Takeshi Noro

Theoretical study on the photoisomerization of azobenzene

Ab initio calculations are performed to elucidate the mechanism of the photoisomerization of azobenzene. We obtain the excitation energies of the S1(n→π*), S2(π→π*), and S3(n2→π*2) states by complete active space self-consistent field (CASSCF) and multireference single double configuration interaction (MRSDCI) calculations. Two-dimensional potential surfaces of the ground- and excited states are obtained at the CASSCF level in order to investigate the isomerization pathways. A conical intersection between the ground state and the S1 state is found near the midpoint of the rotation pathway, and causes a radiationless transition. On the other hand, the S2 state has local minima at the cis and trans structures, so that the isomerization proceeds at the S2 surface following the deexcitation.

Comparison between spin restricted and unrestricted post-Hartree—Fock calculations of effective exchange integrals in Ising and Heisenberg models

Abstract Spin-restricted and unrestricted post-Hartree—Fock calculations were carried out for clusters of triplet methylene and nitroxide radicals. The UHF-based methods such as UMP and QCISD followed by approximate spin projection provide reasonable energy differences between the high-and low-spin states of these species. They are close to the corresponding values from spin-restricted multi-reference (MR) approaches such as CASSCF and second-order (SO) CI. Implications of SOCI and MRSDCI results are discussed in relation to the size inconsistency erros involved in ab initio calculations of weak interaction energies, such as the effective exchange integrals in Ising and Heisenberg models.

Ab initio molecular dynamics simulation of photoisomerization in azobenzene in the nπ∗ state

Photoisomerization mechanism of azobenzene in the lowest excited state S(1)(n pi(*)) is investigated by ab initio molecular dynamics (AIMD) simulation with the RATTLE algorithm, based on the state-averaged complete active space self-consistent field method. AIMD simulations show that cis to trans isomerization occurs via two-step rotation mechanism, accompanying rotations of the central NN part and two phenyl rings, and this process can be classified into two types, namely, clockwise and counterclockwise rotation pathways. On the other hand, trans to cis isomerization occurs via conventional rotation pathway where two phenyl rings rotate around the NN bond. The quantum yields are calculated to be 0.45 and 0.28+/-0.14 for cis to trans and trans to cis photoisomerizations, respectively, which are in very good agreement with the corresponding experimental results.

MO theoretical studies of magnetic interactions in clusters of nitronyl nitroxide and related species

Abstract Ab initio and semi-empirical molecular orbital calculations are carried out for clusters of nitroxide radicals. The intermolecular effective exchange integrals calculated are positive (ferromagnetic) in the case of appropriately stacked forms of these species. Implications of the calculated results are discussed in relation to the ferromagnetic intermolecular interactions observed for nitroxide radicals such as para-nitrophenyl nitronyl nitroxide.

Ab Initio QM/MM Molecular Dynamics Study on the Excited-State Hydrogen Transfer of 7-Azaindole in Water Solution

Ab initio molecular dynamics (AIMD) simulations for the excited-state hydrogen transfer (ESHT) reaction of 7-azaindole (7AI-(H2O)n; n = 1, 2) clusters in the gas phase and in water are presented. The effective fragment potential (EFP) is employed to model the surrounding water molecules. The AIMD simulations for 7AI-H2O and 7AI-(H2O)2 clusters show an asynchronous hydrogen transfer at t approximately 50 fs after the photoexcitation. While the ESHT mechanism for 7AI-H2O in water does not change appreciably compared with that in the gas phase, the AIMD simulations on 7AI-(H2O)2 in water solution exhibit two different mechanisms. Since the tautomer form is lower in energy compared to the normal form in the S1 state, 7AI and (H2O) n fragments separate from each other after the ESHT. With the use of the results of the AIMD trajectories, the minimum energy conical intersection point in the tautomer region has also been located.

Compact and efficient basis sets of s- and p-block elements for model core potential method

We propose compact and efficient valence-function sets for s- and p-block elements from Li to Rn to appropriately describe valence correlation in model core potential (MCP) calculations. The basis sets are generated by a combination of split MCP valence orbitals and correlating contracted Gaussian-type functions in a segmented form. We provide three types of basis sets. They are referred to as MCP-dzp, MCP-tzp, and MCP-qzp, since they have the quality comparable with all-electron correlation consistent basis sets, cc-pVDZ, cc-pVTZ, and cc-pVQZ, respectively, for lighter atoms. MCP calculations with the present basis sets give atomic correlation energies in good agreement with all-electron calculations. The present MCP basis sets systematically improve physical properties in atomic and molecular systems in a series of MCP-dzp, MCP-tzp, and MCP-qzp. Ionization potentials and electron affinities of halogen atoms as well as molecular spectroscopic constants calculated by the best MCP set are in good agreement with experimental values.

Abinitio SCF CI calculations on the ground and π–π* excited states of the pyrrole molecule and its positive ion

Ab initio SCF π‐electron CI calculations are carried out on the ground and lower π–π* excited states of pyrrole and its positive ion. The four singlet states found at 5.9, 6.8, 7.2, and 7.4 eV by experiment are tentatively assigned as the 1B2, 1A1, 1B2, and 1A1 states, respectively. The calculated excitation energies for these four states are 6.0, 6.5, 7.1, and 7.3 eV, respectively. The first excited 1A1 state is a valence type state, whereas the other three states are Rydberg type states. The lowest excited state found at 4.35 eV by experiment is assigned as the 3B2 state, whose calculated excitation energy is 4.27 eV. The three lowest excited states of the positive ion due to π‐electron ionization are also investigated. The calculated ionization potentials are 8.10 (2A2), 8.94 (2B1), and 13.33 eV (2B1). They are in good agreement with experimental values of 8.21, 9.20, and 13.0 eV. The lowest two states are represented by la2→∞ and 2b1 →∞, respectively, whereas the second 2B1 state is a mixture of three...

Contracted polarization functions for the atoms magnesium through argon

Abstract Contracted Gaussian-type function (GTF) sets are developed for polarization functions of the atoms from magnesium to argon. A segmented contraction scheme is used for its compactness and computational efficiency. The contraction coefficients and orbital exponents are determined to minimize the difference from accurate atomic natural orbitals in polarization space. The present polarization functions yield greater than 99.5% of atomic correlation energies predicted by accurate natural orbitals of the same size. Molecular tests of the polarization functions are performed for the P 2 and Si 2 molecules at self-consistent field (SCF) and single and double excitation configuration interaction (SDCI) levels. The present polarization function sets are shown to be superior to both averaged atomic natural orbital (ANO) and Dunning et al.s correlation consistent sets.

Valence and correlating basis sets for the second transition-metal atoms from Y to Cd

Contracted Gaussian-type function sets are developed for the valence 5s and 4d orbitals and for correlating functions of the second transition-metal atoms Y through Cd. A segmented contraction scheme is used for its compactness and efficiency. Contraction coefficients and exponents are determined by minimizing the deviation of the target function from the average of accurate atomic natural orbitals for the three lowest LS states arising from the 5s24d n-2, 5s1 4d n-1, and 5s0d n configurations. The present basis sets give a well balanced description for the three configurations at the Hartree—Fock level, and yield more than 97% of the atomic correlation energies predicted by accurate natural orbitals of the same size.

The ground state of the Fe2 molecule

The Fe2 molecule is a typical transition metal dimer which has a rather large dissociation energy and a small bond distance compared with the inter‐nuclear distance in the crystalline metal. We have investigated the Fe2 molecule with multireference self‐consistent‐field (MCSCF) and multireference configuration interaction (CI) calculations. The dissociation energy (De), the equilibrium nuclear distance (Re), and the zero‐point frequency (ωe) were calculated (with observed in parentheses) as 1.57 (1.30±0.22) eV, 2.06 (1.87 to 2.02) A, and 260.9 (299.6) cm−1, respectively. Thus the agreement between experiment and calculation is very satisfactory, and is a marked improvement on previous theoretical studies. The contribution of the d electrons to the bonding is important and a proper description of correlation effects among the d electrons is indispensable.