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Aims. We study the effect of large scale dynamics on the molecularcomposition of the dense interstellar medium during the transition betweendiffuse to dense clouds. Methods. We followed the formation of dense clouds (onsub-parsec scales) through the dynamics of the interstellar medium at galac-tic scales. We used results from smoothed particle hydrodynamics (SPH)simulations from which we extracted physical parameters that are used as inputsfor our full gas-grain chemical model. In these simulations, the evolution ofthe interstellar matter is followed for ~50 Myr. The warm low-densityinterstellar medium gas flows into spiral arms where orbit crowding producesthe shock formation of dense clouds, which are held together temporarily by theexternal pressure. Results. We show that depending on the physical history ofeach SPH particle, the molecular composition of the modeled dense cloudspresents a high dispersion in the computed abundances even if the localphysical properties are similar. We find that carbon chains are the mostaffected species and show that these differences are directly connected todifferences in (1) the electronic fraction, (2) the C/O ratio, and (3) thelocal physical conditions. We argue that differences in the dynamical evolutionof the gas that formed dense clouds could account for the molecular diversityobserved between and within these clouds. Conclusions. This study shows theimportance of past physical conditions in establishing the chemical compositionof the dense medium.

Influence of galactic arm scale dynamics on the molecular composition of the cold and dense ISM I. Observed abundance gradients in dense clouds