Testing the vibrational exciton and the local mode models on the instructive cases of dicarvone, dipinocarvone, and dimenthol vibrational circular dichroism spectra

Abstract

The vibrational circular dichroism (VCD) spectra of dicarvone (1), dipinocarvone (2), and dimenthol (3) have been recorded in the range 900–3200 cm−1, encompassing the mid‐infrared (mid‐IR), the CO stretching, and the CH‐stretching regions. For compound 3 also, the fundamental and the first overtone OH stretching regions have been investigated by IR/NIR absorption and VCD. Density functional theory (DFT) calculations allow one to interpret the IR and VCD spectra and to confirm the configuration/conformational studies previously conducted by X‐ray diffraction. The most intense VCD signals are associated with the vibrational normal modes involving symmetry‐related groups close to the CC bond connecting covalently the two molecular units. The vibrational exciton (VCDEC) model is fruitfully tested on the VCD data of compounds 1 and 2 for the spectroscopic regions at ~1700 cm−1, and the local mode model is tested on compound 3 at ~3500 and ~6500 cm−1. For compounds 1 and 2 also, ECD spectra are reported, and the exciton mechanism is tested also there, and connections to the VCDEC model are examined.

Solvent Effects and Aggregation Phenomena Studied by Vibrational Optical Activity and Molecular Dynamics: The Case of Pantolactone

“…The carbonyl region is more informative in the IR band-shape and in the presence or absence of VCD signals. The less common OH and CH stretching regions are also examined: good results are obtained for band-shape of the OH absorption bands and for CH stretching VCD and absorption features, despite the fact that anharmonicity has been disregarded in our calculations. − Difficulties are encountered in the VCD OH stretching region, where huge features, highly dependent on minimal structure variations, are calculated: further studies are needed to improve the statistics suggesting how this usually disregarded spectroscopic region is quite challenging for theoretical calculations. Also in this respect we feel that anharmonicity treatment should be undertaken in future work in order to check the influence on IR and VCD intensity.…”

Vibrational Circular Dichroism Detects Symmetry Breaking due to Conformational Mobility in C2-Symmetry Chiral Molecules and Provides Further Insight into Inter-Chromophoric Interactions

“…Thus, for the same reasons discussed above, the typical bisignate aspect of coupled C=O vibrations is absent and only the antisymmetric, lower energy mode shows an intense electric dipole allowed feature. For compound 2, however, the carbonyl stretching rotational strength is quite large [18], despite the similar geometry and the strong similarity in the normal mode pattern in the two molecules: in 2, the presence of nearby double bonds increases the rotational strength of the antisymmetric stretching. In Table 1, we report the observed IR and VCD intensities for the carbonyl stretching region; from the recorded spectra the g factor is about 2 × 10 −5 for compound 1, while it reaches 10 −4 for compound 2.…”

“…Such investigations allow us to learn more about the interaction of the two components by exploiting information obtained in different spectroscopic regions. A particular advantage provided by 1 and 2 is expected from their C2-symmetry, which, in many cases, has been shown to be responsible for quite intense VCD spectra (see for example References [13][14][15][16][17][18][19][20][21]). Two important aspects of this study are: (1) the characterization of conformers of the saturated diketone 1 for which the conformational mobility plays an important role in explaining the VCD data; and (2) analysis of the influence of the diene moiety in 2 which collectively provides a basis for understanding the intense VCD and ECD.…”

Chirality induction and amplification in supramolecular systems exhibiting vibrational optical activity

“…25 With time, various shortcomings of the simple CO model have been indicated. [26][27][28][29] Discrepancies from an ideal degenerate interacting pair of oscillators result in non-conservative couplets that are observed often in real systems. Supramolecular chirality typically brings the dominant contribution to the VOA spectra, for example, the secondary protein structures determine the shapes of VCD and ROA of proteins.…”