Dimitri N. Laikov

High resolution EPR spectroscopy of C60F and C70F in solid argon: Reassignment of C70F regioisomers

Free radicals C(60)F and C(70)F were generated in solid argon by means of chemical reaction of photogenerated fluorine atoms with isolated fullerene molecules (C(60) or C(70)). High resolution anisotropic electron paramagnetic resonance (EPR) spectra of C(60)F and C(70)F at low temperature have been obtained for the first time. The spectrum of C(60)F is characterized by an axially symmetric hyperfine interaction on (19)F nucleus. The hyperfine coupling constants A(iso)=202.8 MHz (Fermi contact interaction) and A(dip)=51.8 MHz (electron-nuclear magnetic-dipole interaction) have been measured for C(60)F in solid argon. Quantum chemical calculations using hybrid density-functional models (either PBE0 or B3LYP) with high-quality basis sets give a theoretical estimate of the hyperfine coupling constants in good agreement with the measurements. The electron spin density distribution in C(60)F is theoretically characterized using the Hirshfeld atomic partitioning scheme. Unlike C(60), five isomers of C(70)F can in principle be produced by the attachment of a fluorine atom to one of the five distinct carbon atoms of the C(70) molecule (denoted A, B, C, D, and E, from pole to equator). The measured high resolution EPR spectrum of the C(70)+F reaction products is interpreted to show the presence of only three regioisomers of C(70)F. Based on the comparison of the measured hyperfine constants with those estimated by the quantum chemical calculation, an assignment of the spectra to the isomers (A, C, and D) is made, which differs strongly from the previous one [J. R. Morton, K. F. Preston, and F. Negri, Chem. Phys. Lett. 221, 59 (1994)]. The new assignment would allow the conclusion that the low-temperature attachment of F atom to the asymmetric C=C bonds of C(70) molecule, namely, C(A)[Double Bond]C(B) and C(D)=C(E), shows remarkably high selectivity, producing only one of the two isomers in each case, A and D, respectively. Theoretical investigation of the reaction mechanism is made, and it shows that the attachment reaction should have no barrier in the gas phase. The thermodynamic equilibration of the C(70)F isomers is excluded by the high activation energy ( approximately 30 kcal/mol) for the F atom shifts. The explanation of the high selectivity presents a challenge for theoretical modeling.

Neglect of four‐ and approximation of one‐, two‐, and three‐center two‐electron integrals in a symmetrically orthogonalized basis

A new integral approximation for use in molecular electronic structure calculations is proposed as an alternative to the traditional neglect of diatomic differential overlap models. The similarity between the symmetrically orthogonalized and the original basis functions (assumed orthonormal within each atomic set but nonorthogonal between different centers) is used to construct a robust approximation for the two‐electron integrals, with the error being quadratic in the deviation between the products of the functions. Invariance properties of this procedure are rigorously proved. Numerical studies on a representative set of molecules at valence‐only minimal basis Hartree–Fock level show that the approximation introduces relatively small errors, encouraging its future application in the semiempirical field.

Excited state geometries within time-dependent and restricted open-shell density functional theories

Abstract Singlet excited state geometries of a set of medium sized molecules with different characteristic lowest excitations are studied. Geometry optimizations of excited states are performed with two closely related restricted open-shell Kohn–Sham methods and within linear response to time-dependent density functional theory. The results are compared to wave-function based methods. Excitation energies (vertical and adiabatic) calculated from the open-shell methods show systematic errors depending on the type of excitation. However, for all states accessible by the restricted methods a good agreement for the geometries with time-dependent density functional theory and wave-function based methods is found. An analysis of the energy with respect to the mixing angle for the singly occupied orbitals reveals that some states (mostly [ n → π ∗ ] ) are stable when symmetry constraints are relaxed and others (mostly [ π → π ∗ ] ) are instable. This has major implications on the applicability of the restricted open-shell methods in molecular dynamics simulations.

A new parametrizable model of molecular electronic structure

A new electronic structure model is developed in which the ground state energy of a molecular system is given by a Hartree-Fock-like expression with parametrized one- and two-electron integrals over an extended (minimal + polarization) set of orthogonalized atom-centered basis functions, the variational equations being solved formally within the minimal basis but the effect of polarization functions being included in the spirit of second-order perturbation theory. It is designed to yield good dipole polarizabilities and improved intermolecular potentials with dispersion terms. The molecular integrals include up to three-center one-electron and two-center two-electron terms, all in simple analytical forms. A method to extract the effective one-electron Hamiltonian of nonlocal-exchange Kohn-Sham theory from the coupled-cluster one-electron density matrix is designed and used to get its matrix representation in a molecule-intrinsic minimal basis as an input to the parametrization procedure--making a direct link to the correlated wavefunction theory. The model has been trained for 15 elements (H, Li-F, Na-Cl, 720 parameters) on a set of 5581 molecules (including ions, transition states, and weakly bound complexes) whose first- and second-order properties were computed by the coupled-cluster theory as a reference, and a good agreement is seen. The model looks promising for the study of large molecular systems, it is believed to be an important step forward from the traditional semiempirical models towards higher accuracy at nearly as low a computational cost.

Ricochet inter-ring haptotropic rearrangement of σ-methyl-(η5-indenyl) chromium tricarbonyls. Experimental kinetic and theoretical DFT study

Abstract σ -Methyl-( η 5 -indenyl) chromium tricarbonyl ( III ) rearranges quantitatively into η 6 -1-endo-methylindene) chromium tricarbonyl ( IV ) in C 6 D 6 solution at 30–60°C. Methyl group attachment to the positions 2 or 3 of indenyl ligand in ( III ) has no influence on the activation parameters of this ricochet inter-ring haptotropic rearrangement ( ΔG # =23.6 kcal mol −1 ; ΔH # =18.9±0.2 kcal mol −1 ; ΔS # =−18.6±0.2 cal K −1 mol −1 ). ( IV ) undergoes further irreversible isomerization at 60–120° into ( ν 6 -3-methylindene) chromium tricarbonyl ( V ) with a higher activation barrier ( ΔG # =28.5±0.1 kcal mol −1 ) via two consecutive [1,5]-sigmatropic hydrogen shifts. The mechanisms of both rearrangements have been studied in detail using density functional theory (DFT) calculations with extended basis sets. Calculations show that the rearrangement ( III ) → ( IV ) proceeds in two steps. Methyl group migration from chromium into position 1 of the indenyl ligand is the rate-determining step leading to the formation of the 16-electron intermediate ( VII ). The calculated activation barrier ( E a =19.6 kcal mol −1 ) is in good agreement with the experimental one. Further rearrangement ( VII ) → ( V ) proceeds via a trimethylenemethane-type transition state ( XVIII ) with an activation barrier 11.8 kcal mol −1 . The coordination of the chromium tricarbonyl group at the six-membered ring has only minor influence on the kinetic parameters of the hydrogen [1,5]-sigmatropic shift in indene.

DFT study of the mechanism of alkane hydrogenolysis by transition metal hydrides. 1. Interaction of silica-supported zirconium hydrides with methane

Model reactions of silica-supported zirconium hydrides (≡Si—O—)3ZrH and (≡Si—O—)2ZrH2 with methane, resulting in cleavage of a C—H bond in the methane molecule and the formation of (≡Si—O—)3ZrCH3 and (≡Si—O—)2Zr(H)CH3 as products were studied using the DFT approach with the PBE density functional. The processes proceed as bimolecular reactions without preliminary formation of agostic complexes. According to calculations, zirconium dihydrides (≡Si—O—)2ZrH2 are more reactive toward the methane C—H bonds than zirconium monohydrides (≡Si—O—)3ZrH. The calculated activation energies of the reactions with participation of zirconium dihydrides (≡Si—O—)2ZrH2 are in better agreement with the known experimental data for the Yermakov—Basset catalytic system.

Communication: Stabilization of radical anions with weakly bound electron in condensed media: A case study of diacetonyl radical anion

The radical anion resulting from electron capture by diacetonyl molecule has been characterized by EPR and optical absorption spectroscopy in glassy ether matrices at 77 K. In non-polar alkane glasses this species was not observed under the same conditions, which confirms the crucial role of matrix interactions in stabilizing this species. Calculations at the MP2 level show the vertical detachment energy to increase gradually from roughly zero for a bare anion to ∼1 eV for the complex involving six ether molecules.

High-resolution electron spin resonance spectroscopy of XeF• in solid argon. The hyperfine structure constants as a probe of relativistic effects in the chemical bonding properties of a heavy noble gas atom

Xenon fluoride radicals were generated by solid-state chemical reactions of mobile fluorine atoms with xenon atoms trapped in Ar matrix. Highly resolved electron spin resonance spectra of XeF* were obtained in the temperature range of 5-25 K and the anisotropic hyperfine parameters were determined for magnetic nuclei 19F, 129Xe, and 131Xe using naturally occurring and isotopically enriched xenon. Signs of parallel and perpendicular hyperfine components were established from analysis of temperature changes in the spectra and from numerical solutions of the spin Hamiltonian for two nonequivalent magnetic nuclei. Thus, the complete set of components of hyperfine- and g-factor tensors of XeF* were obtained: 19F (Aiso=435, Adip=1249 MHz) and 129Xe (Aiso=-1340, Adip=-485 MHz); g(parallel)=1.9822 and g(perpendicular)=2.0570. Comparison of the measured hyperfine parameters with those predicted by density-functional theory (DFT) calculations indicates, that relativistic DFT gives true electron spin distribution in the 2Sigma+ ground-state, whereas nonrelativistic theory underestimates dramatically the electron-nuclear contact Fermi interaction (Aiso) on the Xe atom. Analysis of the obtained magnetic-dipole interaction constants (Adip) shows that fluorine 2p and xenon 5p atomic orbitals make a major contribution to the spin density distribution in XeF*. Both relativistic and nonrelativistic calculations give close magnetic-dipole interaction constants, which are in agreement with the measured values. The other relativistic feature is considerable anisotropy of g-tensor, which results from spin-orbit interaction. The orbital contribution appears due to mixing of the ionic 2Pi states with the 2Sigma+ ground state, and the spin-orbit interaction plays a significant role in the chemical bonding of XeF*.

Activation of C--H bonds in C1-C3 alkanes by titanium(iv) and zirconium(iv) cationic complexes: a DFT study

A DFT study of a model reaction [(η5-C5H5)2MCH3]+ + RH ⇌ [(η5-C5H5)2MR]+ + CH4 (M = TiIV, ZrIV; R = Me, Et, Pr, Pri) was carried out with the PBE density functional. Exchange of σ-bonded ligand proceeds through the formation of agostic complexes [Cp2M(RH)CH3]+ followed by their isomerization into complexes [Cp2M(CH4)R]+via an inner-sphere migration of a hydrogen atom. The calculated rate constants for such migrations involving the primary and secondary C--H bonds of propane molecule differ by 930 times for TiIV complexes and by 47 times for ZrIV complexes, which is due to the effect of steric factors.

Cationic (Azulene)(diene)rhodium(I) Complexes: Spectroscopic, Dynamic, and Catalytic Properties

New [RhI(η5-azulene)(η4-diene)][BF4] complex salts 3–5 (diene=8,9,10-trinorborna-2,5-diene (nbd) and (1Z,5Z)-cycloocta-1,5-diene (cod)) were synthesized according to a known procedure (Scheme 1). All of these complexes show dynamic behavior of the diene ligand at room temperature. In the case of the [RhI(η5-azulene)(cod)]+ complex salts 3 and [RhI(η5-guaiazulene)(nbd)]+ complex salt 4a (guaiazulene=7-isopropyl-1,4-dimethylazulene), the coalescence temperature of the 1H-NMR signals of the olefinic H-atoms was determined. The free energy of activation (ΔG; Table 1) for the intramolecular movement of the diene ligands exhibits a distinct dependency on the HOMO/LUMO properties of the coordinated azulene ligand. The DFT (density-functional theory) calculated ΔG values for the internal diene rotation are in good to excellent agreement with the observed ones in CD2Cl2 as solvent (Table 2). Moreover, the ΔG values can also be estimated in good approximation from the position of the longest-wavelength, azulene-centered UV/VIS absorption band of the complex salts (Table 2). These cationic RhI complexes are stable and air-resistant and can be used, e.g., as precursor complexes in situ in the presence of (M)-6,7-bis[(diphenylphosphino)methyl]-8,12-diphenylbenzo[a]heptalene for asymmetric hydrogenation of (Z)-α-(acetamido)cinnamic acid with ee values of up to 68% (Table 4).