On January 15, 1994, about 2220 Atlantic standard time, a Robinson R22 Beta, N193HC, registered to Initiative Trade Corporation, was substantially damaged during an autorotative landing near Gurabo, Puerto Rico, while on a 14 CFR Part 91 aerial advertising flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The commercial-rated pilot, the sole occupant, was not injured. The flight originated about 2115, from the Fernando Luis Ribas Dominicci Airport, San Juan, Puerto Rico. The pilot stated that during cruise flight during the dark night at 1,800 feet mean sea level, he smelled something burning and the clutch warning light flickered momentarily, then remained illuminated. He waited for 5 seconds then pulled the clutch circuit breaker and initiated an emergency descent for an autorotative landing. When the flight was about 50 feet above ground level he initiated a cyclic flare and about 10 feet above ground level, pulled collective pitch control for landing. The helicopter landed hard on unsuitable terrain. Examination of the helicopter revealed that one of the two vee-belts completely separated and 1/2 of the remaining belt had completely separated. Only 1/2 of one belt remained. According to the maintenance records, the vee-belt tension was checked about 42 hours earlier. The vee-belts had accumulated about 442 hours since new.