Experimental (FT‐IR) and theoretical (DFT) studies on prototropy and H‐bond formation for pyrazine‐2‐amidoxime

Abstract

Abstract The results of the first structural studies (with the use of both experimental and theoretical methods) on pyrazine‐2‐amidoxime ( PAOX ) were shown and discussed. FT‐IR spectra were recorded in different concentrations of the PAOX in apolar solvent to check the possibility of the inter‐ or intramolecular hydrogen‐bond formation. All possible tautomers–rotamers of PAOX were then theoretically considered at the DFT(B3LYP)/6‐311+G** level in vacuo . For selected isomers, calculations were also performed at higher levels of theory {B3LYP/6‐311+G(3df,2p) and G3B3}. Based on the results of DFT calculations, the most stable isomers were found, and their total free energies and infrared spectra were calculated. The energy variation plots for the N8C7N9O10 and N1C2C7N9 dihedral angles were also computed to find two energy barriers, one for E / Z isomerization around the C7N9 double bond and the other one for rotation of the pyrazinyl ring around the C2C7 single bond. The results show that the stability of the PAOX isomers strongly depend on their configuration and orientation of the substituents. The possibilities of inter‐ and intramolecular hydrogen bonds were also experimentally and theoretically checked. Finally, a potential of mean force was determined in CHCl 3 for a dimer of PAOX with hexamethylphosphoramide. Both, experimental and theoretical results are in agreement. Copyright © 2016 John Wiley & Sons, Ltd.