Rep:Mod:Computational Y2 amm416

EX₃

BH₃

3-21G Pre-optimisation Calculation

  Item               Value     Threshold  Converged?
Maximum Force            0.000217     0.000450     YES
RMS     Force            0.000105     0.000300     YES
Maximum Displacement     0.000919     0.001800     YES
RMS     Displacement     0.000441     0.001200     YES
Predicted change in Energy=-1.635268D-07
Optimization completed.
   -- Stationary point found.

6-31G(d,p) (D_3h symmetry constraint applied) Optimisation Calculation

  Item               Value     Threshold  Converged?
Maximum Force            0.000192     0.000450     YES
RMS     Force            0.000126     0.000300     YES
Maximum Displacement     0.000763     0.001800     YES
RMS     Displacement     0.000500     0.001200     YES
Predicted change in Energy=-2.201780D-07
Optimization completed.
   -- Stationary point found.

Frequency Calculation

BH₃ Frequency Calculation .log File

Low frequencies ---   -0.2263   -0.1037   -0.0054   47.9770   49.0378   49.0383
Low frequencies --- 1163.7209 1213.6704 1213.6731

BH3

IR of BH₃

Figure 1: Computed IR Spectrum of BH₃.

Table 1: Table of Computed IR Frequencies and Intensities of BH₃, also showing vibrational modes

In the IR of BH₃, only 3 peaks are present even though there are 6 vibrations. As seen in the Table of IR frequencies, there are 2 sets of 2 generate vibrations: odes 2 and 3 are degenerate (scissoring and rocking) as well as modes 5 and 6 (both asymmetric stretches). Therefore, the degenerate signals will overlap forming a single peak, generating two peaks in the spectrum. The third peak is due to the wagging motion (mode 1). The remaining peak would be the one due to the symmetric stretching, but in this vibrational mode there is no change in net dipole moment; therefore, it is IR inactive.

MO Diagram

Figure 2: MO Diagram of BH₃.

The computed MOs are very similar to the qualitative LCAO analysis. The only MO that is not immediately recongnisable is one of the 2e' antibonding orbitals; the computed MO on the left shows more lobe repulsions than predicted in the qualitative LCAO. Nevertheless, predicting MO shapes and relative sizes qualitatively by linearly combining the AOs gives a very good approximation of the real MOs.

Smf115 (talk) 23:29, 16 May 2018 (BST)Great comment, picking up on the subtle differences and on the good approximation of qualitative MO theory.

Association Energies: Ammonia-Borane

NH3 Optimisation and Frequency Calculation

Optimisation

  Item               Value     Threshold  Converged?
Maximum Force            0.000006     0.000450     YES
RMS     Force            0.000004     0.000300     YES
Maximum Displacement     0.000016     0.001800     YES
RMS     Displacement     0.000011     0.001200     YES
Predicted change in Energy=-1.228228D-10
Optimization completed.
   -- Stationary point found.

Frequency

NH₃ Frequency Calculation .log File

Low frequencies ---   -0.0138   -0.0032   -0.0015    7.0783    8.0932    8.0937
Low frequencies --- 1089.3840 1693.9368 1693.9368

NH3

NH3-BH3 Optimisation and Frequency Calculation

Optimisation

        Item               Value     Threshold  Converged?
Maximum Force            0.000121     0.000450     YES
RMS     Force            0.000057     0.000300     YES
Maximum Displacement     0.000505     0.001800     YES
RMS     Displacement     0.000294     0.001200     YES
Predicted change in Energy=-1.610954D-07
Optimization completed.
   -- Stationary point found.

Frequency

NH₃BH₃ Frequency Calculation .log File

Low frequencies ---   -0.0252   -0.0033   -0.0012   17.0405   17.0427   36.9265
Low frequencies ---  265.7534  632.2124  639.3376

NH3-BH3

Association Energy Calculations

E(NH₃)= -26.61532 a.u.

E(BH₃)= -56.55777 a.u.

E(NH₃BH₃)= -83.22469 a.u.

ΔE=E(NH₃BH₃)-[E(NH₃)+E(BH₃)], where ΔE is the dissociation energy of NH₃BH₃

∴ ΔE = -0.05160 a.u. = - 135 kJmol^-1

Hence, the N-B bond energy is 135 kJmol^-1. This value is 3 times smaller than the energy of the C-C bond in the corresponding molecule of ethane, 402 kJmol^-1.

BBr₃ Optimisation and Frequency Calculation using SCAN Server

Optimisation

        Item               Value     Threshold  Converged?
Maximum Force            0.000008     0.000450     YES
RMS     Force            0.000005     0.000300     YES
Maximum Displacement     0.000036     0.001800     YES
RMS     Displacement     0.000024     0.001200     YES
Predicted change in Energy=-4.190601D-10
Optimization completed.
   -- Stationary point found.

Frequency

BBr₃ Frequency Calculation .log File

Low frequencies ---   -0.0136   -0.0064   -0.0046    2.4367    2.4367    4.8447
Low frequencies ---  155.9631  155.9651  267.7048

BBr3

D-Space Link: DOI:10042/202296

Ionic Liquids: Designer Solvents

Optimisation and Frequency Calculations for [N(CH₃)₄]⁺ and [P(CH₃)₄]⁺

[N(CH₃)₄]⁺

Optimisation

        Item               Value     Threshold  Converged?
Maximum Force            0.000074     0.000450     YES
RMS     Force            0.000027     0.000300     YES
Maximum Displacement     0.000362     0.001800     YES
RMS     Displacement     0.000111     0.001200     YES
Predicted change in Energy=-9.316300D-08
Optimization completed.
   -- Stationary point found.

Frequency

[N(CH₃)₄]⁺ Frequency Calculation .log File

Low frequencies ---   -7.5520   -0.0011   -0.0009    0.0003    6.8978    7.9666
Low frequencies ---  184.2924  289.3429  289.8709

[N(CH3)4]+

[P(CH₃)₄]⁺

Optimisation

        Item               Value     Threshold  Converged?
Maximum Force            0.000030     0.000450     YES
RMS     Force            0.000012     0.000300     YES
Maximum Displacement     0.000107     0.001800     YES
RMS     Displacement     0.000044     0.001200     YES
Predicted change in Energy=-1.742375D-08
Optimization completed.
   -- Stationary point found.

Frequency

[P(CH₃)₄]⁺ Frequency Calculation .log File

Low frequencies ---   -0.0032    0.0017    0.0018   25.3058   25.3058   25.3058
Low frequencies ---  161.2512  195.7467  195.7467

[P(CH3)4]+

Smf115 (talk) 23:26, 16 May 2018 (BST)Great structure information and inclusion of the charges on the complexes

Charge Distribution Analysis

Table 2: Table showing atoms' charges contributions in [N(CH₃)₄]⁺

Figure 3: Diagram showing atoms' charges contributions in [N(CH₃)₄]⁺: negative to positive = red to green

Table 3: Table showing atoms' charges contributions in [P(CH₃)₄]⁺

Figure 4: Diagram showing atoms' charges contributions in [P(CH₃)₄]⁺: negative to positive = red to green

From Table 2, it can be seen that nitrogen is partially negatively charged as well as the carbon atoms. Therefore, the classical model in which nitrogen in a tetrahedral arrangement has a positive charge is actually wrong. The assumed positive charge arises from the fact that nitrogen can form a dative covalent bond and hence become more electrophilic. However, from the charge distribution analysis, the positive charge is entirely located on the hydrogen atoms.

In [P(CH₃)₄]⁺, all the H atoms and the central P atom are partially positive, with the C atoms bearing all the negative charge, as seen from Table 3. Compared to N in [N(CH₃)₄]⁺, P is positively charged because its electronegativity is lower than N's, hence attracts less electron density.

Smf115 (talk) 23:25, 16 May 2018 (BST)Good discussion which could have been improved by considering the C and H charges, particularly the similarity of the charges on H across the molecule.

MO Analysis of [N(CH₃)₄]⁺

Smf115 (talk) 22:54, 16 May 2018 (BST)Good attempt at the LCAOs and ligand FOs however, consideration of the BH₃ orbitals seen in your lectures and how these relate to the CH₃ FO's for the ligands was needed.

Smf115 (talk) 23:34, 16 May 2018 (BST)Overall a good report and a well presented project section with some further thought on the FOs and LCAOs required.