The noncoincidence phenomenon of acetonylacetone CO stretching in a binary mixture and the aggregation-induced split theory

Abstract

This article aims to correlate the noncoincidence effect phenomenon with the aggregation state of acetylacetone CO stretching in a binary mixture. CO stretching noncoincidence effect (NCE) was observed not only between IR and Raman spectra but also between the isotropic and anisotropic Raman spectra of acetonylacetone. The difference in CO stretching wavenumbers of the isotropic and anisotropic Raman spectra (NCE value) in a binary mixture at different concentrations has been calculated. We found that both isotropic and anisotropic Raman wavenumbers of CO stretching increase with the dilution of acetonylacetone by CCl₄ while the NCE value decreases. These noncoincidence and concentration effect phenomena seem to go against the quantum theory. Herein, we proposed an aggregation-induced split (AIS) model to explain the NCE phenomenon and concentration effect. The experimental data were consistent with the DFT calculations performed at the B3LYP-D3/6-311++G (d,p) levels based on the proposed model. The dynamics of transformation from monomers to an aggregated structure can be easily controlled by tuning the concentration. Solvent dependent experiments show that the value of NCE decreased with the increase of the solvent dielectric constant at the same concentration, which is in accordance with Logan's theory.