Visser, Gerrit Jan

English

THE STRUCTURE OF 4,5-DI-T -BUTYLIMIDAZOLE AND 2,3-DI-T -BUTYLQUINOXALINE The synthesis of 0 -di-t-butyl heteroaromatics has involved difficulties, because of steric hindrance between the t-butyl groups. To obtain more information about the strain in these molecules, the structures of two o-di-t-butyl heteroaromatics, viz. 4,5 -di -t-butylimidazole (DTBI) and 2, 3 di-t-butylquinoxaline (DTBC), have been determined by Xray diffraction. Both compounds are monoclinic, space groups P21/n and p2dc respectively, with two independent molecules per cell. The experimental part of the structure determinations is described in chapter 2 (DTBI) and chapter 3 (DTBC). The intensities of the reflexions of DTBC were measured on an automated Nonius diffractometer at room temperature and at low temperature (-155°C), those of DTBI were collected at low temperature only. The structure models were found by means of the symbolic addition method (Karle and Karle, 1966), using the low temperature (L. T.) intensities of DTBI and the room temperature (R. T.) inten- sities of DTBC. The coordinates and anisotropic thermal parameters of the 'heavy' atoms were refined by leastsquares techniques. Hydrogen atoms were introduced with isotropie temperature factors, with fixed parameters for DTBI and variabie parameters for DTBC. The indices R at the end of the refinements are 0.085 (DTBI; 5359 reflexions), 0.072 (DTBC, R. T.; 3296 refl. ) and 0.062 (DTBC, L. T.; 8324 refl.). The estimated standard deviations in bond lengths and angles are 0.005 A and 0.35° for DTBI, 0.005 -0.007 .Ä and 0.4° for DTBC (R. T. ) and 0.0035 .Ä and 0.23° for DTBC (L.T.)..

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   University of GroningenDe conformatie van enige ortho-tertiairbutylverbindingen en van de dithienyllenVisser, Gerrit JanIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1969Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Visser, G. J. (1969). De conformatie van enige ortho-tertiairbutylverbindingen en van de dithienyllen. s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 21-06-2022ee series reflecties voering van de sym­ode (8. Brenner, Na­Iratory , Washington het manuscript werd .ra en Mej. S. J.Kal­elijk. SUMMARY THE STRUCTURE OF 4,5-DI-T -BUTYLIMIDAZOLE AND 2,3-DI-T -BUTYLQUINOXALINE The synthesis of 0 -di- t-butyl heteroaromatics has in­volved difficulties, because of sterk hindrance between the t-butyl groups. To obtain more information about the strain in these molecules, the structures of two o-di-t-butyl he­teroaromatics, viz. 4,5 -di - t-butylimidazole (DTBI) and 2, 3­di-t-butylquinoxaline (DTBC), have been determined by X­ray diffraction. Both compounds are monoclinic, space groups P21/n and p2dc respectively, wUh two independent molecules per cello The experimental part of the structure determinations is described in chapter 2 (DTBI) and chapter 3 (DTBC). The intensities of the reflexions of DTBC were measured on an automated Nonius diffractometer at room temperature and at low temperature (-155°C), those of DTBI were collected at low temperature only. The structure models were found by means of the symbolic addition method (Karle and Karle, 1966), using the low temperature (L. T.) intensities of DTBI and the room temperature (R. T.) inten sities of DTBC. The coordinates and anisotropic thermal parameters of the 'heavy' atoms were refined by least­squares techniques. Hydrogen atoms were introduced with isotropie temperature factors, with fixed parameters for DTBI and variabie parameters for DTBC. The indices R at the end of the refinements are 0.085 (DTBI; 5359 re flexions), 0.072 (DTBC, R. T.; 3296 refl. ) and 0.062 (DTBC, L. T.; 8324 refl.). The estimated standard deviations in bond lengths and angles are 0.005 A and 0.35° for DTBI, 0.005 -0.007 .Ä and 0.4° for DTBC (R. T. ) and 0.0035 .Ä and 0.23° for DTBC (L. T.). The results are discussed in chapter 4. The two crys tallographic non-equivalent molecules of DTBC show an in­teresting difference. In one of the molecules the quinoxa­line group is approximately planar, whereas in the other one this group has a somewhat twisted structure. The non­109 110 planar quinoxaline group lies at shorter distances from its neighbours in the crystal than the planar one. Therefore the deviations from planarity have probably to be ascribed to intermolecular interaction in the crystal; they are not due to sterk hindrance of the t-butyl substituents. A similar phenomenon, although less pronounced, occurs in DTBl. Apart from the differences in conformation mentioned above no essential differences have been observed between the corresponding independent molecules at low temperature, and between the room and low temperature structure of DTBC. In the crystal structure of DTBI the molecules have formed chains in which they are linked by N -H ... N hydrogen bonds of 2.04 and 1.98 R. The conformation of the t-butyl groups is discussed in chapter 5. In DTBC, DTBI and in 1,2,4,5-tetra-t-butyl­benzene (TTBB) investigated by v. Bruijnsvoort, E ilermann, v. d. Meer and Stam (1968) essentially the same conformation has been observed for the o-t-butyl groups. In each of the molecules the o-t-butyl groups are related by pseudo two­fold symmetry and fit together like gear wheels (Fig. 5. Ic and d). The angles around the quarternary carbon atoms are deformed such as to release the strain between the groups. Between the t-butyl groups in DTBC there are six C ... C distances shorter than 3.50 Ä, the remaining distances are larger than 3.90 Ä. For DTBI these distances are 3.67 and 4.00 .Ä re spectively, and for TTBB 3.46 and 3.90 Ä. 01 UlO I.SI1 UIl DTBC +1.01 +=111.1 +.131.1 + ',51> + lil!' Fig.SI The valueS of al' a and epwith thei! deformations. Ar .. aromatic compound.Z The angles 'P and the bonds al and a z (see Fig, SI) are affected most by the presence of the t-butyl groups. The distances from its 1ar one. Therefore l.bly to be ascribed 'ystal; they are not :tituents. A similar occurs in DTBI. )n mentioned above :erved between the ; low te mperature , 'ature structure of the molecules have N-H ... N hydrogen ups is discussed in l, 4, 5-tetra-t-butyl­svoort, Eilermann, same conformation ups. In each of the ted by pseudo two­r wheels (Fig. 5. Ic y carbon atoms are ,etween the groups. lere are six C ... C .ining distances are ances are 3.67 and 6 and 3.90 Ä. ••1l0.S +IO.S· Ar .. aromatic compound. 1 2 (see Fig- SI) are t-butyl groups. The values estimated for the deformations are given in the ure. It appeared to be difficult to get accurate estimates for the deformations, as not in all cases reliable values for corresponding bands in unstrained compounds, e. g. with CH3 in stead of t-butyl, were available. The difference in L1a obtained for TTBB and DTBC has not been explainedz yet. Except for L1az (DTBC) and (DTBI) all L1-va lues can be correlated with force constants from the liter­ature via a simple model given for the strain energy in 0­t-butyl heteroaromatics by Wiebenga and Bouwhuis (1969). In this model the strain energy in the molecule is considered as a function of the distance between the quarternary carbon atoms of the t-butyl groups and the deformations of al' a 2 and <po THE DITHIENYLS The crystal structures of the three isomerie dithienyls C 8 H 6 S 2 have been determined by X-ray diffraction. The main purpose of this study was to check whether or not the molecules in the soUd state would be as is the case for biphenyl (Trotter, 1961). :;Vlany difficulties were encoun­tered during the structure determinations. It appeared to be difficult to grow good crystals of the compounds, the crystals of 2, 2'-dithienyl decomposed during X -ray exposure and those of 2, 3 1 _ and 3, 3' -dithienyl (and possibly also those of 2,2'-dithienyl) appeared to show disorder. None of the structures could therefore be determined with great accu­racy. Nevertheless it could be concluded that all three dithienyls are in the solid state. The bond lengths in the thienyl groups do not differ signi­ficantly from those in thiophene. For 2,2 1 and 3,3 1 -dithienyl it is certain that the majority of the molecules in the crys­tals has the, centrosymmetric, anti conformation although it cannot be excluded that a minority (less than 300/0) is in the syn farm. Disorder as observed in 3, 3 1 -dithienyl ap­peared also to be present in thiophene -3 -carboxylic acid (Hudson and Robertson, 1964). For 2, 3 1-dithienyl disorder is complete and na information is available about the con­formation in the plane of the molecule. The packing of the molecules in the crystals is simil~r 111 112 for the three dithienyls. In all cases the planes of the mole­cules make an angle of approximately 30° with a short crystal axis of approximately 5.5 Ä. Theoretical calculations as done for biphenyl by Casalone, Mariani, Mugnoli and Simonetta (1968) to understand the difference in conformation between the molecules in the gaseaus and in the solid phase, have not been attempted for the dithienyl molecules. 

De conformatie van enige ortho-tertiairbutylverbindingen en van de dithienyllen

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De conformatie van enige ortho-tertiairbutylverbindingen en van de dithienyllen

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