Multiple potential surfaces and movable energy barriers in the chemiluminescent reaction Mn + SnCl₄

Abstract

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* (<<bleu)), <<verb ou ccrougen) + SnCI3. Les donntes indiquent toutes une augmentation importante des basses Cnergies suivie par une baisse aussi rapide; elles diffkrent toutefois par leurs seuils initiaux et par la vitesse B laquelle u(ET) approche de zkro. Une analyse en fonction du modkle i plusieurs raies centrales (Levy, Res. Chem. Kinet. 1, 163 (1993)) indique que chaque fonction d'excitation est unique et qu'elle est causte par la reaction d'un seul ttat rtactif du Mn, probablement le premier ttat mktastable, a6DJ. Ceci suggkre la prtsence d'un cohtr6le important par la symttrie dans le canal d'entrte de le rtaction, au moins que l'atome de Mn approche par l'axe C3 du SnC14, de faqon B ce que la symttrie correspondant B la configuration spaciale de I'tlectron d ctlibataire est maintenue. Dans au moins deux cas, <<bleu* et <<rouge>, I'ttat MnCl* luminescent semble &tre produit sur une surface qui n'est pas en corrtlation directe avec les rtactifs. Lorsque l'tnergie de collision augmente dans chaque cas, I'ttat de transition de la rtaction semble se dtplacer vers I'nvant, dans la vallte d'entrte; ce rtsultat est attribut B un accks limit6 i la vallte de sortie.[Traduit par la rtdaction]

Chemiluminescence in the Reaction of Mn Atoms with CF4

“…Indeed, with its low depletion threshold and high depletion probability, this initial feature is more akin to those Mn-halide excitation functions that show a transition state shift than to those that do not. [9][10][11] The threshold at ∼670 kJ mol -1 suggests the opening up of a new reaction channel, perhaps involving a different reagent state, as does the threshold at ∼1811 kJ mol -1 . However, as pointed out earlier, the σ 3 term could be an artifact due to the rising edge of the beam pulse.…”

“…This behavior has been explained 10,13 in terms of an unseen forward shift in the reaction transition state, from R 0 to a new value R s , at energy E s followed at higher energies by depletion at internuclear distance R d , which here is of magnitude comparable to R s . The forward shift is only revealed once its line-of-centers functionality outstrips that of the initial process, i.e., at higher energies still, when the slope of the yield plot changes from negative to near zero.…”

“…We have rationalized such "simple fall-off" in terms of competition between (a) reaction, with probability P 0 at internuclear distance R 0 and line-of-centers will clearly show an initial linear rise, then a reduced slopesperhaps even zero slopesfrom E 1 . Comparison of the slopes of the two linear regions yields from which a lower limit to either P d or R d /R 0 is obtained, [9][10][11]13 since P d e 1.0 and R d is not expected to be substantially larger than R 0 .…”

Rotational Alignment in MnO*(A6Σ+) from the Reaction Mn + O2

“…In previous studies, [13][14][15][16][17][18][19] two main types of depletion behaviorsso-called "simple" and "complex"shave been observed. The former is characterized most straightforwardly by a two-term expansion of eq 5 or 6, in which σ 1 ∼ -σ 0 .…”

“…32,33 Unlike the present case, all of the latter are extremely endothermic covalent reactions. On the other hand, a 6 D J atoms have been clearly implicated in Mn + SnCl 4 , SiCl 4 , SF 6 , and CF 4 [13][14][15][16] with a probable contribution from z 8 P J atoms to the MnF*(A 7 Π) channel in the SF 6 case.…”