John W. Downing

Solution photochemistry of poly(dialkylsilanes): a new class of photoresists

polysilanes r epresent a new class of polymeric materials of considerable theoretical and practical i nterest. Although formally saturated, they absorb in the near UV and undergo rapid photodegradation . After a review of the present understanding of the e lectronic structure of poly(dialkylsi1a nes) as a function of backbone conformation, we summarize their photophysics and describe recent advances in the understanding of their solution

Photophysics and potential-energy hypersurfaces of permethylated oligosilanes

Information on the potential-energy surfaces of the ground and 1σσ* excited states of oligosilane chains, collected from experimental and theoretical sources, is discussed in terms of electronic structure and the mechanisms of bond and excitation delocalization.

Highly strained single and double bonds

The infrared spectra of the strained single bond compounds, [2.2.lipropellane, {2.1.1]propellaneand [1.1.1]propellane, andthe vibrational and electronic spectra of the strained bridgehead double bond compounds, adamantene and azahomoadamantene, are reported and discussed.

Excited singlet states of pyrene: Polarization directions and magnetic circular dichroism of azapyrenes

Abstract Linear dichroism in polyethylene, polarization of fluorescence excitation, and magnetic circular dichroism of pyrene and three of its aza derivatives have been measured and used to uncover new electronic transitions and to determine polarization directions. The MCD signs are sensitive to structure. The results, including absolute MCD signs, compare well with π-electron calculations. The effects of systematic incorporation of doubly excited configurations in the CI procedure are investigated. Finally, it is shown that the absolute MCD signs, and in particular, their variation with the location of the aza nitrogen, are readily understood in qualitative terms upon inspection of Huckel MOs, permitting facile predictions for other isomers with-out any computations.

Direct observation of a doubly excited state of pleiadene

Abstract Evidence is presented that the second excited state of pleiadene (I) is of doubly excited nature. Such states are of interest in discussions of excited state and ground state reactivities and for further development of calculational methods.

Multidimensional interpolation by polynomial roots

Abstract A new procedure is proposed for interpolation and extrapolation of functions by a root of a low-degree polynomial. Three examples of the application of the procedure are presented. The rms error in representing SCF surfaces ranges from 2 cal/mole (linear HCN) to 0.23 kcal/mole (C 2V MgH 2 triplet).

Bridgehead double bonds

A survey is given of recent progress in our laboratory in the preparation and spectroscopic characterization of compounds containing highly twisted bridgehead CN and CC double bonds.

Selection of configurations for SCF-CI calculations of spectral properties and comparison with TDHF calculations: I. Convergence to CCI results in PPP calculations on S 0?S x transitions in 6?-electron systems

Comparison is presented of energies, oscillator strengths, and polarization directions calculated using the dipole length and dipole velocity formulas for the lowest 4–7ππ* transitions in eight 6π-electron molecules of Cs symmetry using various amounts of CI, from singly excited configurations only (SCI) to complete CI (CCI), and also using the time-dependent Hartree-Fock (TDHF) method. The standard simple PPP approximation was used. For the strongest transitions, SCI and especially TDHF give results in fairly good agreement with CCI. For weaker transitions both SCI and TDHF give similar results, quite different from the CCI solutions. Rational methods for selecting a small number of configurations needed to reproduce correct order of excited states (SECI-1) or a somewhat larger number reproducing correct order, energies, oscillator strengths and polarization directions of transitions (SECI-2) are presented. Both, and particularly the latter, give similar results from both dipole length or dipole velocity formulas when Linderbergs relation is used for matrix elements of the linear momentum operator and in this respect resemble the TDHF and CCI methods for which both formulas necessarily give identical results.

Vibrational transition moment directions in medium-size molecules: Experiment and theory

Abstract Linear dichroism is observed in the infrared spectra of dimethyl ether, isobutene, 2-butyne, and n-butane dissolved in stretched sheets of ordinary and perdeuterated poly-ethylene. For each observed vibrational band the orientation factor has been determined and related to the transition-moment direction for the vibration, providing information on its symmetry species. For n-butane, the angles of the vibrational transition moment directions in the molecular framework have been derived and compared with the angles calculated at various levels of semiempirical and ab initio theory.

MCD of non-aromatic cyclic π-electron systems. Part 1. The perimeter model for antiaromatic 4N-electron [n]annulene biradicals1

The LCAO version of the perimeter model with overlap through second order is used to obtain algebraic solutions for the singlet electronic states of the antiaromatic 4N-electron [n]annulenes of Dnh symmetry. The states of these biradicals are classified and their spectroscopic properties derived. General simple results are given for the signs and magnitudes of the A, B and C terms in magnetic circular dichroism. The effects of perturbations that preserve the antiaromatic biradical or biradicaloid character are considered. Other perturbations will be treated in Parts 2 and 3.