Naslov
Cocrystals of m-halogenopyridines with m- halogenobenzoic acids - controlling structural disorder by halogen bond

Autori
Stilinović, Vladimir ; Bedeković, Nikola

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
27th Slovenian-Croatian Crystallographic Meeting : Book of Abstracts / Leban, Ivan ; Popović, Stanko ; Meden, Anton ; Matković, Dubravka ; Giester, Gerald ; Golobič, Amalija ; Pevec, Andrej ; Počkaj, Marta ; Popović, Jasminka ; Prugovečki, Biserka ; Sabljić, Igor ; Skoko, Željko - Rogaška Slatina, 2019, 25-25

Skup
27th Slovenian-Croatian Crystallographic Meeting (SCCM 2019)

Mjesto i datum
Rogaška Slatina, Slovenija, 19.06.2019. - 23.06.2019

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Nije recenziran

Ključne riječi
halogen bonding

Sažetak
In the majority of everyday structural studies, structural disorder is viewed as an undesirable nuisance, which merely complicates refinement and interpretation of the structural model. However, disorder in crystal structures can have a great effect on the properties of the bulk material, and its study therefore is crucial for elucidating the interconnections between the microscopic structure and macroscopic properties of solid materials. Moreover, the presence, nature, and characteristics of disorder in crystal structures provide invaluable insight into the nature of the solid state, in particularly the molecular dynamics in solids, molecular aggregation upon nucleation and growth of the crystals as well as interplay between the effects of entropy and enthalpy on crystal stability. The latter issue presents one of the fundamental problems of thermodynamic stability of ordered systems. A disordered system is always entropically favoured, and for a system to be ordered, the ordering must be imposed through specific, directional, and strong interactions, which can overcome the entropic contribution by increasing the energy difference between the ordered and disordered system in favour of the (more)ordered one. It follows therefore that in the case of insufficient stabilisation of the ordered structure (e.g. in the absence of strong specific interactions), the disordered structure must be thermodynamically favoured. An interesting system which demonstrates this interrelationship are the cocrystals of m- halogenobenzoic acids with m-halogenopyridines (XbzacH–X’py, X, X’ = Cl, Br, I). All nine cocrystals comprise molecular complexes with the acid carboxyl group hydrogen bonded to the pyridine nitrogen atom. Four of these (with X, X’ = Cl, Br) are isostructural and crystallise in the P21/c space group with Z’ = 1/2 ; the molecules are in positional disorder so that the orientation of the complexes is random, i.e. the complexes are disordered about inversion centres. However, if either one of X or X’ is an iodine atom (a stronger halogen bond donor than Cl or Br), the structure becomes ordered (P21/c with Z’ = 1), as an I∙∙∙O halogen bond is formed with the carboxyl group, causing a difference between the two possible orientations of the hydrogen bonded acid- pyridine complex. In the case when both X and X’ were iodine atoms, however, the structure was again found to be disordered – unlike in the above cases, here the hydrogen bonded acid-pyridine complexes do not interact via I∙∙∙O halogen bonds, but rather by I∙∙∙I halogen bonds, thus no longer inducing an energy difference between the orientations of the complexes, leading again to their randomisation, and thus an increase of entropy. It is therefore evident that the participation of the carboxyl group in the I∙∙∙O halogen bonds with a neighbouring complex is a necessary prerequisite for the ordering of the XbzacH–X’py, crystal structures.

Izvorni jezik
Engleski

Znanstvena područja
Kemija

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