Article pubs.acs.org/jchemeduc
Using Molecular Modeling in Teaching Group Theory Analysis of the Infrared Spectra of Organometallic Compounds Lihua Wang* Department of Chemistry and Biochemistry, Kettering University, Flint, Michigan 48504, United States S Supporting Information *
ABSTRACT: A new method is introduced for teaching group theory analysis of the infrared spectra of organometallic compounds using molecular modeling. The main focus of this method is to enhance student understanding of the symmetry properties of vibrational modes and of the group theory analysis of infrared (IR) spectra by using visual aids provided by computer molecular modeling. It can be implemented in an upper-level inorganic chemistry course. Using molecular modeling techniques, students are able to calculate the IR spectra of organometallic compounds and watch the animated movies of vibrational modes in action. Visual images of vibrational modes help students to identify the characteristic peaks in an IR spectrum and enable them to see the symmetry properties of vibrational modes and understand the physical meaning of the irreducible representations of the vibrational modes. In addition, the visual images of vibrational modes help students to understand why some of the vibrational modes are IR active whereas others are not. Results of sample calculations using both semiempirical and density functional theory methods are discussed. Student evaluations on the teaching method are also presented. KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Computer-Based Learning, Group Theory/Symmetry, IR Spectroscopy, Organometallics, Student-Centered Learning
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spectra by using visual aids provided by computer molecular modeling. Infrared spectroscopy is one of the most important tools in the characterization of organometallic compounds. However, it is not an easy task for beginner undergraduate students to interpret IR spectra of inorganic and organometallic compounds. The interpretation of IR spectra of these compounds requires the use of group theory or the results from group theory analyses, both of which are abstract to undergraduate students who are learning group theory for the first time. It is difficult for them to visualize the vibrational modes of coordination or organometallic compounds to understand the principles of group theory analysis. The new teaching method described in this article uses molecular modeling calculations to show students the visual images of vibrational modes. It helps students to see the symmetry properties of vibrational modes, which greatly enhances their understanding of the principles of group theory analysis of IR spectra and their interest in learning this otherwise difficult and abstract topic.
ith the recent advancement in computational methods and computer technology, the speed of quantum mechanical calculations for inorganic and organometallic compounds involving transition metals has significantly improved. As a result, molecular modeling has been used increasingly in inorganic and organometallic chemistry research and education.1 Several articles using molecular modeling in inorganic classes have been published in this Journal. One of them described an experiment using molecular modeling to study the structures of two related organometallic compounds: (C 6H 6)Cr(CO) 3 and (B 3N 3H 6)Cr(CO) 3. 2 Another article introduced an exercise using the Cambridge Structural Database to study several important inorganic concepts.3 Molecular modeling has also been integrated into an inorganic chemistry laboratory to study the geometry and the molecular orbitals of the products synthesized in the laboratory.4 A more recent article described the use of a molecular modeling exercise to study the pi back-bonding in carbonyl complexes by comparing calculated CO stretching frequencies of related compounds.5 This article describes a new teaching method that uses molecular modeling to help students learn the group theory analysis of infrared (IR) spectra of organometallic compounds, specifically metal carbonyls. The main focus of this method is to enhance student understanding of the symmetry properties of vibrational modes and of the group theory analysis of IR © 2012 American Chemical Society and Division of Chemical Education, Inc.
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COMPUTATION METHODS The calculations were carried out using both Spartan for Windows '04 and '08 software with similar results. The semiempirical method PM3 was selected as the main Published: January 17, 2012 360
dx.doi.org/10.1021/ed200538c | J. Chem. Educ. 2012, 89, 360−364
Journal of Chemical Education
Article
computational method because of its faster computation speed compared to that of other methods and the availability of parameters for transition-metal elements.6 Density functional theory (DFT) calculations (B3LYP and BLYP, 6-31G*) were also performed for some of the compounds for comparison to the PM3 calculations.
Table 1. Examples of Carbonyl Compounds Examined by the Students
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DESCRIPTION AND DISCUSSION Infrared spectroscopy is particularly useful in determining the structures of carbonyl compounds. The most useful characteristic peaks for structural identification are the CO stretching vibrational bands. Most carbonyl compounds exhibit strong and sharp CO stretching vibrational bands, ν(CO), in the range of 2100−1800 cm−1. Because ν(CO) bands are generally free from coupling with other vibrational modes and are not obscured by the presence of other vibrations, studies of ν(CO) often provide valuable information about the structure and bonding of carbonyl compounds. Carbonyl compounds with different symmetries often show different numbers of CO stretching bands in the IR spectra, which are useful for structural identification. An advantage of using CO complexes as examples to teach group theory analysis to beginner students is that CO is a linear ligand that is cylindrically symmetrical around the M−L bond. The linearity avoids complications due to the effect of ligand structure on the symmetry point group of the complex, which may confuse students who are learning group theory analysis of IR spectra for the first time. This teaching method can be implemented either as a classroom activity or as a homework assignment, depending on the number of modeling programs available. Traditional textbook problems of metal carbonyl IR spectra can be used. Due to the limited number of copies (only two) of the software available, it has been implemented in the advanced inorganic chemistry course as a homework assignment along with classroom discussions of the results. Students could access the software on the computers in the departmental student lounge at their convenience during normal business hours by checking out a program key from the departmental office. It took each student about 1−2 h to complete the calculations assigned, identify the CO and CN− vibrational modes by watching the animated movies of the vibrational modes, and draw the pictures of the vibrational modes according to the animations. The group analysis can be performed at a different time without using the software, and the results can be compared with the pictures of the vibrational modes drawn based on the computer calculations. If more copies of the software are available, the exercises can be implemented as a classroom activity in a computer laboratory. The assignment consists of two parts: (1) Exploring the IR Spectra of Carbonyl Compounds of Different Symmetries and (2) Using IR Spectroscopy To Identify Isomers of Carbonyl Compounds.
Compound
Geometry
Symmetry
Cr(CO)6 Ni(CO)4 Cr(C6H6)(CO)3 Fe(CO)5 Mn(CO)5Cl
Octahedral Tetrahedral Tetrahedral (or Pseudo Octahedral) Trigonal Bipyramidal Octahedral
Oh Td C3v D3h C4v
PM3 semiempirical method. In addition, the software could play an animated movie for each vibrational mode. By observing the animated movies, students were able to identify the vibrational modes and IR peaks associated with CO stretching vibrations and draw a picture for each of the vibrational modes. For example, the pictures of CO stretching vibrational modes of Mn(CO)5Cl based on the animated movies generated by the calculated vibrational modes, along with the characters of the corresponding irreducible representations, are shown in Figure 1. Based on these pictures of vibrational modes, students could investigate the effects of the symmetry operations of the point group on each vibrational mode and match that with the characters of the corresponding irreversible representation. By doing so, they were able to gain better understanding of the symmetry properties of the vibrational modes. In addition, the students were able to see the difference between energy degenerate vibrational modes such as the two E vibrational modes of Mn(CO)5Cl, and vibrational modes with the same symmetry but different energy such as the two A1 vibrational modes of Mn(CO)5Cl. Finally, they could use the pictures to examine whether a vibrational mode would change the dipole moment of the molecule, so that they could determine whether it would be IR active. The calculated CO stretching vibrational modes (IR-active modes are identified) of all the compounds studied are listed in Table 2. The results were consistent with that of group theory analysis, which indicated that the semiempirical PM3 method can be used in calculating the CO stretching vibrational modes and in identifying IR-active vibrational modes for these carbonyl compounds. The experimental IR spectra of some of the complexes studied are also listed in Table 2. The calculated IR spectra agreed with the experimental spectra in terms of the number of vibrational modes, the symmetry properties of each vibrational mode, and the number of IR-active modes. The vibrational frequencies of the PM3 calculated IR spectra did not match well with that of the experimental spectra. However, the general trends of the vibrational frequencies of different peaks in the calculated spectra were consistent with the experimental results. Part 2. Using IR Spectroscopy To Identify Isomers of Carbonyl Compounds
In the second part of the assignment, students used IR spectroscopy to distinguish isomers of metal carbonyl compounds. The compounds used were cis- and trans[Fe(CN)4(CO)2]2−,7,8 fac- and mer-[Fe(CO)3(CN)3]−,9 and [Fe(CO)4CN]−10 with CN− at the axial or the equatorial position. Students were given the IR spectrum of one of the isomers in each pair and were asked to explain how to determine which isomer the spectrum belonged to by using both the group theory analysis and molecular modeling technique. They were required to provide the symmetry symbol, draw a picture for each vibrational mode, and explain why it was or was not IR active. These exercises introduced to
Part 1: Exploring the IR Spectra of Carbonyl Compounds of Different Symmetries
Students were asked to determine the vibrational modes and IR-active modes of several carbonyl compounds of different structures and symmetry properties by using group theory analysis and comparing the results with that calculated by the molecular modeling software. Examples of compounds used are listed in Table 1. The equilibrium geometry, vibrational modes, and IR spectrum of each of the compounds were calculated with the 361
dx.doi.org/10.1021/ed200538c | J. Chem. Educ. 2012, 89, 360−364
Journal of Chemical Education
Article
Figure 1. Pictures of CO stretching vibrational modes of Mn(CO)5Cl based on the animated movies generated by the calculated vibrational modes, along with the characters of the corresponding irreducible representations.
Table 2. Calculated and Observed Vibrational Modes of Metal−Carbonyl Compounds CO Stretching Vibrational Modes: Symmetry Symbol (Freq/cm−1) Compound
Point Group of the Molecule
Cr(CO)6 Ni(CO)4
Oh Td
Cr(C6H6)(CO)3 Fe(CO)5 Mn(CO)5Cl
C3v D3h C4v
Calculateda
Experimental
A1g (2231), Eg (2108), T1u (2080, IR active) A1 (2169), T2 (2100, IR active) (calc time:
