IONIZATION-ENERGY OF TERT-BUTYL-D(9) ALCOHOL AND THE APPEARANCE ENERGY OF PROTONATED ACETONE - A NONEQUILIBRIUM DISSOCIATION

In the present photoionization study of tert-butyl alcohol, the parent ion of the deuterated isotopomer [(CD3)(3)-COH] was observed and an i onization energy (IE) of 9.82 +/- 0.02 eV was derived from a step-func tion threshold at 126.25 nm. The appearance energies (AE's) of (CD3)(2 )COH+ and (CH3)(2)COH+ were also determined (at 298 K) to be 9.84 +/- 0.02 and 9.86 +/- 0.02 eV, respectively. From these results, the IE of undeuterated tert-butyl alcohol (which was not observed) was estimate d to be 9.84 +/- 0.03 eV. The difference between the IE and the derive d AE(0) (AE = 10.01 +/- 0.03 eV at 0 K) implies an upper limit of 0.17 +/- 0.04 eV for the barrier to dissociation of (CH3)(3)COH+. A recent evaluation of the proton affinity of acetone was employed to derive a value of 5.40 +/- 0.10 eV for Delta(f)H degrees(0)[(CH3)(2)COH+] and, thence, the enthalpy of reaction, Delta(f)H degrees(0) = 9.87 +/- 0.1 1 eV; and from this, an upper limit of 0.14 +/- 0.11 eV for the revers e barrier of the dissociation was obtained. RRKM calculations were per formed by using the energies derived from the experimental (and estima ted) values. For excitation energies greater than or equal to 0.2 eV a bove threshold, RRKM calculations yield large rate coefficients for di ssociation. Also, the kinetic energy release (KER) was calculated for various models of the dissociation complex. The calculated KER's are c onsistent with experimental values when it is assumed that only 5 of t he 38 vibrational modes of the transition state are active in the rand omization of internal energy.