This prrper is dedicated to Professor John C. Polnnyi on the occasion of his 65th birthday DALE L. HERBERTSON, DAVID A. NEWNHAM, and MARTIN R. LEVY. Can. J. Chem. 72, 850 (1994). A laser-ablated pulsed beam of Mn atoms, in various long-lived states, has been employed to determine excitation functions for the reactions Mn + SnCI4 + MnCI* ("blue," "green," "red") + SnC13. The data all show a sharp rise at low energies, followed by an equally rapid fall, but they differ in the initial thresholds and the rate at which u(ET) approaches zero. Analysis in terms of a multiple line-of-centres model (Levy, Res. Chem. Kinet. 1, 163 (1993)) indicates that each excitation function is unique, and is due to reaction of only one Mn reagent state -most probably the first metastable state, n6DJ. This suggests a strong symmetry control in the entrance channel of the reaction, at least; that is, the Mn atom approaches along the C3 axis of SnCI4, so that the symmetry corresponding to the spatial configuration of the odd d electron is retained. In two cases, "blue" and "red," if not all, the luminescing MnC1" state seems to be produced on a surface that does not directly correlate with the reagents. As collision energy increases in each case, the reaction transition state seems to shift forward into the entrance valley, a result attributed to restricted access to the exit valley. DALE L. HERBERTSON, DAVID A. NEWNHAM et MARTIN R. LEVY. Can. J. Chem. 72,850 (1994). On a utilist un faisceau pulst d'atomes de Mn, obtenus par ablation au laser et prCsents dans divers ttats de longue durte, pour dtterminer les fonctions d'excitation de la rCaction Mn + SnC14 + MnCl* (<
Chemiluminescence in four different MnCl band systems e5Σ+ → a5Σ+ (“blue”), d5Π → a5Σ+ (“green”), c5Σ+ → a5Σ+ (“red”), and b5Π → a5Σ+ (“infrared”) has been observed from the interaction of a laser-ablated beam of Mn atoms with gaseous SiCl4. The initial translational thresholds range from ∼150 to ∼300 kJ mol-1, increasing in the order e5Σ+ < c5Σ+ ∼ d5Π < b5Π. Analysis of the measured excitation functions, in terms of a multiple line-of-centers model [Levy, M. R. Res. Chem. Kinetics 1993, 1, 163], shows that, with the exception of a high-energy c5Σ+ channel contribution to d5Π production and possibly a joint c5Σ+/d5Π production process at low energies, all the reactions take place on distinct potential surfaces. Although, on energetic grounds, the b5Π and c5Σ+ channels could derive from reaction of ground-state Mn(a6S) atoms, the first excited state, a6DJ, is the most likely reagent species in all cases, indicating substantial excess barriers. In contrast to the corresponding SnCl4 reactions, only the c5Σ+ excitation function reveals a shift forward in transition-state location with increasing collision energy. The results have been rationalized in terms of a hierarchy of ionic−covalent curve crossings at short internuclear distances.
Five different MnF* emitting statesA7Π, e5Σ+, d5Π, c5Σ+, and b5Πhave been observed from collisions between SF6 molecules and a pulsed, laser-ablated beam of Mn atoms. The various excitation functions, measured up to nominal collision energy E T 0 = 2000 kJ mol-1, have been modeled by the multiple line-of-centers approach. The analysis indicates that MnF*(A7Σ+) is predominantly formed on a single potential surface, with probable progenitor Mn*(z8PJ); the e5Σ+, d5Π, and c5Σ+ states are formed via three common potential surfaces, with likely reagent state Mn*(a6DJ), while the a6D J state is also likely to be the progenitor of MnF*(b5Π). Partial depletion of the b5Π state at high energy seems to be accompanied by enhanced yield of the other emitters. All reactions seem to be characterized by a shift forward in transition state location with increasing collision energy. The results suggest an electron jump barrier at short internuclear distances, forced outward at higher collision energies due to lack of time for the SF6 to distort to the equilibrium geometry of SF6 -.
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