Information • Textbooks • Media • Resources
Molecular Modeling Exercises and Experiments
Organometallic Computational Exercise: Semiempirical Molecular Orbital Calculations on (C6H6)Cr(CO)3 and (B3N3H6)Cr(CO)3
edited by
Ronald Starkey University of Wisconsin–Green Bay Green Bay, WI 54311-7001
W
James F. O’Brien* Department of Chemistry, Southwest Missouri State University, Springfield, MO 65804;
[email protected] Daniel T. Haworth Department of Chemistry, Marquette University, Milwaukee, WI 53201
Computational packages such as PC Spartan Pro, used in this study, can enhance students’ understanding of structural chemistry and increase their interest in it. The dramatic visualization so easily achieved by PC Spartan Pro makes the structures of organometallic molecules come alive in ways not previously possible. We have used this capability to advantage in introducing the topic of π -bonded rings. Instead of starting the study with ferrocene we have the students do a computational comparison of the structures and bonding in (C6H6)Cr(CO)3 and (B3N3H6)Cr(CO)3. There are sufficient data available on these structures that students can compare their computed results to experimental findings. Initially the geometries of benzene and borazine are optimized. The known similarities (planar hexagonal, nonpolar) and differences (nonzero atomic charges, “lumpy” π system) are reproduced. The appropriateness of the term “inorganic benzene” is discussed. A printout of the coefficients of the molecular orbitals reveals that the HOMO is a π orbital centered on the nitrogen atoms, and the LUMO is a π orbital centered on the boron atoms. On the basis of these π orbitals, predictions are made that nucleophilic attack should occur at the boron atoms and electrophilic attack at the nitrogen atoms. A literature search of the structural data and reaction chemistry of the two molecules is done to compare with the computed data. The next step is to optimize the geometries of (C6H6)Cr(CO)3 and (B3N3H6)Cr(CO)3. Additional differences now arise, as the benzene remains planar but the borazine puckers upon complexation to the Cr(CO)3 fragment. The graphics of PC Spartan Pro allow easily visualization of this pucker, even though it is small. Comparison of the computed vibrational frequencies, the composition of the HOMO and LUMO, and the changes in the atomic charges for the two molecules increases students’ awareness of the structural changes taking place. Time is taken to discuss the hybridization of the boron and nitrogen atoms in the complexed borazine ring. The idea of using the sum of the internal angles in both sp2 and sp 3 six-membered rings is raised. This leads to a geometry optimization of cyclohexane to provide a value for sp3 rings. PC Spartan Pro allows easy readout of the angles once the geometry has been optimized. The sum of the values for the six internal angles in the C6H12 ring is 668°. The sum of the values for the six internal angles in the complexed borazine ring is 718°. Because this value is so close to the 720° found in the perfectly hexagonal benzene and free borazine rings, students generally come to the conclusion 134
that the hybridization in the complexed borazine ring of (B3N3H6)Cr(CO)3 remains sp2. Students are then asked if sp2 hybridization in a sixmembered ring means that the system is aromatic. Opinion is unanimous that it does. They are then referred to recent discussion of this concept in the literature. Students learn how to optimize transition-state structures and verify their existence by having PC Spartan Pro calculate the vibrational frequencies of the molecules. Once this is done for (C6H 6)Cr(CO) 3 and (B3N 3H6)Cr(CO)3, the output contains the thermodynamic data necessary for computation of the activation energies for the rotations of the rings with respect to the Cr(CO)3 fragments. The values found, 0.4 kcal/mol for (C 6 H 6 )Cr(CO) 3 and 18.3 kcal/mol for (B3N3H6)Cr(CO)3 are met with surprise. Discussion now centers on the nature of the bonding of the borazine ring to the chromium atom. The idea of bonding through lone pairs on the nitrogen atoms is considered. This seems an attractive idea until it is remembered that we previously decided that the hybridization is still sp2, whereas the lone pair picture of bonding about the nitrogen atoms requires sp3 hybridization. Some students conclude that the “lumpy” π system is the cause of the much larger activation energy for rotation in (B3N3H6)Cr(CO)3 than in (C6H6)Cr(CO)3. Others decide that the sp3 picture is partially valid. There follows a discussion of the field of π-bonded rings, including ferrocene. Literature sources are provided. Student enthusiasm is one positive aspect of this exercise. Of greater import is the understanding evident in the discussions. Students deal with a number of concepts while considering these two molecules. The visual nature and the hands-on opportunity to use PC Spartan Pro are clearly valuable. The ease of use of the program and the minimal time required for PM3 semiempirical calculations permit students to do their own calculations individually. There is no problem getting the students to do this work; occasionally there is a problem stopping them from doing too much. That is a nice problem to have. Supplemental Material Student handouts and an expanded version of this article are available in this issue of JCE Online. W
This is the first article in the feature column Molecular Modeling Exercises and Experiments. The Mission Statement for this JCE Internet column was published in June 1999 on page 871.
Journal of Chemical Education • Vol. 78 No. 1 January 2001 • JChemEd.chem.wisc.edu
