Inter-ring dihedral angles in polychlorinated biphenyls from photoelectron spectroscopy

Abstract

A proposal that the physiological activity of a chlorinated biphenyl is related to its degree of molecular planarity is explored through experimental measurements of inter-ring dihedral angles in the gas phase. A known correlation between the difference between fourth and first ionization energies of substituted biphenyls (via photoelectron spectroscopy), and the thermally averaged inter-ring dihedral angle as established by electron diffraction techniques, is confirmed and exploited for the present purpose. The expectation, based on both qualitative and quantitative theoretical considerations, that increased ortho substitution causes an increase in inter-ring angle, is confirmed. However, the observation that, for 2,2'-substituted halobiphenyls the only observed conformation is the near-syn form, is not easy to explain, and is discussed in terms of a non-bonding attractive force between halogen atoms. 1292 (1985). Faisant appel aux mesures expCrimentales effectuCes en phase gazeuse des angles ditdres entre les cycles, on a Ctudik I'hypothkse suggCrant que I'activitC physiologique des biphCnyles chlores est relike a son degrC de plankaritt molbculaire, Dans cette Ctude, on a confirm6 et exploit6 la corrklation connue qui existe entre les quatrikme et premikre Cnergies d'ionisation des biphtnyles susbtituks telles que mesurkes (via la spectroscopie photoClectronique) et I'angle dikdre inter-cycle ponder6 thermiquement tel qu'Ctabli par les techniques de diffraction Clectronique. En se basant sur des considkrations thCoriques tant qualitatives que quantitatives, on peut prCvoir qu'une augmentation de la substitution en position ortho provoquera une augmentation de l'angle inter-cycle; nos rksultats confirment cette hypothkse. Cependant, on ne peut pas expliquer facilement le fait que, dans le cas des biphknyles halogCnks substituees en positions 2,2', la forme quasi-syn est la seule conformation observke. On en discute en fonction d'une force attractive non liee entre les atomes d'halogknes.[Traduit par le journal] Introduction Maier and Turner ( I ) showed how the dihedral angle between the two rings of a series of gaseous biphenyls is related to the difference in ionization energies for electrons in the n3 and n6 molecular orbitals. This difference between fourth and first ionization energies was shown (I) empirically to vary linearly with cos 0, where 0 is the inter-ring dihedral angle determined by electron diffraction. The qualitative trends observed were satisfactorily accounted for (1) by simple analogies drawn with the molecular orbitals (MO's) of substituted benzenes, as determined (2) by photoelectron spectroscopy (PES). The linear correlation between AE(n3 -n6) and cos 0 was satisfactorily accounted for (1) by an ingenious analysis of the n-MO's described by a Huckel approach using as basis set the semi-localized n-MO's of the phenyl moieties, in place of the more usual atomic orbitals. Together with a frozen-orbital (Koopmans' theorem) assumption to permit identification of an orbital energy with the n...

A reactivity-selectivity study of the Friedel-Crafts acetylation of 3,3′-dimethylbiphenyl and the oxidation of the acetyl derivatives

“…All fall in the range 37–39 o , which is within the range of values determined for 3,3′-dmbp by photoelectron [46-48] or NMR spectroscopy [49,50]. It is seen also that the C Ar -C Ar -C = O dihedral angle, ϕ, between the carbonyl group and the benzene ring to which it is attached is close to 0 o for 4-Ac and 5-Ac showing that the steric interactions between the 4-Ac group and the neighboring hydrogen and methyl, and between the 5-Ac group and the two neighboring hydrogens, are negligible.…”

Structure-kinetics relationships of Capadenoson derivatives as adenosine A 1 receptor agonists

“…Reference compound 4 has an LE of 0.35, making it a good drug candidate for this metric. From the other compounds in this study, most (16) were in the 0.32 LE 0.38 range. Out of the remaining four, three had a ligand efficiency below 0.30 (8, 9 and 23), and one above 0.4 (10, LE ¼ 0.47).…”

“…An increase in size and lipophilicity in the halogen series also led to similar results (F, 11; Cl, 5; Br, 14; I, 15). In contrast to the analogous cyanopyrimidines, moving the chloro substituent along the ring did not change the affinity much (4-Cl, 5, 5.0 nM; 3-Cl, 16, 12 nM; 2-Cl, 17, 11 nM), which seems to indicate that the dihedral angle between the thiazolyl and phenyl rings does not have a small value (which would be disfavored by the presence of a 2-chloro substituent) [16] in the binding pose. Also, dichlorination on the 3 and 4 position gave a similar affinity (18, 11 nM), which is again in contrast with our previous findings on cyanopyrimidine analogs.…”

A QSAR evaluation of Ah receptor binding of halogenated aromatic xenobiotics.

“…Moreover, these authors were the first to suggest that the absolute planarity of PCBs might not be required for effective binding. This hypothesis was supported by ab initio quantum mechanical calculations and crystallography studies that suggested polarizability and electronacceptor properties of the ligands, rather than size and planarity (or coplanarity), can control the affinity of binding to the AhR (11)(12)(13)(14)(15)(16)(17)(18).…”