Jump to content

JoeWiki1

From ChemWiki

Year 2 Inorganic comp labs

BH3

Method= RB3LYP, Basis set= 6-31G(d,p), Symmetry= D3h


        Item             Value       Threshold   Converged?
Maximum Force            0.000203     0.000450     YES
RMS     Force            0.000098     0.000300     YES
Maximum Displacement     0.000849     0.001800     YES
RMS     Displacement     0.000415     0.001200     YES


Link to frequency file: File:JH BH3 FREQ.LOG

Ng611 (talk) 13:00, 5 June 2019 (BST) This is an opt+freq file, not a pure freq file. You should do the optimization and frequency analysis as separate calculations.

Low frequencies --- -0.4072 -0.1962 -0.0055 25.2514 27.2430 27.2460

Low frequencies --- 1163.1897 1213.3128 1213.3155

Optimised BH molecule


Vibration Data

Vibrations table
Stretch or Bend? Intensity Symmetry IR active? Wavenumber(cm-1)
Bend 92 A2" Yes 1163
Bend 14 E' Yes 1213
Bend 14 E' Yes 1213
Symmetric Stretch 0 A1' No 2581
Asymmetric Stretch 126 E' Yes 2714
Asymmetric Stretch 126 E' Yes 2714

Ng611 (talk) 13:02, 5 June 2019 (BST) are your bending modes in-plane or out-of-plane?


Spectrum shows 3 peaks out of 6 vibrational modes given by BH3 shown in table. 1 is IR in active leaving 5 as it causes no change in dipole in the molecule. Two pairs in the 5 are degenerate meaning they have the same energy and therefore overlap, meaning only 3 peaks show.

[1]

The LCAOs seem very accurate as they depict AOs which combine to form the real MOs very well. Minor issues arise from the fact that it can be said that the overlapping of the orbitals may not be able to be seen fully and the fact that the sizes of the AOs, which represents contribution isn’t consistent. Hydrogen is more electronegative then boron and is therefore lower in energy. Hydrogen should be contributing more to the bonding orbitals and born should be contributing more to the anti-bonding orbitals but for some of them this might not be able to be seen accurately. This shows that the LCAO is a useful tool for finding what a real MO would look like.

Ng611 (talk) 13:04, 5 June 2019 (BST) You're on the right track with this explanation; qualitative MO theory does not always correctly predict orbital contributions. Think about which individual orbital contributions are incorrect and what this tells you.

NH3

Method= RB3LYP, Basis set= 6-31G(d,p)

        Item               Value     Threshold  Converged?
Maximum Force            0.000013     0.000450     YES
RMS     Force            0.000006     0.000300     YES
Maximum Displacement     0.000039     0.001800     YES
RMS     Displacement     0.000013     0.001200     YES


Link to frequency file: File:NH3 OP JH.LOG

Low frequencies --- -8.5628 -8.5571 -0.0047 0.0454 0.1785 26.4189

Low frequencies --- 1089.7603 1694.1865 1694.1865

Optimised NH molecule

NH3BH3

Method= RB3LYP, Basis set= 6-31G(d,p)

        Item               Value     Threshold  Converged?
Maximum Force            0.000122     0.000450     YES
RMS     Force            0.000058     0.000300     YES
Maximum Displacement     0.000514     0.001800     YES
RMS     Displacement     0.000296     0.001200     YES

Link to frequency file: File:NH3BH3 FREQ NEW.LOG

Low frequencies --- -0.0005 0.0003 0.0014 16.7270 18.7414 42.2600

Low frequencies --- 266.2799 632.3010 639.2486

Optimised NH3BH3 molecule

Association energies

E(NH3)= -56.55776863 a.u.

E(BH3)= -26.61532362 a.u.

E(NH3BH3)= -83.22469031 a.u.

ΔE=E(NH3BH3)-[E(NH3)+E(BH3)], Therefore ΔE= (-83.22468960) - [(-56.55776863)+(-26.61532362)]= -0.05159806 a.u. (-134 KJ/Mol)

Ng611 (talk) 13:11, 5 June 2019 (BST) you've got the right value here in a.u. but your conversion is off.

The B-N dative bond is weak and this shown when compared to the Al-N bond which has an energy of 297KJ/Mol [2]

Ng611 (talk) 13:11, 5 June 2019 (BST) Why this particular bond? Can you provide a rationalisation?

NI3

Method= RB3LYP, Basis set= Gen

        Item               Value     Threshold  Converged?
Maximum Force            0.000102     0.000450     YES
RMS     Force            0.000075     0.000300     YES
Maximum Displacement     0.000858     0.001800     YES
RMS     Displacement     0.000629     0.001200     YES

Link to frequency file: File:NEW NI3 JH.LOG

Low frequencies --- -12.3845 -12.3781 -5.6129 -0.0040 0.0194 0.0711

Low frequencies --- 100.9307 100.9314 147.2333


Optimised NI molecule

The optimised B-I bond distance is 2.18 angstrom

Ng611 (talk) 13:21, 5 June 2019 (BST) Bond lengths should be reported to 3 d.p.

Days 2 and 3 Project: Metal carbonyls

This sections of the wiki page focuses on metal carbonyls, more specifically their bond-lengths and CO bond frequencies. The metal-complexes which will be focused on are [Cr(CO)6], [Mn(CO)6]+ and [Fe(CO)6]2+. These were chosen as they come one after another in the d-block so it would be interesting to see how bond-lengths and bond frequencies vary across the period. 2 of the complexes are charged (positively) while one is neutral and this is something which must be taken into account when analysing bond-lengths and bond frequencies. An initial prediction would be that bond length increases across a period (as the metal complexes are becoming more positive meaning less overlap with the CO pi* and overall less back-donation and since back donation strengths the M-C bond less of this means an increases in bond length) and that bond frequencies increases.

[Cr(CO)6]

Method= RB3LYP, Basis set= 6-31G(d,p) CO and LanL2DZ for Cr

Item Value Threshold Converged?

Maximum Force            0.000110     0.000450     YES
RMS     Force            0.000041     0.000300     YES
Maximum Displacement     0.000705     0.001800     YES
RMS     Displacement     0.000334     0.001200     YES


Link to frequency file: File:CR(CO)6 JH 2.LOG

Low frequencies --- -0.0014 -0.0013 -0.0010 11.7482 11.7482 11.7482

Low frequencies --- 66.6574 66.6574 66.6574

Optimised Cr(CO) molecule

[Mn(CO)6]+

Method= RB3LYP, Basis set= 6-31G(d,p) CO and LanL2DZ for Mn

        Item               Value     Threshold  Converged?
Maximum Force            0.000054     0.000450     YES
RMS     Force            0.000024     0.000300     YES
Maximum Displacement     0.000430     0.001800     YES
RMS     Displacement     0.000204     0.001200     YES

Link to frequency file: File:-MN(CO)6-+ OP FREQ JH.LOG

Low frequencies --- -0.0007 0.0006 0.0009 4.7607 4.7607 4.7607

Low frequencies --- 76.3202 76.3202 76.3202


Optimised [Mn(CO)] molecule

[Fe(CO)6]2+

Method= RB3LYP, Basis set= 6-31G(d,p) CO and LanL2DZ for Fe

        Item               Value     Threshold  Converged?
Maximum Force            0.000054     0.000450     YES
RMS     Force            0.000024     0.000300     YES
Maximum Displacement     0.000429     0.001800     YES
RMS     Displacement     0.000200     0.001200     YES


Link to frequency file: File:-FE(CO)6-2+ OP FREQ.LOG

Low frequencies --- -10.5293 -10.5293 -10.5292 -0.0014 -0.0011 -0.0009

Low frequencies --- 82.1285 82.1285 82.1285


Optimised [FE(CO)] molecule

Analysing properties

Bond Length
Metal complex Bond Length(Å)
[Cr(CO)6] 1.915
[Mn(CO)6]+ 1.908
[Fe(CO)6]2+ 1.940

The table above shows the bond metal centre-carbon bond lengths. It shows a decrease moving from Cr to Mn and from Mn to Fe there is an increase. This initial decrease in M-C bond length goes against what was predicted but after speaking to Professor Hunt it turns out a full explanation of this decrease in bond length is too complex for the course and so wont be provided.The later drastic increase is due to a contraction of the d-orbitals causing greater repulsion between the electrons leading to poorer overlap with the CO orbitals meaning less back-bonding and and weaker, so therefore longer, metal-carbon bonds. This agrees with the prediction made earlier.

Vibrations table 2
Metal complex Intensity Vibration type Wavenumber(cm-1)
[Cr(CO)6] 1637 symmetric stretch 2086
[Mn(CO)6]+ 879 symmetric stretch 2199
[Fe(CO)6]2+ 272 symmetric stretch 2297

This table above shows the wave-number and intensity all the IR active symmetric stretches of all the metal complexes CO ligands. As each complex has 13 atoms it is expected to have 3(13)-6= 33 vibrational modes but as shown in the vibrational spectra below many of these modes are IR inactive and have intensities of 0, an example of this is the totally symmetric C-O vibrations meaning they cannot be analysed given the fact that they don't appear. As predicted earlier across the period CO bond frequency would increase and this is due to back donation. Whilst back bonding causes an increase in the M-C bond there is also a increase in the CO bond length. The more positive the metal centre means contraction of the d-orbitals and this means that their is a less overlap between the d-orbital and CO pi* orbital and the less overlap with this orbital the stronger the CO bond.

Cr complex Vibrational spectrum

Mn complex Vibrational spectrum

Fe complex Vibrational spectrum

The images below are three of the molecular orbitals of the [Fe(CO)6]2+ complex with the corresponding LCAO drawn beside them. Also on the diagrams of the LCAOs there are annotations analysing the structure.

bonding MO 36 (eg)

bonding MO 46 (t1g)

Ng611 (talk) 13:20, 5 June 2019 (BST) Does your central metal ion have an s-orbital? It looks more p-like to me...

anti-bonding MO 50 (LUMO/eg)

Ng611 (talk) 13:20, 5 June 2019 (BST) Excepting MO46, your MO analysis was on point, well done!

References

[1]- Hunt. T, [online] Huntresearchgroup.org.uk, Available at: http://www.huntresearchgroup.org.uk/teaching/teaching_comp_lab_year2a/Tut_MO_diagram_BH3.pdf [Accessed 20 May 2019].

[2]- T. L. Cottrell,The Strengths of Chemical Bonds,2nd ed, Butterworth, London, 1958