why 2,6-Di nitro-N,N-dimethyl anilineis more basic than 3,5-dimethyl-N,N-dimethylaniline

In the 3,5-dimethyl isomer, steric effects come into play and influence the acidity.Resonance structures like B in the figure below, that stabilize the phenoxide anion explain why 4-nitrophenol (R=H) is more acidic than phenol. Resonance structure B requires that the nitro group be planar with the aromatic ring. If methyl groups are placed adjacent to the nitro group (R=CH3), the steric interactions between the two methyl groups and the nitro group becomes large and forces the nitro group to rotate out of the aromatic plane, Hence resonance structure B can no longer contribute and the compound is less acidic.