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Molecular mobility and conformational changes in single crystals of triacylmethanes

Over the previous decades the quantity of evidence of the mobility of molecules within the crystal phase increased dramatically. The extensive research of crystal disorder, phase transitions and chemical reactions in the crystalline state have almost completely done away with the classical prejudice of crystals as chemical graveyards, consisting of motionless molecules frozen in a stiff periodical array. Our study over the past several years has been concentrated on the study of triacylmethanes (acyclical 1, 3, 3’ -triketones) in the solid state. The usual conformation of their molecules is such that the triketo group has an approximate C3 symmetry and thus is chiral, with the carbonyl groups twist defining a left (M) or a right (P) helix. In the solution the two enantiomeric conformers easily interconvert, but in the solid state such changes of molecular conformation is not expected. Our studies however demonstrate that, in some cases, the flipping of the triketo group i.e. interconversion of M and P conformers does indeed occur in the crystalline state. In two triacylmethanes we have discovered that undergo single crystal to single crystal phase transformations which include the flipping of the triketo group in a portion of the molecules in the crystal. Tripivaloylmethane (tpm) crystallizes at room temperature with a conformational enantiomeric disorder so that the M and P conformers are placed stochastically throughout the crystal structure. Upon cooling to 110 K, the structure partially orders ; two molecules in the unit cell order into two different conformations of opposite chirality, while the third remains disordered. In order for such a structural change to occur, the conformation of at least one-third of the molecules had to be inverted. An even more evident case of conformational change occurs in (3-cyclopentylpropanoyl)dipivaloylmethane (dp(cpp)m) where at room temperature the structure consists of columns of molecules in which the twist of the triketo group alternates. At ca -22 °C a phase transition occurs so that every alternate molecule changes conformation, rendering the molecular columns homochiral. This change is therefore includes the inversion of the conformation of one-half of all the molecules in the crystal. We have also used a closer inspection of the difference electron map of several other triacylmethanes, which do not exhibit phase transitions of such kind, in order to ascertain whether the conformation changes may occur spontaneously, without leading to phase transition. Our preliminary results indicate that this is so for the majority of aliphatic triacylmethanes.

molecular conformation; single crystal; triacylmethanes

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