Theoretical Study of Mono-terminal Functionalized Oligo Acetylene [CnHn+2X] (n=3-5, X= - COOH, and - PO3H2) in Gas and Aqueous Phases

Background: Gaussian 09 program package was used to optimize the molecular structures and calculate the vibrational wavenumber of cis and trans isomers of oligo acetylene and functionalized oligo acetylene. The geometrical parameters of the more stable conformers are reported and the total energies of the optimized structures were used to calculate the cis-trans relative energies in gas and aqueous phases. Objective: To carryout theoretical investigation of the conformational preference and vibrational spectroscopy of mono-terminal substituted oligo acetylene with carboxylic acid and phosphonic acid groups in gas and aqueous phases. Methods: Computations were performed using Density Functional Theory adopting Becke-3– Lee–Yang–Parr (B3LYP) with the 6-31+G(d,p) standard basis set. The Integral Equation Formalism in the Polarizable Continuum Model was used as a solvation model for the aqueous phase computations. Results: The trans isomers of un-substituted and substituted oligo acetylene are found to be more stable in both gas phase and aqueous phases. The order of stability in gas phase is as follows: CnHn+1 (CO2H)> CnHn+1 (H2PO3)> CnHn+2, (n=3-5). Solvation results for the substituted systems showed that the solvent effect on trans isomers of mono-substituted phosphonic acid is higher than that of mono-substituted carboxylic acid and the solubility increases with the increase in the number of monomer unit. Conclusion: The use of B3LYP/6-31+G(d,p) method shows successful results for the calculated properties of the studied systems which in agreement with results are reported in literature.