Molecules and ions possess magnetic properties determined by the behavior of their electrons. The nitrosyl cation (\(\text{NO}^+\)) is a common, stable species in chemistry, but its magnetic behavior is not immediately obvious. Determining if this ion is attracted to or repelled by a magnetic field requires a deep look into how its electrons are arranged within the molecular framework. This investigation relies on fundamental principles of quantum chemistry.
Defining Paramagnetism and Diamagnetism
Magnetic behavior is fundamentally linked to the spin of electrons and how they occupy specific orbitals. Paramagnetism occurs when a substance contains one or more unpaired electrons. These unpaired spins generate a net magnetic moment, causing the material to be weakly attracted to an external magnetic field.
Diamagnetism, conversely, is the property exhibited by substances where all electrons are completely paired. Since every electron spin is canceled out by its partner, the molecule has no permanent magnetic moment. When placed in an external magnetic field, diamagnetic substances are slightly repelled. The presence or absence of a single unpaired electron dictates a substance’s magnetic classification.
The Role of Molecular Orbital Theory
Understanding the magnetic properties of diatomic species like \(\text{NO}^+\) requires Molecular Orbital Theory (MOT). MOT describes how atomic orbitals combine to form new molecular orbitals extending across the entire molecule. This framework allows scientists to accurately predict the distribution of electrons in complex systems.
When two atoms bond, their atomic orbitals merge to form lower-energy bonding orbitals and higher-energy antibonding orbitals. Electrons preferentially fill the bonding orbitals first, stabilizing the molecule. The distribution of electrons is governed by quantum rules, including the Aufbau principle and Hund’s rule.
The Aufbau principle dictates that electrons fill the lowest available energy levels first. Hund’s rule states that electrons occupy degenerate orbitals singly before pairing occurs. These rules determine the final electron configuration and, significantly, whether any electrons remain unpaired.
The sequence of filling these molecular orbitals is specific for diatomic molecules, following a pattern of sigma (\(\sigma\)) and pi (\(\pi\)) orbitals of increasing energy. By distributing the total number of valence electrons according to these energy levels, one can identify the highest occupied molecular orbital (HOMO). The occupancy of the HOMO ultimately determines the magnetic character of the substance.
Analyzing the Nitrosyl Cation (\(\text{NO}^+\))
To analyze the nitrosyl cation (\(\text{NO}^+\)), the first step is to determine the total count of valence electrons. Nitrogen contributes five valence electrons and Oxygen contributes six, totaling eleven for neutral \(\text{NO}\). Since \(\text{NO}^+\) carries a positive charge, one electron is removed, resulting in a total of ten valence electrons.
These ten valence electrons are distributed into the molecular orbital diagram following the established energy sequence. The electrons fill the lowest available orbitals, pairing up completely. Crucially, the resulting electronic configuration shows no singly occupied orbitals.
This complete pairing of all ten valence electrons is the definitive feature of the \(\text{NO}^+\) ion. The absence of unpaired electrons means that the spin magnetic moments are entirely canceled out. Therefore, the nitrosyl cation exhibits a diamagnetic property and would be weakly repelled when exposed to an external magnetic field.
Context: Comparing \(\text{NO}^+\) to the \(\text{NO}\) Molecule
The magnetic classification of \(\text{NO}^+\) as diamagnetic highlights the profound influence of a single electron on molecular properties. This is best illustrated by comparing the ion to its neutral parent, the nitric oxide (\(\text{NO}\)) molecule. \(\text{NO}\) has one additional electron, totaling eleven valence electrons.
When this eleventh electron is placed into the molecular orbital diagram, it occupies a \(\pi\) antibonding orbital. Because this electron is the sole occupant, it remains unpaired, creating a net magnetic moment. This single unpaired electron causes the \(\text{NO}\) molecule to be paramagnetic, meaning it is attracted to a magnetic field.
The contrasting magnetic behavior between the ten-electron \(\text{NO}^+\) ion and the eleven-electron \(\text{NO}\) molecule demonstrates the power of the quantum model. The loss of just one electron transforms the substance from a paramagnetic radical to a stable, diamagnetic species. This comparison underscores how sensitive a molecule’s physical properties are to its exact electron count.