Hydrogen-Bond-Induced Dual Structural Response in Amide-Functionalized Self-Assembled Monolayers

Internal functionalization can reorganize intermolecular interactions across a self-assembled monolayer (SAM), yet its depth-dependent consequences remain poorly understood. Here, molecular dynamics simulations and vibrational sum-frequency generation (VSFG) spectroscopy reveal a dual structural response in N-(2-mercaptoethyl)heptanamide SAMs on Au(111). A persistent intermolecular hydrogen-bonding network suppresses conformational fluctuations between the Au–S interface and the amide plane. Above this plane, however, the larger volume of the amide increases alkyl-chain separation, weakens van der Waals coupling, and enhances conformational flexibility with temperature. Thus, a hydrogen-bond-stabilized, solid-like lower region coexists with a weakly coupled, liquid-like upper region. Simulated VSFG spectra reproduce the observed terminal CH3 stretching-mode intensities, including the enhanced asymmetric response. This response occurs without a correspondingly large average methyl tilt angle because inequivalent molecular environments produce broad, heterogeneous orientation distributions. Consequently, the conventional assignment of a single methyl tilt angle from VSFG intensity ratio breaks down for internally functionalized SAMs.