Search for Quantum-Chemistry Calculations to Properly Evaluate Strength in the π-Electron Conjugation in a Group of p-Phenylethynyl Compounds

Abstract

We searched for quantum chemical calculations that would properly evaluate strength in the π-electron conjugation of a group of p-phenylethynyl compounds determined by molecular spectroscopy with error ranges. The target molecules are tolane, 1,4-diphenylbutadiyne and 1,4-bis(phenylethynyl)benzene. Quantum chemical calculations were performed with B3LYP, CAM-B3LYP, ωB97X-D, M06-2X, and MP2 for theory, and basis sets were explored among the combinations 6-311++G(d) cc-pVDZ, and aug-cc-pVDZ. Because B3LYP calculation is considered to be unskilled at evaluating dispersion force, a dispersion force correction term is added to B3LYP calculation. The least of square of the difference in the strength between observed and calculated was given by calculations with M06-2X/aug-cc-pVDZ. It has, however, often been reported that M06-2X undergoes inappropriate structural optimization. In calculations for 1,4-diphenylbutadiyne, M06-2X was not giving systematic uniform results. Overall, ωB97X-D was a functional function corrected for long-range and dispersion force correction, and it was considered to give good results together with CAM-B3LYP corrected for dispersion force.