Toward Fully Unsupervised Anharmonic Computations Complementing Experiment for Robust and Reliable Assignment and Interpretation of IR and VCD Spectra from Mid-IR to NIR. The Case of 2,3−Butanediol and Trans-1,2-Cyclohexanediol

The IR and VCD spectra of 2,3-butanediol and trans-1,2-cyclohexanediol from 900 to 7500 cm−1 (including mid-IR, fundamental CH and OH stretchings, and NIR regions) have been investigated by a combined experimental and computational strategy. The computational approach is rooted in DFT computations of harmonic and leading anharmonic mechanical, electrical and magnetic contributions followed by a generalized second-order perturbative (GVPT2) evaluation of frequencies and intensities for all the above regions without introducing any ad hoc scaling factor. After proper characterization of large amplitude motions (LAMs) all resonances plaguing frequencies and intensities are taken into proper account. Comparison of experimental and simulated spectra allows unbiased assignment and interpretation of the most interesting features. The reliability of the GVPT2 approach for OH stretching fundamentals and overtones is confirmed by the remarkable agreement with a local mode model purposely tailored for the latter two...