A quantum chemical study of hydrogen abstraction from silane by methyl radical

557-560The potential profile for the hydrogen abstraction reaction from silane by methyl radical is computed quantum chemically. The all-valence electrons semiempirical SCF MO methods SINDOI and MNDO predict comparable activation barriers of 68.5 and 75.2 kJ mol-1 respectively. The lower activation barrier of the title reaction as compared to that of the thermoneutral reaction CH3 + CH4 → CH4 + CH3 reflects the greater ease of abstracting a hydrogen atom from silane than that from methane. This is in agreement with the lower bond dissociation energy in silane (377.5 kJ mol-1) than that in methane (431.4 kJ mol-1)

The frontier molecular orbital analysis of the phosphorus analogues of cyclopeqtadienyl anion

143-144Quantum chemical calculations at· the semiempirical level reveal that in the phosphorus analogues of cyclopentadienyl anion having three or more in-ring phosphorus atoms, there is a marked lowering of the π* levels as compared to the lowering ofthe highest occupied π levels. This leads to the possibility of having significant interactions between the vacant π* orbitals of these species with the occupied orbitals of other substrates

Ab initio study of boron hydroxy species: B(OH), B(OH)4 and B(OH)3O-

553-557Ab initio SCF studies at STO-3G and 4-31G levels have been performed on B(OH) as well as on the recently reported radicals B(OH)4 and B(OH)3O-. The RHF calculations for B(OH) lead to Td  structure, which is in agreement with the experimental results. The B(OH) radical is found to prefer a C2v geometry rather than Td or D2d structures according to UHF calculations. The Td and D2d structures are 23.7 and 12.6 kcal/mol higher respectively in energy than the C2v structure at the STO-3G level. STO 4-31G calculations show that the corresponding values are 16.3 and 11.5 kcal/ mol respectively for the Td and D2d structures. The UHF calculations using both the basis sets indicate that the two geometries considered for the B(OH)3O- radical anion, Td and C3v, are very close energetically

Intermolecular Interaction in the Formation of Dimers & Excimers of Naphthalene, Anthracene & Pyrene

283-28

Towards non-vertical radical cations: Quantum chemical studies on pentafulvene and triafulvene radical cations

1923-1928Fulvene radical cations are examined by semiempirical AM1 method and by ab initio calculations at HF/6-31G*,MP2/6-31G* and B3LYP/6-31G* levels. The perpendicular form of fulvene radical cations is predicted to be higher in energy than the planar form. Frequency calculations at the optimized semiempirical and ab initio geometries show that the planar arrangements correspond to stable minima while the perpendicular forms are transition states. Further the exocyclic C-C bond, which is a double bond in neutral fulvene molecule,becomes a partial double bond in the planar form and a single bond in the perpendicular form. These observations are also in agreement with previous experimental results. DFT calculations predict that the perpendicular form of fulvene cation sustains ring current while aromaticity is lost in the 6,6-difluoro derivatives. The AM1 calculations repeated, with the three membered ring system triafulvene, with the same set of substituents, also give the same results

Structural Stability in Dimer and Tetramer Clusters of l‑Alanine in the Gas Phase and the Feasibility of Peptide Bond Formation

Stability in low-energy structures of the dimer and tetramer clusters of l-alanine in the gas phase is studied by accurate quantum chemical computations at the DLPNO2013-CCSD­(T) level. It is found that the dispersion interaction energies in the dimer (−0.3 to −0.6 kcal/mol) and in the tetramer (−1.3 to −2.5 kcal/mol) have a small role in the stability of the clusters as compared to the hydrogen bond (HB) energies −4.1 to −14.2 and −32.2 to −40.1 kcal/mol, respectively. The HB energy in the alanine cluster is obtained from the binding energy (BE) of DLPNO2013-CCSD­(T)//B2PLYP/def2-TZVP by subtracting the dispersion interaction energy. Local HB energies deduced from the dimer structures are found to be suitable to estimate total HB energies in similar environments. The BEs of OH···NH and OH···OC bonds are −9.5 and −7.1 kcal/mol, respectively. This suggests that the higher clusters are formed through OH···NH bonds as they confer more stability. Analysis of bonding in the tetramer shows that the low-energy tetramer and higher clusters are formed through the OH···NH mode of hydrogen bonding, unlike the dimer which is formed through the OH···OC bond. Feasibility of the amino acid cluster to function as a precursor for polypeptide formation is examined because the orientation of the OH···NH mode of hydrogen bonding is suitable for chemical condensation. The propensity of forming coiled structures in higher clusters and thus in the polypeptides is examined based on the conformational stability in the tetramer of alanine