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

## TL;DR

This study investigates how dimer formation affects the chiroptical properties of [5]-ladderanoic acid using experimental and theoretical methods.

## Contribution

The study reveals the importance of dimer contributions in interpreting VCD and ORD data for chiral carboxylic acids.

## Key findings

- Dimer contributions are important for reproducing vibrational absorption and VCD spectra with a 30:70 monomer:dimer ratio.
- Dimer contributions do not significantly improve Raman and ROA spectra, indicating a 100:0 monomer:dimer ratio.
- A 70:30 monomer:dimer mixture achieves quantitative agreement with experimental ORD data.

## Abstract

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.