Журнал "Макрогетероциклы"

Valeriy V. Sliznev,^@ and Georgy V. Girichev

Research Institute of Macroheterocyclic Compounds, Ivanovo State University of Chemistry and Technology, 153000 Ivanovo, Russia

^@Corresponding author E-mail: sliznev@isuct.ru

DOI: 10.6060/mhc2011.1.02

The information about geometrical and electronic structure of free molecules M₂Pz, Na₂Pc, K₂Pc is not available. The data concerning structure of Li₂P complex are not definite. The electronic spectra of M₂P, M₂Pz, Na₂Pc and K₂Pc are not reported. Here the systematic study of the electronic and geometrical structures of M₂P, M₂Pz and M₂Pc complexes is performed. Density functional theory (three-parameter hybrid functional B₃LYP), Moller- Plesset perturbation theory (MP₂) and correlation-consisted valence triple-zeta basis sets (cc-pvtz) are used. All calculations are carried out using PC GAMESS program. For all molecules several geometrical configurations with different positions of alkali atoms relatively the macrocycle plane are optimized. The structures with alkali atoms adjacent to the same side of the macrocycle (C_2v symmetry) and with alkali atoms lying inside of the macrocycle (D_2h) correspond to saddle points on PES and possess high relative energies (>150 kJ/mol). More energetically favourable configuration of all complexes excepting Li₂P possesses D_4h symmetry with alkali atoms taking up symmetrical positions at different sides of the macrocycle. According to B₃LYP method the equilibrium structure of Li₂P is the C_2h geometry originating from distorted D_4h configuration. However MP₂ calculation leads to the conclusion about D_4h symmetry of equilibrium structure. Perhaps more sophisticated calculations are needed to determine shape equilibrium configuration in the case of Li₂P. Analysis of all trends is performed for D4h configuration and is based on the data obtained at B₃LYP level. The geometrical parameters of the macrocycle change systematically in the line Li₂L → Na₂L → K₂L (L=P, Pz, Pc): Re(N-Cα) and ϕe(CαNCα) increase by ~0.01 Å and ~2º, respectively; Re(C_α-Cβ) decreases by ~0.01 Å. Re(Cα-Xm) value and the macrocycle cavity size (Re(N⋅⋅⋅N)) increase by 0.011÷0.016 Å and 0.11÷0.15 Å, respectively. Re(Li-Li) does not change practically in the row Li2P → Li2Pz → Li2Pc (increasing -0.006Å). Unlike the Re(Li-Li) trend the magnitudes of Re(Na-Na) and Re(K-K) distances in the similar rows increase by 0.134 and 0.249 Å. Perhaps, so different trends can be explained by decreasing of the macrocycle cavity size in M₂P → M₂Pc → M₂Pz line and increasing of the alkali atom radius. Natural net charges on the alkali atoms in all studied compounds are situated in the small range 0.90 ÷ 0.96 ē. Extra negative charge (~2ē) of the macrocycle is located on four pyrrol cycles in M₂P complexes or on four nitrogen meso-atoms in M₂Pc and M₂Pz complexes. According to NBO analysis the bond orders of M-M and М-N are close to zero. Thus the М-N bond is ionic bond. The strong electrostatic attraction between one M⁺ cation and four N¯ anions results in the contraction of M-M distances in comparison with the same bond length in M₂ molecules. Moreover along with the strong electrostatic attraction there is quite noticeable donor-acceptor interaction between lone pairs of the nitrogen atoms and valence ns-orbital of the alkali atom. Perhaps in the case with small size of ns-orbital and large size of the macrocycle cavity (for example, Li₂P) the alkali atom can be moved toward the nitrogen atom to increase the donor-acceptor interaction. The TDDFT calculation of the transition energies between ground and low-lying excited states shows that electronic spectra in the visible and near UV regions arise from the transition between bonding and anti-bonding π-orbitals of the macrocycle. Indirect effect of the metal atom nature on the electronic spectra results in a shift of the virtual and occupied molecular orbitals. Therefore the electronic spectra depend slightly on a nature of the alkali atom. The comparison with available for Na₂P and K₂P X-ray data shows that the theoretical values of the macrocycle geometrical parameters and the valence angle ϕe(N_xMN_y) are close to the experimental data. Differences of the Re(M-N) distances are 0.049 and 0.056 Å for Na₂P and K₂P, respectively. Probably, the coordination of the metal atoms by two molecules of the solvent together with four nitrogen atoms in crystalline phase leads to the so large differences. The comparison of the observed electronic spectra for disodium and dipotassium porphine derivates and Li₂Pc and calculated data points to a good agreement of experimental and theoretical spectra. Moreover the assignment of some observed bands is performed.

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