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We obtained an ALMA Cycle 0 spectral scan of the dusty LIRG NGC 4418,spanning a total of 70.7 GHz in bands 3, 6, and 7. We use a combined localthermal equilibrium (LTE) and non-LTE (NLTE) fit of the spectrum in order toidentify the molecular species and derive column densities and excitationtemperatures. We derive molecular abundances and compare them with otherGalactic and extragalactic sources by means of a principal component analysis.We detect 317 emission lines from a total of 45 molecular species, including 15isotopic substitutions and six vibrationally excited variants. Our LTE/NLTE fitfind kinetic temperatures from 20 to 350 K, and densities between 10$^5$ and10$^7$ cm$^{-3}$. The spectrum is dominated by vibrationally excited HC$_3$N,HCN, and HNC, with vibrational temperatures from 300 to 450 K. We find highabundances of HC$_3$N, SiO, H$_2$S, and c-HCCCH and a low CH$_3$OH abundance. Aprincipal component analysis shows that NGC 4418 and Arp 220 share very similarmolecular abundances and excitation, which clearly set them apart from otherGalactic and extragalactic environments. The similar molecular abundancesobserved towards NCG 4418 and Arp 220 are consistent with a hot gas-phasechemistry, with the relative abundances of SiO and CH$_3$OH being regulated byshocks and X-ray driven dissociation. The bright emission from vibrationallyexcited species confirms the presence of a compact IR source, with an effectivediameter $<$5 pc and brightness temperatures $>$350 K. The molecular abundancesand the vibrationally excited spectrum are consistent with a youngAGN/starburst system. We suggest that NGC 4418 may be a template for a new kindof chemistry and excitation, typical of compact obscured nuclei (CON). Becauseof the narrow line widths and bright molecular emission, NGC 4418 is the idealtarget for further studies of the chemistry in CONs.

Exploring the molecular chemistry and excitation in obscured luminous infrared galaxies: An ALMA mm-wave spectral scan of NGC 4418