Hydrogen-bonding between the hydrogen peroxide molecule and the hydroperoxy radical (H 2 O 2 –HO 2 ): the global minimum Mohammad Esma € ıl Alikhani a , Vincenzo Barone b, * a Laboratoire de Dynamique, Interactions et R eactivit e UMR 7075, Universit e P. et M. Curie, Bo ^ ıte 49, b^ atiment F74, 4 Place Jussieu 75252 Paris Cedex 05, France b Department of Chemistry, Universit a ‘‘Federico II’’, Complesso Universitario Monte S.Angelo, via Cintia, I-80126 Napoli, Italy Received 25 March 2004; in final form 28 April 2004 Available online 18 May 2004 Abstract Spectroscopic properties of H 2 O 2 –HO 2 have been re-investigated by post Hartree–Fock methods including anharmonic vibra- tional effects through a second order perturbative approach. The results have been compared to recent experimental data. The very low barrier height computed for the conversion of the five-membered complex into its more stable six-membered counterpart (27 cm 1 ) explains why only the six-membered hydrogen-bonded complex is observed in argon matrices. The calculated anharmonic frequencies of the latter complex are in excellent agreement with experimental data. Ó 2004 Published by Elsevier B.V. 1. Introduction The hydroperoxide radical is a key intermediate in several processes of considerable interest for biochem- istry, photochemistry, and atmospheric chemistry. Its ability to react with stable molecules has been evidenced both experimentally [1–6] and theoretically [7,8]. Recently, the infrared spectrum of the H 2 O 2 –HO 2 hydrogen-bonded complex has been recorded using matrix isolation technique [6]. It has been shown that the peroxy radical forms a cyclic complex with hydrogen peroxide. This complex has been also object of a pre- vious quantum chemical study based on DFT (B3LYP) and post Hartree–Fock (QCISD) computations, which revealed the presence of two energy minima corre- sponding to five- and six-membered cycles closed by intermolecular H-bonds [8]. Unfortunately, B3LYP and QCISD methods provide a different stability order for the two cyclic forms. Comparison between the QCISD and experimental red-shift of the OH stretching mode (for both HOOH and HOO units) leads to the conclu- sion that the IR spectrum in argon matrices should be attributed to the six-membered cyclic structure, whereas B3LYP computations are less conclusive. In the present study, we have reexamined the H 2 O 2 – HO 2 complex to show that: 1. The very low activation energy governing intercon- version between five- and six-membered structures explains why only the most stable six-membered form has been characterized experimentally. 2. Anharmonic contributions cannot be neglected for a quantitative comparison between the computed and experimental frequency shifts issuing from H-bond interactions. 3. The electronic contributions to H-bond stability are similar in this open-shell complex and in more con- ventional systems. All the computations have been performed by the GAUSSIAN AUSSIAN 03 package [9] using different computational models. While methods rooted in the density functional theory (DFT) perform a very good job in predicting structures and physicochemical properties of HO 2 [10] and other radicals [11], the situation is different concerning the vibrational properties of hydrogen- bonded complexes. At the same time, the unrestricted Møller–Plesset (UMP2) approach is quite reliable in this connection, in the absence of too strong spin * Corresponding author. Fax: +39081674206. E-mail address: baronev@unina.it (V. Barone). 0009-2614/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.cplett.2004.05.005 Chemical Physics Letters 391 (2004) 134–137 www.elsevier.com/locate/cplett