Rationalising the solid-state properties of dithiadiazolyl radicals using a combined theoretical and experimental approach

Dippenaar, Alwyn Bernard (2018-03)

Thesis (PhD)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: The work describes a series of theoretical and experimental studies carried out to better understand the solid-state properties of 1,2,3,5-dithiadiazolyl (DTDA) radicals. The attempted co-crystallisation of 4-(4'-cyano-2',3',5',6'-tetrafluorophenyl)-1,2,3,5-dithiadiazolyl (2) and 4-(2',6'-difluoro-phenyl)-1,2,3,5-dithiadiazolyl (7) yielded the α-phase of 2, which had previously been shown to only grow on a cold finger at -10 °C. The needle-shaped β-phase crystals of 2 were found to be the first DTDA radical that exhibits flexible properties. The experimental work focussed on the properties of four DTDA radicals. The synthesis of 4-(4'-bromo-2',3',5',6'-tetrafluorophenyl)-1,2,3,5-dithiadiazolyl (3) produced a mixture of the desired product and a novel co-crystal (3-cox). The crystal structure of 3-cox was found to contain dimeric pairs of 3 co-crystallised with starting material, 4-bromo-2,3,5,6-tetrafluoro-benzonitrile. The joint refinement of high-resolution X-ray diffraction (HXRD) and polarised neutron diffraction (PND) data was used to determine the spin density of 4-(2',3',5',6'-tetrafluoro-4'-nitrophenyl)-1,2,3,5-dithiadiazolyl (4). The spin density values were found to be in good agreement with previously determined experimental values, with the majority of the spin density on the sulfur and nitrogen atoms of the heterocyclic radical ring. In the computational part of this study, a series of calculations was performed to investigate why specific DTDA radicals prefer monomeric or dimeric modes of association in the solid state. Geometry optimisations and single point energy calculations showed that dimeric radicals yield attractive energies if the geometry optimisation and single point calculations are performed in the singlet or triplet state, whereas monomeric radicals only interact attractively in the triplet state, irrespective of how the geometry optimisation had been undertaken. Radical 3 was found to be the one exception as it exhibits attractive energies for both monomeric and dimeric modes of association. This agreed with the experimental results as the crystal structures of 3 and 3-cox exhibit monomeric and dimeric modes of association between molecules of 3, respectively. Geometry and interaction energy calculations on substituted radicals showed that the ortho and para groups could reduce the tendency of DTDAs to dimerise. Lastly, the results from the first and second sections were used as a basis for a series of calculations to predict the mode of association in radicals that have not yet been synthesised (‘unknown radicals’). Geometry optimisation and interaction energy calculations were performed on the unknown radicals arranged in eight monomeric or dimeric modes of association. The results were used to identify three of the compounds as likely to associate in the dimeric mode, while three were expected to form monomeric radicals. Crystals were obtained for 4-(3'-fluoro-4'-trifluoromethylphenyl)-1,2,3,5-dithiadiazyl (32) and 4-(4'-bromophenyl)-1,2,3,5-dithiadiazyl (33). The crystal structure of radical 32 shows that the molecules exhibit a trans-cofacial mode of association, which was computationally predicted as the most stable mode of association. Similarly, in the crystal structure of radical 33, the molecules exhibit a cisoid mode of association, which was computationally predicted as one of the most stable modes of association. Taken together, the results from the study show that a combination of theoretical and experimental methods provide a powerful tool for studying the properties of DTDAs.

AFRIKAANSE OPSOMMING: Die werk beskryf 'n reeks teoretiese en eksperimentele studies uitgevoer om die vaste toestand eienskappe van 1,2,3,5-dithiadiazoliel (DTDA) radikale te verstaan. Die eksperimentele werk het gefokus op die eienskappe van vier gesintetiseerde DTDA radikale. Die poging tot ko-kristallisasie van 4-(4-cyaan-2',3',5',6'-tetrafluoorfeniel)-1,2,3,5-dithiadiazoliel (2) en 4-(2',6'-difluoor-feniel)-1,2,3,5-dithiadiazoliel (7) het radikale geprodudeer was geïdentifiseer was as die α-fase van 2, waarvoor dit voorheen bepaal was dat dit slegs groei op n koue vinger by -10 ° C. Die naaldvormige β-fase kristalle van 2 was gevind om the eerste DTDA-radikale te wees wat buigsame eienskappe te vertoon. Die sintese van 4-(4'-broom-2',3',5',6'-tetrafluoorfeniel)-1,2,3,5-dithiadiazoliel (3) radikaal was gevind om 'n mengsel van 3 en 'n nuwe mede-kristal (3-cox) te produseer. Die kristalstruktuur van 3-cox toon ‘n dimeriese paar van 3 wat ko-kristalliseer met 4-broom-2,3,5,6-tetrafluoor-bensonitriel. Die “joint refinement” van HXRD en PND data was gebruik om die spindigtheid van 4-(2',3',5',6'-tetrafluoor-4'-nitrofeniel)-1,2,3,5-dithiadiazoliel (4) te bepaal. Die spindigtheid waardes stem goed ooreen met eksperimentele waardes wat voorheen bepaal was, met die meerderheid van die spindigtheid teenwoordig op die stikstof en swael atome van die heterosikliese ring. In die teoretiese afdeling van hierdie studie was 'n reeks berekeninge uitgevoer om te ondersoek hoekom spesifieke DTDA radikale ‘n monomere of dimeriese vorm van assosiasie verkies. Optimalisering en enkelpunt interaksie-energie berekeninge het getoon dimeriese radikale produseer aantreklike energieë indien die optimalisering en interaksie-energie berekening uitgevoer is in die singulettoestand, terwyl monomeriese radikale reageer slegs in ‘n aantreklike wyse indien hulle uitgevoer word in die triplettoestand, ongeag van die toestand waarin die optimalisering uitgevoer word. Radikaal 3 was die een uitsondering aangesien dit aantreklike energie vertoon vir beide monomeriese en dimeriese vorms van assosiasie. Dit stem goed ooreen met experimentele waardes aangesien die kristalstrukture van 3 en 3-cox beide monomeriese en dimeriese vorms van assosiasie vir 3 vertoon. Optimalisering en enkelpunt interaksie-energie berekeninge op gesubstitueerde radikale het getoon dat orto- en para-substituente die neiging van radikale om te dimeriseer kan verminder. Laastens is die resultate van die eerste en tweede afdelings gebruik as ‘n basis om ‘n reeks berekening op te stel, ten einde te kan voorspel watter vorm van assosiasie verkies sal word deur DTDA radikale wat nog nie gesintetiseer is nie (‘onbekende radikale’). Optimalisering en enkelpunt interaksie-energie berekeninge was uitgevoer op die onbekende radikale wat georiënteer was ten opsigte van agt monomeriese en dimeriese vorms van assosiasie. Drie van die verbindings was geïdentifiseer met die potensiaal om die dimeriese vorm van assosiasie te verkies, terwyl drie ander verbindings die monomeriese vorm van assosiasie sal verkies. Kristalstrukture was verkry vir die radikale 4-(3-fluoor-4-trifluoormetileen)-1,2,3,5-dithia-diazoliel (32) and 4-(4-broom-feniel)-1,2,3,5-dithiadiazoliel (33). Die kristalstruktuur van radikaal 32 vertoon molekules in ‘n “trans-cofacial” vorm van assosiasie. Daar was ook gevind dat hierdie vorm van assosiasie die stabielste vorm is deur middel van berekeninge. Soortgelyk is daar gevind dat die kristalstruktuur van radikaal 33 vertoon molekules in ‘n “cisoid” vorm van assosiasie, wat ook deur berekeninge geïdentifiseer was as een van die stabielste vorms van assosiasie. Ten einde het die resultate in hierdie studie gewys dat 'n kombinasie van teoretiese en eksperimentele metodes 'n kragtige instrument bied om DTDA's te kan bestudeer.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103851
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