<span style="white-space:pre"> </span>Molecular clouds are formed by the compression of atomic gas by interstellar shockwaves. This process is intensively studied using MHD simulations in recent years. While these studies include the formation and destruction of major coolants (e.g. O atom, C+, and CO), they do not include less abundant molecules (e.g. carbon-chains) that are observed and used for chemical diagnosis of molecular clouds. So far more than a hundred molecular species are detected in interstellar clouds, and their abundances and spatial distributions are used to investigate the physical conditions and evolutionary stage of clouds. <div><span style="white-space:pre"> </span>We aim to fully understand the chemistry in the molecular cloud formation process, including the less abundant molecules for the chemical diagnosis. In the present study, we investigate the chemistry in the shock-compressed layer of Av ? a few mag, which corresponds to diffuse clouds, as a first step. We perform 3D MHD simulations of converging atomic gas flow. We analyze the results to derive the 1D mean flow, along which we solve the detailed chemical reaction network. Compared with previous studies on 1D shock model of cloud formation, we cover a wider range of initial gas density and velocity, as well as the angle between the gas flow and magnetic field. </div><div><span style="white-space:pre"> </span>We derive the column densities of various species such as HCO+ and C3H2, and compared the observation of molecular absorption lines of low Av clouds (i.e. diffuse clouds) observed towards field stars and QSRs. While the column densities vary among models, they show reasonable agreement with observations. We also investigate the dependence of chemistry on ionization rate, elemental abundance, and PAH.
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