The present invention is directed to certain novel compounds represented by structural formula I: or a pharmaceutically acceptable salt thereof, wherein R3, R5, R6, R7, R8, R11, R12, R13, Q, W, X, Y and Z are defined herein. The invention is also concerned with pharmaceutical formulations comprising these novel compounds as active ingredients and the use of the novel compounds and their formulations in the treatment of certain disorders. The compounds of this invention are tachykinin receptor antagonists and are useful in the treatment of psychiatric disorders including depression and anxiety.