Structural dimorphism of N-(3-hydroxysalicylidene)-2,4,6-trimethylaniline

The title compound N-(3-hydroxysalicylidene)-2,4,6-trimethylaniline exhibit s dimorphism. The structure of (I) is orthorhombic Pbca with a = 22.222(3), b = 16.200(2), c = 7.596(2) Angstrom, V = 2735(1) Angstrom(3) and Z = 8. T he structure of (II) is monoclinic P2(1)/c with a = 11.734(1), b = 15.734(1 ), c = 7.432(2) Angstrom, beta = 100.97(1)degrees, V = 1347.0(5)Angstrom(3) , and Z = 4. The two structures are analyzed in relation with the possibili ty of an equilibrium between two tautomeric phenol and quinone forms by rap id hydrogen transfer. The molecules associated by intermolecular hydrogen b onds form centrosymmetric dimers. The compounds are thermochromic. The resu lts are compared with the photochromic analog N-(hydroxysalicylidene)-2,4,6 -trimethylaniline (compound III) and with the thermochromic one N-(5-hydrox ysalicylidene)-2,4,6-trimethylaniline (compound IV). It is concluded that t he 3-hydroxyl substituent of the salicylidene moiety enables the intermolec ular hydrogen transfer from oxygen to nitrogen.