Before proceeding further, it should be made clear what is meant by aromaticity. 17 This is one of those concepts such as acidity for which most chemists have an intuitive understanding but which is difficult to define. Benzene is the prototype "aromatic" molecule, and based on it the following attributes of aromaticity have been proposed: (a) aromatic compounds have relatively low reactivity toward electrophiles, (b) aromatic compounds have relatively low energies, (c) aromatic compounds tend to have nearly Table 1. Calculated Energies, 6-311+G* a compound ZPE Hcorr B3LYP MP2 CCSD(T) G2 hydrogen 0.01001 0.01332 -1.17663 -1.14588 -1.15350 -1.16636 ethane (D3d) 0.07463 0.07906 -79.84841 -79.52651 -79.56862 -79.63090 ethene (C2v) 0.05083 0.05481 -78.60836 -78.31651 -78.35424 -78.41593 vinyl anion (Cs) 0.03544 0.03938 -77.94738 -77.65618 -77.69247 -77.76723 propane (C2v) 0.10331 0.10879 -119.17052 -118.70730 -118.76642 -118.85580 isopropyl radical (Cs) 0.08777 0.09402 -118.50556 -118.05394 -118.11076 -118.19887 isopropyl anion (Cs) 0.08563 0.09103 -118.49192 -118.02819 -118.08515 -118.19149 isopropyl cation (C2) 0.08819 0.09407 -118.23376 -117.78742 -117.84558 -117.92706 allyl radical (C2v) 0.06590 0.07067 -117.28948 -116.86957 -116.91667 -117.00589 allyl anion (C2v) 0.06288 0.06823 -117.30326 -116.86722 -116.91515 -117.02516 allyl cation (C2v) 0.06834 0.07310 -116.99373 -116.58010 -116.63165 -116.70870 butane (C2h) 0.13181 0.13854