The Vibrational Raman Optical Activity of β-Propiolactone Derivatives Computed with Coupled-Cluster Theory

Vibrational Raman optical activity (VROA) is an information-rich technique with which to explore the molecular stereochemistry of chiral molecules. The molecular properties needed to model a VROA spectrum can be computed using coupled-cluster (CC) theory. We report VROA spectra computed with CC theory in an origin-independent manner for the first time, applying the approach to several derivatives of β-propiolactone (oxetan-2-one), a strained, four-membered heterocycle with naturally occurring derivatives exhibiting a variety of stereochemistries. The spectra were theoretically benchmarked in several ways. First, a comparison of VROA spectra computed using CC2 theory was made to evaluate the performance of a large number of one-electron basis sets. Second, spectra computed with CC2 and CCSD were compared to gauge the effectiveness of the less costly CC2 method. Third, VROA spectra determined from CC2 chiroptical tensors computed using different geometries and vibrational Hessians were compared with one another. Finally, VROA spectra for β-propiolactones substituted with methyl, ethyl, and propyl groups were produced with CC2 using geometries and Hessians determined by B3LYP/aug-cc-pVTZ computations. For structures monosubstituted at the 3-position (adjacent to the carbonyl), an intense peak occurs at ≈1130 cm-1 corresponding largely to the C-C stretch between the ring and the alkyl substituent. For structures monosubstituted at the 4-position, a similar peak corresponding largely to the C-C stretch between the ring and the substituent appears at ≈1115 cm-1. For disubstituted species of (3S,4R) stereochemistry, a mode involving ring-breathing motion and asymmetrical wagging of the H atoms on C3 and C4 (on the same side of the β-lactone ring) produces a large negative signal at ≈1130 cm-1. The overall spectrum is found to be dominated by the configuration at the 3-position and less sensitive to that at the 4-position. The results suggest that VROA can be productively employed to interrogate the stereochemistry at the 3-position. Unfortunately, no clear VROA signal was identified that characterizes the stereochemistry at the 4-position in disubstituted species.