1: Boron Based Acids
H: Bonding Analysis of BH3
Now you are going to carry out a "Natural Bond Orbital Analysis", this is quite complex and so I am going to provide the input file for you.
- First carry out the calculation:
- copy the input file from here input file, into a text file called (or saved as) "yourname_bh3_nbo.gjf", this is important as without the .gjf extension gaussview will not recognise it
- load the file into gaussview ... don't forget to reset the pull-down menu for "File type" to "Gaussian Input Files (*.gjf *.com)" otherwise gaussview will not see it
- from within gaussview save and submit the file
- Then open the text file associated with this job:
- When the job has compleated the information we need is in the text file "yourname_bh3_nbo.log", find this file and open it with a text editor.
- OR open the file in gaussview and then from the main menu along the top of the screen choose "Results" and then choose "View File"
- This is the actual output from the program guassian, all gausview does is provide a graphical interface for viewing these files, however it is not sophisticated enough to deal with an NBO analysis and so we have to go into the "guts" of the file ourselves.
- The NBO analysis tells us many interesting things, but I will only pick out a few to think about in more detail. Scroll down until you find the following section (this is where the NBO analysis is printed):
******************************Gaussian NBO Version 3.1****************************** N A T U R A L A T O M I C O R B I T A L A N D N A T U R A L B O N D O R B I T A L A N A L Y S I S ******************************Gaussian NBO Version 3.1****************************** - then under the section "Natural populations" find and record the orbital occupation and energy for the B 1s,2s and sp AOs, and the H 1sAO. These are the energies of the atomic orbitals in the environment of the molecule, ie subject to the local electrostatic effects.
NATURAL POPULATIONS: Natural atomic orbital occupancies
- and under the section "Summarry of Natural Population Analysis" find and record the v
Summary of Natural Population Analysis:
- keep scrolling down until you find the section with the headding "Bond orbital/ Coefficients/ Hybrids" this describes the bonding in the compound:
(Occupancy) Bond orbital/ Coefficients/ Hybrids --------------------------------------------------------------------------------- 1. (1.99851) BD ( 1) B 1 - H 2 ( 44.49%) .6670* B 1 s( 33.33%)p 2.00( 66.67%) .0000 .5774 .0000 .0000 .0000 .8165 .0000 .0000 .0000 ( 55.51%) .7451* H 2 s(100.00%) 1.0000 .0000 2. (1.99851) BD ( 1) B 1 - H 3 ( 44.49%) .6670* B 1 s( 33.33%)p 2.00( 66.67%) .0000 .5774 .0000 .7071 .0000 -.4082 .0000 .0000 .0000 ( 55.51%) .7451* H 3 s(100.00%) 1.0000 .0000 3. (1.99851) BD ( 1) B 1 - H 4 ( 44.49%) .6670* B 1 s( 33.33%)p 2.00( 66.67%) .0000 .5774 .0000 -.7071 .0000 -.4082 .0000 .0000 .0000 ( 55.51%) .7451* H 4 s(100.00%) 1.0000 .0000 4. (1.99904) CR ( 1) B 1 s(100.00%) 1.0000 .0000 .0000 .0000 .0000 .0000 .0000 .0000 .0000 5. ( .00000) LP*( 1) B 1 s( .00%)p 1.00(100.00%)- The NBO analysis partitions the electron density of the whole molecule out into atomic like orbitals, which are then used to form 2e-2c bonds, key information about these bonds is printed above.
- For example the first bond (BD) is between boron (atom 1) and hydrogen (atom 2) and 44.49% of the bond is contributed from the B orbitals which have a hybridisation of 33%s+66%p, while 55.51% of the bond comes from the H orbital which is 100%s.
- Thus the B has formed 3 sp3 hybrid orbitals which each interact with the sAO of one hydrogen atom. The last orbital shown (Orbital 5) is the boron lone pair (LP*) 100% p orbital, which is the one sticking up out of the plane. This is formally occupied so it is starred
- Orbital 4 is the core (CR) 1sAO of boron.
- Continue down to see a section "Second Order Perturbation Theory Analysis of Fock Matrix in NBO Basis" this section outlines the interactions between the various MOs (ie mixing). Normally these are interactions from bonding NBOs into non-bonding (like the B lone p orbital) or antibonding orbitals. It does not contain much information for BH3 but is important for the other molecules you will be looking at. If any of the values in the E(2) column are greater than 20 kcal/mol they should be recorded.
Second Order Perturbation Theory Analysis of Fock Matrix in NBO Basis Threshold for printing: .50 kcal/mol E(2) E(j)-E(i) F(i,j) Donor NBO (i) Acceptor NBO (j) kcal/mol a.u. a.u. =========================================================================== within unit 1 - The final section is the "Natural Bond Orbital (Summary)", in this section record the energy and population of the B-H bonds, and the boron lone pair (B(LP*)):
Natural Bond Orbitals (Summary): Principal Delocalizations NBO Occupancy Energy (geminal,vicinal,remote) ==================================================================================== Molecular unit 1 (H3B) - Thus the NBO analysis takes a delocalised MO picture and turns it back into the "organic chemists" picture of 2e-2c bonding.
- Spend some time working through the NBO analysis and thinking about the B-H bonding in this molecule. Include in your presentation a critical analysis of the bonding in BH3. By critical analysis we mean that it is not enough to only report results. The results need to be interpreted and/or evaluated (thats the analysis part), and then compared and/or contrasted with traditional models of bonding (thats the critical part). To help you get started you might want to consider
- relative energy and population of AOs of B and H
- the charge on the B and H
- the character of the bonding and lone-pair orbtials
- the population and energy of the final bonding orbitals
- the extent of delocalisation and mixing
- a comparison with delocalised MOs
- relate vibrational frequencies to bond strengths
- a comparison against the commonly presented description of bonding in these compounds
- If you want to find out more about the NBO analysis (this is OPTIONAL) go to the NBO web-site! The information can be quite technical, but the beginners tutorial should provide a good starting point
- When you are ready close the text window move onto the next step