A set of 36 pnicogen homo- and heterodimers, R3E···ER3 and R3E···E′R′3, involving differently substituted group Va elements E = N, P, and As has been investigated at the ωB97X-D/aug-cc-pVTZ level of theory to determine the strength of the pnicogen bond with the help of the local E···E′ stretching force constants k(a). The latter are directly related to the amount of charge transferred from an E donor lone pair orbital to an E′ acceptor σ* orbital, in the sense of a through-space anomeric effect. This leads to a buildup of electron density in the intermonomer region and a distinct pnicogen bond strength order quantitatively assessed via k(a). However, the complex binding energy ΔE depends only partly on the pnicogen bond strength as H,E-attractions, H-bonding, dipole-dipole, or multipole-multipole attractions also contribute to the stability of pnicogen bonded dimers. A variation from through-space anomeric to second order hyperonjugative, and skewed π,π interactions is observed. Charge transfer into a π* substituent orbital of the acceptor increases the absolute value of ΔE by electrostatic effects but has a smaller impact on the pnicogen bond strength. A set of 10 dimers obtains its stability from covalent pnicogen bonding whereas all other dimers are stabilized by electrostatic interactions. The latter are quantified by the magnitude of the local intermonomer bending force constants XE···E′. Analysis of the frontier orbitals of monomer and dimer in connection with the investigation of electron difference densities, and atomic charges lead to a simple rationalization of the various facets of pnicogen bonding. The temperature at which a given dimer is observable under experimental conditions is provided.

Rhodamine B (RhB) is widely used in chemistry and biology due to its high fluorescence quantum yield. In high concentrations, the quantum yield of fluorescence decreases considerably which is attributed to the formation of RhB dimers. In the present work, a possible mechanism of fluorescence quenching in RhB dimers is investigated with the use of time-dependent density functional theory (TD-DFT). The excited states of monomeric and dimeric RhB species have been studied both in the gas phase and in solution with the use of the TD-BLYP/6-311G* method. Results of the calculations suggest that quenching can occur via an internal conversion to the charge-transfer singlet excited states, which can be followed by an intersystem crossing with the charge-transfer triplet states. A possibility to reduce the loss of the fluorescence quantum yield is discussed.

Vibrational frequencies can be measured and calculated with high precision. Therefore, they are excellent tools for analyzing the electronic structure of a molecule. In this connection, the properties of the local vibrational modes of a molecule are best suited. A new procedure is described, which utilizes local CC stretching force constants to derive an aromaticity index (AI) that quantitatively determines the degree of π-delocalization in a cyclic conjugated system. Using Kekulé benzene as a suitable reference, the AIs of 30 mono- and polycyclic conjugated hydrocarbons are calculated. The AI turns out to describe π-delocalization in a balanced way by correctly describing local aromatic units, peripheral, and all-bond delocalization. When comparing the AI with the harmonic oscillator model of AI, the latter is found to exaggerate the antiaromaticity of true and potential 4n π-systems or to wrongly describe local aromaticity. This is a result of a failure of the Badger relationship (the shorter bond is always the stronger bond), which is only a rule and therefore cannot be expected to lead to an accurate description of the bond strength via the bond length. The AI confirms Clar's rule of disjoint benzene units in many cases, but corrects it in those cases where peripheral π-delocalization leads to higher stability. [5]-, [6]-, [7]-Circulene and Kekulene are found to be aromatic systems with varying degree of delocalization. Properties of the local vibrational modes provide an accurate description of π-delocalization and an accurate AI.

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A set of 42 molecules with N-F, O-F, N-Cl, P-F, and As-F bonds has been investigated in the search for potential bond anomalies, which lead to reverse bond length-bond strength (BLBS) relationships. The intrinsic strength of each bond investigated has been determined by the local stretching force constant obtained at the CCSD(T)/aug-cc-pVTZ level of theory. N-F or O-F bond anomalies were found for fluoro amine radicals, fluoro amines, and fluoro oxides, respectively. A rationale for the deviation from the normal Badger-type inverse BLBS relation is given and it is shown that electron withdrawal accompanied by strong orbital contraction and bond shortening is one of the prerequisites for a bond anomaly. In the case of short electron-rich bonds such as N-F or O-F, anomeric delocalization of lone pair electrons in connection with lone pair repulsion are decisive whether a bond anomaly can be observed. This is quantitatively assessed with the help of the CCSD(T) local stretching force constants, CCSD(T) charge distributions, and G4 bond dissociation energies. Bond anomalies are not found for fluoro phosphines and fluoro arsines because the bond weakening effects are no longer decisive.