Direct Calculations of Mid- and Near-IR Absorption and Circular Dichroism Spectra of Chiral Molecules Using QM/MM...

The infrared (IR) and vibrational circular dichroism (VCD) spectra of (1S)-(-)-β-pinene in the mid- and near- IR frequency regions are numerically simulated by using a time-correlation function theory and mixed quantum/classical simulation method. Anharmonic vibrational dynamics and fluctuating atomic partial charges of the chiral pinene were obtained by carrying out quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations. Thus obtained time-correlation functions of electric and magnetic dipole moments are used to calculate the IR absorption and VCD spectra, and they are directly compared with experimental results. Not only the fundamental transition bands but also first overtone and combination bands in the near-IR frequency region are successfully simulated. It is shown that the polarizable nature of the solute is particularly important in quantitatively reproducing the near-IR spectra, whereas such polarization effects on dipole and rotational strengths of lower-frequency and large-amplitude vibrations are less critical. We anticipate that the present QM/MM MD method in combination with mixed quantum/classical time-correlation function theory to calculate both mid- and near-IR absorption and VCD spectra will be of critical use in interpreting vibrational optical activity properties of even conformationally flexible chiral molecules, such as proteins.