The effect of substituents on electronic states’ ordering in meta-xylylene diradicals: Qualitative insights from quantitative studies

Abstract

Equation-of-motion spin-flip coupled-cluster method with single and double substitutions (EOM-SF-CCSD) is employed to study how substituents affect the electronic states' ordering in meta-xylylene diradicals. The electronegativity of substituents and the incorporation of a heteroatom are found to have a negligible effect. The effect of charges on energy gaps is much more pronounced, in agreement with the proposal of Dougherty and co-workers [J. Am. Chem. Soc. 118, 1452 (1996)]. Resonance structure theory and molecular orbital analysis are employed to explain this phenomenon. The changes in the exocyclic C-C bond length in substituted meta-xylylenes, derived from equilibrium structures calculated by using analytic gradients for the EOM-SF-CCSD method, support the original resonance theory explanation by West et al. However, a similar resonance-theory-based reasoning fails to explain the quantitative difference between positively and negatively charged systems as well as the observed strong stabilization of an open-shell singlet state in the N-oxidized pyridinium analog of meta-xylylene.

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

“…If the selective stabilization of one NBMO is large enough, both non-bonding electrons will tend to occupy that NBMO; and a singlet is likely to be the ground state of the nitrogen-substituted MBQDM derivative. Both Dougherty and coworkers 102 and Wang and Krylov 103 have confirmed this qualitative prediction by carrying out ab initio calculations on the three molecules shown in Fig. 18.…”

Theoretical Design of High‐Spin Polycyclic Hydrocarbons

“…For trimethylenemethane, the UDFT approach gives a vertical (adiabatic) singlet-triplet separation of 1.10 eV (0.99 eV), to be compared with the accurate ab initio result of 1.18 eV (0.94 eV) [63] and the experimental value of 0.79 eV. [64] In meta-xylylene, our S-T energy difference is calculated at 0.61 eV (0.57 eV), to be compared with ab initio values of 0.60 eV (0.49 eV) [65] and experimental adiabatic value of 0.42 eV. [66] Apparently B3LYP, therefore, does not overestimate the magnetic coupling in organic diradicals, as it does in transition-metal complexes.…”

EPR spectroscopic and computational characterization of the 2‐dehydro‐m‐xylylene and 4‐dehydro‐m‐xylylene triradicals

“…The ground states of these diradicals can often be rationalized by considering the spin polarization of the π‐system . In contrast, typical ππ and πππ di‐ and triradicals, such as the non‐Kekule hydrocarbon m ‐xylylene (1,3‐quinodimethane) 1 or 1,3,5‐trimethylenebenzene 2 , show a strong preference for ferromagnetic coupling with robust 3 B 2 and 4 A 2 ground states, respectively, according to their strongly nondisjoint NBMOs (Scheme ) . The singlet–triplet energy splitting of 1 was determined by photoelectron spectroscopy to 9.6 ± 0.2 kcal/mol, in reasonable agreement with vertical and adiabatic excitation energies for the ã 1 A 1 ← X ̃ 3 B 2 transition of 13.8 and 11.3 kcal/mol, respectively, computed at the equation‐of‐motion spin‐flip coupled‐cluster level (EOM‐SF‐CCSD) .…”

“…In contrast, typical ππ and πππ di‐ and triradicals, such as the non‐Kekule hydrocarbon m ‐xylylene (1,3‐quinodimethane) 1 or 1,3,5‐trimethylenebenzene 2 , show a strong preference for ferromagnetic coupling with robust 3 B 2 and 4 A 2 ground states, respectively, according to their strongly nondisjoint NBMOs (Scheme ) . The singlet–triplet energy splitting of 1 was determined by photoelectron spectroscopy to 9.6 ± 0.2 kcal/mol, in reasonable agreement with vertical and adiabatic excitation energies for the ã 1 A 1 ← X ̃ 3 B 2 transition of 13.8 and 11.3 kcal/mol, respectively, computed at the equation‐of‐motion spin‐flip coupled‐cluster level (EOM‐SF‐CCSD) . A detailed matrix‐isolation study of 1 , including infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy, as well as investigations of various photoisomerization processes, became possible only recently, employing 1,3‐bis‐(iodomethyl)benzene as a thermal precursor .…”