Medium Large Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 16
Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict
Air Traffic Control Provided Assistance; Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action
Aircraft; Human Factors; Staffing
Aircraft X was doing VFR air work in the vicinity of MOL VOR at FL145.  He was maneuvering in the area of Aircraft Y who was climbing out of FL140 to FL190 direct LDN.  I attempted to reach Aircraft X a few minutes prior to the event occurring with no response.When it became apparent that the traffic was [now] head on; I gave a 30L turn to Aircraft Y.  After giving that turn Aircraft X reported he was responding to an RA and I saw him descend.  Standard separation was quickly attained by the actions of Aircraft X.Workload and frequency congestion was a factor at the time.  I was working 6 sectors.  The sectors should have been split or a D-side would have been helpful.
ZDC Center Controller reported a loss of separation between two aircraft while working 6 sectors.