Singlet state is attributed to the appearance of a single spectral line. This occurs because all of the electrons are paired. Originally, the singlet state was referred to as a set of particles with a net angular momentum of zero i.e. spin quantum number (s) = 0.
Triplet state, on the other hand, indicates the triple splitting of spectral lines. It happens due to the presence of two unpaired electrons. The spin quantum number (s) = 1, and the allowed values for the spin component are -1, 0, and +1.
By absorbing radiation, an electron in a molecule (singlet ground state) is excited to a higher energy level (excited state). This new excited state can be singlet or triplet.
In a singlet electronic state, all electron spins are paired. This means that the spin of the excited electron is still paired with its ground state counterpart; the two still have opposite spins as per the Pauli exclusion principle. Conversely, in a triplet electronic state, the spin of the excited electron is paired no more with that of the ground state electron; the two electrons now have the same spin instead of opposite and are parallel (in direction and the magnetic field created).
The singlet (ground state) to triplet (excited state) electron transition is a forbidden one and has a lesser chance of happening if a molecule absorbs radiation.
An example is that of carbenes. Carbenes can be singlet or triplet, depending on the electronic spins they possess. Triplet carbenes are paramagnetic with a total spin of 1 while the total spin of singlet carbenes is zero. Triplet carbenes are generally stable in the gaseous state while singlet carbenes occur more frequently in aqueous media.
Additionally, the singlet state of carbenes is more carbocation-like in nature with a trigonal planar geometry and electrophilic character. Comparatively, the triplet state of carbenes is diradical-like in nature with linear geometry.
Difference between singlet and triplet states
| Singlet state | Triplet state |
| A single spectral line occurs | Spectral lines are split into three |
| All of the electrons are paired | Two unpaired electrons are present |
| Spin quantum number (s) = 0 | Spin quantum number (s) = 1 |
| Spin component value, m(s) = 0 | Spin component value(s), m(s) = -1, 0, +1 |
| Most molecules are present in singlet state | Molecular oxygen occurs in triplet state |
Singlet state
Singlet state refers to the appearance of a single spectral line. It is because all of the electrons are paired. Basically, the singlet state refers to the state of a set of particles that are linked and have a net angular momentum of zero, as their total spin quantum number (s) = 0.
In actuality, the only observed fundamental particle with zero spin is the “Higgs boson”. The other singlets are composed of particles with non-zero individual spins (½ or 1).
Singlet refers to those bound systems with zero overall angular momentum that emit photons in a single spectral line. The number of spectral lines (n) is related to the spin quantum number (s) by the formula,
n = 2s + 1
The particles in a singlet state are not required to be bound to each other. The electrons can remain in a shared singlet state even if their separation increases over time but only as long as the angular momentum states remain undisturbed.
The simplest example of a singlet is a set of two ½ spin particles, oriented in opposite directions (anti-parallel).
Triplet state
Triplet state indicates the threefold splitting of spectral lines. It is because of the presence of two unpaired electrons. The spin quantum number (s) equals 1, and the allowed values for the spin component (ms) are -1, 0, and +1.
Most molecules exist in a singlet state but an exception is molecular oxygen (O2), which exists in the triplet state. Although being one of the strongest oxidants thermodynamically, O2 is kinetically unreactive unless it undergoes a forbidden transition into the singlet state, which is done by photochemical or thermal activation.
Related Resources
Singlet and triplet state in fluorescence and phosphorescence
Fluorescence/phosphorescence is the emission of electromagnetic radiation, caused by the excitation of atoms from incident radiation at first. The emission in fluorescence, unlike phosphorescence, is almost instantaneous after the excitation, and so stops as soon as the radiation source is removed.
Fluorescence occurs when an excited atom or molecule relaxes to a lower energy state through the emission of a photon without a change in the electron spin.
Excited singlet state → Ground singlet state
OR
Excited triplet state → Ground triplet state
In phosphorescence, the relaxation occurs along with a change in the electron spin.
Excited triplet state → Ground Singlet state
Concepts Berg
Which state is lower in energy, singlet or triplet?
In terms of stability, the triplet state is lower in energy and more stable than the singlet state. This is so, although most of the molecules normally exist in the singlet state.
Why does the singlet state have more energy than the triplet state?
In the singlet state, as the electrons are paired up, there is more electron repulsion energy acting as a destabilizing factor, as per Hund’s rule. In addition, there is lesser exchange energy which further destabilizes the singlet state, as compared to the triplet state.
What is the difference between triplet oxygen and singlet oxygen?
Oxygen exists in several allotropic forms. One of them is singlet oxygen that has all its electrons spin-paired, so the total spin quantum number is zero. In the triplet oxygen, two electrons are present unpaired and the spin quantum number is 1.
Which is more stable, triplet or singlet carbene?
In general, triplet states are more stable than singlet states as the electrons remain unpaired in degenerate orbitals, and so is the case with carbenes as well. When compared, triplet carbenes are more stable in the gaseous state while singlet carbenes occur more frequently in aqueous media.
Why is oxygen a triplet?
Oxygen exists in the triplet state because the triplet state is lower in energy and more stable than the singlet state. Moreover, oxygen has degenerate orbitals available to accommodate two electrons with the same spin.
What is triplet state absorption?
When an electron absorbs radiation in the singlet ground state, it is excited to a higher energy level, and an excited singlet or triplet state forms. In the case of the triplet state, it is called triplet state absorption where the electron is excited as well as its spin reversed.
Why is intersystem crossing forbidden?
As energy is required to reverse the spin of the electron and it is not always feasible for the intersystem crossing. However, it is favorable when the vibrational energy levels of the excited states overlap.
Excited singlet state vs triplet state.
All of the electron spins are paired in the singlet state. This means that the spin of the excited electron is still paired with the ground-state electron; the two electrons still have opposite spins as per the Pauli exclusion principle. Alternatively, in a triplet electronic state, the spin of the excited electron is no longer paired with that of the ground state electron; the two electrons now have the same spin instead of the opposite.
Singlet and triplet states in fluorescence.
Fluorescence is a type of emission spectroscopy where the excited electron returns to the ground state without a change in its spin. The transition that usually takes place is from the singlet excited state to the singlet ground state.
Why are multiplet states different from singlet states?
In the singlet states, all of the electrons are paired up, unlike multiplet ones. This affects various chemical and physical properties of molecules.
Why are singlet to triplet transitions forbidden?
Singlet to triplet transitions are forbidden because these transitions are a type of intersystem crossing which are less probable because, in addition to exciting the electron from the ground state, its spin also needs to be altered (reversed).
Why is the ground state of an atom always a singlet?
It is true that most of the time the ground state is singlet as the ground state electrons are usually paired. But it is not always the case.
What is the difference between fluorescence and phosphorescence?
In fluorescence, the excited electron returns to the ground state without a change in its spin i.e. excited singlet state → ground singlet state transition normally takes place. Whereas, in phosphorescence, the electron’s spin is reversed in going from the initial excited state to the final ground state i.e. excited triplet state → ground singlet state transition takes place.
Why do electrons having opposite spins attract?
Because the electrons spin in opposite directions, the magnetic field these rotating electric charges create is in the opposite direction, and this causes the force of attraction between them.
References
- Modern Quantum Chemistry: Introduction to Advanced Electronic Structure Theory by Attila Szabo
