METHYLENE ROCKING IN CYCLOBUTANEDICARBOXYLIC ACID AT 4.2 K - THE GEOMETRY OF A 4-MEMBERED RING IN ITS GROUND-STATE

The geometry of the four-membered ring in 2,2,4,4-deuterated 1,1-cyclobutanedicarboxylic acid in its lowest vibrational state was elucidated from a single-crystal ENDOR detected NMR (EDNMR) study at 4.2 K of the β-deuteron quadrupole coupling tensors. These data, combined with the location of the carbon atoms obtained from a 20 K X-ray study of the same material (Santarsiero, 1990), provide a direct measurement of the geometry of the ring in its ground state. The D-C-D bond angle was found to be 109.5 ± 0.5°, with the two methylene groups tilted (rocked) by 4.4 ± 0.5° toward each other. The ratio of the rocking angle to the pucker angle, 0.21 ± 0.01, is identical with the value obtained from a gas electron diffraction study of cyclobutane (Egawa, 1987). The remarkable agreement between these very different experiments supports the view that the ring atoms in this “highly strained” molecule retain their usual sp3 symmetry, with all four orbitals simply pivoting as a unit to minimize the energy. A C-H bond length of 1.097 ± 0.005 Å (obtained from the Γ H-H dipolar coupling tensor) is also consistent with this view of the ring atoms. The average value of e2qQ/h is 176 ± 2 kHz with η ≈ 0.03, in agreement with other measurements of carbon-bonded deuterons. © 1990, American Chemical Society. All rights reserved.