Methyl group conformation-determining intermolecular C–HO hydrogen bonds: structure of N-methyl-2-pyrrolidone

The structure of N-methyl-2-pyrrolidone (NMP; 1-methyl-2-pyrrolidinone) was determined at 168 K. Crystal data: m.p. 249 K, monoclinic P2₁/c, a = 6.221 (3), b = 12.076 (2), c = 7.529 (4) Å, β = 111.03 (2)°, V = 527.9 (4) ų, Z = 4, D_x = 1.247 Mg m⁻³, μ = 0.082 mm⁻¹, R = 0.058 for 1190 unique reflections. It adopts a slightly puckered ring conformation in the solid state, which is intermediate between a twist and an envelope conformation. The N-methyl group takes part in two almost linear intermolecular C—HO hydrogen bonds to neighbouring keto groups [d(HO) = 2.57 (2)/ 2.65 (2) Å, d(CO) = 3.552 (2)/3.482 (2) Å, angle (C—HO) = 160 (1)/148 (2)°], which cause a notable deviation from its usually preferred conformation with one C—H bond being almost eclipsed to the adjacent N—C amide bond. These findings are substantiated by force-field and high-level ab initio calculations on isolated NMP molecules.