LIF spectrum for the localised S0 → S1(ππ*) excitation in the H-bonded anthranilic acid dimer: Symmetry breaking or coupling of vibrations.

This study aims to investigate the impact of the π â†’ π* excitation localised in one monomer on the equilibrium geometry and oscillations of the AA dimer. Several low-frequency vibrations appear in pairs in the LIF spectrum because oscillations involving intermolecular hydrogen bonds are coupled, generating approximately symmetric and antisymmetric combinations (especially the COOH rocking modes, LIF: 295 and 301 cm-1). Furthermore, quantitative evaluation based on the TDDFT(B3LYP) results indicates that a dozen among 90 intramolecular oscillations are strongly coupled. In contrast, most vibrations are decoupled or weakly coupled, since they involve remote parts of the monomers. This makes several single vibrations active in the LIF spectrum (including the bending mode of the NH···O intramolecular hydrogen bond associated the strongest vibronic band 442 cm-1), while the other in each pair remains inactive. The reason for decoupling of oscillations and symmetry breaking is that the π â†’ π* electronic excitation is entirely localised within one of the monomers, which makes them no longer equivalent in terms of geometry and dynamics. Additionally, the excitation of one monomer induces strengthening and shortening by 6 pm of only one intermolecular hydrogen bond linking the carboxylic groups of both molecules. This causes the 1.7° in-plane distortion of the dimer and lowering of its symmetry to Cs group (from C2h for the S0 state). The distortion induces the activity of two low-frequency in-plane intermolecular vibrations, i.e. the geared oscillation (LIF: 58 cm-1) and the shearing motion (99 cm-1) of the monomers.

Tuning the metal-ligand bond in the σ-complexes of stannylenes and azabenzenes.

The metal-ligand bond in a set of 60 σ-complexes has been investigated by electronic structure computations. These σ-complexes originate from the unique combination of 12 stannylenes (SnX2 ) with five azabenzene ligands (pyridine, pyrazine, pyrimidine, pyridazine, and s-triazine), where the nitrogen center of the ligand acts as σ-donor and the tin(II) center as σ-acceptor in a 1:1 fashion. The Sn ← N bond and the total interaction between the stannylene and azabenzene moieties of the σ-complexes are characterized in depth to relate the Sn ← N strength to the substitution pattern at SnX2 and to the number and the positioning of N atoms in the azabenzenes. Such X substituents as (iso)cyano and trifluoromethyl groups enhance the interaction strength, while the presence of alkyl, phenyl, and silyl substituents in SnX2 diminishes the stability of σ-complexes. A gradual weakening of the total interaction is associated with the growing number of N atoms in the azabenzenes, while the N-atom positioning in pyridazine is particularly effective in strengthening the interaction with stannylenes. Variations in the Sn ← N bond strength usually follow those in the total interaction between the moieties but the interacting quantum atoms picture of Sn ← N reveals certain intriguing exceptions.

Electron density characteristics in bond critical point (QTAIM) versus interaction energy components (SAPT): the case of charge-assisted hydrogen bonding.

Charge-assisted hydrogen bonds (CAHBs) of N-H···Cl, N-H···Br, and P-H···Cl type were investigated using advanced computational approach (MP2/aug-cc-pVTZ level of theory). The properties of electron density function defined in the framework of Quantum Theory of Atoms in Molecules (QTAIM) were estimated as a function of distance in H-bridges. Additionally, the interaction energy decomposition was performed for H-bonded complexes with different H-bond lengths using the Symmetry-Adapted Perturbation Theory (SAPT). In this way both QTAIM parameters and SAPT energy components could be expressed as a function of the same variable, that is, the distance in H-bridge. A detailed analysis of the changes in QTAIM and SAPT parameters due to the changes in H···A distance revealed that, over some ranges of H···A distances, electrostatic, inductive and dispersive components of the SAPT interaction energy show a linear correlation with the value of the electron density at H-BCP ρ(BCP). The linear relation between the induction component, E(ind), and ρ(BCP) confirms numerically the intuitive expectation that the ρ(BCP) reflects directly the effects connected with the sharing of electron density between interacting centers. These conclusions are important in view of charge density studies performed for crystals in which the distance between atoms results not only from effects connected with the interaction between atomic centers directly involved in bonding, but also from packing effects which may strongly influence the length of the H-bond.

Kinetics of 1,5-hydrogen migration in alkyl radical reaction class.

Kinetics of the 1,5-intramolecular hydrogen migration in the alkyl radicals reaction class has been studied using the reaction class transition state theory combined with the linear energy relationship (LER) and the barrier height grouping (BHG) approach. The high pressure limits of the rate constants for the reference reaction of 1-pentyl → 1-pentyl, calculated by the Canonical Variational Transition State Theory (CVT) with the Small Curvature Tunneling (SCT), are taken from the literature. Direct comparison with available experimental data indicates that the RC-TST/LER, where only reaction energy is needed, can predict rate constants for any reaction in this reaction class with excellent accuracy. Specifically for this reaction class, the RC-TST/LER method has less than 65% systematic errors in the predicted rate constants when compared to explicit rate calculations.

Kinetics of thermoneutral intermolecular hydrogen migration in alkyl radicals.

High pressure limits of thermal rate constants of intramolecular hydrogen migrations, particularly 1,3 to 1,6 H-shift in propyl, butyl, pentyl and hexyl radicals, respectively, were calculated using the canonical variational transition state theory (CVT) with a multi-dimensional small-curvature tunneling (SCT) correction over the temperature range of 300-3000 K. The CCSD(T)/cc-pVDZ//BH&HLYP/cc-pVDZ method was used to provide necessary potential energy surface information. Rate constants for these reactions were used to extrapolate rate constants for reactions of larger alkyls where experimental data are available using the Reaction Class Transition State Theory (RC-TST). Excellent agreement with experimental data confirms the validity of the RC-TST methodology and the accuracy of the calculated kinetic data in this study.

Intramolecular double proton transfer from 2-hydroxy-2-iminoacetic acid to 2-amino-2-oxoacetic acid.

The double intramolecular proton transfer process which transforms 2-hydroxy-2-iminoacetic acid into 2-amino-2-oxoacetic acid is analyzed. The MP2 and DFT calculations were performed for different tautomers and conformers of these species. Pople style (6-311++G(d,p), 6-311++G(3df,3pd)) and Dunning type (aug-cc-pVTZ) basis sets were applied. O-H...O, N-H...O, and O-H...N intramolecular hydrogen bonds were also analyzed with the use of Quantum Theory of "Atoms in Molecules" (QTAIM) as well as Natural Bond Orbitals (NBO) method. Different criteria of the hydrogen bonding existence were applied here for intramolecular interactions. It was found that some of N-H...O hydrogen bonds may be treated as blue-shifting ones.

Kinetics of 1,4-hydrogen migration in the alkyl radical reaction class.

The kinetics of the 1,4-intramolecular hydrogen migration in the alkyl radicals reaction class has been studied using reaction class transition-state theory combined with the linear energy relationship (LER) and barrier height grouping (BHG) approach. The rate constants for the reference reaction of n-C(4)H(9) were obtained by canonical variational transition-state theory (CVT) with the small curvature tunnelling (SCT) correction in the temperature range 300-3000 K with potential-energy surface information computed at the CCSD(T)/cc-pVDZ//BH&HLYP/cc-pVDZ level of theory. Error analyses indicate that RC-TST/LER, where only reaction energy is needed, and RC-TST/BHG, where no other information is needed, can predict rate constants for any reaction in this reaction class with excellent accuracy. Specifically, for this reaction class the RC-TST/LER method has less than 65% systematic errors in the predicted rate constants, while the RC-TST/BHG method has less than 80% error when compared to explicit rate calculations.