Matrix-Isolated Diglycolic Anhydride: Vibrational Spectra and Photochemical Reactivity S. Jarmelo,* ,† I. D. Reva, † L. Lapinski, ‡ M. J. Nowak, ‡ and R. Fausto † Department of Chemistry, UniVersity of Coimbra, 3004-535 Coimbra, Portugal, and Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland ReceiVed: June 25, 2008; ReVised Manuscript ReceiVed: August 11, 2008 The structure of diglycolic anhydride (1,4-dioxane-2,6-dione; DGAn) isolated in a low-temperature argon matrix at 10 K was studied by means of FTIR spectroscopy. Interpretation of the experimental vibrational spectrum was assisted by theoretical calculations at the DFT(B3LYP)/aug-cc-pVTZ level. The optimized structure of the isolated DGAn molecule adopts an envelope conformation, which was found to resemble closely the structure of DGAn in a crystal. The UV-induced (λ > 240 nm) photolysis of the matrix-isolated compound was also investigated. In order to identify the main species resulting from irradiation of the monomeric DGAn, a comparison between the DFT(B3LYP)/aug-cc-pVTZ calculated spectra of the putative products and the experimental data was carried out. The observed photoproducts can be explained by a model involving four channels: (a) 1,3-dioxolan-4-one + CO; (b) CO 2 + CO + oxirane; (c) formaldehyde + ketene + CO 2 ; (d) oxiran-2-one + oxiran-2-one. As a whole, the experiments indicated that the C-O-C bridge, connecting the two CdO groups, is the most reactive fragment in the molecule excited with UV light. This observation was confirmed by the natural bond orbital (NBO) analysis revealing that the most important NBO interactions are those between the carbonyl groups and the adjacent C-O and C-C bonds. Introduction Glycolic acid (GA) is the simplest of the R-hydroxy car- boxylic acids. This compound has been the subject of numerous investigations due to its important role in essential biological processes (e.g., the photorespiratory carbon oxidation cycle in higher plants and algae or the oxalate metabolism in human beings) 1 and industrial uses, in particular in the dermatology and cosmetics industry. 2 GA has also been used as a prototype molecule for a large family of organic acids that have recently been discussed in terms of their connection to both atmospheric chemistry and spectroscopy, 3,4 and it was found in the organic content of aerosols in a polluted troposphere. 3 GA has been recently studied in our laboratory at the monomeric level. 5 Because it has dual chemical functionality, with both alcohol (-OH) and acid (-COOH) functional groups on a very small molecule, GA possesses unique chemical attributes, as well as typical alcohol and acid chemistry. It is a useful intermediate for organic synthesis, in a range of reactions including redox, 6 esterification, 7 and long chain polymerization. 8,9 Polyglycolic acid (PGA) is a biodegradable thermoplastic polymer and the simplest linear aliphatic polyester. It can be prepared starting from GA by means of polycondensation 9 or ring-opening polymerization. 8 In the first half of the last century, research on materials synthesized from GA was abandoned because the resulting polymers were too unstable for long-term industrial uses. 10 However, this instability, leading to biodeg- radation, has proven to be extremely important in medical applications. Polymers prepared from GA have found a multi- tude of uses in the medical industry, beginning with the biodegradable sutures first approved in the 1960s. 10 Since that time, diverse products based on GA have been accepted for use in medicine. Oligomers of glycolic acid can be seen as precursors for polymers as well as their decomposition intermediates. One of the simplest oligomers of GA is diglycolic acid (DGAc; HOOC- CH 2 -O-CH 2 -COOH). The thermally induced reactivity of DGAc was recently investigated by Vinciguerra et al. 11 They found that in the 200-250 °C range DGAc undergoes simple fragmentation, forming the corresponding anhydride, with loss of a water molecule. On the other hand, diglycolic anhydride (1,4-dioxane-2,6-dione; DGAn), having the six-membered cyclic structure, is thermally more stable than DGAc and evaporates at nearly the same temperatures without degradation. It was found that the general photoreaction pathways of cyclic anhydrides in the vapor 12 and liquid 13 phases concern decar- bonylation and decarboxylation. However, diglycolic anhydride has not been studied from this viewpoint, neither experimentally nor theoretically. In the current work, the infrared spectrum of DGAn isolated in an argon matrix at 10 K was studied in the 4000-400 cm -1 region. In addition, the UV-induced photochemistry of the matrix-isolated DGAn was also investigated. The assignment of the experimental vibrational spectra was assisted by calcula- tions of the IR spectra of DGAn and of the putative photoprod- ucts of its phototransformations. To the best of our knowledge, this is the first report on these subjects. Experimental and Computational Methods Diglycolic anhydride (1,4-dioxane-2,6-dione; purity g 90%) was purchased from Sigma-Aldrich. Prior to the matrix-isolation experiment, a glass tube with solid DGAn was connected to the vacuum chamber of the cryostat and evacuated by pumping for 1 h. This allowed an additional purification of the compound by removing volatile impurities. Matrixes were prepared by deposition of DGAn vapor onto a cold CsI window (10 K) directly attached to the cold tip of a continuous flow liquid helium cryostat. The vapor of the compound was deposited from a glass tube at room temperature * Corresponding author. E-mail: sjarmelo@qui.uc.pt. † University of Coimbra. ‡ Polish Academy of Sciences. J. Phys. Chem. A 2008, 112, 11178–11189 11178 10.1021/jp805603b CCC: $40.75  2008 American Chemical Society Published on Web 10/14/2008 Downloaded by PORTUGAL CONSORTIA MASTER on July 13, 2009 Published on October 14, 2008 on http://pubs.acs.org | doi: 10.1021/jp805603b