A theoretical study of the chiroptical properties of molecules with isotopically engendered chirality

Abstract

There has been a considerable interest in the chiroptical properties of molecules whose chirality is exclusively due to an isotopic substitution and numerous examples for the electronic circular dichroism (CD) spectra of isotopically chiral systems have been reported in literature. Four different explanations have been proposed for the mechanism as to how the isotopic substitution induces a chiral perturbation of the otherwise achiral electronic wave function; however, up to now no conclusive answer has been given about the dominating effect responsible for the experimental observations. In this study we will present, for the first time, fully quantum-mechanical calculations of the CD spectra of three different molecular systems with isotopically engendered chirality. As examples, we consider the spectra of organic molecules with ketone and alpha-diketone carbonyl and diene chromophores. The effect of vibronic couplings for the reorientation of the electric and magnetic transition dipole moments is taken into account within the Herzberg-Teller approximation. The ground and excited state geometries and vibrational normal modes are obtained with (time-dependent) density functional theory [(TD)DFT], while the vibronic coupling effects are calculated at the TDDFT and density functional theory/multireference configuration interaction (DFT/MRCI) levels of theory. Generally, the band shapes of the experimental CD spectra are reproduced very well, and also the absolute CD intensities from the simulations are of the right order of magnitude. The sign and the intensity of the CD band are determined by a delicate balance of the contributions of a large number of individual vibronic transitions, and it is found that the vibrational normal modes with a large displacement are dominant. The separation of the calculated CD spectrum into the different contributions due to the overlap of the in-plane and out-of-plane components (regarding the symmetry plane of the unsubstituted molecule) of the electric and magnetic transition dipole moments yields information about the influence of the vibronic coupling effects for the reorientation of the corresponding transition dipole moments. In conclusion, the calculations clearly show that vibronic effects are responsible or at least dominant for the chiroptical properties of isotopically chiral organic molecules.

The prediction of the circular dichroism of the benzene chromophore: TDDFT calculations and sector rules

“…For each compound, the two average spectra are almost coincident in the 1 L a region, which is at odds with experimental results at different temperatures; most probably the effective conformational ensemble populated at room temperature is not adequately represented by the 3-5 minima considered in the calculations. On the other hand, calculated averages of 1 L b bands are strongly affected by temperature; in this region, however, appropriate inclusion of vibronic effects 26,27 would be obviously needed for a correct prediction.…”

“…28 As basis sets, we used the triple-f basis set with polarization functions TZVP, the double-f basis set with polarization and diffuse functions ADZP, 29 and the correlation consistent triple-f basis set with polarization and diffuse functions aug-cc-pVTZ. 28 As anticipated above, the vibronic coupling 26,27 was not considered. All the above functionals were able to reproduce the CD spectrum of 4 in terms of sign and relative intensity of bands (Fig.…”

“…24 Here we report the CD spectra of the smaller homologues 25 (R)-1-3 (see Chart) based on a similar chiral pattern, i.e., the a-phenylethyl with an ethyl (1), n-propyl (2), isopropyl (3), and tert-butyl (4) group attached at the chiral center; we then check the consistency of 1 L a and 1 L b Cotton effects measured for 1-3 with the relative sector rules. Second, we test the performance of TDDFT method (without including vibronic structure calculations) 26,27 in the prediction of CD spectra of compounds 1-4.…”

Excited States from Time‐Dependent Density Functional Theory

“…113 Several studies include solvent effects, [114][115][116][117][118][119][120][121][122] one example being the behavior of metal ions in explicit water. 123 TDDFT in the realm of linear response can also be used to examine chirality, [124][125][126][127] including calculating both electric and magnetic circular dichroism, 26,[128][129][130][131][132] and it has been applied to both helical aromatics 133 and to artemisinin complexes in solution. 134 There exist applications in materials 135,136 and quantum dots, 137 but, as discussed below, the optical response of bulk solids requires some nonlocal approximations.…”

Electronic Circular Dichroism

“…However, specific interactions such as hydrogen bonds may affect overall ECD spectra, for example, of amino acids and oligopeptides to a large extent and imply the inclusion of explicit solvent molecules [52,53], although their impact may be minimal for less polar compounds [54]. Whenever possible and desired, vibronic coupling [55][56][57] and/or solvent models [58] (see also Chapter 5) may be introduced in ECD calculations although they will considerably increase the computational time.…”