Observation of H₂O and D₂O water dimers in CCl₄ at room temperature via Matrix Isolation Spectroscopy

The vibrational spectrum of a single molecule is typically investigated in the gas phase, however, the spectrum is crowded by combination bands with rotational transitions and not all molecules can be prepared in the gas phase. Room-temperature matrix isolation spectroscopy (MIS) is a tool that utilizes a low solubility of the solute molecule as well as weak interactions between solute and solvent molecules to obtain a spectrum of isolated solute molecules. Water (H₂O and D₂O) was loaded on top of the nonpolar solvent CCl₄, and the in situ inclusion process was monitored in the CCl₄ phase by infrared absorption spectroscopy. These in situ spectra showed non-linear deviations from the monomer spectrum over time attributed to formation of water dimers. The water dimer spectrum was extracted from the time-series of the spectra using 2D correlation spectroscopy and principal component analysis (PCA). These dimer peaks from the 2D correlation and PCA analysis compared to those obtained from DFT calculations using the CC-VSCF approximation showed good agreement and allowed the assignment of the dimer modes. Changes from the monomer to the dimer spectra as well as the difference between H₂O and D₂O dimer spectra were discussed in terms of hydrogen bonding.