1,4-Dimethylenespiropentane: A Unique Model System for Studying Fermi Resonance in Raman Optical Activity

The C2‐symmetric title compound, a small, rigid hydrocarbon molecule with two distinct, strongly interacting vibrational chromophores, represents a unique model system for testing computational approaches to Raman optical activity (ROA) beyond the isolated molecule and the harmonic approximation. We show that the experimental Raman and ROA spectra are marked by the presence of strong Fermi resonances, and that the shape and relative size of the bands which we attribute to such resonances strongly depend on the solvent environment of the molecule. On the other hand, the dominant computed ROA couplet, arising from the CC stretching vibrations, is absent in the spectra measured in the condensed phase. The strong dependence of the computed size of the couplet on the description of the diffuse part of the electron distribution suggests that this absence is due to nonspecific interactions in the condensed phase. Compare and contrast: Experimental Δ curves give the ratio of Raman optical activity to Raman scattering. Curves measured for backscattering for neat (−)‐(M)‐ (see picture, red) and (+)‐(P)‐1,4‐dimethylenespiropentane (black) on two different instruments are essentially mirror images of each other, which demonstrates the high instrumental precision of the measurements and the similar enantiomeric purity of the samples.