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tokens: to interpret the OPR values of H ( Crabtree et al. 2011c , b ) and subsequently studied in more detail for the specific system ( Gomez-Carrasco et al. 2012 ). The mechanistic distinctions were also recently considered for observations of the OPRs of HO ( Herbst 2015 ) and HCl ( Neufeld et al. 2015 ), providing new insights to explore. A rigorous determination of the mechanisms in a particular system requires a full quantum scattering treatment, but as pointed out by Gomez-Carrasco et al. (2012) , full quantal computations are rarely achievable because of their complexity and the computational cost. The next best approach is the semi-classical trajectory method, in which atoms and molecules undergo classical trajectories on a quantum mechanical PES. It is this approach that is used in this paper. Of course, both destruction and conversion processes must also be considered in determining OPRs, and such processes are also contained in our chemical simulations. In the remainder of the paper, we first consider in Sect. 3 the astronomical observations leading to new OPR measurements for HCl in an extragalactic absorber. Then we discuss in Sect. 4 our modeling approach and present its outcomes for the scrambling versus hydrogen abstraction/proton hop mechanisms. Section 5 describes our semi-classical treatment of reaction ( 4 ), which models show to be the dominant formation process for HCl. Finally, Sect. 6 summarizes our results and draws the main conclusions of the present study. 3. Observations 3.1. Description of the observations The ortho and para forms of HCl were observed with the Atacama Large Millimeter/submillimeter Array (ALMA) during Early Science cycle 2, in summer 2014. Two different tunings were observed to cover the – transition of ortho-HCl (rest frequencies 189 GHz, redshifted to 100 GHz, within ALMA Band 3) and the – transitions of para-HCl (rest frequencies 485 GHz, redshifted to 257 GHz, within ALMA Band 6), respectively, as listed Table 1 . The array was maintained in a configuration providing a synthesized beam resolution of better than 0.5, sufficient to resolve the two point-like lensed images of PKS 1830211, separated by 1. The bandpass calibration was done on the bright quasar 1924292, and the gain calibration was solved by regular visits on 18322039 during the observations. The correlator was set up with spectral windows of 1.875 GHz, providing a velocity resolution of 1.3 km s (Band 6) and 2.9 km s (Band 3), after Hanning smoothing. The data calibration was performed within CASA 1 following a standard procedure. A further run of self-calibration using the bright continuum of PKS 1830211 helped us to significantly improve the data quality. The final spectra were extracted toward both lensed images of PKS 1830211, using the CASA-python task UVMULTIFIT ( Martí-Vidal et al. 2014 ), by fitting a model of two point sources to the interferometric visibilities. The ALMA spectra of HCl toward PKS 1830211 are shown in Fig. 1 . The