The NMR data for compound I4 recorded at 40 °C were taken from Ref. . The experimental observable J tot was computed by measuring the distance between the two outer peaks of the multiplet (see Figure ) using the MestReNova software (version 14.3.3). The spectra and MestReNova files are available in the ETH Research Collection (DOI: 10.3929/ethz-b-000710355).