Theoretical study of the structure and thermochemical proprieties of adducts of the gas phase reaction of NH2CO[rad] with HCO[rad] possibly formed under atmospheric conditions on the prebiotic Earth

Abstract

In the present work, the thermochemical properties of seventeen adducts, expected for the reaction of NH2CO with HCO, are studied using the CBS-Q energies of the isodesmic/isodesmotic and the atomization reactions as well as the group additivity method. The good agreement between the values of ΔfHo298.15, calculated using the different methods and the quantum mechanical approaches are observed.According to the values of ΔfHo298.15, calculated in the present work, the formation of trans- and cis-diones (trans- and cis-1NH2(O)CC(O)H) is thermochemically more favorable than the formation of carbene esters (1NH2C(O)OCH and 1NH2COC(O)H). Nevertheless, in all those cases, the reactions of formation of these adducts in the ground singlet state are exothermic. The formation of adducts in the triplet state is favorable only in the case of the trans- and cis-diones (trans- and cis-3NH2(O)CC(O)H). The formation of peroxides (NH2COOCH) as products of this reaction is found to be impossible for both the singlet and triplet states.The mean values of ΔfH̅o298.15(trans-1NH2(O)CC(O)H)iso=−293.6±4.3kJ/mol and ΔfH̅o298.15(cis-3NH2(O)CC(O)H)iso=−6.7±3.2kJ/mol of the most thermochemically favorable adducts, respectively, in the lowest singlet and triplet states have been calculated, respectively, using the CBS-Q enthalpies of seven and three isodesmotic reactions. The first of these two values also agrees well with the values of ΔfHo298.15(trans-1NH2(O)CC(O)H)Benson=−299.5kJ/mol and ΔfHo298.15(trans-1NH2(O)CC(O)H)atom=−302kJ/mol, calculated, respectively, using the group additivity method and thermochemistry of the atomization reaction, as well as with tabulated values of ΔfHo298.15 of others trans-diones (H(O)CC(O)H, CH3(O)CC(O)H, CH3(O)CC(O)CH3, HO(O)CC(O)OH, HO(O)CC(O)NH2, NH2(O)CC(O)NH2) and with the energy of dissociation of their (O)CC(O) bond (BDE).The exothermic formation of the (O)CC(O) bond have also been determined for the several trans-diones (H(O)CC(O)H, CH3(O)CC(O)H, CH3(O)CC(O)CH3, HO(O)CC(O)OH) in the lowest triplet state. Moreover, the good linear correlation between the values of BDE((O)CC(O)) and energy of S-T gap of the corresponding trans-diones is observed for all considered trans-diones, excluding only Cl(O)CC(O)Cl. The reactions of the cis-3NH2(O)CC(O)H with the main stable components of the prebiotic atmosphere of Earth, such as CH4 and NH3, are assumed on the basis of their thermochemistry.The values of ΔfHo298.15, calculated in the present work, have also been used for the estimation of the increment values for several groups containing carbene that are not available in the literature.Based on the scan of the potential energy surface, it is concluded that the formation of trans-1NH2(O)CC(O)H is barrierless and the value of rate constant of it formation must be in the range of (3±2)×10−11cm3/(molec. s). Asa result, this reaction pathway should be one of the main channel for the sink of NH2CO radicals under the atmospheric conditions of the prebiotic Earth, and leads to the formation of relatively stable cis- and trans-1NH2(O)CC(O)H.