How to determine whether a organic compound is diamagnetic or not? I have faced several questions regarding the diamagnetic behaviour of certain organic compounds. I know that one has to check the electronic configuration of the atoms in the compound and see if there is an unpaired electron or not? Of all aromatic compounds (benzene, cyclopropene cation, cyclopentene anion) that I have come across one thing was common they were all diamagnetic. I have the following question in my mind 


Are all aromatic compounds necessarily diamagnetic and if it is true what may be the proper reasoning for that?
Moreover, the compounds which are stabilised by resonance (hyper conjugation,+M effect) are necessarily diamagnetic. I have this doubt because by the electronic configuration tertiary butane cation should be paramagnetic ($2p^1$ configuration) but is still diamagnetic.




The above is the structure of tertiary butyl cation. It is clear that if we consider the electronic configuration of the central carbon it
 is $1s^{2}2s^{2}2p^{1}$.So it must be paramagnetic. This is where my confusion is.
If all the electrons are spin paired then the molecule will be in a singlet state$^*$ and will be diamagnetic, if they are not so paired then it will be  will be paramagnetic. The rules are the same whether or not the molecule is 'organic'.

Most 'normal' organic compounds, e.g. methane, benzene, etc. have paired electrons in each of the orbitals from the lowest one to the highest occupied one (HOMO), and thus are diamagnetic. The orbitals including the LUMO and up are unoccupied.

Radicals such as $\ce{CH3\cdot}$ have an unpaired electron (doublet state) and so are paramagnetic. 

If a biradical is formed e.g. carbenes (see IUPAC Gold Book 'biradical' for an example structure) then it will depend upon whether the spins are paired in a singlet state (diamagnetic) or parallel as in a triplet state which will be paramagnetic. Often singlet and triplets are close in energy with the triplet being lowest. Similarly, dioxygen has two unpaired electrons and forms a triplet ground state so is paramagnetic. Nitric oxide  has one unpaired electron and is paramagnetic.

If a molecule is put into an electronically excited state by absorbing a photon then both singlet and triplet states can be produced, however, theses are short lived and decay back to the ground state.

$^*$(singlet /doublet/triplet refer to spin multiplicity, spin quantum number  $\pm 1/2$ and total spin e.g. $S=s_1+s_2$ gives multiplicity g  as $g=2S+1$)

example of biradical from IUPAC Gold Book.