Suitability of the Kihara Potential To Predict Molecular Spectra of Linear Polyatomic Liquids S. Calero, ²,‡ B. Garzo ´ n, § S. Jorge, ‡ J.A. Mejı ´as, ² J. Tortajada, ‡ and S. Lago* ,² Facultad de Ciencias Experimentales, Carretera de Utrera, Km 1, UniVersidad Pablo de OlaVide, 41013 SeVille, Spain; Departamento Quı ´mica Fı ´sica, Facultad de Ciencias Quı ´micas, UniVersidad Complutense, 28040 Madrid, Spain; and Departamento Ciencias Ba ´ sicas, Facultad de Ciencias Experimentales y Te ´ cnicas, UniVersidad San Pablo CEU, Urbanizacio ´ n Monteprı ´ncipe, Boadilla del Monte, 28660 Madrid, Spain ReceiVed: NoVember 4, 1999; In Final Form: February 4, 2000 Gibbs ensemble Monte Carlo (GEMC) simulations of linear molecular liquid chlorine and carbon disulfide are performed to determine the vapor-liquid equilibrium (VLE) of these fluids and the corresponding intermolecular potential parameters. The intermolecular potential function considered is the Kihara potential with a point multipole added. Molecular dynamics runs of liquid nitrous oxide, chlorine, and carbon disulfide are then presented to obtain angular velocity autocorrelation functions of different orders. Using these functions, transport coefficients as well as correlation times are calculated. Next, the band shapes of molecular vibration spectra are obtained as Fourier transforms of the autocorrelation functions. Agreement between experiment and simulation is excellent for all the calculated properties of chlorine, showing that the intermolecular potential presented here is extremely accurate as well as simple. Results for carbon disulfide and nitrous oxide range from very good to acceptable for different correlation times and transport properties and they are discussed in detail in the text. I. Introduction Intermolecular forces and statistical mechanics are two general key terms used in relation to liquid state physical properties. Thermodynamic and dynamic properties of liquids composed of monatomic molecules are well described by simple inter- molecular potentials, 1 such as Lennard-Jones, square-well, or Stockmayer potentials. The situation is not so clear in the case of polyatomic liquids, where intermolecular forces depend on the mutual orientations of molecules. In this case, the most popular intermolecular potential is the n-center Lennard-Jones potential, written as a sum of individual Lennard-Jones-like contributions for each pair of atoms or molecular sites. 2 This potential is particularly useful when interpreting X-ray or neutron diffraction 3,4 data, but its complexity rapidly grows with the number of chemically different atoms included in the molecule. So, for such a simple molecule as ethanol, the number of different site-site interactions can be as high as 21. In this and in more complicated cases, the potential becomes unman- ageable and drastic simplifications, either by reducing the number of sites or by other means, are necessary. To model some substances, alternative ways may sometimes include different potential functions such as the Gay-Berne potential 5,6 or the Kihara potential. 7 The latter has been widely used by several groups 8-13 to obtain thermodynamic properties, notably the vapor-liquid equilibrium of models 14 and actual systems of linear or pseudolinear molecular liquids. 15 Moreover, a variety of theories 16-21 and simulation 22-24 techniques have been used. The Kihara potential is also very popular in several fields related to chemical engineering to calculate properties of gas hydrates 25-29 as well as transport properties and some simple dynamic properties, 30-31 and it has been used recently in different theoretical frameworks. 32-34 Indeed, a similar potential has been suggested for helical molecules such as proteins. 35 In spite of its wide use, we are aware of only one paper using this potential for spectroscopic applications specifically concerning the problem of stability of solid phases of linear molecules. 36 The main aim of this paper is to evaluate how well this particular potential works to predict some typical spectroscopic liquid state properties, as in the Raman spectrum of linear nonpolar molecules such as Cl 2 , or in the infrared spectrum of molecules such as N 2 O or S 2 C. In all these cases, we consider that intermolecular and intramolecular vibrations are well separated and we restrict the study to the highest frequency intramolecular vibration in polyatomic molecules. The inter- molecular interaction in the liquid causes a band broadening and frequency shift with respect to the ideal gas phase spectrum. The band broadening is related to the Fourier transform of the different order Legendre polynomials of molecular angular velocity (AVCF), and the line width is related to the correlation time 37 characterizing the decay of the AVCF. This information is readily obtained from our molecular dynamics simulations. The frequency shift due to the condensed phase effect is estimated in a simple way, considering the perturbation due to the intermolecular potential on the total molecular potential. Parameters for N 2 O are taken from a previous paper, 38 and we have performed additional Gibbs ensemble Monte Carlo (GEMC) simulations for S 2 C and Cl 2 to obtain the VLE using a Kihara plus a multipole-multipole interaction, as described in section II. Once the intermolecular potential parameters were obtained, we carried out molecular dynamics simulations at the temper- * To whom correspondence should be addressed. Phone: 00-3495- 4349309. Fax: 00-3495-4349238. E-mail: slagara@dex.upo.es. ² Universidad Pablo de Olavide. ‡ Universidad Complutense. § Universidad San Pablo CEU. 5808 J. Phys. Chem. B 2000, 104, 5808-5815 10.1021/jp993904j CCC: $19.00 © 2000 American Chemical Society Published on Web 05/27/2000