Electronic Spectra of Adsorbed Molecules: Stable Carbonium Ions on Silica Alumina

Generation and Reactions of Organic Radical Cations in Zeolites

“…319 This blue species attracted much attention but proved to be enigmatic. 257,[320][321][322][323][324][325][326][327][328][329][330] ESR integration showed an insignificant spin population, casting doubt on its assignment to a radical cation; the concentration dependence suggested some type of dimer. 320,321 Alternative structures, such as a π-complex between 103 and a Lewis acid site or a carbenium ion were also proposed over the years.…”

“…257,[320][321][322][323][324][325][326][327][328][329][330] ESR integration showed an insignificant spin population, casting doubt on its assignment to a radical cation; the concentration dependence suggested some type of dimer. 320,321 Alternative structures, such as a π-complex between 103 and a Lewis acid site or a carbenium ion were also proposed over the years. [331][332][333][334][335][336] Shortly after Rooney and Pink's work the first synthetic zeolites were reported.…”

“…A species of λ max 615 nm was reported also upon adsorption of 103 onto a 88% Ca 2+ -exchanged Y zeolite (and again erroneously assigned to 103 •+ ), 312,318 based on the qualitative observation of a "strong ESR signal with a g value of 2.00235"; 340 this result directly contradicts the report of insignificant spin populations. 320,321 Oxygen was deemed essential for the generation of the blue species; 103 only produced "a very light green color" on CaY activated under nitrogen. 310 Interestingly, the blue color (λ max 620 nm) of bis-(p-dimethylamino)-103 paled upon degassing and was regenerated upon introduction of air; this process was reversible for at least six cycles.…”

“…434,[522][523][524][525][526] In this context, we reiterate that the recent significant progress in organic radical cation chemistry in zeolites has its roots in the seminal studies of Rooney, Leftin, Hall, and others in the 1960s on amorphous silicas and aluminas. 257,[319][320][321][322][323][324][325][326][327][328][329][330][527][528][529] The adsorption of electron-rich arenes, alkenes, heterocycles, etc. on acid solids spontaneously generated radical cations by ET from the organic substrate to the support.…”

NMR Investigations of Arylmethyl Carbocation Precursors Adsorbed on Zeolite HY

“…Most early studies of the ionization of carbocation precursors in the solid state depended on optical spectroscopies, such as IR − and UV − spectroscopies. During the past 3 decades, the most useful and definitive data bearing on this field of “stable” carbenium ions are those obtained from nuclear magnetic resonance studies, which have included important 13 C NMR investigations in very strong liquid acids .…”