Third Year Computational Laboratory Report - Module 2: Bonding (Ab innitio and density functional molecular orbital)
Analysis of BH3
BH3 Modelling And Optimisation
Gaussian Optimised BH3
The bond length of B-H was pre-set to be 1.5 Å, then the Gaussian calculation was run, with the key information listed below:
Table 1: Optimisation of BH3
File Type
.log
Calculation Type
FOPT
Calculation Method
RB3LYP
Basis Set
3-21G
Final Energy / a.u.
-26.4623
RMS Gradient Norm / a.u.
0.0002
Dipole Moment / Debye
0.00
Point Group
D3h
Job Time / s
16.0
Final B-H Bond Length / Å
1.19
Final B-H Bond Angle / o
120.0
Optimisation of the geometry of BH3 converged to an energy minimum after the 4th step as shown in the Total energy diagram below. The change in energy difference, i.e. the gradient of the energy graph, became close to zero from the 4th step, which means the energy has finally reached the energy minimum and kept constant since then.
Molecular Orbital Diagram And Visualisation of BH3
BH3 charge distribution
BH3 Vibrational Analysis
Table 2: Vibrational analysis of BH3
No.
Form of the vibration
Description
Frequency / cm-1
Intensity
Point Group
1
Out of plane Wagging: All 3 H atoms move up/down together in a concerted motion, while the B atom moves in the opposite direction with a smaller amplitude
1144
93
A2"
2
In plane Scissoring: 2 H atoms move in a scissoring motion, while the B atom moves slightly away from the scissoring H's. The last H atom remains stationary.
1204
12
E'
3
In plane rocking: A BH2 unit rocks in one direction, and the other H atom wags sideways in the BH3 plane, with a larger amplitude.
1204
12
E'
4
Symmetric stretch: 3 H atoms move in and out together in a concerted manner. The B atom remains stationary.
2598
0
A1'
5
Asymmetric stretch: 1 H atom moves out while the other H atom moves in. The B atom moves very slightly, while the last H atom is fixed in position.
2737
104
E'
6
Asymmetric stretch: 1 H atom moves in, while the other 2 H atoms move out together, in a concerted motion. The movement occurs on the BH3 plane (2D vibration).