Abstract. Structural chemistry is a branch discipline of structure, properties and application of chemical substance, which is expounded from the perspectives of both electron factor and space factor. Structural chemistry touches upon various fields and needs higher mathematics as well as contents of multiple chemistry major courses as supports, and also has a very abstract molecular model, and hence is with strong theoretical, experimental and application values and requires the students to have a quite strong ability in space thinking. In this paper, the authors use the origami model to replace Kekle model and Stuart model and easily resolve the difficult teaching points of solid fabrication and synthesis stereo control. It is one of convenient and feasible teaching means and can be used at any time and place, and also can highlight the positions of four vertices of tetrahedron of carbon chemistry in space. Hence, the accurate concept can be obtained, the teaching time can be shortened, and the teaching objective can be achieved.
Key words: Structural Chemistry; Model Teaching; Phenanthroline-iron
1. Introduction
Structure chemistry is a discipline mainly to research the microstructures of atoms, molecules, crystalloid and other substances as well as the relationship between structure and properties. It includes two most important parts: (1) motion laws of electron; (2) research molecules and crystal structures through the motion laws of microscopic particles. Considering using the traditional origami technique, the homemade molecular model is simple and convenient, and every student can start work personally and can make the three-dimensional graphs visual and specific. Therefore, as an auxiliary teaching method, it can exert a very good teaching effect.
2. Ways to Make Basic Polyhedron
In structure chemistry, the polyhedrons which can be seen commonly in polyatomic structure model include regular tetrahedron and octahedron, as well as triangular bipyramid, square bipyramid and other molecular models. In this paper, the ways to make the origami of regular tetrahedron and octahedron are introduced first in the following.
2.1.Way to make regular tetrahedron
Many molecules and ions are structured with a regular tetrahedron or approximately to regular tetrahedron. In structure chemistry, the molecules such as CH4 and SO42- which are hybridized by and the coordination compounds such as TiCl4, FeCl4- and VCl4 whose electron numbers are and are
structured with tetrahedron. The operation steps of using origami technique to make a regular tetrahedron model are shown in figure 1.
(1)Get a sheet of paper (length: 23cm; width: 3.5cm), longitudinally 50% fold the left section (length: 4cm) and then unfold it (see figure 1 (a)).
(2)Fold the point 0 at the left of the paper on the midcourt line, and hence the dents 1 2 appear on the paper (see figure 1 (b)).
(3)Let dents 1 2 overlap the top edge of the paper, forming the dents 2 3 (see figure 1 (c)).
(4)Let dents 2 3 overlap the bottom edge of the paper, forming the dents 3 4(see figure 1 (d)).
(5)Repeat the steps (3) and (4) and form other dents as shown in figure 1 (e).
(6)Overlap dents 1 2 with dents 5 6, and make the first four equilateral triangles at the left end of the paper form a tetrahedron skeleton.
(7)Use transparent adhesive tape to adhere the joints of the equilateral triangles, to get a regular tetrahedron model.
2.2. Way to make regular octahedron
AL6 coordination compounds which have been known now are almost structured with a regular octahedron or approximately to a regular octahedron, and their space stereogram is shown in figure 2.
Coordination compounds such as [Cr(CN)6]4-, [Co(NH3)6]3+ and AlF63- (including 12 valence electrons), TeCl62-, SbBr63- and XeF6 (including 14 valence electrons) are all structured with an octahedron. The transition metal ions including d4, d5, d6 and d7 can free two d tracks to join the hybridization of d2sp3 and form six tracks which are directed at the hybridization of d2sp3 of vertices of regular octahedron. These free hybridization tracks can form covalent coordination bonds with the lone pair electrons of ligands (e.g. CN- and NH3) together.
Now, the origami technique is used to make the molecular model of regular octahedron, and the specific steps are shown in figure 3.
(1) Make lines by folding on a piece of paper (length: 49cm; width: 3.5cm); these lines make the paper form 23 regular triangles; keep in mind that 8 9, 9
10, … 15 16 and 16 17 are convexes, and other lines are dents (see figure 3 (a)).
(2) Overlap 1 2and 7 8, and hence form a regular octahedron skeleton (see figure 3 (b)).
(3) Overlap Δ 7 8 9 and Δ 1 2 3, and then overlap Δ 8 9 10 and Δ 7 8 9, and hence make Δ between Δ 1 2 3 and Δ 8 9 10 (see figure 3 (c)).
(4) Wind the left paper on the regular octahedron skeleton until Δ 15 16 17 and Δ 9 10 11 meet with each other; keep in mind that 16 17 are convexes, and , 18 19,…, 24 25 are dents (see figure 3 (c)).
(5)Use dents 16 17to make and
overlapped, and then press Δ 15 16 17 lightly，and hence make Δ 16 17 18 between Δ 15 16 17 and
(see figure 3 (d)).
(6) Wind the left paper on the core of the regular octahedron skeleton, and finally insert Δ 23 24 25 into 15
17, to get the regular octahedron model (see figure 3 (e)).
Figure 3: the Made Process of Octahedron Model
3.Selection of Phenanthroline-iron (II) Ion Model
In the above, the ways to make the regular octahedron and octahedron are introduced. And the molecular (ion) models of many complex compounds can be combined with other molecular models base the above methods. At class, the phenanthroline-iron (II) ion model was selected. The phenanthroline-iron (II) ions are the reagent which is often used in the chemical analyses exams of the schools where the authors work.
The red phenanthroline-iron (II) ion association complex can be formed by adding the color reagent phenanthroline into the iron (II) ion solution. Also, the concentration of the iron (II) ion solution can be measured through visible spectrophotometry. Actually, the students are not stranger to the complex, but many of them do not know well the coordination between the central ion and ligand of this complex as well as the spatial configuration. Therefore, through the making of the phenanthroline-iron (II) ion model, the students get a better understanding of it.
4.Making of the Phenanthroline-iron (II) Ion Model
Through the origami method, the students made a phenanthroline-iron (II) ion model themselves. First of all, the regular octahedral model for the hybridization track of the central iron (II) ions was made. Then, the phenanthroline molecule plane conformation was drawn on cardboard with pen, and then was cut along the lines with scissor. The nitrogen atoms of three phenanthroline molecules were flipped on the six vertices of regular octahedron by pairs, forming the phenanthroline-iron (II) ion model (see Figure 4).
5.Analysis on the Phenanthroline-iron (II) Ion Mode
The phenanthroline-iron (II) ion complex is structured with octahedrons. Through the physical model, the students very intuitively saw the structure of the octahedral complex, and therefore got an understanding of the ligand of the octahedral complex. The energy levels of the five d tracks of the central ions were split: and
were at a higher energy level; , and were at a lower level; the energy absorption of electrons can make a d-d transition occur, and the green light was absorbed, showing the complementary color (red). And through the molecular model, it can easily allow students rotate the model themselves and analyze the molecular point groups of this complex.
The recognition on the molecular point groups is one of the major difficulties of structural chemistry. The basic rotation operations were carried out on this ion model. The model was rotated by 120 ° and was finally recovery as same as the original model; a three-rotation axis (C3) was found, and also the C2 axis that was perpendicular to C3 axis was special and was easily ignored by the students. However, the reference model was rotated by 180°on the plane perpendicular to C3 axis and was finally recovery as same as the original model, in which the students are easy to find there were three C2 axes on the plane perpendicular to C3 axis, so toothpicks were used to insert the model and express one C3 axis and three C2 axes. Therefore, phenanthroline-iron (II) ions belong to the D3 point group.
6. Conclusion
Through the classroom practice of the homemade molecular model, the ability of the students in practices is exercised; the space imagination ability of the students is strengthened; the roles of students change from the objects to the subjects, and also the students can make the models themselves at extracurricular activities. In short, the homemade molecular model, as an auxiliary teaching method, will have a very good effect in stimulating the interest of the students in study, inspiring the students to have rational thinking, and improving the quality of teaching.
7. References
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