ACR BE99 DEPARTING OTH ANNOUNCES TKOF ON WRONG CTAF FREQ AND ENCOUNTERS A C414 COMMUTER DEPARTING OPPOSITE DIRECTION.

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AFTER LNDG I HAD TO CALL ZSE TO CANCEL IFR ON 121.4. CTAF IS 122.7. UPON NEXT LEG TO BROOKINGS; I SELF-ANNOUNCED ON ZSE FREQ INADVERTENTLY. I DISCOVERED THE WRONG FREQ DURING OUR SOP XCHK ON TKOF AND QUICKLY CHANGED AND ANNOUNCED TO ON CTAF. ZSE FREQ AT OTH IS A REPEATER STATION THAT DOES NOT ALWAYS RECEIVE/XMIT. I LOOKED ALL THE WAY ONTO THE OTHER SIDE OF THE RWY; AND SAW THAT IT WAS CLR BEFORE WE INITIATED TKOF. TAXI TO RWY 4; ANNOUNCED TAXI AND DEP INSTRUCTIONS INADVERTENTLY ON WRONG FREQ (121.4 ZSE) NOT CTAF 122.7. I TAXIED ONTO RWY 4 AND DIDN'T SEE ANY OTHER TFC ON RWY. ON THE TKOF ROLL; I DISCOVERED THE FREQ ERROR AND QUICKLY ANNOUNCED ON CTAF AT 90 KIAS WE SAW ACFT Y ON RWY 22. WE ROTATED AND MADE R TURN AWAY FROM RWY ENVIRONMENT. SEPARATION APPROX 3000 FT BTWN BOTH ACFT AT TIME OF ROTATION. SUPPLEMENTAL INFO FROM ACN 594211: AFTER LNDG AT OTH (NORTH BEND) WE HAD TO SWAP FREQ FROM CTAF (122.4) TO ZSE (121.4) TO CANCEL IFR FLT PLAN. UPON START OF NEXT LEG; WE TAXIED ANNOUNCING INTENTIONS INADVERTENTLY ON 121.4 (CTR). THE CAPT WAS TAXIING AND COMMUNICATING ON THE RADIO. HE DID NOT NOTICE THE WRONG FREQ WAS SELECTED UNTIL WE STARTED TKOF ROLL. ONCE HE REALIZED THE MISTAKE; HE IMMEDIATELY CHANGED FREQ TO CTAF. I DID NOT DOUBLECHK THE FREQ HE WAS TALKING ON. I DID NOT SEE THE OTHER ACFT AT THE OTHER END OF THE RWY BEFORE TAKING THE RWY. I ONLY NOTICED THE OTHER AIRPLANE AFTER STARTING TKOF ROLL. BEFORE TKOF: TAXIED TO RWY 4 FROM FBO. ANNOUNCED TAXI/DEP INTENTIONS INADVERTENTLY ON WRONG FREQ (121.4 ZSE); NOT CTAF 122.7. TAXIED ONTO RWY 4; DID NOT SEE ANY OTHER TFC ON RWY AT THIS TIME. ON TKOF ROLL; WE DISCOVERED FREQ ERROR AND QUICKLY ANNOUNCED DEP ON CTAF; THEN ACCELERATED TO 90 KIAS AND SAW OTHER AIRPLANE OPPOSITE DIRECTION ON RWY 22. WE ROTATED AND MADE R TURN; AWAY FROM RWY ENVIRONMENT. THIS WAS SAFEST COURSE OF ACTION! SEPARATION APPROX 3000 FT BTWN BOTH ACFT AT TIME OF INCIDENT.