Corrigendum

Abstract

Vertical and horizontal rows ofTable 1 were exchanged. The correct Table is as shown below.

Heterocycles with 7 and 8π electrons. Models for flavin and pteridine intermediates and candidates for novel inorganic rings

“…Increased bending of the six-membered ring on introduction of methyl substituents at the ring carbon centers causes low field 1H- [78] and lsC-NMR shifts [182] -in contrast to what would be expected from the inductive effect of a methyl group. NMR data also confirm that there is a general low field shift within these molecules due to the geometry change and that there is no delocalization of the electron density to the methyl substituents [78].…”

“…Among the most conspicuous properties of the organometal-stabilized 1,4-dihydropyrazines are their varying colours which correlate to the photoelectronspectroscopically determined ionization energies [78,90] and to the molecular structure ( Figure 7): whereas the distinctly non-planar permethyl derivative 8 is colourless [78], the almost planar unsubstituted compounds are bright yellow (trialkylsilyl species [171,183]) or even orange-red (the trimethylgermanium compound 7 [90]). Although part of this absorption in the visible spectrum is due to tails from very intense bands in the UV region, the planar organometallic systems are distinguished by weak (~~I0 M-lcm -1) absorptions around 400 nm [182]. These bands are attributed to a*(Si-C) ~ ~ charge transfer transitions for the following reasons: these molecules have high-lying, "antibonding ~ occupied molecular orbitals [40,50], in fact, their ionization potentials around 6 eV [78,90] indicate that they are among the most electron rich non-transition metal compounds studied to date [40,78].…”

“…Figure 9). The replacement of CH by N is not only useful in order to understand the function of each individual center; supported by perturbation calculations [223], it can also provide guidelines for a systematic design of "a~a ~-znd ~deaza~-modified coenzymes which have recently been the focus of intense research and which often exhibit markedly different reactivity and physiological effects [28,33,[166][167][168][169].…”

“…Calculated and experimental ground state basicities of the individual nitrogen centers are known [224,225], however, the 1,4-diazine nitrogen atoms remain the centers with high ~ orbital coefficients in the lowest unoccupied MO [223], i.e. offering the best opportunity for ~ back bonding with a metal fragment.…”

“…(4)) 1,3-sigmatropic shift [202[, reductive silylation of these molecules yields the 87c electron species, the 5,8-dlhydro forms in pteridlne n,~mherlng system, as iso!able molecular products [223]. The series (Figure 9) of N-silylated dihydro forms includes a first "genuine ~, i.e.…”