Equilibrium Molecular Structure of 3,3,6-trimethyl-1,5-diazabicyclo[3.1.0]hexane: the joint analysis of the gas-phase electron diffraction data and quantum chemical simulations

The equilibrium structure of the 3,3,6-trimethyl-1,5-diazabicyclo[3.1.0]hexane (TMDABH) molecule was studied for the first time by means of gas-phase electron diffraction (GED) supplemented with quantum chemical calculations. TMDABH exists as a mixture of two conformers of Cs point group symmetry, namely, a chair and a boat. The fractions of these two forms are 74 and 26% respectively, if the reference approximation was obtained at the MP2/aug-cc-PVTZ level, and 81 and 19%, if the reference approximation was generated at the DFT-B3LYP/cc-pVTZ level. The agreement between the theoretical and experimental molecular intensities is characterized by Rf disagreement factors of 5.06 and 6.93%, respectively. Based on the more accurate MP2/aug-cc-PVTZ quantum chemical approximation, the energy difference between the global minimum, which corresponds to the chair conformer of TMDABH, and the local minimum of the boat on the potential energy surface (PES) was found to be 1.39 kcal/mol. Moreover, NMR, IR, and Raman spectroscopic studies were carried out. According to the joint analysis of the data obtained, the most important equilibrium parameters of the chair and boat TMDABH conformers were determined to be as follows (bond lengths in Å, angles in degrees, for boat form in square brackets, Cs symmetry): N1N5 = 1.554(2) [1.520(2)], C2C3 = 1.525(2) [1.539(2)], N1C2C3 = 105.7(14) [108.8(14)], θ = C2-C3-C4/C2-N1-N5-C4 = 37.4(6) [−16.6(6)], φ = N1-C6-N5/C2-N1-N5-C4 = 72.9(3) [73.9(3)]. Comparison of especially NN bond lengths reveals a strong dependence on the bicyclic system conformation.