The Alternant Hydrocarbon Pairing Theorem and All-Valence Electrons Theory. An Approximate LCOAO Theory for the Electronic Absorption and MeD Spectra of Conjugated Organic Compounds. 1.

The significance of the pairing theorem for alternant hydrocarbons is demonstr ated by a variety of experimental data, but standard all-valence electrons methods based on the »Neglect of Differential Overlap« (NDO) approximation fail to predict the observed pairing properties for this class of compounds. In this communication, a new all-valence electrons method is presented, using a basis of L6wdin orthogonalized atomic orbitals. This »Linear Combination of Orthogonalized Atomic Orbitals« (LCOAO) method is designed to predict the electronic absorption and magnetic circular dichroism data for organic :n: systems with particular emphasis on the pairing properties of alternants. Results for phenanthrene and hiphenylene are discussed

Prediction of OH stretching frequencies for intramolecular OH··O linkages

The OH stretching frequencies for a series of compounds with intramolecular OH-O hydrogen bonds are computed with the DVPT2 and GVPT2 anharmonic models of Barone andassociates using B3LYP-D3 density functional theory. The anharmonic models systematically underestimate the observed OH stretching wavenumbers for systems with medium to strong hydrogen-bonding, apparently due to the inability of approaches based on 2nd-order perturbation theory (PT2) to account accurately for the properties of large-amplitude vibrational motions. Nevertheless, the computed wavenumbers correlate linearly with the observed values. But it is noteworthy that the wavenumbers obtained with standard harmonic analyses lead to even better correlations than those computed with the expensive anharmonic models, resulting in linear regression equations of great predictive value (standard deviations close to 20 cm–1). For example, effective OH stretching wavenumbers for 2-acetylmalonamide enol and 2-cyanomalonamide enol are predicted between 2300 and 2400 cm–1. In contrast to the linear correlations observed for intramolecular OH∙∙∙O hydrogen bonds, intramolecular NH-O linkages are characterized by corresponding non-linear relationships

Molecular vibrations of acetylacetone enol.:Infrared polarization spectroscopy and theoretical predictions

The IR polarization spectrum of acetylacetone enol (AAe, (3Z)-4-hydroxy-3-penten-2-one) was recorded in the region 2000 – 450 cm–1 using stretched polyethylene as an anisotropic solvent. The measured orientation factors were consistent with Cs molecular symmetry of AAe and provided an experimental distinction between in-plane and out-of-plane polarized spectral features. The results suggest the assignment of at least one previously unrecognized fundamental transition. Excellent agreement with presently and previously observed transition wavenumbers in the investigated region was obtained by theoretical predictions in the harmonic approximation using B3LYP-D3 Density Functional Theory (DFT); calculated diagrams are provided for all 39 normal modes of AAe. The prediction of in-plane transition moment directions was found to be very sensitive to calculational details. The wavenumbers computed with the anharmonic models VPT2 (2nd-order Vibrational Perturbation Theory) and DVPT2 (Deperturbed VPT2) were consistent with those of the harmonic analysis, but the predictions obtained using GVPT2 (Generalized VPT2) deviated significantly