It is generally assumed that molecular structures concentrate in low-energy regions of conformational space in the solid state as they do in the gas phase. On the other hand, conformations that do not correspond to an intramolecular energy minimum are observed from time to time in the solid state; their occurrence is attributed to crystal-packing effects. We have recently encountered an interesting case while studying the crystal structure of the complexes [Co(phen)3][Cd2Cl7].3H2O and [Co(phen)3] [Cd2Br7].2H2O, containing two new discrete anions [Cd2X7]3- where X = Cl/Br.These anions have a ‘boomerang’ shape. In a preliminary attempt to investigate the factors effecting the molecular conformation, DFT calculations at the GGA-PW21/DNP level of theory on [Cd2X7]3- ions (X= Cl or Br) has been performed both in vacuum and in simulated water environment using the COSMO model. In vacuum, both molecules assume a linear geometry around the bridging atom, with Cd-X-Cd angles very close to 180°, while in simulated water the optimized geometries are similar to those found in crystals. A more detailed computational study has shown that while the molecules in vacuum are definitely more stable in the ‘linear’ conformation, in water they experience a very flat potential energy surface, which means that practically all conformations are allowed. Actually, the Metal-Halogen-Metal angle in analogous [Hg2X7]3- halomercurate ions of known structure (six entries in the Cambridge Structural Database) shows a great variability assuming values in the wide range 98.5 -180.0 degrees. The crystal environment seems therefore to be the ultimate factor determining the final conformation.
The influence of crystal structure on molecular conformation: a case study
FERRETTI, Valeria;
2011
Abstract
It is generally assumed that molecular structures concentrate in low-energy regions of conformational space in the solid state as they do in the gas phase. On the other hand, conformations that do not correspond to an intramolecular energy minimum are observed from time to time in the solid state; their occurrence is attributed to crystal-packing effects. We have recently encountered an interesting case while studying the crystal structure of the complexes [Co(phen)3][Cd2Cl7].3H2O and [Co(phen)3] [Cd2Br7].2H2O, containing two new discrete anions [Cd2X7]3- where X = Cl/Br.These anions have a ‘boomerang’ shape. In a preliminary attempt to investigate the factors effecting the molecular conformation, DFT calculations at the GGA-PW21/DNP level of theory on [Cd2X7]3- ions (X= Cl or Br) has been performed both in vacuum and in simulated water environment using the COSMO model. In vacuum, both molecules assume a linear geometry around the bridging atom, with Cd-X-Cd angles very close to 180°, while in simulated water the optimized geometries are similar to those found in crystals. A more detailed computational study has shown that while the molecules in vacuum are definitely more stable in the ‘linear’ conformation, in water they experience a very flat potential energy surface, which means that practically all conformations are allowed. Actually, the Metal-Halogen-Metal angle in analogous [Hg2X7]3- halomercurate ions of known structure (six entries in the Cambridge Structural Database) shows a great variability assuming values in the wide range 98.5 -180.0 degrees. The crystal environment seems therefore to be the ultimate factor determining the final conformation.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
