Reappraisal of the spin-forbidden unimolecular decay of the methoxy cation†

Abstract

The mechanism of the unimolecular loss of H₂ from triplet methoxy cations (³CH₃O⁺) is revised.

References

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4. The experiments were carried out with a four-sector ZAB-2F-HF/AMD604 mass spectrometer (ref. 5). Methoxy cations were generated by charge reversal (CR) of the corresponding anions [ref. 2(d)]. Hydroxymethyl cations were formed by electron ionization of labelled ethanols. The ions’ connectivities were checked by collisional activation (CA) and CR/CA experiments [refs. 2(e), 3(b)]. The metastable ion decomposition products were monitored in MI and CR/MI experiments by scanning the fourth sector.
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7. The MI mass spectra of [C,H₃,O]⁺ and [C,D₃,O]⁺ were also recorded, and the results are consistent with the present analysis.
8. KIE₁ and KIE₂ are product determining. Intermolecular kinetic isotope effects have not been taken into account, but, of course, would not affect the qualitative conclusions made here.
9. The effect of different internal energies of non-ergodicity on KIE₂ is probably small; an enormous change would anyway be needed to modify the conclusions outlined in Scheme 2.
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14. Calculations at the UMP2 and B3LYP levels of theory were performed with GAUSSIAN94 [ref. 15(a)] and the 6-311+G(d,p)(BSI) basis set. The MECPs were located using our recently described method (ref. 12), and the gradients at these points were inspected to verify that they connect to the relevant minima. The CCSD(T) calculations at B3LYP optimized geometries were performed in MOLPRO [ref. 15(b)] with the cc-pVTZ basis set, omitting the d functions on hydrogen (BSII). At this level, the MECPs were optimized using the hybrid method of ref. 12 with B3LYP/BSI gradients.
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18. The relative slope of the two surfaces at the MECP also affects the surface hopping probability. This factor is very similar at the two MECPs.
19. Spin crossover is also important in transition and chemistry. See, for example S. Shaik, M. Filatov, D. Schröder and H. Schwarz, Chem. Eur. J., 1998, 4, 193.
20. Proton transfer reactions involving spin changes were recently reported: J. Hu, B. T. Hill and R. Squires, J. Am. Chem. Soc., 1997, 119, 11 699; P. A. Janaway, M. Zhing, P. P. Gater, M. L. Chabinye and J. I. Brennan, J. Am. Chem. Soc., 1997, 119, 11 697.