VESSALLY, ESMAIL, et al. “Singlet-Triplet Energy Gaps in Divalent C2H2M and C2H6M (M = C, Si, Ge, Sn and Pb)”. Asian Journal of Chemistry, vol. 20, no. 2, Mar. 2010, pp. 1121-9, https://asianpubs.org/index.php/ajchem/article/view/12493.