Scheme of the CH₃CN and CH₂DCN chemistry. Upper panel: different routes of formation on the grain surfaces (left) and in the gas phase (right). Lower panel: two possibilities for when CH₃CN and CH₂DCN formed, during the cold prestellar core phase (left) and during the protostellar warm phase (right). In the first case, high CH₂DCN/CH₃CN ratios (≥0.1) are expected, while in the second case the ratios are low (≤0.1).