Computational study on structures, isomerization and dissociation of [Si, N, C, O]⁺ isomers

A detailed singlet potential energy surface (PES) of the [Si, N, C, O]⁺ system including 13 isomers and 15 transition states is investigated by means of the MP2 and QCISD(T) (single-point) methods. At the final QCISD(T)/6-311 + G(3df)//MP2/6311G(d) level with zero-point energy inclusion, SiNCO⁺ 1 is found to be thermodynamically the most stable isomer followed by SiOCN⁺ 2, OSiCN⁺ 3, OSiNC⁺ 4 and SiCNO⁺ 5 at 40.90, 70.00, 70.43 and 87.72 kcal mol^-1, respectively. For isomer 1, the isomerization to the species, 2, 3 and 4 can very favorably compete with its direct dissociation, and thus, together with the structural and bonding analysis, the very recent mass spectrometric experiment can reasonably be interpreted. Furthermore, our results indicate that while 1 is kinetically the most stable, the species, 3, 4 and 5 are also shown to reside in very deep potential wells. Therefore, in addition to the mass spectroscopically characterized isomers 1 and 3, the species 4 and 5 should also be experimentally observable. However, observation of the second low-lying isomer 2 seems less likely due to its easy conversion to isomer 1 via a four-membered ring Si-shift process.