Calculating rate constants for intersystem crossing and internal conversion in the Franck-Condon and Herzberg-Teller approximations.

Effective and fast algorithms for calculating rate constants for internal conversion (IC) and intersystem crossing (ISC) in the Franck-Condon and Herzberg-Teller approximations have been developed and implemented. The methods have been employed for calculating IC and ISC rate constants for the pyrromethene-567 dye (PM567), hetero[8]circulene (4B) and free-base porphyrin (H2P). The fluorescence quantum yields obtained by comparing calculated rate constants for the radiative and non-radiative processes are in good agreement with experimental data.

A new look at acid catalyzed deacetylation of carbohydrates: A regioselective synthesis and reactivity of 2-O-acetyl aryl glycopyranosides.

In the present work we report that acetyl groups of per - acetylated aryl glycosides have different reactivity during the acidic deacetylation using HCl/EtOH in CHCl3, which leads to preferential deacetylation at O-3, O-4 and O-6. Thereby, the one-step preparation of 2-O-acetyl aryl glycosides with simple aglycon was accomplished for the first time. It was proved that the found reagent is to be general and unique for the preparation of series of 2-О-acetyl aryl glycosides. We have determined the influence of both carbohydrate moiety and the aglycon on the selectivity of deacetylation reaction by kinetic experiments. Using DFT/B3LYP/6-31G(d,p) and semi-empirical АМ1 methods we have found that the highest activation barrier is for 2-О-acetyl group. This completely explains the least reactivity of 2-О-acetyl group.

Ab initio 3D potential energy and dipole moment surfaces for the CH4-Ar complex: Collision-induced intensity and dimer content.

We present new three-dimensional potential energy surface (PES) and dipole moment surfaces (DMSs) for the CH4-Ar van der Waals system. Ab initio calculations of the PES and DMS were carried out using the closed-shell single- and double-excitation coupled cluster approach with non-iterative perturbative treatment of triple excitations. The augmented correlation-consistent aug-cc-pVXZ (X = D,T,Q) basis sets were employed, and the energies obtained were then extrapolated to the complete basis set limit. The dipole moment surface was obtained using aug-cc-pVTZ basis set augmented with mid-bond functions for better description of exchange interactions. The second mixed virial coefficient was calculated and compared to available experimental data. The equilibrium constant for true dimer formation was calculated using classical partition function based on the knowledge of ab initio PES. Temperature variations of the zeroth spectral moment of the rototranslational collision-induced band as well as its true dimer constituent were traced with the use of the Boltzmann-weighted squared induced dipole properly integrated over respective phase space domains. Height profiles for N2-N2, N2-H2, CH4-N2, (CH4)2, and CH4-Ar true bound dimers in Titan's atmosphere were calculated with the use of reliable ab initio PESs.

Computational and experimental studies of the electronic excitation spectra of EDTA and DTPA substituted tetraphenylporphyrins and their Lu complexes.

Ethylendiaminetetraacetic acid (EDTA) substituted and diethylenetriaminopentaacetic acid (DTPA) substituted aminated free-base tetraphenylporphyrins (H2ATPP) and the corresponding lutetium(III) complexes have been studied computationally at the density functional theory (DFT) and second-order algebraic diagrammatic construction (ADC(2)) levels using triple-ξ basis sets augmented with polarization functions. The molecular structures were optimized using Becke's three-parameter hybrid functional (B3LYP). The electronic excitation spectra in the range of 400-700 nm were calculated using the ADC(2) and the linear-response time-dependent DFT methods. The calculated spectra are compared to those measured in ethanol solution. The calculated excitation energies agree well with those deduced from the experimental spectra. The excitation energies for the Qx band calculated at the B3LYP and ADC(2) level are 0.20-0.25 eV larger than the experimental values. The excitation energies for the Qy band calculated at the B3LYP level are 0.10-0.20 eV smaller than the ADC(2) ones and are thus in good agreement with experiment. The calculated excitation energies corresponding to the Bx and By bands are 0.10-0.30 eV larger than the experimental values. The excitation energies of the Bx and By bands calculated at the B3LYP level are in somewhat better agreement with experiment than the ADC(2) ones. The calculated and measured band strengths largely agree.

Static hyperpolarizability of the van der Waals complex CH(4)-N(2).

The static first hyperpolarizability of the van der Waals CH(4)-N(2) complex was calculated. The calculations were carried out in the approximation of the rigid interacting molecules for a broad range of intermolecular separations (R = 6-40 a(0)) and for six configurations at CCSD(T) level of theory using the correlation consistent aug-cc-pVTZ basis set with the basis set superposition error correction. It was shown that the long-range classical approximation, including the terms up to R(-6), is in a good agreement with ab initio calculations for R > 11 a(0). It was found out that for the family of most stable configurations of the complex, the first hyperpolarizability invariants practically do not change (the changes are less than 0.1%). Under forming the stable van der Waals CH(4)-N(2) complex, the intensity and degree of depolarization of the hyper-Rayleigh scattering are noticeable decreased (by ∼10%) to be compared with the free CH(4) and N(2) molecules.

Computational studies of photophysical properties of porphin, tetraphenylporphyrin and tetrabenzoporphyrin.

The molecular photonics of porphyrins are studied using a combination of first-principle and semi-empirical calculations. The applicability of the approach is demonstrated by calculations on free-base porphyrin, tetraphenylporphyrin, and tetrabenzoporphyrin. The method uses excitation energies and oscillator strengths calculated at the linear-response time-dependent density functional theory (TDDFT) or the corresponding values calculated at the linear-response approximate second-order coupled-cluster (CC2) levels. The lowest singlet excitation energies obtained in the TDDFT and CC2 calculations are 0.0-0.28 eV and 0.18-0.47 eV larger than the experimental values, respectively. The excitation energies for the first triplet state calculated at the TDDFT level are in excellent agreement with experiment, whereas the corresponding CC2 values have larger deviations from experiment of 0.420.66 eV. The matrix elements of the spin-orbit and non-adiabatic coupling operators have been calculated at the semi-empirical intermediate neglect of differential overlap (INDO) level using a spectroscopic parameterization. The calculations yield rate constants for internal conversion and intersystem crossing processes as well as quantum yields for fluorescence and phosphorescence. The main mechanism for the quenching of fluorescence in tetraphenylporphyrin and tetrabenzoporphyrin is the internal conversion, whereas for free-base porphyrin both the internal conversion and the intersystem crossing processes reduce the fluorescence intensity. The phosphorescence is quenched by a fast internal conversion from the triplet to the ground state.

Theoretical investigation of the ethylene dimer: interaction energy and dipole moment.

The interaction potential energy and the interaction-induced dipole moment surfaces of the van der Waals C(2)H(4)-C(2)H(4) complex has been calculated for a broad range of intermolecular separations and configurations in the approximation of rigid interacting molecules. The calculations have been carried out using high-level ab initio theory with the aug-cc-pVTZ basis set and within the framework of the analytical description of long-range interactions between ethylene molecules. Binding energy for the most stable configuration of the C(2)H(4)-C(2)H(4) complex was calculated at the CCSD(T)/CBS level of theory. The harmonic fundamental vibrational frequencies for this complex were calculated at the MP2 level of theory.

Dipole moment surface of the van der Waals complex CH4-N2.

The interaction-induced dipole moment surface of the van der Waals CH(4)-N(2) complex has been calculated for a broad range of intermolecular separations R and configurations in the approximation of the rigid interacting molecules at the MP2 and CCSD(T) levels of theory using the correlation-consistent aug-cc-pVTZ basis set with the basis set superposition error correction. The simple model to account for the exchange effects in the range of small overlap of the electron shells of interacting molecules and the induction and dispersion interactions for large R has been suggested. This model allows describing the dipole moment of van der Waals complexes in analytical form both for large R, where induction and dispersion have the key role, and for smaller R including whole ranges of their potential wells, where the exchange effects are important. The proposed model was tested on a number of configurations of the CH(4)-N(2) complex and was applied for the analytical description of the dipole moment surface for the family of the most stable configurations of the CH(4)-N(2) complex.

Static polarizability surfaces of the van der Waals complex CH4-N2.

The static polarizability surfaces of the van der Waals complex CH(4)-N(2) have been calculated for a broad range of intermolecular separations and configurations in the approximation of rigid interacting molecules. The calculations have been carried out at the CCSD(T) and MP2 levels of the theory using the aug-cc-pVTZ basis set with the BSSE correction and within the framework of the classical long-range multipolar induction and dispersion interactions. It was shown that the results of analytical polarizability calculations for the CH(4)-N(2) complex are in a good agreement with the ab initio polarizabilities in the outer part of the van der Waals well on the complex potential surface. Ab initio calculations of the polarizability tensor invariants for the complex being in the most stable configurations were carried out. The change in the polarizability of CH(4)-N(2) due to the deformation of the CH(4) and N(2) monomers at the formation of the complex was estimated. In the framework of the analytical approach the polarizability functions alpha(ii)(R) of the free oriented interacting molecules CH(4) and N(2) were calculated.

Theoretical investigation of the potential energy surface of the van der Waals complex CH4-N2.

The interaction potential energy surface of the van der Waals CH(4)-N(2) complex has been calculated for a broad range of intermolecular separations and configurations in the approximation of rigid interacting molecules at the CCSD(T) and MP2 levels of theory using the correlation consistent aug-cc-pVTZ basis set. The BSSE correction was taken into account for all the calculations. The most stable configurations of the complex were found. Binding energies were calculated in the CBS limit with accounting for the molecular deformations. The harmonic and anharmonic fundamental vibrational frequencies and rotational constants for the ground and first excited vibrational states were calculated for the most stable configurations at the MP2 level of theory with BSSE correction. Fitting parameters were found for the most stable configuration for the Lennard-Jones and Esposti-Werner potentials.