1 H - NMR ( 400 MHz , DMSO - d 6 ) δ : 8 . 19 ( 1 H , s ) , 7 . 75 ( 2 H , d , J = 8 . 8 Hz ) , 7 . 62 ( 2 H , d , J = 8 . 8 Hz ) , 7 . 11 ( 2 H , d , J = 8 . 8 Hz ) , 6 . 81 ( 2 H , d , J = 6 . 4 Hz ) , 3 . 92 ( 2 H , s ) , 3 . 71 ( 3 H , s ) .