The Role of Anharmonicity in the HC*D Chromophore in Vibrational Circular Dichroism Spectra and Optical Rotation Data

The vibrational circular dichroism (VCD) spectra in the CD-stretching region of (R)-(-)-neopentyl-1d-halides (Cl and Br), (R)-(+)-1-exo-d₁-camphor, and (S)-(+)-1-phenylethane-1-d₁ (and (S)-(+)-1-phenylethane-1,2,2,2-d₄) are correctly interpreted by properly including anharmonicity, both mechanical and electrical, with DFT calculations. This result is noteworthy, since the spectroscopic range containing the CD-stretching transitions is always rich in features attributed to overtone and combination transitions. The importance of the reported simulations is thought to go beyond the cases discussed here and to be applicable to other molecules containing the HC*D fragment and, more generally, CD, a point that is rising in importance after the FDA's approval of deuterated drugs. Furthermore, proper treatment of anharmonicity helps in the interpretation of other observables, like e.g., specific optical rotations (OR).