Min H. Lien

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 calculations on the singlet and triplet energy surfaces for CsiH2

Abstract Singlet and triplet calculations, including configuration interaction, are reported for H 2 CSi, HCSiH and CSiH 2 , and for the transition state on both surfaces.

A theoretical study of the cyano and isocyano groups as α-substituents in alkyl carbanions and carbenium ions

Abstract Structures optimised at the 3-21G level are reported for RCN and RNC, where R is CH 2 − , CH 3 CH − , (CH 3 ) 2 C − , CH 3 , CH 3 CH 2 , (CH 3 ) 2 CH, CH 2 + , CH 3 CH + and (CH 3 ) 2 C + . Inclusion of diffuse functions does not change significantly the geometry of CH 2 CN − and CH 2 NC − . Geometries optimised at the double-zeta level are also reported for some of the smaller molecules. The cyano group is strongly stabilising in carbanions, with the effect being largest for primary anions and smallest for tertiary. In carbenium ions the α-cyano group is destabilising at all levels of calculation (3-21G, double-zeta and double-zeta plus polarisation) with the destabilisation increasing with the size of the basis set. The α-isocyano substituent is stabilising in both carbanions and carbenium ions, and is more effective at stabilising ions than the α-fluoro substituent.

The proton affinity of SiNH and its formation from SiNH2+ in the gas phase

SiNH2+ has been formed in the gas phase by the rapid reaction of Si+(2P) with NH3. Experimental and theoretical studies of the proton affinity of SiNH indicate that this ion is protonated at N, so that deprotonation produces selectively the isomer SiNH in partially ionized interstellar environments.

Ab initio calculations on tautomers of CSiH–, CSiH2, and CSiH3+. Examples of the tendency of silicon to avoid formation of multiple bonds with carbon

Ab initio calculations on the energy hyper-surfaces for CSiH–, CSiH2, and CSiH3+ reveal one, two, and three minima, respectively, with structures in which all the hydrogens are bonded to carbon being the most stable on each surface.

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.

An AB initio molecular orbital study of ten isomers on the C2SiH4 energy surface

Abstract Double-zeta basis set calculations employing gradient techniques have been used to locate minima on the C 2 SiH 4 energy hypersurface. 3-silapropyne is the most stable molecule and structures containing divalent silicon are more stable than those containing carbon-silicon multiple bonds. Inclusion of polarisation functions is most important for cyclic structures.

A quantum chemical study on electrophilic addition: Part I. Reaction of fluorine with ethylene

Non-empirical SCF-MO calculations were carried out on two limiting structures of C2H4F+, corresponding to the cyclic and open valence tautomers, both of which are possible reaction intermediates of the electrophilic addition reaction of F2 to CH2 =CH2. It was found that both species had thermodynamic stability, corresponding to two distinct minima on the energy surface. However, the 2-fluoroethyl carbonium ion showed a greater stability than the fluoronium ion by about 10 kcal/mole.

An ab initio study of the structure of the 2-chloroethyl radical

Minimal and split-valence shell basis set calculations, both with and without d orbitais, predict the radical centre to be pyramidal, with the planar radical only 0.3 kcal mol−1 higher. The barrier to internal rotation is 2 kcal mol−1. There is no evidence of bridging from chlorine.

Optimum Gaussian basis set for the Bromine atom. Ab initio calculations on the HBr molecule

An optimum (15s12p2d) Gaussian basis set was obtained for the Bromine atom by minimizing the open shell energy functional. In the minimization procedure the method of conjugate gradients was applied. The optimum (15s12p2d) basis set was contracted to an [8s6p2d] “double zeta” quality basis set and this contracted set was tested on the HBr molecule.