Practice question

Shielding occurs when electron cloud circulates under external field, inducing local opposing magnetic field that reduces effective field at nucleus. Greater electron density enhances this diamagnetic shielding, so nucleus requires lower applied radiofrequency to achieve resonance, appearing at lower ppm toward right side, termed upfield. Conversely, electronegative neighbors withdraw electrons, causing deshielding and downfield shift to higher ppm. Protein spectra illustrate this: aliphatic methyls near 1 ppm upfield, aromatic residues near 7 ppm downfield, reflecting distinct electronic and anisotropic environments.