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The molecular and crystal structure of the title compound was determined by the single-crystal X-ray analysis. The crystal is orthorhombic with a space group Pbca, a=15.161 (9), b=25.805 (5), c=9.948 (1) Å, and Z=8. The 4-nitrophenyl ring is nearly on a same plane with the imidazole ring, while the other two benzene rings make angles of 26.5° and 47.7°.

Benzylic acid rearrangements in the solid state were studied and some of them were found to proceed faster than in solution. Although the rearrangement which is initiated by an attack of OH− and then proceeds via radical intermediate was clarified to be similar to that in solution, the effect of alkali metal hydroxide on the rearrangement in the solid state was different from that in solution.

4,5-Bis-(4-methoxyphenyl)-Z-(4-nitrophenyl)imidazolium acetate dihydrate (1 ) in the crystalline state exhibits a sensitive colour change from orange-yellow to red on treatment by various methods (drying, trituration, or heating). The colour change was reversible below 70 "C but above 70 "C the change was irreversible and the colour deepened. Compound (1) was deprotonated with the loss of one mole of water of crystallization under treatment and can revert to the original by absorption of one mole of water. On heating to 100 "C, (1) lost all solvent of crystallization to give a deep red material (2). (2) Was considered to be 4,5-bis-(4-methoxyphenyl) -2-(4-nitropheny1)imidazole which was linked by hydrogen bond from the imino group in one molecule to the nitro oxygen in another. Compound (2) reverted to (1 ) in saturated AcOH-H,O vapour.