An empirical proton NMR shielding equation for alkenes based on ab initio calculations

Nuclei of hydrogen atoms located over a carbon-carbon double bond in the pr esence of a strong magnetic field experience a perturbed magnetic field cau sed primarily by the magnetic anisotropy of the pi bond. However, the commo nly used theoretical model for predicting the shielding effect of an alkene double bond on hydrogen nuclei is sometimes inconsistent with the observed proton NMR chemical shifts in structures that have covalently bonded hydro gens located over a carbon-carbon double bond. We have used the ab initio g auge including atomic orbital (GIAO) method to calculate isotropic shieldin g values and to determine the proton NMR shielding increments for a simple model system: methane held at various positions over ethene. These shieldin g increments calculated for one proton of methane have been mapped as a fun ction of their position in Cartesian coordinates relative to the center of ethene. A mathematical function has been fit to this three-dimensional shie lding increment surface at each of four distances from the face of the ethe ne molecule. Additionally, a single mathematical equation has been develope d for predicting the shielding caused by the carbon-carbon double bond in e thene. In contrast to the traditionally employed shielding model, our resul ts predict deshielding for protons within 3 Angstrom above the center of a carbon-carbon double bond, consistent with experimental observations in sev eral molecular systems. The NMR shielding increments predicted by this equa tion are compared to observed shielding increments in some test alkenes.