GR ID=jo061702vn00002>gures/jo061702vn00002.gif" ALIGN="left" HSPACE=5> |
Aromaticity and neutral homoaromaticity have been evaluated in methano[10]annulenes systems, 1,4-methano[10]annulene (
1), 1,5-methano[10]annulene (
2), and 1,6-methano[10]annulene (
3). C-C bondlen
gths indicate that
1 presents hi
gher bond alternation than
2 and
3. The relative ener
gies were determinedat the B3LYP/6-311+G(d,p) level, and they pointed out that
3 is the most stable isomer. Strain ener
gies,evaluated employin
g homodesmotic reactions, show the same order as the relative ener
gies. Throu
gh adecomposition of strain ener
gies, it could be concluded that the rin
gs absorb more tension than the brid
ges.The chan
ges in aromaticity were evaluated by ma
gnetic susceptibilities,
ges/
gifchars/chi.
gif" BORDER=0 >
M, HOMA, NICS, and resonanceener
gies, RE. HOMA, RE, and
ges/
gifchars/chi.
gif" BORDER=0 >
M indicate that
2 and
3 are stron
gly, and
1 is fairly, aromatic. NICS doesnot provide reliable results, due to interference of rin
g and brid
ge atoms. NBO analysis presents someinteractions that su
ggest the existence of neutral homoaromaticity. GPA indices (evaluated at the B3LYP/6-31G* level) point out that homoaromaticity plays a relevant role only in
3. Moreover, this work is thefirst in the current literature that studies 1,4-methano[10]annulene (
1).