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Time-dependent density functional theory (TD-DFT) has been applied to investigate the relevant excited states involved in the photodissociation of hydroxocobalamin (HOCbl). The TD-DFT/BP86/6-31G(d) and TD-DFT/BP86/6-311G(d,p)//BP86-631G(d) calculations suggest that the photoscission of Co–OH bond is mediated by the repulsive 1(n + dCo → σ∗Co–OH) singlet state that drops in energy along the reaction stretch coordinate and yields the products of photodissociation i.e., Cob(II)alamin and hydroxyl radical. The channel involving the repulsive triplet state may also be a possible mechanistic pathway. The present calculations thus may help in enhancing our understanding about the photophysical behavior of HOCbl which has implications in the investigation of biological complexes such as DNA and RNA.

Density functional theory has been applied to the analysis of NB–Co–CR interligand bonding in octahedral complexes of CoIII. Employing B-[CoIII(corrin)]-R+ models, it is shown that a change in the electron donating (or withdrawing) character of alkyl ligands (R) lengthens (or shortens) both interligand Co–CR and Co–NB bonds in accordance with the anomalous (or inverse) trans effect. Six molecular orbitals are required for the succinct description of this phenomenon, which lies in an unusual combination of a poor σ/π-donor of the base (B) and an unusually strong σ-donor of the alkyl group (R).

Density functional theory (DFT) is applied to the calculation of activation of the Co–C R bond in models of vitamin B 12 , B–[Co III (corrin)]–R. It is shown that there is a positive correlation of the bond lengths between the Co atom and the two axial ligands, B and R. The electron donation from axial ligands to the cobalt atom either by electron donating substituents or by a properly oriented external electric field caused by external electric charges is argued to be the main trigger for the activation of the Co–C R bond.

This Letter introduces a systematic development of a quantitative vibrational force field (FF) capable of modeling vibrational spectra of cobalt corrinoids (B 12 derivatives). The scaled quantum mechanical (SQM) method is used to refine density functional theory (DFT) based force constants calculated at the B3LYP level of theory. The reliability of the DFT–SQM FF for cobalt corrinoids is tested on a six-coordinate methylcobalamin Im–[Co III –corrin]–CH 3 model. It is shown that the computed frequencies and Raman intensities permit detailed vibrational assignment. This analysis illustrates the utility of DFT–SQM FF in making detailed connections between the structure of cobalt corrinoids and their vibrational spectra.

The ground state geometry, force field, and vibrational intensity data of free-base porphine were determined by density functional theory. Calculations were carried out with the Becke-Lee-Yang-Parr composite exchange-correlation functional and 6-31G ∗ basis set. The calculations predict a stable structure of D 2h symmetry. The resulting force field was transformed to internal coordinates and scaled by transferable scale factors from our previous work. There is very good agreement between the calculated and experimental frequencies and intensities, and between experimental and simulated spectra. The assignment of the in-plane modes is discussed and some reassignments are proposed. Finite field perturbation theory was used to obtain the Raman intensities.

A form of open shell perturbation theory that is invariant with respect to orbital degeneracy is proposed. This new methodology is based entirely on the spin-restricted formalism. Preliminary numerical results are presented and compared with other formulations of perturbation theory.

A modified form of multi-reference second-order perturbation theory is considered. The nature of the poor convergence of Moller-Plesset perturbation theory for systems like cyano radical or He+2, He2+2, O2+2 cations is analysed. Additionally, isomerization of the formaldehyde radical cation and the electronic ground state of ozone are considered in detail. To all of the above cases, a modified form of multi-reference second-order perturbation theory is applied and results re-examined.