Quantifying Inter- and Intramolecular Interactions in Liquids with Correlated Vibrational Spectroscopy: Case Study of CCl ₄ and CH ₃ CN

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.