CASPT2N/CASSCF

Jensen calculated the energy surface for the walk reaction in the parent case at various levels of theory and found the inversion transition state to be 8 kcal/mol lower than a retention transition state with CASSCF/3-21G. Subsequent calculations at the CASPT2N/CASSCF(8/8)/c-31G(d) level revealed 11.1 kcal/mol difference in the inversion/retention transition states. 

Fig. 8 CASPT2N/6-311G(2d,p) relative energies of species involved in the ring expansion of phenylnitrene (PN). Energies are for CASSCF/6-31G geometries and include ZPE corrections." ...

Energies computed at CASPT2N/6-31G //CASSCF(8,8)/6-31G, CASPT2N/cc-pVDZ//, and CASSCF(8,8)/6-31G values are within parentheses. 

Questions have surrounded the nature of the intermediate(s), diyl, zwitterion, or cyclic allene. The most recent experiments suggest that two pathways are involved in methanol solvent since there is a dependence of the product ratio on methanol concentration, but calculations even with high solvent dielectric constant do not allow the potential energy of the zwitterion to be within 20 kcal/mol of the biradical. Further, the non-planar and therefore enantiomeric cyclic allene was calculated to be approximately only 7 kcal/mol less stable than the singlet biradical at the CASPT2N 6-31G7/CASSCF(2,2)/6-31G" level. Thus the diyl and the allene were suggested to be responsible for the radical and zwitterionic products, respectively. 

Computational efforts at the CASSCF (8,8), CASPT2N, or (U)B3LYP level of theory using a 6-3IG basis set and zero point corrections found, relative to 1,2,6-heptatriene, an intermediate biradical at 14.3 kcal/mol and two transition states, one forming the intermediate at 25.9 kcal/mol and the one forming product at 22.6 kcal/mol. In addition, a pathway was found that connected the two transition states, and was 9.1-9.6 kcal/mol above the intermediate. This enthalpy surface... 

Certain features of this energy surface were confirmed, more or less, by very high level calculations (CASPT2N from CASSCF(8/8)/6-31G wave functions) which also carefully delineated the reaction pathways for both the thermal and photochemical isomerizations of the (CH)s hydrocarbons." Perhaps the most informative aspect of the calculations is the finding that cis,cis,cis,trans-CXyT is only 20 kcal/mol less stable than all cis-COT (Scheme 9.22). Further, it cyclizes... 

Powell, H.K. and Borden, W.T, CASSCF and CASPT2N calculations predict singlet cyclopentan-2-one-l,3-diyl is stable toward closure to bicyclo[2.1.0]pentan-5-one,/. Org. Chem., 60,2654-2655, 1995. 

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