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The equilibrium torsion angles of 2-arylpyrroles in the liquid and solid phases were estimated by UV spectroscopy. In solution, compounds comaining no substituents in positions I, 3, and 2" possess an average torsion anne of 24 ~ those containing one substituent have an angle of 29 ~ and in the case of two and three substituents, the angles are 53 ~ and 65 ~ respectively. Phase transitions lead to tlattenmg of the molecules ha almost all cases. The average torsion angles in the compounds with no substituents in positions 1, 3, and 2" decrease by -5 ~ on passing from the gas to the liquid state and by -25 ~ on going from the liquid to the solid state. The geometric parameters of 2-arylpyrroles with one or two substituents in positions 1, 3, and 2" axe less sensitive to phase transitions, while trisubstituted derivatives even retain their equilibrium conformations upon phase transitions.Key words: equilibrium conformation of molecules, UV spectroscopy, semiempirical MO calculations, 2-arylpyrro[es.Of conformationaily mobile molecules with extended r~-systems, diphenyl and its derivatives have been structuraUy studied in greatest detail The equilibrium geometry of these compounds depends markedly on the nature of the substituents and on the physical state. For example, the equilibrium torsion angle in diphenyl varies from 44 ~ in the gas phase I to 30 ~ (or 20~ 2'3 in the hquid phase and to 0 ~ in the cr~3tal. 4 The conformations of a fairly large number (-20) of diphenyl derivatives having no substituents in the ortho-positions of the phenyl groups are susceptible to similar influences. 5 This is due to the fact that rotation of phenyl groups requires relatively little energy, which can be made up by intermolecular interactions. 6 The conformations of other linked cyclic systems also depend on the physical state. One of the few known examples of this dependence is 3,3 "-dithienyl, whose equilformm torsion angle is 30 ~ in the gas phase, 7 22 ~ in the nematic phase, s and 0 ~ in the crystal. 9 Structural studies of the vast majority of compounds of the diphenyl type have been carried out for one of the physical states, l~In particular, we determined the equilibrium torsion angles for a series of 2-phenylpyrroles (2-PP) in the gas phase by photoelectron spectroscopy. 13-16 Almost all of these compounds were found to be non-planar. In the present work, the equih'brium conformations of 2-arylpyrroles in solution and in a solid matrix were studied by UV spectroscopy.+Deceased. ExperimentalElectronic absorption spectra were recorded on a Specord M-40 spectrophotometer at 22--25 ~C. The concentrations of compounds in hexane and acetonitrlle amounted to 10 -4-10 -3 tool L -l. The spectra in the solid phase were recorded in poly-(1-vinyi-2-methylimidazole) (PVMD films and in KCI discs. The first procedure makes it possible to obtain more precise information on the positions of long-wavelength bands (Fig. 1); addition of PVMI has no effect on the UV spectra of the compounds in solution. However, this procedure is inapplicabl...