Appearance Colorless gas | ||
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Triplet oxygen, systematically but less commonly, 1,2-dioxidanediyl, is normal, gaseous oxygen (O2, dioxygen) in its ground state. It is therefore classified as an inorganic chemical, and more specifically as a particular electronic state of one allotrope of the inorganic chemical element, oxygen. In this particular state, according to one type of modern bonding theory, the electron configuration of the oxygen molecule has two electrons occupying two molecular orbitals (MOs) of equal energy (that is, degenerate MOs), therefore remaining unpaired. These orbitals are classified as antibonding and are of higher energy, so the resulting bonding structure between the oxygen atoms is weakened (i.e., is higher in energy)—for instance, it is higher in energy than the bonding in dinitrogen, where the corresponding antibonding orbitals are empty. The spectroscopic molecular term symbol for triplet (ground state) oxygen is 3Σ−
g.
Contents
Spin
The s = 1⁄2 spins of the two electrons in degenerate orbitals gives rise to 2 × 2 = 4 independent spin states in total. Exchange interaction splits these into a singlet state (total spin S = 0) and a set of 3 degenerate triplet states (S = 1). In agreement with Hund's rules, the triplet states are energetically more favorable, and correspond to the ground state of the molecule with a total electron spin of S = 1. Excitation to the S=0 state results in much more reactive, metastable singlet oxygen.
Because the molecule in its ground state has a non-zero spin magnetic moment, oxygen is paramagnetic; i.e., it can be attracted to the poles of a magnet. The Lewis structure O=O does not accurately represent the diradical nature of molecular oxygen; molecular orbital theory must be used to adequately account for the unpaired electrons. Triplet oxygen is better described by a bond order of one and two halves instead of two to better reflect its unpaired bonding electrons. This allows for easier reasoning of the bond length.
Reaction
The unusual electron configuration prevents molecular oxygen from reacting directly with many other molecules, which are often in the singlet state. Triplet oxygen will, however, readily react with molecules in a doublet state, such as radicals, to form a new radical. Conservation of spin quantum number would require a triplet transition state in a reaction of triplet oxygen with a closed shell (a molecule in a singlet state). The extra energy required is sufficient to prevent direct reaction at ambient temperatures with all but the most reactive substrates, e.g. white phosphorus. At higher temperatures or in the presence of suitable catalysts the reaction proceeds more readily. For instance, most flammable substances are characterised by an autoignition temperature at which they will undergo combustion in air without an external flame or spark.