Tr(B = 0 T) divided by the in-field transmission Tr(B). Because absorption and transmission are related by Tr =log I I 0 , the sign of increased transition strength is the same for both experimental and calculated spectra. Therefore, we must conclude that the transition intensity increases upon application of a field, which is a strong indication that the experimental spectral features are indeed essentially due to electric-dipole-induced vibrational transitions that are perturbed by coupling to spin excitations. This is corroborated by the fact that the experimental features for 1A are rather strong, which suggests that they are electric-dipole (i.e., largely vibrational), rather than magnetic-dipole (i.e., large magnetic resonance) in origin, because the transition rate of the latter is orders of magnitude smaller than that of the former. Secondly, calculated magnetic resonance spectra (employing unphysically large transition dipole moments), using the same phonon grid model shows the opposite behaviour, i.e., decreased absorption upon application of a magnetic field (Fig. ). We have also tested the influence of polarization of the absorption bands, which potentially has an impact. Our simulations show that polarization only has a very small imfluence on the result (see supporting information section S4.4) and we therefore do not consider this effect any further.