Non‐Ergodic Behavior of Excited Radical Cations in the Gas Phase

Abstract

poster at the Symposium des Arbeitskreises (' : *) proportions. Furthermore, the kinetic energy KSAM der GDCh-Fachgruppe ,,Analytische Chemie" in Martinsried (Germany), March 1981.[2] Syntex P2, four-circle diffractometer with modified LT-I cooling unit, Mo,,,, 20,,,, =60", 762 reflections, of which 722 observed (Fo>3uF), program system EXTL, R , =0.026.[3] The crystal structure of [(HOC2H,),NH]+SH-(unpublished) shows the release, T, for the I3CH3-and "CH3-eliminations, would be expected to be closely similar. If, however, the internal energy of (2,* is distributed non-statistically ciation (i. e. the behavior is non-ergodic), then-despite the equivalence Of both methyl group-the two I3CH3 and to same geometry for the singly protonated molecule. One of the central questions in reaction dynamics today is whether highly excited molecules behave ergodically['], i. e. whether unimolecular dissociations are slower than intramolecular vibrational energy redistribution. It has been shown for neutral molecules[21 that energy randomization is completed within a few picoseconds and that molecules behave non-ergodically only under special conditions['"]. If similar non-ergodic behavior were found in ionic systems, then one of the basic assumptions of the quasi-equilibrium theory ( Q E P of mass spectra'31 would be violated.Evidence for the non-statistical behavior of the enol radical cation of acetone ( I ) , through measurements of kinetic energy release distributions T of the dissociation fragments, has recently appearedl4]. In that study and in previous ones[51, [2H]-labeled isotopomers of (1) were employed and there was the ever present possibility of kinetic isotope effects. In order to circumvent this problem, we have now prepared the [13C]-labeled isotopomers (la) and (lb). We report here on the unimolecular dissociation of this radical cation in the gas phase and present evidence indicating that this system is characterized by non-ergodic behavior.The study of ['HI-labeled ions (l), strengthened by thermochemical has shown that the CH3-cleavage from metastable ions (1) (lifetimes = lo-' s) does not take place by direct a-cleavage ((1)#(3) + CH3), but by dissociation to ion (5) following a rate determining [1,3]-Hshift ((1)-(4)+(2)) ( Fig. 1). A highly excited intermediate acetone radical cation (2)* is formed, whose lifetime has been estimated to be 5 x If (2)" were to behave ergodically, one would expect, on the basis of the chemical equivalence of the two methyl groups, that the two specifically ['3C]-labeled ions (la) and ( l b ) would eliminate the radicals I3CH; and "CH; in d4].

Control of Product Distribution and Mechanism by Ligation and Electric Field in the Thermal Activation of Methane

“…In 7,the emerging CH 3 C is only loosely coordinated to the cluster via the H-atom of the newly formed OH group.I ng oing from 7 to all energetically conceivable neutral products CH 3 C,O H C,a nd CH 3 OH, various reaction pathways were considered (for details,s ee Figures 2a nd S4). As the intramolecular vibrational energy redistribution may not necessarily be complete, [14] the vibrational mode along the reaction coordinate, for example,t he collinear triad O···H···C, in 7 is expected to pop-up the loosely bound CH 3 C directly with the concomitant generation of the product ion P4 (À77.5 kJ mol À1 ). Ther ate constant for this process is expected to be orders of magnitude larger than those for the formation of [(CH 3 CN)Zn(CH 3 )] + / OHC or [(CH 3 CN)Zn]C + /CH 3 OH according to the PESs shown in Figure S4.…”

Efficient Room‐Temperature Methane Activation by the Closed‐Shell, Metal‐Free Cluster [OSiOH]+: A Novel Mechanistic Variant

“…Thus, the formations of [SiOH] + + and [SiOD] + + from [OSiOH] + + /CD 4 are expected to occur because the rebound of the CD 3 group can involvee ither the -OH or the -OD unit. However,i nt he absence of as ignificant secondary KIE, which can only be expected when the hybridization in the reaction center is changed in the TS compared to the adduct [22] (and this is not the case for reaction step 2!3), the experimentally observed ratio [SiOH] + + :[SiOD] + + of 1:1.8 is not compatible with this reasonings ince a1 :1 ratio is expected under the assumptions that 1) both hydroxy groups are indistinguishable, 2) the intramolecular energy redistributiono ft he rovibrationally hot intermediates 2 and 3 is complete (ergodic behavior [23] ), and 3) dynamic effects do not matter. [24] Thus, other processes must be taken into account that explain the preferred formation of [SiOD] + + from the [OSiOH] + + /CD 4 couple.…”

Lactone enol cation‐radicals: gas‐phase generation, structure, energetics, and reactivity of the ionized enol of butane‐4‐lactone

“…Unimolecular dissociations of simple enol cation‐radicals, e.g. ethenol (acetaldehyde enol),8–16 1‐propen‐1‐ol (propionaldehyde enol),17–22 1‐propen‐2‐ol (acetone enol),23–28 butene‐2‐ols (methyl ethyl ketone enols),29–35 buta‐1,3‐dien‐2‐ol (methyl vinyl ketone enol),36 buta‐1,3‐dien‐1‐ol (crotonaldehyde enol),37 but‐1‐en‐3‐yne‐2‐ol38 and 1‐phenylethen‐1‐ol (acetophenone enol)39–41 have been studied in detail previously using a variety of ion chemistry methods, as reviewed. 6,7 Enol cation‐radicals derived from carboxylic acids and esters, such as 1,1‐dihydroxyethene (enol of acetic acid),42–48 1‐hydroxy‐1‐methoxyethene (enol of methyl acetate)49–57 and 1‐hydroxy‐1‐ethoxyethene (enol of ethyl acetate),58 have also been studied and found to have distinct chemistries in the gas phase.…”