The pilot reported that prior to the flight the fuel tanks were "topped off with fuel" and after being distracted, a second walk around the airplane was not properly conducted. The pilot further reported that after about an hour of flight the engine lost power and he was forced to maneuver for an emergency landing. During the emergency landing, the pilot reported that his initial approach was to an interstate, but due to thick traffic he made a turn towards a landfill and impacted terrain. The airplane sustained substantial damage to the fuselage. The pilot reported that after the impact the passenger received a telephone call from the departure airport fuel service attendant stating that the airplane's left fuel cap was on the airport ramp. During a postaccident examination by the pilot, it was revealed that the left fuel cap was missing. The pilot reported that there were no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.