An ab Initio Valence Bond Study on Cyclopenta-Fused Naphthalenes and Fluoranthenes

Abstract

To probe the effect of external cyclopenta-fusion on a naphthalene core, ab initio valence bond (VB) calculations have been performed, using strictly atomic benzene p-orbitals and p-orbitals that are allowed to delocalize, on naphthalene (1), acenaphthylene (2), pyracylene (3), cyclopenta[b,c]-acenaphthylene (4), fluoranthene (5), and cyclopenta[c,d]fluoranthene (6). For the related compounds 1-4 and 5,6 the total resonance energies (according to Pauling's definition) are similar. Partitioning of the total resonance energy in contributions from the possible 4n + 2 and 4n π-electron conjugated circuits shows that only the 6π-electron conjugated circuits (benzene-like) contribute to the resonance energy. The results show that cyclopenta-fusion does not extend the π system in the ground state; the five-membered rings act as peri-substituents. As a consequence, the differences in (total) resonance energy do not coincide with the differences in thermodynamic stability. Notwithstanding, the relative energies of the Kekulé structures can be estimated using Randić's conjugated circuits model.

Consecutive Reaction Mechanism for the Formation of Spore Photoproduct in DNA Photolesion

“…The Coulombic repulsion energy arising from electrons of opposite spin is minimized. As a result, the biradical species in its singlet and triplet state are nearly degenerate in principle. , Indeed, as shown in the calculated potential energy profiles of Figure , the singlet biradical SI is equal to the triplet biradical TI in energy, from which the singlet–triplet surface intersection results. To obtain the singlet–triplet interaction profile, the PES was scanned from TS1 to TI by varying the C7′–H8′ distance from a configuration which is close to the TS1 (the initial optimized value is 1.76 Å in that structure) with a step length of 0.6 Å and optimizing the remaining coordinates.…”

Electric and Magnetic Properties Computed for Valence Bond Structures: Is There a Link between Pauling Resonance Energy and Ring Current?

“…The weights of the structures calculated for this model are 0.646 for 3a, 0.173 for 3b/3c, and 0.008 for 3d (Scheme 1), which is remarkably close to those obtained for pyracylene itself (3a: 0.642, 3b/3c: 0.175, and 3d: 0.008). 26 The relative energies of the structures obtained for the H 14 model (3a: 0.0 kcal/mol, 3b/3c: 62.8 kcal/mol, and 3d: 165.0 kcal/mol) are also in agreement with those found in the π-VB calculation on 3 (3a: 0.0 kcal/mol, 3b/3c: 52.6 kcal/mol, and 3d: 167.6 kcal/mol). Coincidentally, the Pauling resonance energy of 15.28 kcal/mol found for H 14 is similar to the value of 10.6 kcal/mol calculated for pyracylene.…”

“…23,24 Deformation of benzene to "localized" benzene leads to a loss of resonance energy, 22 whereas the magnetic properties are hardly influenced. 23,25 Pyracylene is interesting in the sense that the cyclopentafusion of the naphthalene core has a negligible effect on the Pauling resonance energy, 26 whereas the magnetic properties change considerably as soon as the second pentagon is attached to acenaphthylene. 27,28 These observations already suggest that a discrepancy exists between the magnetic and energy criteria for aromaticity.…”

The application of cholesky decomposition in valence bond calculation

“…However, valence bond (VB) theory still attracts the attentions of many chemists and is considered as one of modern chemical bonding theories, because of its ability to provide intuitive insights for chemical problems . Since the 1980s, more and more ab initio VB methods and programs have been developed and widely applied to explore the nature of chemical bond …”