Azumao Toyota

Tetrahydrides of third-row transition elements: Spin-orbit coupling effects on the stability of rhenium tetrahydride

The potential energy surfaces of low-lying states in rhenium tetrahydride (ReH(4)) were explored by using the multiconfiguration self-consistent field (MCSCF) method together with the SBKJC effective core potentials and the associated basis sets augmented by a set of f functions on rhenium atom and by a set of p functions on hydrogen atoms, followed by spin-orbit coupling (SOC) calculations to incorporate nonscalar relativistic effects. The most stable structure of ReH(4) was found to have a D(2d) symmetry and its ground state is (4)A(2). It is found that this is lower in energy than the dissociation limit, ReH(2)+H(2), after dynamic correlation effects are taken into account by using second-order multireference Møller-Plesset perturbation (MRMP2) calculations. This reasonably agrees with previous results reported by Andrews et al. [J. Phys. Chem. 107, 4081 (2003)]. The present investigation further revealed that the dissociation reaction of ReH(4) cannot occur without electronic transition from the lowest quartet state to the lowest sextet state. This spin-forbidden transition can easily occur because of large SOC effects among low-lying states in such heavy metal-containing compounds. The minimum-energy crossing (MEX) point between the lowest quartet and sextet states is proved to be energetically and geometrically close to the transition state for the dissociation reaction on the potential energy surface of the lowest spin-mixed state. The MEX point (C(2) symmetry) was estimated to be 9184 cm(-1) (26.3 kcal/mol) higher than the (4)A(2) state in D(2d) symmetry at the MRMP2 level of theory. After inclusion of SOC effects, an energy maximum on the lowest spin-mixed state appears near the MEX point and is recognized as the transition state for the dissociation reaction to ReH(2)+H(2). The energy barrier for the dissociation, evaluated to be MEX in the adiabatic picture, was calculated to be 5643 cm(-1) (16.1 kcal/mol) on the lowest spin-mixed state when SOC effects were estimated at the MCSCF level of theory.

The doublet instabilities of the Hartree-Fock solutions for the cation and anion radicals of fulvalene systems

The doublet instabilities of the restricted Hartree-Fock (RHF) solutions for the ion radicals of fulvalene systems, some of which have been known to show the lattice instabilities, are examined in the framework of the semiempirical SCF MO approximation. The RHF solutions for the anion radical of heptafulvalene and the cation radical of pentafulvalene are found to be doublet unstable at the conventional D2h nuclear arrangement. We calculate the broken-symmetry charge-density-wave (CDW) solutions lower in energy than the usual symmetry-adapted RHF solutions and examine their properties. It is shown that the energy lowerings due to the CDW solutions are very small in both the ion radicals, but their density matrix elements are remarkably different from those for the symmetry-adapted RHF solutions. It is also shown that the proton hyperfine splittings for the anion radical of heptafulvalene calculated assuming the CDW solution are in good agreement with the experimental values. Finally, we refer to the relationship between the doublet instability of the RHF solution and the lattice instability.

Thermal and photochemical behaviour, structure and bonding of the stereoisomers of 6,7,8,9,14a,14b-hexahydrodipyrido[1,2-a:2′,1′-c][1,4]diazocine

The meso and (±) isomers of the title compound have been prepared by reduction of 1,1′-(butane-1,4-diyl)dipyridinium dibromide with sodium amalgam. The meso form converts thermally into the (±) form, which reverts photochemically into the meso form through the diradical intermediate. The diradical generated by photolysis was characterized by EPR and absorption spectroscopy. MNDO calculations indicate that the energetically most favourable meso and (±) conformers possess an extremely long C–C single bond of ca. 1.60 A at the position joining the two 1,2-dihydropyridine rings, the latter being lower in energy than the former by 5.7 kcal mol–1. The available experimental facts are reasonably accounted for by the theoretical results.

The length of a CC single bond joining two 1,2-dihydropyridine rings in 6,7,12a,12b-tetrahydrodipyrido[1,2-a:2′,1′-c]pyrazine and its congeners

Abstract We have carried out full geometry optimizations using the MNDO MO method on conformers of the titled molecule and those of its congeners. It is found that a dl form is more stable than the corresponding meso form by 1.2–5.7 kcal mol −1 . Further, the CC single bond which joins the two 1,2-dihydropyridine rings is found to be unusually long, especially for the meso conformer of the higher members, ca. 1.60 A. A perturbational MO analysis reveals that a through-bond interaction is operative between the two non-conjugated C=C-C=C-N π systems via the CC single bond. Besides, the C atoms of the CC single bond are shown to carry large positive charges, and a fairly good linear correlation is found between the CC single bond distance and the magnitude of the net atomic charge. The origin of bond elongation common to the meso and dl conformers of the present molecules is taken as the combined effects due to the through-bond and coulombic interactions.

Structural properties of 3,10-diazadispiro[5.0.5.3]pentadeca-1,4,8,11-tetraene and its homologues: a generation of an unusually long CC single bond by through-bond interaction

Abstract Full geometry optimizations using the MNDO MO method have been carried out on the title molecule and its homologues. The CC single bond joining the two 1,4-dihydropyridine rings is found to be unusually long, especially for 3,10-diazadispiro-[5.0.5.2]tetradeca-1,4,8,11-tetraene, 1.618 A. A perturbational MO analysis clearly reveals that a through-bond interaction is operative between the two non-conjugated π systems via the CC single bond. The origin of bond elongation common to the molecules is taken as the combined effects due to the through-bond interaction and the inter-ring steric repulsion. The available experimental facts are reasonably accounted for by the theoretical results.

The electronic structure of the heptafulvalene trianion radical—The doublet instability of the Hartree-Fock solution and related phenomena

The restricted Hartree-Fock (RHF) solution for the trianion radical of heptafulvalene, obtained using the open-shell SCF formalism of the Pariser-Parr-Pople method, is found to be doublet unstable at the conventionalD2h nuclear arrangement. We calculate the broken-symmetry charge-density wave (CDW) solution and examine its properties. It is shown that the proton hyperfine splittings for the trianion radical calculated from the CDW solution are in good agreement with the experimental values. Further, we briefly refer to the possibility of the lattice instability of the trianion radical from the fully-symmetricalD2h to theC2v nuclear arrangement.

The nonsinglet instabilities of the hartree-fock solutions for nonalternant hydrocarbons

The nonsinglet instabilities of the restricted Hartree-Fock (RHF) solutions for the nonalternant hydrocarbons, propalene, pentalene, heptalene and nonalene, that have been known to show the lattice instabilities (bond-length alternations) when treated within a semiempirical RHF approximation are examined. By examining the energy lowerings due to the appearance of the spin-density-wave (SDW) solutions, or better, of the singlet projected SDW solutions, we show that the treatment of the lattice instability based on the RHF theory is valid at least in these molecules. On the basis of the energy lowerings ΔE(ERHF-ESDW), we discuss-the dependence of the electron correlation energies on the number of π-electrons.

Multiconfiguration Self-Consistent Field Study on Formonitrile Imine and N-Substituted Nitrile Imines HCN2–R: Energy Component Analysis of the Pseudo-Jahn–Teller Effect

Stable geometrical structures for formonitrile imine (1) and N-substituted nitrile imines HCN2-R (R = Li, BeH, BH2, CH3, CN, CCH, C6H5, NH2, OH, and F) (2-11) were examined by using the multiconfiguration self-consistent-field (MCSCF) method followed by second-order configuration interaction (SOCI) calculations and second-order multiconfiguration quasi-degenerate perturbation theory (MCQDPT2) calculations, together with the aug-cc-pVTZ basis sets. The results show that 1 suffers a pseudo-Jahn-Teller (JT) distortion from a linear C∞v structure to a C1 structure via a planar bent Cs structure. Each of the others is found to undergo pseudo-JT distortion from a symmetrical structure to a planar bent Cs structure for 2, 3, and 7 and to a C1 structure for 4, 5, 6, 8, 9, 10, and 11. At the stationary structures of 1-11, the structural characteristics were briefly discussed in terms of allenic and propargylic. To elucidate the nature of pseudo-JT distortions, energy component analyses were carried out at the MCSCF+SOCI level of theory at all of the stationary structures for the relevant molecules. In most of the molecules examined, pseudo-JT stabilizations were classified into two groups, one in which the stability arises from a lowering of the energy of the attractive term Ven and the other in which the stability results from a lowering of the energy of the repulsive terms Vnn and Vee. In addition to the above two groups, it was also found that the following three groups are responsible for the pseudo-JT stabilizations in a certain stage of the structural changes. Namely, one is a lowering of the energy of the term Vee observed in 6, another is a lowering of the energy of the terms Vee and Ven observed in 9-11, and the other is a lowering of the energy of the terms Ven and Vnn observed in 10. These energetic behaviors were accounted in terms of an elongation or a contraction of the molecular skeleton, a migration of electrons from one part of the molecule to other parts, and the combined effects arising from these two factors.

Numerical Estimation of the Pseudo-Jahn–Teller Effect Using Nonadiabatic Coupling Integrals in Monocyclic and Bicyclic Conjugated Molecules

The pseudo-Jahn-Teller (pJT) effect in monocyclic and bicyclic conjugated molecules was investigated by using the state-averaged multiconfiguration self-consistent field (MCSCF) method, together with the 6-31G(d,p) basis sets. Following the perturbation theory, the force constant along a normal mode Q is given by the sum of the classical force constant and the vibronic contribution (VC) resulting from the interaction of the ground state with excited states. The latter is given as the sum of individual contributions arising from vibronic interactions between the ground state and excited states. In the present work, each VC was calculated on the basis of nonadiabatic coupling (NAC) integrals. Furthermore, the classical force constant was estimated by taking advantage of the VC and the force constant obtained by vibrational analyses. For pentalene and heptalene, the present method seems to overestimate the VC in absolute value because of the small energy gap between the ground state and the lowest excited state. However, we are confident that the VC and the classical force constant for the other molecules are reasonable in magnitude in comparison with available literature information. Thus, it is proved that the present method is applicable and useful for numerical estimation of pJT effect.

Bond Distortions in Dibenzo[a,g]-s-indacene and Its Congeners

Semiempirical MO calculations predict that dicyclopenta[a,g]-, dicyclopenta[a,h]-, dibenzo[a,g]-, dicyclohepta[a,g]-, and dicyclohepta[a,h]-s-indacene undergo no bond distortions, retaining the full molecular symmetry groups, whereas dibenzo[a,h]-s-indacene suffers a pseudo Jahn–Teller bond distortion from C2v to Cs.