Time‐resolved IR studies of α‐lactones

Abstract

epoc ABSTRACT: A series of -lactones were generated from the reaction of phenylchlorocarbene, 4-nitrophenylchlorocarbene, diphenylcarbene, bis(4-nitrophenyl)carbene and bis(4-methoxyphenyl)carbene with carbon dioxide and examined by nanosecond time-resolved infrared (TRIR) spectroscopy. Estimated second-order rate constants for the reaction of these carbenes with carbon dioxide indicate that more nucleophilic carbenes react at faster rates, in agreement with previous low-temperature matrix experiments. Spectral TRIR data confirms that the structure oflactones is dependent both on substituents at the -carbon and on solvent polarity, with electron-donating substituents and polar solvents favoring a zwitterionic ring-opened structure as opposed to the three-membered ring oxiranone form. B3LYP calculations using self-consistent reaction field (SCRF) methods also provide support for these experimental investigations.

Infrared spectroscopic confirmation of α‐lactone formation in the dissociation of a gaseous amino acid

“…α‐Lactones are highly reactive molecules that have been proposed as short‐lived intermediates in a variety of chemical reactions, often involving α‐substituted carboxylates or α,β‐unsaturated acids, in the photolysis of cyclic peroxides and α‐halocarboxylic acids, in the oxidation of ketenes, or in the gas‐phase dissociation of α‐substituted carboxylic acids or in the decomposition of amino acids, both in solution and in the gas phase . They have also been proposed as intermediates in atmospheric oxidation reactions in mass spectrometry and as products of the reaction of carbenes with CO 2 . There has long been discussion regarding the electronic structure of α‐lactones, and whether they are best considered to be cyclic, canonical structures or zwitterionic, although those discussions generally relate to the structure in solution.…”

“…Despite being highly reactive, α‐lactones have been investigated spectroscopically by using matrix isolation, gas‐phase and solution‐phase time‐resolved IR . Whereas the gas‐phase and solution‐phase spectroscopy studies detect α‐lactones as transient intermediates, they have been isolated as persistent species under low‐temperature, matrix‐isolation conditions.…”

“…α‐Lactones have spectroscopic properties that allow them to be readily detected. In particular, the carbonyl stretching frequency in the range 1900–1930 cm −1 lies in a region of the IR spectrum that is typically inactive, and therefore is highly characteristic.…”

Mass spectrometric study of the decomposition pathways of canonical amino acids and α‐lactones in the gas phase

“…[23,74] They have also been proposed as intermediates in atmospheric oxidation reactions [75,76] in mass spectrometry, [77][78][79][80] and as products of the reaction of carbenes with CO 2 . [81][82][83][84][85] Despite being highly reactive, α-lactones have been investigated spectroscopically by using matrix isolation, [82][83][84] gas phase, [86] and solution-phase time-resolved IR. [85] There has long been a discussion regarding the electronic structure of α-lactones, and whether they are best considered to be cyclic, canonical structures [56,[58][59][60] or zwitterionic, [55] although those discussions generally relate to the structure in solution.…”

“…[81][82][83][84][85] Despite being highly reactive, α-lactones have been investigated spectroscopically by using matrix isolation, [82][83][84] gas phase, [86] and solution-phase time-resolved IR. [85] There has long been a discussion regarding the electronic structure of α-lactones, and whether they are best considered to be cyclic, canonical structures [56,[58][59][60] or zwitterionic, [55] although those discussions generally relate to the structure in solution. The calculated structures of gas-phase α-lactones have exceptionally long (~1.55 Å) and weak [87] C α -O bonds, and even in the gas phase, there is considerable ionic character.…”