Quantum chemical characterization of the $${\tilde{X}}(^{1} A_{1} )$$ X ~ ( 1 A 1 ) , $${\tilde{a}}(^{3} B_{1} )$$ a ~ ( 3 B 1 ) , $${\tilde{A}}(^{1} B_{1} )$$ A ~ ( 1 B 1 ) and $${\tilde{B}}(2{}^{1}A_{1} )$$ B ~ ( 2 1 A 1 ) states of diiodomethylene and the enthalpies of formation of diiodomethylene, iodomethylene and iodomethylidyne