ABSTRACT
[reaction--see text] Despite the increasing reactivity from benzene to heptacene, the acene resonance energies per pi electron are nearly constant. The reactivities (computed activation energies) of the individual acene rings correlate with the reaction energies and depend on the product stabilities. Nucleus-independent chemical shifts (NICS; note the sizes of the red dots, above) indicate that the more reactive inner rings actually are more aromatic than the less reactive outer rings and even more aromatic than benzene itself.
ABSTRACT
[structure: see text] Analysis of the basic pi-aromatic (benzene) and antiaromatic (cyclobutadiene) systems by dissected nucleus-independent chemical shifts (NICS) shows the contrasting diatropic and paratropic effects, but also reveals subtleties and unexpected details.
ABSTRACT
Computations on 2,6-dibromo-4-tert-butyl-2',6'-bis(trifluoromethyl)-4'-isopropyldiphenylcarbene (1) using ab initio and density functional theory methods underscore the unusual stability of the triplet over the singlet state. At the B3LYP/6-311G(d,p) level, the triplet state had a slightly bent central C-C-C bond angle of 167 degrees, whereas this angle in the singlet was 134 degrees. The B3LYP singlet-triplet splitting (12.2 kcal/mol) was larger than that of the parent molecule (5.8 kcal/mol), diphenylcarbene (2), which also has a triplet ground state. The energy of a suitable isodesmic reaction showed the triplet and singlet states of (1) to be destabilized, by 6.3 and 12.5 kcal/mol, respectively, due to the combined effects of the CF3, Br, and alkyl substituents. The linear-coplanar form of (3)(1), which might facilitate dimerization or electrophilic attack at the more exposed diradical center, was prohibitively (35.9 kcal/mol) higher in energy. Our results confirm Tomioka's conclusion that the triplet diarylcarbene, ortho-substituted with bulky CF3 and Br substituents, is persistent due to steric protection of the diradical center. Dimerization and other possible reaction pathways are inhibited, not only by the bulky ortho substituents but also by the para alkyl groups. The increase in stability of the triplet ((3)(1)) state relative to the singlet ((1)(1)) state does not influence the reactivity directly.
ABSTRACT
A series of charged and neutral four-center n-electron (4c-ne, n = 1-4) molecules based on the adamantane framework, but which include combinations of boron, nitrogen, and phosphorus atoms at bridgehead positions, were studied computationally at the B3LYP/6-31G* level of density functional theory (DFT). The three-dimensional aromaticity, observed earlier for the 1,3,5,7-bisdehydroadamantane dication (1), is found to be general for 4c-2e electron systems. The degree of electron delocalization, evaluated by energetic, geometric, and various magnetic criteria, is quite independent of the molecular symmetry (point groups vary from Td to Cs), the degeneracy of the orbitals, the molecular charges, and the nature of the atoms participating in the delocalized bonding. Although the multiple positive (e.g., in 1 and some of the heteroatom systems) and multiple negative charges are strongly repulsive, the rigid adamantane frameworks help hold the bridgehead atoms within bonding distances with the fewer available electrons. The corresponding 4c-1e doublets are approximately half as aromatic as the 4c-2e singlets based on the same criteria. However, the three-electron systems may either adopt distorted but still four-center delocalized structures, or alternative 3c-2e two-dimensional arrangements in which the fourth bridgehead atom is more distant. There is no need to derive special rules for each point group for 4c-ne systems. Although the three-dimensional stabilization is computed to be quite appreciable, ranging between 10 and 50 kcalmol(-1), this delocalization energy is generally not sufficient to overcome distortion due to strain in higher homologues of 1 and in analogous noncage systems. Among the various 4c-2e homoadamantanedehydro dications studied, only the 1,8-dehydrohomoadamandiyl-3,6-dication forms a three-dimensional aromatic system.
ABSTRACT
The viability of molecules with planar hexacoordinate carbon atoms is demonstrated by density-functional theory (DFT) calculations for CB6(2-), a CB6H2 isomer, and three C3B4 minima. All of these species have six pi electrons and are aromatic. Although other C3B4 isomers are lower in energy, the activation barriers for the rearrangements of the three planar carbon C3B4 minima into more stable isomers are appreciable, and experimental observation should be possible. High-level ab initio calculations confirm the DFT results. The planar hexacoordination in these species does not violate the octet rule because six partial bonds to the central carbons are involved.
