Oleksandr Loboda

Linear and Nonlinear Optical Properties of (60)Fullerene Derivatives

Using a wide variety of quantum-chemical methods we have analyzed in detail the linear and non-linear optical properties of [60]fullerene-chromophore dyads of different electron-donor character. The dyads are composed of [60]fullerene covalently linked with 2,1,3-benzothiadiazole and carbazole derivatives. Linear scaling calculations of molecular (hyper)polarizabilities were performed using wave function theory as well as density functional theory (DFT). Within the former approach, we used both semiempirical (PM3) and ab initio (Hartree-Fock and second-order Møller-Plesset perturbation theory) methods. Within the latter approach only the recently proposed long-range (LRC) schemes successfully avoid a large overshoot in the value obtained for the first hyperpolarizability (β). Calculations on model fullerene derivatives establish a connection between this overshoot and the electron-donating capability of the substituent. Substitution of 2,1,3-benzothiadiazole by the triphenylamine group significantly increases the electronic first and second hyperpolarizabilities as well as the two-photon absorption cross section. For [60]fullerene-chromophore dyads we have, additionally, observed that the double harmonic vibrational contribution to the static beta is much larger than its electronic counterpart. The same is true for the dc-Pockels β as compared to the static electronic value, although the vibrational term is reduced in magnitude; for the intensity-dependent refractive index the vibrational and electronic terms are comparable. A nuclear relaxation treatment of vibrational anharmonicity for a model fulleropyrrolidine molecule yields a first-order contribution that is substantially more important than the double harmonic term for the static β.

Ab initio calculations of zero-field splitting parameters

We present calculations of electron spin-spin (SS) coupling strengths evaluated as expectation values over multi-configuration and restricted high-spin self-consistent field wave functions. Togethe ...

Ab initio calculations of zero-field splitting parameters in linear polyacenes

Abstract The results of ab initio calculations of zero-field splitting (ZFS) parameters are presented for the linear polyacenes from benzene to pentacene. We show how the electron spin–spin (SS) parameters can be efficiently obtained from restricted high-spin open-shell wave functions (ROHF), and present calculations of these, comparing with the results of a recent multi-configurational self-consistent field approach. The SS parameters are obtained from electron SS coupling strengths evaluated as expectation values over the wave functions and from state-to-state spin–orbit (SO) interactions. The results for the two lowest triplet states of naphthalene demonstrate that excellent values can be obtained even using moderate basis sets in the wave function, indicating that this technique can be used to obtain reliable ZFS parameters of aromatic compounds. Electron correlation is, however, not negligible; by accounting for full π-electron correlation the ZFS parameters are in considerably better agreement with experiment than the ROHF results. The ROHF method still reproduced the qualitative trend in the polyacene series in which the ZFS parameters are reduced with increasing size of the π-conjugation. We confirm that the SS coupling contributions completely determine the D and E parameters for the lowest triplet state of the linear polyacenes and that the SO coupling contributions are small. Geometry optimization of the lowest triplet state was found to be fairly significant for the calculated D and E values; these were reduced by about 30% and 40%, respectively, when the geometry was changed from the ground-state singlet to the triplet-excited state optimized geometry, with the latter values being in better agreement with experiment. The present calculations predict that the second triplet state of naphthalene is very unusual, as it has a negative zero-field splitting, implying an altered ordering of the spin sublevels compared to what is common in aromatic systems.

CASSCF calculations of triplet state properties: applications to benzene derivatives

We illustrate the capability of the complete active space self-consistent field method by Roos and co-workers for calculations of triplet state properties. We report phosphorescence lifetimes, zero-field splitting parameters, and nuclear quadrupole coupling constants for the lowest triplet state of a variety of benzene derivatives.

Electronic and vibrational linear and nonlinear polarizabilities of Li@C60 and [Li@C60]+

Electronic and vibrational nuclear relaxation (NR) contributions to the dipole (hyper)polarizabilities of the endohedral fullerene Li@C60 and its monovalent cation [Li@C60]+ are calculated at the (U)B3LYP level. Many results are new, while others differ significantly from those reported previously using more approximate methods. The properties are compared with those of the corresponding hypothetical noninteracting systems with a valence electron transferred from Li to the cage. Whereas the NR contribution to the static linear polarizabilities is small in comparison with the corresponding electronic property, the opposite is true for the static hyperpolarizabilities. A relatively small, but non‐negligible, NR contribution to the dc‐Pockels effect is obtained in the infinite frequency approximation.

Revealing water’s secrets: deuterium depleted water

BackgroundThe anomalous properties of water have been of great interest for generations of scientists. However the impact of small amount of deuterium content which is always present in water has never been explored before. For the first time the fundamental properties of deuterium depleted (light) water at 4°C and 20°C are here presented.ResultsThe obtained results show the important role of the deuterium in the properties of bulk water. At 4°C the lowest value of the kinematic viscosity (1.46 mm2/s) has been found for 96.5 ppm D/H ratio. The significant deviation in surface tension values has been observed in deuterium depleted water samples at the both temperature regimes. The experimental data provides direct evidence that density, surface tension and viscosity anomalies of water are caused by the presence of variable concentration of deuterium which leads to the formation of water clusters of different size and quantity.ConclusionsThe investigated properties of light water reveal the origin of the water anomalies. The new theoretical model of cluster formation with account of isotope effect is proposed.

Singlet-triplet transitions in three-atomic molecules studied by time-dependent MCSCF and density functional theory

Singlet–triplet transition moments and phosphorescence lifetimes have been calculated for the three-atomic molecules HCN, O3, H2O, H2S, GeF2, GeCl2 and GeBr2 by time-dependent density functional theory (DFT) utilizing quadratic response functions in order to qualify DFT which recently has become available for studies of this kind [TUNELL, I., Rinkevivius, Z., VAHTRAS, O., SALEK, P., HELGAKER, T., and ÅGREN, H., 2003, J. chem. phys., 119, 11024]. Comparison with ab initio and experimental data indicates that DFT exhibit results of similar quality as explicitly correlated methods which indicates that it indeed is a viable approach for singlet–triplet transitions. O3 provides an intriguing example in that a systematic investigation of the singlet–triplet transition moment of its Wulf band indicates a clear advantage of the DFT technique despite the multiconfigurational character of the electronic structure of this molecule. The electronic spin–spin coupling and the hyperfine nuclear coupling constants have also been calculated in order to further characterize the triplet state in the spectra of the investigated systems.

Ab initio study of nonhomogeneous broadening of the zero-field splitting of triplet guest molecules in diluted glasses

Nonhomogeneous broadening of phosphorescence lines and microwave signals in optical detection of magnetic resonance (ODMR) has been calculated using multiconfigurational self-consistent field wave functions and the polarized continuum model. The solvent effects on the zero-field splitting (ZFS) parameters in the low-lying triplet states of aza-aromatic molecules are found to be linearly dependent on the solvent-induced shifts in the phosphorescence frequency in agreement with experimental data. The main contribution to the ZFS originates in the dipolar interaction of the two electron spins, the spin–spin coupling. The spin–orbit coupling (SOC) contribution to the ZFS parameter is much larger for the 3nπ* state of pyrazine compared to the 3ππ* states of quinoline. The second-order SOC contribution to the splitting of the 3nπ* state in the pyrazine molecule does not show any appreciable dependence on the dielectric constant of the solvent. This raises doubts about earlier theories for explaining the inhomog...

Quantum-chemical studies on Porphyrins, Fullerenes and Carbon Nanostructures

Porphyrins.- Exohedral Metallofullerenes.- Nonlinear optical properties of fullerene derivatives.- Enhohedral metallofullerenes.- Fullerene-Porphyrin Dyads.- Linear Scaling Methodology.

Multiple Additions of Palladium to C60

Abstract Density functional theory calculations on exohedral metallofullerenes Pd n C60 show that the palladium–fullerene bond energy remains essentially constant for n=1−6. A novel Pd2(η2−C60) structure with the two metal atoms bridging over a six‐membered ring has been identified as the most stable arrangement of two palladium atoms on the surface of C60, although entropy considerations suggest that both isolated atoms and weakly bonded metal aggregates may exist in equilibrium. Both metal atoms benefit from η2 coordination at (6–6′) junctions as well as some metal–metal interaction. Binding of Pd atoms to the fullerene is preferred over palladium dimerization.