As per my understanding, noncovalent interactions play a crucial role in determining the stability of molecular crystals at ambient as well as at high pressure. When pressure increases, the molecular entities become close to each other either they can form a closed pack structure due to cooperative nature or they under go a structural phase transition due to anti-cooperative behavior of two (or more) noncovalent interactions. How one can explain or validate this behavior in molecular crystals under high pressure.
Dear Yedukondalu Neelam
In general, the mechanism you described not obligatory right. Increasing hydrostatic (equal to all direction) pressure sometimes similar to decreasing temperature. So molecular entities become close to each other not only with increasing pressure but with decreasing temperature too.
Dear Professor,
Thank you for your reply. Yes, I agree with you. My question is how to predict or understand the cooperative or anti-cooperative nature with pressure/temperature in molecular crystals?
Dear Yedukondalu Neelam:
If I am understanding your question correctly, understanding or explaining the above mentioned behaviors are possible by structural characterization techniques such as X-ray diffraction, X-ray absorption spectroscopy (especially the near-edge part of the spectra, the XANES, which is more sensitive to the 3D topology around the absorber atom), Raman and Infra-red spectroscopy etc. Some of these techniques require synchrotron radiation, and some of them are possible in usual laboratories.
Features in a diffractogram or spectrum shift uniformly if there is only densification, or show abrupt changes if there are structural phase transitions.
Dear Yimin Mijiti,
To my knowledge from the literature, yes, one can use X-ray diffraction, IR and Rama spectroscopic techniques to determine the structural phase transitions in molecular crystals (X. Tan et al, J. Phys. Chem. B, 2012, 116, 14441−14450; A. Katrusiak et al, Cryst. Growth & Des., 2010, 10, 3461-3465; Q. Li et al, J. Chem. Phys., 2012, 137, 184905; S. Li et al, J. Phys. Chem. B, 2011, 115, 8981-
8988; S. Li et al, J. Phys. Chem. B, 2012, 116, 3092-3098; M. Pravica et al, Phys. Chem. B, 2016, 120, 2854−2858 and Y. Wang et al. Phys. Chem. C, 2016, 120, 29510−29519) as they do for normal (non-molecular) crystals. However, cooperative/anti-cooperative nature of two/more non-covalent interactions in a molecular crystal leads to structural phase transitions under high pressure/temperature. Moreover, cooperativity in non-covalent interactions in case of molecules and clusters are extensively studied (https://pubs.acs.org/doi/abs/10.1021/cr500344e and references therein). How to predict/determine the magnitude of the cooperativity in case of molecular crystals?
The effect of high pressures/low temperatures in molecular crystals have a slightly different behavior. The cooperative nature is like sponge, this effect is directly see in elastic modulii (i.e in C11, C22 and C33). The anti-cooperative nature leads to tensile slip among the molecules, these are directly effected by the elastic modulii of the crystallographic planes (i.e in C44, C55 and C66). Measure the elastic modulii for applied pressures and identify the cooperative/anti-cooperative nature and the change by calculation the percentage from the ambient phase.
Yes, this is one way of understanding the strength of intermolecular interactions along crystallographic axis and planes and it can be correlated with the compressibility of axes with pressure/temperature. I have done this correlation for couple of molecular crystals [10.1039/C5CP04294A ; https://doi.org/10.1063/1.4959900]. Apart from this, can we calculate the contribution of each noncovalent interaction present in a system at a given pressure and see how this changes with pressure?
That is a good work, you have seen the collective effect of intermolecular interactions. You can quantify the one-to-one interaction by measuring the interatomic distances for the applied pressure/temperature. The abrupt changes will help you to notify the direct effect. This you already did in your articels.
OR,
one has to formulate a model Hamiltonian to see the one-to-one effect, just write a simple Hamiltonian (similar to tight binding model) and diagonalise it in a MATLAB for eigen values and fit the energy curve for each pressure/temperature. This is how one can investigate.
- Badges
- Science topic