Oscillations in the stability of consecutive chemical bonds revealed by ion-induced desorption.

While it is a common concept in chemistry that strengthening of one bond results in weakening of the adjacent ones, no results have been published on if and how this effect protrudes further into the molecular backbone. By binding molecules to a surface in the form of a self-assembled monolayer, the strength of a primary bond can be selectively altered. Herein, we report that by using secondary-ion mass spectrometry, we are able to detect for the first time positional oscillations in the stability of consecutive bonds along the adsorbed molecule, with the amplitudes diminishing with increasing distance from the molecule-metal interface. To explain these observations, we have performed molecular dynamics simulations and DFT calculations. These show that the oscillation effects in chemical-bond stability have a very general nature and break the translational symmetry in molecules. Self-assembled monolayers (SAMs) (1) are a nanotechnolog- ical system, in which molecules are chemically bonded to a substrate in an ordered and oriented fashion. The influence of the strength of the molecule-substrate bond on the structure and stability of SAMs is still poorly understood even for the most simple system of methanethiol on Au- (111). (2) As it is known from other fields of chemistry, the formation of a strong bond (in this case to the surface) should lead to a weakening of the adjacent bonds within the molecules. Thus, it would be interesting from a very basic point of view as well as technologically relevant to determine these influences. We decided to take a new approach by exploiting static secondary-ion mass spectrometry (SSIMS) in combination with the molecular dynamics (MD) simulation of such experiments and density functional theory (DFT) calcula- tions. As model SAMs we have selected two homologous series of the general form CH3-(C6H4)2-(CH2)n-S(Se)/Au- (111), (BPnS(Se), n = 1-6), for which either sulfur atoms or selenium atoms (BPnSe) act as a binding groups to the Au(111) substrate. Previous microscopic (3-5) and spectroscop- ic (6-9) studies demonstrated that the BPnS(Se)/Au(111) pack- ing structure, unlike other SAMs, remains virtually the same for the S and Se analogues within both series. This means that intermolecular interactions in these SAMs are identical when comparing the respective members of both families and thus the only source of difference in stability between them is the binding atom (S vs. Se). Analysis of such SAMs gives a direct way to trace molecule-substrate interface energetics. Positive and negative SSIMS mass spectra are presented in Figure S1 in the Supporting Information. The most