Computational NMR Study of Benzothienoquinoline Heterohelicenes

Early NMR studies of several heterohelicenes containing an annular nitrogen atom and a thiophene ring in their structure suggested the possibility of the lengthening of the carbon-carbon bonds in the interior of the helical turn of the molecule based on the progressive upfield shift of ¹³C resonances toward the center of the helical turn. We now report a comprehensive analysis of the optimized geometry and a comparison of the calculated vs. observed ¹H and ¹³C NMR chemical shifts of nineteen representative benzothienoquinoline heterohelicenes. As was initially hypothesized on the basis of the progressive upfield shift of carbon resonances toward the center of the interior helical turn, the present computational study has demonstrated that carbon-carbon bonds indeed have more sp3 character and are longer than normal sp² bonds to accommodate the helical twist of the molecule, as expected.