Crystals of N-(trifluoromethyl)formamide, C2H2F3NO, (I), N-(2,2,2-trifluoroethyl)formamide, C3H4F3NO, (II), and 2,2,2-trifluoroethyl isocyanide, C3H2F3N, (III), were grown in situ on an X-ray diffractometer and analysed by single-crystal X-ray diffraction methods at low temperatures. Crystal data: (I) orthorhombic, P212121, a = 4.547 (2) Å, b = 5.947 (3) Å, c = 14.731 (9) Å, V = 398.3 (4) Å3, Z = 4, M
r
= 113.05, T = 143 K, D
x
= 1.885 Mg m−3; (II) monoclinic, P21/n, a = 4.807 (1) Å, b = 16.707 (3) Å, c = 6.708 (1) Å, β = 109.90 (1)°, V = 506.6 (2) Å3, Z = 4, M
r
= 127.07, T = 141 K, D
x
= 1.666 Mg m−3; (III) orthorhombic, P212121, a = 5.668 (2) Å, b = 9.266 (3) Å, c = 8.626 (2) Å, V = 453.0 (2) Å3, Z = 4, M
r
= 109.06, T = 163 K, D
x
= 1.599 Mg m−3. The results showed that in the crystal both formamides (I) and (II) are exclusively present in the form of the Z isomer, although measurements of solutions of (I) have shown that the E isomer prevails [Lentz et al. (1987). Angew. Chem. 99, 951–953]. In addition ab initio calculations for (I) predicted the E isomer to be the more stable one. In compound (III) the isocyanide group is staggered with respect to the trifluoroethyl group. In the crystal packing of (I) and (II) intermolecular N—H\cdotsO hydrogen bonds generate infinite chains. In (I), these chains are linked to form sheets by C—H\cdotsO contacts. In the crystal structure of (III) each isocyanide dipole is surrounded by four electronegative F atoms with intermolecular C\cdotsF contacts between 3.4 and 3.5 Å.
I. The low temperature photochemistry of iron pentacarbonyl and disubstituted acetylenes, II. The X-ray structure determination of trans-6,8-dibromo-1,2,3,4,4a,9a-hexahydro-4a,9-dimethylcarbazole