Bond-angle dependence of 13C NMR chemical shifts in some carbon sp 3 systems

Abstract

Using the mathematical approach as proposed in a preceding study on 31P chemical shifts, it is found that the 13C chemical shift for molecules of the type X′CX 3, where X and X′ are different monofunctional substituents, can be related to the XCX bond angle and Z′, the number of surrounding electrons at the 13C nucleus. Taking this number to be equal to the carbon atomic number Z for the molecules under study, except those containing substituents such as Br, SCH 3, Cl, OH, OCH 3, and F for which Z′ = Z − 1, not only is a satisfactory agreement between XCX bond-angle values calculated from experimental chemical-shift data and observed values obtained, but also an expected feature concerning the dependence of the chemical shift on the effect of substituents, including their electronegativity as a major factor, appears to be discernible.