Minimum Energy Reaction Pathway Research Articles

Ab initio variational transition state theory calculations for the O+NH2 hydrogen abstraction reaction on the 4A′ and 4A″ potential energy surfaces

We report high level ab initio calculations that characterize the quartet potential energy surfaces 4A″ and 4A′ for the O+NH2 direct hydrogen abstraction reaction. Minimum energy reaction pathways have been computed on both surfaces at the full-valence complete active space self-consistent field level using a correlation consistent polarized valence double zeta basis set. Energies along these reaction pathways have been further refined by multireference configuration interaction calculations with a correlation-consistent polarized valence triple zeta basis set. Canonical variational transition state theory calculations using the ab initio potential energy surface information as input and incorporating tunneling through the ground state vibrationally adiabatic potential energy curves indicate that below about 2000 K, the O+NH2 reaction is dominated by addition/(isomerization)/dissociation pathways, and for temperatures below about 1000 K, even the OH+NH product channel is dominated by the addition/isomerization/dissociation route.

Unimolecular dissociation of tetramethylsilane radical cation: apparent absence of an ion/neutral complex intermediate

Quantum chemistry calculations and experimental investigations of the unimolecular dissociation step of the molecular radical cation of tetramethylsilane losing a methyl radical to give the m/z 73 trimethyl silyl cation have been carried out. There is an apparent lack of an ion/neutral complex in the minimum energy reaction pathway, where in quaternary carbon compounds there is evidence of such complexes. It is suggested that at interfragment separation distances at which an ion/neutral complex would be expected the electrostatic interactions are “swamped” by the strength of the conventional covalent bonding interactions which are an order of magnitude greater than in the quaternary carbon systems at similar separations.

AM1, PM3 and MNDO study of S N2 reaction of methylhydroperoxy anion with alkyl chlorides

The reactions of the methylhydroperoxide anion with various alkyl chlorides proceeding according to S N2 mechanism were investigated by means of semiempirical AM1, PM3 and MNDO methods. The minimum energy reaction pathways and structures of stationary points were calculated. The reactivities of the alkyl chlorides used as electrophiles were estimated based on the S N2 reaction energy barrier. The results of the AM1, PM3 and MNDO calculations were compared.

The gas phase chemistry of the formic acid radical cation [HCOOH] [rad]+ . Mechanism for exchange of the hydrogen atoms: a quantum chemical investigation

The CH 2O 2 + potential energy surface has been (re)explored by ab initio quantum chemical calculations executed at the SD-CI/6-31G**//UHF/4-31G level of theory. Earlier labelling experiments had indicated that prior to the loss of H the hydrogen atoms in ionized formic acid, HCOOH + , can become positionally equivalent via an unknown pathway. Using ab initio molecular orbital calculations we have located a minimum energy reaction pathway for hydrogen exchange reactions which takes place close to the dissociation level and proceeds via flexible ion/dipole complexes. Along this path three equilibrium structures of different atom connectivity are found: HCOOH + , 1, ionized formic acid; two forms of the surprisingly stable ion/dipole complex HO ·HCO +, 2, 3; and the distonic ion H 2OCO + , 4. Ion 1 is separated from ion 2 by a barrier of 142 kJ mol −1 and this barrier corresponds to a 2A′ → 2A″ surface crossing. The barrier for the proton transfer 3 → 4 is only 25 kJ mol −1 relative to 3; the transformation HCOOH + → H 2OCO + , formally a 1,2-hydrogen shift, takes place via the ion/dipole complex HO ·HCO +, H 2OC + is the reacting configuration for decarbonylation and this reaction is calculated to take place at the dissociation limit in full agreement with experiment.

Explorations on the multidimensional potential energy surface of a chiral stationary phase

AbstractThe multidimensional potential energy surface for a soluble analog of a chiral phase is computed with MM2 and MNDO. Minimum energy conformations are located. The minimum energy reaction pathway between these forms is located, and the templating ability of these phases is described.

Rehybridization and the Chemical Reaction: POAV and 3D-HMO Analysis

In this study chemical reactions are examined from a new standpoint — rehybridization. With the possible exception of oxidation-reduction processes, all chemical reactions involve rehybridization. Bondmaking and bond-breaking processes are nothing more than changes in hybridization. ln the present study two categories of pericyclic reactions are examined with the π-orbital axis vector (POAV) analysis and-the three-dimensional (3D) HMO theory. The first makes use of a series of related molecular structures derived from X-ray crystallography which provide information on the reaction pathway between the valence tautomers 1,6-methano[10]annulene (1a) and the bisnorcaradiene (1b). The second category involves structures calculated by Dewar and coworkers along the minimum energy reaction pathway of the Cope rearrangement of 1,5-hexadiene (2). The analysis provides a clear delineation of the structural and electronic changes which accompany chemical reaction. The sense of the π-orbital axis vectors at the bridgehead atoms is shown to be the determining feature in dividing the potential surface of 1 between bridged-[10]annulene (1a) and bisnorcaradiene (1b).

Conformational analysis of the covalent pirkle chiral stationary phases.

The multidimensional potential energy surface for a model Pirkle chiral stationary phase is computed with MM2. Five minimum energy conformations are located, the minimum energy reaction pathway between the three lowest energy forms is described and the ability of these chiral phases to act as templates is discussed.