A compound of structural formula IIIa which is radiolabelled with 11 C, 18 F, 15 O or 13 N: where Ar is selected from the group consisting of phenyl, indole, indazole, or biphenyl, where each is optionally substituted with 1 to 3 groups of R a; R a is selected from the group consisting of halogen, hydroxyl, -C 1-6 alkyl, which is unsubstituted or substituted with one or more halogen, -C 3-6 cycloalkyl, -NR 2 C R 2, -C N(R 2) 2, -C(R 2) 2 OR 2, -C R 2, -NO 2, -CN, -N(R 2) 2, -C OR 2, -OR 2, -(CH 2) n C 5-10 heterocyclyl, -(CH 2) n C 6-10 aryl, and -(CH 2) n C 5-10 heteroaryl, wherein said heterocyclyl, aryl, and heteroaryl are each optionally substituted with 1 to 3 groups of (a) halogen, (b) hydroxyl, (c) -C 1-6 alkyl, (d) -CN, (e) -(CH 2) n CF 3, or (f) -C 6-10 aryl; R 2 is selected from the group consisting of hydrogen, hydroxyl, -C 1-6 alkyl, which is unsubstituted or substituted with one or more halogen, -(CH 2)CF 3, -(CH 2) n F, -C 3 - 10 cycloalkyl, -C 6 - 10 aryl, -C 5-10 heteroaryl, and -C 5 - 10 heterocyclyl, wherein said cycloalkyl, aryl, heterocycyl, or heteroaryl is each optionally substituted with 1 to 3 groups of Ra and n is independently 0 to 4 or a pharmaceutically acceptable salt thereof. where Ar is selected from the group consisting of phenyl, indole, indazole, or biphenyl, phenyl, indole, indazole, or biphenyl, where each is optionally substituted with 1 to 3 groups of R a; R a is selected from the group consisting of halogen, hydroxyl, -C 1-6 alkyl, which is unsubstituted or substituted with one or more halogen, -C 3-6 cycloalkyl, -NR 2 C R 2, -C N(R 2) 2, -C(R 2) 2 OR 2, -C R 2, -NO 2, -CN, -N(R 2) 2, -C OR 2, -OR 2, -(CH 2) n C 5-10 heterocyclyl, -(CH 2) n C 6-10 aryl, and -(CH 2) n C 5-10 heteroaryl, wherein said heterocyclyl, aryl, and heteroaryl are each optionally substituted with 1 to 3 groups of (a) halogen, (b) hydroxyl, (c) -C 1-6 alkyl, (d) -CN, (e) -(CH 2) n CF 3, or (f) -C 6-10 aryl; halogen, hydroxyl, -C 1-6 alkyl, which is unsubstituted or substituted with one or more halogen, -C 3-6 cycloalkyl, -NR 2 C R 2, -C N(R 2) 2, -C(R 2) 2 OR 2, -C R 2, -NO 2, -CN, -N(R 2) 2, -C OR 2, -OR 2, -(CH 2) n C 5-10 heterocyclyl, -(CH 2) n C 6-10 aryl, and -(CH 2) n C 5-10 heteroaryl, wherein said heterocyclyl, aryl, and heteroaryl are each optionally substituted with 1 to 3 groups of (a) halogen, (b) hydroxyl, (c) -C 1-6 alkyl, (d) -CN, (e) -(CH 2) n CF 3, or (f) -C 6-10 aryl; (a) halogen, (b) hydroxyl, (c) -C 1-6 alkyl, (d) -CN, (e) -(CH 2) n CF 3, or (f) -C 6-10 aryl; R 2 is selected from the group consisting of hydrogen, hydroxyl, -C 1-6 alkyl, which is unsubstituted or substituted with one or more halogen, -(CH 2)CF 3, -(CH 2) n F, -C 3 - 10 cycloalkyl, -C 6 - 10 aryl, -C 5-10 heteroaryl, and -C 5 - 10 heterocyclyl, wherein said cycloalkyl, aryl, heterocycyl, or heteroaryl is each optionally substituted with 1 to 3 groups of Ra and hydrogen, hydroxyl, -C 1-6 alkyl, which is unsubstituted or substituted with one or more halogen, -(CH 2)CF 3, -(CH 2) n F, -C 3 - 10 cycloalkyl, -C 6 - 10 aryl, -C 5-10 heteroaryl, and -C 5 - 10 heterocyclyl, wherein said cycloalkyl, aryl, heterocycyl, or heteroaryl is each optionally substituted with 1 to 3 groups of Ra and n is independently 0 to 4 or a pharmaceutically acceptable salt thereof.