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Another Look at the Mechanism of the Concerted 1,3-Dipolar Cycloaddition of Fulminic Acid to Acetylene.

The transition structure and energy barrier for the concerted addition of fulminic acid to acetylene, a prototypical 1,3-dipolar cycloaddition, have been determined using various molecular orbital and density functional theory methods (MP2, CCSD(T), G2(MS), G2(CC), CASSCF/CASPT2, and B3LYP) with the aim of obtaining accurate energetics and finding an economical but reliable approach for treating larger substituted systems. Although the activation energy is not particularly sensitive to the geometries employed, it is strongly dependent on the treatment of dynamical electron correlation. The approximate G2(MS) appears to be an efficient and reliable treatment. Both CCSD(T) and CASPT2 results agree with each other, suggesting that the energy barrier for the HCNO + HCCH addition amounts to about 14 kcal/mol. The electronic mechanism of the cycloaddition has also been probed further using DFT descriptors, as well as an analysis of the CAS-LMO-CI wave functions. The hardness profile along the minimum energy path shows a minimum in the saddle region, but the position of its minimum is somewhat shifted toward the product side compared to the maximum in energy profile. The variation of the coefficients of the excited configurations in the CAS wave function along the reaction path suggests that the transition state does correlate with a substantial electron movement from the O to the N of HCNO. The O thus behaves as a new bond acceptor center and the C as a new bond donor, in contrast with a picture previously derived from either the net charges distribution, or the motion of the centroids of Hartree-Fock based localized orbitals accompanying the nuclear approach of both reaction partners, or a spin-coupled valence bond analysis.