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7.5: Exercise 2 - Molecular Orbitals of Formaldehyde

  • Page ID
    371863
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    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.

    1. Click C0yEUEJjgCfM5w8ZvgV7jNOkAqiBopWJ0eyGRssP3gkg5ZdyM03A9IR9RxeHNJgd-M153UzV1zNiddT356pzka81MKt5X2cjO7ShjmNUH2VyO0h_vK3RoSa9W6YhUXZIifaoNbpy7Fym-tQdhtw to start a new file.
    2. 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.
    Screen Shot 2022-03-28 at 10.25.31 AM.png
    Figure \(\PageIndex{1}\): Building Formaldehyde. (CC-NC-BY-DUKE CHEM)
    1. Click the SIYq1FQSJpHiSldKBEzn-9VHXdwrlOPfWUGbC720MURhVinVapMnyjYcZ806yxH14XGeY2IvBFp3lPiQ_c9S77oMnDuYzPB5cpt6cJOD7GeGi3DCmxgnZugoIqGHhetbWKcz4E7NS0S1M7aPC4k minimize button (lower right, below Clipboard).
    2. 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.
    3. Choose Submit from the Setup menu. Click on the OK button. Save the file with the suggested filename, or choose one of your own.
    4. Once the calculations have completed, determine the calculated equilibrium values for the C–O and C–H bond lengths, and for the u8eby8iKNjBsVzIbXynq2LENjEo1A7GtXGNdQdzaBeUgfGDeySucsZxsqGTPytw2jakTPEgDF_6rRoKEa0ErkzXMiz2O967lVn47X-QaU_GtZjQOWyEyoB2r3EOF4jt9MZ6mZjVwUoOGGUsQ7KsHCH and HIpJKP3tu8J4yN6_X8BbNd8XeOFKGTzrRRvWd8tIXHzuShP_PnVcFYMA6HrGSvfdZpjEsJ0m39sgTjlX5_RbJRupYHMJiQ4VodhtRUeMzye139uJh_9c3vj9bZAccm10-dqLjlZFTw8CjPMizd8HCO bond angles, by using Geometry, Measure Distance/Measure Angle. Record these values in your lab notebook. Compare your calculated values with literature values.
    Note

    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

    Calc

    Lit

    Calc

    Lit

    Calc

    Lit

               
    Reference
    1. Inspect the Output of this job using the CCAfpsE6xY8jnfmgJgnVrTO4smgRRRef5AefvlEDmDlCuPoBipIWGSAhqSYFM3U1TfxQoPX4aaJHed0qu5Sw-YOOp0j-6SOptOa6OKecv4Kf-K1hnHqAh8aj6C4_s16n7Twpsf6ImlF3-m0o92k 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.
    2. Click the VWYTMstXjd5na4l-IpJ1TXJw03QHupu9cqhMZoNGfNFPsS8Ff9JAVGxtzcqCYiIIjeD3QX_67ExAhNSknE-y7sN7LS3zutlu4I6wy16DuT4Sq6a91lIV2Dsbe02l7gCsigTdoFQqUz0wZthZvRE 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).
    3. In the same way, visualize the LUMO surface. How might you describe this molecular orbital?
    4. 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.
    Note

    Not every MO is shown in the diagram generated by clicking the VWYTMstXjd5na4l-IpJ1TXJw03QHupu9cqhMZoNGfNFPsS8Ff9JAVGxtzcqCYiIIjeD3QX_67ExAhNSknE-y7sN7LS3zutlu4I6wy16DuT4Sq6a91lIV2Dsbe02l7gCsigTdoFQqUz0wZthZvRE_h0u6 button. You can calculate the surface for any desired MO by clicking the y213o7bFQS3CxskyjQwGn18prR6iJ3-hC5Ww6hA-FFpvmOvfa7xmLSPp5CdhmX_MDUrzM-7FTO8lBjItCubSejGSZu_UHODTes6Q587oBAGWWd8YwmxrbkmBW6qxho3bcdxrlj-r9TcQtHfdMNc 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.

    Note

    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)

     


    7.5: Exercise 2 - Molecular Orbitals of Formaldehyde is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.