CBS-QB3 methods

The rate-determining step of this second reaction mechanism is the isomerization of the cyclic diradical to tropone, since the TS connecting the intermediate to the final product lies 84.0 kcal/mol (from CBS-QB3 method) above the starting o-QM. 

The unimolecular reactions of CH3CH2CH2O2 were studied in detail (Fig. 6) complete potential energy surfaces were generated using both DFT [B3LYP/ 6-31+G(d,p) and mPWlK/6-31+G(d,p)] and CBS-QB3 methods. As expected, 1,5-H transfer [Equation (34)] occurs with the lowest barrier, followed by simultaneous 1,4-H transfer and HO2 expulsion [Equation (31)]. The overall decompositions of each H-atom transfer product (i.e., each QOOH radical) were modeled. It... 

The CBS methods are available as keywords in the Gaussian program. The CBS-Q, CBS-q, and CBS-4 methods are incorporated in Gaussian 9475 and later versions. The CBS-QB3 method is available in Gaussian 98."... 

A similar pattern is seen for the two isomers of 1,3-diazetidine. For example, (1) for the trans-isomer, the CBS-QB3 method shows two variances from the monotonically increasing frequencies (2) because of their symmetries (both molecules have a C2 axis), many of the vibrations of both the conformers are predicted to have a very low or exactly zero intensity and (3) only half or less of the 24 normal modes of vibration have significant intensity. 

Activation energies of pericyclic reactions Performance of DFT, MP2, and CBS-QB3 methods for the prediction of activation barriers and reaction energetics of 1,3-dipolar cycloadditions, and revised activation enthalpies for a standard set of hydrocarbon pericyclic reactions ... 

Three functionals, B3PW91, MPW1PW91, and PBEIPBE in combination with 6-31-l-G, 6-311-l-G and AUG-cc-pVTZ basis sets were used in a DFT study of the one-electron reduction of CF3X (X = Br, Cl, I) accompanied by C-X bond dissociation. The dissociation energies and electron affinities were compared with results obtained with the Gn and CBS-QB3 methods. ... 

Gimer V, KhuongKS, Leach AG, Lee PS, BartbergerMD, HoukKN A standard set of peri-cyclic reactions of hydrocarbons for the benchmarking of computational methods the performance of ab initio, density functional, CASSCF, CASPT2, and CBS-QB3 methods for the prediction of activation barriers, reaction energetics, and transition state geometries, / Phys Chem A 107(51) 11445—11459, 2003. 

Pathways and reaction barriers for the unimolecular decomposition of MB were explored with CBS-QB3 methods by El-Nahas et al. in 2007. Ah and Vioh " have recendy used ab initio and DFT methods to reexamine MB decomposition. Their bond energies and reaction barriers were generally in good agreement with those of El-Nahas. They have used the computations to compute high-pressure and pressure-dependent kinetics for 10 possible decomposition pathways. They noted the importance of entropic factors in addition to the energetics in determining the kinetics. 

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