In: Chemistry
If electron density around H atom is decreased by any adjacent environment then that H atom is said to be deshielded and it resonates at much higher frequency than norma, hence that particular proton appear in the spectrum at much downfield relative to internal standard reference Tetra methyl silane (TMS) calibrated as 0.00 ppm.
When H atom attaced atom (either C or N or O) as in C-H, O-H, N-H also have a strong electron withdrawing atom (such as F, Cl, phenyl ring, -OCOR group) or group then the electron density around proton decreases, converesly if electron donating atoms or groups such as alkyl, -OR, -NHCOR are present (sometimes depending upon the situation -X (halogens) also will behaves as electron donating type due to +M (mesomeric effect) effect, the the electron density around proton increases thereby that proton will be shielded and appear as upfield chemical shift in delta ppm (parts per million) scale.
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