Ab initio and DFT benchmark study for the calculations of isotopic shifts of fundamental frequencies for 2,3-dihydropyran

A systematic investigation has been carried out to assess the performance of various exchange-correlation energy density functionals coupled with various basis sets to optimize structural parameters and calculate vibrational frequencies corresponding to (C5H8O)-C-12-O-16 and (C5H8O)-C-12-O-18 isotopomers of 2,3-dihydropyran (DHP) in order to estimate the isotopic shift. For its accurate determination, anharmonic molecular vibrational frequencies have been calculated by employing both, the vibrational self-consistent field (VSCF) method, and the second-order perturbative correction on top of VSCF (VSCF-PT2). We have tested pure DFT GGA functionals (BP86 and PBE), hybrid DFT GGA functionals (B3LYP and X3LYP), pure DFT meta-GGA functionals (TPSS and revTPSS), hybrid DFT meta-GGA functionals (TPSSh and M06), and double hybrid DFT functionals (B2K-PLYP and B2T-PLYP). For the purpose of comparison, the geometry of DHP has been optimized using the MP2 and CCSD methods, and frequencies have been calculated employing MP2 method. From the analysis of all the computed vibrational frequencies, the highest isotopic shift has been found to be similar to 15 cm(-1) corresponding to one of the vibrational mode that comprises of =C-1-O-2 stretch, C-3-O-2 stretch, and H-C-6=C-1 bend. Therefore, the present work definitely puts forward the laser-based enrichment process for oxygen-18 isotope separation involving DHP as a working molecule.