Cloud-cloud collision as drivers of the chemical complexity in Galactic
  Centre molecular clouds
  G+0.693-0.03 is a quiescent molecular cloud located within the Sagittarius B2
(Sgr B2) star-forming complex. Recent spectral surveys have shown that it
represents one of the most prolific repositories of complex organic species in
the Galaxy. The origin of such chemical complexity, along with the small-scale
physical structure and properties of G+0.693-0.03, remains a mystery. In this
paper, we report the study of multiple molecules with interferometric
observations in combination with single-dish data in G+0.693-0.03. Despite the
lack of detection of continuum source, we find small-scale (0.2 pc) structures
within this cloud. The analysis of the molecular emission of typical shock
tracers such as SiO, HNCO, and CH$_3$OH unveiled two molecular components,
peaking at velocities of 57 and 75 km s$^{-1}$. They are found to be
interconnected in both space and velocity. The position-velocity diagrams show
features that match with the observational signatures of a cloud-cloud
collision. Additionally, we detect three series of class \rom{1} methanol
masers known to appear in shocked gas, supporting the cloud-cloud collision
scenario. From the maser emission we provide constraints on the gas kinetic
temperatures ($\sim$30-150 K) and H$_2$ densities (10$^4$-10$^5$ cm$^{-2}$).
These properties are similar to those found for the starburst galaxy NGC253
also using class \rom{1} methanol masers, suggested to be associated with a
cloud-cloud collision. We conclude that shocks driven by the possible
cloud-cloud collision is likely the most important mechanism responsible for
the high level of chemical complexity observed in G+0.693-0.03.
