  We carried out observations of a peculiar comet, C/2016 R2 (Pan-STARRS),
using a low resolution spectrograph mounted on the 1.2m telescope at Mount Abu
Infrared Observatory, India. The comet was observed on two dates in January
2018, when it was at a heliocentric distance of 2.8 AU. Study based on our
observations revealed that the optical spectrum of this comet is quite unusual
as compared to general cometary spectra. Most of the major cometary emissions
like C$_{2}$, C$_{3}$ and CN were absent in comet C/2016 R2. However, the comet
spectrum showed very strong emission bands from ionic species like CO$^{+}$ and
N$_{2}^{+}$. A mean N$_{2}$/CO ratio of 0.09 $\pm$ 0.02 was derived from the
spectra and an extremely low depletion factor of 1.6 $\pm$ 0.4 has been
estimated for this ratio as compared to the solar nebula. We have also detected
minor emission features beyond 5400 Angstroms, albeit marginally. The column
densities of CO$^{+}$ and N$_{2}^{+}$ were calculated from their emission
bands. The optical spectrum suggests that the cometary ice is dominated by CO.
The low depletion factor of N$_{2}$/CO ratio in this comet, as compared to the
solar nebula and the unusual spectrum of the comet are consequences of
distinctive processing at the location of its formation in the early solar
nebula.
