with n1 ≈ -4.0 (R 2 = 0.99). This distance dependence is reminiscent of the energetic contribution from dipolequadrupole interactions, which have a 𝑑 -4 distance dependence. The result is therefore in line with previous interpretations of the ∆𝜈̅ 𝐻𝐵 describing higher (difference) multipole terms not included in the dipolar VSE equations (eqs. 1a/b). When evaluating side-on HBs (θ = 70°), we note a more complicated asymptotic distance dependence with a minimum at roughly 3.5 Å (as noted above), at which point ∆𝜈̅ 𝐻𝐵 is about -5 cm -1 ; this is followed by a gradual increase of ∆𝜈̅ 𝐻𝐵 at larger d, becoming nearly negligible around 5.0 Å. We modelled this distance dependence with a Buckinghamlike function (R 2 = 0.89), ). See Table for the complete list of optimized parameters.