because Δ𝛼 is typically experimentally negligible. Several vibrational modes, such as the carbonyl (C=O) stretch, have become very useful VSE sensors because they behave according to eq. 1b. As such, they have enabled the assessment of electric field strengths for HBs and other noncovalent interactions in the condensed phase. The nitrile (C≡N) stretch is the most commonly used vibrational probe, since it appears in an uncluttered region of the infrared (IR) spectrum and because nitriles are easily introduced into biological environments like proteins (via drugs or noncanonical amino acids) or chemical settings like surfaces. Despite its popularity, C≡N frequency tuning can exhibit complicated behavior that does not always follow the VSE (Fig. ). In aprotic environments, the C≡N stretch shows a linear 𝜈̅ /𝐹 ⃗ -behavior as described by eq. 1b. However, in H-bonding environments, anomalous frequency shifts are observed which are inconsistent with eq. 1b. Further, this anomalous behavior cannot be explained by relevant quadratic electric field contributions due to Δ𝛼, that is, eq. 1a also cannot describe the frequency tuning. Instead, a description of nitrile frequencies requires the introduction of an additional variable called the HB blueshift ∆𝜈̅ 𝐻𝐵 , to account for 'C≡N⋅⋅⋅H' interactions: 𝜈̅ (𝐹 ⃗ , ∆𝜈̅ 𝐻𝐵 ) = 𝜈̅ (𝐹 ⃗ ) + ∆𝜈̅ 𝐻𝐵 .