Raman, infrared and NMR spectra, vibrational assignments and quantum mechanical calculations of centrosymmetric 3,6-Dicholoro-1,2,4,5-tetrazine

The Raman spectrum (2000−100 cm⁻¹) of 3,6-Dicholoro-1,2,4,5-tetrazine (DCTZ, C₂N₄Cl₂) in solid phase has been recorded. Moreover, the mid-IR (2000–400 cm⁻¹) and ATR-IR (2000-480 cm⁻¹), ¹³C (0–200 ppm) and ¹⁵N (0–440 ppm) NMR spectra were recorded. Initially, optimization followed by frequency calculations were carried out for DCTZ restricted to C_i and D_2h point groups to work out the least energy conformer with real frequencies. DFT(B3LYP) and MP2(full) computations utilizing 6-31G(d) and 6–311++G(d,p) basis sets favor C_i symmetry conformer by 781/598 cm⁻¹ (9.3/7.2 kjmol⁻¹) and 745/698 cm⁻¹ (8.9/8.3 kjmol⁻¹). The simulated spectra are well compiled to the experimentally recorded IR, Raman, ¹³C and ¹⁵N regardless of few coincident vibrational frequencies that belong to D₂h symmetry conformer. Thus, we have carried out additional CASTEP solid state optimization for one molecule per unit cell (C_i symmetry) employing Materials Studio using DFT-PBE method. Similar calculations were completed for D₂h molecular symmetry for one molecule and repeated molecules per unit cell in which C_i symmetry is favored by 341 cm⁻¹ (4.1 kjmol⁻¹). It is worth mentioning that computations for both C_i and D_2h conformers produce real vibrational frequencies. Assisted by normal coordinate analysis, potential energy distributions (PEDs), complete and confident vibrational assignments are provided herein for C_i conformer. The results are conferred herein are compared with comparable molecules whenever applicable.