p give rise to non-ideality. In some cases, when

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The
structure and dynamics of molecular liquids have been of interest for
a long time. Intermolecular interactions, especially hydrogen bonding
brings about many interesting properties in self-associated liquids.
Binary mixtures of two molecular liquids will have various such
interactions that give rise to non-ideality. In some cases, when this
non-ideality is large, we observe the formation of an azeotrope. In
this study, we try to understand this non-ideality at a molecular
level, using Optical Kerr Effect (OKE) spectroscopy, broadband (1-10
THz) THz-Time domain spectroscopy (THz-TDS) along with temperature
dependent Nuclear Magnetic Resonance spectroscopy (NMR) of the
azeotropic as well as other composition mixtures of benzene and
methanol.

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With
OKE spectroscopy we can probe both, the collective orientational
diffusion and the intermolecular dynamics in liquids. From previous
OKE studies and simulation on benzene and other molecular liquids, we
can see that the Reduced Spectral Densities (RSDs) lack distinct
features and can be broadly categorized into two broad categories,
rectangular and triangular. This categorization depends mostly on the
molecular shapes and not so much on the local structural or
polarizability changes. The benzene spectra show a bimodal character.
Simulations have shown that while the high-frequency shoulder is
attributed purely to rotational motion, the low-frequency mode has
contributions from the collective translational and the rotational
modes. The addition of polar methanol to benzene not only changes the
shape of the RSD but also shifts it to lower frequencies. The bimodal
character of the RSD is less pronounced in the mixtures than in
benzene. The azeotrope and the manually mixed mixture have similar
“triangular” RSDs. We did not observe a linear trend of the RSDs
with the mole fraction of benzene. A detailed analysis of the RSDs
can indicate the rate of structural fluctuations in the
benzene-methanol liquid mixtures.

Vibrational
and NMR studies have shown that the formation of the methanol benzene
azeotrope, weakens the hydrogen bond network. Intermolecular forces
between the benzene molecules have also been reduced significantly.
Methanol disrupts the stacking of benzene which is also evident from
the depression in the boiling points.

OKE
spectroscopy along with other complementary techniques is a powerful
tool that can be used to understand the ultrafast liquid state
dynamics. The terahertz region gives us information on the vibrations
of a large number of atoms and molecules interacting with each other.
These are the collective modes which are sensitive to both
intermolecular and intramolecular perturbations in the system.
Studying these low-frequency modes can be useful, in investigating
the intermolecular dynamics in a binary liquid system.

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