4.a. Quantum Numbers  
J  Resultant total angular momentum quantum number, excluding nuclear spins. 
N  Rotational angular momentum quantum number, excluding electron and nuclear spins, in the case where electron spin is present. 
K_{}, K_{+}  Projection of J (or N) on the symmetry axis in the limiting prolate or oblate symmetric top. 
F_{1}  Resultant angular momentum quantum number including nuclear spin for one nucleus. 
F  Resultant total angular momentum quantum number. 
A, E  Torsional symmetry substates representing irreducible representations of the symmetry group of the rotationinternal rotation Hamiltonian. 
ν_{1},ν_{2},ν_{3}  Vibrational modes (ν) and quantum numbers (υ) 
U or L  Upper or lower energy level or transition frequency. 
′ or ″  Prime or double prime is used to distinguish the upper (′) and lower (″) energy states in a transition. They occur as superscripts on the quantum numbers. 
ℓ  Quantum number for vibrational angular momentum. 
I or (I_{i})  Angular momentum quantum number of nuclear spin for one (or i^{th}) nucleus. 
4.b. Molecular Constants  
A, B, C  Rotational constants (MHz). These are related to the principal moments of
inertia: 
Bbar equals (B + C)/2.  
τ, Δ, δ, D  Quartic centrifugal distortion constants (MHz or kHz). 
H, h  Sextic centrifugal distortion constants (MHz or kHz). 
L, ℓ, G, g  Octic centrifugal distortion constants (MHz). 
I_{α}  Moment of inertia of the methyl top around internal rotation axis (u Å^{2}) 
ρ  Internal rotation interaction constant ρ = [Σ_{x}(λ_{x} I_{α} /I_{x})^{2}]^{1/2}. 
λ_{a}, λ_{b}, λ_{c}  Direction cosines between the internal rotation axis and the principal axes a,b,c, respectively. 
α  Angle of rotation around internal rotation axis. 
F  Internal rotation dynamical constant (GHz) F = h/8π^{2}r I_{α}. 
V_{3}  Threefold component of torsional barrier potential V = V_{3}(1cos 3α)/2. 
s  Reduced barrier height s = 4V_{3} / 9F. 
r  r = 1  Σ_{x}(λ_{x}^{2}I_{α} / I_{x}). 
Θ  Angle between CH_{3} symmetry axis and aprincipal axis. 
ω_{1}(s)  Fourier coefficient. 
Δ_{0}  Internal rotation interaction constant (eq 13) 
µ_{a} , µ_{b} , µ_{c}  Components of the electric dipole moment along the a or b or cprincipal axes. 
α_{v} , γ_{v}_{v}  Rotationvibration coefficients in the power series representing B_{v} (see text). 
q_{v}  ℓdoubling constant (MHz). 
σ_{} , σ_{⊥}  Components of the magnetic shielding tensor which are parallel and perpendicular to the molecular axis, respectively. 
Q  Molecular quadrupole moment relative to the center of mass (esu · cm2). 
α_{} , α_{⊥}  Electric polarizability anisotropy (cm^{3}). 
χ_{⊥} , χ_{}  Components of the magnetic susceptibility tensor which are respectively perpendicular and parallel to the molecular axis (erg/G^{2} · mol). 
χ_{xx}  χ_{yy}  Magnetic susceptibility anisotropy. 
g_{⊥} , g_{}  Components of the molecular G tensor which are respectively perpendicular and parallel to the molecular axis. g_{⊥} is sometimes denoted g or g_{J} for linear molecules in the ground state (µ_{N}). 
g_{xx}  g_{yy}  Anisotropy of the molecular G tensor perpendicular to the molecular axis (µ_{N}). 
c_{x} or M  Spin rotation constant related to nucleus X (kHz). 
S_{XY}  Spinspin interaction constant between nucleus X and nucleus Y (kHz). 
α_{P} , β_{P}  Ωtype doubling parameters (eq 14) (eq 15) 
p_{eff}  Λtype doubling constant in the ^{2}Π_{1/2} state (MHz). 
a, b, c, d  Magnetic hyperfine coupling constants (MHz) where,
(eq 16) (eq 17) (eq 18)
(eq 19) Here µ_{B} is the Bohr magneton, µ_{N} is the nuclear magneton and g_{N} is the nuclear gvalue. 
A  Spinorbit coupling constants defined by the power series expansion,

4.c. Other  
#  A hash mark located in front of the uncertainty denotes that the frequency is the hyperfine free center of the frequency. 
(...)  Parentheses in the numerical listings contain measured or estimated uncertainties. For example, the value 1.407(83) should be interpreted as 1.407±0.083. Thus the value in parentheses refers to the last significant digits given. 
a, b, c  Designate principal axes corresponding to A, B, and C, respectively. 
