Harmonic and Anharmonic Features of IR and NIR Absorption and VCD Spectra of Chiral 4-X-[2.2]Paracyclophanes

Abstract

The vibrational absorption spectra and vibrational circular dichroism (VCD) spectra of both enantiomers of 4-X-[2.2]paracyclophanes (X = COOCD3, Cl, I) have been recorded for a few regions in the range of 900-12000 cm(-1). The analysis of the VCD spectra for the two IR regions, 900-1600 cm(-1) and 2800-3200 cm(-1), is conducted by comparing with DFT calculations of the corresponding spectra; the latter region reveals common motifs of vibrational modes for the three molecules for aliphatic CH stretching fundamentals, whereas in the mid-IR region, one is able to identify specific signatures arising from the substituent groups X. In the CH stretching region between 2900 and 2800 cm(-1), we identify and interpret a group of three IR VCD bands due to HCH bending overtone transitions in Fermi resonance with CH stretching fundamental transitions. The analysis of the NIR region between approximately 8000 and approximately 9000 cm(-1) for X = COOCD3 reveals important features of the aromatic CH stretching overtones that are of value since the aromatic CH stretching fundamentals are almost silent. The intensifying of such overtones is attributed to electrical anharmonicity terms, which are evaluated here by ab initio methods and compared with literature data.

Calculations of overtone NIR and NIR-VCD spectra in the local mode approximation: Camphor and Camphorquinone

“…In addition, in order to implement the above-described computational method, frequencies and matrix elements must be determined: we assume as mechanical harmonic frequencies, v 0n , those obtained by a standard harmonic calculation with selective deuteration [2][3][4] and the correct values for overtones are calculated by including the anharmonicity parameter x n in the expression of vibrational energy eigenvalues…”

“…As previously done [2][3][4], the same value x n = 65 cm À1 , taken from the analysis of experimental overtone absorption spectra, is assumed for every local mode n (i.e. for every CH group) and is applied to both (1S)-Camphor and (1S)-Camphorquinone.…”

“…For this reason in Refs. [2][3][4] we simulated the NIR and NIR-VCD spectra at Dv = 3 of Camphor and Camphor-related molecules or of 4-X- [2.2]paracyclophanes in the following way. We first calculated frequencies, dipole strengths and rotational strengths for all Dv = 1 transitions associated with isolated CH stretching modes (the rest of the CH bonds being deuterated) in the harmonic approximation, as implemented in GAUSSIAN03 [16] at the DFT level.…”

“…With that approach we could reproduce fairly well the general aspect of NIR absorption and NIR-VCD experi-mental spectra with the y-axis of the calculated spectra in arbitrary units. In the case of 4-X-[2.2]paracyclophanes [4] (see also Ref. [3]), we explicitly calculated transition moments for the Morse wavefunctions, and could provide absolute values also for the y-axis.…”

“…Besides, in Ref. [4] we have evaluated some anharmonic terms in the APTs (atomic polar tensors) and thereby have significantly improved the calculations of NIR-VCD and NIR absorption spectra; however we did not consider there anharmonic corrections to the magnetic dipole moment terms (we notice, en passant, that the idea of using Morse local modes for interpreting NIR-VCD spectra had been suggested by Polavarapu [17] and is discussed also in Ref. [18]).…”