Table 1. IR shifts of C O and N–H stretching modes, melting points, and orthogonal angle α (∠X···O···N, with X = Br, I) of 1 and co-crystals 3a–f .

	ν C O (cm–1)	ν N–H (cm–1)	m.p. (°C)	α (∠X···O···N) (°)
1	1634	3290	12–20	—
3a	1635	3292	18	98.5(3)
3b	1629	3289	35	91.9(7)
3c	1625	3291	75	81.2(9)
3d	1616	3281	67	82.9(3); 89.0(2) a
3e	1617	3282	76	81.9(4)
3f	1607	3300	87	77.9(5)

^a Two symmetry-independent molecules.