Abstract
High-quality crystals of a certain polymorphic form of thiobarbituric acid containing both keto and enol tautomers in the asymmetric unit were obtained. High-resolution X-ray diffraction data up to sinθ/λ = 1.0 Å−1 were collected and subsequently successfully used for the refining of the multipolar model of electron density distribution. The use of a crystal containing both ketone and enol forms allowed a direct comparison of the topological analysis results and a closer look at the differences between these two forms. The similarities and differences between the deformation densities, electrostatic potentials, Laplacian maps and bond characteristics of the tautomers were analysed. Additionally, the spectrum of the intermolecular interactions was identified and studied from classical, relatively strong N-H···O and O-H···O hydrogen bonds through weaker N-H···S hydrogen bonds to weak interactions (for instance, C-H···O, C-H···S and N···O). The results of these studies point toward the importance of including both the geometrical features and the details of the electron density distribution in the analysis of such weak interactions.
Highlights
Tautomerism occurs when two or more constitutional isomers exist in a dynamical equilibrium
The first example of tautomerism in a solid state was a case of keto-enol tautomers of acetyldibenzomethane isolated by Claaisen in 1896 [2]
Intermolecular Interactions It was found that thiobarbituric acid had a very rich collection of polymorphic forms; this was related to the presence and availability of very good donors and acceptors of hydrogen bonds in the molecule allowing for the constructions of different stable crystal architectures
Summary
Tautomerism occurs when two or more constitutional isomers (with different connectivities) exist in a dynamical equilibrium. These forms can, (relatively) interconvert by transferring an atom (mainly hydrogen–prototrophic tautomerism) or even a group of atoms from one position of the molecule to another. This description does not introduce any clearly defined boundaries between isomers and tautomers. A certain type of this phenomenon, keto-enol tautomerism, seems to be the most appropriate for structural studies due to the slow transformation from one form to another and the possibility of the isolation of individual forms of tautomers [1]. Sfceacrtciveealnydmtahkeesse hsitgruhc‐rtuesroelsuotifotennreoscecaurrchinimthpeofsosrimbleo.f a solid solution, which effectively makesThheigrhe-arreesotlwuotiopnrirnecsiepaarlchreiamsopnosssfiabvloe.uring this choice: first, TBA can be obtained as wellT‐dheefrieneadre, sttwabolepsriinncgilpeaclryresataslosn, ms feaevtoinugritnhge cthriitsecrihoonicoef:hfiigrsht‐,rTeBsoAluctaionnbdeaotabtcaoilnleecdtiaosn waenldl-dsecfionnedd,,TstBaAbliesskingolwe ncrtyosthaalsv,emaeveetrinygritchhe (cirfinteortiothneorfichhigehst-)recsoolleucttiioonndoaft“atcaoultloecmtieornic apnodlysmecornpdh,sT”B[A7]iisnkcnluodwingtoonheavceryasvtaelrystruicchtu(irfenooft athpeurriecheensot)l cfolrlmec,titohnreoefc“rtyasuttaolmsterruic‐ ptoulryems orfpkhest”o [i7so] mineclrusdainndg one cory‐csrtaylstsatrluocftuarkeeotof /aepnoulrefoernmol, wfohrmic,hthterneedctroybsteatlhsetrtuhceturmreos‐ odfykneatomiiscoamllyerms aonsdt sotnaeblceof-ocrrymsta(altorfoaokmettoe/menpoelrafoturmre,)wanhdichintcelnuddetothbe thhieghtheesrtmnuodmybnearmo-f ichayldlyromgoesnt bstoanbdles fionrtmhe(actryrostoaml sterumcptueraet[u7r]e.)Aadndditiinocnluadlley,tthheiohbigahrbesittunruicmabciedr iosfthhyedbrioogloeng‐ biocanldrseilnevtahnetccroymstaplosutrnudc,tuorfete[n7]u. sAeddinitimoneaalsluy,rtihnigobthaerbliptuidricpaecriodxids athtieonbipolroogdiucacltsreinlevcealnlst, ctoismsupeosu,nbdo,doyftfelnuiudseadnidnfmooedas[u8r,9in].g the lipid peroxidation products in cells, tissues, body fluids and food [8,9]
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