İlker Özkan

Franck-Condon principle for polyatomic molecules: Axis-switching effects and transformation of normal coordinates

Abstract A rigorous derivation of the relationship between the vibrational coordinates belonging to two different electronic states of a polyatomic molecule is given. Contrary to common supposition, it is shown that this transformation is generally nonlinear and nonorthogonal. The nonlinearity arises because each electronic state has its own molecule-fixed axis system, and hence axis-switching effects, first discussed by Hougen and Watson ( Canad. J. Phys. 43, 298–320 (1965)), must be taken into account. Explicit expressions giving the transformation to first order in the vibrational variables are presented, and it is shown that the transformation may be linearized for small-amplitude vibrations. As an example, the bent, symmetric XY 2 molecule is treated in detail. Axis-switching effects are suggested as providing an alternative mechanism to the Herzberg-Teller theory of vibronic activity of nontotally symmetric modes.

Nucleus-independent chemical shift evaluation for benzo- and dibenzo-fused pyrrole, furan and thiophene derivatives

Abstract Aromaticity of pyrrole, indole, isoindole, indolizine, carbazole, furan, benzofuran, isobenzofuran, dibenzofuran, thiophene, benzothiophene, benzo[c]thiophene and dibenzothiophene has been examined via their nucleus-independent chemical shift values (NICS(0) and NICS(1)) calculated at GIAO-HF/6-31G*//B3LYP/6-31G* and GIAO-HF/6-31+G*//B3LYP/6-31G* levels.

Theoretical Study of Thermal Rearrangements of 1-Hexen-5-yne, 1,2,5-Hexatriene, and 2-Methylenebicyclo(2.1.0)pentane

In this research, a comprehensive theoretical investigation of the thermal rearrangements of 1-hexen-5-yne, 1,2,5-hexatriene, and 2-methylenebicyclo[2.1.0]pentane is carried out employing density functional theory (DFT) and high level ab initio methods, such as the complete active space self-consistent field (CASSCF), multireference second-order Møller-Plesset perturbation theory (MRMP2), and coupled-cluster singles and doubles with perturbative triples [CCSD(T)]. The potential energy surface (PES) for the relevant system is explored to provide a theoretical account of pyrolysis experiments by Huntsman, Baldwin, and Roth on the target system. The rate constants and product distributions are calculated using theoretical kinetic modelings. The rate constant for each isomerization reaction is computed using the transition state theory (TST). The simultaneous first-order ordinary-differential equations are solved numerically for the relevant system to obtain time-dependent concentrations, hence the product distributions at a given temperature. Our computed energy values (reaction energies and activation parameters) are in agreement with Roths experiments and the product distributions of Huntsmans experiments at 340 and 385 °C with various reaction times, while simulated product fractions are in qualitative accordance with Baldwins experiment.

Intermolecular acetaldehyde and dimethoxymethane formation mechanisms via ethenol and methoxymethylene precursors in reactions of atomic carbon with methanol: a computational study.

Atomic carbon, a reactive intermediate abundant in the interstellar medium (ISM) can participate in various energetically demanding reactions in its extremely long living (69 min) first excited singlet state ((1)D). Several studies on reactions of oxygen containing species with carbon atoms have been reported, however mechanistic details of the title reaction remain obscure. We report here quantum chemical studies on reactions of methanol with (3)P and (1)D carbon atoms at the CCSD(T)/cc-pVTZ level of theory, with which experimentally well known facile CO production, intermolecular acetaldehyde formation, and intermolecular dimethoxymethane production mechanisms are explained. Energetics of the fragmentation, O-H insertion, C-H insertion, and O-C insertion channels on the triplet and singlet surfaces are studied. The CO production mechanism by C ((1)D) is identified as an oxygen abstraction and a triplet PES seems non-operative. Presenting novel features for the intermolecular reaction channels, current findings may be applicable to C + ROR reactions.

Substituent effects on the transannular ring closure of 2,4-cyclooctadienones to 5,5-fused ring systems: an AM1 study

Abstract Substituent effects on the transannular ring closure of 2,4-cyclooctadienones to 5,5-fused ring systems were theoretically investigated by using AM1 method at the restricted HF level. As anticipated by elementary considerations, it was found that electron-donating substituents at positions 2 and 4 of cyclooctadienone lower the activation barrier, and make the transannular cyclization feasible. Substituents elsewhere on the ring including position 3 do not have a significant effect on the rearrrangement, their influence being mainly steric.

The energy spectrum of a spin-1/2 ladder with mixed interactions

We studied the energy spectrum of two coupled XY spin-1/2 chains with different exchange integrals and inter-chain interaction of Ising type. The thorough analytical treatment of two-magnon states was given for finite and infinite lattice strips. To study the case of macroscopic number of inverted spins we used the corresponding density matrix renormalization group (DMRG) simulation. We also derived the self-consistent Bardeen-Cooper-Schrieffer (BCS) estimates for the lowest energies of the system. The comparison of the BCS and the DMRG estimates has shown incorrect behaviour of the BCS approximation for energy as a function of the intra-chain exchange integrals. To use the analytical results for two-magnon bound states we proposed a simple approximate formula for the lowest n-magnon energies of an infinite ladder with a more correct dependence on model parameters.

The ground state spin ordering and lowest energy excitations of a model organic ferrimagnet-polyallyl spin chain

Abstract We studied zero-temperature magnetic ordering in a model organic ferrimagnet- polyallyl spin-1/2 chain as a function of the exchange integral λ describing the interaction between spins of radical centers and the main chain. Exact diagonalization and density matrix renormalization group (DMRG) methods demonstrated that for small applied fields, the difference in zero-temperature magnetizations of the sublattices reaches a maximum value at λ =0.70. It was shown that there is a critical value of magnetic field, above which this ferrimagnetic ordering is destroyed. We also present the results of DMRG calculations that show the gap character of excitations without decrease of the total spin for λ ⩾0.4.