Computational studies B3LYP

It has been shown that different methods may ascribe different bond lengths to the 0-0 and C—O bonds and that the medium and substituents affect the electronic behaviors of carbonyl oxides." For example, recent computational studies (B3LYP/6-31 + G (d, p)) of carbonyl oxides, syn- and awri-methyl carbonyl oxides and dimethylcarbonyl oxides in gas and solution reveal that dipolar character increases with the number of methyl groups, and the ionic configuration is stabilized in a polar medium. These effects result in a weakened 0 0 bond and an increased double-bond character in the CO bond. ... 

A thorough computational study of this process has been carried out using B3LYP/ONIOM calculations.31 The rate-determining step is found to be the formation of the rhodium hydride intermediate. The barrier for this step is smaller for the minor complex than for the major one. Additional details on this study can be found at ... 

There have been a number of computational studies of the epoxidation reaction. These studies have generally found that the hydrogen-bonded peroxy acid is approximately perpendicular to the axis of the double bond, giving a spiro structure.75 Figure 12.8 shows TS structures and Ea values based on B3LYP/6-31G computations. The Ea trend is as expected for an electrophilic process OCH3 < CH3 CH = CH2 < H < CN. Similar trends were found in MP4/6-31G and QCISD/6-31G computations. 

AMI semi-empirical and B3LYP/6-31G(d)/AMl density functional theory (DFT) computational studies were performed with the purpose of determining which variously substituted 1,3,4-oxadiazoles would participate in Diels-Alder reactions as dienes and under what conditions. Also, bond orders for 1,3,4-oxadiazole and its 2,5-diacetyl, 2,5-dimethyl, 2,5-di(trifluoromethyl), and 2,5-di(methoxycarbonyl) derivatives were calculated <1998JMT153>. The AMI method was also used to evaluate the electronic properties of 2,5-bis[5-(4,5,6,7-tetrahydrobenzo[A thien-2-yl)thien-2-yl]-l,3,4-oxadiazole 8. The experimentally determined redox potentials were compared with the calculated highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) energies. The performance of the available parameters from AMI was verified with other semi-empirical calculations (PM3, MNDO) as well as by ab initio methods <1998CEJ2211>. 

Calculations at the B3LYP/6-31G level on structures 15 to 19 shows that the cis- 1,5-p-H-bridged structure 16 is the global minimum. This result is similar to that found in a recent computational study by Siehl (26). In Table 1, the relative energies of 15 to 19 are compared, where the c/s-l,5-p-H-bridged structure 16 is set as an arbitrary zero reference point for these energy comparisons. 

In marked contrast to the generally accepted mechanism, the involvement of a radical pair produced by an alkene-induced 0—0 bond homolysis was suggested by Minisci and coworkers . In a combined experimental and theoretical study Curci, Houk and coworkers sought to differentiate between a radical pathway and the commonly accepted concerted mechanism. Both product and kinetic smdies tended to exclude a radical pathway. Computational studies at the B3LYP/6-31G level on the epoxidation of isobutylene with DMDO predicted an activation energy = 15.3 kcalmor ) significantly lower... 

No theoretical studies were reported in the previous volume <1996CHEC-11(7)89> for these 5-5 bicyclic systems. Since then a number of computational studies have been carried out. The structure and properties of cyclic sulfites 3a and 3b and thionosulfites 3c and 3d have been reported <2003HAC587, 2005HCA1451>. Density functional theoretical (DFT) calculations using the Becke s exchange with Lee, Yang, and Parr correlation functional (B3LYP/6-31G level) have shown that the cyclic sulfite 3b is more stable than its diastereomer 3a. 

Some of us addressed the mechanistic study of its intricate reaction mechanism (71) at B3LYP level coupled with PCM single-point calculations in order to take into account the solvent effects. The results of the computational study are summarized in... 

The homolytic bond dissociation energies (BDEs) of phenohc O—H bonds has been the subject of a computational study focusing on substituent effects by ab initio and density functional theory (DFT) methods.6 Consistent overestimation of the BDEs by MP2 and MP4 calculations was associated with spin contamination in the reference UHF wave functions, whilst the DFT calculations (particularly the B3LYP/6-31G level of theory) were relatively unaffected. Ab initio calculations of the photosensitized C—C BDEs of /f-phenethyl ethers has revealed a significant configurational... 

Computational study at B3LYP/6-31G showed three possible pathways. The first one was the concerted process that directly gives the ene product, 25. The TS, 23, is 23.8 kcalmol-1 higher in energy than 22. Biradical intermediate, 24, for a stepwise process, which is formed with an inward Me rotation optimized. However, the TS leading to 24 could not be obtained, and all attempts to locate the TS converged on 23. Alternative stepwise process with outward Me rotation gave a TS which is 8.8 kcal mol 1 less stable than 23. 

Computational study at (U)B3LYP/6-31G for the reaction of 27 (R = Z = H) showed that the activation barrier (AE + ZPE correction) for the diradical (31) formation (path b) is 40.2kcal mol, which is 2.1 kcalmol-1 lower than that for the diazenyl diradical (30) formation step (path a).44 The barrier from 31 to 29 is very small (0.6 kcal mol ), whereas the barrier from 31 to 28 is 7.0 kcal mol-1. Direct formation of 28 from 27 via a [2 + 2 + 2] reaction did not give TS. Thus, the calculations suggested that the reaction of 27 (R = Z = H) would give 29 as a major product via path b. This is consistent with experimental report, which showed that the denitrogenation of the parent 27 nearly exclusively gave spiropentane.45... 

Computational studies [89] on the model reactions shown in Scheme 22 at the RHF/6-31G, MP2/6-31G, B3LYP/6-31G in vacuo and including solvent effects via SCRF methods showed that both processes take place via stepwise mechanisms similar to those found for the reaction between ketenes and imines. In both cases the nucleophilic attack of the iminic moiety was the rate-limiting step, the electrocyclic ring closure of intermediates (93, 96) being much faster. However, the first... 

The calculations were performed employing either pure ab initio Hartree-Fock (HF) methods, or hybrid HF-DFT functionals, in particular B3LYP [22]. The hybrid functionals have several advantages. One is that they are commonly applied with great success in computational studies of molecules and clusters, thus making it possible to benefit from the gathered experience from molecular studies. Another is their recently noted ability to accurately model band gaps in semiconductor compounds [57]. 

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