  This work proposes deuteronated PAH (DPAH+ ) molecules as a potential carrier
of the 4.4 and 4.65 {\mu}m mid infrared emission bands that have been
observationally detected towards the Orion and M17 regions. Density Functional
Theory calculations have been carried out on DPAH+ molecules to see the
variations in the spectral behaviour from that of a pure PAH. DPAH+ molecules
show features that arise due to the stretching of the aliphatic C-D bond.
Deuterated PAHs have been previously reported as carriers for such features.
However, preferred conditions of ionization of PAHs in the interstellar medium
(ISM) indicates the possibility of the formation of DPAH+ molecules. Comparison
of band positions of DPAH+ s shows reasonable agreement with the observations.
We report the effect of size of the DPAH+ molecules on band positions and
intensities. This study also reports a D/H ratio ([D/H]sc ; the ratio of C-D
stretch and C-H stretch bands per [D/H]num ) that is decreasing with the
increasing size of DPAH+ s. It is noted that large DPAH+ molecules (no. of C
atoms ~ 50) match the D/H ratio that has been estimated from observations. This
ratio offers prospects to study the deuterium abundance and depletion in the
ISM.
