LESS THAN STANDARD SEPARATION BETWEEN CORP ACFT AND ACR. OPERATIONAL ERROR.

Narrative

CORP X WAS HDG APPROX N AT 13000' AND ACR Y NNEBOUND AT 11000'. WHEN THE 2 ACFT WERE 5 NM LAT SEPARATED I INSTRUCTED CORP X TO CROSS THE SHELBYVILLE VOR DME AT AND MAINTAIN 6000'. CORP X LEFT 13000 FOR 6000'; STILL DIVERGING AWAY FROM ACR Y AT 11000'. AT THIS TIME I THOUGHT I HAD ASSIGNED A HDG OF 050 DEGS TO ACR Y; WHICH WOULD HAVE PROVIDED LAT SEP. THE 050 DEG HDG WAS GIVEN TO ANOTHER ACR Y COMPANY JET ABOUT 4 MINS BEFORE. ACR Y REACHING OKIES INTXN ON THE SHELBYVILLE 1 ARR ROCKET TRANSITION TURNED TO THE NNW. WHEN I REALIZED ACR Y TURNED I ISSUED INSTRUCTIONS FOR ACR Y TO TURN RIGHT TO A 090 DEG HDG; AND CORP X TO TURN LEFT TO A HDG OF 280 DEGS. C/A DID NOT ACTIVATE UNTIL AFTER THE TURNS WERE ISSUED TO EACH ACFT. THIS INCIDENT OCCURRED IN THE SHELBYVILLE LOW SECTOR AT THE MEMPHIS ARTCC CENTER. I HAD APPROX 16 ACFT ON FREQ. THE 2 ACFT CAME WITHIN 4.8 NM OF EACH OTHER WITH APPROX 500' VERT SEP AND INCREASING (MINOR ERROR). I MADE A MISTAKE WHEN I THOUGHT I HAD ACR Y ASSIGNED 050 DEG HDG.