Abstract
The infrared (IR) and vibrational circular dichroism (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 near-infrared regions) have been investigated by a combined experimental and computational strategy. The computational approach is rooted in density functional theory (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, 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 regions. Together with the specific interest of the studied molecules, our results confirm that an unbiased assignment and interpretation of vibrational spectra for flexible medium-size molecules can be achieved by means of a nearly unsupervised reliable, robust, and user-friendly DFT/GVPT2 model.
Highlights
An accurate description of the nuclear dynamics of bound states in molecular systems is pivotal for the reproduction of molecular spectra, especially in the infrared (IR) region of the electromagnetic spectrum
Despite some remarkable achievements, the development of efficient yet accurate methodologies to go beyond the harmonic approximation is still a challenging issue when one or more among the following features must be taken into account: (i) high number of vibrational degrees of freedoms; (ii) presence of large-amplitude motions (LAMs); and (iii) presence of many symmetry elements
IR, NIR, vibrational circular dichroism (VCD), and NIR− VCD spectra have been recorded and computed; the anharmonic effects in the calculation of experimental observables have been included by means of two different methodologies: the generalized vibrational second-order level of perturbation theory (GVPT2) approach implemented in the Gaussian[14] suite of programs by two of the present authors[15,16]
Summary
An accurate description of the nuclear dynamics of bound states in molecular systems is pivotal for the reproduction of molecular spectra, especially in the infrared (IR) region of the electromagnetic spectrum. The two enantiomeric forms of 2,3-butanediol were characterized by means of several experimental techniques:[20−27] IR (in solution[21,22,25] and in the gas phase23), VCD,[21,22] photoelectron circular dichroism,[26] and microwave[24] spectra have been published.
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