Rep:Mod:caw116

2018-05-11T13:07:12Z

Caw116: /* BH3 */

== BH3 ==

'''B3LYP/6-31G'''

''Summary of computed values''

[[File:Caw116_bh3_opt_summary_pic.jpg | thumb | Figure 1: Summary of optimized BH3 |centre|500px]]

''Item table''

[[File:Caw116_bh3_opt_energy_stuff_png.PNG | thumb | Figure 2: Item table of the optimized BH3| centre | 500px]]

''Frequency analysis''

[[CAW116 BH3 OPT SYM OPT 2.LOG| text to display]]

[[File:Caw116_bh3_opt_low_freq.PNG | thumb | Figure 3: Frequency analysis | centre | 500px]]

''Jmol''

<jmol><jmolApplet>
<title>BH3</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>Caw116 bh3 frequency.txt</uploadedFileContents>
</jmolApplet></jmol>

''Vibrations''

{| class="wikitable" border="1"
|+ Vibrational modes of BH3
! Mode!! Frequency in cm-1 !! Intensity !! Type !! Activity
|-
| 1 || 1163 || 93 || symmetrical bend || Yes
|-
| 2 || 1213 || 14 || symmetrical bend || Yes
|-
| 3 || 1213 || 14 || asymmetrical bend || Yes
|-
| 4 || 2582 || 0 || symmetrical stretch || No
|-
| 5 || 2715 || 126 || asymmetrical stretch || Yes
|-
| 6 || 2715 || 126 || symmetrical stretch || Yes
|}

''Corresponding IR-spectrum ''

[[File: Caw116_bh3_spectrum.PNG | thumb | Figure 4: IR-spectrum of BH3 | centre | 500px]]

From the table it can be seen that the molecule exhibits 6 vibrational modes, however, mode 4 does not show a change in dipole moment, which makes it IR-unactive.

''MO-diagram of BH3''

[[File:Caw116 bh3 modiagramwithpic.png | thumb | Figure 5: MO-diagram of BH3 | centre | 500px]]

''Based on: Dr. Patricia Hunt, 2018, Imperial College London, "Lecture 4 Tutorial Problem Model Answers", p. 2 ''

The computed MO's largely resemble the theoretical MO's, however, the computed ones show a greater degree of overlap and diffusion than the theoretical ones. This may mainly be due to the theoretical ones being supposed to be clear and easily readable, so that the computed ones are actually closer to reality. Qualitative MO theory still shows great usefulness and accuracy when it comes to predicting the appearance of MO's.

==NH3==

'''B3LYP/6-31G'''

''Summary of computed values''

[[File:Caw116 nh3 opt summarytable.PNG | thumb | Figure 6: Summary of optimized NH3 |centre|500px]]

''Item table''

[[File:Caw116 nh3 opt energy stuff.PNG | thumb | Figure 7: Item table of the optimized NH3| centre | 500px]]

''Frequency analysis''

[[File:Caw116 nh3 freq itemtable.PNG| thumb | Figure 8: Frequency analysis | centre | 500px]]

''Jmol''

<jmol><jmolApplet>
<title>NH3</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>CAW116 NH3 OPT.LOG</uploadedFileContents>
</jmolApplet></jmol>

== BH3NH3 ==

'''B3LYP/6-31G'''

''Summary of computed values''

[[File:Caw116 nh3bh3 opt summarytable.PNG | thumb | Figure 9: Summary of optimized Caw116 nh3bh3 opt item.PNG |centre|500px]]

''Item table''

[[File:Caw116 nh3bh3 opt item.PNG | thumb | Figure 10: Item table of the optimized Caw116 nh3bh3 opt item.PNG| centre | 500px]]

''Frequency analysis''

[[Media:''Frequency analysis''| Caw116 bh3 frequency.txt]]

[[File:Caw116 nh3bh3 opt low freq.PNG| thumb | Figure 11: Frequency analysis | centre | 500px]]

''Jmol''

<jmol><jmolApplet>
<title>NH3BH3</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>CAW116 NH3BH3 OPT SYM.LOG</uploadedFileContents>
</jmolApplet></jmol>

''Energy analysis''

E(BH3)= -26.61532362 a.u.
E(NH3)= -56.55776536 a.u.
E(BH3NH3)= -83.22468897 a.u.

Association energy AE = E(BH3NH3) - (E(NH3)+E(BH3))

= -83.22468897 a.u. -(-56.55776536 a.u.+-26.61532362 a.u.)
= - 0.05159999 a.u.
= -135.4499738 kJ/mol

The dissociation energy corresponds to the bond strength and is a positive value so

'''Bond strength = 135.4499738 kJ/mol'''

This bond strength is relatively low, compared to other organic molecules like methane or ammonia, whose bond strength lies above 400 kJ/mol.

==BBr3==

B3LYO/6-31G(d,p)LANL2DZ

''Summary of computed values''

[[File:Caw116 bbr3 summarytable.PNG | thumb | Figure 12: Summary of optimized BBr3 |centre|500px]]

''Item table''

[[File:Caw116 bbr3 energystuff.PNG | thumb | Figure 13: Item table of the optimized BBr3| centre | 500px]]

''Frequency analysis''

[[Media:''Frequency analysis''| Caw116 bh3 frequency.txt]]

[[File:Caw116 bbr3 lowfreq.PNG | thumb | Figure 14: Frequency analysis | centre | 500px]]

''Jmol''

<jmol><jmolApplet>
<title>BBr3</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>CAW116 BBR3 OPT 3 SYM.LOG</uploadedFileContents>
</jmolApplet></jmol>

''Vibrations''

==Project: Aromaticity==

===Benzene===

'''B3LYP/6-31G'''

''Summary of computed values''

[[File:Caw116 benzene opt summarytable.PNG | thumb | Figure 15: Summary of optimized benzene |centre|500px]]

''Item table''

[[File:Caw116 benzene opt energystuff.PNG | thumb | Figure 16: Item table of the optimized benzene| centre | 500px]]

''Frequency analysis''

[[Media:''Frequency analysis''| Caw116 bh3 frequency.txt]]

[[File:Caw116 bbr3 lowfreq.PNG| thumb | Figure 17: Frequency analysis | centre | 500px]]

''Jmol''

<jmol><jmolApplet>
<title>Benzene</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>CAW116 BENZENE OPT SYM.LOG</uploadedFileContents>
</jmolApplet></jmol>

===Borazine===

'''B3LYP/6-31G'''

''Summary of computed values''

[[File:Caw116 borazine summarytable.PNG | thumb | Figure 18: Summary of optimized borazine|centre|500px]]

''Item table''

[[File:Caw116 borazine energystuff.PNG | thumb | Figure 19: Item table of the optimized borazine| centre | 500px]]

''Frequency analysis''

[[Media:''Frequency analysis''| Caw116 bh3 frequency.txt]]

[[File:Caw116 borazine lowfreq.PNG| thumb | Figure 20: Frequency analysis | centre | 500px]]

''Jmol''

<jmol><jmolApplet>
<title>Borazine</title>
<color>black</color>
<size>200</size>
<uploadedFileContents>CAW116 BORAZINE OPT SYM.LOG</uploadedFileContents>
</jmolApplet></jmol>

== NBO charge analysis ==

[[File:Caw116 borazine chargedistribution.PNG | thumb | Figure 21: The charge distribution of borazine | 300 px | centre ]]

[[File: Caw116 benzene chargedistribution.PNG | thumb | Figure 22: The charge distribution of benzene | 300 px | centre ]]

In benzene the charges are equally distributed throughout the molecule, with each carbon carrying a charge of -0.239 NBO charges and each hydrogen carrying a charge of 0.239 NBO charges. This is due to the carbon having a higher electronegativity value (2.5) than hydrogen (2.1). Thus, carbon pulls the electron closer than hydrogen.

In contrast to benzene, the charge in borazine is not as equally distributed. Here, each nitrogen carries are charge of -1.102 NBO charges, each boron a charge of 0.747 NBO charges, each hydrogen bound to a nitrogen a charge of -0.077 NBO charges and each hydrogen bound to boron a charge of 0.432 NBO charges. This can again be traced back to the difference in electronegativity of the different atoms. Boron has the lowest electronegativity (2.0), so it carries a partial positive charge and the adjacent hydrogen a small partial negative charge. Nitrogen has a very high electronegativity value (3.0) so that it carries the highest partial negative charge. Accordingly, the adjacent hydrogens carry a relatively high partial positive charge.

== MO comparison ==

{| class="wikitable" border="1"
|+ Comparison of the 3 MO's in benzene borazine
! !! Benzene!! Borazine !! Comparison
|-
| 1 || [[File:Caw116 benzene mo10.PNG | thumb | 200px]] || [[File:Caw116 borazine mo12.PNG | thumb | 200px ]] || MO10 of benzene and MO12 of borazine correspond to antibonding orbitals of the respective molecules. As they lie in the same plane as the molecule, they arise from the overlap of the sigma interactions of the different bonds. Here, the electron-withdrawing effect of boron can be observed quite well by comparing the size of the node to the size of the observed node on the opposite nitrogen.
|-
| 2 || [[File:Caw116 benzene mo17.PNG | thumb | 200px]] || [[File: Caw116 borazine mo17.PNG | thumb | 200px]] || MO17 show the symmetric bonding orbitals of both molecules which arise from the overlap of the pi-bonding orbitals of the C-C and B-N bonds respectively. These MO's contribute significantly to the aromatic character of the moelcules.
|-
| 3 || [[File:Caw116 benzene mo21.PNG | thumb | 200px]] || [[File: Caw116 borazine mo21.PNG | thumb | 200px]] || MO21 shows the HOMO of both molecules. It shows bonding character and is due to the overlap of the pi-orbitals of the different bonds. The difference in charge distribution in comparison to benzene can here be observed very well by the non-symmetrical orbitals on borazine being due to the strong electron-withdrawing effect of nitrogen.
|}

== The concept of aromaticity ==

In general, aromatic compounds ought to obey Huckel's rule to be called aromatic. This rule states that "planar, fully conjugated and monocyclic systems", which have 4n+2 pi electrons in the valence bonding MO's, are called aromatic. [J. Clayden et al, 2012, Oxford University Press, "Organic Chemistry - Second edition", p.161] However, there are some examples in which the compound shows aromatic character although its structure cannot be described as planar. An example is when benzene is cooled down to 20K so that it changes to its crystalline state and thus also change its conformation to the chair conformation. [M.Palusiak, T. M. Krygowski, 2007, Chemistry - A European Journal, Vol.13 "Application of AIM Parameters at Ring Critical Points for Estimation of pi-Electron Delocalization in Six-Membered Aromatic and Quasi-Aromatic Rings", p.7996-8006] Planarity is therefore destroyed. In its planar conformation however, its pz-orbitals (the orbitals perpendicular to the plane of the ring) merge to give a plane of electron density above and below the ring, stabilising the compound and leading to equal bond lengths between the carbons (cf MO17 shown above). This spreading out of electron density over the ring, resembles reality. In theory, the described overlap of the pz-orbitals would lead to donut-shape-like rings of electron density above and below the molecule, leaving a hole in the middle. This is why, singly stating that aromaticity is due to the overlap of the pz orbitals is not a good method to describe the aromatic character.

2018-05-10T14:53:25Z

Caw116: