Properties of Stable Organic Bond-Stretched Non-Lewis Molecules

Abstract

The conditions required for the existence of a stable bond-stretched singlet isomer of hetero derivatives of bicyclo[2.1.0]pentane (which is a cyclopentane-1,3-diyl derivative) are discussed. Such species are non-Lewis systems with a ruptured C-C bond (formally diradicals), in which two electrons occupy the nonbonding orbital. A high-level calculation shows that in contrast with the carbon substituted compounds, in which the open form is a transition state between two classical-bonded closed bicyclic forms, in the heterosubstituted molecules, the open form is calculated to be a stable minimum. The ionization potentials of the open forms are considerably lower than those of their bicyclic isomers and also of regular organic radicals/diradicals. Nitrogen atoms are found to be more effective than oxygen or sulfur in stabilizing the open isomer. In this case, the open isomer is calculated to be a little more stable than the bicyclic compound, and a barrier of approximately 40 kcal/mol is computed for the ring closing reaction. Thus, the open isomer is both thermodynamically and kinetically stable. This result rationalizes some experimental observations that indicated the existence of non-Lewis singlet species.

Chemical reaction with two different elementary transition states

“…(a) Open ylide—bicyclo[2.1.0]‐pentane ring closure; (b) MO correlation diagram; (c) two reaction paths 2D domain between closed and open ylide forms (CAS calculations; and (d) three dominant VB forms—1,3‐DIRADICAL and two mirror ylide (zwitterionic) forms—YLIDE L and YLIDE R .…”

“…c and d). The EWF along two reaction coordinates loop {Closed form → TS L → Open form → TS R → Closed form} preserves a sign, which was checked by analysis of a contribution of the leading configurations at different points on the loop . No CI inside this 2D domain.…”

“…c). 1,3‐Singlet diradical describes a closed form, but it contribute also to the open form—1,3‐DIRADICAL . In‐phase combination of two mirror zwitterionic (YLIDE L + YLIDE R ) forms is a dominant contribution to the open form.…”

“…1,3-Singlet diradical describes a closed form, but it contribute also to the open form-1,3-DIRADICAL. [51] In-phase combination of two mirror zwitterionic (YLIDE L 1 YLIDE R ) forms is a dominant contribution to the open form. The transition from the open to the closed isomers leads to distortion along the antisymmetric Q 2 coordinate, the loss of effective YLIDE L $YLIDE R resonance, Scheme 3.…”

Theoretical study on stability and nonlinear optical properties of tetrahydropyrrole diradical and its isoelectronic systems in different electronic states

“…The molecules studied in this paper are called non-Lewis systems and have a ruptured CC bond (they are formally diradicals), in which two electrons occupy the nonbonding orbital. Some studies of the energy and stability of such system have been reported [24][25][26]. Depending on the different interactions between the two radical electrons, the system forms one of three possible electronic states.…”