Enthalpy of vaporization of a molecule can be calculated when a chemical reaction occurs. Here is an example:
(CH3-H) + (Cl-Cl) --> (CH3-Cl) + (H-Cl)
(H=104) + (H=58) ---> (H=83.5) + (H=103)
DeltaH = (104+58)-(83.5+103) => -24.5 KCal/mole
Knowing the molecular mass, you can now calculate the KiloCalories.
I hope this helps,
Eugene Dakin Ph.D., P.Chem.