sp2 carbons are more downfield in NMR spectroscopy compared to sp3 carbons primarily due to the effects of double bonding. The presence of a double bond creates a phenomenon known as diamagnetic anisotropy, which leads to the formation of a 'ring current.' This current generates areas of shielding and deshielding around the double bond. The position of protons relative to these zones significantly influences their chemical shifts, resulting in the observed downfield shift for sp2 carbons. The shorter bond lengths in sp2 hybridization contribute to this effect, as the electrons are more localized, affecting the magnetic environment experienced by nearby protons.
