Generalized valence bond study of rotational singlet structures and pi bond energies for systems containing C==C, Si==Si, and C==Si double bonds

1998 ◽  
Vol 76 (4) ◽  
pp. 477-482 ◽  
Author(s):  
Youliang Wang ◽  
Raymond A Poirier
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Bond Energy ◽  
Valence Bond ◽  
Double Bonds ◽  
Bond Energies ◽  
Fluorine Substitution ◽  
Generalized Valence Bond ◽  
Pi Bonds ◽  
Optimized Geometries

Ab initio GVB(6/12)/6-31G** calculations were performed on A2X==YB2 (A, B = H, F; X, Y = C, Si) to obtain the optimized geometries for planar and twisted singlet structures, and to also calculate pi bond energies (rotational barriers). The nature of C-C, Si-Si, and C-Si pi bonds has been investigated. The results show that the C-C pi bond energy (E pi (ethene) = 65.4 kcal/mol) decreases with increasing fluorine substitution. The pyramidalization at the carbon or silicon center for the twisted structures decreases the pi bond energies in the substituted ethenes and their silicon counterparts. The Si-Si (E pi (disilene) = 23.2 kcal/mol) and C-Si (E pi (silaethene) = 31.6 kcal/mol) pi bonds become much weaker. Fluorine substitution stabilizes both the diradical and the dipolar twisted singlet structures.Key words: pi bond energy, ab initio calculations, generalized valence bond, fluorine substitution, disilene, and silaethene.

On the generalized valence bond description of the anomeric and exo-anomeric effects: an ab initio conformational study of 2-methoxytetrahydropyran

2005 ◽  
Vol 340 (13) ◽  
pp. 2171-2184 ◽  
Author(s):  
Rodrigo S. Bitzer ◽  
André G.H. Barbosa ◽  
Clarissa O. da Silva ◽  
Marco A.C. Nascimento
Keyword(s):  
Ab Initio ◽  
Valence Bond ◽  
Conformational Study ◽  
Generalized Valence Bond ◽  
Anomeric Effects

A study of hydrogen abstraction reactions by halogen atoms with HFCO and HClCO: determination of transition state structures, barrier heights, and vibrational frequencies

1993 ◽  
Vol 71 (1) ◽  
pp. 135-140 ◽  
Author(s):  
J. S. Francisco ◽  
N. Mina-Camilde
Keyword(s):  
Transition State ◽  
Ab Initio Calculations ◽  
Ab Initio ◽  
Chlorine Atom ◽  
Fluorine Atom ◽  
Hydrogen Abstraction ◽  
Barrier Heights ◽  
Optimized Geometries ◽  
Good Agreement

Theoretical barriers and reaction energetics were determined using ab initio calculations at the PMP4/6-311 + +G** level with UMP2/6-311G** optimized geometries for hydrogen abstraction reactions of HFCO and HClCO by fluorine and chlorine atoms. The largest barriers are predicted for fluorine and chlorine atom abstractions of hydrogen from HFCO. Fluorine and chlorine atom abstractions of hydrogen from HClCO are predicted to be barrierless. The predicted barrier for fluorine atom abstraction of hydrogen from HFCO is calculated to be 1.3 kcal mol−1 and is in good agreement with the experimental value of 1.8 ± 0.4 kcal mol−1.

On the possible formation of Si=O, Si=S, and Si=Se double bonds via the reaction of silylenes with oxirane, thiirane, and selenirane, respectively. An ab initio theoretical study

2000 ◽  
Vol 78 (11) ◽  
pp. 1496-1510 ◽  
Author(s):  
Yitzhak Apeloig ◽  
Stepan Sklenak
Keyword(s):  
Ab Initio Calculations ◽  
Ab Initio ◽  
Gas Phase ◽  
Theoretical Study ◽  
Single Step ◽  
Membered Ring ◽  
Basis Set ◽  
Double Bonds ◽  
Ring Compounds ◽  
Encounter Complex

The reactions between silylene, H2Si, and the three-membered ring compounds, oxirane (9), thiirane (10), and selenirane (11), which provide possible routes to Si=X (X = O, S, Se) double bonds were studied by ab initio calculations at the MP2 and QCISD correlated levels of theory employing the polarized 6-31G** basis set. The calculations show that the three reactions are all highly exothermic, (–61.8, –45.2, –52.9 kcal/mol at MP2/6-31G**//MP2/6-31G** for X = O, S, Se, respectively). In the gas phase, at 0 K, these reactions are predicted to be also spontaneous (i.e., the calculated transition states are lower in energy than the reactants). At 298 K, entropy contributions result in small barriers on the ΔG surface for X = O, S (7.4 kcal/mol for both reactions at QCISD (full)/6-31G**), but for X = Se the reaction remains spontaneous. Thus, the calculations suggest that these reactions are viable routes for the preparation of compounds with Si=X (X = O, S, Se) double bonds. The first step in all the reactions is the barrier-less formation of an encounter-complex between the silylene and the X atom of the precursor. For X = O, S, these complexes are predicted to be sufficiently stable to be observed in a matrix. The reaction steps which follow depend on X; for X = O fragmentation of the silylene-XC2H4 complex proceeds in two steps via a biradical intermediate, while for X = S and X = Se the fragmentation occurs in a single step. The first ab initio calculations for H2Si=Se are reported.Key words: silylene, silanone, silanethione, silaneselone.

An Ab Initio Study of the Energies of Coherent Interfaces Formed between bcc Iron and Carbides or Nitrides of Transition Metals

2007 ◽  
Vol 124-126 ◽  
pp. 1625-1628 ◽  
Author(s):  
Woo Sang Jung ◽  
Soon Hyo Chung ◽  
Heon Phil Ha ◽  
Ji Young Byun
Keyword(s):  
Ab Initio ◽  
Bond Energy ◽  
Nearest Neighbor ◽  
Interface Energy ◽  
Lattice Plane ◽  
Ab Initio Study ◽  
Bond Energies ◽  
Bcc Iron ◽  
Before And After ◽  
Coherent Interfaces

An ab initio study was carried out on interface energies at coherent interfaces between bcc Fe and MXs (NaCl structure, M = Ti, Zr, Hf, V, Nb, Ta, X=C, N). The interface energies have positive values for carbides and nitrides of group IVB metals (Ti, Zr, Hf), while they have negative values for carbides and nitrides of group VB metals (V, Nb, Ta). Influence of bond energy was estimated using the discrete lattice plane/nearest neighbor broken bond (DLP/NNBB) model. It was found that the dependence of interface energy on the type of carbides and nitrides was closely related to changes of the bond energies between Fe, M and X(=C, N) atoms before and after formation of the interfaces Fe/MX.

Sign in / Sign up

Export Citation Format

Share Document