A full conformational space analysis of bilirubin

Ab initio methods were utilized in a gas-phase systematic conformational search of bilirubin conformers. The whole molecule was divided into four fragments. Most stable conformers of them were employed to build 196 conformers of the complete bilirubin molecule. Initial geometries were optimized using HF/3-21G level of theory and the minimum energy conformers were then reoptimized at B3LYP/6-31G(d) level. Ridge-tile conformer was the most stable one, in perfect agreement with X-ray data. We found that while tetrapyrrole backbone shows some flexibility, propionic acid side chains have a greater influence in bilirubin conformation because they can interact through different hydrogen bond patterns with the backbone and between them.     

The electronic structure and one-electron properties of BH computed from SCF and CI wave functions

The SCF and CI wave functions for BH, obtained in calculations described in detail elsewhere [2], are compared through their electron distributions and electron moments.Taken in part from a Ph.D. thesis submitted to the University of Toronto in 1971.     

Vitamin E models

The molecular conformations of shortened molecular models of vitamin E (tocopherol and tocotrienol) and their sulfur and selenium congeners were studied computationally at the DFT level of theory [B3LYP/6-31G(d)]. The sequence of stabilization by the various heteroatoms was found to be the following: O∼Se > S. On the basis of the present structural results it seems that the seleno-congener of vitamin E is a distinct possibility. Received 9 May 2002 / Received in final form 9 July 2002 Published online 13 September 2002     

A non-empirical molecular orbital study of valence tautomers of C2H3N

The valence tautomers of C₂H₃N have been examined by non-empirical molecular orbital calculations using two split-valence shell basis sets. All geometries were fully optimized using the 4–31G basis set and these structures were then used in 6–31G basis set calculations. The order of stability of the three possible cyclic isomers is 1-azirine > cyclic carbene > 2-azirine. The profiles for conversion of vinylmethylene into cyclopropene, vinylnitrene into 1-arizine, and iminomethylene into 2-azirine have all been shown to have barriers.     

Uniform quality gaussian basis sets for molecular calculations. IV. Gradient and charged optimized basis sets for CH4

Comparison of the molecular Q-optimized and molecular gradient optimized carbon basis sets for CH 4 showed that molecular Q optimization is an excellent substitute to the more expensive molecular gradient optimization. The parameter Q of the Q optimization is related to the population (i.e., net charge) on the atom.     

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

An optimum (15 ^s 12 ^p 2 ^d ) 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 (15 ^s 12 ^p 2 ^d ) basis set was contracted to an [8 ^s 6 ^p 2 ^d ] “double zeta” quality basis set and this contracted set was tested on the HBr molecule.     

The ring-opening of an unsymmetrical tetrahedral intermediate - 2-hydroxy-1,3-oxathiolane

Ab initio calculations were performed on 2-hydroxy-1,3-oxathiolane: and the two products of its breakdown: Complete geometry optimizations were performed at minimal (STO-3G) and split-valence (3-21G) basis set levels. In addition, a single point calculation was performed at 6-31G* level withd orbitals added on sulfur only. The conformation of the oxathiolane intermediate and its stability relative to the breakdown products was investigated. The STO-3G basis set gave an envelope form while 3-21G gave the twist form of the five-membered ring as the most stable. For all three basis sets the ester product was more stable than thioester.     

An ab initio molecular orbital study of the C2F2 species: The difluorovinylidene → difluoroacetylene rearrangement

An ab initio MO study, using medium size Gaussian basis sets has been made of vinylidene carbene, acetylene and the isomeric cyclic intermediate presumed to represent the transition state of their interconversion reaction, along with their perfluorinated analogs. The total energies of the acetylenes are lower than the vinylidenes and the estimated activation energy of their interconversion is considerably higher for the fluorinated molecules. The results are in line with experimental observations.