New Approach for Determining the Conformational Features of Pseudorotating Ring Molecules Utilizing Calculated and Measured NMR Spin-Spin Coupling Constants

A new method is developed to determine the most stable conformations of puckered rings by a comparison of measured and calculated SSCCs (DORCO: determination of ring conformations). The DORCO method extensively uses the ring puckering coordinates to express the properties of a pseudorotating puckered ring. In the case of NMR spin-spin coupling constants (SSCCs) J, this leads to an extension of the Karplus relationship to puckered rings. For five-membered rings with puckering coordinates q and φ, the Karplus relationships adopt the form nJ(q,φ) or nJ(φ). It is shown in this paper that functions 3J(HCCH)(φ) calculated with coupled perturbed density functional theory for a parent molecule can also be used for derivatives of the parent molecule and help to determine their conformational probability distribution function F(φ). For this purpose, the DORCO procedure is developed to use functions nJ(φ) of the parent molecule in connection with measured SSCCs of the substituted compound. DORCO was tested for tetrahydrofuran and two of its derivatives. The most stable conformations were determined by an accuracy of 4% or better.