No abstract

Potential energy surfaces for photodissociation reaction NO2→NO(2Π)+O(3P) have been studied by ab initio calculations. The effect of spin–orbit interaction on the potential surface features was studied near the product region. All the 12 potential surfaces asymptotically correlated to the NO(2Π)+O(3P) limit were obtained by the state-averaged complete-active-space-self-consistent-field (CASSCF) method. The adiabatic potential surfaces including the spin–orbit interaction were constructed using the full Breit–Pauli Hamiltonian. It was found that the lowest two states are attractive, while all the other states are repulsive. Assuming that NO2 undergoes the photodissociation on the ground state surface, we obtained the bending-rotation energy levels along the dissociation coordinate, and the transition state for each bending level was determined. The potential barriers for the vibrationally adiabatic energy curves were consistent with the recent experiments. Using a simplified model based on the infinite order sudden approximation (IOSA) and the Franck–Condon approximation, the product fine structure distribution was estimated, which is in good agreement with the experimental results.

Equation-of-motion coupled cluster with singles and doubles (EOM-CCSD) method has been applied to exciton states of polyethylene using ab initio crystal Hartree-Fock method with one-dimensional periodic boundary condition. Full transformation of two-electron integrals from atomic-orbital basis to crystal-orbital basis has been performed for EOM-CCSD calculations. In order to make transformed integrals to have correct properties of translational symmetry, a lattice summation scheme has been proposed. The EOM-CCSD excitation energies have been obtained for the lowest singlet and triplet exciton states of polyethylene. The excitation energies converge with system size much faster than oligomer calculations using n-alkanes. Quasiparticle energy-level calculations by second-order many-body perturbation theory and by solving the inverse Dyson equation have also been performed to obtain exciton binding energies. Basis set dependencies on excitation energy, quasiparticle band gap, and exciton binding energy have been investigated. At the 6-31+G level, the excitation energy of the lowest singlet-exciton state and its binding energy are calculated to be 8.1 and 3.2 eV, respectively. The calculated excitation energy is well comparable with the corresponding experimental value, 7.6 eV.

No abstract

When diacetylene RÀCtCÀCtCÀR 0 molecules are arranged in a bulk crystal, a LangmuirÀBlodgett (LB) film, or a selfassembled monolayer (SAM) etc. and irradiated with UV light, 1,4-polymerization takes place topochemically to yield a polydiacetylene (PD) (dCRÀCtCÀCR 0 d) n . 1À4 The π-conjugated systems have attracted attention on account of the electrical and optical properties. 5,6 Though a correlation between the isomerization and color of PDs had been a controversial issue, the color transition is correlated to a change in the conjugation length 7 or/ and that in the conformation of the substituents R and R 0 containing methylene sequences. 8,9 For such fundamental studies, the selection of simple substituents is crucial to elucidate the PD electronic and geometric structures interrelated closely.Most theoretical studies 10 are limited to unsubstituted PDs (R = R 0 = H), which are difficult to obtain in a laboratory owing to the extreme reactivity of the monomer, although there are a few first-principles theoretical studies on PDs with realistic substituents. 11,12 An alternative to the hydrogen is a n-alkyl group: one might assume that polyalkadiyne retains simplicity to directly link the real system with the theoretical consideration.To our knowledge, however, poly(2,4-hexadiyne) (R = R 0 = CH 3 ) is probably the only substituted PD for which the first-principles calculations have been reported. 10,11 From the experimental point of view, on the contrary, the aggregates of diacetylene molecules having alkyl groups are usually prepared by wet techniques. This means that R 0 inevitably contains a polar group at the end and complicates the intermolecular interactions and the molecular behavior upon polymerization. In addition, if the PD chains of an ordinary LB film 3 or a SAM 4 are buried in the close assembly of "standing" alkyl chains, they are hardly probed by extraordinarily surface-sensitive techniques such as Penning ionization electron spectroscopy (PIES) 13,14 and scanning probe microscopy. 15,16 For the direct observation of the local electronic and geometric structures, lying PD chains exposed outside the film are required, and atomic sash (AS) on a graphite (0001) surface 17À22 is appropriate for this purpose. ABSTRACT: Atomic sash (AS) is a single sheet of a sashlike macromolecule comprising a column of alkyl chains bridged by a polydiacetylene (PD) chain [H. Ozaki et al. J. Am. Chem. Soc. 1995, 117, 5596À5597]. The AS is produced by the intramonolayer polymerization of 17,19-hexatriacontadiyne molecules laid flat on a graphite (0001) surface under ultrahigh vacuum. In an AS conformer initially formed at low temperature (AS-I), the PD chain and the R carbon atoms of the alkyl chains are raised higher than other carbons of methylenes in contact with the substrate; with rising temperature, the AS-I is transformed to another conformer AS-II, in which all the carbon atoms are placed in a common plane [O. Endo et al. J. Am. Chem. Soc. 2004, 126, 9894À9895]. A simplified model is constructed in this study ...