Appendix I: Equilibrium Structures for the three conformers of allyl-NCO (conf. I, II and III) at the B3LYP-D3(BJ)/cc-pVQZ level of theory.Appendix II: Basis set benchmark using the B3LYP-D3(BJ) method.Appendix III: Measured line lists for conf. III and conf. I, including their parent and minor isotopologues ( 13 C, 15 N and 18 O).Appendix IV: Kraitchman coordinates for conf. I and conf. III.

The pure rotational spectrum of allyl isocyanate (CH 2 = CHCH 2 NCO) was studied using chirped pulse and Balle-Flygare Fourier Transform microwave (FTMW) spectroscopy. Besides the previously reported gauche conformer, a the lowest energy conformer was identified for the first time with the assistance of the quantum-chemical calculations performed at the B3LYP-D3(BJ) and MP2 levels of theory with Dunning's cc-pVQZ basis set. The assignments were confirmed by the resolved hyperfine structure due to the 14 N quadrupole moment and the spectra of the corresponding 13 C, 15 N and 18 O singly substituted isotopologues in natural abundance. Rotational transitions of the most stable conformer revealed a tunneling splitting due to the interconversion motion between its two mirror images, and the tunneling path was established theoretically. In addition, benchmark calculations of various density functionals with and without dispersion corrections were carried out to investigate the effect of the short-range dispersion energy on the conformational structures.