A series of alkyl groups CH₃-, CD₃, CT₃  on benzene ring represents

(1) As atomic mass of isotopes of Hydrogen increases, [H¹, D², T³] bond vibration decreases (C – H > C –D > C-T)

i.e., vibrational frequency increases, and hence bond dissociation energy increases,

hence the delocalization of α – C – H  σ-bond with benzene π-bond decreases, Therefore Hyperconjugation effect decreases. That is electron donation effect by Hyper Conjugation decreases.

As bond length decreases, electron density over the bond increases, hence electron density around the carbon increases, electronegativity of carbon is greater than that of Hydrogen, electron density at the carbon increases. Hence, electron donating to the benzene ring increases, ie., I.E. strength increases.