Some periodic trends, molecular structure, normal coordinate analysis of 1,3,5-trioxane, -trithiane and -triselenane: computational and vibrational studies

The Raman (3200-100 cm−1) and infrared spectra (3200-200 cm−1) of solid C3H6O3 (TO) and C3H6S3 (TS) have been recorded and only the chair conformation is present. Furthermore, utilizing the CH stretching bands at the infrared spectrum of the gaseous phase, the CHax and CHeq distances are found to be 1.088 and 1.107 Å, respectively. The structural parameters and conformational stabilities for 1,3,5-C3H6X3 series (where X=O, S and Se atoms) have been obtained from density functional theory at the Becke3-LYP gradient-corrected functional (DFT-B3LYP) and from MP2 level with full electron correlation. These calculations have been extended up to 6-311++G(d,p) basis set to include polarization and diffusion functions. All computational results and vibrational analysis are in favor of the chair conformation (C3v), whereas the boat (C1) and Planar (D3h) forms have been excluded owing to the predicted imaginary wavenumber(s). According to 6-311++G(d,p) basis set, the ring size and the tendency of the ring to undergo flattening is found to be directly proportional to the atomic size of the substituted hetero atoms (X) of the chair. The calculated DFT-B3LYP scaled quantum chemistry (QC) force fields at 6-31G(d) basis set lead to a number of revised assignments for certain vibrational modes and it appears to give quite accurate Raman spectra for the investigated chalcogenanes. The estimated bond lengths, bond angles, rotational constants, Raman activities dipole moment and unscaled force constants are compared with either theoretical and/or experimental results whenever possible.