Rotational barriers ab initio computations

Rotational Barriers Ab Initio Computations Rotational Barriers Barrier Origins. 

Combined Quantum Mechanical and Molecular Mechanical Potentials Hyperconjugation M0ller-Plesset Perturbation Theory Ifatural Bond Orbital Methods Rotational Barriers and Molecular Mechanics Corrections Rotational Barriers Ab Initio Computations Spectroscopy Computational Methods Structural Similarity Measures for Database Searching. 

Finally, experimental measurement as well as ab initio computation show that the methyl rotational barrier is also higher in the cis than the tram conformation. These results are shown in Table 17. 

The more lipophilic thioamide unit is also characterized by the poor H-bond acceptor nature of the sulfur and by its larger covalent radius (1.04 vs 0.74 A for O). The thioamide unit mainly adopts a Z-planar conformation, with a rotation energy barrier averaging about 23 vs 18 kcal-mol-1 for amides. 11 A recent ab initio computational study 12 suggests that conformational perturbation in linear thioamides is more likely effected in the C-terminal side. 

Senning and co-workers investigated tetrafluorothiirane and isomers by means of ab initio computations <2003JFC99>. They concluded that the thiirane 51, which is known, should interconvert with its isomers, trifluoro-thioacetyl fluoride 52 (also known) and trifluoroethenesulfenyl fluoride 53. The latter could exist as two conformers 53 and 54 with a calculated rotation barrier of about 37 kJ mol . The authors determined likely mechanisms for interconversions of the isomers. 

Rotational Barrier in Ethylene. It is well known that the rotational barrier of the ethylene molecule cannot be adequately described by a single reference Hartree-Fock calculation SCF calculations on this level resulted in values of 126 kcal/mole (30) and 129 kcal/mole (31) whereas the experimental value is 65 kcal/mole (32). Open-shell ab initio calculations of double zeta+polarization quality give the more acceptable value of 48 kcal/mole (33). Inclusion of correlation such as in CEPA calculations yield theoretical results within the experimental error bar (34), albeit at a considerable computational cost. 

One of the main aims of such computations is the prediction and rationalization of the optoelectronic spectra in various steric and electronic environments by either semiempirical or ab initio methods or a combination of these, considering equilibrium structures, rotation barriers, vibrational frequencies, and polarizabilities. The accuracy of the results from these calculations can be evaluated by comparison of the predicted ionization potentials (which are related to the orbital energies by Koopman s theorem) with experimental values. 

Ab initio Calculations on Molecules containing Five or Six Atoms Table 6 Computed energies and barriers to rotation for N204... 

Structural parameters for the planar and orthogonal forms of H2BNH2 have been computed from ab initio calculations.151 The best estimate of the internal rotation barrier is 33.3 kcal mol-1. 

All MO calculations, both semiempirical and ab initio, have demonstrated that the barrier to aZ>3h-C4v-D3h interconversion, the Berry pseudorotation process, is small for acyclic phosphoranes (see also Section 2.1.). Estimates of 1.4 (extended Hiickel MO34 ), 4.8 (ab initio35 ) and 3.5 kcal/mol (CNDO/231 ) have been obtained for PFS. The barrier computed from the ab initio calculation increases to 8.5 kcal/mol if the 3c -orbitals are not included. The turnstile rotation mechanism traverses a much higher energy barrier 10.0 (extended Hiickel MO), 18.1 (ab initio) and 9.1 kcal/mol (CNDO/2). On the basis of these calculations, the Berry pseudorotation mechanism must be the preferred explanation of pentatopal isomerisation in acyclic phosphoranes (see Section 2.1.). 

The earlier determinations of the aggregation state and the discussions of its influence on the rotational barrier have recently been questioned 29). Cryoscopic measurements show conclusively that allyllithium is a dimer in THF at —108 °C. Furthermore, the ab initio rotational barrier computed for isolated allylsodium 23 b (11.5 kcal/ mol)30) is the same as the experimentally observed value (11.5 kcal/mol)24) though the latter value must refer to a solvated species. It is hence concluded that the discrepancy between the ab initio calculated barrier for isolated allyl// / /urn (17.7 kcal/mol) and the experimental value in THF solution (10.7 kcal/mol, see Table 2) is due to dimerization. MNDO calculations indeed indicate an asymmetric dimer structure of allyllithium. The asymmetric structure is consistent both with earlier32 and more recent29> NMR results. 

It is not always necessary to detail the electronic behavior of materials an accurate understanding of the atomic interactions is often sufficient to describe the phenomenon of interest with reasonable accuracy. In contrast to ab initio methods, molecular mechanics is used to compute molecular properties, which do not depend on electronic effects. These include geometry, rotational barriers, vibrational spectra, heats of formation, and the relative stability of conformers. As the calculations are fast and efficient, molecular mechanics can be used to examine systems containing thousands of atoms. However, unlike ab initio methods, molecular mechanics relies on experimentally derived parameters so that calculations on new molecular structures may be misleading. 

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