Electronic Basis of the Comparable Hydrogen Bond Properties of Small H 2 CO/(H 2 O) n and H 2 NO/(H 2 O) n Systems (n ) 1, 2) C. Houriez, ² N. Ferre ´ ,* ,² J.-P. Flament, ‡ M. Masella, § and D. Siri ² UMR CNRS 6517 “Chimie, Biologie, Radicaux Libres”, UniVersite ´ de ProVence, Faculte ´ de Saint-Je ´ ro ˆ me, Case 521, AVenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France, UMR CNRS 8523, Laboratoire de Physique des Lasers, Atomes et Mole ´ cules, UniVersite ´ de Lille 1, Ba ˆ timent P5, 59655 VilleneuVe d’Ascq, France, and Laboratoire de Chimie du ViVant, SerVice d’Inge ´ nierie Mole ´ culaire des Prote ´ ines, Institut de Biologie et de Technologies de Saclay, Commissariat a ` l’Energie Atomique, Centre de Saclay, 91191 Gif-Sur-YVette Cedex, France ReceiVed: July 2, 2007; In Final Form: August 28, 2007 The electronic and structural properties of dihydronitroxide/water clusters are investigated and compared to the properties of formaldehyde/water clusters. Exploring the stationary points of their potential energy surfaces (structurally, vibrationally, and energetically) and characterizing their hydrogen bonds (by both atoms in molecules and natural bond orbitals methods) clearly reveal the strong similarity between these two kind of molecular systems. The main difference involves the nature of the hydrogen bond taking place between the X-H bond and the oxygen atom of a water molecule. All the properties of the hydrogen bonds occurring in both kind of clusters can be easily interpreted in terms of competition between intermolecular and intramolecular hyperconjugative interactions. 1. Introduction Nitroxides are spin-doublet radicals, whose single electron is mainly described by the π* orbital of the N-O bond. They exhibit rather long half-life times and their electron paramagnetic resonance (EPR) spectra are highly sensitive to molecular mobility and environment. 1 That explains why they are widely used as spin probes to investigate the properties of biopolymers and nanostructures, 2 as well as controlling species in the living radical polymerization. 3 Moreover, nitroxides can be produced by the attachment of a transient free radical to a nitrone. In that case, the EPR spectrum of the resulting nitroxide is characteristic of both the nitroxide and the free radical. Such a procedure, referred to as “spin-trapping”, is commonly used for monitoring reactions involving reactive radicals at concentrations too low for direct observations (such as the active forms of oxygen 4 ). As in nearly every field of chemistry, hydrogen-bonding plays also a key role in the understanding of the EPR characteristics of the spin probe in solution. For instance, Barone and co-workers, who have shown a long standing interest for the computations of organics EPR spectra in condensed phase (cf., e.g., their recent review 5 ), demonstrated the necessity of accounting explicitly for the interactions of the free radicals with the solvent molecules to compute accurate hyperfine coupling constants. By contrast with the massive amount of experimental and theoretical results regarding hydrogen bonding among water molecules (whose current knowledge is still far from being complete 6 ), only a few theoretical results are available concern- ing the interactions between nitroxides and water (see ref 5 and references therein): they all concern some particular structures, which cannot be used as such to draw a clear picture of these interactions. In particular, most of the theoretical studies devoted to nitroxide/water aggregates have focused on their global minimum and essentially ignored the rest of their potential energy surface (PES). However, it has been shown that the understanding of high-resolution experiments concerning small water aggregates (ranging from the dimer to the hexamers) needs to consider the rearrangement pathways connecting their global minimum. 7 Hence, reliable theoretical investigations of hydrogen- bonded systems have to focus not only on structures corre- sponding to minima but also on important stationary points of their PES. Hence, the primary, but not sole, goal of this work is to theoretically investigate at different levels of theory the proper- ties of several structures of the H 2 NO/H 2 O dimer and of the H 2 NO/(H 2 O) 2 trimer, corresponding to either minima or saddle points. This will provide further insight into hydrogen bonding involving nitroxides. Moreover, nitroxides can be seen as carbonyl compounds with an extra electron in a π* orbital. That suggests that the properties of hydrogen bonds involving either a NO or a CO moiety are expected to be similar. To test this hypothesis, we have also investigated at the same levels of theory all the corresponding formaldehyde/water structures drawn by substituting H 2 NO by H 2 CO. To draw reliable conclusions, several properties have been considered, such as interaction energies, vibrational spectra, geometrical parameters as well as topological properties of the electronic density along the hydrogen bond axes. Particular attention was also devoted to evaluate the energetic incidence of cooperative effects on the heterotrimers, which are known to strongly affect the properties of hydrogen-bonded clusters by enhancing their hydrogen bond network. Anticipating the results, we show that carbonyl/ and nitroxide/ water clusters mainly differ by the hydrogen bond taking place between the X-H bond of the H 2 XO moiety (X ) C, N) and the water oxygen atom, whereas the properties of all the * To whom correspondence should be addressed. ² Universite ´ de Provence. ‡ Universite ´ de Lille 1. § Institut de Biologie et de Technologies de Saclay. 11673 J. Phys. Chem. A 2007, 111, 11673-11682 10.1021/jp075136z CCC: $37.00 © 2007 American Chemical Society Published on Web 10/18/2007