Keith Yates

Theoretical Study on the Proton Affinity of Small Molecules Using Gaussian Basis Sets in the LCAO–MO–SCF Framework

The proton affinities of certain small molecules have been calculated as an energy difference between the parent molecule and the protonated species. Various size Gaussian basis sets were used to see how the calculated proton affinities approximate the experimental values as the wavefunctions approached the Hartree–Fock limit. The correlation between experimental and calculated proton affinities was excellent with the extensive basis sets.

A nonempirical molecular orbital study on the acidity of the carbon–hydrogen bond

The geometries of acetylene, ethylene, methane, ethane, ketene, allene, propyne, acetonitrile, ketenimine, aminoacetylene, hydroxyacetylene, and all the possible anions formed by removal of a proton from these molecules have been carefully optimized using nonempirical molecular orbital calculations employing the split‐valence shell 4‐31G basis set. These geometries have then been used in 6‐31G basis set calculations. The computed order of gas phase acidity found for these molecules is hydroxyacetylene≳ketenimine≳ketene ≳acetonitrile≳aminoacetylene≳acetylene≳propyne≳allene ≳ethylene≳ethane≳methane.

Ab Initio LCAO–MO–SCF Calculations on Formic Acid, Formate Ion, and Protonated Formic Acid

The site of protonation of formic acid as well as the proton affinities of the formate ion and formic acid have been estimated using ab initio LCAO–MO–SCF calculations with Gaussian basis sets. The three carbonyl protonated formic acid conformers were found to be more stable than the hydroxy protonated isomer for all basis sets. In agreement with experimental observation, the large basis set calculations gave the cis–trans conformer to be the most stable.

GROUND-AND EXCITED-STATE DIPOLE MOMENTS OF SOME NITROAROMATICS : EVIDENCE FOR EXTENSIVE CHARGE TRANSFER IN TWISTED NITROBENZENE SYSTEMS

Electro‐optical absorption measurements have been made on four model nitroaromatics to determine the effect of twisting of the donor–acceptor single bond on the charge‐transfer characteristics in the Franck–Condon excited states. Observed ground‐ and excited‐state dipole moments of nitromesitylene, which has been treated experimentally as the nonplanar analogue of planar nitrobenzene, indicate that electronic excitation of twisted nitrobenzene results in a nearly full unit charge transfer from donor (benzene) to the acceptor (nitro) group (Δμ=18.3 D). On the other hand, in planar nitrobenzene and nitronaphthalene the charge transfer is more delocalized over the whole molecular skeleton, resulting in normal changes in dipole moment (Δμ=5–10 D). In the analogous anthracene system, i.e., 9‐nitroanthracene, the charge transfer upon electronic excitation is extremely low (Δμ=1.7 D), which is reflected by its very small change in the dipole moment. Therefore, it is evident that the charge‐transfer processes in ...

Nonempirical LCAO MO SCF Calculation on Acetylene, Vinylidene Carbene, and the Vinyl Cation

Nonempirical LCAO MO SCF calculations using Gaussian‐type functions have been used to predict the geometries of acetylene, vinylidene carbene, π‐protonated acetylene, and the vinyl cation. The computed geometry and molecular ionization potential of acetylene give good agreement with experimental data. The vinyl cation is predicted to be more stable than π‐protonated acetylene by about 18 kcal/mole.

The electronic spectra of carbonyl compounds—I. Spectra and molecular orbital calculations for α-silyl and α-germyl ketones

Abstract The electronic spectra of R 3 XCOR′ molecules (X = C, Si, Ge; R, R′ = Ph, Me) have been measured in a variety of solvents over the range 20,000–55,000 cm −1 . The large red shifts of the n → π* bands and the small red shifts of the π → π* bands which arise from α-silicon or α-germanium substitution are considered in terms of the σ-donor and π-acceptor properties of these two atoms relative to carbon. Both simple and extended Huckel molecular orbital calculations have been made on this type of system in an attempt to explain the observed effect. The calculated results support the conclusion that it is the σ-donor properties of silicon and germanium which are primarily responsible for the observed spectral shifts, and that any possible p π — d π interactions are of minor importance.

The electronic spectra of carbonyl compounds—II: Spectra and molecular orbital calculations for typical acetyl and benzoyl systems

Abstract The electronic and steric effects of a substituent X in typical acetyl (CH 3 COX) and benzoyl (PhCOX) systems have been investigated experimentally (ultraviolet spectra) and theoretically (LCAO-MO calculations). The good semiquantitative agreement between calculated and observed results (based on a study of twenty-six different carbonyl derivatives) allows some general conclusions to be drawn concerning the effects of substituents on the electronic transitions.

Photogenerated Carbonium Ions and Vinyl Cations in Aqueous Solution

Article de synthese sur la photogeneration de divers carbocations dans des conditions exceptionnellement douces, a partir des substrats: alcenes, alcynes et allenes aromatiques en particulier

An abinitio study of the rearrangement of carbonyl compounds to oxacarbenes

As the first part of a systematic theoretical study of oxacarbene rearrangement, ab initio SCF MO calculations employing a split valence shell basis set have been carried out for the model conversion of formaldehyde to the corresponding oxacarbene. The various cross‐sections of the reaction hypersurface were obtained by complete geometry optimization. The results suggest that, at least for the present model system, the reaction takes place preferentially in the first triplet (T1) state via a concerted pathway.

Quantum chemical studies on electrophilic addition

The geometries of the 2-hydroxyethyl and isomeric oxiranium cations have been fully optimized using ab initio molecular orbital calculations employing the split valence shell 4-31G basis set. These species are possible intermediates in both the electrophilic addition of OH⊕ to ethylene and in the acid catalysed ring opening of oxirane. The optimized structures were then used to compute more accurate wave functions using Dunnings double-zeta basis set, and with this large basis set the bridged oxiranium ion was found to be the more stable by 7.2 kcal/mole. The barrier to interconversion of these two C2H4OH⊕ ions was computed to be 25.0 kcal/mole above the oxiranium ion.