Fenchone, Camphor, 2-Methylenefenchone and 2-Methylenecamphor:  A Vibrational Circular Dichroism Study

Abstract

We report and discuss the infrared (IR) vibrational circular dichroism (VCD) spectra of the enantiomeric pairs of the olefin derivatives of fenchone (1,3,3-trimethyl-2-methylenebicyclo[2.2.1]heptane) and camphor (1,7,7-trimethyl-2-methylenebicyclo[2.2.1]heptane), respectively, together with those of the parent molecules. The VCD spectra were taken in three spectral regions: the mid-IR region, encompassing the fundamental deformation modes, the region of CH-stretching fundamental modes and the NIR-region between 1100 and 1300 nm, which corresponds to the second CH-stretching overtone. The VCD and absorption spectra in the first two regions are analyzed by use of current density functional theory (DFT) calculations. The NIR region is analyzed by a protocol that consists of the use of DFT-based calculations and in assuming local mode behavior: the local mode approach is found appropriate for interpreting the absorption spectra and, for the moment, acceptable for calculating NIR-VCD spectra. The analysis of the first region allows us to track the contribution of the C=O group in the vibrational optical activity of C-C stretching modes; notable differences are indeed found in olefins and ketones. On the contrary, in the other two regions the VCD spectra of olefins and ketones are more similar: in the normal mode region of CH stretching fundamentals the spectra are determined by the mutual orientation of the CH bonds; in the second overtone local mode region olefins and ketones signals show some differences.

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

“…This is intuitively acceptable, since stretching CH bonds give rise to larger variation of polarity on the CH bonds and thus to larger z components in the electric dipole moment, while the same charge rearrangement generates larger magnetic dipole moment perpendicular to the CH bond. Since x and y components of AAT are comparatively large, also x and y components of APT should not be neglected, even if small, since the dot product of APT and AAT is needed in calculating rotational strengths, unlike what we had done earlier [2][3][4]. To make our considerations more quantitative, we give in Table 1 the values of APT and AAT at z = 0 and their first derivatives with respect to z still at z = 0.…”

“…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.…”

“…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.…”