Sometimes you run across a paper that is surprising for a strange reason: hasn’t this work been done years before? That was my response to seeing this paper on the structure of gauche-1,3-butadiene.¹ Surely, a molecule as simple as this has been examined to death. But, in fact there has been some controversy over whether the cis or gauche form is the second lowest energy conformation. Computations have indicated that the cis form is a transition state for interconverting the two gauche isomers, but experimental confirmation was probably so late in coming due to the small amount of the gauche form present and its small dipole moment.

This paper describes Fourier-transform microwave (FTMW) spectroscopy using two variants: cavity-enhanced FTMW combined with a supersonic expansion and chirped-pulse FTMW in a cryogenic buffer gas cell. In addition, computations were done at CCSD(T) using cc-pCVTZ through cc-pCV5Z basis sets and corrections for perturbative quadruples. The computed structure is shown in Figure 1. In addition to confirming this non-planar structure, with a C-C-C-C dihedral angle of 33.8°, they demonstrate the tunneling between the two mirror image gauche conformations, through the cis transition state.

Figure 1. Computed geometry of gauche-1,3-butadiene.

References

1. Baraban, J. H.; Martin-Drumel, M.-A.; Changala, P. B.; Eibenberger, S.; Nava, M.; Patterson, D.; Stanton, J. F.; Ellison, G. B.; McCarthy, M. C., "The Molecular Structure of gauche-1,3-Butadiene: Experimental Establishment of Non-planarity." Angew. Chem. Int. Ed. 2018, 57, 1821-1825, DOI: 10.1002/anie.201709966.

InChIs

1,3-butadiene: InChI=1S/C4H6/c1-3-4-2/h3-4H,1-2H2
InChIKey=KAKZBPTYRLMSJV-UHFFFAOYSA-N

Corannulene 1 is an interesting aromatic compound because it is nonplanar, having a bowl shape. [6]helicene is an interesting aromatic compound because it is nonplanar, having the shape of a helix. Kato, Segawa, Scott and Itami have joined these together to synthesize the interesting quintuple helicene compound 3.¹

The optimized structure of 3 is shown in Figure 1. They utilized computations to corroborate two experimental findings. First, the NMR spectra of 3 shows a small number of signals indicating that the bowl inversion should be rapid. The molecule has C₅ symmetry due to the bowl shape of the corannulene core. Rapid inversion makes the molecule effectively D₅. (The inversion transition state is of D₅ symmetry, and would be a nice quiz question for those looking for molecules of unusual point groups.) The B3LYP/6-31G(d) computed bowl inversion barrier is only 1.9 kcal mol^-1, significantly less that the bowl inversion barrier of 1: 10.4 kcal mol^-1. This reduction is partly due to the shallower bowl depth of 3 (0.572 Å in the x-ray structure, 0.325 Å in the computed structure) than in 1 (0.87 Å).

Figure 1. Optimized structure of 3.

Second, they took the enhanced MMMMM-isomer and heated it to obtain the thermodynamic properties for the inversion to the PPPPP-isomer. (The PPPPP-isomer is shown in the top scheme.) The experimental values are ΔH^‡ = 36.8 kcal mol^-1, ΔS^‡ = 8.7 cal mol^-1 K^-1, and ΔG^‡ = 34.2 kcal mol^-1 at 298 K. They computed all of the stereoisomers of 3 along with the transition states connecting them. The largest barrier is found in going from MMMMM–3 to MMMMP–3 with a computed barrier of 34.5 kcal mol^-1, in nice agreement with experiment.

References

1. Kato, K.; Segawa, Y.; Scott, L. T.; Itami, K., "A Quintuple [6]Helicene with a Corannulene Core as a C₅-Symmetric Propeller-Shaped π-System." Angew. Chem. Int. Ed. 2018, 57, 1337-1341, DOI: 10.1002/anie.201711985.

InChIs

1: InChI=1S/C20H10/c1-2-12-5-6-14-9-10-15-8-7-13-4-3-11(1)16-17(12)19(14)20(15)18(13)16/h1-10H
InChIKey=VXRUJZQPKRBJKH-UHFFFAOYSA-N

2: InChI=1S/C26H16/c1-3-7-22-17(5-1)9-11-19-13-15-21-16-14-20-12-10-18-6-2-4-8-23(18)25(20)26(21)24(19)22/h1-16H
InChIKey=UOYPNWSDSPYOSN-UHFFFAOYSA-N

3: InChI=1S/C80H40/c1-11-31-51-41(21-1)42-22-2-12-32-52(42)62-61(51)71-63-53-33-13-3-23-43(53)44-24-4-14-34-54(44)64(63)73-67-57-37-17-7-27-47(57)48-28-8-18-38-58(48)68(67)75-70-60-40-20-10-30-50(60)49-29-9-19-39-59(49)69(70)74-66-56-36-16-6-26-46(56)45-25-5-15-35-55(45)65(66)72(62)77-76(71)78(73)80(75)79(74)77/h1-40H
InChIKey=XYUIBQJVZTYREY-UHFFFAOYSA-N