AN ACR FO (PNF) SET THE WRONG VOR FREQ FOR A DIRECT NAV CLRNC AND CAPT (PF) FOLLOWED WITHOUT VERIFYING. ARTCC INTERVENED.

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AFTER DEPARTING THE CHICAGO AREA AND SWITCHING TO THE ENRTE NAV FACILITY; I ACCIDENTALLY SET OXI (115.6) INSTEAD OF GIJ (115.4). AS I WAS MOVING THE AUDIO LEVER UP TO IDENT GIJ; I WAS STOPPED BY A CALL FROM ATC WHO WAS ASSIGNING A VECTOR; REQUESTING AN INCREASED RATE OF CLB; AND CHANGING THE CLRED ALT. THERE WERE SUBSEQUENTLY NUMEROUS VECTORS BTWN 040 DEGS AND 090 DEGS AND A HDOF TO CTR. BEING BUSY WITH COMS; I DID NOT REMEMBER TO GO BACK AND IDENT GIJ. WHEN WE WERE CLRED DIRECT TO GIJ; WE THOUGHT THE LONG VECTORS HAD TAKEN US SOMEWHAT N; BUT WERE STILL SURPRISED BY THE S HDG REQUIRED TO GO TO WHAT WE THOUGHT WAS GIJ. WHILE WE WERE TRYING TO SORT THIS OUT; ATC TOLD US TO TURN L TO 020 DEGS TO GO DIRECT GIJ. AT THAT TIME I CHKED THE RADIOS AND SAW THAT THE CAPT AND I BOTH HAD THE WRONG NAV FREQ SET. WE FOLLOWED THE VECTOR; TUNED AND IDENTED GIJ AND RESUMED NORMAL NAV ON J146. I THINK THE HIGH VOLUME OF COMS AND THE FREQUENT CHANGE IN VECTOR HDGS HAD A LOT TO DO WITH OUR FAILURE TO RECOGNIZE OUR ERROR EARLIER. IN OUR POST FLT DISCUSSION; THE CAPT AND I AGREED THAT IT WOULD HAVE BEEN BETTER FOR THE PF TO SET HIS RADIO TO ORD TO MEET THE ORD DEP RESTR; AND FOR THE PNF TO SET THE FIRST ENRTE NAV FIX PRIOR TO DEP WHEN THINGS ARE LESS HECTIC AND THERE IS TIME TO VERIFY THE FREQ. TO OUR KNOWLEDGE; THERE WAS NO TFC CONFLICT AS A RESULT OF THIS NAV ERROR. SUPPLEMENTAL INFO FROM ACN 314582: SHORTLY AFTER TKOF; I LOOKED AT THE FO'S #2 NAV AND SAW THE FREQ 115.6 SET ON HIS RADIO. I WRONGLY ASSUMED THAT WAS THE CORRECT FREQ FOR GIJ; 115.4; AND I SET 115.6 ON MY #1 NAV RADIO. AFTER NUMEROUS VECTORS; ZAU GAVE US A 180 DEG HDG; DIRECT GIJ; ON COURSE. SW OF GIJ; ZAU PICKED US UP AND ASSIGNED A VECTOR BACK TO GIJ. PREVENTIVE ACTION: BACK TO BASICS; TUNE AND IDENT; SET THE PNF'S NAV TO THE FIRST ENRTE FIX FREQ ON THE GND BEFORE ENTERING THE BUSY DEP ENVIRONMENT.