Equilibrium molecular structure of benzamide from gas-phase electron diffraction and theoretical calculations

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The molecule structure of benzamide has been determined in the gas phase by electron diffraction using results from quantum chemical calculations for constraints in the analysis. The root-mean-square amplitudes of vibration and harmonic shrinkage corrections were calculated taking into account nonlinear relations between Cartesian and internal vibrational coordinates at the level of first-order perturbation theory. The corresponding r h1 structure agrees well with the results of B3LYP/cc-pVTZ calculations. To take into account vibrational effects, the corrections to the experimental r a bond lengths, the values of the differences r e − r a were obtained using quadratic and cubic force constants from ab initio calculations at the MP2/cc-pVTZ level of theory). The equilibrium structure from the experiment agrees well with the results of the ab initio calculations. The following structural parameters were obtained (bond lengths in Angstroms and bond angles in degrees with 3σ in parentheses): r(C=C)av = 1.392(1), r(C–C) = 1.502(4), r(C–N) = 1.367(1), r(C=O) = 1.224(3), r(C–H) = 1.084(4), ∠C6C1C2 = 119.8, ∠CCN = 116.8(10), ∠CCO = 120.8(11), ∠C2C1C7O = 18.4(27). The C=O bond is shorter by 0.02 A, and the C–N bond is longer by 0.03 A than the corresponding bonds in the crystal. These differences are ascribed to the effect of intermolecular hydrogen bonding on the structure in the crystal.