title: Quantum-chemical analyses of aromaticity, UV spectra, and NMR chemical shifts in plumbacyclopentadienylidenes stabilized by Lewis bases.
abstract: We carried out a series of zeroth-order regular approximation (ZORA)-density functional theory (DFT) and ZORA-time-dependent (TD)-DFT calculations for molecular geometries, NMR chemical shifts, nucleus-independent chemical shifts (NICS), and electronic transition energies of plumbacyclopentadienylidenes stabilized by several Lewis bases, (Ph)2 ((t) BuMe2 Si)2 C4 PbL1 L2 (L1, L2 = tetrahydrofuran, Pyridine, N-heterocyclic carbene), and their model molecules. We mainly discussed the Lewis-base effect on the aromaticity of these complexes. The NICS was used to examine the aromaticity. The NICS values showed that the aromaticity of these complexes increases when the donation from the Lewis bases to Pb becomes large. This trend seems to be reasonable when the 4n-Huckel rule is applied to the fractional π-electron number. The calculated (13)C- and (207)Pb-NMR chemical shifts and the calculated UV transition energies reasonably reproduced the experimental trends. We found a specific relationship between the (13)C-NMR chemical shifts and the transition energies. As we expected, the relativistic effect was essential to reproduce a trend not only in the (207)Pb-NMR chemical shifts and J[Pb-C] but also in the (13)C-NMR chemical shifts of carbons adjacent to the lead atom.
authors.affiliation: Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachi-Oji, Tokyo, 192-0397, Japan; JST, CREST, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan.
authors.forename: Toshiaki
authors.initials: T
authors.lastname: Kawamura
authors.forename: Minori
authors.initials: M
authors.lastname: Abe
authors.forename: Masaichi
authors.initials: M
authors.lastname: Saito
authors.forename: Masahiko
authors.initials: M
authors.lastname: Hada
keywords: NMR; Pb; ZORA; aromaticity; chemical shift; five-membered ring; lead; nuclear spin-spin coupling; plumbole; relativistic effect