Kinetically stable high-energy isomers of C14H12 and C12H10N2 derived from cis-stilbene and cis-azobenzene

Journal of Molecular Modeling, Sep 2010

Following on from our recent enforced geometry optimization (EGO) investigation of isomerization in cis-stilbene (J Comput Chem, in press) we report the discovery of two interesting new, symmetrical “fused sandwich” isomers of both cis-stilbene and the related cis-azobenzene. The isomers were obtained by applying external forces to pairs of carbon atoms from each of the benzene rings in cis-stilbene and cis-azobenzene simultaneously, and are all at least 100 kcal mol-1 higher in energy than the starting material. Each new structure was characterized as a minimum by vibrational analysis. Despite their high energy, all of the new isomers appear to be kinetically stable with respect to rearrangement back to cis-stilbene or cis-azobenzene, respectively.

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Kinetically stable high-energy isomers of C14H12 and C12H10N2 derived from cis-stilbene and cis-azobenzene

Jon Baker Krzysztof Wolinski 0 ) Department of Chemistry, Maria Curie-Sklodowska University , pl. Maria Curie-Sklodowska 3, 20-031 Lublin, Poland Following on from our recent enforced geometry optimization (EGO) investigation of isomerization in cisstilbene (J Comput Chem, in press) we report the discovery of two interesting new, symmetrical fused sandwich isomers of both cis-stilbene and the related cis-azobenzene. The isomers were obtained by applying external forces to pairs of carbon atoms from each of the benzene rings in cis-stilbene and cis-azobenzene simultaneously, and are all at least 100 kcal mol-1 higher in energy than the starting material. Each new structure was characterized as a minimum by vibrational analysis. Despite their high energy, all of the new isomers appear to be kinetically stable with respect to rearrangement back to cis-stilbene or cis-azobenzene, respectively. - In previous publications we reported the discovery of several potentially kinetically stable new isomers of C14H12 and C12H10N2 derived from cis-stilbene (1) [1] and cis-azobenzene (2) [2] (see Fig. 1), respectively. They were obtained using enforced geometry optimization (EGO), in which pairs of carbon atoms, one from each of the two phenyl rings, were pushed together along the line joining the two atomic centers, by means of an applied external force [3]. This force is added to the normal gradient vector computed at that geometry at each cycle of a geometry optimization and typically results in bond formation between the atoms involved. All atoms in the system adjust their relative positions so as to counter the external force and the final result of the optimization is usually a new structure, albeit one that is highly strained. Whether the new structure is stable, i.e., represents a local minimum on the potential energy surface, can be determined by carefully reoptimizing with the external force removed. If the new structure is stable, it will relax but remain substantially intact; if not it usually reverts back to the starting structure. All the kinetically stable isomers found in our earlier work were obtained by forcing together only a single pair of carbon atoms and all lay energetically no higher than 90 kcal mol-1 from the starting material for the stilbene isomers, and under 30 kcal mol-1 in the case of cisazobenzene. In this communication we report the discovery of some interesting high-energy symmetric fused sandwich isomers of both stilbene and azobenzene obtained by forcing together multiple pairs of carbon atoms (in this case all six) from each of the two phenyl rings. Results and discussion The standard methodology used in this work is density functional theory (DFT) [4, 5] using the B3LYP hybrid Fig. 1 Schematic showing structure and atom labeling of cis-stilbene (1) and cis-azobenzene (2) exchange-correlation functional [6, 7] with the 6-31G* basis set [8]. All calculations were carried out using the PQS program package incorporating the PQSMol graphical user interface for post-job visualization and display [9]. All (a) C14H12-Cs (b) C14H12-C2v stationary points found at B3LYP/6-31G* were reoptimized at MP2/6-311G** to ensure that the energetics remained stable. The new isomers, which we call C14H12-Cs, C14H12-C2v, C12H10N2-Cs and C12H10N2-C2v are shown in Fig. 2. They were all obtained by pushing together all symmetry equivalent pairs of carbon atoms from the two phenyl rings in cis-stilbene and cis-azobenzene, respectively. As denoted by the names, the new isomers have either Cs or C2v symmetry. The optimization history for this procedure starting from cis-stilbene using an applied force of 0.1 au is depicted in Scheme 1, below. As can be seen, the energy initially rises to a maximum, then falls and then starts to oscillate. The calculation did not in fact converge, but simply stopped after reaching the maximum allowed number of optimization cycles. Apparently there is simply too much internal strain in the system for it to settle down. If the geometry corresponding to the highest energy oscillation is taken and allowed to relax (i.e., is reoptimized after removing the external force) then the C2v minimum results; if the lower energy structure is relaxed then the result is the Cs minimum. The situation is similar for cis-azobenzene. As shown in Fig. 2 the new isomers are symmetrical fused sandwich compounds in which the former phenyl rings lay one on top of the other (or side-by-side depending on ones point of view). In the C2v isomers, every carbon atom in each of the phenyl rings forms a bond to its (c) C12H10N2-Cs (d) C12H10N2-C2v Fig. 2 High energy isomers of C14H12 and C12H10N2 obtained via an enforced geometry optimization by pushing together all symmetry related pairs of carbon atoms in the phenyl rings of cis-stilbene and cis-azobenzene, respectively Scheme 1 EGO optimization history Fig. 3 Transition states for decomposition of the new Cs and C2v isomers of C14H12 and C12H10N2, (...truncated)


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Jon Baker, Krzysztof Wolinski. Kinetically stable high-energy isomers of C14H12 and C12H10N2 derived from cis-stilbene and cis-azobenzene, Journal of Molecular Modeling, 2010, pp. 1335-1342, Volume 17, Issue 6, DOI: 10.1007/s00894-010-0835-0