Measuring the Heat Capacity of Large Isolated Molecules via Gas-Phase Collisions: C60

We present a novel method to measure the heat capacity of isolated molecules or clusters. Neutral molecules emerge from an effusive source at known temperature. They are heated in a single sticking collision with an atomic ion of known kinetic energy; the breakdown curve of the adduct ion is measured as a function of collision energy. The procedure is repeated for different source temperatures. The heat capacity of the neutral molecule equals the change in the breakdown energy divided by the change in source temperature. The method avoids potential sources of systematic errors inherent to other approaches that involve multiple collisions. The accuracy of the method is demonstrated by colliding an effusive beam of C60 with a monoenergetic beam of Na+ which produces endohedral Na@C60+. The value obtained for the heat capacity of C60 at 535 ± 35 °C agrees with theoretical ones within the experimental uncertainty of 11%.