To reconcile the excellent match for F92oCNF and the MOFs but the disparity for F28oCNF and oTN in solvents, we must take into consideration the time scales under which HBs fluctuate for both groups. In F92oCNF (Fig. ), the C≡N is engaged in an intra-protein HB: we detect extended periods of uninterrupted H-bonding and narrow HB distance and angle distributions in AMOEBA MD simulations (see SI Sections 7 and 8), indicating this HB experiences long residence times and minimal geometrical fluctuations. Because of this weakly fluctuating (rigid) C≡N---HO-T90 interaction, the ∆𝜈̅ 𝐻𝐵 distribution (Fig. ) directly reflects on the HB geometry as derived by our model in eq. 7 and Fig. . The same argument holds true for the MOFs, where the HB geometry is locked in place by the framework. These cases are classified as the inhomogeneous limit in IR spectroscopy, i.e. where IR spectra directly reflect the distribution of instantaneous vibrational frequencies. Instead, for F28oCNF and oTN in solvents, the H-bonding with bulk solvent is highly fluctuating, characterized in MD by short H-bonding residence times and broad HB distance/angle distributions (SI Sections 7 and 8). If these fluctuations are faster than the difference in the vibrational frequencies between the fluctuating sub-states (a vibrational frequency difference of ~ 20 cm -1 corresponds to a time scale of ~ 2 ps), the sub-states are not resolved in the IR spectrum but instead motionally narrowed towards one IR band with an averaged peak position (as occurs in coalescence in nuclear magnetic resonance) ; lifetimes for H-bonding and non-H-bonding nitrile states were extracted from MD simulations and qualitatively support this possibility (Fig. and Table ).