Structure and Intermolecular Dynamics of Liquids:  Femtosecond Optical Kerr Effect Measurements in Nonpolar Fluorinated Benzenes

Abstract

A comparative study of the collective polarizability anisotropy dynamics of benzene, 1,3,5-trifluorobenzene, and hexafluorobenzene was carried out by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 45 fs laser pulses. The OHD-RIKES data were analyzed by using a modeldependent approach and a Fourier transform approach, which yields a spectral density for the liquid. From an analysis of the long-time tails, collective reorientation times of 2.45 ( 0.06, 9.05 ( 0.10, and 13.5 ( 0.1 ps were obtained respectively for C 6 H 6 , 1,3,5-C 6 F 3 H 3 , and C 6 F 6 . The spectral densities are narrower for 1,3,5-C 6 F 3 H 3 and C 6 F 6 than for C 6 H 6 by roughly a factor of 2. Information about the intermolecular vibrational modes of the liquid is contained in the reduced spectral density which is obtained by subtracting the tailmatched diffusive reorientational response from the OHD-RIKES data. In the case of C 6 H 6 and C 6 F 6 , the intermolecular spectral density can be decomposed into at least two broad overlapping bands. In contrast, the intermolecular spectral density for 1,3,5-C 6 F 3 H 3 is characterized by a single band. These spectra are rationalized in terms of the librational dynamics of perpendicular dimers in liquid C 6 H 6 and C 6 F 6 and parallel dimers in liquid 1,3,5-C 6 F 3 H 3 .

Optically-heterodyne-detected optical Kerr effect (OHD-OKE): Applications in condensed phase dynamics

“…For example McMorrow and coworkers assigned the low-frequency shoulder observed in the spectral density of liquid benzene (which is well ®tted by equation (19) [44]) to a dimer structure [27]. They reported that the shoulder disappeared on dilution of benzene in CHCl 3 , consistent with a shift in a monomer±dimer equilibrium.…”

“…However, the shift observed was somewhat larger than predicted on the basis of equation (22). Neelakandan et al showed that the observed librational frequencies of benzene and hexa¯uorobenzene could be well understood on the basis of the harmonic model [44]. In addition they showed that the librational frequency of 1,3,5-…”

“…Clearly some means of deconvoluting these contributions is desirable, and this inevitably involves a degree of curve ®tting. Chang and Castner [38], in a procedure subsequently adopted by a number of groups [33,41,44,45,47,55], showed that Im‰D 0 …! †Š can often be well represented by the sum of an antisymmetrized Gaussian…”

“…BL , the exponent ¬ and the relative amplitude g 1 =b. Although this ®tting procedure is partly empirical (see below), this pair of functions has been remarkably successful at describing the ultrafast spectral densities of a large number of liquids [33,38,39,41,44,45,47,55].…”

Interpretation of the low frequency response of aqueous solutions: A molecular dynamics analysis

“…The changes in the positions and widths of the two vibrational bands in the regions of 180 cm À1 and 50 cm À1 provide a basis for the quantitative analysis of the influence of the solute. The same approach was applied to the analysis of the low-frequency response obtained by the optical Kerr effect [8][9][10].…”