Norrish Type I Cleavage of Aliphatic Carboxylic Acids and Esters in Solution. A CIDNP.‐study

Abstract

SummaryUV.-irradiation of pivalic and isobutyric acid and their methyl esters as well as of a-hydroxy isobutyric and malonic acid in solution lead to chemically induced dynamic nuclear polarization (CIDNP.) of parent compounds and of various reaction products. C1DNP.-effects and product distributions confirm a-cleavage of the C(u), CO bond to be the major mode of photodecomposition. Scavenger experiments indicate that decomposition of the molecules from a triplet excited state is roughly equally or more probable than from a singlet excited state. Quantum yields of educt disappearance are also given for the methyl esters of pivalic and isobutyric acid.

Photochemical free radical formation from aliphatic dicarboxylic acids in solution as studied by electron spin resonance spectroscopy

“…However, judged from the observed relative steady state radical concentrations this process occurs with a much lower quantum yield than the other two. This finding parallels our previous results obtained for the photochemical decay processes of aliphatic carboxylic esters [3] [4]. There, a partial decay via 0, CO-bond cleavage 2 could be directly proven by ESR.-detection of the primarily formed acyl type radical R'CO.…”

“…Because of fast relaxation due to spin-rotation interaction [ 111 the acyl radical is expected to exhibit a very broad resonance line at g Y 2.0006 at room temperature [12] which at our low steady state radical concentrations should be undetectable in the ESR.-spectrum. The reactive OH radical, although not (4) and in this way generate the observed species R = HOOCCH,CHCOOH.…”

“…There, a partial decay via 0, CO-bond cleavage 2 could be directly proven by ESR.-detection of the primarily formed acyl type radical R'CO. For the methyl ester of pivalic acid this reaction occurred with about a factor 10 lower quantum yield than the C, CO-bond rupture 1 [4]. With respect to the partitioning ratio of a-cleavage and intermolecular photoreduction the dicarboxylic acids behave in an analogous way to the monocarboxylic acids and esters 131.…”

“…Since ketones undergo a-cleavage and photoreduction most effectively from nn*-states it may be inferred that the photodecomposition of carboxylic compounds occurs from the singlet nn*-state. However, the C1DNP.-pattern obtained from the cleavage reaction 1 of some acids and esters revealed that this process occurred at least in part from a triplet state radical precursor [4].…”

Radical–Triplet Pair Interactions as Probes of Long-Range Polymer Motion in Solution

“…40 It is likely that the release of ring strain energy contributes to this rather short lifetime. Additionally, the Norrish I α-cleavage reaction is known to be facile in aliphatic esters, 41 and similar radicals (alkyl and oxo-acyl) have been detected in high concentrations by SSEPR, 42,43 suggesting a fast cleavage rate at least comparable to aliphatic ketones (5 × 10 8 s −1 at room temperature). 44 We carried out a quenching experiment on PMMA in propylene carbonate using sorbic acid as the quencher and monitoring the integrated TREPR signal for the Stern− Volmer plot (data not shown).…”