Influence of Internal Bond Rotation on Ultrafast IR Anisotropy Measurements and the Internal Rotational Potential

Abstract

Measurement of molecular orientation relaxation using ultrafast infrared (IR) pump−probe experiments is widely used to understand the properties of liquids and other systems. In the simplest situation, the anisotropy decay is a single exponential reflecting diffusive orientational relaxation. However, the anisotropy decay is frequently biexponential. The faster component is caused by solvent caging restricting angular sampling until constraint release permits all angles to be sampled. Here, we describe another mechanism that limits the range of sampling, i.e., sampling of a restricted range of angles via internal bond reorientation on a rotational potential surface with barriers. If the internal angular sampling occurs faster than the entire molecule's diffusive orientational relaxation, it will produce a fast component of anisotropy decay with a cone angle determined by the shape of the internal rotation potential. We studied four molecules to illustrate the effects of internal bond rotations on anisotropy decay. The molecules are p-chlorobenzonitrile, phenylselenocyanate, phenylthiocyanate, and 2-nitrophenylselenocyante in the solvent N,Ndimethylformamide. The CN stretch is used as the IR chromophore. p-Chlorobenzonitrile does not have internal rotation; its anisotropy decays as a single exponential. The other three have bent geometries and internal rotation of the moieties containing the CN occurs; the anisotropies decay as biexponentials. The faster of the two decays can be understood in terms of motions on the rotational potential surface. A method is developed for extracting the intramolecular rotational potential surface by employing a modification of the harmonic cone model, and the results are compared to density functional theory calculations.

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“…The extent of the anisotropy provides a measurement of the angular range sampled by the probes. When the probe molecules are free to rotate, e.g., in a bulk liquid, all angles can be sampled, and the anisotropy decays from the theoretical maximum of 0.4 to 0 . However, when there are physical restrictions that block complete angular sampling, i.e., when the probe molecules are confined to FVEs, the anisotropy will decay to a nonzero offset because only a limited range of angles can be sampled.…”