As such, we systematically explore HB blueshifts of the C≡N probe herein with the aim to find a simple, analytical expression for this observable. Towards this goal, we combined results from density functional theory (DFT) and the AMOEBA polarizable force field to generate a calibration for the vibrational response of the nitrilecontaining molecule o-tolunitrile (oTN; see Fig. ). In this approach, DFT was used to obtain C≡N vibrational frequencies and TDMs in a large set of purely electrostatic and H-bonding environments (~ 1000 conditions) including point charges (Fig. ) and water and methanol (MeOH) molecules (Fig. ), respectively. Then, the corresponding electric fields exerted on the C≡N were derived from the AMOEBA force field. We attempted to recapitulate the DFTbased frequencies using the VSE (eq. 1a), which was (expectedly) unsuccessful due to the HB blueshift; in contrast, DFT TDMs are well-described by their corresponding VSE equation (eq. 3a), highlighting the different frequency/TDM behaviors that were experimentally observed (Fig. ). We modelled the DFT-derived HB blueshift as a function of HB distance and angle and successfully formulated a quantitative "HB blueshift-vs-HB geometry" relationship. We demonstrate the applicability of Figure . VSE modelling of DFT-based transition dipoles and vibrational frequencies (eq. 3a and eq. 1a, respectively; exact analytical forms are shown in eq. S1 and S2) for oTN's C≡N stretching mode in purely electrostatic environments with point charges and in Hbonding environments with water (A, B, C) and methanol (D, E, F). A, D: Correlation plots between modelled and DFT-based transition dipoles demonstrate eq. 3a accurately describes nitrile environments with purely electrostatic perturbations (red triangles in A and D), water as a HB donor (black circles in A), and methanol as a HB donor (blue squares in D). Fitting parameters for all three environments are reported in Table . B, E: Correlation plots between modelled and DFT-based vibrational frequencies indicate that eq. 1a only applies to nitriles under purely electrostatic perturbations (red triangles in B and E; fitting parameters reported in Table ); when water or methanol are HB donors (black circles in B and blue squares in E, respectively), no correlation (R 2 < 0) is found. The HB blueshift, ∆𝜈̅ 𝐻𝐵 , is determined as the difference between the DFT-predicted frequency and the black line representing ideal correlation, as indicated by the double headed horizontal arrows. C, F: 2D heat plots of ∆𝜈̅ 𝐻𝐵 dependence on heavy atom HB distance and angle [d(C≡N---Owater/MeOH) and θ(C≡N---Owater/MeOH)].