D. L. Eng-Wilmot

Structures of two isomeric hydroxamic acids: N-methyl-p-toluohydroxamic acid (MTH) and N-(4-methylphenyl)acetohydroxamic acid (MPA).

MTH, C9H11NO2, Mr = 165.19, monoclinic, P2(1)/n, a = 7.106 (2), b = 10.211 (3), c = 11.962 (2) A, beta = 97.99 (2) degrees, V = 859.5 A3, Z = 4, Dx = 1.276 Mg m-3, lambda(Mo K alpha) = 0.71069 A, mu = 0.054 mm-1, F(000) = 352, T = 138 K, final R = 0.042 for 1687 reflections with I greater than or equal to 2 sigma(I). MPA, C9H11NO2, Mr = 165.19, orthorhombic, Pbca, a = 9.300 (3), b = 9.463 (4), c = 19.340 (6) A, V = 1702.0 A3, Z = 8, Dx = 1.29 Mg m-3, lambda(Cu K alpha) = 1.5418 A, mu = 0.665 mm-1, F(000) = 704, T = 138 K, final R = 0.044 for 1362 reflections with I greater than or equal to 3 sigma(I). The hydroxamate group in each compound assumes the trans conformation as observed in other secondary hydroxamic acids. The interchange of the C and N substituents influences the planarity and dimensions (C = O, C-N bonds) of the hydroxamate moiety. In MTH the hydroxamate group shows significant deviation from planarity; the r.m.s. deviation of the four atoms, O = C-N-O(H), is 0.094 A, whereas in MPA it is only 0.004 A. This nonplanarity is due largely to out-of-plane bending at N, chi N = 23.9 (1) degree, as compared to chi N = 0.1 degree for MPA. The phenyl ring in MPA is nearly coplanar with the hydroxamate plane (dihedral angle of 10.3 degrees), but is significantly rotated from the hydroxamate plane in MTH (dihedral angle of 43.8 degrees). Electronic differences in the two molecules are considered.