Electrical and mechanical anharmonicities from NIR‐VCD spectra of compounds exhibiting axial and planar chirality: The cases of (S)‐2,3‐pentadiene and methyl‐d₃ (R)‐ and (S)‐[2.2]paracyclophane‐4‐carboxylate

Abstract

The IR and Near infrared (NIR) vibrational circular dichroism (VCD) spectra of molecules endowed with noncentral chirality have been investigated. Data for fundamental, first, and second overtone regions of (S)-2,3-pentadiene, exhibiting axial chirality, and methyl-d 3 (R)-and (S)-[2.2]paracyclophane-4-carboxylate, exhibiting planar chirality have been measured and analyzed. The analysis of NIR and IR VCD spectra was based on the local-mode model and the use of density functional theory (DFT), providing mechanical and electrical anharmonic terms for all CH-bonds. The comparison of experimental and calculated spectra is satisfactory and allows one to monitor fine details in the asymmetric charge distribution in the molecules: these details consist in the harmonic frequencies, in the principal anharmonicity constants, in both the atomic polar and axial tensors and in their first and second derivatives with respect to the CH-stretching coordinates. Chirality 23:841-849, 2011. V

Chiroptical Signatures of Planar and Central Chirality in [2]Paracyclo[2](5,8)quinolinophane Derivatives

“…A compariwww.eurjoc.org son of the experimental VCD spectra of the two diastereomers (R p ,R)-4 and (R p ,S)-4 (X = iPr) with the corresponding calculated VCD spectra is shown in Figure 4, and the experimental VCD spectra of the two diastereomers (S p ,R)-5 and (S p ,S)-5 (X = tBu) are compared with the corresponding calculated VCD spectra in Figure 5. In Figure 6 and 7 the experimental VCD spectra of the two dia- Our previous studies on paracyclophanes [30,32,33] showed that vibrational features associated with the quinolinophane moiety should be expected above 1500 cm -1 , whereas the region below 1500 cm -1 is more affected by the aliphatic why quinolinophane is so unusual. Related to each band there are several vibrational transitions, in particular, the band at approximately 1590 cm -1 contains three modes, whereas the bands at approximately 1560 and 1500 cm -1 contain two modes each.…”

“…Our previous studies on paracyclophanes [30,32,33] showed that vibrational features associated with the quinolinophane moiety should be expected above 1500 cm -1 , whereas the region below 1500 cm -1 is more affected by the aliphatic substituents. We found that the quinolinophane moiety provides an unambiguous signature of the planar chirality, and this was common to all thirteen compounds 1-8; the spectrum consists of a triplet of VCD bands at approximately 1590, 1560, and 1500 cm -1 with (+,+,-) signs for (R p ) and (-,-,+) signs for (S p ).…”

“…However, precise calculations of the VCD spectra taking into account anharmonicity effects, mostly Fermi resonance, should be effected. [30,32,33] Such calculations could be achieved by using updated DFT technology, [34][35][36] but the topic is beyond the scope of the present work (for more information, we refer to Figure SI Further information on this matter may be gained by analyzing the VCD spectra in the mid-IR region (IR data, together with calculated IR and VCD spectra of relevant conformers are shown in Figures SI-2-13 of the Supporting Information). The comparison of experimental (top) and calculated (lower) VCD spectra of 1 (left) and 2 (right), exhibiting opposite planar configuration is shown in Figure 2.…”

“…However, precise calculations of the VCD spectra taking into account anharmonicity effects, mostly Fermi resonance, should be effected. [30,32,33] Such calculations could be achieved by using updated DFT technology, [34][35][36] but the topic is beyond the scope of the present work (for more information, we refer to Figure SI-1 of the Supporting Information, showing the IR spectra in the C-H stretching region of all compounds in Scheme 1).…”

“…alize, [28] and they are substantially influenced by vibronic contributions. [29] VCD has already been shown to play an essential role when two chirality centers or two different sources of chirality, such as planar and central chirality, [30,31] are simultaneously present in the same molecule; in this case, one form of chirality might obscure the other when the survey is based on ECD, as occurs in (R p ,R)and (R p ,S)-1-([2.2]paracyclophan-4-yl)-2,2,2-trifluoroethanol, which exhibits both planar and central chirality. [31] In such a situation, the use of VCD proved decisive to determine the absolute configuration of the side chain stereogenic carbon, as was subsequently confirmed by X-ray scattering studies; most compounds depicted in Scheme 1 present similar problems.…”