Chemical shifts of exchangeable and non-exchangeable protons in double-helical B DNA fragments were collected from the literature , subjected to statistical analysis and tabulated. The tables are arranged according to the central residue involved (A, G, C and T) and to the 5 0 -and 3 0 -flanking residues (triplet model). The standard deviations in the mean (90% confidence limits) are of the order of 0.02-0.04 ppm for most protons. In addition, data on terminal residues were collected (terminal doublets) and it is shown that these shifts play a key role in the empirical determination of physically meaningful shielding/deshielding increments induced by the 5 0 -and 3 0 -flanking bases in triplets. The r.m.s. error of the shifts predicted with the aid of our new incremental scheme is 0.01-0.03 ppm. The analysis throws new light on the mechanism involved in the fraying process.
Abstract-13C n.m.r. chemical shifts of a number of 1,l-disubstituted ethylenes are presented. Moreover, effects of changing temperatures on the 13C n.m.r. chemical shifts of some of these compounds as well as of three normal alkanes are given. These variations in chemical shifts are attributed to varying amounts of sterically induced shifts in the different conformational equilibria. In addition to the well-known 1,4 interaction between two alkyl groups shielding effects on the carbon atoms of the connecting bonds are also proposed. No definite explanation of this effect is presented at this time. It is further shown that no simple correlations exist between 13C n.m.r. chemical shifts and calculated total charge densities at this level. Instead, the experimental results in 1-alkenes are rationalized by assuming a linear dependence of the 13C n.m.r. chemical shifts of C-1 and C-2 via rehybridizations on changes in bond angles for small skeletal deformations caused by steric interactions. These changes in geometries, as well as conformational energies in three 1-alkenes, were calculated by means of V F F calculations. Finally, upfield shifts for both C-2 and C-4 are proposed for those conformations of 1-alkenes in which the C-3-C-4 group interacts with the p,-orbital of C-2.
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