Infrared and NMR spectra, tautomerism, vibrational assignment, normal coordinate analysis, and quantum mechanical calculations of 4-amino-5- pyrimidinecarbonitrile

The infrared (4000-200 cm^-1) spectrum for 4-amino-5- pyrimidinecarbonitrile (APC, C₅H₄N₄) was acquired in the solid phase. In addition, the ¹H and ¹³C NMR spectra of APC were obtained in DMSO-d₆ along with its mass spectrum. Initially, six isomers were hypothesized and then investigated by means of DFT/B3LYP and MP2(full) quantum mechanical calculations using a 6-31G(d) basis set. Moreover, the ¹H and ¹³C NMR chemical shifts were predicted using a GIAO approximation at the 6-311+G(d,p) basis set and the B3LYP method with (and without) solvent effects using PCM method. The correlation coefficients showed good agreement between the experimental/ theoretical chemical shift values of amino tautomers (1 and 2) rather than the eliminated imino tautomers (3-6), in agreement with the current quantum mechanical calculations. Structures 3-6 are less stable than the amino tautomers (1 and 2) by about 5206-8673 cm^-1 (62.3-103.7 kJ/mol). The MP2(full)/6-31G(d) computational results favor the amino structure 1 with a pyramidal NH₂ moiety and calculated real vibrational frequencies, however; structure 2 is considered a transition state owing to the calculated imaginary frequency. It is worth mentioning that, the calculated structural parameters suggest a strong conjugation between the amino nitrogen and pyrimidine ring. Aided by frequency calculations, normal coordinate analysis, force constants and potential energy distributions (PEDs), a complete vibrational assignment for the observed bands is proposed herein. Finally, NH₂ internal rotation barriers for the stable non-planar isomer (1) were carried out using MP2(full)/6-31G(d) optimized structural parameters. Our results are discussed herein and compared to structural parameters for similar molecules whenever appropriate.