7.5: Exercise 2 - Molecular Orbitals of Formaldehyde
- Page ID
- 371863
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Exercise 2 - Molecular Orbitals of Formaldehyde
This experiment explores the capability of Spartan® to calculate Molecular Orbitals and construct corresponding surfaces. The formaldehyde molecule is used as a model.
- Click
to start a new file.
- Build H2CO. Select Carbonyl from the Groups menu and double-click in the working area. You may want to rotate the molecule on the screen for a better view.
- Click the
minimize button (lower right, below Clipboard).
- Select Setup, Calculations from the menu. Choose Equilibrium Geometry at Ground State in Gas, with Hartree-Fock, 6-31G*. Total charge should be Neutral, and Unpaired Electrons should be 0. Click OK.
- Choose Submit from the Setup menu. Click on the OK button. Save the file with the suggested filename, or choose one of your own.
- Once the calculations have completed, determine the calculated equilibrium values for the C–O and C–H bond lengths, and for the
HCH and
HCO bond angles, by using Geometry, Measure Distance/Measure Angle. Record these values in your lab notebook. Compare your calculated values with literature values.
The Computational Chemistry Comparison and Benchmark Database (CCCBDB) at NIST is one suggested place to look for this type of information. Choose an appropriate search string in your favorite web browser. For example: formaldehyde bond lengths and angles NIST. Look for the hit directing you to the CCCBDB site.
|
H2CO |
C=O length |
HCO angle |
HCH angle |
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|
Calc |
Lit |
Calc |
Lit |
Calc |
Lit |
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| Reference | ||||||
- Inspect the Output of this job using the
button. Find the Molecular Orbital Energies section under the Summary tab. Calculate the energy difference between the HOMO and LUMO and record this in your lab notebook.
- Click the
button to see an abbreviated MO diagram on the left side of the screen. From here you can click on an orbital to view it on the molecule. In this way find and visualize the \( \pi \)-bonding MO in H2CO. You can change the appearance of the orbital display by changing the Style from Solid to Mesh to Transparent, etc. (at the lower right corner of the screen).
- In the same way, visualize the LUMO surface. How might you describe this molecular orbital?
- Now visualize the HOMO. How might you describe this one? Take a look at some of the other orbitals and record any other interesting observations (with images) in your lab notebook.
Not every MO is shown in the diagram generated by clicking the button. You can calculate the surface for any desired MO by clicking the
button, and More Surfaces. Then, from the Surface dropdown menu select HOMO{-},N or LUMO{+},N, where N is how many orbitals above the LUMO or below the HOMO needed to get to the orbital you wish to see. Click on Apply, and then OK. Put a check mark in the box next to the selected orbital and then return to the Spartan window to see the result.
This section of your lab notebook should include:
- A table recording your calculated bond lengths and angles for formaldehyde and a comparison to literature values
- Calculation of the energy difference between the HOMO and the LUMO
- Images and descriptions of the HOMO, LUMO, and any other interesting orbitals of your choice.
(See your ELN template for details)