Water-assisted isomerization of the [H, C, N, O] system.

Ten minima of [H, C, N, O] isomers were investigated in gas phase and water solution using the polarizable continuum model at the CCSD(T)//M06-2X/6-311 + G(3df,2p) level of theory. The results show that the stability order of all isomers in water solution is HNCO > HOCN > HCNO > HONC > Y-OC(H)N > cycl-OCN(H)-a ≈ cycl-OCN(H)-b > cycl-NCO(H) > HNOC > HCON, i.e., the same as that in the gas phase. Potential energy surfaces of [H, C, N, O] system isomerization were constructed in gas phase and in water solution, showing that the isomerization of [H, C, N, O] isomers in gas phase is unfavorable because of the high barrier height. Interestingly, although the water solvent has a little impact on the isomeric mechanism, water molecules (H2O)n(n = 1-3) acting as catalyst dramatically lower the barrier height in the hydrogen transfer processes (HCNO → HONC, HNCO → HOCN, and HCON → HNOC). Water is the most abundant compound in the interstellar area. These results give new insight into the mechanism of [H, C, N, O] system isomerization in interstellar gas. Enthalpies of formation of the isomers were predicted at the CBS-QB3, G4MP2 and W1U levels.