Category: 2001

The remotely substituted 5-exo-bicyclo[2.1.1]hexan-2-one system is introduced as a new probe to study long range electronic effects on π-face selectivity during hydride reduction and a systematic computational study demonstrates good predictability at the semi-empirical level.

Hybrid density functional theory (DFT) calculations at the B3LYP/cc-pVDZ level have been performed on a series of heterobuckybowls, 3X, C₁₈X₃H₆ (X = O, NH, CH₂, BH, S, PH, PH₃, Si, SiH₂, and AlH). The minimum energy conformations and the transition states for bowl-to-bowl inversion, where the geometry is bowl shaped, are computed and characterized by frequency calculations. The geometries of heterotrindenes, 2X, C₁₂X₃H₆ (X = O, NH, CH₂, BH, S, PH, PH₃, Si, SiH₂, and AlH), were obtained, and the bond length alternation (Δ) in the central benzenoid ring shows remarkable sensitivity as a function of substituent with a wide range of fluctuations (−0.014 to +0.092 Å). The Δ computed in 2BH was found to be comparable with the highest bond alternation reported to date in benzenoid frameworks. The inversion dynamics of these heterobowls and their bowl depths were fit to a mixed quartic/quadratic function. The size of the heteroatom seems to exclusively control the bowl depth and rigidity as well as the synthetic feasibility. In contrast, the bond length alternation seems to be controlled by electronic factors and not by the size of the substituted atom either in trindenes or in heterosumanenes. The thermodynamic stability of this class of compounds is very much comparable with trithiasumanene (3S), which has been synthesized recently. The chemical hardness (η) was measured to assess the stability of the heterosumanenes. The strain energy buildup in a sequential ring closure strategy along two synthetic routes, namely a triphenylene route and a trindene route, were explored, and the trindene route was found to be highly favorable for making such compounds compared to the triphenylene route. However, in both routes the ease of the synthetic feasibility increases as the size of the heteroatom increases.

Exploratory semi-empirical SCF calculations are done on mono, di, tri, tetra, and pentabenzocorannulenes to assess the effect of sequential benzannulation to the corannulene moiety. Computed results predict a gradual reduction in the bowl-to-bowl inversion barrier in each step of the successive benzannulations and eventually for pentabenzocorannulene the barrier is only 1.4 kcal mol⁻¹ at the MNDO level. This is also followed by gradual flattening of the central corannulene skeleton and the strain energy in benzannulated corannulenes is found to be more due to the steric repulsions between the peri-hydrogens due to the tessellation of six-membered rings. Molecular mechanics calculations are performed to evaluate the strain energy of the various benzocorannulenes. The strain energy build-up in a sequential bridging starting from pentabenzocorannulene en route to C₄₀H₁₀ are done. AM1 consistently overestimates the inversion barrier and based on the previous related calculations MNDO is adjudged to be the better choice to employ on this class of compounds. The sequential strain energy build-up is also evaluated starting from the pentabenzocorannulene, C₄₀H₂₀, to a very deep bowl, C₄₀H₁₀, which is the main structural motif of C₆₀.

The synthetically elusive C_3v symmetric sumanene (C₂₁H₁₂), a key structural motif of C₆₀, was subjected to a detailed computational study, exploring the structure, bowl-to-bowl inversion dynamics, vibrational spectra, and some other physicochemical properties. Hartree−Fock (HF), pure (BLYP, BP86, and BPW91), and hybrid density functional (B3LYP, B3P86, and B3PW91) calculations were done with an array of basis sets (STO-3G, 3-21G, 6-31G*, 6-31G**, 6-311G*, 6-311G**, 6-311+G*, 6-311++G*, cc-pVDZ, and cc-pVTZ). The effect of a basis set higher than double-ζ quality and the inclusion of dynamic correlation on the geometry and bowl-to-bowl inversion barrier was insignificant. The B3LYP or HF method with the cc-pVDZ or 6-311G* basis set gave satisfactory results. The previously computed modified neglect of diatomic overlap (MNDO) value of 24.2 kcal/mol for the bowl-to-bowl inversion was found to be too high, and a revised value of 16.9 kcal/mol was obtained by the B3LYP/cc-pVTZ//B3LYP/cc-pVDZ method. Consequently, the computed results indicate that sumanene (2) is not locked in the bowl geometry and that a definitive bowl-to-bowl inversion should exist at room temperature. The highest level of theory used in the study (B3LYP/6-311G**) yields values of 1.14 Å, 2.45 D, and 98.8° for the bowl depth, dipole moment, and π-orbital axis vector angle at the hub carbon for sumanene, respectively. Interesting temperature dependency of inversion dynamics is predicted near room temperature.

Ab initio (HF, MP2, and CCSD(T)) and density functional theory computed results on the equilibrium geometries, relative stabilities, strain energies, and vibrational spectra of the nine possible valence isomers of pyridine are reported. Although some aza-benzvalenes (V1N and V3N) lie lower in energy than Dewar pyridines (D1N and D2N), the strain energies for the latter are lower. Relative stabilities of the valence isomers, thermodynamic stability, and skeletal rigidity are comparable to those of benzene valence isomers.

Quantum mechanical calculations predict that larger heteroatom substituents on the periphery increase the feasibility of the crucial third ring closure in sumanene and are responsible for the accompanying modulations in the curvature, rigidity, stability and some of the physicochemical properties of the resulting heterosumanenes. Systematic application of semiempirical, ab initio, and DFT methods reveal that the qualitative trends obtained and our principal conclusions are independent of level of theory, albeit with minor quantitative differences.

The structures and energetics of nine valence isomers of phosphinine, (CH)₅P, have been investigated by ab initio (HF, MP2 and CCSD(T)) and hybrid density functional (B3LYP) methods. The relative stability ordering of the (CH)₅P isomers is similar to those of (CH)₆. Strain energies are evaluated for all the non-planar isomers based on the sum of standard bond strengths, taking the planar resonance stabilized isomer, phosphinine as the reference. Lower magnitudes of the frequencies corresponding to the first few normal modes compared to their benzene isomers account for smoother isomerization reactions among them.

Two possible synthetic routes, namely a triphenylene route and a trindene route for the synthesis of sumanene, C₂₁H₁₂ (2CH₂) and trithiasumanene, C₁₈S₃H₆ (2S) are analyzed in detail. Theoretical calculations unequivocally predict that the synthesis of sumanene may be easily achieved through a trindene route.

Here is a clue to achieve the synthesis of sumanene. Theoretical calculations predict that the synthesis of sumanene can be achieved if one starts from appropriately substituted trindene.