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Non-classical structures of organic compounds are Non-classical structures of organic compounds are defined as molecules containing non-tetrahedral tetracoordinate defined as molecules containing non-tetrahedral tetracoordinate and/or hypercoordinate carbon atoms. The evolution of the views and/or hypercoordinate carbon atoms. The evolution of the views on this subject is considered and the accumulated theoretical and on this subject is considered and the accumulated theoretical and experimental data on the structures and dynamic transformations experimental data on the structures and dynamic transformations of non-classical organic compounds are systematised. It is shown of non-classical organic compounds are systematised. It is shown that computational analysis using the methods and the software that computational analysis using the methods and the software potential of modern quantum chemistry has now acquired high potential of modern quantum chemistry has now acquired high predictive capacity and is the most important source of data on the predictive capacity and is the most important source of data on the structures of non-classical compounds. The bibliography includes structures of non-classical compounds. The bibliography includes 227 references. 227 references.

Ab initio (RHF/LanL2DZ, MP2(fc)/LanL2DZ, MP2(fc)/6-31G**, and MP2(fc)/6-311++G**) calculations were performed for a series of β -chalcogenovinylaldehydes, 1,6-dioxa-6a-chalcogenopentalenes, and bimolecular complexes of formaldehyde with chalcogen hydrides and chlorides. The calculations reproduce well the existence and experimentally observed structural peculiarities of the intra- and intermolecular Ο - > chalcogen attractive interactions that stabilize the hypervalent T-shaped bond configuration at a chalcogen atom. These interactions increase in the order S, Se, Te and with the increasing electronegativity of a substituent attached to the chalcogen center. The ab initio calculations performed predict the existence of sufficiently stable bimolecular complexes H2CO . . .XR1R2 (X = S, Se, Te; R1, R2 = H, Cl) with a complexation energy comparable to the energy of a strong hydrogen bond.Key words: ab initio calculations, chalcogen-containing compounds, intramolecular coordination.

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