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Early observations of comet C/2016 R2 (PanSTARRS) have shown that thecomposition of this comet is very peculiar. We obtained high resolution spectraof the comet in February when it was at 2.8 au from the Sun. We used the UVESspectrograph of the ESO VLT, complemented with narrow-band images obtained withthe TRAPPIST telescopes. We detect strong emissions from the ions$\mathrm{N_2^+}$ and $\mathrm{CO^+}$, but also $\mathrm{CO_2^+}$, emission fromthe CH radical, and much fainter emissions of the CN, $\mathrm{C_2}$, and$\mathrm{C_3}$ radicals which were not detected in previous observations ofthis comet. We do not detect OH or $\mathrm{H_2O^+}$, and derive an upper limitof the $\mathrm{H_2O^+/CO^+}$ ratio of 0.4, implying that the comet has a lowwater abundance. We measure a $\mathrm{N_2^+/CO^+}$ ratio of $0.06\pm0.01$. Thenon-detection of $\mathrm{NH_2}$ indicates that most of the nitrogen content ofthe comet lies within $\mathrm{N_2}$. Together with the high$\mathrm{N_2^+/CO^+}$ ratio, this could indicate a low formation temperature ofthe comet, or that the comet is a fragment of a large differentiated KuiperBelt object. The $\mathrm{CO_2^+/CO^+}$ ratio is $1.1\pm0.3$. We do not detect$\mathrm{^{14}N^{15}N^+}$ lines, and can only put a lower limit on the$\mathrm{^{14}N/^{15}N}$ ratio measured from $\mathrm{N_2^+}$ of about 100,compatible with measurements of the same isotopic ratio for $\mathrm{NH_2}$ andCN in other comets. Finally, in addition to the [OI] and [CI] forbidden lines,we detect for the first time the forbidden nitrogen lines [NI] doublet at519.79 and 520.04 nm in the coma of a comet.

High resolution optical spectroscopy of the $\mathrm{N_2}$-rich comet C/2016 R2 (PanSTARRS)