This Letter analyses the hydrogen bond donor properties of the HOO radical. The NH3-HOO radical is examined by means of ab initio as well as topological methods. The obtained results are in good agreement with the conclusions given in an experimental paper devoted to the examination of this complex by means of IR spectroscopy in an argon matrix. In particular, present calculations confirm an open structure of this complex and the assignment of the observed bands. Anharmonic IR spectra for the complex are calculated. The necessity of distinction between geometry and structure of the complex is emphasized as the complex has an open structure and cyclic geometry. (c) 2005 Elsevier B.V. All rights reserved.