Examples of electron withdrawing substituents are -OH, -OCOR, -OR, -NO2 and halogens. These cause a downfield shift of approximately 2–4 ppm for H atoms on Cα and of less than 1–2 ppm for H atoms on Cβ. Cα is an aliphatic C atom directly bonded to the substituent in question, and Cβ is an aliphatic C atom bonded to Cα. Carbonyl groups, olefinic fragments and aromatic rings contribute "sp2" hybridized carbon atoms to an aliphatic chain. This causes a downfield shift of 1–2 ppm at Cα.