Finite-field method calculations. IV. Higher-order moments,dipole moment gradients,polarisability gradients and field-induced shifts in molecular properties: Application to N2, CO,CN−, HCN and HNC

In this paper, theoretical methods developed in III are applied in calculating polarisabilities, polarisability gradients and field-induced shifts, by the finite-field method. Values of dipole moment gradients and higher-order moments, calculated from the unperturbed wavefunctions, are also reported. Results for N₂, CO, CN^?, HCN and HNC have been obtained at the SCF level; some CI results for the N₂ polarisability components and moments and for the dipole moment gradients of HCN are also given. The calculated polarisability gradients and dipole moment gradients have been used to estimate the Raman scattering intensities and depolarisation ratios and the IR absorption intensities. Model calculations of field-induced shifts in bond length, vibrational levels, spectroscopic constants, force constants and dipole moment gradient are reported for N₂ and CO.The discrepancy between the SCF and experimental bond dipole moment gradients for HCN, previously noted in the literature, has been re-examined and resolved by our CI results.     

Oscillator strengths and radiative lifetimes for C2: Swan, Ballik-Ramsay, Phillips, and d 3Pig<--c 3Sigmau+ systems

High level ab initio calculations, using multireference configuration interaction (MRCI) techniques, have been carried out to investigate the spectroscopic properties of the singlet A 1Piu<--X 1Sigmag+ Phillips, the triplet d 3Pig<--a 3Sigmau Swan, the b 3Sigmag-<--a 3Piu Ballik-Ramsay, and the d 3Pig<--c 3Sigmau+ transitions of C2. The MRCI expansions are based on full-valence complete active space self-consistent-field reference states and utilize the aug-cc-pV6Z basis set to resolve valence electron correlation. Core and core-valence correlations and scalar relativistic energy corrections were also incorporated in the computed potential energy surfaces. Nonadiabatic and spin-orbit effects were explored and found to be of negligible importance in the calculations. Harmonic frequencies and rotational constants are typically within 0.1% of experiment. The calculated radiative lifetimes compare very well with the available experimental data. Oscillator strengths are reported for all systems: fv'v", where 0相似文献   

Comparison of the mass transfer in totally porous and superficially porous stationary phases in liquid chromatography

The characterization of mass-transfer processes in a chromatographic column during a separation process is essential, since the influence of the mass-transfer kinetics on the shape of the chromatographic band profiles and on the efficiency of the separation is crucial. Several sources of mass transfer in a chromatographic bed have been identified and studied: the axial dispersion in the stream of mobile phase, the external mass-transfer resistance, intraparticle diffusion, and the kinetics of adsorption–desorption. We measured and compared the characteristics and performance of a new brand of shell particles and those of a conventional brand of totally porous silica particles. The shell stationary phase was made of 2.7-μm superficially porous particles (a 1.7-μm solid core is covered with a 0.5-μm-thick shell of porous silica). The other material consisted of totally porous particles of conventional 3.5-μm commercial silica. We measured the first and second central moments of the peaks of human insulin over a wide range of mobile phase velocities (from 0.02 to 1.3 mL/min) at 20°C. The plate height equations were constructed and the axial dispersion, external mass transfer, as well as the intraparticle diffusion coefficients were calculated for the two stationary phases.     

An ab initio SCF calculation of the dipole-moment derivatives and infrared-absorption intensities of the water-dimer molecule

The dipole-moment derivatives and infrared-absorption intensities of the water dimer including several deuterated species have been calculated using ab initio SCF techniques. The results are compared with the analogous, quantities for monomeric water. In addition to the highly enhanced intensity of the intramolecular OH stretch most intermolecular modes that occur in the 90–600 cm^?1 region are also found to be very intense. An electrostatic model for the water dimer has been explored with a view to developing a possible scheme for the calculation of infrared intensities of larger clusters. As a result of the significant exchange and charge-transfer effects such a model is found to be unreliable in describing the dipole-moment derivatives that directly involve the hydrogen bond.     

Mixed basis functions in molecular quantum mechanics

A series of calculations have been carried out for the linear system of four equidistant hydrogen atoms for an internuclear separation of 1.7 a.u. The configuration interaction technique was used, the orbital basis consisting of a mixed set of 1s Slater and floating spherical Gaussian functions. The results obtained are encouraging although the effects of unoptimized non-linear parameters are noticeable.

---

Zusammenfassung Es wurde eine Reihe von Rechnungen für ein lineares System von vier äquidistanten Wasserstoffatomen (Kernabstand jeweils 1.7 a.u.) durchgeführt. Dabei benutzt man die Konfigurationswechselwirkungs-Technik, die Orbitalbasis bestand aus einem gemischten Satz von 1s Slater- und floating sphärischen Gauß-Orbitalen. Die Resultate sind ermutigend, obwohl die Effekte der nicht optimierten nichtlinearen Parameter merklich sind.

     

Protonation-induced paramagnetism. Structures and stabilities of six- and seven-coordinate complexes of Os(II) in singlet and triplet states: a density functional study

Li, Yeh, and Taube in 1993 (J. Am. Chem. Soc. 1993, 115, 10384) synthesized a number of complexes which can be formally regarded as protonated Os(II) species. Some of these were paramagnetic, in contrast to the diamagnetism of the closed shell 5d(6) Os(II) ions. This intriguing phenomenon is investigated theoretically using density functional theory. The geometries, stabilities, and electronic structures of a series of six- and seven-coordinate osmium complexes were studied in gas phase and aqueous solution using the B3P86 functional, in conjunction with the isodensity-polarized continuum model of solvation. The general formula for these complexes is [Os(NH(3))(4)H(L(1)(x)())(m)()(L(2)(y)())(n)()](()(x)()(+)(y)()(+3)+), where L(1) and L(2) = H(2)O, NH(3), CH(3)OH, CH(3)CN, Cl(-), and CN(-), which could be regarded as protonated Os(II) species or hydrides of Os(IV), although according to this work the osmium-hydrogen interaction is best described as a covalent Os(III)-H bond, in which the hydrogen is near-neutral. The ground states are generally found to be singlets, with low-lying triplet excited states. Solvation tends to favor the singlet states by as much as approximately 18 kcal mol(-)(1) in the 3+ ions, an effect which is proportional to the corresponding difference in molecular volumes. To have realistic estimates of the importance of spin-orbit coupling in these systems, the spin-orbit energy corrections were computed for triplet [Os(NH(3))(4)](2+), [Os(NH(3))(4)H](3+), and [Os(NH(3))(4)H(H(2)O)](3+), along with gas-phase Os and its ions as well as [Os(H(2)O)(6)](3+). The seven-coordinate triplet-state complex [Os(NH(3))(5)H(CH(3)OH)](3+), which had been successfully isolated by Li, Yeh, and Taube, is predicted to be a stable six-coordinate complex which strongly binds to a methanol molecule in the second coordination shell. The calculations further suggest that the singlet-triplet splitting would be very small, a few kilocalories per mole at most. The geometries and the electronic structures of the complexes are interpreted and rationalized in terms of Pauling's hybridization model in conjunction with conventional ligand field theory that effectively precludes the existence of true seven-coordinate triplet-state complexes of the above formula.     

Finite-field method calculations. III. Dipole moment gradients,polarisability gradients and field-induced shifts in bond lengths,vibrational levels,

In this paper, the finite-field method is applied in calculating polarisabilities and, by extension, polarisability gradients. The method is also used to examine the shifts in a number of properties induced by electric fields of given strength. Model calculations at the SCF and Cl levels are reported for LiH. Estimates of the Raman scattering intensity and depolarisation ratio, obtained from the calculated polarisability gradient components, are presented. The dipole moment gradient, which is related to the infrared absorption intensity, is also reported. The properties for which field-dependent shifts are given comprise bond length, vibrational levels, spectroscopic constants, harmonic force constant, first anharmonic constant and dipole moment gradient.     

Oxidation of CO by SO2: a theoretical study

The elementary reaction SO(2) + CO --> CO(2) + SO((3)Sigma) (1) and the subsequent reaction SO((3)Sigma) + CO --> CO(2) + S((3)P) (2) have been studied by the application of the Gaussian-3//B3LYP quantum chemical approach to characterize the potential energy surfaces and transition state kinetic analysis to derive rate coefficients. Reaction 1 is found to take place via two transition states (TS), a cis-OSOCO TS and a trans-OSOCO TS. Reaction via the cis-TS is concerted and takes place on a singlet surface. Intersystem crossing to the final products occurs after passage through the barrier on the singlet surface. The trans-TS leads to a very weakly bound singlet OSOCO intermediate that then passes through a second TS (on the triplet surface) to form the products. Reaction 2 takes place on triplet surfaces. There is a concerted reaction through a cis-SOCO TS and a weakly bound trans-SOCO has also been identified. Reaction 2 is analogous to the reaction CO + O(2)((3)Sigma) --> CO(2) + O((3)P) (3), and this reaction has been reinvestigated at a similar level of theory and the rate coefficient derived by quantum chemistry is compared with experiment. The sensitive effects of trace impurities such as H(2), H(2)O, and hydrocarbons on the accurate experimental determination of the rate coefficient of reaction 3 is discussed. Using rate coefficients for reactions 1 and 2 obtained via quantum chemical calculations, we have been unable to model the extent of decomposition of SO(2) measured in a shock tube study of reaction between SO(2) and CO [Bauer, S. H.; Jeffers, P.; Lifshitz, A.; Yadava, B. P. Proc. Combust. Inst. 1971, 13, 417]. In light of the known sensitivity of reaction 3 to trace impurities, we have incorporated trace amounts of H(2), CH(4), or H(2)O, together with our rate coefficients for (1) and (2), in a kinetic model of Alzueta et al. [Combust. Flame 2001, 127, 2234], which is then shown to be able to substantially model the SO(2) data of Bauer et al. In the course of this modeling study we also computed heats of formation for a number of sulfur-containing small molecules: HS, HSO, HSOH, HOSO, HS(2), HSO(2), HOSO(2), HOSOH, and HOSHO.     

Seasonal Variations of Polycyclic Aromatic Hydrocarbons in Air Particulate Extracts

Aroma-active volatile organic compounds (VOCs) present in brandies produced in the Slovak Republic have been identified by gas chromatography–olfactometry (GC–O) and GC–MS. GC–O data treatment was based on a concept of detection frequency. Direct injection of samples was used due to its simplicity and the fact that determined composition is that of the original samples. This is especially important for correlation of results obtained by sensory evaluation and gas chromatographic measurements. Among 200 organic compounds found by GC only 71 showed olfactive properties at the given concentration levels. In order to identify aroma-active substances, linear retention indices (LRI) have been calculated and compared with available LRI libraries. The most aroma-active compounds have been found in samples of Frucon brandy, while Vinovica showed their minimal content. The results obtained by gas chromatography have been compared with sensory evaluation of brandy odor.