CASPT2 calculations

The first step — cyclization of lb to the azirine 2b — is predicted to be the rate-determining step. The CASPT2 calculated barrier of 9.2 kcal/mol is... 

At the CASSCF(8,8)/6-31G level, 1 A"-4b is predicted to be a transition state for the enantiomerization of 3b. The CASPT2 calculated barrier for this process is ca. 21 kcal/mol (Fig. 12). This predicted value for the barrier to enantiomerization of 3b is essentially the same as that calculated for the racemization of 3a,55,57 which, as already noted, is approximately half of the experimental value of ca. 42 kcal/mol for racemization of allene.73... 

The CASSCF/CASPT2 calculations were performed with an active space including the five nd, the (n + l)s, the three (n+ l)p orbitals, and a second set of nd orbitals to account for the double shell effect. The importance of including a second 3d shell in the active space was detected in an early study of the electronic spectrum of the nickel atom [2]. This had already been suggested from MRCI results [1]. The results obtained by RT at about the same time indicated that such effects are effectively accounted for when a method is used that includes cluster corrections to all orders, like the QCI method used by them [3]. This result will hold true also for the less approximate coupled cluster method CCSD(T). 

Also the CASSCF/CASPT2 calculations were performed in D2h symmetry and the orbitals rotations were restricted such that mixing between different angular... 

Overall, DFT methods are user-friendly and helpful in identifying reactive species based on their IR features. For relative energies of the various electronic states, CASSCF and, in particular, CASPT2 calculations are recommended. Especially the latter method is expected to be semiquantitatively correct. It should be noted, though, that in several cases, DET relative energies come surprisingly close to the CASPT2 results. 

Bernhardsson and coworkers have recently used CASPT2 calculations (electron-correlation correction to the CAS wave function) to model carbonyl oxides in solution. Solvation effects in acetonitrile solvent also suggest that the zwitterionic form would be favored with an elongation of the 0—0 bond length and a decrease in the C—O bond. Ab initio calculations have been recently reported for monofluorocarbonyl oxide , diflu-orocarbonyl oxide , methylcarbonyl oxide and cyclopropenone carbonyl oxide. In the recent literature the idea that carbonyl oxide can be an important source of OH radicals has also been presented. ... 

Another route to the formation of carbonyl oxides is the reaction of methylene (CH2) with dioxygen. This oxidative process involving methylene with O2 is one of the most important reactions in the combustion of unsaturated hydrocarbons. The reaction between CH2 (X Bi) 4- O2 has been studied by Anglada and BofilP in the gas phase by carrying out CASSCF and CASPT2 calculations with the 6-31G(d,p) and 6-31H-G(3df,2p) basis... 

T. Bally, S. Bernhard, S. Matzinger, J.-L. Roulin, G. N. Sastry, L. Truttmann, Z. Zhu, A. Marcinek, J. Adamus, R. Kaminski, J. Gebicki, F. Williams, G.-F. Chen, and M. P. Fulscher, The Radical Cation of i-yn-Tricyclooctadiene and Its Rearrangement Products, Chem. Eur. J. 2000, 6, 858. A combination of CASPT2 calculations of UV spectra and B3LYP and CCSD(T) calculations of PESs is used to identify the intermediates formed sequentially in the rearrangement of the radical cation of the syn dimer of cyclobutadiene and to understand why they differ from those formed from the anti dimer. 

The CASPT2 calculations predict a transition to a I Bi excited state at 581 nm with a very small oscillator strength (1.6 x 10 ). This transition could be assigned to a very weak band with a maximum around 540 nm (Fig. 2). As in the case of long-wavelength transition in PN it consists of the same electronic configurations (7r(la2) n (3b]) and it (3b,) n (2a2)). 

CASSCF and CASPT2 calculations both overestimate the stability of the open-shell electronic structure of singlet nitrenes 8c-f by about 3 kcal/moP as in the case of parent The ring opening is also computed to require... 

Very recent sophisticated CASSCF/CASPT2 calculations (61) shed some light on this problem. Radon et al. (61) performed an analysis of the number of configurations and calculated the weight of the given resonance structures. Consequently, FeP-NO can best be characterized as mixture of Fe(II)-NO0 and Fe(III)-NO resonance structures. The Fe(I)-NO+ electronic structure participates only with a few %. 

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