The heats of formation of tert-butyl isocyanide and other alkyl isocyanides by photoelectron photoion coincidence spectroscopy

Threshold photoelectron photoion coincidence (TPEPICO) spectroscopy has been used to investigate the dissociation energy of tert-butyl isocyanide (t-C4H9NC+) ion. The 0 K dissociation limit of CN. + C4H9+ (10.943 +/- 0.010 eV) is determined from the breakdown diagram. The ionization energy of tert-butyl isocyanide is measured by threshold photoelectron spectra (TPES) and is 10.757 +/- 0.005 eV. These values are used to obtain the 298 K heat of formation of tert-butyl isocyanide molecule (88.1 +/- 3.7 kJ/mol). The 298 K heats of formation of CH3CH2NC and (CH3)(2)CHNC are 144.8 +/- 8 kJ/mol and 116.5 +/- 8 kJ/mol, respectively, determined by a linear interpolation between the heats of formation of CH3NC and t-C4H9NC. These results are in excellent agreement with density functional and ab initio theoretical values obtained from isomerization energies between the alkyl cyanides and the isocyanide heats of formation. Aside from CH3NC, these are the first experimental heats of formation for the alkyl isocyanide molecules.