SMT CHARTER DID NOT RESPOND NORMALLY ON ROTATION. TKOF ABORTED; RAN OFF RWY.

Narrative

I ARRIVED AT IML TO WAIT FOR THE REMAINING PAX. I PREFLTED THE ACFT AND FOUND THE ACFT AIRWORTHY. THE FINAL PAX ARRIVED AT XA55. I IMMEDIATELY CALLED COLUMBUS; NE; CLRNC VIA PHONE TO GET MY IFR CLRNC. I WAS GIVEN A 1/2 HR WINDOW FOR DEP. ALL 4 PAX WERE LOADED AND I WAS READY FOR ENG START. I HAD TROUBLE STARTING THE R ENG ON THE SMT. I HAD BOTH ENGS RUNNING AND WARMING UP. I WAITED BEFORE STARTING THE BEFORE TKOF CHKLIST ONLY TO ALLOW THE R ENG TO WARM UP MORE. AT THIS POINT I TAXIED ONTO RWY 13 AND DID THE BEFORE TKOF PROCS WHICH INCLUDED A RUNUP OF THE ENGS -- EVERYTHING WAS NORMAL. I STARTED THE TKOF ROLL; AT 100 KTS I BEGAN TO ROTATE. THE REACTION WAS A SLIGHT INCREASE IN NOSE ATTITUDE FOLLOWED BY AN ABRUPT DECREASE IN ATTITUDE. I BELIEVE THE NOSEWHEEL NEVER DID LEAVE THE GND. I ABORTED THE TKOF SENSING THAT THIS REACTION WASN'T NORMAL. HOWEVER; DUE TO THE SLICK CONDITIONS I WASN'T ABLE TO STOP ON THE REMAINING RWY. THE ACFT EXITED THE RWY AT THE END AND CAME TO A REST WITH THE NOSE GEAR COLLAPSED APPROX 200 FT BEYOND THE END OF THE RWY. THE NOSEWHEEL COLLAPSED DUE TO HITTING AN EARTH MOUND IN THE FIELD BEYOND THE END OF THE RWY. ALL 4 PAX WERE OK AND WE EXITED THE ACFT THROUGH THE MAIN ENTRANCE WITHOUT A PROB. DUE TO THE NATURE OF THE DAMAGE IT IS VIEWED AS NOT SUBSTANTIAL; THEREFORE; THIS HAS BEEN CLASSIFIED AS AN INCIDENT.