M. V. Makarova

On the Degenerate Tautomerism

Tautomerism is a phenomenon of dynamic equilibrium between structural isomers (tautomers). This type of isomerism is characterized by fast reversible spontaneous interconversion of structural isomers. This interconversion is facilitated by low or permeable energy barriers between the molecule energy minima. Under the conditions of thermodynamic equilibrium, tautomers co-exist in comparable concentrations if their energy difference is low enough. If the energy difference is zero, tautomerism is referred to as a degenerate tautomerism [1]. The migration of a mobile hydrogen atom between the oxygen atoms of unsymmetrical intramolecular hydrogen bond in cis-enol of malonic dialdehyde (3

Tautomers of 4,5-dihydroxy-1,8-bis(dimethylamino)naphthalene and related boron compounds: A quantum-chemical study

The quantum-chemical method PBE0/cc-pV(D,T)Z was applied to determine structural parameters, relative energies, and dipole moments of the prototropic tautomers of 4,5-dihydroxy-1,8-bis(dimethylamino) naphthalene. According to the calculations, this compound can exist in the gas phase as low-polar molecules with one asymmetric intramolecular hydrogen bond. In a polarizable medium a pair of zwitter-ionic tautomers dominate of similar energy with two asymmetric intramolecular hydrogen bonds. These tautomers and a hypothetic heterocyclic analog containing a BH2+ group instead of a proton between the nitrogen atoms are characterized by a high value of dipole moment and first hyperpolarizability. Mutual transformation of enantiomers (mirror antipodes) when transferring a proton from the hydroxy group to a neighboring atom in the O-boron-containing zwitter-ion is an example of strictly degenerate prototropic tautomerism.

1,2,3-Oxadiazole rings in the aromatic compounds: A quantum-chemical investigation

Quantum chemical method PBE0/cc-pVTZ in conjunction with the polarizable continuum model (PCM) was applied to the calculation of the equilibrium structural parameters of the benzene and naphthalene 1,2,3-oxadiazole derivatives, as well as relevant diazocarbonyl compounds both in the free state and dissolved in dichloromethane. The calculated ionization potentials of benzo-1,2,3-oxadiazole (9.49 eV), 6-diazocyclohexa-2,4-dien-1-one (8.16 eV) and (6-oxocyclohexa-2,4-dien-1-ylidene)carbonyl (8.34 eV) differ from the experimental values no more than by 0.09 eV. The calculations indicate the possible existence of naphtho [2,3-d:6,7-d′]bis(1,2,3-oxadiazole) in the gas phase and in aprotic solvents.

Quantum chemical study of tautomerism of bullvalene, bullvalenol, and the corresponding ketone

Using the quantum chemical method PBE0/cc-pVTZ, we determined structural parameters and dipole moments of bullvalene (tricyclo[3.3.2.04.6]deca-2,7,9-triene), 1-, 2-, 3-, and 4-hydroxytricyclo[3.3.2.04.6]-deca-2,7,9-trienes, and isomeric 2- and 3-ketones transforming into each other through the keto-enol tautomerism and Cope rearrangement with the triatomic ring migration. The dominant tautomer is 3-ketone containing a triatomic ring conjugated with the carbonyl group.

Solvent induced quadrupole polarization of the molecules of diazo and diazocarbonyl compounds: Quantum-chemical investigation

Equilibrium structural parameters, atomic charges, and quadrupole moments of the molecules of 2,4,6-tris(diazo)cyclohexane-1,3,5-trione, 3,6-bis(diazo)cyclohexane-1,2,4,5-tetraone and its anion, isomeric 3,6-bis(diazonium)cyclohexanediondioles and 4,4′-bis(diazonium)-1,1′-biphenyl in a vacuum and in dichloromethane were calculated by the quantum chemical method PBE0/cc-pVTZ using the polarizable continuum model (PCM).The p-quinoid dication is more favorable than o-quinoid dication by 20 kcal mol-1 in a vacuum and by 13 kcal mol−1 in dichloromethane.The norm of the quadrupole moment of the centrosymmetric molecules in a polarizable medium increases while that of the dications decreases. The quadrupole polarization of molecules and molecular ions by the solvent does not change their symmetry point group.

Quantum-chemical study of barbaralone in singlet and triplet states

Equilibrium structural parameters of barbaralone molecule in the S0, T1, and T2 states have been determined by DFT (U)PBE0/cc-pVTZ method. The calculated barrier of the Cope rearrangement in the singlet state (9.4 kcal/mol) was close to the experimental activation energy. The equilibrium structure in the T1 state (29.3 kcal/mol) was similar to the S∩ intermediate structure. The S0 → T2 excitation (69.2 kcal/mol) was accompanied by extension of the carbon-oxygen bond (1.207 Å → 1.284 Å) and decreasing the molecule dipole moment (3.51 D → 2.77 D).

Quantum chemical study Ca@C60 and Sc+@C60 endo complexes in the gas phase and pyridine

DFT (U)PBE0/cc-pVDZ calculations were carried out to determine the structural parameters of the endo complexes Ca@C60 and Sc+@C60 in the gas phase and in pyridine. The (3A1//C3v)-Ca@C60 and (3A1//C3v)-Sc+@C60 triplets are higher in energy than the (1A1//C2v)-Ca@C60 and (1A1//C2v)-Sc+@C60 singlets by 0.21 and 2.61 kcal/mol in the gas and by 0.30 and 2.67 kcal/mol in pyridine. The dipole moments of the zwitter ions (1A1//C2v)- and (3A1//C3v)-Ca@C60 are 0.86 and 0.99 D (1.44 and 1.77 D in pyridine). The β decay 45Ca → 45Sc+ is accompanied by population of the 3d orbital of the endo atom and strengthening of its bond with carbon. The 1A′//Cs, 3A1//C2v, and 3A1//C5v electronic states correspond to low (not higher than 0.54 kcal/mol) barrier of calcium migration inside the cavity. The delocalization of the endo atom provides a highly symmetric mixed state of the fluctuating endo complex. The Stone-Wales (1A1//C2v)-(Ca, Sc+)@C60 rearrangement increases the energy of the endo complex by 9.9 (Ca) and 3.1 kcal/mol (Sc+).

Quantum-chemical study of tautomers of reduced forms of anthraquinone

The quantum-chemical method B3LYP/6-311G(d,p) was used to calculate structural parameters of four conformers of 9,10-dihydroxyanthracene, two conformers of 9-hydroxyanthrone, and the corresponding anions, dianion, and radical anion. The energy of 9,10-dihydroxyanthracene in a gas phase is higher and in aqueous solution lower than the energy of 9-hydroxyanthrone. The dianion can exist exclusively in a polarizable medium.

Quantum chemical study of polycyclic derivatives of cyclohexa-1,3,5-triene

A possibility of the existence of cyclohexa-1,3,5triene derivatives has been proved by the synthesis of compound I [1]. In this work, the quantum chemical calculations of the structural parameters, relative energies, and vibration spectra of similar compounds were performed by PBE0/cc-pVTZ and RHF/3-21G methods using a GAUSSIAN-03 software [2]. The following values were obtained by the PBE0 method. degenerate Cope rearrangements [3,4]. The degenerate Cope rearrangements in the hypothetical isomers III of dedihydrobullvalene C10H8 fragments must be accompanied by oscillations of the bond lengths in the central rings. The relative energy (kcal mol) of the isomers decreases as the alternation of these bond lengths reduces: 33.7 (IIIa) > 22.6 (IIIb) > 7.6 (IIIc) > 0.0 (IIId). Isomer IIId has the smallest bonds alternation in the central ring (1.38 and 1.41 A). For isomer IIIb the obtained bonds lengths are 1.36 and 1.43 A, and for IIIc, 1.37 and 1.42 A. DOI: 10.1134/S1070363213040294

“Vertical” and “Adiabatic” proton affinity

The protonation of some Brønsted bases is accompanied by a significant structural reorganization leading to a decrease in the rate of the processes of addition and detachment of the mobile protons. Such cases are considered as a combination of high thermodynamic basicity with low nucleophilicity [1] or with low kinetic basicity [2]. However, by analogy with the “vertical” ionization potential, which characterizes the process of the instantaneous removal of an electron, and the “adiabatic” ionization potential, which takes into account the difference between the equilibrium structural parameters of the cation and the parent molecule, it is possible to distinguish the “vertical” and “adiabatic” proton affinity (VPA and APA). In this report the quantum chemical method PBE0 using the cc-pVTZ orbitals for the nitrogen and mobile hydrogen and the cc-pVDZ orbitals for the carbon and for hydrogen atoms strongly bound to carbon and Gaussian-03 software [3] were applied to calculate the VPA and APA values for the quinuclidine and 1,8-bis(dimethylamino)naphthalene molecules.