Alan T. Yeates

Vibronic contribution to static molecular hyperpolarizabilities

A simplified semiempirical scheme (AM1) is developed as a handy tool to calculate vibrational corrections to hyperpolarizabilities of moderately large planar conjugated systems. In formulating the scheme Born-Oppenheimer (BO) approximation is assumed and only those normal modes are considered which correspond to vibration along the direction of the field. Ab initio calculations are performed to calibrate and justify the various steps involved in the scheme. Using this approach vibronic corrections to the third-order polarizabilities γ are computed for a number of systems including the polyenes C 2 n H 2 n +2 , n = 3, 4 and 5. The corrections are large and may explain some of the discrepancies that are found to exist between experiment and theory.

Fulleride Based Ionic Liquids

A room temperature fulleride-based ionic liquid has been developed and characterized. Utilizing the trihexyl(tetradecyl) phosphonium cation, the ionic liquid [PC6C6C6C14]3[C60]Cl was synthesized, and distinctly shows spectroscopic characteristics of the [C60] anion. The glass transition of the ionic liquid occurs well below room temperature at -56°C, but it still retains an extremely high viscosity at room temperature. Attempts were made to obtain the pure fulleride salt [PR4]3[C60], but nearly always results in the binary fulleride-chloride salt.

Virtual Synthesis of Sub-Nanoscale Materials With Prescribed Physical Properties

Properties of electronic energy spectra of several small virtual clusters (known as small quantum dots, or QDs) composed of In, Ga and As atoms are investigated for the further use in nanoheterostructure (NHS) units of pre-designed electronic properties. Modern quantum statistical physics methods relate these properties to electronic transport properties of such systems and therefore, lead to realization of a virtual (i.e., fundamental theory- based, computational) approach to synthesis of sub-nanoscale electronic materials with pre-designed electronic properties[1].

Band-gap calculations for squarelene-based polymers

Abstract The electronic structures and band-gaps, modeled as the energy difference between ground singlet state and excited triplet state, for a series of oligomers of a polymer based on squarelene and fused thiophenes were investigated. AM1 and PM3 semi-empirical, Extended Hiickel Theory (ETH), Hartree-Fock (HF) and second-order Moller-Plesset perturbation (MP2) ab initio, numerical atomic orbital basis set density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations are employed. The results indicate that a narrow gap less than 0.5 eV could be reached in a long polymer .

Magneto-optical Properties of Small Atomic Clusters of Ga and In with As, V and Mn

The Hartree-Fock (HF) method is used to synthesize virtually (i.e., fundamental theory-based, computationally) small stable atomic clusters of Ga and In with As and V, and an In-based cluster with As and Mn. The electronic energy level structures (ELSs), optical transition energies (OTEs), and charge/spin density distributions of these clusters have been analyzed. It has been shown that the spin of such clusters is collectivized, and that this collectivization is responsible for a dramatic drop in the clusters’ OTEs as compared to those of similar pyramidal clusters that do not contain “magnetic” atoms.

A study of iodine-doped transpolyacetylene

Abstract With a view to elucidate and develop a model for the mechanism of conduction in doped transpolyacetylene (t-PA) we have carried out an ab initio study of the ground and excited state of transoctatetraene in the presence of an Iodine atom. While the ground state is calculated as characterized by a slightly distorted alternating bond geometry with Iodine remaining largely neutral atop the plane of the polyene near roughly the midpoint of chain axis, the excited state has the geometry characterized by a solitonic distortion with the octatetraene carrying a net positive charge close to .7e. The bandgap (HOMO-LUMO gap) is computed to be nearly .8 eV. Moreover, the excited state energy appears very insensitive to the position of Iodine along the axis of the octatetraene chain. As an extension of an earlier calculation on the transport of charge by a charged soliton, a similar calculation is performed on a C 8 Hi 10 + “kink” travelling along the backbone of C 12 H 14 + yielding very similar results. We have extended the ab initio calculations also to the infinite chain.

Linear molecular-mass dependence of second hyperpolarizabilities: lack of polar enhancement in nonconjugated molecules

Second hyperpolarizabilities (γ) of a series of R–X molecules (R = H, Me, Et, Pri, But; X =–NCHO, OH, Me) have been calculated at various ab initio levels; surprisingly, γ increases nearly linearly with molecular mass, is minimally affected by the functional group X, and is slightly greater for alkanes.