Properties of Stable Organic Bond-Stretched Non-Lewis Molecules † Shmuel Zilberg,* Ehud Tsivion, and Yehuda Haas* Department of Physical Chemistry and the Farkas Center for Light Induced Processes, the Hebrew UniVersity of Jerusalem, Jerusalem, Israel ReceiVed: December 13, 2007; ReVised Manuscript ReceiVed: June 10, 2008 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 ∼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. I. Introduction The quest for stable molecules having a singlet diradical structure led in recent years to the production, crystallization, and characterization of several inorganic species in crystalline form 1–3 and in solution. 4 As summarized in an extensive recent review, 5 these species turn out to have considerable charge- transfer character, leading to their designation as diradicaloids. Here, we discuss the properties of some formal organic diradicals that are calculated to be quite stable under ambient conditions. Their stability is due to polar structures that contribute significantly to the electronic wavefunction of the ground-state singlet. In the context of this paper, bond-stretched isomers are two isomers interconnected by a reaction in which a single bond is formed and separated by a large barrier from the open form side. Figure 1 shows the structures of some cyclopentane-1,3-diyl diradicals that were examined as possible stable singlet diradi- cals. Buchwalter and Closs showed in 1975 6 that triplet diradicals of cyclopentane-1,3-diyl (Ia) can be prepared and observed by ESR. The triplet-singlet gap (ΔE ST ) was calculated to be a few kilocalories/mole, the triplet being more stable. The singlet diradical was taken as the transition state between the two closed forms of the bicyclic compound. Borden et al. 7 proposed that the singlet diradical can be stabilized by substitut- ing the two geminal hydrogen atoms with fluorine ones (Ib). The idea was that the fluorine atoms will act as electron acceptors and help delocalize the electronic charge in the singlet state. Indeed, the singlet of 1b was calculated to lie well below the triplet at all geometries, but it turned out to be a transition state between the two classical closed structures. 7 Other modes of substitution were proposed theoretically and tested experi- mentally; Adam et al. 8 presented evidence for the existence of a reactive intermediate Ic, though it could not be observed directly. In an effort to stabilize carbon-based diradicals, the Z ) SiH 3 group was proposed 9 as a potent stabilizer of singlet diradicals with respect to the triplet one; the calculation showed that the singlet is indeed lower than the triplet, but only by about 2 kcal/mol. The singlet open form of Id was calculated to be 20.8 kcal/mol higher, with a barrier of 9.5 kcal/mol from the open form side. 9 The relatively large stabilization was assigned to strong hyperconjugation due to electron donation from the high lying C-Si sigma orbitals. Similar computational results were reported by Abe et al. 10 We propose an approach that leads to substantially better stabilization of a singlet state of the open form in comparison to the corresponding triplet and more importantly, to the singlet of the closed form isomer in which the two atoms, C1 and C3, are covalently bonded. In other words, the system is shown to have two stable, iso-energetic bond-stretched singlet isomers, separated by a substantial barrier. It was recently shown 11 that partial intramolecular charge transfer can lead to substantial stabilization of singlet ground- state diradicals and a corresponding increase of the singlet-triplet gap for molecules iso-electronic with the dianions of antiaro- matic hydrocarbons. The formal diradicals 2,5-diheterosubsti- tuted-pentalenes and 1,5-diheterosubstituted-cyclooctatetraenes were theoretically predicted to be stable persistent non-Kekule ` molecules. In this communication, we further extend the idea and show that some non-Lewis heterosubstituted molecules are predicted to be as stable as their classical Lewis-type isomers. The relationship between these isomers is discussed within the concept of bond-stretched isomers as introduced by Stohrer and Hoffmann: 12 two stable conformations related by a simple bond stretching. These are the normal classical bonded form and a stabilized diradical form. Several bond-stretched isomer systems have been reported. 1–5 The stability of the novel bond-stretched isomers reported in this communication can be traced to the participation of at least four electrons in the bonding, which in VB language is explained by resonance stabilization of several diradical, zwitterion and ylide-type structures. Since the stability of some of the open forms of these systems is comparable to † Part of the “Sason S. Shaik Festschrift”. * Corresponding authors. J. Phys. Chem. A 2008, 112, 12799–12805 12799 10.1021/jp711705v CCC: $40.75  2008 American Chemical Society Published on Web 07/31/2008