Nitrobenzene anti-parallel dimer formation in non-polar solvents

作者：Shikata Toshiyuki^*; Sakai Yuji; Watanabe Junji

来源：AIP Advances, 2014, 4(6): 067130.

DOI：10.1063/1.4884393

We investigated the dielectric and depolarized Rayleigh scattering behaviors of nitrobenzene (NO2-Bz), which is a benzene mono-substituted with a planar molecular frame bearing the large electric dipole moment 4.0 D, in non-polar solvents solutions, such as tetrachloromethane and benzene, at up to 3 THz for the dielectric measurements and 8 THz for the scattering experiments at 20 degrees C. The dielectric relaxation strength of the system was substantially smaller than the proportionality to the concentration in a concentrated regime and showed a Kirkwood correlation factor markedly lower than unity; g(K) similar to 0.65. This observation revealed that NO2-Bz has a tendency to form dimers, (NO2-Bz)(2), in anti-parallel configurations for the dipole moment with increasing concentration of the two solvents. Both the dielectric and scattering data exhibited fast and slow Debye-type relaxation modes with the characteristic time constants similar to 7 and similar to 50 ps in a concentrated regime (similar to 15 and similar to 30 ps in a dilute regime), respectively. The fast mode was simply attributed to the rotational motion of the (monomeric) NO2-Bz. However, the magnitude of the slow mode was proportional to the square of the concentration in the dilute regime; thus, the mode was assigned to the anti-parallel dimer, (NO2-Bz)(2), dissociation process, and the slow relaxation time was attributed to the anti-parallel dimer lifetime. The concentration dependencies of both the dielectric and scattering data show that the NO2-Bz molecular processes are controlled through a chemical equilibrium between monomers and anti-parallel dimers, 2NO(2)-Bz. (NO2-Bz)(2), due to a strong dipole-dipole interaction between nitro groups.

• 出版日期2014-6