The H-1 NMR chemical shift for the hydroxy proton of 4-(dimethylamino)-2 '-hydroxychalcone in chloroform: A theoretical approach to its inverse dependence on the temperature

[GRAPHICS] The inverse dependence of the chemical shift on the temperature experimenta lly found for the phenolic proton of 4-(dimethylamino)-2'-hydroxychalcone ( DMAHC) is theoretically studied. As the temperature decreases, the solvent dielectric constant epsilon increases and the zwitterionic resonance form i s more stabilized. Electronic calculations at the DFT level of theory were performed by immersing the solute DMAHC in chloroform cavities of different epsilon values, The values of the calculated chemical shifts for DMAHC as a function of epsilon show that the growing contribution of the zwitterioni c structure justifies the experimental results.