This invention provides compounds of formula I: wherein a, b, c, m, p, s, t, W, Ar1, X1, R1, R2, R3, R4, R6, and R7 are as defined in the specification. The compounds of formula I are muscarinic receptor antagonists. The invention also provides pharmaceutical compositions containing such compounds, processes and intermediates for preparing such compounds and methods of using such compounds to treat pulmonary disorders.