Theoretical Studies of Furan and Thiophene Nanothreads: Structures, Cycloaddition Barriers and Activation Volumes

In this theoretical study we examine several aspects of the formation, structure, and stability of the most ordered nanothreads yet made, those derived from furan and thiophene. First, we look at the enthalpic consequences and activation barriers of the first two steps of oligomerization by a Diels-Alder mechanism. The ca. 20 kcal/mol difference in the synthetic pressures (furan lower) is explainable in terms of greater loss of aromaticity by the thiophene. Subsequent steps have understandably lower barriers. We show explicitly how pressure affects the reaction profiles, operating through the volume decrease in the transition state and onward to the product molecule. The interesting option of polymerization proceeding in one or two directions opens up the possibility of polymers with two opposing and cumulative dipole moments. The computed activation volumes are consistently more negative for likely initial furan (compared with thiophene) polymerization steps, in accord with the lower onset pressure of furan polymerization. In the second part of our study we examine the energetics of the likely polymers. Three ordered polymer structures compete in enthalpy -- a syn one, with all O/S on the same side, an anti one, S/O alternating, and a syn-anti isomer, with segments of four monomers repeating. The syn polymer, if not allowed to distort, is at high enthalpy relative to the other two. The origin of the destabilization is apparent, being S/O lone-pair repulsion, understandably greater for S than O at the 2.8/2.6Å separation. Set free, the syn isomers curve or arc, in two- or three-dimensional (helical) ways, whose energetics are traced in detail. The syn polymer can also stabilize itself by the thread twisting into zig-zag or helical enthalpic minima. Release of strain in a linear thread as the pressure is relaxed to 1 atm, with consequent thread curving, is a likely mechanism for the observed loss of crystalline order in the polymer as it is returned to ambient pressure.