NMR detection of the strained metallacycles in organolithiums: theoretical study.

For the first time through quantum chemistry methods, the effective use of 1JCLi spin-spin coupling constants as descriptors for assessing the formation of strained metallacycles is demonstrated. Both acyclic organolithiums and 3- to 7-membered metallacycles are examined. 80 organolithium compounds, including both monomeric and dimeric species, with ligands containing fluorine, nitrogen, oxygen, and carbon (in the form of carbanions), are tested. In general, the 1JCLi values below 12 Hz for monomeric species and below 6 Hz for dimeric species serve as clear indicators of strained monomeric metallacycle formation (for 6Li nuclei). The primary contributor to the overall 1JCLi value is the Fermi-contact term, which correlates directly with the carbon-lithium interatomic distance and allows to distinguish between dimers and monomers.

Choice of computational protocol for carbon-lithium spin-spin coupling constants 1 JCLi.

In this work, we tested various computational schemes for calculations of 1 JCLi constants with a high accuracy. On the example of six organolithium reagents (phenyllithium monomer and dimer, monomer s-butyllithium, monomers of 1- and 2-lithionaphthalenes, and a methyllithium tetramer), the following aspects are discussed: (i) the role of a model system geometry, (ii) influence of solvent effects, and (iii) the choice of a functional and basis set. Practical recommendations for calculation of 1 JCLi with an accuracy ±2 Hz are formulated.

Hydration of selenolate moiety: Ab initio investigation of properties of O-H⋯Se(-) hydrogen bonds in CH3 Se(-)⋯(H2 O)n clusters.

This work is devoted to investigations of the influence of O-H···Se(-) hydrogen bonds on the electronic shells of selenolate R-Se(-) fragment (R═CH3 ). The geometric, energetic and nuclear magnetic resonance (NMR) spectral parameters for various conformers of CH3 Se(-)⋯(H2 O)n clusters with n = 0-6 were calculated at CCSD/aug-cc-pVDZ level of theory. For selenolate anion CH3 Se(-) solvation free energy was calculated, and for water media it is equal to -71.41 kcal/mol. For O-H···Se(-) hydrogen bonds the proportionality coefficients between QTAIM parameters at (3; -1) bond critical point and the strength of an individual hydrogen bond ∆E were proposed. It was shown, that despite a relative weakness of O-H···Se(-) hydrogen bonds, the outer electronic shell of the selenium atom changes significantly upon formation of each hydrogen bond. This, in turn, cause the dramatic change of NMR parameters of selenium nuclei.

On the influence of water molecules on the outer electronic shells of R-SeH, R-Se(-) and R-SeOH fragments in the selenocysteine amino acid residue.

In this computational work (MP2/aug-cc-pVTZ) we investigated the features of the outer electronic shells of R-SeH, R-Se(-) and R-SeOH fragments (R = CH3), which can be considered as simplified models for the forms of the active centres of glutathione peroxidases GPx along their catalytic pathway (reduction of peroxides). It is shown that the preferential direction of a nucleophilic attack on the R-Se(-) fragment by a peroxide molecule is determined by the presence of the electron-depleted region of the selenium atom in front of the C-Se bond and nucleophilic attack can be facilitated by the solvation of R-Se(-) by water molecules. Such solvation does not block the direction of potential nucleophilic attack and also leads to the increase of the maximal value of the molecular electrostatic potential on the selenium atom. It was shown that the 77Se NMR chemical shift is sensitive both to the oxidation state and the hydration state of the selenium-containing fragment.

Sterically Facilitated Intramolecular Nucleophilic NMe2 Group Substitution in the Synthesis of Fused Isoxazoles: Theoretical Study.

The influence of steric repulsion between the NMe2 group and a second ortho-(peri-)substituent in the series of 1-dimethylaminonaphthalene and N,N-dimethylanilene ortho-oximes on the ease of the NMe2 group's intramolecular nucleophilic substitution is studied. Possible reaction intermediates for three mechanisms are calculated (ωB97xd/def-2-TZVP), and their free Gibbs energies are compared to model reaction profiles. Supporting experiments have proved the absence of studied reactivity in the case of simple 2-dimethylaminobenzaldoxime, which allowed us to establish reactivity limits. The significant facilitation of NMe2 group displacement in the presence of bulky substituents is demonstrated. The possibility of fused isoxazoles synthesis via the intramolecular nucleophilic substitution of a protonated NMe2 group in the aniline and naphthalene series is predicted.

Stretching force constants as descriptors of energy and geometry of F···HF hydrogen bonds.

In this work applicability of proton donor group stretching vibration force constants ks and intermolecular stretching force constants kσ for evaluations of hydrogen bond strength and geometry are discussed. For a set of 30 complexes with F···HF hydrogen bonds in a wide range 0.5-48 kcal/mol by means of quantum chemical calculations equilibrium geometries, complexation energies, vibrational frequencies and corresponding force constants were calculated (MP2/aug-cc-pVTZ). It is shown, that properties of a hydrogen bond are more strictly correlated with the values of force constants than with vibrational frequencies. Easy-to-use equations for estimations of hydrogen bond energy ∆E and geometry (rFH, rFF) based on ks and kσ values are proposed.