Don't ditch the NMR just yet: Using atomic force microscopy as an aid to solving problematic structures

Abstract

Images of chemical structures abound in the literature, and we use different formats to explain different features of the molecules in question. Recent advances in atomic force microscopy (AFM), using a carbon monoxide terminated tips have brought about a spectacular enhancement in the resolution attainable.1 It is now feasible to distinguish chemical features such as atoms, bond lengths and bond angles at the Ångstrom scale. The images observed are very close to textbook stick structures and we postulated that this might be useful to assist organic structure determination. To test the possibility of using this technique for this purpose, we used a planar molecule to obtain NMR and AFM data and assembled the substructures guided by the AFM image (Figure 1).2 Two solutions were deemed possible, and one was excluded based on density functional theory calculations leaving only one structure remaining. More recent studies have dealt with non-planar structures as well as the combined use of NMR, AFM and computer assisted structure elucidation to solve structures. The presentation will include the amount of material needed to perform such studies as well as the current limitations of the method. Finally, prospects for the future will be discussed for the integration of AFM into the structure determination armoury.