Axial atomic mass

Structural data from X-ray- and electron-diffraction studies, mass spectra, and MO calculations were analyzed for R3SiCo(CO)4 (R = H, F, Cl) and Me3SiMn(CO)5 it was believed that there is partial double-bond character between Si and Co and that there may be intramolecular interaction between the axial silicon atom and the equatorial carbonyl groups 30, 212). 

The relative stereochemistry of hyperaspine 93 was determined by 2-D NMR spectroscopic and mass spectrometry (MS) methods. It has a m-fused bicyclic conformation 93a <2001TL4621>. The trans-fused one is disfavored by an axial pentyl group at C-8 and by a destabilizing dipole-dipole interaction between the N- and O-atoms, which does not exist in the alternative //.(-conformation. The geminal coupling constant of C( 1 )H2 in 93 (11.0 Hz), and that of its 6-hydroxy derivative (11.2 Hz), indicates that they exist preferentially in / //-conformations, whereas their 6-epimers adopt trans-conformations (9.3 and 8.4 Hz, respectively) <2005EJ01378>. Nuclear Overhauser enhancement spectroscopy (NOESY) studies also confirmed the stereochemistry of 93 by the marked nuclear Overhauser effect (NOE) correlation between H-3 and H-4a <20030L5063>. 

The Moura group propose the formation of a stable cyano-bridged diiron derivative following ferricyanide oxidation of the superoxide reductases (SORs) from Treponema pallidum and from Desulfovibrio vulgaris.10 Spectroscopic evidence for a ferrocyanide adduct with the bridge (SOR)Fe-NsC-Fen(CN)5 came from mass and IR measurements, where the Fe co-ordination is made up of four equatorial histidine N atoms and an axial cysteinyl S atom, and it is proposed... 

The spectrum of [Au7(PPh3)7][OH] shows two doublets thought to correspond to the axial and equatorial Au atoms of a pentagonal bipyramidal cluster. However, the area ratio of the two subspectra is approximately 1.4 1 instead of the expected 0.4 1. Even when the expected relative intensities are treated to take the effective masses of the recoiling units into account, as described by Parish, Salter, and co-workers (111), the expected ratio increases to only 0.48 1. 

The stereochemistry of enolisation has been mostly examined in cyclic systems where the relative positions of the enolisable hydrogen atoms are fixed. Over the last decade, these studies have benefited from important improvements in the experimental methods, namely mass spectrometry and nmr spectroscopy. Of great interest is the comparison of the relative mobilities of diastereoisomeric axial and equatorial protons from ketones in the cyclohexane series. Indeed, since axial a(C—H) bonds of rigid cyclohexanones are closer than equatorial a(C—H) bonds to the desirable conformation in which the breaking C—H bond is perpendicular to the direction of the C=0 bond, it can be expected that the axial a(C—H) bond-breaking is easier than that of the equatorial one. 

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