An experimental and computational investigation of organic molecular cocrystals

Carstens, Thalia (2017-12)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: Cocrystals have historically provided a unique platform for studying and tuning structure-property relationships of materials. In this study, co-crystallisation was specifically investigated as a possible way to prevent unwanted dimerisation in dithiadiazolyl (DTDA) radicals. Attempts were made to co-crystallise DTDAs with one another, as well as with TEMPO radical, p-benzoquinone (BQ) and 2,3-dichloro-5,6-dicyanobenzoquinone (CCBQ). A cocrystal between the TEMPO radical and CCBQ, {TEMPO}{CCBQ}, was successfully obtained by both co-grinding of the co-formers and recrystallisation from methanol. Molecular gas-phase and periodic binding energy calculations on the three known DTDA cocrystals showed that the heterodimers (cocrystals) were consistently more stable than the homodimers of the co-formers. Using the known cocrystal results as a benchmark, similar calculations carried out on a series of experimentally attempted co-former combinations indicated that some combinations may warrant further investigation. The DTDA radical 4’-(2,6-difluorophenyl)-DTDA (3) is known to be polymorphic, and conversion between these polymorphs was observed in this study. Differential scanning calorimetry (DSC) identified a phase change from 3γ to 3β upon heating, and a transformation of 3α to 3γ was observed over time at room temperature, however this has not yet been observed using thermal analysis. The effect of a change in halogen substituent on the structures of a series of {p-halophenol}{p-benzoquinone} ({XP}{BQ}, where X is a halogen) cocrystals has been investigated. Two new cocrystals, {4-fluorophenol}{BQ} ({FP}{BQ}) and {4-iodophenol}{BQ} ({IP}{BQ}), were found in this study, where the chloro- and bromo-derivatives are previously known. Both 1:1 and 2:1 cocrystals of {FP}{BQ} was obtained concomitantly. The 1:1 form, {1FP}{BQ}, also has two polymorphs where α-{1FP}{BQ} and β-{1FP}{BQ} were obtained by recrystallisation and sublimation, respectively. {IP}{BQ} was only found to crystallise as the 2:1 form. This form also has two polymorphs, with α-{2IP}{BQ} and β-{2IP}{BQ} obtained from recrystallisation and sublimation, respectively. Although the previously-known cocrystals were found to be isostructural to one another, none of the newly obtained forms or polymorphs were isostructural to each other or to the known cocrystals.

AFRIKAANSE OPSOMMING: Kokristalle het histories ‘n unieke platform geskep vir die bestudering en afstemming van materiale se struktuur-eienskap verhoudings. In hierdie studie is kokristallisasie spesifiek ondersoek as 'n moontlike manier om ongewenste dimerisasie in dithiadiazolyl radikale (DTDA) te voorkom. Daar is gepoog om DTDA's met mekaar te kristalliseer, sowel as met TEMPO radikaal, p-bensokinoon (BQ) en 2,3-dichloor-5,6-disiano bensokinoon (CCBQ). 'n Kokristal tussen die TEMPO radikaal en CCBQ {TEMPO}{CCBQ} is suksesvol verkry deur beide mede-maling van die mede-vormers, en herkristallisasie uit metanol. Molekulêre gasfase en periodiese bindingsenergie berekenings op die drie bekende DTDA-kokristalle het getoon dat die heterodimere (kokristalle) konsekwent meer stabiel is as die homodimere van die mede-vormers. Deur gebruik te maak van die bekende kokristal resultate as 'n maatstaf, het soortgelyke berekenings wat uitgevoer is op 'n reeks eksperimentele kokristal pogings aangedui dat sommige van dié kombinasies verdere ondersoek kan regverdig. Die DTDA radikaal 4’-(2,6-difluorofeniel)-DTDA (3) word geken om polimorfies te wees, en in hierdie studie is die verandering tussen dié polimorfe waargeneem. Differensiële skanderingskalorimetrie (DSK) het 'n faseverandering van 3γ tot 3β geïdentifiseer tydens verwarming, en 'n transformasie van 3α tot 3γ is mettertyd waargeneem by kamertemperatuur. Maar laasgenoemde is nog nie met behulp van termiese analise bevestig nie. Die effek van 'n verandering in halogeensubstituent op die strukture van 'n reeks {p-halofenol}{p-benzoquinone} ({XP}{BQ}, waar X 'n halogeen is) kokristalle is ondersoek. Twee nuwe kokristalle, {4-fluorofenol}{BQ} ({FP}{BQ}) en {4-jodofenol}{BQ} ({IP}{BQ}), is in hierdie studie gevind waar die chloor- en broomderivate vooraf bekend is. Beide 1:1 en 2:1 kokristalle van {FP}{BQ} is gelyktydig verkry. Twee polimorfe van die 1:1-vorm, {1FP} {BQ}, het ook verskyn, waar α-{1FP}{BQ} en β-{1FP}{BQ} onderskeidelik deur herkristallisasie en sublimasie verkry is. {IP}{BQ} het slegs gekristalliseer as die 2:1-vorm. Hierdie vorm het ook twee polimorfe, waar α-{2IP}{BQ} en β-{2IP}{BQ} onderskeidelik verkry is uit herkristallisasie en sublimasie. Alhoewel die voorheen bekende kokristalle bevind is om isostruktureel te wees teenoor mekaar, was geen van die nuut verkrygde vorms of polimorfe bevind om isostruktureel teenoor mekaar, of teenoor die voorheen bekende kokristalle te wees nie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102605
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