Correlation between Hydrogen Bond Strength and Temperature: A Quantitative Single-Molecule Study over a Broad Temperature Range.

Temperature is widely acknowledged as a crucial factor influencing the strength of hydrogen bonds (H-bonds). While the qualitative relationship between temperature and H-bond strength is well-established, the quantitative details of this relationship remain largely unexplored. Variable-temperature single-molecule force spectroscopy in vacuum (VT-Vac-SMFS) now provides a direct and accurate method to quantify the relationship between temperature and H-bond intrinsic strength (i.e., the H-bond strength without interference from other external factors). Herein, poly(hydroxyethyl methacrylate), a model polymer capable of forming H-bonds between side chains, was used to examine variations in H-bond intrinsic strength across a temperature range of 261 to 363 K by VT-Vac-SMFS. The experimental data reveal a significant decline in H-bond intrinsic strength as the temperature increases. Based on theoretical analysis, we propose, for the first time, a nonlinear correlation between H-bond intrinsic strength (ΔG*) and temperature with an empirical equation: ΔG* = 7.88 - 1.34ln(T - 251.64). This formula enables the prediction of H-bond intrinsic strength at various temperatures within a reasonable range, which provides insights into the precise control of H-bond strength through temperature regulation. Although the formula may not be universally applicable, this pioneering work represents an upgrade in our understanding of this fundamental relationship from a qualitative to a quantitative perspective.