Chemistry of naphthazarin derivatives 13. Conformational analysis of 3-(alk-1-enyl)-2-hydroxy-1,4-naphthoquinones by quantum chemistry methods

The molecular structures of various conformers of 2-hydroxy-1,4-naphthoquinone; 3-(alk-1-enyl)-2-hydroxy-1,4-naphthoquinones; 2,5,8-trihydroxy-1,4-naphthoquinone; and 3-(alk-1-enyl)-2,5,8-trihydroxy-1,4-naphthoquinones were studied by density functional theory (B3LYP/6-31(d), B3LYP/6-31(d, p)) and ab initio (MP2/6-31G, MP2/6-31(d)) methods. The strengths of the intramolecular hydrogen bonds formed by the β-hydroxy group with the O atom at C(1) and with the double bond π-electrons of the alkenyl substituents in the quinonoid rings were estimated. The compounds studied mainly exist as rotamers with the former-type hydrogen bonds. The splitting of the quinonoid bands of the stretching vibrations of the β-hydroxy group in the IR spectra of 3-(alk-1-enyl)-2-hydroxy-1,4-naphthoquinones and 3-(alk-1-enyl)-2,5,8-trihydroxy-1,4-naphthoquinones in hexane solutions is due to the existence of rotamers formed upon internal rotation of the alkenyl substituent. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1667–1673, October, 2006.