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Knorr, Rudolf; Boehrer, Petra (2018): Demonstration of NC(sp(2))-centered stereofluctuations in a guanidine derivative and its cation. In: Journal of Physical Organic Chemistry, Vol. 31, No. 8, e3833
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Abstract

NMR line shapes and coalescences of pairs of diastereotopic reporter nuclei (H-1 and C-13) at suitable molecular positions were used to measure the stereofluctuation rate constants of an N-aryl cycloguanidine model compound of the type aryl-N[N(CH3)CH2](2) (7 in Scheme 1) in toluene and in CDCl3 as the solvents. The key criterion is coincidence or noncoincidence of the rate constants, which revealed that both free N=C double-bond rotation and free N-aryl single-bond rotation occur much more slowly than in-plane, lone-electron pair inversion at nitrogen, which is fast even at -99 degrees C. The related cycloguanidinium model compound (6 in Scheme 1) cannot perform such an N-inversion but simulates it at roughly 35 degrees C in CDCl3 and in Cl2CD-CDCl2 through a less rapid N=C half-rotation that is coupled with a correlated, counter-sense N-aryl half-rotation, whereas both free N=C double-bond rotation and free N-aryl rotation are again significantly slower.