Dynamics of hydration water in CaCl 2 complexes M. Plazanet a, * , I. Glaznev b , A.G. Stepanov b , Yu.I. Aristov b , H. Jobic c a Institute Laue Langevin, 6 Rue Jules Horowitz, BP 156, 38042 Grenoble, France b Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt Akademika Lavrentiava 5, Novosibirsk 630090, Russia c Institut de Recherches sur la Catalyse, CNRS, 2 Avenue A. Einstein, 69626 Villeurbanne, France Received 5 October 2005; in final form 8 November 2005 Available online 9 December 2005 Abstract We report in this Letter the measurement of unusually well-defined vibrational modes of water. Calcium chloride forms complexes made of different amounts of water, namely 1/3, 2 or 4 water per CaCl 2 molecule. Based on the crystallographic structure, we performed normal mode calculations from first principles, and simulated the neutron scattering spectra. A very good agreement between calculation and experiment, without any parameter refinement, confirms and completes the intuitive assignment of vibrational excitations of water. A closer look at the calculation enables us to investigate the anharmonicity of the system and the dispersion of the excitations along the hydrogen bonds. Ó 2005 Elsevier B.V. All rights reserved. 1. Introduction Because of its ubiquitous character, and the complexity of its behaviour, water is the subject of continuous studies. The vibrational modes of water in different crystalline forms have been extensively studied, but due to disorder, the modes are generally not well defined, and precise assignment of each expected mode is not reached in most cases [1]. Complexed water is another case: systems are more able to form ordered crystalline phases and give rise to better defined vibrational modes. Hundreds of examples investi- gated can be found in the review [2], or a more recent example, particularly well ordered is given by the family of M 2 [FeCl 5 H 2 O] (M = K, Rb, Cs, NH4) [3]. We report here the dynamical behaviour of water com- plexed with CaCl 2 , a complex that is often encountered in biological systems as well as in synthetic material. CaCl 2 forms a crystalline compound in different stoichiometric proportions, here the 1/3, 2 and 4 H 2 O per CaCl 2 com- plexes are investigated. This system is, to our knowledge, one of the better resolved inelastic neutron scattering measurement of water librations. 2. Experimental details The density of states have been measured by neutron scattering at the ILL (Grenoble, France), using two differ- ent techniques. The GDOS on the anti-Stokes side was measured at 300 K, on the cold time-of-flight spectrometer IN5 [4]. With an incident wavelength of 10 A ˚ , the resolution degrades drastically with the increasing energy transfer, equals 0.4 meV at 10 meV and 3.6 meV at 50 meV. The S(2H,x) data were reduced using standard procedure to get the GDOS quantity. The Stokes side of the spectra have been measured on IN1-BeF [5] at 10 K, i.e., using hot neutrons of variable incident energy (10–440 meV), and measuring the constant final energy of 4 meV (beryllium filter cut-off). This instru- ment achieves an energy resolution of about 5% of the energy transfer. The Stokes side is labelled on the figures as positive energy transfer and the anti-Stokes is set to negative values. 0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.11.069 * Corresponding author. Fax: +33476207688. E-mail address: plazanet@ill.fr (M. Plazanet). www.elsevier.com/locate/cplett Chemical Physics Letters 419 (2006) 111–114