Elusive Forms and Structures of <i>N</i>-Hydroxyphthalimide:  The Colorless and Yellow Crystal Forms of <i>N</i>-Hydroxyphthalimide

A comprehensive structural characterization of the colorless and yellow forms of <i>N</i>-hydroxyphthalimide (NHP), the deuterated form (NDP), and the ethoxylated form (ethoxy-NHP) has been carried out using single-crystal X-ray diffraction, FTIR and Raman spectroscopies, and scanning electron microscopy. Both NHP and NDP forms crystallize in the monoclinic space group (<i>P</i>2<sub>1</sub>/<i>c</i>, No. 14). The various forms of NHP differ in the way in which the molecules adjoin one another through their <i>N</i>-hydroxyl groups and how the carbonyls of the isoindole-1,3-dione ring differ through intermolecular hydrogen bonding. Although the hydrogen bonding about the <i>b</i> axis is virtually the same, the isoindole-1,3-dione ring experiences different twists for the two NHP forms. Both the colorless and yellow forms of NHP exhibit strong intermolecular hydrogen bonding between O(3) and H(1). In the yellow form, the <i>N</i>-hydroxyl group is significantly out of the plane (∼1.19°), but the <i>N</i>-hydroxyl group in the colorless form is only ∼0.06° out of the plane. Both forms of NHP reveal an infinite chain of intermolecular hydrogen-bonded molecules in the direction of the <i>b</i> axis; however, the molecules are ordered differently within the unit cells. The hydrogen-bond geometry for the yellow form of NHP is O(2)−H(1)···O(3), with an angle of 185°, intermolecular distances of O(2)···O(3) = 2.68 Å and H(1)···O(3) = 1.70 Å, and an intramolecular hydrogen bond of O(1)···H(1) = 1.17 Å. The colorless form of NHP shows an intermolecular hydrogen-bond geometry between O(3) and H(1) with a distance of 1.78 Å; the O(2)−O(3) distance is 2.71 Å. The O(2)−H(1)···O(3) angle is 159°, and the intramolecular distance is O(1)···H(1) = 0.97 Å. The <i>N</i>-ethoxy derivative of NHP crystallizes in an orthorhombic space group (<i>Pnma</i>, No. 62) and exhibits no hydrogen bonding, displaying a strong head-to-tail stacking of the planar rings along the needle axis direction.