Part 1: Boron Based Acids

C: Optimize a Molecule of BH₃

• Steps for carrying out a calculation
  • Now we are ready to set up the commands that tell the program how to do the calculation, we have to tell it
    • the method: B3LYP
    • the basis set: 3-21G
    • and what type of calculation to do: OPT (for optimization)

    The method BLYP determines the type of approximations that are made in solving the Schrodinger equation. The basis set determines the accuracy, 3-21G has a very low accuracy, however this means the calculations should be very quick. In carrying out an optimization we are determining the optimium position of the nuclei for a given electronic configuration, this is the first step for any quantum chemical calculation.

  • From the main menu along the top of the screen (in gaussview) choose "Calculation" and then choose "Gaussian":

    

  • a new palette will appear the calculation palette. It opens with the "job type" tab open. Use the pull down menu under "energy" to choose "optimization" (ignore the other stuff that appears)

    

  • then click on the "method" tab to open it, use the pull down menu under "Hartree-Fock" to pick "DFT", some of the entries will change, find the pull-down menu "LSDA" and choose "B3LYP" (don't change anything else!)
  • then click on the "title" tab to open it, and type "BH3 optimization"
  • then click on the "link 0" tab to open it, and after "%chk=" type "yourname_bh3_opt", make sure you type the underscores as the program is sensitive to extra spaces, replace "yourname" with your first name ... remember NO extra spaces!! For example you might have "sally_bh3_opt"
  • check that the first few lines in the image is similar to that in the image below;

    

  • Then press submit (button is in the bottom left of the palette):

    

  • a new window will pop-up and telling you that you must save the file first, press "save":

    

  • a new window will then pop-up and ask you for a file name, navigate to the folder you created at the begining and then use exactly the same name as you put in the "%chk=" part, ie "yourname_bh3_opt", then press "save":

    

  • a new window will then pop-up and ask you if you want to submit the job, press submit!

    

  • close the molecule window, this is important as the "finished" molecule will look very similar to your starting one and many students accidentally take the wrong molecule for the following bits!
  • wait for the message "Gaussian Job Completed ...", and click yes for opening a results file.

    

  • a new window will open, make sure that the file type is for "Gaussian Output Files (*.out *.log) and select your .out or .log file, and then click open. A new molecule window should apear with your optimized molecule in it.

    

  • Finding out geometric information
  • on the comands palette click on the button with a question-mark in front of a ruler. This is the inquiry button and will tell you bond distances if you click on two atoms, and bond angles if you click on three atoms
  • use the inquiry button to determine the B-H bond distance and the H-B-H bond angle, note these numbers down, as they are the "optimised" bond distance and angle, you will need to report these in your presentation
  • When you are ready close the molecule window move onto the next step