&Pyrazincarboxamide crystallizes in the space group P1 with unit-cell dimensions a= 5"728 (2), b---5-221 (3), c=9"945 (6)A, ~=96.81 (5), fl=97"27 (4), y= 106.22 (4) °. The structure was first solved on the basis of a three-dimensional Patterson synthesis and later confirmed by the method of symbolic addition. Refinement was achieved by means of the full-matrix least-squares method, including anisotropic temperature factors and the use of a weighting scheme. The final value of the R1 index is 0"061. The molecules are closely planar and connected by hydrogen bonds across the centres of symmetry to form dimers, which are held together in the lattice by van der Waals forces.
fl-Pyrazincarboxamide crystallizes in the space group P21/c with unit-cell dimensions a= 14.372 (7), b= 3"711 (3), c= 10.726 (5) A. and fl= 101.92 (5) °. The structure was solved on the basis of 813 observed independent reflexions by the aid of the symbolic addition procedure and refined by the full-matrix least-squares method, including anisotropic temperature factors and the use of a weighting scheme, to a final R1 index of 0"055. The molecules have a closely planar pyrazine ring, and the carboxamide group, which is also planar, deviates 3.2 ° from the plane of the ring. The molecules exist as dimers formed by hydrogen bonds between carboxamide groups, the dimers being linked together in chains by other weaker hydrogen bonds between the carboxamide groups.
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