Gas-phase molecular structure of nicotinamide studied by electron diffraction combined with MP2 calculations

The molecular structure of nicotinamide has been determined at 467 K by gas electron diffraction combined with MP2/ 6-31G** calculations. In the analysis of diffraction data, the potential function for torsion of the pyridine ring was assumed to be V(φ) = V₁(1 - cos φ)/2 + V₂(1 - cos 2φ)/2 + V₄(1 - cos 4φ)/2, where φ denotes the torsional angle around the C_ring-C(=0) bond. The following potential parameters (V/kJ mol^-1) and structural parameters of (E)-nicotinamide (r_g/Å and ∠_α^o) have been obtained: V₁ = 5_-7⁺¹⁰, V₂ = 11.5 (assumed), V₄ = -9(4), 〈r(C_ring-C_ring)〉 = 1.403(4), 〈r(N_ring-C_ring)〉 = 1.328(6), r(C_ring-C(=O)) = 1.498(8), r(C=O) = 1.216(5), r((O=)C-N) = 1.362(12), ∠C_ringN_ringC_ring = 117.5(2), 〈∠C_ringC_ringC_ring〉 = 118.0 (dependent parameter), 〈∠N_ringC_ringC_ring〉 = 124.3 (dependent parameter), ∠C_ringCO = 121.9(6), ∠C_ring- C(=O)N = 115.0 (dependent parameter), ∠C_cisCC(=O) = 120.5(17), where C_cis denotes the ring carbon atom at the cis position against the nitrogen atom of the NH₂ group. Parenthesized numbers denote the estimated limits of error and the angle bracket denotes average values. These structural parameters are close to the corresponding parameters of pyridine and acetamide except for r(N_ring-C_ring) and ∠C_ringN_ringC_ring. The N_ring-C_ring and C=O bonds are shorter than the corresponding parameters in the crystal by 0.01 Å and the (O=)C-N bond is longer than that in the crystal by 0.03 Å. These differences are ascribed to the effect of intermolecular hydrogen bonding on the structure in the crystal.