ChemInform Abstract: Activation of Alkanes by Cr+: Unique Reactivity of Ground-State Cr+(6S) and Thermochemistry of Neutral and Ionic Chromium-Carbon Bonds.

Abstract

Guided ion beam mass spectrometry is used to study the reactions of ground-state Cr{sup +}({sup 6}S) with propane, butane, methylpropane, dimethylpropane, and selectivity deuteriated propane and methylpropane. Thermal energy reactions of Cr{sup +}({sup 6}S) with 4-octyne are also investigated. Ground-state Cr{sup +} ions undergo no bimolecular reactions at thermal energies with any of the alkanes, but do react at elevated kinetic energies. The only products formed at thermal energy in the alkane systems are the collisionally stabilized adduct complexes. Approximate lifetimes for these adducts are determined. Analyses of the endothermic processes in the alkane systems yields 298 K bond energies for several chromium-ligand species. These include the neutral and ionic chromium methyl species [D{degrees}(Cr-CH{sub 3}) = 37.9 {+-} 2.0 kcal/mol and D{degrees}(Cr{sup +}-CH{sub 3}) = 30.3 {+-} 1.7 kcal/mol], several other chromium ion-alkyl species [D{degrees}(Cr{sup +}-C{sub 2}H{sub 5}) = 35.0 {+-} 2.1 kcal/mol, D{degrees}(Cr{sup +}-1-C{sub 3}H{sub 7}) = 32.1 {+-} 1.4 kcal/mol, D{degrees} (Cr{sup +}-2-C{sub 3}H{sub 7}) = 28.5 {+-} 1.3 kcal/mol], and chromium ion-vinyl and -various alkylidenes [D{degrees}(Cr{sup +}-C{sub 2}H{sub 3}) = 59.0 {+-} 2.3 kcal/mol, D{degrees}[Cr{sup +}=CHCH{sub 3}] = 52 {+-} 3 kcal/mol, D{degrees}[Cr{sup +}=CHCH{sub 2}CH{sub 3}] = 36 {+-} 3 kcal/mol, D{degrees}[Cr{sup +}=C(CH{sub 3}){sub 2}] = 39more » {+-} 3 kcal/mol]. The observed reactivity requires unusual reaction mechanisms that are discussed in detail. 60 refs., 9 figs., 4 tabs.« less