Oxygen: A Mysterious Amalgam of Exceptional Inter-contradictory Features

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Author: Abdelkader Bouaziz

SALAM Research Laboratory – Polymer Science and Engineering

Sougueur Division – Organic Chemistry, Sougueur 14003 Ibn Khaldun University of Tiaret, Algeria

Since the emergence of polymer science and materials, oxygen is involved positively and/or negatively into the normal course of many processes, such as the radical polymerization, where due to the bi-radical nature of the ground state structure of oxygen (˙O-O˙), inhibition and retardation are imparted against both radical polymerization and curing reactions. It adds to that, the low reactivity types of generated reactive oxygen species (ROS), such as ROO˙. On the other contrary side, oxygen may be a source of activation or even initiation for some other radical polymerization reactions.

Reactive Oxygen Species (ROS) can react with different macromolecules in the body causing irreversible damage to DNA, proteins and lipids.

Oxygen is really considered at once a disease and a medication; it is implied in many areas of chemistry, biology, corrosion, food processing and others. Oxygen therapy is a growing branch in clinical medicine, where many diseases/activities are reduced or suppressed by just intensifying oxygen inside the body or in its surrounding. Anoxemia (oxygen deficiency) by its three common types symptomatology (Anoxic, stagnant, and anemic), where stagnant anoxemia is behind heart failure. ROS can react with many different macromolecules in the body causing irreversible damage to DNA, proteins and lipids. As a whole, the normal function of the organs depends on an appropriate supply of oxygen. This short communication briefly presents the different states of oxygen and ROS, as to exemplify their influences in polymer chemistry, health and biology of livings, and diseases and medication science.

Major ROS include superoxide anion radical (O2), hydrogen peroxide (H2O2), singlet oxygen (1O2), and hydroxyl radical (OH).

ROS are generated via chemical, biochemical, and physical routes, or by their combination.  Major ROS are: superoxide anion radical (O2), hydrogen peroxide (H2O2), singlet oxygen (1O2), and hydroxyl radical (OH). A deep and broad understanding of the different states of oxygen and their influences is necessary.

Excited electronic states (S*) are due to the promotion of an electron from the ground state (G) distribution, changing the electron density configuration to one of higher energy. The excited singlet state is generally short-lived (10-8-10-9 sec) with the valence electrons having opposite spins. An excited triplet state is longer-lived (milliseconds or more) with the two electrons having parallel spins.

Since the singlet and triplet states or molecules are of different multiplicity (i.e., spin quantum numbers S = 0 and 1, therefore 2S + 1 = 1 and 3respectively), the transitions between them are strictly forbidden. Oxygen exists in three distinct states, where the bi-radical triplet structure being the ground state, which is the one abundantly available in the breathing air. The other states are the excited ones.

The origin of these different states is in the concept of atomic orbitals and the Pauli’s exclusion principle in filling these orbitals with 8e of oxygen, as it is represented by the electronic configuration ls22s22p4. Since the 2p orbitals are only partially filled, there are three different ways to arrange the electrons in the 2px, 2py, and 2pz orbitals (see Figure-1).

Figure-1: Schematic representation of three different ways to arrange the electrons in the 2px, 2py, and 2pz orbitals.

ROS are generated via chemical, biochemical, and physical routes, or by their combination.  Major ROS are: superoxide anion radical (O2), hydrogen peroxide (H2O2), singlet oxygen (1O2), and hydroxyl radical (OH).

Hund’s rule states that the ground-state configuration corresponds to an orbital occupancy that gives the highest multiplicity, it rules out that either the second or the third configurations in the figure as the ground state, because each of these configurations has a multiplicity of 1 as compared to a multiplicity of 3 for the first configuration. The ground state of atomic oxygen, therefore, has two unpaired electrons, and is designated as the 3P (“triplet P”) state. Of the two remaining configurations, the second is lower in energy based on Hund’s second rule which states that, if the multiplicity is the same, the configuration with the highest total orbital angular momentum (L) will have the lower energy. Because the second configuration has the higher L value, it is lower in energy. This configuration is the first excited state of atomic oxygen, designated as the 1D (“singlet O”) state. The third configuration is the second excited state, designated 1S.

As a conclusion, matter is full of paradigms; it may associate, at the same time, opposite properties and behaviors. Mastering and understanding the relationships between structure and properties is the secret key behind the prediction of presumed behaviors.


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