Predictive Quantum Chemistry: A Step Toward 'Chemistry Without Test Tubes'

摘要

The merits of the claims made in two recent papers entitled "First generation of pentazole (HN_5, pentazolic acid), the final azole, and a zinc pentazolate salt in solution: A new N-dearylation of l-(p-methoxyphenyl) pyrazoles, a 2-( p-methoxyphenyl) tetrazole and application of the methodologytol-( p-methoxyphenyl) pentazole" (R. N. Butler, J. C. Stephan and L. A.Burke, J. Chem. Commun. 2003, 1016-1017) and "First generation of the pentazolate anion is solution is far from over" (T. Schroer, R. Haiges, S. Schneider and K. O. Christe, Chem. Commun. 2005,1607-1609) are verified by predictive quality theoretical methods. Knowing whether the CF_3OH in HF solution undergoes protonation to form CF_3[OH_2]~+ is critical to the success of the recently proposed synthetic route to form the prototype perfluorinated alcohol, CF_3OH. Chirstie and co-workers first considered the ~(13)C and ~(19)F shielding constants to distinguish CF_3OH and CF_3[OH_2]~+, but it turns out that they both have similar chemical shifts. Furthermore, they noted that the computed ~(13)C chemical shifts differ by 11 ppm from the measured ones and claimed that "These findings presented a dilemma because1 either experimental or the calculated shifts has to be seriously flawed and, therefore chemical shifts alone it was impossible to decide whether CF_3OH in liquid HF is protonated or not". Instead of chemical shifts, they propose to use ~(13)C-~(19)F MMR spin-spin coupling constants and argue that the observed 20 Hz difference of ~1J(~(13)C-~(19)F) to the increase in the covalent character upon protonation. The reported discrepancy in computed and measured chemical shifts is reexamined and the spin-spin coupling constants results are verified by the predicative-level calculations.