Electron momentum spectroscopy of 1-butene: a theoretical analysis using molecular dynamics and molecular quantum similarity.

The results of experimental studies of the valence electronic structure of 1-butene by means of electron momentum spectroscopy (EMS) have been reinterpreted on the basis of molecular dynamical simulations in conjunction with the classical MM3 force field. The computed atomic trajectories demonstrate the importance of thermally induced nuclear dynamics in the electronic neutral ground state, in the form of significant deviations from stationary points on the potential energy surface and considerable variations of the C-C-C-C dihedral angle. These motions are found to have a considerable influence on the computed spectral bands and outer-valence electron momentum distributions. Euclidean distances between spherically averaged electron momentum densities confirm that thermally induced nuclear motions need to be fully taken into account for a consistent interpretation of the results of EMS experiments on conformationally flexible molecules.

Statistical Validation of Absolute Configuration Assignment in Vibrational Optical Activity.

Chiroptical spectroscopy usually requires theoretically computed spectra to assist in the elucidation of the absolute configuration of samples for which experimental spectra have been recorded. Due to the inherently different nature of these two types of spectra, perfect agreement is quasi impossible. Several methods exist to quantify the degree of similarity between the two spectra, but rather limited work has been done to evaluate the robustness of the similarity between theory and experiment. In this work, a novel method is described to determine the statistical significance of the numerical degree of similarity between experimental and calculated vibrational circular dichroism spectra and to offer valuable support for performing absolute configuration assignments. The approach is successfully applied to a number of quinolizidine alkaloids.

Performance of DFT Methods in Momentum Space: Quantum Similarity Measures versus Moments of Momentum.

The quality of momentum space electron densities obtained from a large array of density functionals is investigated through careful numerical comparison with the density obtained using reference CCSD calculations. Using a test set of 68 closed-shell molecules in their ground state and 77 different computational methods, including coupled cluster, MP2 perturbation theory, Hartree-Fock, and a total of 74 DFT functionals, including long-range corrected functionals, we confirm that DFT momentum densities generally show poorer agreement with the reference than MP2 densities. The performance of DFT functionals varies significantly with only 8 DFT functionals outperforming Hartree-Fock with respect to electron momentum densities and their spherically averaged counterparts.

Electron delocalization and aromaticity in low-lying excited states of archetypal organic compounds.

Aromaticity is a property usually linked to the ground state of stable molecules. Although it is well-known that certain excited states are unquestionably aromatic, the aromaticity of excited states remains rather unexplored. To move one step forward in the comprehension of aromaticity in excited states, in this work we analyze the electron delocalization and aromaticity of a series of low-lying excited states of cyclobutadiene, benzene, and cyclooctatetraene with different multiplicities at the CASSCF level by means of electron delocalization measures. While our results are in agreement with Baird's rule for the aromaticity of the lowest-lying triplet excited state in annulenes having 4nπ-electrons, they do not support Soncini and Fowler's generalization of Baird's rule pointing out that the lowest-lying quintet state of benzene and septet state of cyclooctatetraene are not aromatic.