Equilibrium molecular structure and spectra of 6-methyl-1,5-diazabicyclo[3.1.0]hexane: joint analysis of gas phase electron diffraction, quantum chemistry, and spectroscopic data

摘要

The equilibrium geometry of the boat conformation (C-s point group symmetry) of the 6-methyl-1,5-diazabicyclo[3.1.0]hexane (MDABH) molecule, absolutely dominating under normal conditions, was studied by the gas-phase electron diffraction (GED) method at 20 degrees C with the involvement of NMR, IR, and Raman spectroscopic data and quantum chemical calculations. The potential function of ring-puckering deformation for the MDABH bicyclic system was calculated at the MP2/aug-cc-pVTZ and B3LYP/cc-pVTZ levels. It was found by MP2 calculation that the total energy of the boat conformation is 3.52 kcal mol(-1) lower than that of the chair conformation. For the first time, we recorded the IR and Raman spectra for liquid samples of MDABH and assigned their peculiarities only to boat conformation vibrations using the Pulay technique of scaling quantum chemical force fields. In the case of the chair form, transferability of the refined scale factors was used for reliable prediction of the location of its fundamental frequencies. According to the joint structural analysis of the above data, the most important equilibrium geometric r(e)-parameters for the boat conformation of the MDABH molecule were determined to be (bond lengths in angstrom; angles in degrees, C-s symmetry): C2N1 = 1.466(2), C2C3 = 1.523(2), N1N5 = 1.512(2), C6N1 = 1.440(2), C6C7 = 1.487(2), angle C2N1N5 = 106.1(2), angle N1C2C3) = 110.2(4), angle C2C3C4 = 99.9(4), angle N1N5C6 = 58.3(1), angle N1C6N5 = 63.3(1), angle N1C6C7 = 114.9(6), angle C6N1C2 = 111.8(1), angle N5N1C2C3 = 17.3(1), angle N1C2C3C4 = -26.8(2), theta = C2C3C4/C2N1N5C4 = -26.2(3), phi = N1C6N5/C2N1N5C4 = 74.0(1). Comparison of these and earlier results showed that the NN bond length in the diaziridine ring is very weakly dependent on the cis- or trans-arrangement of substituents at the nitrogen atoms.