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Abstract— N‐retinylidene‐butyl‐l‐aminium tetrafluoroborate (NRBH+BF4‐) was used to determine the frequencies of the free (non‐hydrogen‐bonded) v(N+‐H) and v(C=NH+) vibrations and their anharmon‐icity constants. For this purpose the Fourier‐transform IR and near‐IR (overtone) spectra were recorded. The visible‐UV spectra were also taken. The results locate the free v(N+‐H) band at about 3250 ± 50 cm‐1; the vibration has normal anharmonicity while v(C=NH+) is very nearly harmonic. The UV and IR spectra show that the Schiff base is protonated, but that the proton is only very weakly hydrogen‐bonded to the anion. Therefore, in this respect, NRBH+BF4‐ is a good model for the hydrogen‐bonding environment of the region around the nitrogen of rhodopsin chromophores.