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The molecular structure and conformational properties of tryptophan have been investigated by gas-phase electron diffraction and theoretical methods. Quantum chemical calculations realized by program Gaussian 03 (B3LYP/cc-pVTZ) have been predicted the existence of six conformers at the temperature of experiment (T = 495 K). The ability of gas-phase electron diffraction method to distinguish the structure of conformers was estimated theoretically. Conformers have different orientations of carboxylic and amine group, backbone and indole fragment to each other. These conformers can be divided on two groups: distinguishable parameters (with different torsion angle C(OOH)-C(HNH2)-C(H2)-C(ind)) and weekly distinguishable ones (with different torsion angles H-N-C-C and H-O-C-C) by gas-phase electron diffraction. The molecular parameters of the conformers were determined. The conformers have intramolecular hydrogen bonding of the H2N···HO. The analysis of the gas-phase electron diffraction data have been carried out assuming the saturated vapor of tryptophan at T = 495 K consists of mixture at least of two conformers with lowest energy. It was shown that optimal ratio between conformers I : II was 50 : 50, respectively. The geometrical parameters of amino acids molecules (glycine, alanine, tryptophan) obtained by gas-phase electron diffraction were compared. The influence of the intramolecular hydrogen bond was established onto the structural parameters of the backbone of the above amino acids molecules.

The variants of structural organization in the systems “mesogen – nonmesogen” are considered. The systems contain p-n-propyloxycinnamic acid (A), as a mesogenic component, and nonmesogenic Ph–X–Ph compounds: phenyl benzoate (B, where X = –COO–), azobenzene (C, where X = –N=N–) and N-benzylideneaniline (D, where X = –CH=N–). Quantum-chemical modeling of possible structural units in such systems has been performed. It was shown that all assumed A∙∙∙X(Ph)2 H-complexes do not have electronic and geometric anisotropy and have a lower intermolecular interaction energy than the cyclic dimer of acid A∙∙∙A. The calculated values of the Gibbs free energy of complexation reactions also indicate a low probability of the formation of A∙∙∙X(Ph)2 type H-complexes. It is noted that the “length” of the A∙∙∙A dimer is comparable with the doubled “length” of Ph–X–Ph molecules, which, like the acid dimer, have a rod-like structure favorable for the formation of nematic and smectic LC phases. Based on the analysis of the quantum chemical calculations, it was assumed that Ph–X–Ph can be embedded between acid cyclic dimers A∙∙∙A and can facilitate reduce intermolecular interactions in the system, which reduces the temperature of Cr–LC transitions. The proposed structural organization of systems A: Ph–X–Ph is confirmed by an experimental IR spectrum for a similar system, in which the bands corresponding to the vibrational frequencies of the acid dimer and to individual molecules of alkyloxy substituted phenyl benzoate B are recorded.

Quantum chemical calculations of the geometric structure, force fields and harmonic vibration frequencies of the molecule Mn(acac)3 for electronic states with multiplicities M = 1, 3 and 5 were performed using the GAUSSIAN 09 program in the framework of density functional theory (DFT/UB3LYP) with correlation-consistent valence three-exponential basis functions cc-pVTZ. The structure with high-spin state S=2 (symmetry of electronic state 5B) possesses the lowest energy and it is characterized by C2 symmetry. The coordination polyhedron MnO6 possesses the shape of “elongated octahedron”. The high-spin state 5A is characterized by structure of compressed octahedron. The distortion of octahedral structure of coordination polyhedron in the states 5A and 5B is significant, and this fact testifies to the strong Jahn-Teller effect, or vibronic effect, in 5E electronic state. The calculations for low-spin state S=0 are notable for some specifics. The optimization resulted in C2 symmetry of molecule having the symmetry of electronic state 1B. The bond distances Mn-O within 0.001 Å were equal to values obtained for structure with D3 symmetry with S=1. This result corresponds to the situation if two electrons occupy different 1e orbitals possessing opposite spins. The spin states 3A2 and 1B lie higher than the high-spin state by 5.2 and 17.3 kcal/mol, respectively. The structural features are explained well in a framework of simple crystal field theory indicating that d-orbitals of Mn3+ ion undergo the significant influence of ligand field.Forcitation:Berger R.J.F., Girichev G.V., Giricheva N.I., Petrova A.A., Sliznev V.V., Tverdova N.V. Molecular structure of manganese tris-acetylacetonate in different spin states. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2017. V. 60. N 4. P. 47-53.

The molecular structure of tris(acetylacetonate)iron, Fe(O2C5H7)3, has been determined by synchronous gas-phase electron diffraction and mass spectrometric experiment along with quantum chemical calculations at the DFT/UB3LYP/cc-pVTZ theory level. The best agreement between theoretical and experimental intensity curves of scattered electrons was obtained for the high spin model with the symmetry D3 of molecule. The DFT calculations also resulted in high spin electronic state 6A1 of the molecule Fe(O2C5H7)3 as the ground one at the geometrical configuration of D3 symmetry.Forcitation:Petrova A.A., Tverdova N.V., Giricheva N.I. Girichev G.V. Molecular structure of tris(acetylacetonate)iron studied by gas-phase electron diffraction and DFT calculations. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2017. V. 60. N 3. P. 97-99.