Molecular Motion in Liquids: Rotational and Vibrational Relaxation in Highly Polar and Strongly Associated Systems

The characteristics of the rotational‐vibrational relaxation of acetonitrile, benzonitrile, 2‐propanol, and p‐dioxane were observed by determining the correlation functions of several infrared and Raman rotation‐vibration bands of these liquids. Benzonitrile and 2‐propanol were also investigated as dilute solutions in CCl₄ and in CS₂, whereas p‐dioxane was also observed in solution with a variety of electron‐donating, electron‐accepting (among them chloroform, water), and nonpolar molecules. The results show that the intermolecular forces need about ${\text{0.1 × 10}}^{\text{−12}}{\text{–0.2 ×}}^{\text{−12}}\hspace{0.17em}\sec$ to cause an observable effect on the molecular motion. Regardless of the polarity or association of the liquids or solutions investigated, rotational relaxation is the predominant mode of decay for times less than ${\text{0.5 × 10}}^{\text{−12}}\hspace{0.17em}\sec$ after the onset of observation: The $\mathrm{C}\equiv \mathrm{N}$ stretch of benzonitrile and the investigated modes of p‐dioxane are, probably, exceptions. It is also shown that the vibrational correlation functions do not decay purely exponentially but have Gaussian‐Lorentzian shape‐as the rotational correlation functions. As a consequence, vibrational lifetimes cannot predict how the initial rotational decay is affected by fast vibrational energy transfer. The results show that rotation of acetonitrile is strongly hindered in those motions which tend to tilt the permanent dipole moment axis but that the molecules undergo rotational jumps by about 0.5–1.2 rad around this axis. Rotational motion of benzonitrile is strongly hindered in all directions, but fast vibrational decay makes interpretation difficult. The long‐time decay (beyond ${\text{0.8 × 10}}^{\text{−12}}\hspace{0.17em}\sec$ ) of the $\mathrm{C}–\mathrm{C}$ stretch of 2‐propanol is mainly governed by vibrational decay; for shorter times, rotational decay is predominant. Molecular relaxation in neat p‐dioxane and its solutions seems to be influenced mainly by vibrational relaxation.