How would you characterized the benzotrithiophene 1? Is it planar? How about when methyl groups are attached (2)? Are these compounds aromatic? A joint computational/experimental study by Wu and Baldridge has tackled these questions.1

It turns out that both of these compounds are non-planar and have C2 symmetry. Now, 1 is very nearly planar. But 2 is decidedly non-planar. The MO6-2X/DZ(2d,p) structures are shown in Figure 1. The central 6-member ring has long bonds and expresses some bond alternation: the C-C distance for the bond between the thiophene rings is 1.469 Å and that of the bond shared by the two rings is 1.451 Å The exocyclic bonds are short, 1.375 Å. This appears to be [6]-radialene-like. NICS computations confirm this notion. The NICS(0) value for the central ring of 1 and 2 is -1.6, significantly less negative than the value in benzene of -7.2. The NICSzz values also reflect non-aromatic character of the central ring. The central ring is non-aromatic.



Figure 1. MO6-2X/DZ(2d,p) structures of 1 and 2.1


(1) Wu, T.-T.; Tai, C.-C.; Lin, W.-C.; Baldridge, K. K., "1,3,4,6,7,9-Hexamethylbenzo[1,2-c:3,4-c:5,6-c]trithiophene: a twisted heteroarene," Org. Biomol. Chem., 2009, 7, 2748-2755, DOI: 10.1039/b902517k.


1: InChI=1/C12H6S3/c1-7-8(2-13-1)10-4-15-6-12(10)11-5-14-3-9(7)11/h1-6H

2: InChI=1/C18H18S3/c1-7-13-14(8(2)19-7)16-10(4)21-12(6)18(16)17-11(5)20-9(3)15(13)17/h1-6H3