Dynamics of sodium-doped polyacetylene

The Journal of Chemical Physics, 1993

The low-frequency « 20 meV) polarized vibrational density of states in crystalline polyacetylene has recently been derived from inelastic neutron scattering data [

International Journal of Quantum Chemistry, 1992

The vibronic interaction in an (A)zN chain (e.g., polyacetylene) has been studied within the Hiickel framework. A Hiickel framework scheme for calculating the parameters of molecular dynamic structure, i.e., the linear and quadratic orbital vibronic constants (ova), has been presented. Selection rules for the ovcs in this scheme have been obtained and discussed by using graph theory and group theory, under various boundary conditions. A Hiickel noncoupling rule has been concluded and discussed. The dimerization of polyacetylene has then been discussed based on the molecular dynamic structure. It has been shown that for a finite undoped (A)~N chain the occupied orbital energy E , at the unified configuration has a ilegative slope in direction of Qzn. As a result, there exists a net Hellmann-Feynman force leading to dimerization. When the chain length goes to infinity, the slopes and forces tend to zero. However, a significant negative curvature in potential surface may occur in the direction of Q2,v, due to the two-phonon coupling of the rr-electrons, which could also induce the dimerization. These can be interpreted as the results of the hidden C~N + Z symmetry and the imaginary degeneracy in an (A)zN chain, according to the graph theory for molecular orbitals. Thus, the dimerization of an (A)~N chain actually destroys its hidden symmetry of CAN+2 and reduces its imaginary degeneracy.

Macromolecules, 1991

Molecular mechanics and molecular dynamics calculations have been applied to several substituted polyacetylenes. Results for the barrier height for single-bond rotation for polyacetylene were compared to experimental values and quantum mechanics results for low molecular weight models of polyacetylene in order to check the methodology. As side groups increase in number and size in polypropyne, polybutyne, and poly(tert-butylacetylene), the backbone conformation becomes increasingly nonplanar. For poly[ 1-(trimethylsily1)-1-propyne] barriers for rotation about a backbone single bond are approximately 40 kcallmol. Thus the chain conformation achieved during polymerization will be maintained, though fluctuations in dihedral angles in narrow potential energy wells do allow significant changes in the chain contour.

Physical review. B, Condensed matter, 1992

Symmetry-breaking structural and charge oscillations are calculated for (C~H~) lattices (p =5 to 9) and used to predict Coulombic expansion coefficients, x-ray photoelectron spectra, and crystallographic features that can be compared with experimental results. Extension of these predictions to other dopant concentrations is provided by derived analytical expressions. The parameters for the infinite lattices are derived from oligomer-ion results obtained from modified neglect of differential overlap calculations.

The Journal of Chemical Physics, 1994

Within the double harmonic oscillator approximation, ab inirio vibrational contributions to the static electric dipole polarizability have been computed for the all-truns polyacetylene chains homologous series. Atomic basis set and electron correlation effects have been studied on ethylene and trans-butadiene as well as the use of semiempirical Hamiltonians. By using the 6-3 lG* atomic basis set within the Hartree-Fock and the second order Mdller-Plesset procedures as well as by using semiempirical Austin model 1 Hamiltonians, the evolution with chain length of the vibrational polarizability per structural unit has been investigated and compared to the electronic contribution. Although smaller than the electronic contributions (a;ibration~ 10% oelecuonic), the longitudinal component to the polarizability presents a similar exaltation as chain length grows, but a slower saturation to an asymptotic value per unit cell. Inclusion of electron correlation via the second order MBller-Plesset technique turns out to reduce the longitudinal component calculated at the Hartree-Fock level, but to increase the transversal and perpendicular components. Whereas it reproduces correctly the evolution with chain length of the vibrational polarizability tensor components, the Austin model 1 technique underestimates the longitudinal term and overestimates the perpendicular term. The major contribution to the vibrational polarizability results from large charge fluxes associated with asymmetric stretching motions of the carbon backbone and with the torsion motions presenting very low vibrational frequencies.

Journal of Chemical Theory and Computation, 2007

Electronic excitations of polyene cations with chain lengths of up to 101 CH units were investigated as model systems for lightly doped polyacetylene (PA). Since high level ab initio calculations such as complete active space perturbation theory (CASPT2) are limited to systems with about 14 CH units, the performances of time-dependent Hartree-Fock (TDHF) and time-dependent density functional theory (TDDFT) were evaluated. It turned out that TDDFT excitations energies are much more accurate for polyene cations than for neutral polyenes. The difference between TDHF and TDDFT excitation energies for the first allowed excited state of C 49 H 51 + is only 0.30 eV with pure DFT and 0.21 eV with a hybrid functional. For open-shell systems, pure DFT is found to be superior to DFT-hybrid functionals because it does not suffer from spin-contamination. Pure TDDFT excitation energies and oscillator strengths for small openshell polyene cations compare well with high level ab initio results. Excitation energies are found to be almost independent of the geometry, i.e., the size of the defect. Localization of the defect, however, shifts oscillator strengths from the HOMO-LUMO transition to higher lying excited states of the same symmetry. Lightly doped PA is predicted to exhibit several strong absorptions below 1 eV.

Chemical Physics Letters, 1988

The QCFF/PI method results of the CC stretch force constant calculations in the ground electronic state of trans-and cispolyacetylene are reported and the force fields of the two isomers are discussed in terms of interactions between local oscillators. * Issued as NRCC No. 29378. ' Research Associate.

Physical review, 1989

A matrix formulation of the renormalization-decimation method is given to evaluate analytically the average density of vibrational states for randomly disordered polymeric chains. The method is preliminarily applied to the distribution of isotopic defects in trans-polyacetylene. Good qualitative agreement with the available experimental data is obtained.

Chemical Physics Letters, 2013

Real-time vibrational spectra in a polyacetylene derivative, poly[o-TFMPA([o-(trifluoromethyl) phenyl]acetylene)] in a broad electronic spectral region were observed using a sub-7-fs laser. Using the frequencies and initial phases of vibrational modes obtained by the spectroscopy, the assignment of the wavepackets was made. From the first moment, Huang-Rhys parameters were determined for six most prominent modes, which characterize the potential hypersurface composed of multi-dimensional vibrational mode spaces.

The Journal of …, 2012

The vibrational contribution to static and dynamic (hyper)polarizability tensors of polyacetylene are theoretically investigated. Calculations were carried out by the finite field nuclear relaxation (FF-NR) method for periodic systems, newly implemented in the CRYSTAL code, using the coupled perturbed Hartree-Fock scheme for the required electronic properties. The effect of the basis set is also explored, being particularly important for the non-periodic direction perpendicular to the polymer plane. Components requiring a finite (static) field in the longitudinal direction for evaluation by the FF-NR method were not evaluated. The extension to that case is currently being pursued. Whereas the effect on polarizabilities is relatively small, in most cases the vibrational hyperpolarizability tensor component is comparable to, or larger than the corresponding static electronic contribution.