The Methylene and Dichloromethylene Singlet-Triplet Energy Gap

Following directly on the success of determining the bond angle in triplet methylene, computational chemists turned to the size of the methylene singlet-triplet energy gap. Methylene is a ground-state triplet X Bx) and its first excited state is the d state. Prior to 1972, experiments placed the value of their energy 

This calm was shattered by the first direct measurement of the singlet-triplet gap of methylene by Lineberger in 1976. Laser photoelectron spectrometiy of CH2 provided a spectrum interpreted as transitions to singlet and triplet CH2. Despite some difficulty in arriving at the 0-0 transition, Lineberger concluded that was equal to 19.6 kcal mol , far above the theoretical (or previous experimental) predictions. However, since this was the first direct observation of the gap, it was considered to be the most accurate determination yet. 

TABLE 5.2 Singlet-Triplet Energy Gap (kcal mol ) of Methylene 

65 kcal mor ) or from the quartet state of its isomeric species ( C=Cl-Cr C=Cl-Cr 59 kcal mor ). 

---