A density functional and Hartree-Fock study of pyran and 2,4-pentadienals

The geometries and vibrational frequencies of pyran and its variants obtained by ring dissociation at the CO bond (2,4-pentadienals) are studied using density functional theory (DFT; VWN, BLYP) and Hartree-Fock (HF; 6–31G(d), MP2/ / 6–G(d)) calculations. The BLYP, 6–31G(d) and MP2/ /6–31G(d) geometry optimizations predict an open form as the most stable configuration, while VWN determines the cyclic form to be most stable. DFT calculations of the vibrational frequencies of 2,4-pentadienal are in good agreement with IR data, favoring the conformer identified by our energy calculations.