J . Phys. Chem. 1989, 93, 3635-3639
3635
Reactivity and Collisional Deexcitation of a Metastable State of Mn' Fred Strobel and D. P. Ridge* Department of Chemistry and Biochemistry and Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 1971 6 (Received: September 19, 1988; In Final Form: November 8, 1988)
Evidence is presented that electron impact on Mn2(CO)loproduces a metastable state of Mn+ that is 22.4 & 1.4% of the Mn+ formed. The excited state reacts with IC1 to produce more MnI+ than does the ground state. Charge-exchangereactions indicate the state to be the % state. The lifetime of the state is measured to be greater than 5.8 & 0.7 s. The metastable state of Mn+ is shown to undergo collisional deexcitation with methane at a rate of (1.88 & 0.35) X lo-' molecules-l cm3 s-' to form ground-state Mn+. The mechanism for this process is discussed in terms of an oxidative addition of methane to the metal followed by reductive elimination of excited methane. Simple molecular orbital arguments support that mechanism. A lower limit of 50.5 & 0.1 kcal mol-' is obtained for D(Cr+-CI), D(Mn+-I), and D(Mn+-CI).
Introduction Examination of the chemistry of alkanes with electronically excited atomic transition-metal ions raises a number of interesting Electron impact on Cr(C0)6, for example, has been shown to produce a metastable state of Cr+ which reacts with alkanes.'-, Electron impact on Mn2(CO)loalso forms excited as evidenced by ion beam studies of endothermic reactions with H2, but ion cyclotron resonance (ICR) studies detect no evidence that the excited state reacts with alkanes.' These results could be explained by a Mn+ excited-state lifetime longer than the microsecond time scale of the beam experiment but shorter than the millisecond time scale of the ICR experiment. However, the low-lying states of Mn+ shown in Table I are expected to have very long lifetimes (seconds), since transitions to the ground states are both spin and parity f ~ r b i d d e n . ~ .This ~ suggests that the excited Mn+ fails to react with alkanes for energetic or dynamical reasons. We report here an investigation of the thermal ion-molecule reactions of Mn+ formed by thermal surface ionization and by electron impact on Mn2(CO)lo. The observed chemistry verifies the existence of an excited state in the population of ions formed by electron impact, establishes the state's relative population, and limits the state's energy and lifetime. The interaction of the excited state with alkanes is shown to involve efficient energy transfer which precludes other reactions.
TABLE I: Some Electronic States of Mn+"
state
7s 5s
3d6
5G
3d54s
0.00 1.17 1.78 3.41
TABLE 11: Relative Rate Constants for the Reactions of Metal Ions with IC1
metal ksI(MCI+) ksr(MI+) kEI(MC1+) kBI(MI+) Mn' 1.000 f 0.147 0.256 f 0.030 0.965 f 0.066 0.511 f 0.030 Cr+ 0.688 f 0.046 0.059 f 0.031 0.625 f 0.068 0.288 f 0.038 gas inlets. This minimizes interference from pyrolysis products in the observed chemistry. It also minimizes reactions of ions with gases admitted to the source and ionization of gases admitted to the analyzer. The tandem arrangement thus facilitates unambiguous identification of the ions and neutrals involved in observed reactions. The energy of the ionizing electron beam in the present experiments was 70 eV. A rhenium filament extending through the backplate of the ICR cell provides for thermal surface ionization. Thermal surface ionization results from heating the rhenium filament in the presence of 10-5-104 Torr of Cr(C0)6 and Mn(CH,(C,H,))(CO),. Mn(CH,(C,H,))(CO), was used for the surface ionization because it has a higher vapor pressure than Mn2(CO)lo. It is assumed that surface ionization produces metal ions in a Boltzmann distribution of electronic states at the temperature of the filament. The distribution of states from surface ionization has not been directly measured, but experiments by Armentrout and co-workers'," on a number of systems strongly suggest the assumption of a Boltzmann distribution to be correct. The temperature of the filament was not measured in these experiments but could not be greater than the melting point of rhenium, 3180 K.12 This implies that the ratio of first excited state to ground state population is
