The usual rules for constructing Lewis structures must be modified to accommodate molecules like triplet dioxygen or nitric oxide that contain 2c-3e bonds. In the case of triplet dioxygen, each 2c-3e bond consists of two electrons in a π u bonding orbital and one electron in a π g antibonding orbital to give a net bond order contribution of 1 / 2. Under a molecular orbital theory framework, the oxygen-oxygen bond in triplet dioxygen is better described as one full σ bond plus two π half-bonds, each half-bond accounted for by two-center three-electron (2c-3e) bonding, to give a net bond order of two (1+2× 1 / 2), while also accounting for the spin state ( S = 1). Molecular orbital theory must be used to correctly account for the observed paramagnetism and short bond length simultaneously. This indicates that triplet oxygen has a higher bond order. is also inadequate, since it implies single bond character, while the experimentally determined bond length of 121 pm is much shorter than the single bond in hydrogen peroxide (HO–OH) which has a length of 147.5 pm.Thus, the Lewis structure O=O with all electrons in pairs does not accurately represent the nature of the bonding in molecular oxygen. Lewis structure Pauling's Lewis structure for triplet dioxygen.īecause 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. Excitation to the S = 0 state results in much more reactive, metastable singlet oxygen. 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. Exchange interaction splits these into a singlet state (total spin S = 0) and a set of 3 degenerate triplet states ( S = 1). The s = 1⁄ 2 spins of the two electrons in degenerate orbitals gives rise to 2 × 2 = 4 independent spin states in total. Spin The valence orbitals of molecular oxygen (middle) in the ground state, the electrons in the π* orbitals have their spins parallel. The molecular term symbol for triplet oxygen is 3Σ − These half-filled orbitals are antibonding in character, reducing the overall bond order of the molecule to 2 from a maximum value of 3 (e.g., dinitrogen), which occurs when these antibonding orbitals remain fully unoccupied. In accordance with Hund's rules, they remain unpaired and spin-parallel and account for the paramagnetism of molecular oxygen. According to molecular orbital theory, the electron configuration of triplet oxygen has two electrons occupying two π molecular orbitals (MOs) of equal energy (that is, degenerate MOs). Molecules of triplet oxygen contain two unpaired electrons, making triplet oxygen an unusual example of a stable and commonly encountered diradical: it is more stable as a triplet than a singlet. Triplet oxygen, 3O 2, refers to the S = 1 electronic ground state of molecular oxygen (dioxygen).
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