Solid State Nuclear Magnetic Resonance 25 (2004) 209–215 Solid state and solution 43 Ca NMR of calcium peroxides involved in the disproportionation of hydrogen peroxide by calcium hydroxide Arlette Trokiner, a, Aure´lie Bessie`re, a Rene´ Thouvenot, b Damien Hau, c Jean Marko, c Ve´ronique Nardello, c Christel Pierlot, c and Jean-Marie Aubry c a Laboratoire de Physique du Solide, UPR CNRS A05, ESPCI, 10 Rue Vauquelin, F-75231 Paris Cedex 05, France b Laboratoire de Chimie Inorganique et Mate´riaux Mole´culaires, UMR CNRS 7071, UPMC, 4 place Jussieu F-75252 Paris Cedex 05, France c Laboratoire d’Oxydation et de Formulation, UMR CNRS 8009, ENSCL, BP 108, F-59652 Villeneuve d’Ascq Cedex, France Received March 3, 2003; revised September 11, 2003 Abstract In order to get some insight into the mechanism of the disproportionation of hydrogen peroxide catalyzed by calcium hydroxide, 43 Ca NMR spectra of enriched samples of calcium peroxides and of their precursors have been studied in both solution and solid state. This study demonstrates that no well-defined peroxidized calcium species are formed in solution, showing that the catalytic role of calcium is likely restricted to the solid state. Most of the calcium compounds that could be involved in the catalytic process have been investigated with solid state NMR. The shift and quadrupolar parameters of Ca(OH) 2 , CaO 2  8H 2 O and CaO 2  2H 2 O 2 are reported for the first time. These parameters are different enough to allow the quantitative analysis of a complex mixture of these compounds by NMR. r 2003 Elsevier Inc. All rights reserved. Keywords: 43 Ca NMR in solid state and solution; Calcium peroxides; Hydrogen peroxide; Catalysis 1. Introduction Hydrogen peroxide is a thermodynamically unstable molecule, which readily disproportionates into water and ground-state molecular oxygen, 3 O 2 , in the presence of a wide variety of inorganic compounds. In 1985, Aubry established that about 30 inorganic oxides, hydroxides or oxo-anions are able to direct this catalytic process in aqueous solution towards the production of the first excited singlet state of oxygen, 1 O 2 (Eq. (1)) [1]. 2H 2 O 2 water  ! catalyst 2H 2 O þ a 1 O 2 þð1  aÞ 3 O 2 : ð1Þ In contrast to ground-state oxygen, 1 O 2 has found considerable synthetic utility since it can undergo selective reactions with many electron-rich molecules [2–5]. This highly reactive and selective oxidant is usually generated by photosensitization. Chemical sources of 1 O 2 , involving readily available precursors such as H 2 O 2 , could be attractive alternatives since they do not require photochemical reactors. It was shown that the disproportionation of H 2 O 2 in the presence of calcium (II) generates 1 O 2 [1]. The precursor of 1 O 2 might be the calcium peroxide diperoxohydrate CaO 2  2H 2 O 2 since it gives 1 O 2 on thermolysis [6,7]. The mechanism of the catalytic reaction appears very puzzling from a theoretical point of view, since one can wonder how the Ca 2+ ion, with its highly stable oxidation state, could be involved in the redox process leading to the disproportionation of H 2 O 2 . In order to gain some insight into the mechanism of this process and to get direct evidence of the inter- mediates involved in the catalytic reaction, it is tempting to use metal-NMR spectroscopy since this technique proved to be powerful in the cases of molybdate and tungstate anions. For example 183 W and 95 Mo NMR clearly demonstrate that the diperoxotungstate WO 2 (O 2 ) 2 2 and the triperoxomolybdate, MoO(O 2 ) 3 2 are precursors of 1 O 2 in the solution [8,9]. Elemental calcium has only one nonradioactive isotope, 43 Ca (spin I ¼ 7=2), that can be detected by ARTICLE IN PRESS  Corresponding author. Fax: 33-1-4079-4425. E-mail address: arlette.trokiner@espci.fr (A. Trokiner). 0926-2040/$-see front matter r 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.ssnmr.2003.09.002