MD simulations were performed with the FC AMBER ff99SB-ILDN FF and the POL AMOEBABIO18 FF, which differ in the extent of their electrostatic descriptions: while AMBER only considers atomic partial charges, AMOEBA uses charges, dipoles, quadrupoles, and polarizabilities on each atom to describe electrostatic potentials more accurately. Starting with high resolution crystal structures (Figure ), we performed AMBER and AMOEBA simulations to produce 200 ns and 100 ns trajectories in aggregate per variant, respectively, and extracted FC≡N,MD's every 10 ps (Table ,B black traces; details for extraction of MD electric �ields are provided in Section S1). The distributions of the variants' electric �ields demonstrate considerable variety in terms of shape, width, and center positions, and several distributions depend strongly on the choice of FF. Accordingly, we analyzed the origins of the MD FC≡N distributions by characterizing the nitrile's environment as H-bonding or non-H-bonding using a heavyatom nitrile-H-bond donor-acceptor distance cutoff of 4.0 AÅ (dNX in Figure ; see Figures for details on choices of H-bond geometric cutoffs). The non-H-bonding and Hbonding populations separate into distinct, nearly Gaussian electric �ield distributions that are centered at more positive and more negative FC≡N,MD's, respectively (Table ,B gray and magenta histograms; distributions with a 3.5 AÅ cutoff are effectively unaltered, shown in Figure ), as previously observed for FC MD-based sub-populations of a nitrilecontaining inhibitor for human aldose reductase. F62oCNF displays a symmetric MD electric �ield distribution, and it is accordingly involved in little to no nitrile H-bonding with both FFs (≤ 1.8%; Table ). Only F96oCNF's FC MD distribution is similarly symmetric, and it also had little H-bonding (3.5%; Table ). All other electric �ield distributions exhibit either asymmetry or bimodality, and the corresponding nitriles experienced H-bonding in 23 -74% of MD frames, with POL MD predicting up to 6.5times higher H-bonding probability than FC MD (Table ). Similar to the H-bonding fractions, the median Hbonding/non-H-bonding FC≡sN,MD's are sensitive to the FF. The median FC≡N,POL MD's are consistently more negative than the FC≡N,FC MD's by factors of 2 -3 and 2 -6 for the H-bonding and non-H-bonding populations, respectively (Table ). While the MD electric �ield distributions demonstrate variety in their shape, the observed room temperature IR spectra of all variants were well-�it with a single, symmetric band (spectra shown in Figure ), indicating an apparent discrepancy between the calculations and experiment. Since the asymmetries/bimodalities for F28oCNF, F92oCNF, and F96oCNF's calculated electric �ield distributions arise from the presence of both H-bonding and non-H-bonding nitrile populations (Table ), one possible explanation is that these species are in chemical exchange and that they exchange quickly enough to appear as a single IR band at room temperature. Indeed, linear IR spectra of a nitrile experiencing varying molecular environmentsrapid exchange between H-bonding and non-H-bonding interactions in this case -can demonstrate a single or multiple bands depending on how quickly the electric �ields �luctuate in comparison with the nitrile's dephasing lifetime. To address this possibility, we analyzed the nitrile's H-bonding state and electric �ields as functions of time in both FC and POL MD (Figure ) and determined the lifetimes of the H-bonding and non-H-bonding states (Figures ; Tables ). We then employed a twostate �irst order exchange model to describe the transition between these states and found that the exchange rate between the protic and aprotic nitrile populations is > 0.1 ps -1 (Table ). This rate translates to a lifetime of < 10 ps, which is comparable to the timescale of vibrational dephasing for p-cyanophenylalanine (~4 ps; no lifetime could be found for oCNF) such that a single symmetric IR lineshape is feasible. In addition, the H-bonding/non-Hbonding lifetimes from the exchange model can be used to predict the nitrile H-bonding fractions (Table ). The exchange model correctly predicts POL MD has more Hbonding and the results deviate from the H-bonding fractions in Table by an RMSD of only 11%, suggesting that the length of our MD simulations was suf�icient to approach equilibrium statistics of the nitriles' environments.