31 May 2013

Lithium Diazenide Surprise!

Literature Discussion

Submitted by Maggie Geselbracht, Reed College
Categories
Description: 

Students in a sophomore-level inorganic chemistry course were asked to read the paper “High-Pressure Synthesis and Characterization of the Alkali Diazenide Li2N2” (Angew. Chem. Int. Ed. 2012, 51, 1873-1875. DOI: 10.1002/anie.201108252) in preparation for a class discussion.  For many students, this was a first exposure to reading the primary literature. 

In this paper, the authors describe the surprising stability of lithium diazenide, Li2N2.  In contrast with the increasing stability of peroxides and superoxides with the heavier alkali metals, the first alkali metal diazenide that was isolated was not the rubidium or cesium salt but rather the lithium salt (hence the surprise)!  The black, metallic-looking Li2N2 was synthesized at high pressure and high temperature by the decomposition of lithium azide, LiN3.  The crystal structure of Li2N2 was determined by Rietveld refinement of powder X-ray diffraction data and relevant bond distances were compared to diazene, H2N2, and the alkaline earth diazenides, CaN2, SrN2, and BaN2.  The authors also report evidence from infrared spectroscopy of nitrogen-nitrogen bonds in Li2N2.   Electronic band structure calculations suggest metallic behavior for Li2N2 and antibonding characteristics for the conduction electrons consistent with the molecular orbital description of the isolated diazenide ion.

AttachmentSize
Microsoft Office document icon LitDisc_Li2N2.doc41.5 KB
File Li2N2.pdb3.2 KB
Learning Goals: 

After reading and discussing this paper, a student will be able to:

  • Articulate the underlying problem and motivations driving the research presented in the paper.
  • Explain the collection of experimental data that lead to figures and tables presented in the paper.
  • Describe the important conclusions of the paper and the evidence presented by the authors to support these conclusions.
  • Gain confidence reading a paper from the primary literature in inorganic chemistry.
  • Describe periodic trends in the descriptive chemistry observed for the alkali metals with oxygen and for the alkali metals with nitrogen.
  • Interpret images of a unit cell to deduce stoichiometry and notable coordination features of an extended structure.
  • Draw the molecular orbital diagram of a homonuclear diatomic species and use the MO diagram to predict the bond order and magnetic properties of the diazenide ion.
Implementation Notes: 

Students in a sophomore-level inorganic chemistry course read the paper "High-Pressure Synthesis and Characterization of the Alkali Diazenide Li2N2" prior to a class discussion, held during a conference session.  A link to the pdf file was posted to the course Moodle, and students were asked to complete the short list of discussion questions (attached below as a Word document) prior to coming to conference.  I also used the Jmol resource embedded in a course Moodle page to display the unit cell of the Li2N2 structure (the pdb file that I used as the input for Jmol is attached below.) On the Moodle page with the Jmol structure, I told students, “Sorry, I don't know how to make Jmol draw the boundaries of the unit cell!  To guide your eye, there are lithium atoms (in lavendar) at all 8 corners of the unit cell.”  This Jmol model was intended to help students answer questions 4 and 5 in the attached list of questions in addition to Figure 2 from the paper.

I used this learning object during the past two years at different times during my course.  In Spring 2012, the paper was assigned in week 4 of the semester, after covering periodic trends in atomic properties, ionic structures, unit cells, stoichiometries of extended structures, basic X-ray powder diffraction, and Lewis structures.  In Spring 2013, the paper was assigned in week 9 of the semester, after MO theory of diatomics was also covered in addition to the previous topics.  The attached discussion questions were from the Spring 2013 version.

During conference, we used the discussion questions as a guide to walk through the paper.  I explained to students how to access the Supporting Information for a paper and the varying utility of this information, depending on the article.  In this case, the Supporting Information is quite helpful, and we talked through key figures and tables in both the paper and in the Supporting Information.  We also discussed the synthesis, and I answered some questions on the characterization methods, explaining briefly what Rietveld refinement is and how the results in Table 1 and Figure 1 are correlated.  For more information on the Rietveld refinement method, see the Web Resource on Powder Diffraction Crystallography Instructional Materials.  Finally, we took the last 10 minutes of conference, and I made all the students get up and in small groups, draw the MO diagram and molecular orbital surfaces for (N2)2– on the white boards around the room.  I circulated and made comments and corrections to each group on their MO diagrams.  

I also used this paper for one student as part of our Junior Qualifying Exam in Inorganic Chemistry.  In this format, the student has 3 days to read the paper and learn about it before taking a 30 minute oral exam, which takes the form of a question-and-answer based discussion of the paper.  For the Junior Qual, we spent most of the time discussing the diffraction analysis of the material, molecular orbital theory of the diazenide ion, and a brief discussion of the properties of lithium diazenide.

Finally, although I have not done so yet, I intend to use this paper in my Advanced Inorganic Chemistry course where we cover the electronic band structure of extended solids in more detail. Our discussions in Advanced I-chem are based on Roald Hoffmann’s book, Solids and Surfaces: A Chemist’s View of Bonding in Extended Structures.  The details of the Li2N2 electronic band structure calculations provided in the Supporting Information for this paper are an excellent illustration of many of the key concepts presented in Hoffmann’s book.

Time Required: 
50 minutes
Evaluation
Evaluation Methods: 

During conference, I gave students direct feedback on the MO diagrams and cartoons of MO surfaces their groups had drawn on the board.  As this conference was scheduled the day before a problem set was due, this feedback was helpful to students in gaining confidence drawing MO diagrams of diatomics. 

I collected the discussion questions and graded these on a 10 point scale, 1.5 points for question 1, 0.5 point for question 3, 3 points for question 7, 1 point for questions 2, 4, 5, and 6, and 1 point for effort.

Evaluation Results: 

Out of the 15 students that turned in written answers, 6 (40%) earned 9-10 points, 5 students (33%) earned 6.5-7 points, another 2 students (13%) earned 5-5.5 points, and 2 students (13%) earned < 5 points.  On the MO diagrams, I told them I did not care if they invoked sp­-mixing or not in their diagrams, but that they needed to be consistent with their choice.  In other words, if the 3 sg energy level was shown above the 1 πu energy levels in their diagram, then I expected to see evidence of sp­-mixing in their molecular orbital surface cartoons.  About half of the students chose to use sp­-mixing in their diagrams, but most of them did not show evidence of sp­-mixing in the molecular orbital surfaces.  Others that chose to invoke sp­-mixing did not shift the MO energy levels correctly; the common mistake was elevating the 2 su* energy level above the 1 πu energy levels rather than the 3 sg energy level.  Nearly all students that have read this paper find it approachable and understand the major conclusions and how these were reached after the discussion.

 

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