formation (ΔH⁰) of the intermolecular complexes of SiF₄ with aniline derivatives depend not only on the inductive and resonance effects, but also on the polarizability of substituents which can be characterized by the σ_α constants.     

Vibrational deactivation of CO(v = 1) by p-H2. The importance of the higher-order multipole moments

Detailed calculations show that the agreement with the experimental rate constant for the vibrational deactivation of CO by p-H₂ is improved by including the octopole moment and the hexadecapole moment of CO. Previous calculations included only the dipole and the quadrupole moments. The present theoretical rates are 10–30 percent lower than the experimental values.     

4,5-Dicyanoimidazole-promoted synthesis of dinucleoside polyphosphates and their analogs

A general and efficient P(V)–N activation method for the preparation of symmetrical and asymmetrical dinucleoside polyphosphates (Np_nN′s, n=2–4) and P²,P³-CX₂-dinucleoside tetraphosphates (X=H, F, and Cl) has been established. Twenty-two dinucleoside polyphosphates and their phosphonate analogs were synthesized from nucleoside 5′-phosphoropiperidates with 4,5-dicyanoimidazole as the activator in good to high yields.     

Theoretical study of the low-lying electronic states of ZnO and ZnS

Theoretical spectroscopic constants (r_e, D_e) and dipole moments (μ, ∂μ/∂r) are determined for the ¹σ⁺, ^1,3Π and ³σ⁺ states of ZnO and ZnS, using extended Gaussian basis sets and incorporating correlation using both configuration- interaction and coupled pair (CPF) methods. Relativistic corrections (Darwin plus mass velocity), included using first-order perturbation theory, are relatively small. At the CPF level, both ZnO and ZnS have ¹Σ⁺ ground states, with the ³Π state lying 209 and 2075 cm⁻¹ higher, respectively. The ³σ⁺ state lies about 1.5 eV higher in ZnO and 2.1 eV higher in ZnS. The ^1,3Π states are relatively close together since the exchange splitting is small with the σ electron localized on Zn and the π electron on oxygen (or sulfur).     

NMR studies of mixed-ligand iron(III) dithiocarbamates

¹H NMR studies of mixed-ligand iron (III) dithiocarbamates have been carried out using the following ligands: N,N-diethyldithiocarbamate, morpholinyl-N-, and piperidyl-N-carbodithioate. The ligand exchange equilibria gave all species of the general formula Fe(dtc)_n(dtc′)_3?n, where n = 0-3 with nearly random statistical distribution of Fe(Et₂dtc)_n(morphdtc)_3?n complexes. Magnetic moments of the mixed-ligand complexes have been determined. Both the magnetic moment and isotropic shift temperature dependences confirmed the cross-over properties of these mixed-ligand complexes.     

The use of scaled moments of inertia in experimental structure determinations of polyatomic molecules

Recently developed procedures for determining near-r _e structures (r _m ^p )from scaled ground-state moment of inertia data have been extended to general polyatomic molecules. A simple and general empirical method for correctingl _m ^p moments for the large vibration-rotation effects of deuterium substitution has been proposed and illustrated for a variety of polyatomic molecules. The correction requires utilization of a bond elongation parameter,δr _D ,whose experimental value is found to be approximately 0.003 Å. A theoretical justification of the approximation has been provided for linear triatomic molecules.     

The use of the CNDO method in spectroscopy

The Pariser approximation for the two center Coulomb repulsion integrals Γ _rshas been replaced by the Nishimoto-Mataga approximation in the original CNDO/S method. This modification has significantly improved the calculated position of the benzene ¹ B _1u ←¹ A _1g(¹ L _a ) electronic transition in benzenoid compounds. The calculation of transition moments of n — π ^* transitions is also considered. These moments vanish formally in any theory employing the ZDO approximation since integrals of the form 〈2s¦er2p〉 vanish even when the 2s and 2p atomic orbitals are on the same center. In this work the ZDO approximation is abandoned in the evaluation of the electronic transition moment resulting in calculated intensities for n — π ^*, ¹W←¹A, transitions which are in good agreement with experiment.     

Correlations of substituent parameters with the carbonyl stretching force constant in arenetricarbonylchromium complexes

Correlations are established between selected substituent parameters (σ_I,σ_P, σ_R^P,σ_P^O,σ_R^O) and the carbonyl stretching force constant, k(CO), for 28 mono and poly-substituted tricarbonylchromium-complexed arene compounds. On the basis of the statistical results it is concluded that the overall electronic substituent effect transmitted to the carbonyl groups involves both mesomeric and inductive mechanisms. Within the restricted domain, including substituent group and benzene ring, transmission proceeds largely by resonance, with a minor inductive (through-bond and field) effect operative in the same domain. Further transmission from the substituted arene ring to the chromium atom predominantly involves an inductive mechanism. This result, in support of existing literature data, suggests appreciable participation of the ring carbon σ framework in the metal—ring bond formation.     

An analytical determination of the proton second moment from tunnelling methyl groups,together with a numerical estimate of its modification due to the experimental signal-to-noise ratio

The proton second moments M₂, from both a unidirectional and an isotropic distribution of tunnelling methyl groups, are evaluated analytically in terms of the individual transitions of the proton spectrum. They are shown, in principle, to be independent of the tunnelling splitting, Δ, of the methyl group, and in the latter case, equal to the polycrystalline van Vleck rigid lattice second moment. In practice, the effects of a finite signal-to-noise ratio modify the observed M₂ and in order to account for this, second moments are evaluated numerically as a function of signal to noise ratio, for methyl groups with tunnelling splittings in the range Δ < E_D to Δ >E_D, where E_D is the intra methyl-group nuclear dipolar energy. These effects, which are most significant when Δ ≈ E_D, are linked to the experimental situation by calculating, in a representative case, the increase in M₂ with decreasing temperature, due to the inherent temperature dependence of both the spectrum and the signal-to-noise ratio. As a result the unequivocal importance of accounting for the signal-to-noise ratio when extracting tunneling splitting information from second moment data is clearly seen for that very range of methyl group barriers which can best be studied by spectral experiments.