The molecular structures, conformations and force fields of bis(chloroimino)butanedinitrile as studied by gas-phase electron diffraction and quantum chemical calculations

Quantum chemical calculations using levels up to MP2(Full)/aug-cc-pVTZ have been applied. B3LYP calculations using the 6-31G* basis set reveal that there are four conformations of bis(chloroimino)butanedinitrile. The planar anti-ZZ conformer with C2h symmetry is the most stable conformer. The non-planar EE conformer with C2 symmetry, the non-planar EZ conformer with C1 symmetry and the non-planar ZZ conformer with C2 symmetry are 16.8, 22.7, 27.2 kJ/mol, respectively, less stable than the planar anti-ZZ conformer according toB3LYP/6-31G* calculations. Calculated frequencies for the planar anti-ZZ conformer have been compared with observed frequencies, and some reassignments have been proposed. l models have been used in the gas-phase electron diffraction analysis. The most reliable results are expected to be obtained using a dynamic model where the large amplitude motion is simulated by a harmonic angular motion using a Gaussian distribution about the central CC bond. Only the planar anti-ZZ conformer was used in the final refinements due to the high energy difference to the other conformers. The most important bond distances (ra, Ångstrom) and bond angles (∠α, degrees) are [GED/MP2(Full)/aug-cc-pVTZ]: rC1C2 = [1.509(15), 1.460], rC2 = N3 = [1.295(6), 1.292], rN3Cl5 = [1.706(5), 1.696], rC2C7 = [1.434(11), 1.421], rC7N9 = [1.165(4), 1.170], ∠C1C2N3 = [114.5(11), 115.6], ∠C2N3Cl5 = [115.0(4), 115.0], ∠C1C2C7 = [118.8(8), 118.5], ∠C2C7N9 = [178.2(15), 177.4]. The dihedral angle N3C2C7N9 is 0°, i.e. the cyano groups are bended towards the Cl atom. Error estimates from electron diffraction are given as: σr = 2.5[σlsq2 + (0.001r)2]½ for bond distances and σ∠ = 2.5σlsq for bond angles.