The crystal and molecular structure of 2-(2',4'-dinitrobenzyl)pyridine

Abstract

When the pale yellow crystals of 2-(2',4'-dinitrobenzyl)pyridine (DNBP, C6H3(NOz)zCH2CsNH4) are irradiated with light of wavelength 4000 A or less, a deep blue substance is produced by what is believed to be a reversible tautomeric reaction. The haft-life of the excited form is several hours in the crystal but only a few seconds, or much less, in various solvents. Crystals of the isomeric 4-substituted compound do not show this long half-life. DNBP forms monoclinic crystals in space group P2~/c, with a0= 10.276, b0= 15.198, co= 7708/~,//= 101.84 °, and with four molecules in the unit cell. Nearly complete integrated Weissenberg f i l l data were collected with the use of Cu Kct radiation. The structure was solved by direct methods and refined by full-matrix least squares to R= 0.073. Small corrections for molecular libration are included in the final geometry. The intramolecular angle between the planes of the two aromatic rings is 1140. The closest approach of a methylene hydrogen atom to an electronegative atom is intramolecular to an oxygen atom (at 2.4/~) on the o-nitro group. This nitro group is rotated 32 o from the plane of the benzyl ring; the other is twisted 12 o from the benzyl plane. These nitro groups undergo appreciable torsional oscillation about the C-N bonds, with approximate libration amplitudes of 8 ° and 10 °. The shortest intermolecular distance from a methylene hydrogen atom to an electronegative atom is 2.6 A (H. • • O).