Effect of methyl substitution on the crystal structures of complexes of quinones with hydroquinones. Crystal structures of the 1 : 1 complex of 2,3,5,6-tetramethyl-1,4-benzoquinone with 2,3,5,6-tetramethylbenzohydroquinone and of the 1 : 2 complex of 2,3,5,6-tetramethylbenzoquinone with hydroquinone. Correlation of Fourier transform infrared spectra with structure