Question

Which of the underlined protons in each pair absorbs farther downfield in NMR spectrum `CH_3OCH_3` or `CH_3SCH_3`?

Answer 1

The hydrogen in Ether (`CH_3OCH_3`) will absorb farther downfield due to the de-shielding effect of the polar C-O bond

Since O is more polar than S, it will inductively draw more electron density from the methyl group. The result is a more exposed H nucleus (remember that electron density creates a local magnetic field that can oppose the applied, external magnetic field used in NMR).

So, with less electron density, the hydrogens in ether will be more deshielding, resulting in a downfield shift in an 1H NMR