Satoshi Yabushita

Generalized Hartree-Fock natural-orbital configuration-interaction (GHF NO CI) approach to unstable molecules: trimethylene

Abstract A configuration interaction (CI) method by use of natural orbitals (NO) derived from the generalized Hartree-Fock (GHF) solutions is proposed and applied to trimethylene as an example. It is found that the GHF NO CI method is equivalent to the traditional restricted HF (RHF) CI approach in the domain of weak correlation, and is similar to the generalized VB (GVB) CI method in the strong-correlation region. The GHF NO CI method thus seems to be promising for the study of the electronic structure of unstable molecules which are characterized by orbital and spin degeneracies.

Ab initio UHF and UHF NO CI approaches for quasi-degenerate systems: methylene peroxide (CH2OO)

Abstract The ab initio UHF and multi-reference (MR) UHF natural-orbital (NO) CI methods have been applied to elucidate geometrical structures and electronic energy levels of methylene peroxide which is regarded as a typical example of a quasidegenerate system. The results by MR UHF NO CI are in good agreement with those of the GVB CI method. It is shown that the present CI is closely related to the extended Hartree-Fock CI approach, which is equivalent to the GVB CI or MC SCF CI for diradical species.

Geometry optimization of the ring-opened oxirane diradical: mechanism of the addition reaction of the triplet oxygen atom to olefins

Abstract Geometry optimizations of several low-lying diradical states of the ring-opened oxirane (·CH 2 CH 2 O·) were performed by using the energy gradients of the UHF MINDO/3, STO-3G and 4-31G solutions. Both the STO-3G and 4-3 IG methods predict that the most stable form is the triplet state of the non-twisted σπ conformation in which the unpaired spins localized on the terminal carbon and oxygen atoms are oriented perpendicularly to each other. The singlet σσ diradical state in which both the radical-site p orbitals are coplanar with the molecular framework is only 2.3 (STO-3G) and 1.2 (4-31G) kcal/mol less stable than the triplet σπ diradical state. It is found that the geometry of the singlet σσ diradical is unique in that the C-C-O angle is noticeably small as compared with various other diradical states. Implications of these results to the mechanism of the oxirane-forming O( 3 P) + C 2 H 4 reaction are discussed.

Unrestricted Hartree-Fock (UHF) calculations of singlet and triplet diradicals: Nitrene peroxide (HNOO)

It is shown that the eight different diradical states of nitrene peroxide (HNOO) are well represented by the spin density wave (SWD) solutions which are equivalent to the unrestricted Hartree–Fock (UHF) solutions for this radical. (AIP)

Geometry optimizations of unstable intermediates by the generalized hartree-fock (GHF) method

Abstract The geometry optimization procedure by use of the generalized Hartree-Fock (GHF) solution is proposed and applied to trimethylene diradical, cyclopentadienyl cation and benzynes as examples. It is found that the trans -bent conformation is the most stable for trimethylene. The bond-length alternation plays an imortant role in the stabilization of the latter two systems. The GHF plus geometry optimization method seems to be promising for studies of the geometrical and electronic structures of such unstable intermediates as diradicals.

Geometry optimizations of the dioxetane, perepoxide and 1,4-diradicals for the ethylene plus molecular oxygen system: mechanism of photooxygenation of olefins

Abstract Geometry optimization of perepoxide, 1,4-diradicals and dioxetane for the ethylene plus molecular oxygen system is performed using the energy gradients of the HF 4-31G and STO-3G solutions. Perepoxide is less stable than the singlet (σπ) diradical by ≈24 k cal/mole at the 4-31G level, incompatible with the GVB CI plus thermochemical estimations. The rotational barrier of the terminal methylene group around the C-C bond is small.

Zwitterionic mechanisms for photooxygenation reactions of n-activated c-c double bonds: full geometry optimizations of the diradical and zwitterionic intermediates by ab initio SCF method

Abstract Geometry optimizations of perepoxide, 1,4-diradical, zwitterion and dioxetane for the 1,1-diaminoethylene plus singlet molecular oxygen system were performed using the energy gradients of the HF 4-31G and STO-3G solutions. The zwitterion (ZW) is more stable than the perepoxide and σπ-diradical (DR) intermediates (at the 4-31G level), supporting the previous ZW mechanism for photoovygenation reactions of N-activated C-C double bonds

DODS natural-orbital CI (DODS-NO CI) approach to the excited states of unstable molecules: twisted substituted ethyelenes

Abstract The electronic structures of the excited states of a few substituted ethylenes in the planar and perpendicularly twisted forms were investigated by the RHF CI and DODS-NO CI methods in the INDO approximation. It was found that the latter methods provides reasonable descriptions of the lower-lying excited states of both conformations. The limited DODS-NO CI method thus seems to be reliable enough for thereotical investigations of the excited states of unstable molecules.