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The title compound, C16H9NO4, also known as the 3-benzoylpyridinium betaine of squaric acid, exhibits a dipolar electronic ground-state structure with a positively charged pyridinium fragment and a negatively charged squarate moiety. In the molecule, the two aromatic rings are twisted by 56.03 (2)° relative to one another. The three-dimensional packing of the molecules is stabilized by C—H··O· short contacts. [ABSTRACT FROM AUTHOR]

The mol­ecule of the title compound, C21H22N2O2, is nearly planar, except for the C(CH3)2 group on the cyclo­hexene ring. Molecules are connected via electrostatic C⋯O contacts, forming zigzag pseudo-chains along the c axis. [ABSTRACT FROM AUTHOR]

In the title compound, 3,4-dioxo-2-(pyridinium-1-yl)cyclobut-1-enolate, C9H5NO3, molecules are connected three-dimensionally through nonclassical C—H...O and π–π interactions [3.220 (3) Å] between the oppositely charged squarate and pyridinium fragments. Classical hydrogen-bonding interactions are not observed. In the unit cell, only half an independent molecule is present and a twofold rotation axis passes through the pyridinium ring and the opposite CO group. [ABSTRACT FROM AUTHOR]

Cations of the title compound, C5H11N2O+·C4HO4−, are connected by inter­molecular N(amide)— H⋯O(amide) hydrogen bonds into helical chains, whereas anions are inter­linked by strong O(hydrox­yl)—H⋯O hydrogen bonds into chains. These two distinct chains are interconnected by both N(amide)—H⋯O and N(ammonium)—H⋯O hydrogen bonds into a three-dimensional framework. One of the ammonium protons is involved in a three-centre hydrogen bond. [ABSTRACT FROM AUTHOR]