Analytical Instrument Documents

Structural analysis by NMR can provide not only a planar molecular structure but also three-dimensional structural information. In this Note, we describe a method for obtaining information on dihedral angles by using ¹H-¹H coupling constants (J_HH values). For example, hydrogen atoms attached to a cyclohexane ring are either located in axial or equatorial positions in respect to the cyclohexane ring (Fig. 1). The dihedral angles between vicinal protons are known to be ∠H_ax-C-C-H_ax ≈ 180°, ∠H_ax-C-H_eq ≈ 60°, and ∠H_eq-C-C-H_eq ≈ 60°. If we look at the Karplus curve shown in Fig. 2, we can see that ³J_HH of around 4 Hz can be expected in the case of the dihedral angle of 60°, while ³J_HH of around 13 Hz corresponds to the dihedral angle of 180°. In reality, ³J_HH values depend on substituents attached to the cyclohexane ring in substituted cyclohexanes, so the analysis is not straightforward, but the basic trend of having a larger J-value for a 180° dihedral angle compared to a 60° dihedral angle remains unchanged. Therefore, from the value of ³J_HH of the methylene protons, it is possible to differentiate between the dihedral angle of 60° or 180°.

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