Symmetry

31 Jan 2020

Advanced Inorganic Chemistry

Submitted by Terrie Salupo-Bryant, Manchester University

21 Jan 2020

Inorganic Chemistry 2020

Submitted by Adam R. Johnson, Harvey Mudd College

9 Oct 2019

Fourier Transform IR Spectroscopy of Tetrahedral Borate Ions

Submitted by Zachary Tonzetich, University of Texas at San Antonio

Evaluation Methods:

The students perpare laboratory reports displaying their data in proper format with each peak labeled. The report must also contain answers to all of the quetions posed in the manual. Student performance and learning is assessed by the qualtity of their written reports and by a separate quiz covering aspects of vibrational spectroscopy. Teaching assistans also ensure that students' data acquisition is performed in a satisfactory manner during the laboratory period.

Evaluation Results:

Students typically have great difficulty connecting the idea of normal modes, their symmetries, and why we observe IR peaks. They approach IR spectroscopy in much the same way they do NMR spectroscopy (i.e. methane shows four equivalent C-H bonds so I expect one C-H stretching motion) leading to serious misconceptions. This laboratory was designed in part to dispell these misconceptions. Question 1 addresses this issue most directly and many of the class answer incorrectly.

The questions in the laboratory involving harmonic oscialltor analysis are generally more straightforward for students as they just need to use the correct equations. Most of the class answers these correctly.

Likewise, students generally understand that vibrational frequencies are inversely proportional to the mass of the atoms involved in the vibration and are there able to make connections between the observed spectra of BH₄^-, BD₄^- and BF₄^-.

Aspects of functional group analysis are more familiar to students and they generally have little trouble assigning the spectrum of tetraphenylborate.

Description:

This experiment was developed for an upper division Instrumental Analysis course to give students additional experience with infrared (IR) spectroscopy beyond the routine functional group identification encountered in undergraduate Organic Chemistry courses. It shares some aspects with the analysis of gas phase rovibrational spectra typically performed in Physical Chemistry courses, but places a greater emphasis on more practical considerations including data acquisition (using ATR) and interpretation. The molecular ions used in the experiment also demonstrate tetrahedral symmmetry which allows for topics in Group Theory to be exploited.

The experiment has students record the spectra of several tetrahedral borate ions including the isotopomers NaBH₄ and NaBD₄. The students then analyze their data in the context of the symmetry of normal modes, the harmonic osciallator model, comparisons with Raman spectra, and functional group composition. Post lab questions guide students through each of the topics and ask them to make quantative and qualitative predictions based on their data and theoretical models of molecular vibration.

Course Level:

Upper Division

Prerequisites:

First Semester Inorganic Chemistry / Foundation Course in Inorganic Chemistry

Organic Chemistry

Corequisites:

Physical Chemistry: Quantum Mechanics

Subdiscipline:

Molecular Structure and Bonding

Spectroscopy and Structural Methods

Learning Goals:

-Students should be able to understand the relationship between molecular structure, normal modes, and peaks in the IR spectrum. This is a major misconception with students as they tend to believe that the presence of four B-H bonds in the borohydride ion will neccessary mean that four peaks (or one since they are equivalent) will be observed by IR. Unlike NMR spectroscopy, there is no 1:1 correspondence between the number of equivalent bonds and the number of peaks observed in the spectrum.

-Students should also be able to apply their knowledge of theoretical models (quantum harmonic oscillator) to quantitaively intrepret IR spectra and predict the energy of transitions that cannot be observed due to instrumental limitations.

-Students should be able to understand at a qualitative level how the masses of atoms affect the energy of molecular vibrations.

Topics Covered:

Group theory & applications

Molecular structure

Physical methods / analytical techniques

Symmetry

Equipment needs:

The only required piece of equipment beyond the chemicals is an infrared spectrophotometer. At our institution we use an ATR element to acquire the data, but KBr pellets or nujol mulls should work equally well. All chemicals were purchased from Sigma-Aldrich and are of reasonable price.

Related activities:

Isotopic labeling and reduced mass calculations for IR spectroscopy

CompChem 05: Infrared, Thermochemistry, UV-Vis, and NMR

Vibrational Modes and IR Spectra using Character Tables

Good resource for Teaching IR with Symmetry Elements

Synthesis and Molecular Modeling of Sodium Tetrathionate

Implementation Notes:

See attached file with more details. The data acquisition is very straightforward if ATR sampling is employed. Students need only use the instrument for about 15 - 20 minutes to record all four samples.

Time Required:

30 minutes to 2 hr depending upon the number of students.