# Quantifying Inter- and Intramolecular Interactions in Liquids with Correlated Vibrational Spectroscopy: Case Study of CCl4 and CH3CN

## TL;DR

This paper uses a new spectroscopy method to study molecular interactions in liquids, revealing how molecules in CCl4 and CH3CN behave and arrange.

## Contribution

The paper advances correlated vibrational spectroscopy and applies it to quantify inter- and intramolecular interactions in liquid systems.

## Key findings

- CCl4 shows no intermolecular coupling, indicating short-range and isotropic interactions.
- CH3CN exhibits intermolecular coupling with an effective average orientational angle of ~102° between adjacent dipoles.
- Fermi resonance in CCl4 reveals strong intramolecular coupling and phase relationships.

## Abstract

Correlated vibrational spectroscopy (CVS) is a hyper-Raman-based
vibrational spectroscopy that retrieves separate spectra of individual
(self-correlated, SC) and interacting (cross-correlated, CC) molecules.
The spectra are recorded in the >40 cm–1 THz/mid-IR
frequency range and contain modes that are IR and/or Raman active.
Here, we further develop CVS and apply it to investigate intra- and
intermolecular interactions using room temperature liquid carbon tetrachloride
(CCl4), a nonpolar liquid, and acetonitrile (CH3CN), a polar liquid, as case studies. CVS spectra of CCl4 display no intermolecular coupling, confirming the isotropy and
short-range nature of the molecular interactions. Strong intramolecular
coupling is observed on the Fermi resonance, and the relative phase
between the participating modes is determined based on the intensities
in the experimental spectra. CVS spectra of acetonitrile display intermolecular
coupling of the CN mode vibrations, whose cross-correlated
out-of-phase signature is evidence for near-perpendicular pair arrangements.
Performing a theoretical analysis of the CVS response, an equation
for the effective average orientational angle between CN groups
of adjacent liquid molecules is developed and solved. The effective
average orientational angle between adjacent acetonitrile dipoles
is ∼102° ± 2°, which is close to a head-to-tail
arrangement.