As expensive and rare transition metal catalysts become less sustainable, the organocatalysts recognized by the 2021 Nobel Prize in Chemistry are becoming increasingly attractive alternatives. Common organocatalyst cores include urea, thiourea, and squaramide. The naturally occurring biological molecule urea is well-studied, but theory and experiment disagree about urea’s structure. High level computations have located two nonplanar geometries: the anti-conformation with oppositely pyramidalized nitrogens and the syn-conformation with similarly pyramidalized nitrogens. However, experimental microwave studies could not locate the syn-conformation, drawing its existence into question. Thiourea and squaramide similarly show both computed anti- and syn-conformations. Their overall similarity to urea implies that their syn-conformations may also be false ground states. This study computationally examines the structure of squaramide to consider its similarities and differences to that of urea and to better understand its syn-conformation. We used the M06-2X and HCTH407 functionals along with second-order Møller-Plesset theory (MP2) with the jul-, aug-, and maug-cc-pV[D, T, Q]Z basis sets for geometry optimization, energy evaluation, and calculation of anharmonic frequencies. Inertial defects were computed using both harmonic and anharmonic frequencies. The syn-conformation of squaramide appears as an energy minimum at some levels of theory but cannot be located with others. Additionally, the inertial defects showed odd trends, which may point to the syn-conformation as an unrealistic computational artefact. M06-2X and MP2 show good agreement on the structure and energy of squaramide, but only MP2 produced inertial defects that agree with experiment. Other functionals, including M06-2X, produced uninterpretable trends, likely due to their lack of calibration for this process. This suggests that MP2 is the most reliable functional of those tested for inertial defects and similar determinations. Our work provides insight into the structure of squaramide and critiques of how various functionals perform with these types of calculations.

Structural analysis and inertial defects of dual hydrogen bond donor squaramide

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