As in the case of the ^{3}Σmolecules, the energy levels of a ^{2}Πmolecule exhibit the additional splittings due to the electron spin and orbital angular momentum interactions. In order to describe the rotational spectra of this class. Hund's coupling case (a) is employed as a starting point. The rotational levels are defined with the quantum number Ω, the absolute value of the projection of the total electronic angular momentum on the molecular axis, with the quantum number J which represents the total angular momentum from rotation and electronic motion, and with the parity. For NO the parity, + or , of the levels split by Ωdoubling follows the notation of ref. [12]. Although the parity is not known for the other ^{2}Π diatomic molecules, it is necessary to distinguish transitions of
The electric dipole transitions are given by the following selection rules:
The rotational energies, derived from the observed rotational transitions, can be described with the Hamiltonian [12]:
$$\begin{eqnarray*} {\cal H}~=~B~(\textbf{J}^2~~L_z^2~+~\textbf{S}^2)~+~AL_zS_z~~2B \textbf{J} \cdot \textbf{S} \\
+~(B~+~A/2)~(L_+S_~+~L_S_+)~~B(J_+L_~+~J_L_+)~+~ \gamma(\textbf{J  S}) \cdot \textbf{S} \end{eqnarray*}$$
(eq 20)
where a moleculefixed cartesian coordinate system, with the zaxis along the molecular axis, is employed. The operators, L_{z}, L_{+}, and L_ are the three spherical components of the electronic orbital angular momentum; S_{z}, S_{+}, and S_ are the equivalent operators for electron spin and J_{z}, J_{+}, J_ for the total angular momentum. The parameters B, A, and γ are functions of the internuclear distance and, thus, may be defined in terms of a power series in
$$\zeta = \frac{rr_{\rm e}}{r_{\rm e}}$$
as:
$$\begin{eqnarray*}
B&=&B_{\rm e}(1  2\zeta + 3\zeta^3 + ...) ~ ,\\
A&=&A_{\rm e} + A_{(1)} \zeta + A_{(2)} \zeta^2 + ... ~ ,\\
\gamma&=&\gamma_{\rm e} + \gamma_{(1)} \zeta + ... ~ ,
\end{eqnarray*}$$
(eq 20a)
The eigenvalue solution of the Hamiltonian above is normally achieved by a perturbation method which takes into account the mixing of various vibrational states, and the mixing of various electronic states with the ground state. In this way centrifugal distortion terms, the vibrational dependence of the molecular parameters, luncoupling and Λ or Ωdoubling can be determined.
There are a variety of possible approximations employed to describe the observed microwave spectra. The method used depends on how close the angular momenta coupling in a specific: molecule corresponds to Hund's coupling case (a). Formulations employed for intermediate coupling cases, like that for OH and NO, are given in ref. [13] and [14]. The determinable parameters are
The appropriate formulation for coupling cases close to Hund's case (a), e.g., ClO and NS, are given in ref. [16]. The determinable parameters are
The rotational constant
The hyperfine coupling Hamiltonian given in ref. [10] is evaluated in ref. [14] to first order for the magnetic and nuclear electric quadrupole interactions. Although the first order perturbation treatment is adequate for the interpretation of the microwave spectra, the more detailed analysis in ref. [12] is necessary to adequately describe the radiofrequency spectrum of NO. The determinable parameters are the magnetic coupling constants a, b, c, and d, as well as the quadrupole coupling constant, eQq, which is proportional to the electric field gradient at the nucleus in the direction of the molecular axis, and
$$e Q \bar q ~ ,$$
which is proportional to the field gradient perpendicular to the molecular axis. In molecules with coupling cases close to case (a), the determinable parameters are functions of combinations of the constants a, b, c, and d.
Symbols  Definitions  

Effective rotational constants in the 

Centrifugal distortion correction constants in the 

Ωdoubling parameters, $$ \alpha_p = 4\Sigma(1)^S {\displaystyle ~ \frac (eq 21) $$ \beta_p = \Sigma(1)^S {\displaystyle ~ \frac (eq 22) 

p_{eff}  Λtype doubling constant in the 

Magnetic hyperfine coupling constants (MHz) where,
$$ a = 2\mu_{\rm B} g_{\rm N}\mu_{\rm N} \langle 1/r^3\rangle $$ (eq 23) $$ b =  \mu_{\rm B}g_{\rm N}\mu_{\rm N} ~\left\langle (eq 24) $$ c = 3\mu_{\rm B}g_{\rm N}\mu_{\rm N} ~\left\langle (eq 25) $$ d = 3\mu_{\rm B}g_{\rm N}\mu_{\rm N} ~\left\langle (eq 26) 

Here _{μB} is the Bohr magneton, _{μN} is the nuclear magneton, and g_{N} is the nuclear gvalue.  
eQq  Quadrupole coupling constant along the molecular axis, where $$q = \left\langle {\displaystyle \frac{3\cos^2\chi1}{r^3}}\right\rangle ~ (MHz).$$  
$$e Q \bar q$$  Quadrupole coupling constant perpendicular to the molecular axis, where $$\bar q = \left\langle {\displaystyle \frac{3\sin^2\chi}{r^3}}\right\rangle ~ (MHz).$$  
A^{ }  Spinorbit coupling constant defined by the power series, expansion, $$A = A_{\rm e} + A_{(1)} \zeta + A_{(2)} \zeta^2 + ... ~ .$$  
_{γ}  Spinrotation coupling constant defined by the power series $$\gamma = \gamma_{\rm e} + \gamma_{(1)} \zeta + ... ~ .$$ 