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The two lowest singlet excitation energies of 26 2-imidazolyl-2-thiazolylazo compounds have been investigated by ab initio methods within the second-order polarization propagator approximation (SOPPA). Various combinations of 4- and 5-substituents at both the imidazole and thiazole units have been studied for obtaining pi --> pi* excitation energies corresponding to an absorption maximum in the red or infrared regions. In many cases, pi --> pi excitation energies below 2 eV (corresponding to wavelengths longer than 600 run) are found for molecules that possibly may be synthesized. For the molecules included in this study, the longest wavelength was calculated to be 1049 nm. (C) 2001 Elsevier Science B.V. All rights reserved.

We review two novel developments in theoretical and computational approaches to chiroptical spectroscopy, namely the study of the rotatory strength tensor governing anisotropic circular dichroism, and the use of quantum mechanical response theory for incorporating the effect of electron correlation in ab-initio calculations of electronic circular dichroism. Large scale ab-initio calculations of the rotatory strength tensor, using the random phase approximation, are reported for the beta-gamma unsaturated ketone norbornenone and for the helically twisted hexahelicene molecule, and the predicted anisotropy of the circular dichroism is illustrated for a number of excitations in the two molecules using a graphical approach. For the hexahelicene molecule, the computed scalar rotatory strengths and rotatory strength tensors are used to generate theoretical circular dichroism spectra. The agreement between the theoretical isotropic circular dichroism spectrum and the corresponding experimental solution spectrum is quite good, and the theoretical circular dichroism spectra for light propagating along the principal axes of the molecular inertia tensor show striking variations in signs and magnitudes relative to the isotropic spectrum.

Predictions of the unpolarized vibrational absorption and vibrational circular dichroism (VCD) spectra of the chiral molecule 6,8-dioxabicyclo[3.2.1]octane (1) are reported. Harmonic force fields and atomic polar tensors are obtained using the density functional theory (DFT), MP2 and SCF methodologies, and the 3-21G and 6-31G* basis sets. Three functionals, LSDA, BLYP, and B3LYP, are used in DFT calculations. Atomic axial tensors are obtained using the Distribution Origin gauge; distributed atomic axial tensors are calculated using gauge-invariant atomic orbitals (GIAOs) at the SCF level of approximation. The quality of the predicted spectra is highly dependent on the methodology and the basis set is employed. Spectra calculated using 6-31G* MP2 and DFT/B3LYP force fields are very similar and in excellent agreement with experimental spectra. 6-31G* SCF, DFT/LSDA and DFT/BLYP calculations are in significantly worse agreement with experiment, as are 3-21G MP2 and DFT/B3LYP calculations. When both accuracy and computational effort are considered, at this time, predictions of vibrational absorption and VCD spectra of molecules comparable to 1 in size are optimally performed using 6-31G* or equivalent basis sets and (i) harmonic force fields and atomic polar tensors calculated via DFT and a hybrid density functional; (ii) Distributed Origin gauge atomic axial tensors; and (iii) SCF GIAO-based distributed atomic axial tensors.