Fourier transform ion cyclotron resonance (FTICR) mass spectrometry has been used to examine the reactions of Sc(OCD3)(2)(+) with water, ethanol, and l-propanol. Sigma-bond metathesis resulting in the elimination of CD3OH is the initial reaction observed, with further solvation of the metal center and subsequent elimination of hydrogen occurring as additional reaction channels. These processes are facile at room temperature and involve little or no activation energy. Measured equilibrium constants for the reaction Sc(OCD3)(2)(+) + ROH reversible arrow CD3OScOR+ + CD3OH with R = H, ethyl, and n-propyl are 0.013 +/- 0.004, 0.5 +/- 0.15, and 0.7 +/- 0.2, respectively. For the reaction ROScOCD3+ + ROH reversible arrow Sc(OR)(2)(+) + CD3OH with R = H and ethyl the measured equilibrium constants are 0.013 +/- 0.004 and 0.3 +/- 0.1, respectively. Delta S is estimated for these processes using theoretical calculations and statistical thermodynamics, and in conjunction with the measured equilibrium constants we have evaluated Delta H for these reactions and the relative and absolute bond strengths of the Sct-OR bonds, R = I-I, methyl, ethyl, and n-propyl. The relative bond strengths, D-298(0)(CD3OSc+-OR)-D-298(0)(CD3OSc+-OCD3), for R = H, methyl, ethyl, and n-propyl are + 11.9, 0, - 0.1, and - 1.4 kcal mol(-1), respectively. The absolute bond strengths for HOSc+-OCD3, CD3OSc+-OCD3, CD3OSc+-OC2H5, CD3OSc+-OCH2CH2CH3, and H5C2OSc+-OC2H5 are 115.0, 115.0, 114.9, 113.6, and 114.7 kcal mol(-1), respectively. Theoretical calculations with an LAV3P* ECP basis set at the level of localized second-order Moller-Plesset perturbation theory were performed to evaluate Delta S and Delta G for the specific equilibria Sc(OH)(2)(+) + CD3OH reversible arrow CD3OScOH + H2O, CD3OScOH + CD3OH reversible arrow Sc(OCD3)(2)(+) + H2O, and Sc(OCD3)(2)(+) + C2H5OH reversible arrow CD3OScOC2H5+ + CD3OH. The theoretically determined Delta G values agree reasonably well with the experimentally determined Delta G values. In accordance with earlier theoretical predictions, these metathesis reactions are consistent with an allowed four-center mechanism similar to that of a 2(sigma) + 2(sigma) cycloaddition. (C) 1999 Elsevier Science B.V.
