Collision-induced absorption in liquid carbon tetrachloride

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Abstract

Molecular dynamics (MD) simulation has been used for the computation of the far infrared absorption coefficient of CCl4, with an excellent accord with experimental results at all frequencies. Nonadditive polarization interactions have been implemented with the chemical potential equalization method, which requires the gas phase values of the octupole moment and mean polarizability tensor. It is shown that the absorption coefficient constitutes an exacting probe of interaction potentials, strongly favoring a physically reasonable set of atomic charges.

Introduction

Far infrared absorption in nonpolar liquids stems from collision-induced dipole moments [1]. The most thoroughly studied molecules in this connection have been carbon disulfide, carbon tetrachloride and benzene, as they are representative of different molecular symmetries [2]. Presently, the accord between numerical results and experiment cannot be considered as fully satisfactory. We have for instance that in a recent study of carbon disulfide and benzene, the computed values were 20–30% lower than experiment [3]. Actually, it is common to scale the absolute height of the computed spectrum to that of the experimental one. The case of carbon tetrachloride is not different, with an additional redshift of the computed spectrum and a nonnegligible overdamping at high frequencies [4]. Only theoretical calculations using physically motivated model correlation functions have provided acceptable accord between theory and experiment for tetrahedral molecules [5].

Molecular dynamics (MD) calculations of the absorption coefficient have usually employed point multipoles located at the molecular centers [4], [6], [7], [8], following the approach characteristic of the gas phase. However, Davies et al. [9] pointed long ago the difficulties that this approximation may have at the high densities characteristic of the liquid state, and strongly advocated for the use of models based on partial charges located on atomic sites plus molecular polarizability. Moreover, even in the works using distributed partial charges, it has been customary to only include two body interactions [3], [4], [6], [7], [8], [10], while it is reasonable to assume that nonadditive effects might be particularly relevant for nonpolar liquids. The chemical potential equalization method (CPE) [11], in which partial charges are allowed to fluctuate, looks like the most appropriate way to address all these issues simultaneously. This approach, which includes nonadditive contributions, can be easily tailored to yield the gas phase multipole moment of interest and the polarizability tensor. Moreover, carbon tetrachloride is ideally suited for this method due to its spherical symmetry, which allows to reproduce any induced dipole moment varying the site charges, without recourse to additional point dipoles. For these reasons, we assess here the performance of CPE for the computation of the absorption coefficient in this liquid.

Section snippets

Theory and computational methods

The basic ingredient for the calculation of the absorption coefficient is the time correlation function of the total dipole moment of the sampleφ(t)≡〈M→(t)·M→(0)〉.It is known [12] that in the computation of dielectric properties from MD simulations, special attention has to be paid to boundary conditions. In this work we have used the Ewald sum and thus, we have for the imaginary component of the dielectric functionϵ″(ω)=4πω3VkBT∫0∞cos(ωt)φ(t),from which the absorption coefficient can be easily

Results and comparison with experiment

The total dipole correlation function (φ(t)) is displayed in Fig. 1. It shows a smooth decay, approaching zero in about 2 ps. There is no long time tail nor any backscattering, in line with the features deduced from the experimental measures [21]. This function can be split into self- and cross-correlation contributions, between molecular dipole momentsφ(t)=N〈m→(t)·m→(0)〉+∑i=1i≠jN∑j=1N〈mi→(t)·mj→(0)〉≡φself(t)+φcross(t),included in Fig. 1. As it has also been found in similar systems [8], the

Concluding remarks

The excellent accord attained with realistic parameters suggests that the present approach might allow a reliable and detailed study of induced absorption. In this connection we just mention a few possibilities, like the effect of temperature and pressure variations [24], the role of local field effects, and within the broad field of inhomogeneous systems, some puzzling features in the absorption spectrum of CHCl3–CCl4 mixtures. Of course, it will also be necessary to check if this agreement

References (35)

  • Th. Dorfmüller et al.

    Chem. Phys.

    (1986)
  • C.G. Joslin et al.

    Chem. Phys. Lett.

    (1989)
  • M. Neumann et al.

    Chem. Phys. Lett.

    (1984)
  • J.P. Ryckaert et al.

    J. Comp. Phys.

    (1977)
  • H.-J. Arning et al.

    Chem. Phys.

    (1982)
  • L. Potthast et al.

    Chem. Phys.

    (1986)
  • Ph. Marteau et al.

    J. Mol. Liq.

    (1989)
  • S.K. Garg et al.

    J. Chem. Phys.

    (1968)
  • G.W. Chantry et al.

    Nature

    (1967)
  • Y. Fujita et al.

    J. Chem. Phys.

    (1995)
  • P.A. Madden et al.

    Mol. Phys.

    (1983)
  • W.A. Steele

    Mol. Phys.

    (1985)
  • J. Samios et al.

    Mol. Phys.

    (1986)
  • G.J. Davies et al.

    J. Chem. Soc. Faraday II

    (1973)
  • S.-B. Zhu et al.

    Mol. Phys.

    (1988)
  • A.K. Rappé et al.

    J. Phys. Chem.

    (1991)
  • R. Chelli et al.

    J. Chem. Phys.

    (1999)

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