Polimorfizem dihidrata acetilen dikarboksilne kisline

magistrsko delo

Natalija Klančnik (Author), Amalija Golobič (Mentor), Franc Požgan (Thesis defence commission member), Franc Perdih (Thesis defence commission member)

Abstract

V magistrskem delu sem s pomočjo rentgenske difrakcije, diferenčne termične kalorimetrije (DSC) in IR spektroskopije preučevala polimorfizem ob ohlajevanju in/ali devteriranju dihidrata acetilen dikarboksilne kisline (ADAD). V literaturi so to spojino skupaj z bolj znanim dihidratom oksalne kisline že opisovali zaradi preučevanja vpliva temperature in devteriranja na vodikove vezi. V primeru ADAD so opazili, da pride z nižanjem temperature do vsaj enega faznega prehoda, vendar s tedanjo opremo niso uspeli določiti kristalne zgradbe nizkotemperaturnih oblik, so pa določili lege nevodikovih atomov v strukturi pri sobnih pogojih. Z rentgensko difrakcijo in DSC analizo sem ugotovila, da na temperaturnem območju med 293 in 150 K tako v primeru protične kot devterirane ADAD obstajata dve polimorfni kristalni obliki α in β, pri čemer ob ohlajevanju prva preide v drugo v primeru protične ADAD pri približno 4,9 °C, sicer pa je fazni prehod enatiotropen s histerezo. Oba polimorfa imata monoklinsko osnovno celico in prostorsko skupino št. 14. Razlike v dimenzijah parametrov osnovne celice in v praškovnem difraktogramu potrjujejo, da gre za polimorfni kristalni obliki. Opravila sem rentgensko strukturno analizo pri 293, 263 in 150 K in označila ustrezne strukture ADAD za protično obliko z α, β1 in β2 ter za devterirano ADAD z αD, β1D in β2D. V vseh treh strukturah α, β1 in β2 se v osnovni celici nahajata dve planarni, centrosimetrični molekuli kisline in štiri molekule vode. Vsaka molekula vode je preko obeh vodikovih atomov donor ene srednje močne in ene šibke intermolekularne vodikove vezi, hkrati pa je akceptor močne vodikove vezi, ki jo donira OH skupina iz kislinske karboksilne skupine. Na prvi pogled sta bili strukturi obeh polimorfnih oblik precej različni, vendar se je po transformaciji osnovne celice in ustrezni izbiri asimetrične enote v β obliki izkazalo, da je razporejanje molekul v obeh polimorfnih oblikah sorodno, predvsem je enaka shema povezovanja z intermolekularnimi vodikovimi vezmi. Ob faznem prehodu pride le do spremembe v medsebojni orientaciji molekul, vodikove vezi pa se ne prekinejo. Ob ohlajevanju postanejo te vezi le nekoliko močnejše. Enake zaključke lahko podamo, če primerjamo med sabo tri določene strukture devterirane ADAD. Ob devteriranju se zaradi nekoliko večje prostornine devterija glede na protij nekoliko poveča volumen osnovne celice, vezne razdalje med nevodikovimi atomi so praktično enake. Opazimo lahko običajni geometrijski izotopski efekt, to pomeni manjše povečanje kontaktnih razdalj med kisiki iz vodikovih vezi, ne pride pa do izotopskega polimorfizma, kot pri dihidratu oksalne kisline. Tudi izračunani indeksi izostrukturnosti potrjujejo izostrukturnost devteriranih oblik z ustrezno protično. IR spektri protične in devterirane ADAD, posneti pri sobni in nizkih temperaturah, so v skladu z rezultati rentgenske strukturne analize.

Keywords

acetilen dikarboksilna kislina dihidrat;vodikova vez;geometrijski izotopski efekt;polimorfizem;izostrukturnost;rentgenska strukturna analiza;magistrska dela;

Data

Language:	Slovenian
Year of publishing:	2020
Typology:	2.09 - Master's Thesis
Organization:	UL FKKT - Faculty of Chemistry and Chemical Technology
Publisher:	[N. Klančnik]
UDC:	548.33(043.2)
COBISS:	26937091
Views:	453
Downloads:	117
Average score:	0 (0 votes)
Metadata:

Other data

Secondary language:	English
Secondary title:	Polymorphism of acatylenedicarboxylic acid dihydrate
Secondary abstract:	In my master's thesis, I studied polymorphism with cooling and/or deuterization of acetylenedicarboxylic acid dihydrate (ADAD) by X-ray diffraction, differential thermal calorimetry (DSC) and IR spectroscopy. ADAD, along with the better known oxalic acid dihydrate, has already been described in the literature studying the effect of temperature and deuterization on hydrogen bonds. In the case of ADAD, at least one phase transition was observed at decreasing of temperature, but the crystal structure of low-temperature modifications could not be determined with the equipment of the time. The positions of nonhydrogen atoms in the structure were determined at room conditions. By X-ray diffraction and DSC analysis, I found that in the temperature range between 293 and 150 K, in the case of both protic and deuterated ADAD, there are two polymorphic crystalline forms α and β. In the case of cooling of protic ADAD α transforms to β at about 4,9 °C. The phase transition is enantiotropic and show hysteresis. Both polymorphs have a monoclinic unit cell and space group no. 14. The differences in the dimensions of their unit cell parameters and of their powder diffractogram confirm that α and β are polymorphic modifications. I performed X-ray structural analysis at 293, 263, and 150 K and labelled the appropriate ADAD structures for the protic form with α, β1, and β2, and for deuterated ADAD with αD, β1D, and β2D. In all three structures α, β1 and β2, there are two planar, centrosymmetric acid molecules and four water molecules in the unit cell. Each water molecule is a donor of one medium and one weak intermolecular hydrogen bond through both hydrogen atoms. Each water molecule is also an acceptor of a strong hydrogen bond donated by the OH group from the acid carboxylic group. Initially the structures of the two polymorphs seemed quite different, but after unit cell transformation and appropriate selection of the asymmetric unit in β, the arrangement of molecules in both polymorphs turned out to be similar. During the phase transition, there is only a change in the mutual orientation of the molecules, while the hydrogen bonds are not broken. At cooling these bonds become somewhat stronger. The same conclusions can be drawn by comparing the three structures of deuterated ADAD. During deuteration the volume of the unit cell slightly increases, due to the slightly larger volume of deuterium relative to the protium. The bond distances between nonhydrogen atoms are practically the same. The usual geometric isotopic effect can be observed, i.e. a small increase in the contact distances between the oxygen from the hydrogen bonds, but no isotopic polymorphism occurs as with oxalic acid dihydrate. The calculated isostructurality indices also confirm the isostructurality of the deuterated forms with the corresponding protic. The IR spectra of protic and deuterated ADAD recorded at room and low temperatures are in the agreement with the results of X-ray structure analysis.
Secondary keywords:	X-ray structure analysis;polymorphism;isostructurality;geometric isotope effect;
Type (COBISS):	Master's thesis/paper
Study programme:	1000375
Embargo end date (OpenAIRE):	2021-08-20
Thesis comment:	Univ. v Ljubljani, Fak. za kemijo in kemijsko tehnologijo, smer Kemija
Pages:	52 str.
ID:	12058468