Ab initio GB study of prebiotic synthesis of purine precursors from aqueous hydrogen cyanide: dimerization reaction of HCN in aqueous solution

Abstract

Ab initio MO GB theory which includes the continuum model of solvent effect using generalized Born formula has been applied to the dimerization reaction of HCN in aqueous solution which is the starting step in prebiotic synthesis of purine precursors from aqueous hydrogen cyanide. Three steps considered were: (i) the reaction of HCN and H 2O to produce the CN − anion, (ii) the reaction of CN − with HCN to give the NC–CHN − anion, and (iii) the addition of a proton to the anion to give iminoacetonitrile. The formation of CN − ion from HCN in aqueous solution requires 15.1 kcal/mol (the experimental value estimated from the dissociation constant of HCN in water is 14.8 kcal/mol). The reaction of CN − with HCN requires the activation energy of 32.2 kcal/mol (MP2/6-31++G ∗∗//HF/6-31++G ∗∗) to give the dimer. This barrier height is reduced to 26.1 kcal/mol when HCN is associated with H 3O +. In the presence of NH 3 in aqueous solution, CN − is produced easily by the reaction of HCN and NH 3 with a low activation energy of 4.3 kcal/mol. It was shown that the formation of CN − becomes easier in ammoniacal solution, and the dimerization occurs efficiently in aqueous solutions which contain NH 3.