1,720,987 research outputs found
CS-INDO/CI calculation of the singlet and triplet spectra of naphthalene and phenanthrene
No full textCS-INDO/CI calculation of the singlet and triplet spectra of naphthalene and phenanthren
Regarding enumeration of molecular isomers
No full textIn this paper, a simple theoretical approach to counting of substitution isomers is described. It is based on Polya's theorem and on point groups as recently described by us [Baraldi, I.; Fiori, C.; Vanossi, D. J. Math. Chem. 1999, 25, 23-30] and extended in this paper. Several applications are developed that range from molecules without symmetry to molecules with icosahedral symmetry (Ih) The problem of the appearance of stereoisomers is also analyzed
On the character tables of finite point groups
No full textA new method for the "complete" construction of the character tables of all the finite point groups is presented. In an explicit way, only the character tables of the five nonisomorphic rotational groups (Cn, Dn, T, O, I) have been derived, because those of the remaining groups can be obtained through isomorphisms and direct products. The characters of the 1-dimensional irreducible representations are calculated using the generators and the defining relations of the groups, while for the multi-dimensional ones explicit formulas are given
Potential-energy curves of the torsional mode of 1,1′-binaphthyl in the ground and lowest excited singlet states. A CS–INDO/CI study
No full textThe application of a recently developed theoretical method, CS–INDO/CI, to the study of the sections of the potential-energy surfaces of the ground and four lowest excited singlet states of 1,1′-binaphthyl corresponding to the coordinate of twisting around the inter-ring 1,1′-bond has proved to be successful in explaining the measured spectroscopic and photophysical properties of this molecule. All the changes observed in its spectroscopy and photophysics on going from low-temperature rigid solutions to room-temperature fluid solutions (strong bathochromic shift of the fluorescence maximum, change in the Snâ†�S1 absorption spectrum, shortening of the fluorescence lifetime by an order of magnitude) are qualitatively accounted for in terms of the calculated potential-energy curves. In particular, the S1 state experiences a drastic change in its electronic nature (from Lb to La) when the interplanar angle is allowed to relax from the near-orthogonal ground-state conformation to the trans twisted S1 equilibrium conformation (φ≈ 130 °). A comparison with the results of previous theoretical approaches stresses the reliability of the CS–INDO/CI method and the wealth of information on potential-energy surfaces which can be achieved through its application
Conformations and barriers to internal rotation in trans-diarylethylenes: Theoretical investigation using a new INDO-type method (C-INDO)
No full textC-INDO, a new INDO-based technique specially devised for conformational studies of conjugated systems, is used to investigate conformational equilibria in ground state trans-diarylethylenes, as revealed by a number of emission spectroscopic observations (both steady-state and time resolved). Conformations, ΔE and barriers to internal rotation are provided for all the possible rotamers of 1-StN (1-styrilnaphthalene), 2-StN, 2,2′-DNE (2,2′-dinaphthylethylene) and 1,2′-DNE. Energetic parameters are used to predict the relative abundances of the distinct rotameric species at equilibrium as well as to estimate the rate at which equilibria are established (at different T). The results prove to be a consistent basis on which experimentally observed behaviour can be rationalized
Rotamerism and electronic spectra of aza-derivatives of stilbene and diphenylbutadiene. A combined experimental and theoretical study
No full textThe experimental results on the rotameric equilibrium and electronic spectra of aza-derivatives of trans-stilbene and 1,4-diphenylbutadiene, have been rationalized by a theoretical study which combines simple ab initio calculations of molecular energies for the ground state with a theoretical analysis of the splitting of the conjugation band developed at CS INDO Cl level. All results indicate that the stable conformer of each ortho aza-derivative is that corresponding to A species. As suggested by the H-1-NMR experiments, the ab initio geometry of ZE-2-pyridylphenylbutadiene is consistent with the presence of the N.H intramolecular hydrogen bond. As regards the Franck-Condon excited states of aza-derivatives, our theoretical results show that the first singlet excited state has (pi(H), pi(L)(*)) character in all compounds except for E-4,4'-dipyridylethene, where S-1 has (n, pi*) character in non-polar solvent. In this last compound, the theoretical study of solvatochromism indicates a crossing between the (1)(n, pi(L)(*)) and (1)(pi(H), pi(L)(*)) states which occurs in solvents of high polarity. The inclusion of the most important. doubly- and triply-excited configurations in the CI calculations shows that the (1)A(g)(-) excited state is above the spectral region analyzed. (C) 2002 Elsevier Science B.V. All rights reserved
On the cycle index of point groups
No full textIn this brief work we express the cycle index of the molecular point groups as a function of a limited number of initial geometrical parameters. Such parameters are the number m of elements composing the domain D of sites of substitutions in the molecule belonging to the point group G, and the numbers (n(C-n),n(sigma),...) of sites of D lying on the symmetry elements (C-n,sigma,...) for G
Theoretical conformational analysis of p-, m-, and o-terphenyl
No full textA ground state conformational analysis has been carried out on p-, m-, and o-terphenyl using a recently developed semiempirical calculation method (CS-INDO). The potential energy surface has been calculated as a function of the torsion angles of the two terminal phenyls relative to the central one. Two nearly isoenergetic rotamers are found to exist for both p- and m-terphenyl, at geometries corresponding to D2, and C2h symmetries for p-terphenyl, C2, and Cs symmetries for m-terphenyl. The torsion angles are ca. 35° in all cases. The barriers separating the two minima of each potential surface are low enough to allow a rapid interconversion of the rotamers at room temperature in the gas phase (they are in “dynamic” equilibrium). o-Terphenyl presents only one equilibrium geometry. These findings are compared with the results of previous experimental and theoretical studies, which are extensively reviewed
Solvent influence on absorption and fluorescence spectra of merocyanine dyes: a theoretical and experimental study
No full textThe solvaton-CS INDO model, previously successfully used to describe the solvatochromic properties of merocyanines, has been extended to the study of the solvent influence on the fluorescence spectra (fluorosolvatochromism) of these dyes. A ketocyanine (M1) and a stilbazolium betaine (M2) were chosen as representatives of positively and negatively solvatochromic behaviours, respectively. The gap of experimental knowledge concerning the emission properties of M2 was filled by a spectrofluorometric analysis in a set of solvents covering a large range of the E-T(30) scale. Solvato- and fluorosolvatochromism were described by calculating the S-0 (eq.) --> S-1 (Franck-Condon) and S-1 (eq.) --> S-0 (Franck-Condon) transition energies as a function of a polarity factor related to the static dielectric constant of the solvent, and ranging from 0 to 1. The absorbing S-0 (eq.) and emitting S-1 (eq.) units (solute molecule + solvent cage) were approximated using the S-0 and S-1 geometries of the unsolvated molecule and the respective charge distributions fitted to the current value of k(epsilon). The calculation results fully confirm that S-0 and S-1 states of merocyanines can be viewed as a mixture of a neutral and a zwitterionic structure whose composition is controlled by the solvent polarity. The plots of the calculated spectral data (absorption and emission maxima and corresponding Stokes shifts) vs k(epsilon) are in fairly good agreement with those of the experimental data over almost the entire range of the normalized E-T(N) values, thus showing that specific solvent interactions are at least partly simulated within the solvaton-CS INDO scheme. The methodological prerequisites for a correct prediction of solvatochromic shifts are recalled with reference to previous conflicting theoretical interpretations
Correlated second-order perturbation studies of spin-spin nuclear coupling constants from CS-INDO molecular orbitals: polymethine compounds
No full textA second-order double perturbation treatment, using both the electronic con elation and Fermi contact operators as perturbation terms, has been performed for the evaluation of the indirect couplings of protons and carbon-13 nuclei of even- and odd-membered polymethine chains (conjugated polyenes and cyanines), in the frame of the CS-INDO molecular orbital method. The effect of electronic correlation on the values computed for the contact terms produces a significant increase of the (1)J and (3)J couplings and a decrease of the (2)J couplings, sometimes leading to negative coupling constants, but the nucleus s-density parameter s(N)(2)(0) of hydrogen or carbon- 13 cannot be reduced to a unique value, valid everywhere. (C) 1998 Published by Elsevier Science B.V. All rights reserved
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