Abstract
Over the past two decades, the vibrational Stark effect has become an important tool to measure and analyze the in situ electric field strength in various chemical environments with infrared spectroscopy. The underlying assumption of this effect is that the normal stretching mode of a target bond such as CO or CN of a reporter molecule (termed vibrational Stark effect probe) is localized and free from mass-coupling from other internal coordinates, so that its frequency shift directly reflects the influence of the vicinal electric field. However, the validity of this essential assumption has never been assessed. Given the fact that normal modes are generally delocalized because of mass-coupling, this analysis was overdue. Therefore, we carried out a comprehensive evaluation of 68 vibrational Stark effect probes and candidates to quantify the degree to which their target normal vibration of probe bond stretching is decoupled from local vibrations driven by other internal coordinates. The unique tool we used is the local mode analysis originally introduced by Konkoli and Cremer, in particular the decomposition of normal modes into local mode contributions. Based on our results, we recommend 31 polyatomic molecules with localized target bonds as ideal vibrational Stark effect probe candidates.
Highlights
The Stark effect refers to the response of a spectroscopic transition to an applied electric field.It was discovered by Stark in 1913 observing that an applied electric field causes a splitting in the absorption lines of hydrogen [1]
(i) we evaluate the performance of commonly used vibrational Stark effect (VSE) probes in comparison with a number of potential vibrational probe candidates, using the characterization of normal mode (CNM) method
These polyatomic probe molecules contain C=O, C≡N, S=O or other chemical bonds, whose stretching vibrations are considered as decoupled from other local vibrational modes. (ii) We analyze how the atomic masses influence the performance of selected VSE probes and discuss the feasibility of improving a vibrational probe by isotopic substitution. (iii) In addition, we analyze how the performance score of a VSE probe depends on the density functional used for the calculation. (iv) Practical suggestions on the ideal
Summary
The Stark effect refers to the response of a spectroscopic transition to an applied electric field. It was discovered by Stark in 1913 observing that an applied electric field causes a splitting in the absorption lines of hydrogen [1]. The vibrational Stark effect (VSE) describes the perturbation of a vibrational frequency by an electric field [2,3,5,12] according to Equation (1):. The electric field strength is in general below 100 MV/cm; the quadratic term with regard to ∆α in Equation (1) can be neglected, so that the change in the vibrational frequency ∆ν = ν − ν0 directly correlates with the change in the strength of the electric field ~F [5]
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