How Important Are Dimers for Interpreting the Chiroptical Properties of Carboxylic Acids? A Case Study with [5]-Ladderanoic Acid.

Chiroptical spectroscopy is sensitive to the formation of intermolecular interactions for chiral molecules. Experimental Vibrational Circular Dichroism (VCD), Vibrational Raman Optical Activity (ROA), and Optical Rotatory Dispersion (ORD) data of (-)-[5]-ladderanoic acid in chloroform have been analyzed using theoretical predictions for both monomeric and dimeric structures of (R)-[5]-ladderanoic acid to better understand their utility for the interpretation of experimental data. B3LYP, B3PW91, and M06-2X functionals, with and without dispersion corrections, and the 6-31+G(2d,p) basis set were used for theoretical predictions. It is found that dimer contributions are important to better reproduce the experimental vibrational absorption and associated VCD spectra, and a monomer:dimer ratio of 30:70 is indicated at the B3LYP level. However, no significant improvement is evident from dimer contributions to reproduce the experimental Raman and associated ROA spectra, and a monomer:dimer ratio of 100:0 is indicated at the B3LYP level. Boltzmann population-weighted specific rotations are predicted to be negative for both monomeric and dimeric conformations of (R)-[5]-ladderanoic acid, and quantitative agreement with the experimental ORD of (-)-[5]-ladderanoic acid is obtained with a 70:30 mixture of monomers and dimers.