Theoretical vibrational spectra of organic ring compounds compared to experiment

Abstract

AbstractWe have studied vibrational infrared (IR) and Raman spectra of several organic ring compounds. These include first cyclobutanecarboxaldehyde (CBA) and cyclohexanecarboxaldehyde (CHA) using density functional theory (DFT) together with a Becke‐3 exchange and Lee–Yang–Parr correlation (B3LYP) functional. DFT in these systems gives a stable and low‐energy gauche‐axial (g‐ax) conformer for both of the molecules together with the experimentally found equatorial conformations. However, on the basis of comparison of the theoretical vibrational spectra of the conformational mixture at 300 K with experimental spectra, the presence of the g‐ax conformer could be ruled out because it would yield extra lines not present in the corresponding experimental spectra. Thus, we concluded that DFT yields properties related to energy derivatives such as minimum geometries and vibrational spectra in fair agreement with experiment, while total energy differencies are not trustworthy when they are small, especially if small linear dependencies in the atomic basis sets applied appear. However, in that case we found that Møller–Plesset perturbation theory of second order (MP2) applied to CBA has the same problems and even does not predict the absolute minimum on the potential hypersurface correctly. Therefore, we applied DFT also to biphenyle (BP) and terphenyle (TP). Again, in BP and TP DFT turned out to yield reliable structures that as far as the important torsional angle between the rings is concerned are even better than MP2. Also, the vibrational spectra are in fair agreement with experimental ones. In BP restricted Hartree–Fock (RHF), DFT and experimental IR spectra all show two intense absorptions around 700 cm−1, while MP2 gives only one intense absorption band in this region. DFT also reproduces the position of these two lines well. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004