Far-UV laser flash photolysis in solution. A time-resolved spectroscopic study of the chemistry of 1,1-dimethyl-1,3-(1-sila)-butadiene

摘要

Laser flash photolysis (193 nm) of 1,1-dimethyl-(1-sila)cyclobut-2-ene in hexane solution leads to the formation of a transient species (tau similar to 1.5 mu s at 23 degrees C) assigned to 1,1-dimethyl-1,3-(1-sila)butadiene on the basis of its UV absorption spectrum (lambda(max) = 312 nm), and reactivity toward methanol (k(MeOH) = (3.6 +/- 0.1) x 10(9) M-1 s(-1); k(H)/k(D) = 1.3 +/- 0.1), ethanol (k(EtOH) = (2.41 +/- 0.06) x 10(9) M-1 s(-1)), tert-butanol (k(BuOH) = (1.8 +/- 0.1) x 10(9) M-1 s(-1); k(H)/k(D) = 1.5 +/- 0.1), and oxygen (k(O2) = (3.0 +/- 0.9) x 10(8) M-1 s(-1)). Experiments using isooctane and acetonitrile as solvent are also described. In acetonitrile, the Lifetime of the silene is shortened considerably compared to hydrocarbon solvents, presumably due to quenching by adventitious water. In isooctane, under conditions of low excitation intensity, the siladiene decays with clean pseudo-first-order kinetics and a maximum lifetime of similar to 5 mu s at 23 degrees C. The decay rate constant varies only slightly with temperature over the 20-60 degrees C range, leading to Arrhenius activation parameters of E-a = 0.5 +/- 0.2 kcal/mol and log A = 5.7 +/- 0.2. While steady state irradiation experiments suggest that in the absence of silene traps the predominant fate of the silabutadiene is thermal ring closure to regenerate the precursor, it is concluded that the rate constants and activation parameters for decay of the siladiene measured by flash photolysis represent a composite of those due to thermal electrocyclic ring closure (with E-a > similar to 3 kcal/mol) and reaction with adventitious quenchers (probably water, with E-a < 0). The measured Arrhenius parameters for reaction of the siladiene with methanol in isooctane (E-a = -2.6 +/- 0.3 kcal/mol and log A = 7.6 +/- 0.3) are consistent with this proposal. The potential and limitations of the use of 193-nm laser excitation for flash photolysis studies in solution are discussed.