  Nitriles play a key role as molecular precursors in prebiotic experiments
based on the RNA-world scenario for the origin of life. These chemical
compounds could have been partially delivered to the young Earth from
extraterrestrial objects, stressing the importance of establishing the
reservoir of nitriles in the interstellar medium. We report here the detection
towards the molecular cloud G+0.693-0.027 of several nitriles, including cyanic
acid (HOCN), and three C$_4$H$_3$N isomers (cyanoallene, CH$_2$CCHCN; propargyl
cyanide, HCCCH$_2$CN; and cyanopropyne (CH$_3$CCCN), and the tentative
detections of cyanoformaldehyde (HCOCN), and glycolonitrile (HOCH$_2$CN). We
have also performed the first interstellar search of cyanoacetaldehyde
(HCOCH$_2$CN), which was not detected. Based on the derived molecular
abundances of the different nitriles in G+0.693-0.027 and other interstellar
sources, we have discussed their formation mechanisms in the ISM. We propose
that the observed HOCN abundance in G+0.693-0.027 is mainly due to surface
chemistry and subsequent shock-induced desorption, while HCOCN might be mainly
formed through gas-phase chemistry. In the case of HOCH$_2$CN, several
grain-surface routes from abundant precursors could produce it. The derived
abundances of the three C$_4$H$_3$N isomers in G+0.693-0.027 are very similar,
and also similar to those previously reported in the dark cold cloud TMC-1.
This suggests that the three isomers are likely formed through gas-phase
chemistry from common precursors, possibly unsaturated hydrocarbons (CH$_3$CCH
and CH$_2$CCH$_2$) that react with the cyanide radical (CN). The rich nitrile
feedstock found towards G+0.693-0.027 confirms that interstellar chemistry is
able to synthesize in space molecular species that could drive the prebiotic
chemistry of the RNA-world.
