A CRJ900 ACCIDENTALLY SWITCHED FREQ BECAUSE THE VHF FREQ SWITCH IS SIMILAR TO AND VERY NEAR THE XPONDER IDENT BUTTON.

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IT WAS A DARK; RAINY NIGHT WITH LOW CEILINGS. DUE TO A RAPIDLY APCHING VOID TIME; TOWER CLRED US FOR AN IMMEDIATE TKOF ON RWY 12 AND ASSIGNED US TO FLY RWY HEADING AND CONTACT DEP. UPON CONTACTING DEP WE WERE TOLD TO IDENT. WHEN I REACHED DOWN TO PUSH THE IDENT BUTTON ON THE RADIO TUNING UNIT (RTU) I ACCIDENTALLY PUSHED THE FREQUENCY FLIP BUTTON AND UNKNOWINGLY SWITCHED BACK TO THE TWR FREQ. I THEN NOTICED AN ACFT ON TCAS THAT APPEARED TO BE ON THE ILS APCH TO RWY 30 AND DSNDING IN THE OPPOSITE DIRECTION OF OUR DEP TRACK. HEARING NOTHING FROM WHAT I THOUGHT WAS DEP I MADE A QUERY ABOUT THE HEAD-ON TFC. I THEN SUDDENLY REALIZED WHAT HAPPENED AND SWITCHED BACK TO DEP. WE WERE IMMEDIATELY ISSUED INSTRUCTIONS TO TURN 100 DEGS TO THE R. I NOTICED THAT THE ACFT ON THE APCH WAS FIVE MI AND AT THE SAME ALT AND CLOSING RAPIDLY. AS WE CLRED THE AREA TO THE W; APCH ISSUED INSTRUCTIONS TO THE OTHER ACFT TO REJOIN THE LOC AND RE-CLRED HIM FOR THE APCH. I OBVIOUSLY MISSED A VECTOR WHILE I WAS OFF FREQ AND CAUSED THE OTHER ACFT TO ALMOST GAR. I BELIEVE THAT ATC FELT PRESSURE TO RELEASE US DUE TO THE VOID TIME AND WAS TRYING TO GET US AIRBORNE EVEN THOUGH THERE WAS AN ACFT BEGINNING THE APCH IN THE OPPOSITE DIRECTION. THE TIMING OF ACCIDENTALLY SWITCHING OFF FREQUENCY COULDN'T HAVE BEEN AT A WORSE TIME. IF I DIDN'T NOTICE THE OTHER ACFT ON TCAS I PROBABLY WOULD NOT HAVE DISCOVERED MY MISTAKE RIGHT AWAY. THIS SCENARIO COULD HAVE BEEN AVOIDED IF WE HAD TAKEN OFF ON ANOTHER RWY OR BEEN MADE TO WAIT FOR THE OTHER ACFT TO LAND REGARDLESS OF THE VOID TIME. AT 1500 FT AGL IN IMC AT NIGHT IS A BUSY TIME WITH CALLOUTS; FREQ CHANGES; ETC AND AN EASY TIME TO GET DISTRACTED. THE IDENT BUTTON ON THIS ACFT IS LOCATED RIGHT NEXT TO AND IS ALMOST THE SAME SHAPE AS THE FREQ FLIP BUTTON. IN THE PAST I HAVE WITNESSED OTHER PLTS MAKE THE SAME MISTAKE I MADE BUT NEVER AT SUCH A CRITICAL TIME. RELOCATING BUTTONS AWAY FROM THE FREQ FLIP BUTTON WOULD HELP AVOID THIS HAPPENING AGAIN. I'LL BE MORE AWARE OF WHICH BUTTON IS BEING PUSHED IN THE FUTURE.