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Comparatively Speaking: Singlet vs. Triplet State of UV Filters
By: Anthony J. O'Lenick Jr., Siltech LLC, and Craig Bonda, the HallStar Company
Posted: October 20, 2010
In organic photochemistry, the terms singlet and triplet refer to the configuration of two coupled electrons in the outer or valence orbital of a molecule and, by extension, to the two unpaired electrons in a radical pair. In the singlet state, shown in Figure 1, the two electrons are in a “spin paired” or “anti-parallel” configuration, meaning that both are spinning about opposite vectors.
A vector is a geometric object in the shape of a line with both a magnitude (its length) and a direction. Think of each electron as a ball spinning in a clockwise (when viewed from below) direction about a vector, with the direction of the vector indicated by the arrow at its tip. In addition to spinning about a vector, the electrons “precess” around an axis, labeled “z” in Figure 1. Precession is analogous to the motion of a gyroscope or, if you will, a wobbly top.
While precessing, the vector arrow traces out a circular path about the z axis. The spinning, precessing electrons exert a force called angular momentum along the z axis, either up or down depending on the orientation of the arrow. In the opposite direction, they exert a magnetic force called a magnetic moment.
In the singlet state, the two electrons not only spin about opposite vectors, they also precess “out of phase,” meaning their vector arrows are always opposed. As a result, the angular momenta and magnetic moments exerted by the two electrons cancel each other out. Thus, there is no net spin, no net angular momentum, and no net magnetic moment. This is the single (one and only) configuration in which two electrons can occupy the same atomic orbital, and is known as the singlet state.
In the triplet state, shown in Figure 2, the two electrons are always “orbitally unpaired.” There are three possible configurations of the triplet state. In one, found to the left of Figure 2, both electrons are spinning about “up” vectors and precessing “in phase,” meaning their vector arrows are always pointed in the same direction. In the opposite case, to the right in Figure 2, both electrons are spinning about down vectors and again precessing “in phase.” The third triplet configuration, found in the center of Figure 2, has the two electrons spinning about opposite vectors but precessing “in phase.” Though there are subtle differences between them, all three produce a positive net spin and angular momentum. Most importantly, all three configurations are “spin unpaired” and look and behave as highly reactive radicals.