Intramolecular Kinetic Carbon Isotope Effect in the Gas Phase Decomposition of Deuteriooxalic Acid

Reactive intermediates in 4He nanodroplets: Infrared laser Stark spectroscopy of dihydroxycarbene

“…The absence of the cis,cisrotamer is apparently due to the low internal energy content of gas-phase trans,cis-HOCOH, which cannot rotationally interconvert prior to being captured by a He droplet. These results strongly support the oxalic acid decomposition mechanism proposed by Lapidus et al [68][69][70][71] It is more difficult to rationalize the extent to which formic acid is produced in these experiments. In the Ar matrix study, Schriener et al reported a 1:5 ratio of dihydroxycarbene to formic acid upon pyrolysis of oxalic acid.…”

“…Again, there is no evidence for the cis,cis-rotamer in He droplets, and there was also no evidence for it in the Ar matrix. 36 These observations are consistent with a unimolecular decomposition mechanism of oxalic acid, [68][69][70][71][72] in which a concerted hydrogen migration and C-C bond cleavage produces dihydroxycarbene and CO 2 . Assuming this mechanism, it is expected that the thermal extrusion of CO 2 from the cTc oxalic acid isomer produces trans,trans-HOCOH, whereas the cTt isomer decomposes to trans,cis-(see Fig.…”

Spectroscopic Identification of Dihydroxycarbene

“…There is no indication of the presence of an s-cis,s-cis form, as expected from its lower stability and the mechanism of its generation (see above). Remarkably, the thermal reaction (during HVFP) leads to methyl formate (6), while irradiation at 254 nm produces acetic acid (7).…”

“…Under these conditions the decarboxylation reactions were not complete, starting materials were also present in the matrix and the yields of the matrix-isolated carbenes were low. The main reaction products in the matrix were the more-stable tautomers formic acid (3) and methyl formate (6). Subsequent photolysis of the matrix with a low-pressure mercury lamp at a wavelength of 254 nm (which corresponds to the absorption wavelengths of 1) for 4 min induced specific photochemical transformations.…”

“…Wobbe and Noyes apparently presented the first kinetic study of this reaction as early as 1926, [3] and numerous publications followed by Clark [4] in the 1950s as well as by Lapidus et al in the 1960s. [6] None of these, however, mention the possible involvement of free carbene 1 along the decomposition pathway to give predominantly CO 2 and H 2 ; the reason for this is that carbenes were just being established as viable chemical entities at that time. Lapidus et al proposed the direct decomposition of 2 to CO 2 and H 2 by an intramolecular hydrogen transfer that implies the formation of a divalent species, although it was neither drawn nor explicitly stated.…”