Simulations of solid-state vibrational circular dichroism spectroscopy of (S)-alternarlactam by using fragmentation quantum chemical calculations.

Simulations of vibrational circular dichroism (VCD) spectroscopy of optical active aggregates of chiral molecules in the amorphous solid encounter great difficulties in the description of complicated intermolecular interactions by using the conventional quantum mechanical (QM) methods. The fragmentation approach is applied to calculate the VCD spectra of the covalently bonded oligomers and nonbonded molecular aggregates of (S)-alternarlactam, a new fungal cytotoxin with cyclopentenone and isoquinolinone scaffolds. Starting from the statistically averaged configurations that are sampled from the molecular dynamic simulations, the target oligomers or packing systems are divided into several fragments with a proper treatment of boundary effects on the separated segments. Each fragment is embedded in the background point charges centered on the distant atoms to simulate the long-range electrostatic interactions. The total VCD signals are assembled from the rotational strength of all the fragments. Test calculations on the σ-bonded oligomers and molecular aggregates using fragmentation method show good agreement with the conventional QM results. The packing effects on the infrared (IR) absorption and VCD spectroscopies of amorphous (S)-alternarlactam solid are investigated with density of 0.5 and 0.8 g/cm(3), respectively. The fragment-based VCD calculations on (S)-alternarlactam aggregates give a better agreement with experimental spectra than the Boltmann-weighted spectra of various possible monomeric, dimeric, and trimeric configurations. Hydrogen-bonded networks are the dominant packing configurations at the density of 0.5 g/cm(3). The (C═)O···H-N hydrogen-bonding interactions result in the signal splitting of IR and VCD spectra at the C═O stretching vibrational regions. When the density is increased to 0.8 g/cm(3), π-π stacking turns to be the dominating intermolecular interaction pattern. The computational cost of fragmentation calculation scales linearly with the number of the molecular fragments, facilitating the future applications to a wide range of the large-sized chiral systems.