The (2:1) complex of picric acid with tetramethylpyrazine: The structure, IR spectra and tunnel splitting of methyl groups

The crystal structure of 2:1 picric acid (PAH) with tetramethylpyrazine (TMP) was determined by using X-ray diffraction studies. Two equivalent +NH⋯O− hydrogen bonds are formed with the length of 2.601 Å (calculated 2.640 Å). In the IR spectra very broad doublet at ca. 2000 and 2400 cm−1 is observed, which can be interpreted as due to symmetric and asymmetric +NH vibrations. In neutron backscattering two tunnel splittings are observed, in agreement with the symmetry of 2:1 assemblies. At 4 K the tunnel peaks are located at 3.17 and 4.24 μeV.