Orientation of OH terminal groups in oligo(ethylene glycol)-terminated self-assemblies: Results of ab initio modeling

Abstract

Ab initio optimization of periodic arrays of HS(CH2) 3CONH-EG3 [EGn = (CH2CH 2O)nH], which are used to model real self-assembled monolayers (SAMs) terminated by oligomers of ethylene glycol (OEGs), favors a molecular geometry with a strong distortion of the OH tail group. The distortion effect is conformation dependent and appears exclusively in assemblies of helical OEGs. Helical SAM structures with the O-H bond directed towards the substrate surface exhibits a lower energy than the corresponding all trans SAMs with OH group oriented towards the vacuum interface. In view of strict limitations in molecular complexity and basis set for this kind of calculations, we tested the obtained molecular geometry as the starting point for performing optimization of the HS(CH2)15CONH-EG6 molecule with the use of BP86/6-31G* method/basis set provided by Gaussian03. It is found that the observed trend in OH tail group orientation survives and is consistent with a wealth of experimental data available for this type of SAMs. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.