Quetiapine Dimer Impurity; 11,11'-(1,4-Piperazinediyl)bis-dibenzo[b,f][1,4]thiazepine; 11,11'-(piperazine-1,4-diyl)bis(dibenzo[b,f][1,4]thiazepine); 1,4-Bis(dibenzo[b,f][1,4]thiazepin-11-yl)piperazine;