Solvent-Induced Infrared Frequency Shifts in Aromatic Nitriles Are Quantitatively Described by the Vibrational Stark Effect

Abstract

The physical properties of solvents strongly affect the spectra of dissolved solutes, and this phenomenon can be exploited to gain insight into the solvent-solute interaction. The large solvatochromic shifts observed for many dye molecules in polar solvents are due to variations in the solvent reaction field, and these shifts are widely used to estimate the change in the dye’s dipole moment upon photoexcitation, which is typically on the order of ~1-10 Debye. In contrast, the change in dipole moment for vibrational transitions is approximately two orders of magnitude smaller. Nonetheless, vibrational chromophores display significant solvatochromism, and the relative contributions of specific chemical interactions and electrostatic interactions are debated, complicating the interpretation of vibrational frequency shifts in complex systems such as proteins. Here we present a series of substituted benzonitriles that display widely varying degrees of vibrational solvatochromism. In most cases, this variation can be quantitatively described by the experimentally determined Stark tuning rate, coupled with a simple Onsager-like model of solvation, reinforcing the view that vibrational frequency shifts are largely caused by electrostatic interactions. In addition, we discuss specific cases where continuum solvation models fail to predict solvatochromic shifts, revealing the necessity for more advanced theoretical models that capture local aspects of solute-solvent interactions.

Detection of electron tunneling across plasmonic nanoparticle–film junctions using nitrile vibrations

“…Assuming Stark tuning rate (Δ μ CN ) to be 0.61 cm −1 /(MV/cm) for aromatic nitrile group 63 and a linear Stark shift relation Δ ν = −Δ μ CN · Δ E , we can calculate the strength of local electric fields: −41.0 MV/cm and +41.0 MV/cm from the blue (Δν=+25cm −1 ) and red (Δν=-25 cm −1 ) shifted nitrile frequencies, respectively. The observed shifts are larger than those induced by solvent effects (<15 cm −1 red shift) 34 and ion-pairing environments (<16 cm −1 red shift) 37 , indicating larger local fields within these NPoM gaps. Compared with the ensemble measurement, the nitrile peaks in single isolated NPoMs are more asymmetric (Fig.…”

“…The VSE occurs when the local electric field perturbs the electronic environment of a chemical bond and results in a change in its vibrational energy, which can be directly measured with infrared or Raman spectroscopy. 33 Due to these advantages, the VSE has been used to quantify local electric fields in a diverse range of chemical environments including organic solvents 34, 35 , ionic liquids 36, 37 , catalysts 31, 38 , proteins 39, 40 and biomembranes 41 .…”

Site-Specific Infrared Probes of Proteins

“…Next, we describe examples showing how the C≡N stretching vibration can be used to interrogate the local electric field environment of proteins and, in particular, the role of electrostatics in determining the catalytic activity of enzymes (83–85). In one study, Herschlag and coworkers (86) used Vibrational Stark Effect (VSE) spectroscopy with the SCN probe, in conjunction with NMR measurements, to characterize how modulating the charge of a bound ligand (i.e., phenol) affects the local electrostatic environment in the active site of the bacterial enzyme ketosteroid isomerase.…”

Electrostatics of liquid interfaces

“…20,30,31 We illustrate the application of the sum rules in Eq. (36) to numerical simulations of cavities in dipolar fluids. 17 We consider polarization of the dielectric by an ion with charge q placed at the center of the cavity of radius a. Dielectric models suggest that the product of the surface charge density with the surface area S = 4πa 2 remains constant…”