The Heat of Dissociation of Non-Polar Molecules

Heat of dissociation from band spectra data.—The limiting amount of vibrational energy which a diatomic molecule can possess is given by $E_n=h\int o^{n_o}{\omega }^n\mathrm{dn},$ where ${\omega }^n$ is the frequency of vibration, as a function of the vibrational quantum number $n$, and $n_o$ is the value of $n$ for ${\omega }^n=0\mathrm{}$. For non-polar molecules $n_o$ is finite, for polar molecules it is infinite. The ${\omega }^n:n$ curve for the normal state of each of the molecules here discussed is strictly linear, over the known range, and its linear extrapolation to ${\omega }^n=0\mathrm{}$, for essentially non-polar molecules, seems to give the true value of $E_n$ to within half a volt. Since increasing the vibrational energy until the two nuclei dissociate, results in the formation of two normal atoms, $E_n=D$, the heat of dissociation. When a molecule dissociates, while excited by $E_e$ units of electronic energy, the products of dissociation seem in some cases to include at least one excited atom. The total energy required for the dissociation thus exceeds the true heat of dissociation by the amount of the resulting atomic excitation. The ${\omega }^n:n$ curves, and the correlated sets of energy levels are shown for all known band systems of ${\mathrm{O}}_2$, ${\mathrm{O}}_2^{+}$, ${\mathrm{N}}_2$, ${\mathrm{N}}_2^{+}$, CO, C${\mathrm{O}}^{+}$, and NO.