[Jfalken]

What other functional group has the same characteristics as the nitro NO2 group?
What I am looking for is strong electron negativity and it should be about the same size too.

CF3 should be stronger in regards to electron withdrawing ability but I believe it is fairly larger too.
How about NF2, should be somewhat smaller (F being smaller than O) but should have more electron withdrawing density than NO2, any other ideas?

[wildfyr]

Look up some tables of Hammett sigma values for some ideas.

[rolnor]

CN, CHO.

[Enthalpy]

Organic acid groups, including here carbonates, give molecules a dipole moment comparable to nitro.

Whether this is called "electron negativity", I can't tell.