M. Ibrahim, Mohammad, et al. “Crystal Structure and Density Functional Calculation of (E)-4-Hydroxy-3-Methyl-N’- (thiophen-2-Ylmethylene)-1,4-Dihydroquinoxaline-2-Carbohydrazide Radical”. Asian Journal of Chemistry, vol. 24, no. 7, Mar. 2012, pp. 2926-30, https://asianpubs.org/index.php/ajchem/article/view/9263.