Center for Relativistic Laser Science

ALTA 2015

Summary with reference:

Solvatochromism of small vibrational infrared (IR) probes have been widely used to probe local molecular environments when either dissolved in bulk liquid or inserted in biomolecules and bioorganic materials. One of the most direct and easy-to-measure observables related with probe-environent interaction is the vibrational frequency change with respect to reference state upon perturbation in the environment. [1] Therefore, understanding the underlying mechanisms of frequency shifts is absolutely crucial to interpret changes of experimental spectra. Since some correlations of vibrational frequencies and electrostatic potential or field were generally observed for many vibrationally localized oscillators, the great majority of theoretical and experimental studies was focused on electrostatic interaction between IR probe and its environment, concluding that this is the dominant factor governing vibrational solvatochromism. In effect, the probes like carbonyl (CO), nitrile (CN), thiocyanate (SCN) and azide (N3) functional groups have been extensively used to probe electrostatic potentials and fields based on vibrational frequency shifts. [1]

However, vibrational solvatochromism was shown to arise from the change in solute-solvent intermolecular interaction potential along the vibrational coordinates. [2-3] It is well known that electrostatics is not the only component responsible for intermolecular interactions. Therefore, a variety of effects contribute to vibrational frequency shifts.

To address this problem, we have been developing a first-principles theory of vibrational solvatochromism [4-6] that is based on coarse-grained models.[3] Our theory describes vibrational frequency shift as a sum of five terms originating from the five fundamental types of interactions between molecules that can be found in nature: Coulomb electrostatics, induction, dispersion, exchange-repulsion and charge-transfer. Our models were applied for amide I mode in N-methylacetamide (NMA) and also CN stretch mode in MeSCN that sense the local molecular environments (from bulk aprotic solutions through heterogeneous H-bonding environments). From our results it is clear that almost all of the above mentioned effects (except charge-transfer) are very important and non-negligible. However, in some cases very good correlations of absorption frequencies with electric fields can still be observed because of the relative strength and substantial directionality of Coulomb frequency shift component [6]. Nevertheless, we already detected important cases when electrostatic models fail because of short-range exchange-repulsion and dispersion interactions. In particular, significant blue shifts in CN stretch mode due to H-bonding were found.

References:
[1] H. Kim et al., Chem. Rev. 113, 5817 (2013)
[2] Cho, M., J. Chem. Phys. 130, 094505 (2009)
[3] Cho, M., J. Chem. Phys. 118, 3480 (2003)