# Rigid rotor

The rigid rotor is a mechanical model that is used to explain rotating systems. An arbitrary rigid rotor is a 3-dimensional rigid object, such as a top. To orient such an object in space three angles known as Euler angles are required. A special rigid rotor is the linear rotor which requires only two angles to describe its orientation. An example of a linear rotor is a diatomic molecule. More general molecules like water (asymmetric rotor), ammonia (symmetric rotor), or methane (spherical rotor) are 3-dimensional, see classification of molecules.

## Linear rotor

The linear rigid rotor model consists of two point masses located at fixed distances from their center of mass. The fixed distance between the two masses and the values of the masses are the only characteristics of the rigid model. However, for many actual diatomics this model is too restrictive since distances are usually not completely fixed. Corrections on the rigid model can be made to compensate for small variations in the distance. Even in such a case the rigid rotor model is a useful point of departure (zeroth-order model).

### Classical linear rigid rotor

The classical linear rotor consists of two point masses ${\displaystyle m_{1}}$ and ${\displaystyle m_{2}}$ (with reduced mass ${\displaystyle \mu ={\frac {m_{1}m_{2}}{m_{1}+m_{2}}}}$) each at a distance ${\displaystyle R}$. The rotor is rigid if ${\displaystyle R}$ is independent of time. The kinematics of a linear rigid rotor is usually described by means of spherical polar coordinates, which form a coordinate system of R3. In the physics convention the coordinates are the co-latitude (zenith) angle ${\displaystyle \theta \,}$, the longitudinal (azimuth) angle ${\displaystyle \varphi \,}$ and the distance ${\displaystyle R}$. The angles specify the orientation of the rotor in space. The kinetic energy ${\displaystyle T}$ of the linear rigid rotor is given by

${\displaystyle 2T=\mu R^{2}{\big [}{\dot {\theta }}^{2}+({\dot {\varphi }}\,\sin \theta )^{2}{\big ]}=\mu R^{2}{\big (}{\dot {\theta }}\;\;{\dot {\varphi }}{\Big )}{\begin{pmatrix}1&0\\0&\sin ^{2}\theta \\\end{pmatrix}}{\begin{pmatrix}{\dot {\theta }}\\{\dot {\varphi }}\end{pmatrix}}=\mu {\Big (}{\dot {\theta }}\;\;{\dot {\varphi }}{\Big )}{\begin{pmatrix}h_{\theta }^{2}&0\\0&h_{\varphi }^{2}\\\end{pmatrix}}{\begin{pmatrix}{\dot {\theta }}\\{\dot {\varphi }}\end{pmatrix}},}$

Scale factors are of importance for quantum mechanical applications since they enter the Laplacian expressed in curvilinear coordinates. In the case at hand (constant ${\displaystyle R}$)

${\displaystyle \nabla ^{2}={\frac {1}{h_{\theta }h_{\varphi }}}\left[{\frac {\partial }{\partial \theta }}{\frac {h_{\varphi }}{h_{\theta }}}{\frac {\partial }{\partial \theta }}+{\frac {\partial }{\partial \varphi }}{\frac {h_{\theta }}{h_{\varphi }}}{\frac {\partial }{\partial \varphi }}\right]={\frac {1}{R^{2}}}\left[{\frac {1}{\sin \theta }}{\frac {\partial }{\partial \theta }}\sin \theta {\frac {\partial }{\partial \theta }}+{\frac {1}{\sin ^{2}\theta }}{\frac {\partial ^{2}}{\partial \varphi ^{2}}}\right].}$

The classical Hamiltonian function of the linear rigid rotor is

${\displaystyle H={\frac {1}{2\mu R^{2}}}\left[p_{\theta }^{2}+{\frac {p_{\varphi }^{2}}{\sin ^{2}\theta }}\right].}$

### Quantum mechanical linear rigid rotor

The linear rigid rotor model can be used in quantum mechanics to predict the rotational energy of a diatomic molecule. The rotational energy depends on the moment of inertia for the system, ${\displaystyle I}$. In the center of mass reference frame, the moment of inertia is equal to:

${\displaystyle I=\mu R^{2}}$

where ${\displaystyle \mu }$ is the reduced mass of the molecule and ${\displaystyle R}$ is the distance between the two atoms.

According to quantum mechanics, the energy levels of a system can be determined by solving the Schrödinger equation:

${\displaystyle {\hat {H}}\Psi =E\Psi }$

where ${\displaystyle \Psi }$ is the wave function and ${\displaystyle {\hat {H}}}$ is the energy (Hamiltonian) operator. For the rigid rotor in a field-free space, the energy operator corresponds to the kinetic energy[1] of the system:

${\displaystyle {\hat {H}}=-{\frac {\hbar ^{2}}{2\mu }}\nabla ^{2}}$

where ${\displaystyle \hbar }$ is reduced Planck constant and ${\displaystyle \nabla ^{2}}$ is the Laplacian. The Laplacian is given above in terms of spherical polar coordinates. The energy operator written in terms of these coordinates is:

${\displaystyle {\hat {H}}=-{\frac {\hbar ^{2}}{2I}}\left[{1 \over \sin \theta }{\partial \over \partial \theta }\left(\sin \theta {\partial \over \partial \theta }\right)+{1 \over {\sin ^{2}\theta }}{\partial ^{2} \over \partial \varphi ^{2}}\right]}$

This operator appears also in the Schrödinger equation of the hydrogen atom after the radial part is separated off. The eigenvalue equation becomes

${\displaystyle {\hat {H}}Y_{\ell }^{m}(\theta ,\varphi )={\frac {\hbar ^{2}}{2I}}\ell (\ell +1)Y_{\ell }^{m}(\theta ,\varphi ).}$

The symbol ${\displaystyle Y_{\ell }^{m}(\theta ,\varphi )}$ represents a set of functions known as the spherical harmonics. Note that the energy does not depend on ${\displaystyle m\,}$. The energy

${\displaystyle E_{\ell }={\hbar ^{2} \over 2I}\ell \left(\ell +1\right)}$

is ${\displaystyle 2\ell +1}$-fold degenerate: the functions with fixed ${\displaystyle \ell \,}$ and ${\displaystyle m=-\ell ,-\ell +1,\dots ,\ell }$ have the same energy.

Introducing the rotational constant B, we write,

${\displaystyle E_{\ell }=B\;\ell \left(\ell +1\right)\quad {\textrm {with}}\quad B\equiv {\frac {\hbar ^{2}}{2I}}.}$

In the units of reciprocal length the rotational constant is,

${\displaystyle {\bar {B}}\equiv {\frac {B}{hc}}={\frac {h}{8\pi ^{2}cI}},}$

with c the speed of light. If cgs units are used for h, c, and I, ${\displaystyle {\bar {B}}}$ is expressed in wave numbers, cm−1, a unit that is often used for rotational-vibrational spectroscopy. The rotational constant ${\displaystyle {\bar {B}}(R)}$ depends on the distance ${\displaystyle R}$. Often one writes ${\displaystyle B_{e}={\bar {B}}(R_{e})}$ where ${\displaystyle R_{e}}$ is the equilibrium value of ${\displaystyle R}$ (the value for which the interaction energy of the atoms in the rotor has a minimum).

A typical rotational spectrum consists of a series of peaks that correspond to transitions between levels with different values of the angular momentum quantum number (${\displaystyle \ell }$). Consequently, rotational peaks appear at energies corresponding to an integer multiple of ${\displaystyle 2{\bar {B}}}$.

### Selection rules

Rotational transitions of a molecule occur when the molecule absorbs a photon [a particle of a quantized electromagnetic (em) field]. Depending on the energy of the photon (i.e., the wavelength of the em field) this transition may be seen as a sideband of a vibrational and/or electronic transition. Pure rotational transitions, in which the vibronic (= vibrational plus electronic) wave function does not change, occur in the microwave region of the electromagnetic spectrum.

Typically, rotational transitions can only be observed when the angular momentum quantum number changes by 1 (${\displaystyle \Delta l=\pm 1}$). This selection rule arises from a first-order perturbation theory approximation of the time-dependent Schrödinger equation. According to this treatment, rotational transitions can only be observed when one or more components of the dipole operator have a non-vanishing transition moment. If z is the direction of the electric field component of the incoming electromagnetic wave, the transition moment is,

${\displaystyle \langle \psi _{2}|\mu _{z}|\psi _{1}\rangle =\left(\mu _{z}\right)_{21}=\int \psi _{2}^{*}\mu _{z}\psi _{1}\,\mathrm {d} \tau .}$

A transition occurs if this integral is non-zero. By separating the rotational part of the molecular wavefunction from the vibronic part, one can show that this means that the molecule must have a permanent dipole moment. After integration over the vibronic coordinates the following rotational part of the transition moment remains,

${\displaystyle \left(\mu _{z}\right)_{l,m;l',m'}=\mu \int _{0}^{2\pi }\mathrm {d} \phi \int _{0}^{\pi }Y_{l'}^{m'}\left(\theta ,\phi \right)^{*}\cos \theta \,Y_{l}^{m}\,\left(\theta ,\phi \right)\;\mathrm {d} \cos \theta .}$

Here ${\displaystyle \mu \cos \theta \,}$ is the z component of the permanent dipole moment. The moment ${\displaystyle \mu }$ is the vibronically averaged component of the dipole operator. Only the component of the permanent dipole along the axis of a heteronuclear molecule is non-vanishing. By the use of the orthogonality of the spherical harmonics ${\displaystyle Y_{l}^{m}\,\left(\theta ,\phi \right)}$ it is possible to determine which values of ${\displaystyle l}$, ${\displaystyle m}$, ${\displaystyle l'}$, and ${\displaystyle m'}$ will result in nonzero values for the dipole transition moment integral. This constraint results in the observed selection rules for the rigid rotor:

${\displaystyle \Delta m=0\quad {\hbox{and}}\quad \Delta l=\pm 1}$

### Non-rigid linear rotor

The rigid rotor is commonly used to describe the rotational energy of diatomic molecules but it is not a completely accurate description of such molecules. This is because molecular bonds (and therefore the interatomic distance ${\displaystyle R}$) are not completely fixed; the bond between the atoms stretches out as the molecule rotates faster (higher values of the rotational quantum number ${\displaystyle l}$). This effect can be accounted for by introducing a correction factor known as the centrifugal distortion constant ${\displaystyle {\bar {D}}}$ (bars on top of various quantities indicate that these quantities are expressed in cm−1):

${\displaystyle {\bar {E}}_{l}={E_{l} \over hc}={\bar {B}}l\left(l+1\right)-{\bar {D}}l^{2}\left(l+1\right)^{2}}$

where

${\displaystyle {\bar {D}}={4{\bar {B}}^{3} \over {\bar {\boldsymbol {\omega }}}^{2}}}$
${\displaystyle {\bar {\boldsymbol {\omega }}}}$ is the fundamental vibrational frequency of the bond (in cm-1). This frequency is related to the reduced mass and the force constant (bond strength) of the molecule according to
${\displaystyle {\bar {\boldsymbol {\omega }}}={1 \over 2\pi c}{\sqrt {k \over \mu }}}$

The non-rigid rotor is an acceptably accurate model for diatomic molecules but is still somewhat imperfect. This is because, although the model does account for bond stretching due to rotation, it ignores any bond stretching due to vibrational energy in the bond (anharmonicity in the potential).

## Arbitrarily shaped rigid rotor

An arbitrarily shaped rigid rotor is a rigid body of arbitrary shape with its center of mass fixed (or in uniform rectilinear motion) in field-free space R3, so that its energy consists only of rotational kinetic energy (and possibly constant translational energy that can be ignored). A rigid body can be (partially) characterized by the three eigenvalues of its moment of inertia tensor, which are real nonnegative values known as principal moments of inertia. In microwave spectroscopy—the spectroscopy based on rotational transitions—one usually classifies molecules (seen as rigid rotors) as follows:

• spherical rotors
• symmetric rotors
• oblate symmetric rotors
• prolate symmetric rotors
• asymmetric rotors

This classification depends on the relative magnitudes of the principal moments of inertia.

### Coordinates of the rigid rotor

Different branches of physics and engineering use different coordinates for the description of the kinematics of a rigid rotor. In molecular physics Euler angles are used almost exclusively. In quantum mechanical applications it is advantageous to use Euler angles in a convention that is a simple extension of the physical convention of spherical polar coordinates.

The first step is the attachment of a right-handed orthonormal frame (3-dimensional system of orthogonal axes) to the rotor (a body-fixed frame) . This frame can be attached arbitrarily to the body, but often one uses the principal axes frame—the normalized eigenvectors of the inertia tensor, which always can be chosen orthonormal, since the tensor is Hermitian. When the rotor possesses a symmetry-axis, it usually coincides with one of the principal axes. It is convenient to choose as body-fixed z-axis the highest-order symmetry axis.

One starts by aligning the body-fixed frame with a space-fixed frame (laboratory axes), so that the body-fixed x, y, and z axes coincide with the space-fixed X, Y, and Z axis. Secondly, the body and its frame are rotated actively over a positive angle ${\displaystyle \alpha \,}$ around the z-axis (by the right-hand rule), which moves the ${\displaystyle y}$- to the ${\displaystyle y'}$-axis. Thirdly, one rotates the body and its frame over a positive angle ${\displaystyle \beta \,}$ around the ${\displaystyle y'}$-axis. The z-axis of the body-fixed frame has after these two rotations the longitudinal angle ${\displaystyle \alpha \,}$ (commonly designated by ${\displaystyle \varphi \,}$) and the colatitude angle ${\displaystyle \beta \,}$ (commonly designated by ${\displaystyle \theta \,}$ ), both with respect to the space-fixed frame. If the rotor were cylindrical symmetric around its z-axis, like the linear rigid rotor, its orientation in space would be unambiguously specified at this point.

If the body lacks cylinder (axial) symmetry, a last rotation around its z-axis (which has polar coordinates ${\displaystyle \beta \,}$ and ${\displaystyle \alpha \,}$ ) is necessary to specify its orientation completely. Traditionally the last rotation angle is called ${\displaystyle \gamma \,}$.

The convention for Euler angles described here is known as the ${\displaystyle z''-y'-z}$ convention; it can be shown (in the same manner as in this article) that it is equivalent to the ${\displaystyle z-y-z}$ convention in which the order of rotations is reversed.

The total matrix of the three consecutive rotations is the product

${\displaystyle \mathbf {R} (\alpha ,\beta ,\gamma )={\begin{pmatrix}\cos \alpha &-\sin \alpha &0\\\sin \alpha &\cos \alpha &0\\0&0&1\end{pmatrix}}{\begin{pmatrix}\cos \beta &0&\sin \beta \\0&1&0\\-\sin \beta &0&\cos \beta \\\end{pmatrix}}{\begin{pmatrix}\cos \gamma &-\sin \gamma &0\\\sin \gamma &\cos \gamma &0\\0&0&1\end{pmatrix}}}$

Let ${\displaystyle \mathbf {r} (0)}$ be the coordinate vector of an arbitrary point ${\displaystyle {\mathcal {P}}}$ in the body with respect to the body-fixed frame. The elements of ${\displaystyle \mathbf {r} (0)}$ are the 'body-fixed coordinates' of ${\displaystyle {\mathcal {P}}}$. Initially ${\displaystyle \mathbf {r} (0)}$ is also the space-fixed coordinate vector of ${\displaystyle {\mathcal {P}}}$. Upon rotation of the body, the body-fixed coordinates of ${\displaystyle {\mathcal {P}}}$ do not change, but the space-fixed coordinate vector of ${\displaystyle {\mathcal {P}}}$ becomes,

${\displaystyle \mathbf {r} (\alpha ,\beta ,\gamma )=\mathbf {R} (\alpha ,\beta ,\gamma )\mathbf {r} (0).}$

In particular, if ${\displaystyle {\mathcal {P}}}$ is initially on the space-fixed Z-axis, it has the space-fixed coordinates

${\displaystyle \mathbf {R} (\alpha ,\beta ,\gamma ){\begin{pmatrix}0\\0\\r\\\end{pmatrix}}={\begin{pmatrix}r\cos \alpha \sin \beta \\r\sin \alpha \sin \beta \\r\cos \beta \\\end{pmatrix}},}$

which shows the correspondence with the spherical polar coordinates (in the physical convention).

Knowledge of the Euler angles as function of time t and the initial coordinates ${\displaystyle \mathbf {r} (0)}$ determine the kinematics of the rigid rotor.

### Classical kinetic energy

The following text forms a generalization of the well-known special case of the rotational energy of an object that rotates around one axis.

It will be assumed from here on that the body-fixed frame is a principal axes frame; it diagonalizes the instantaneous inertia tensor ${\displaystyle \mathbf {I} (t)}$ (expressed with respect to the space-fixed frame), i.e.,

${\displaystyle \mathbf {R} (\alpha ,\beta ,\gamma )^{-1}\;\mathbf {I} (t)\;\mathbf {R} (\alpha ,\beta ,\gamma )=\mathbf {I} (0)\quad {\hbox{with}}\quad \mathbf {I} (0)={\begin{pmatrix}I_{1}&0&0\\0&I_{2}&0\\0&0&I_{3}\\\end{pmatrix}},}$

where the Euler angles are time-dependent and in fact determine the time dependence of ${\displaystyle \mathbf {I} (t)}$ by the inverse of this equation. This notation implies that at ${\displaystyle t=0}$ the Euler angles are zero, so that at ${\displaystyle t=0}$ the body-fixed frame coincides with the space-fixed frame.

The classical kinetic energy T of the rigid rotor can be expressed in different ways:

• as a function of angular velocity
• in Lagrangian form
• as a function of angular momentum
• in Hamiltonian form.

Since each of these forms has its use and can be found in textbooks we will present all of them.

#### Angular velocity form

As a function of angular velocity T reads,

${\displaystyle T={\frac {1}{2}}\left[I_{1}\omega _{x}^{2}+I_{2}\omega _{y}^{2}+I_{3}\omega _{z}^{2}\right]}$

with

${\displaystyle {\begin{pmatrix}\omega _{x}\\\omega _{y}\\\omega _{z}\\\end{pmatrix}}={\begin{pmatrix}-\sin \beta \cos \gamma &\sin \gamma &0\\\sin \beta \sin \gamma &\cos \gamma &0\\\cos \beta &0&1\\\end{pmatrix}}{\begin{pmatrix}{\dot {\alpha }}\\{\dot {\beta }}\\{\dot {\gamma }}\\\end{pmatrix}}.}$

The vector ${\displaystyle {\boldsymbol {\omega }}=(\omega _{x},\omega _{y},\omega _{z})}$ contains the components of the angular velocity of the rotor expressed with respect to the body-fixed frame. It can be shown that ${\displaystyle {\boldsymbol {\omega }}}$ is not the time derivative of any vector, in contrast to the usual definition of velocity.[2] The dots over the time-dependent Euler angles indicate time derivatives. The angular velocity satisfies equations of motion known as Euler's equations (with zero applied torque, since by assumption the rotor is in field-free space).

#### Lagrange form

Backsubstitution of the expression of ${\displaystyle {\boldsymbol {\omega }}}$ into T gives the kinetic energy in Lagrange form (as a function of the time derivatives of the Euler angles). In matrix-vector notation,

${\displaystyle 2T={\begin{pmatrix}{\dot {\alpha }}&{\dot {\beta }}&{\dot {\gamma }}\end{pmatrix}}\;\mathbf {g} \;{\begin{pmatrix}{\dot {\alpha }}\\{\dot {\beta }}\\{\dot {\gamma }}\\\end{pmatrix}},}$

where ${\displaystyle \mathbf {g} }$ is the metric tensor expressed in Euler angles—a non-orthogonal system of curvilinear coordinates

${\displaystyle \mathbf {g} ={\begin{pmatrix}I_{1}\sin ^{2}\beta \cos ^{2}\gamma +I_{2}\sin ^{2}\beta \sin ^{2}\gamma +I_{3}\cos ^{2}\beta &(I_{2}-I_{1})\sin \beta \sin \gamma \cos \gamma &I_{3}\cos \beta \\(I_{2}-I_{1})\sin \beta \sin \gamma \cos \gamma &I_{1}\sin ^{2}\gamma +I_{2}\cos ^{2}\gamma &0\\I_{3}\cos \beta &0&I_{3}\\\end{pmatrix}}.}$

#### Angular momentum form

Often the kinetic energy is written as a function of the angular momentum ${\displaystyle \mathbf {L} }$ of the rigid rotor. With respect to the body-fixed frame it has the components ${\displaystyle \quad L_{i}}$, and can be shown to be related to the angular velocity,

${\displaystyle \mathbf {L} =\mathbf {I} (0)\;{\boldsymbol {\omega }}\quad {\hbox{or}}\quad L_{i}={\frac {\partial T}{\partial \omega _{i}}},\;\;i=x,\,y,\,z.}$

This angular momentum is a conserved (time-independent) quantity if viewed from a stationary space-fixed frame. Since the body-fixed frame moves (depends on time) the components ${\displaystyle \quad L_{i}}$ are not time independent. If we were to represent ${\displaystyle \mathbf {L} }$ with respect to the stationary space-fixed frame, we would find time independent expressions for its components.

The kinetic energy is expressed in terms of the angular momentum by

${\displaystyle T={\frac {1}{2}}\left[{\frac {L_{x}^{2}}{I_{1}}}+{\frac {L_{y}^{2}}{I_{2}}}+{\frac {L_{z}^{2}}{I_{3}}}\right].}$

#### Hamilton form

The Hamilton form of the kinetic energy is written in terms of generalized momenta

${\displaystyle {\begin{pmatrix}p_{\alpha }\\p_{\beta }\\p_{\gamma }\\\end{pmatrix}}\ {\stackrel {\mathrm {def} }{=}}\ {\begin{pmatrix}\partial T/{\partial {\dot {\alpha }}}\\\partial T/{\partial {\dot {\beta }}}\\\partial T/{\partial {\dot {\gamma }}}\\\end{pmatrix}}=\mathbf {g} {\begin{pmatrix}\;\,{\dot {\alpha }}\\{\dot {\beta }}\\{\dot {\gamma }}\\\end{pmatrix}},}$

where it is used that the ${\displaystyle \mathbf {g} }$ is symmetric. In Hamilton form the kinetic energy is,

${\displaystyle 2T={\begin{pmatrix}p_{\alpha }&p_{\beta }&p_{\gamma }\end{pmatrix}}\;\mathbf {g} ^{-1}\;{\begin{pmatrix}p_{\alpha }\\p_{\beta }\\p_{\gamma }\\\end{pmatrix}},}$

with the inverse metric tensor given by

${\displaystyle {\scriptstyle \sin ^{2}\beta }\;\;\mathbf {g} ^{-1}=}$
${\displaystyle -\hbar ^{2}}$