Crystals and molecular structures

Abstract

The thesis entitled “The Crystal and Molecular Structures of Push–Pull Ethylenes” deals with the structure and conformation of eight push–pull ethylenes. Ethylenes substituted with electron?donating and electron?accepting groups in the vicinal positions are known as push–pull or polarised ethylenes. The conjugative interaction between the donor and acceptor groups through the ethylene bond results in lengthening of the C=C bond and lowering of the barrier to rotation about this bond. The donor–ethylenic carbon bond and acceptor–ethylenic carbon bonds are shortened, resulting in an increase in the barrier to rotation about these bonds. In sterically hindered polarised ethylenes, the molecules are twisted about the C=C bond. The effect of the electron?donating and electron?withdrawing substituents on the C=C bond length and the twist about the C=C bond has been investigated for eight suitable push–pull ethylenes, and this study forms the subject matter of the thesis. Chapter I presents a brief review of the results available from X?ray analysis, NMR studies, and theoretical calculations on such push–pull ethylenes.

Chapter II Chapter II deals with the crystal and molecular structures of 1?benzoyl?1?cyano?2?dimethylamino?2?methylthioethylene (A) (Fig. 1) and 1,1?dicyano?2?dimethylamino?2?methylthioethylene (B) (Fig. 2). The crystals of A are monoclinic, space group P2?/c with a = 7.214(1), b = 8.935(5), c = 20.243(6) Å, ? = 99.42(2)°, and Z = 4. The compound B crystallises in the orthorhombic space group Pna2? with a = 7.899(3), b = 8.670(2), c = 12.956(6) Å and Z = 4. Both structures were solved by direct methods. Structure A refined to an R?factor of 4.2%, and structure B refined to 4.1%. The central C=C bonds in both structures are significantly longer—1.414(5) Å in A and 1.397(8) Å in B—than the C=C bond in ethylene. The molecules are twisted about the C=C bond, the twist angles being 38.3° in A and 21.7° in B. In both cases, atom C(2) deviates from the plane of the bonded atoms by 0.066(4) Å in A and 0.056(6) Å in B. Semi?empirical conformational energy calculations were performed for B to determine its conformation in the free state.

Chapter III Chapter III deals with the structural details of 1,1?bis?dimethylamino?2,2?dicyanoethylene (C) (Fig. 3), 1,3?dimethyl?2?(dicyanomethylene)?imidazolidine (D) (Fig. 4), and 1,3?dimethyl?2?(dicyanomethylene)?hexahydropyrimidine (E) (Fig. 5). Compound C crystallises in the orthorhombic space group Pcab with a = 7.876(2), b = 14.430(4), c = 16.312(5) Å and Z = 8. Crystals of D are monoclinic, space group P2?/a with a = 7.965(1), b = 16.232(2), c = 7.343(1) Å, ? = 113.54(1)°, Z = 4. Crystals of E are orthorhombic, space group P2?cn with a = 7.983(3), b = 8.075(2), c = 14.652(3) Å and Z = 4. Structures C, D, and E were solved by direct methods and refined to R?factors of 4.9%, 5.8%, and 5.2% respectively. In all three structures, the electron?withdrawing groups are cyano groups, and the electron donors are nitrogen atoms (in rings for D and E). Conjugation lengthens the C=C bonds, but the C=C bond lengths in C and D are identical [1.407(4) Å and 1.407(3) Å]. The C=C bond length in E is 1.429(6) Å, significantly longer than in D, suggesting that the hexahydropyrimidine ring is a better donor than the imidazolidine ring. The twist angles about the C=C bond are: 29.3° in C, 20.2° in D, and 31.5° in E. Conformational energy calculations for D and E are discussed in relation to their solid?state conformations. Dipole moments calculated by the CNDO/2 method agree well with experimental values.

Chapter IV Chapter IV deals with the crystal and molecular structures of one planar push–pull olefin: 2?(diacetylmethylene)?imidazolidine (F) (Fig. 6), and two highly twisted push–pull ethylenes: 1,3?dimethyl?2?(diacetylmethylene)?imidazolidine (G) (Fig. 7), and 1,3?dimethyl?2?(acetyl?carbomethoxymethylene)?imidazolidine (H) (Fig. 8). Compound F crystallises in the triclinic space group P1 with a = 5.596(2), b = 6.938(3), c = 10.852(4) Å, ? = 75.64(3)°, ? = 93.44(3)°, ? = 95.47(3)°, Z = 2. Compound G is triclinic, space group P1? with a = 7.731(2), b = 3.580(2), c = 11.033(3) Å, ? = 97.66(2)°, ? = 98.86(2)°, ? = 101.78(2)°, Z = 2. Compound H crystallises in monoclinic space group P2?/n with a = 12.028(2), b = 7.163(2), c = 15.187(5) Å, ? = 91.88(2)°, Z = 4. Structures F, G, and H were solved by direct methods and refined to R?factors of 4.4%, 5.7%, and 5.6% respectively. The C=C bond lengths in G and H are 1.451(3), 1.468(3), and 1.464(2) Å respectively. In addition to conjugation, the twist about the C=C bond (72.9° in G and 62.6° in H) appears to contribute slightly to the bond lengthening. The very low twist (5°) about the C=C bond in molecule F is due to intramolecular N–H…O hydrogen bonding. In the crystal structures of G and H, there is an extensive network of intermolecular hydrogen bonds involving water molecules. Semi?empirical conformational energy calculations yield twist angles close to the observed values for G and H. For F, the calculated twist angle without including intramolecular hydrogen bonding is 22°, whereas including the hydrogen bond gives 5°, in good agreement with experimental data.

Chapter V In Chapter V, the results obtained for the eight push–pull ethylenes are briefly discussed. An attempt has been made to rationalise the variation of the C=C bond length with the electron?withdrawing capacities of the substituents. A correlation between the C=C bond length and the twist angle has been observed. The lists of observed and calculated structure factors for all the structures are placed at the end of the last chapter. References appear in alphabetical order. Based on the work reported here, the papers already published and those under preparation are listed below.