Identification of hydrogen bond characterizations of isomeric 4Bpy and 2Bpy by STM

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Abstract
With submolecular resolution, scanning tunneling microscopy (STM) has been used extensively to study adsorption phenomena of organic molecules on surfaces. In this paper, we report a STM study of the isomeric compound of 4,4′‐bipyridyl (4Bpy) and 2,2′‐bipyridyl (2Bpy) co‐adsorbed with stearic acid on a highly oriented pyrolytic graphite (HOPG) surface. The 4Bpy and 2Bpy are isomeric compounds, and their only difference is the positions of the nitrogen atoms in each molecule. When Bpy molecules are mixed with stearic acid, the nitrogen atoms of Bpy can hydrogen bond with the COOH group of stearic acid, and hence the positions of the nitrogen atoms determine the hydrogen bond structure. It is observed that in the 4Bpy – stearic acid system the angle of the 4Bpy to the stearic acid molecular axis is ∼165°, whereas in the 2Bpy – stearic acid system the angle of the 2Bpy to the stearic acid molecular axis is ∼125°. The distances between two neighbouring ‘bright’ rows in an STM image are different in these two systems. Thus, by using this hydrogen bond method we can distinguish the 4Bpy molecule from its isomeric 2Bpy molecule. Furthermore, as reported previously, we observe the enantiomeric molecular patterns in both 4Bpy and 2Bpy systems. Copyright © 2001 John Wiley & Sons, Ltd.