Diradicals and Diradicaloids in Natural Orbital Functional Theory

• Published in: Chemphyschem Vol. 12; no. 6; pp. 1061 - 1065
• Main Authors: Lopez, Xabier, Ruipérez, Fernando, Piris, Mario, Matxain, Jon M., Ugalde, Jesus M.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 18.04.2011; WILEY‐VCH Verlag; Wiley
• Subjects: Ab initio calculations; Atomic and molecular physics; Calculations and mathematical techniques in atomic and molecular physics (excluding electron correlation calculations); computational chemistry; correlation energies; electronic structure; Electronic structure of atoms, molecules and their ions: theory; Exact sciences and technology; Physics; radicals; Self-consistent-field methods; CAS SCF method; Electronic structure; Correlation energy; radicals; Theoretical study; Wave functions; correlation energies; Ab initio calculations; computational chemistry; Isomers
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201100136

Natural orbital functional (NOF) theory in its PNOF3 and PNOF4 implementations is used to investigate the electronic structure and energetics of selected diradicals and diradicaloids: the trimethylenmethane (TMM) diradical and the imino‐allyl (IMA) and oxyallyl (OXA) diradicaloids. These compounds are taken as paradigmatic cases of molecules with full and partial diradical character, and are considered as a case study to illustrate the potentiality of Piris NOF (PNOF) to yield the correct treatment of near‐degeneracy effects in diradicals. The degree of diradical character is determined through the inspection of the corresponding natural orbital occupation numbers. In addition, the energetics of these compounds with respect to their cyclic isomers (methylenecyclopropane, MCP, iminocyclopropane, ICP, and cyclopropanone, OCP) is investigated. We have found that PNOF4 shows a promising description of these species, and yield the correct trends in occupation numbers as compared with wave‐function methods such as CASSCF. Therefore, PNOF4 can give a correct description of near‐degeneracy effects, and hence, is a promising method to treat diradicals and diradicaloids in chemistry, a delicate type of molecular systems to simulate by theoretical methods. Natural orbital functional (NOF) theory in its PNOF3 and PNOF4 implementations is used to investigate the electronic structure and energetics of selected diradicals and diradicaloids taken as paradigmatic cases of molecules with full and partial diradical character (see picture). It is found that these methods can give a correct description of near‐degeneracy effects, and hence, are promising for the treatment of diradicals and diradicaloids in chemistry.