MO-Theoretical Description of Electronic Structures of Tricentric Bisdesmiphiles in the Ground and Excited States

Abstract

A systematic MO-theoretical description of electronic structures of tricentric bisdesmiphiles in the ground state is presented in response to contradictory arguments on the biradical character of the species by Huisgen, Firestone, and others. The approximately spin-projected but size-consistent (AP) UHF and UMP methods remedy artifact arising from the spin contamination effect involved in the singlet UHF and UMP wavefunctions, giving reasonable a different-orbitals-for-different-spins (DODS) MO description for oxygenated dipoles such as O3 and CH2OO and 1,3-biradicals without octet stabilization such as OCH2O and CH2CH2CH2. The RHF method provides well-accepted MO description for diszonimum and nitrilium betaines having little biradical character, and the RMP method works well even for species with weak biradical character. They are successfully applied to calculating relative energies between nonradical ring- and biradical open-forms of tricentric species, singlet–triplet energy gaps of tricentric biradicals, and high- and low-spin energy gaps of polyradicals constructed of 1,3-biradical units. Reliability of the projected UHF and UMP scheme for biradical species was confirmed by the complete active space (CAS) configuration interaction (CI) by the use of UHF natural orbitals (UNO). The UNO CASCI calculations are also carried out both for the ground and excited of 1,3-biradicals. The energy gradient technique based on UNO CASSCF is applied to the full geometry optimizations of tricentric systems, followed by the UNO CASSCF MP2 calculations for dynamical correlation corrections. On the other hand, the UNO MR SDCI method by the use of the UNO CASCI reference gives a well-balanced MO-theoretical description of the ground and excited states. Comparisons of different definitions of biradical character are given in relation to theoretical explanations of 1,3-dipoles and 1,3-dipolar reactions. It is shown that the present MO-theoretical description based on both delocalized and localized natural orbitals is wholly compatible with experimental data obtained by Huisgen and others.

Symmetry and broken symmetries in molecular orbital descriptions of unstable molecules II. Alignment, flustration and tunneling of spins in mesoscopic molecular magnets

“…4. The sulfur ylides and/or oxygenated dipoles (1) with planar conformations are well-known 1,3-dipolar compounds with a singlet ground state [3]. The reactivity of these species was investigated in an earlier paper [1].…”

“…However, electron transfer from the sulfur (or oxygen) atom to the left or right radical orbital is feasible to provide zwitterionic structures 3 and 4. The singlet state is therefore described by the superposition of these three con®gurations [3]. This con®guration-mixing stabilizes the singlet state: the strong p bond between the cation radical of the central hetero atom and the terminal radical is formed in the case of a zwitterion because of the strong pp-pp overlap.…”

“…The LoÈ wdin-type spinprojection scheme [38] is utilized for this purpose; however, it often provides wrong t values because of the approximations introduced [38]. Alternately, approximate spin projections [3,29,30] are feasible for G(U)HF-based and spin-polarized DFT methods to calculate potential curves for dissociations from near closed-shell to the localized limit. To this end, we have considered an approximate but size-consistent spinprojection procedure, where the denominator in Eq.…”

“…Earlier [1], we throughly examined the biradical character of 1,3-dipolar species [3]. Here, let us ®rst consider dimers, trimers, and clusters (x 11) of 1,3-dipolar species as illustrated in Fig.…”

“…For example, the instability of chemical bonds was extensively studied in relation to nonconcerted behavior observed in various gas-phase reactions of unstable species such as carbenes, nitrenes, oxygen atoms, and multi-center biradicals. The orbital symmetry control for 1,3-dipolar reactions was often lost because of signi®-cant biradical character of 1,3-dipolar species such as oxygenated dipole, though the reactions are formally symmetry-allowed [1,3]. The orbital symmetry breakings and electron localizations are also recognized for many binuclear metal complexes [4], organometallic conjugated systems [5], metal clusters and amorphous materials [6], which often exhibit high catalytic activity.…”