Imidazole and 1-Methylimidazole Hydrogen Bonding and Nonhydrogen Bonding Liquid Dynamics: Ultrafast IR Experiments

• Published in: The journal of physical chemistry. B Vol. 123; no. 9
• Main Authors: Shin, Jae Yoon, Wang, Yong-Lei, Yamada, Steven A., Hung, Samantha T., Fayer, Michael D.
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 06.02.2019
• Subjects: Diffusion; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Liquids; MATERIALS SCIENCE; Molecules; Noncovalent interactions; SOLAR ENERGY; Solvents
• ISSN: 1520-6106; 1520-5207

Dynamics of imidazole (IM) and 1-methylimidazole (1-MeIM) in the liquid phase at 95 °C were studied by IR polarization selective pump-probe (PSPP) and two-dimensional IR (2D IR) spectroscopies. The two molecules are very similar structurally except that IM can be simultaneously a hydrogen bond donor and acceptor and therefore forms extended hydrogen bonded networks. The broader IR absorption spectrum and shorter vibrational lifetime of the vibrational probe, selenocyanate anion (SeCN–), in IM vs. 1-MeIM indicate that stronger hydrogen bonding exists between SeCN– and IM. Molecular dynamics (MD) simulations support the strong hydrogen bond formation between SeCN– and IM via the HN moiety. SeCN– makes two H-bonds with IM; it is inserted in the IM H-bonded chains rather than being a chain terminator. The strong hydrogen bonding influenced the reorientation dynamics of SeCN– in IM, leading to more restricted short time angular sampling (wobbling-in-a-cone). The complete orientational diffusion time in IM is 1.7 times slower than in 1-MeIM, but the slowdown is less than expected, considering the 3-fold larger viscosity of IM. The jump reorientation mechanism accounts for the anomalously fast orientational relaxation in IM, and the MD simulations determined the average jump angle of the probe between hydrogen bonding sites. Spectral diffusion time constants obtained from the 2D IR experiments are only modestly slower in IM than in 1-MeIM in spite of the significant increase in viscosity. Here, the results indicate that the spectral diffusion sensed by the SeCN– has IM hydrogen bond dynamics contributions not present in 1-MeIM.