Аннотация:Experimental and theoretical studies of the structure of organic molecules containing small cyclic fragments (such as cyclopropane and cyclobutane rings) are of great interest due to the uniqueness of some of their physical and chemical properties.Previously [1], it was shown that the molecule of cyclobutanecarboxaldehyde (CBCA) is a complicated object for theoretical and experimental investigation of its conformational and vibrational dynamics. The data obtained on the structure of the molecule potential energy surfaces (PESs) suggest the complex nature of the conformational dynamics of this molecule even during transitions to lower vibrational levels: the minimal energy pathways may have a marked curvature or breakings; also, the form of the vibrations can be significantly affected by the behavior of the kinetic energy of the molecule.In the present work, the authors attempt to qualitatively characterize internal rotation in some model systems containing cyclobutene ring and compare the results with previously obtained for CBCA molecule. Two isomers of C5H6O, namely cyclobut-1-ene-1-carbaldehyde (CB1CA) and cyclobut-2-ene-1-carbaldehyde (CB2CA) were chosen as the objects of investigation. The conformational behavior and geometrical structure of the presented two related molecules were studied using the different methods of quantum chemistry, one- and two-dimensional sections of the PESs along the coordinates of the internal rotation and the closest to it non-planar deformational vibration of the formyl (CHO) group were constructed and analyzed. The possibility of kinematic coupling of these two vibrations and the complexity of their shapes are considered in particular detail. The kinematic interaction of these two low-frequency vibrations of the same symmetry can significantly depend on the relative position of the formyl group and the cycle in the case of CB1CA and CB2CA molecules. This relationship is much more complicated in the case of CBCA molecule.Such studies are necessary for modeling the IR spectra and obtaining accurate theoretical estimates of thermodynamic functions sensitive to the quality of the description of the low-frequency region of the vibrational spectra.
