Rep:Mod:jun0404

• Moleclar Orbital Calculations

This is the lowest energy valence MO of energy -1.19651 a.u..It is a totally bonding orbital arsing from the through bond overlap of the 2s orbitals of the Nitrogen and Carbon centres.

This is a triply degenerate valence MO of energy -0.92556 a.u..Although a node is present at the nitrogen due to the 2p orbital contribution,it is not significant since it is found on the atom. Therefore this can be classified as a mainly bonding orbital due to all the through-bond directional in phase overlap of the p and s orbitals on the nitrogen, carbons and hydrogens.

This is a triply degenerate valence MO of energy -0.58031 a.u..There is no contribution from the nitrogen in the particular MO. Again despite the presence of several nodes, since most of the are due to the 2p orbitals at the carbon centres they are not very significant. Some through space antibonding character is also present between lobes on different atom which is also quite small. Therefore, looking also at the relative energy of the orbital, it can be classified as a non-bonding orbital due to the directional, though-bond in phase overlap of p and s orbitals on the methyl groups, that do not involve the nitrogen.

Charge Distribution Calculations

A full NBO charge analysis was performed with a range of -0.6 to +0.6

LEFT:Charge distribution for [N(CH3)₄]⁺
RIGHT: Charge distribution for [P(CH3)₄]⁺.

[N(CH3)₄]⁺ charges(DEBYE):

• Nitrogen: -0.295
• Hydrogen: 0.269
• Carbon: -0.483

[P(CH3)₄]⁺ charges(DEBYE):

• Phosphorous: 1.667
• Hydrogen: 0.298
• Carbon: -1.060

The formal charge on the Nitrogen or Phosphorous is merely a convention, assuming
that electrons in all chemical bonds are shared equally between atoms, regardless of
relative electronegativity. It is also assumed that electrons are localised in the bonds
and thus formal charges in the valence bond model keep track of the electrons around the
atom, in this case N or P. However, as shown in both molecules the charge is delocalised
in the entire molecule .
Specifically in [N(CH3)₄]⁺ the positive charge is found on the electropostive protons, while
the nitrogen, being an electronegative element is negatively charged. This is the opposite of what
of what VSEPR and localisation of bonds predict, exactly because in real molecules, molecular
orbitals that span all over the entire molecule are present, leading to delocalisation of the
charge.
When compared to [N(CH3)₄]⁺ , the charge separation in the C-P bond is greater than in the N-P bond
due to the greater difference in electronegativity between C&P(𐤃elec=0.49) and C&N(𐤃elec=-0.36). Also
the charge on the phosphorous is now positive since phosphorous is more electropostive than nitrogen.
Therefore in this case the charge is greatly found on the phosphorous atom and partly in the hydrogens.