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 f ulminic acid to acetylene, a prototypical 1,3-dipolar cycloaddition, have b een determined using various molecular orbital and density functional theor y methods (MP2, CCSD(T), G2(MS), G2(CC), CASSCF/CASPT2, and B3LYP) with the aim of obtaining accurate energetics and finding an economical but reliabl e approach for treating larger substituted systems. Although the activation energy is not particularly sensitive to the geometries employed, it is str ongly dependent on the treatment of dynamical electron correlation. The app roximate G2(MS) appears to be an efficient and reliable treatment. Both CCS D(T) and CASPT2 results agree with each other, suggesting that the energy b arrier for the HCNO + HCCH addition amounts to about 14 kcal/mol. The elect ronic 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 saddl e region, but the position of its minimum is somewhat shifted toward the pr oduct side compared to the maximum in energy profile. The variation of the coefficients of the excited configurations in the CAS wave function along t he reaction path suggests that the transition state does correlate with a s ubstantial electron movement from the O to the N of HCNO. The O thus behave s 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 ac companying the nuclear approach of both reaction partners, or a spin-couple d valence bond analysis.