Sum-over-States Calculation of the Specific Rotations of Some Substituted Oxiranes, Chloropropionitrile, Ethane, and Norbornenone

Abstract

A sum-over-states approach has been applied to the calculation of the specific rotations of several substituted oxiranes, 2-chloropropionitrile, and 30 degrees-rotated ethane. In each case, the first few excited states proved to have only a relatively small effect on the calculated specific rotation. It was necessary to use a very large number of excited states in order to achieve convergence with the results of the more direct linear response method. However, the latter does not give information on which excited states are important in determining the specific rotation. Norbornenone is unique in that its greatly enhanced specific rotation as compared to norbornanone is associated with the low-energy n-pi* transition. The C=C bond orbitals interact with the C=O in the LUMO, and a density difference plot for going from the ground state to the first excited state clearly shows the perturbation of the C=C.

Time‐Dependent Density Functional Theory for Calculating Origin‐Independent Optical Rotation and Rotatory Strength Tensors

“…This finding is in qualitative agreement with an SOS analysis by Wiberg et al, [43] who showed with B3LYP/ aug-cc-pVDZ computations that the (partial) isotropic OR resulting from just the first excitation is more than 40 % larger in magnitude than the converged full linear response value. That is, higher excitations dampen the isotropic OR, and this dampening effect must also influence the characteristics of the OR tensor.…”

“…The (perhaps fortuitously) good performance of B3LYP/aug-cc-pVDZ calculations for this molecule has been noted previously. [43,44] Wiberg et al [43] pointed out that much of the large magnitude of the optical rotation of 2 can be attributed to the intense CD of the longestwavelength excitation and the coupling of the C=O and the C=C chromophores. See also a discussion by Moscowitz from 1962.…”

Absolute Configuration Assignment from Optical Rotation Data by Means of Biphenyl Chiroptical Probes

“…This is obviously impossible and practical reasons dictate that only a finite number of electronic transitions can be used. In fact, it has been observed that the low‐lying valence and Rydberg transitions usually are not of major importance for evaluation of OR in the non‐resonant long wavelength region and the use of the first 25–30 transitions is generally sufficient . The computation on the first 30 excited states also allows us to take into account the contribution of transitions occurring below 200 nm, not experimentally visible because of the solvent cut‐off.…”

Toward a general mixed quantum/classical method for the calculation of the vibronic ECD of a flexible dye molecule with different stable conformers: Revisiting the case of 2,2,2‐trifluoro‐anthrylethanol

“…Before concluding this section, we notice that the method presented here shares some analogies with what proposed in the case of hindered rotations to obtain vibrational averages of molecular properties like the optical rotatory dispersion, which is connected to ECD by the Kramers‐Kronig relation . In Mort and Autschbach, however, only the soft mode is considered, while the other vibrational modes are not explicitly included in the calculation, being not important for a Boltzmann average.…”