Perhaps the most direct measurement of s character in a bonding orbital between hydrogen and carbon is via the H&minus;C coupling constants determined from NMR spectra. Theory predicts that J values correlates with s character. In particular, the one bond C-H coupling constant J is related to the fractional s character of the carbon hybrid orbital used to form the bond through the empirical relationship , where is the s character.  (For instance the pure sp hybrid atomic orbital found in the C-H bond of methane would have 25% s character resulting in an expected coupling constant of 500 Hz × 0.25 = 125 Hz, in excellent agreement with the experimentally determined value.)
As the electronegativity of the substituent increases, the amount of p character directed towards the substituent increases as well. This leaves more s character in the bonds to the methyl protons, which leads to increased J coupling constants.