The sustainability issues with rare earth transition metal catalysts are becoming increasingly clear due to their cost and negative environmental impacts. In response, asymmetric organocatalysts have become more attractive alternatives, garnering significant interest. Common organocatalyst cores include the small molecules urea, thiourea, and squaramide. Urea has been well-studied as a naturally occurring biological molecule, but disagreement over its gas phase structure persists between experiment and computation. Prior computational work has identified two ground states of gaseous urea: the anti-conformation, with oppositely pyramidalized nitrogens, and the syn-conformation, with similarly pyramidalized nitrogens. Our work establishes that the syn-conformation is a false energetic well that does not physically exist. As a similar molecule, squaramide computationally optimizes to the same anti- and syn- conformations. This work examines the energy and structural properties of both scaffolds to determine if squaramide shares urea’s single ground state as well. We used the M06-2X and HCTH407 density functionals along with second-order Møller-Plesset theory (MP2) and the Dunning basis sets for geometry optimization, energy evaluation, and calculation of anharmonic frequencies. The majority of our calculations identify the syn-conformation not as a ground state, but as an anti-to-anti transition state with a relatively weak activation barrier of 0.1 kcal/mol. Those that establish a syn-ground state have a similarly low difference in energy between the two conformations. We also located an anti-to-anti transition state through C-N bond rotation, but a similar one could not be located for the syn-to-syn equivalent. Additionally, the calculated inertial defects for the anti-conformation follow relatively linear trends, but the syn-conformation produces nonsensical values that strongly indicate squaramide, like urea, has only one ground state in the gas-phase.

Energetic structural exploration of organocatalyst core squaramide: Transition state analysis

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