Nitrobenzene anti-parallel dimer formation in non-polar solvents

Abstract

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 °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; gK ∼ 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 ∼7 and ∼50 ps in a concentrated regime (∼15 and ∼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, 2NO2-Bz ↔ (NO2-Bz)2, due to a strong dipole-dipole interaction between nitro groups.

Orientation correlation and local field in liquid nitrobenzene

“…The high frequency dielectric constant ε ∞ entering the analysis can be estimated by extrapolating the optical refractive index (ε ∞ = n 2 = 2.3 at 20 • C), 43 by extrapolating dielectric response measurements for ν < 14 GHz (ε ∞ = 3.3 at 21.4 • C), 63 or by fitting dielectric response measurements for ν < 3 THz (ε ∞a = 3.50 at 20 • C). 61 This last experiment reaches the high frequency limit for all orientation relaxation modes, and ε ∞a includes the directly measured 29 cm −1 librational contribution in addition to the vibrational and electronic polarizability contributions. In that work, 61 the value µ = 4.00 D for NB was obtained from measurements of the dielectric increment for dilute solutions of NB in the nonpolar solvents CCl 4 and benzene, and then the value g K = 0.65 for neat NB was obtained using Eq.…”

“…61 This last experiment reaches the high frequency limit for all orientation relaxation modes, and ε ∞a includes the directly measured 29 cm −1 librational contribution in addition to the vibrational and electronic polarizability contributions. In that work, 61 the value µ = 4.00 D for NB was obtained from measurements of the dielectric increment for dilute solutions of NB in the nonpolar solvents CCl 4 and benzene, and then the value g K = 0.65 for neat NB was obtained using Eq. (1) assuming that µ for NB is the same in the pure liquid and in dilute solution.…”

“…(1) with ρ, µ, and, g K from the MD simulation combined with ε ∞ = 3.50 from dielectric measurements. 61 The results of these calculations are compared with experimental measurements in Table IV. Good agreement with experiment for density ρ, dielectric constant ε s , and correlation time τ 1 is obtained for the CGenFF model, especially using correlation function expressions for g K and ε. This agreement indicates that orientation correlation is determined by short-range forces and the permanent dipole moment, and is insensitive to the molecular polarizability.…”

Квазимакроскопические Приграничные Структуры В Непростых " Жидкостях: Эксперимент И Модель

“…3 в [26], а также рис. 5 в [27]). Такие димеры могут объединяться в нитевидные структуры -олигомеры (рис.…”

“…1 -не полярны. Важно подчеркнуть, что существование таких димеров как устойчивых конструкций зафиксировано в экспериментах с нитробензолом (см., например, [27,28]).…”