Photo-Induced Hydrogen Exchange Reaction between Methanol and Glyoxal: Formation of Hydroxyketene

• Published in: Chemphyschem 2008-08, Vol.9 (12), p.1774-1780
• Main Authors: Mielke, Zofia, Mucha, Małgorzata, Bil, Andrzej, Golec, Barbara, Coussan, Stephane, Roubin, Pascale
• Format: Article
• Language: English
• Subjects: ab initio calculations; Chemistry; Exact sciences and technology; General and physical chemistry; hydrogen transfer; IR spectroscopy; matrix isolation; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics)
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200800249

We study the structure and photochemistry of the glyoxal–methanol system (G–MeOH) by means of FTIR matrix isolation spectroscopy and ab initio calculations. The FTIR spectra show that the non‐hydrogen‐bonded complex, G–MeOH‐1, is present in an inert environment of solid argon. MP2/aug‐cc‐pVDZ calculations indicate that G–MeOH‐1 is the most stable complex among the five optimized structures. The interaction energy partitioned according to the symmetry‐adapted perturbation theory (SAPT) scheme demonstrates that the dispersion energy gives a larger contribution to the stabilization of a non‐hydrogen‐bonded G–MeOH‐1 complex than compared to the hydrogen‐bonded ones. The irradiation of G–MeOH‐1 with the filtered output of a mercury lamp (λ>370 nm) leads to its photo‐conversion into the hydroxyketene–methanol complex HK–MeOH‐1. The identity of HK–MeOH‐1 is confirmed by both FTIR spectroscopy and MP2/aug‐cc‐pVDZ calculations. An experiment with deuterated methanol (CH3OD) evidences that hydroxyketene is formed in a photo‐induced hydrogen exchange reaction between glyoxal and methanol. The pathway for the photo‐conversion of G–MeOH‐1 to HK–MeOH‐1 is studied by a coupled‐cluster method [CR–CC(2,3)]. The calculations confirm our experimental findings that the reaction proceeds via hydrogen atom exchange between the OH group of methanol and CH group of glyoxal. Methanol please! The molecular glyoxal–methanol complex, isolated in solid argon and irradiated with a mercury lamp (λ>370 nm), converts into a hydroxyketene–methanol complex (see scheme). The reaction involves exchange of two hydrogen atoms between the two complex subunits.