Theoretical investigation of the conformational preference and spectroscopic analysis of cinnamoyl chloride and cinnamide

The rotational isomers of cinnamoyl chloride and cinnamide have been computed using the B3LYP method and the 6-311++G (d, p) basis set to calculate their molecular structure, conformational stability, and spectroscopic properties. Computations revealed that the s-cis rotamers of both compounds are the most stable rotamers in both the gas phase and in the solution. The s-trans rotational barrier is 8.76 kcal/mol and 5.63 kcal/mol for cinnamoyl chloride and cinnamide, respectively. Replacing the Cl group of the cinnamoyl chloride with the NH₂ group in the cinnamide has reduced the highest occupied molecular orbital – lowest unoccupied molecular orbital (HOMO-LUMO) gap. The solvation effects on the conformational stability of rotamers have been studied in nine solvents (heptane, chloroform, tetrahydrofuran, dichloroethane, acetone, ethanol, methanol, dimethyl sulfoxide, and water) using the integral equation formalism of the polarizable continuum model. The chemical shifts of ¹³C and ¹H NMR spectra have been simulated in the gas phase, dimethyl sulfoxide (DMSO), and chloroform using the gauge-independent atomic orbital (GIAO) method. The UV absorption spectral analysis has been computed in different solvents (chloroform, methanol, and water), and atomic charges calculated. The examination of vibrational wave numbers and their corresponding assignments revealed excellent agreement between simulated and experimental infrared spectra for the studied molecules.