Applications of electronic structure computations to the solution of chemical problems
MetadataShow full item record
This dissertation reports computational studies of endohedral hydrocarbon cage complexes, the aromaticity of polybenzenoid and saturated hydrocarbons and the 2- pyridinethiol/2-pyridinethione tautomeric preference. A variety of density functional theory (DFT) and ab initio quantum chemical techniques have been used throughout the dissertation research, however, B3LYP in conjunction with the 6-31G(d) and 6- 311+G(d,p) basis sets are most prevalent because of their efficiency and typically excellent performance. Herein the following major conclusions are reported: (A) Of the X@C20H20 (X = H, He, Ne, Ar, Li, Li+, Na, Na+, Be, Be+, Be2+, Mg, Mg+, Mg2+) endohedral dodecahedrane complexes, only Li+@C20H20 (Ih; -12.7 kcal/mol), Be+@C20H20 (C5v; -1.3 kcal/mol), Be2+@C20H20 (C5v; -236.3 kcal/mol) and Mg2+@C20H20 (Ih; -118.0 kcal/mol) zero point-corrected inclusion energies of are exothermic relative to their isolated components. However, all the endohedral dodecahedrane complexes are higher in energy than their corresponding exohedral isomers. M@C20H20 (M = Li, Na, Be, Mg) species possess lower first ionization potentials than the Cs atom (3.9 eV) and, therefore, are “superalkalis”. (B) Small hydrocarbon complexes (X@cage) incorporating cagecentered endohedral atoms and ions (X=H+, H, He, Ne, Ar, Li0,+, Be0,+,2+, Na0,+, Mg0,+,2+) have been studied at the B3LYP/6-31G(d) hybrid HF/DFT level of theory. No tetrahedrane (C4H4, Td) endohedral complexes are minima, not even with the very small hydrogen atom or beryllium dication. Cubane (C8H8, Oh) and bicyclo[2.2.2]octane (C8H14, D3h) minima are limited to encapsulating species smaller than Ne and Na+. Despite its intermediate size, adamantane (C10H16, Td) can enclose a wide variety of endohedral atoms and ions including H, He, Ne, Li0,+, Be0,+,2+, Na0,+ and Mg2+. In contrast, the truncated tetrahedrane (C12H12, Td) encapsulates fewer species while the D4d symmetric C16H16 hydrocarbon cage (see Abstract graphic) encapsulates all but the larger Be, Mg and Mg+ species. The host cages have more compact geometries when metal atoms, rather than cations, are inside. This is due to electron donation from the endohedral metals into C-C bonding and C-H anti-bonding cage molecular orbitals. The relative stabilities of endohedral minima are evaluated by comparing their energies (Eendo) with the sum of their isolated components (Einc = Eendo – Ecage – Ex) and compared to their exohedral isomer energies (Eisom = Eendo – Eexo). Although exohedral binding is preferred to endohedral encapsulation without exception (i.e. Eisom is always exothermic), Be2+@C10H16 (Td; –235.5 kcal/mol), Li+@C12H12 (Td; 50.2 kcal/mol), Be2+@C12H12 (Td; –181.2 kcal/mol), Mg2+@C12H12 (Td; –45.0 kcal/mol), Li+@C16H16 (D4d; 13.3 kcal/mol), Be+@C16H16 (C4v; 31.8 kcal/mol), Be2+@C16H16 (D4d; –239.2 kcal/mol) and Mg2+@C16H16 (D4d; –37.7 kcal/mol) are relatively stable as compared with experimentally known He@C20H20 (Ih), which has an Einc = –37.9 kcal/mol and Eisom = –35.4 kcal/mol. Overall, endohedral cage complexes with low parent cage strain energies, large cage internal cavity volumes and a small, highly charged guest species are the most viable synthetic targets. (C) The geometries of four different series of D6h-symmetric polybenzenoid hydrocarbons (PBH) up to and including C222H42 have been optimized at the B3LYP/6- 31G(d) level of theory. Excluding C48H24 and C138H42, which have D3d minima due to 1,5 H...H repulsions between adjacent perimeter rings, optimized geometries are planar D6h minima. Nucleus Independent Chemical Shifts (NICS), at the same level, indicate the presence of individual aromatic rings, which correspond to Clar’s qualitative sextets rule (Clar, E. The Aromatic Sextet; Wiley: London, 1972). NICS and the Clar valence electron topologies agree perfectly in the molecule plane; however, the NICS values computed in parallel planes further away from the molecular surface converge (see Abstract graphic), indicating the presence of a uniform magnetic shielding field. For each series, PBH total NICS values (i.e. the sum of NICS values for all rings in the PBH) correlate linearly with the number of carbon atoms, indicating constant magnetic field development within a series. The C-C lengths depend on their proximity to the more olefinic rich molecular perimeters. However, the large PBH (³C48H24) internal C-C distances converge to ~1.426 Å. In agreement with Clar’s rule, HF/6-31G(d)//B3LYP/6-31G(d) vertical ionization potentials and B3LYP/6-31G(d) HOMO-LUMO gaps are largest within the “fully benzenoid” series, where all carbon atoms are members of a single sextet. The largest members of the four series studied are predicted to exhibit semiconducting properties. (D) Dissected nucleus-independent chemical shift (NICS) analyses of cycloalkanes and cage hydrocarbons reveal diatropic ring currents in three (3MR) and five (5MR) membered rings, but paratropic behavior of four (4MR) membered rings. The large CC bond shielding effects of the archetypal s-aromatic, cyclopropane, are magnified in tetrahedrane and other 3MR-containing cages. The remarkable deshielding of the cyclobutane CC(s) bonds is general: cubane and related 4MR structures are strongly paratropic (i.e. s-antiaromatic). The respective 3MR (diatropic) and 4MR (paratropic) PP bonds in P4 (Td) and P8 (Oh) exhibit similar behaviour. (E) Finally, 2-pyridinethiol (Cs; 2SH) is 2.61 kcal/mol more stable than 2-pyridinethione (Cs; 2S) (CCSD(T)/ccpVTZ// B3LYP/6-311+G(3df,2p)+ZPE), while cyclohexane solvent-field relative energies (IPCM-MP2/6-311+G(3df,2p)) favor 2S by 1.96 kcal/mol. This is in accord with the absence of spectroscopic evidence (nS-H stretch) for the thiol in toluene, C6D6, heptane, or methylene chloride solutions. Although the thione is not aromatic, it is stabilized by thioamide resonance. In solvent, the large 2S dipole, 2-3x greater than 2SH, favors the thione tautomer and in conclusion, 2S is thermodynamically more stable than 2SH in solution.