Methoxy is a doublet radical in a degenerate electronic gound state with ²E_3/2 below ²E_1/2. The parameter set was taken from
(1) J.-T. Spaniol, O. Chitarra, O. Pirali, M.-A. Martin-Drumel and H. S. P. Müller, 2025, J. Phys. Chem. A 129, accepted.
A large fraction of the data, between about 352 and 854 GHz are from that work. Additional millimeter wave data up to 199 GHz were published by
(2) Y. Endo, S. Saito, E. Hirota, 1984, J. Chem. Phys. 81, 122.
Further millimeter and submillimeter wave data between 242 and 372 GHz were taken from
(3) T. Momose, Y. Endo, E. Hirota, T. Shida, 1988, J. Chem. Phys. 88, 5338.
A small number of transition frequencies were published by
(4) J. Laas and S. L. Widicus Weaver, 2017, Astrophys. J. 835, 46;
several of these last transitions are the same as the ones from (3), some are new. A small number of transition frequencies from (3) and (4) were treated slightly differently than reported because of blending, and two transition frequencies from (4) were omitted because of large residuals.
The calculated transition frequencies should be reliable for at least most observational purposes; frequencies with calculated uncertainties exceeding 0.2 MHz should be viewed with increasing caution.
The quantum numbers in the standard entry are N, K, v, t, and F, where v indicates the l components, and t is an aggregate spin number with

 t / N – F / J – F / sym / I_tot
  0 / –2.0 / –1.5 / 0 / 1.5
  1 / –1.0 / –0.5 / 2 / 0.5
  2 / –1.0 / –0.5 / 4 / 0.5
  3 / –1.0 / –0.5 / 0 / 1.5
  4 / –1.0 / –1.5 / 0 / 1.5
  5 /   0.0 /   0.5 / 2 / 0.5
  6 /   0.0 /   0.5 / 4 / 0.5
  7 /   0.0 / –0.5 / 2 / 0.5
  8 /   0.0 / –0.5 / 4 / 0.5
  9 /   0.0 /   0.5 / 0 / 1.5
10 /   0.0 / –0.5 / 0 / 1.5
11 /   1.0 / –0.5 / 2 / 0.5
12 /   1.0 / –0.5 / 4 / 0.5
13 /   1.0 / –1.5 / 0 / 1.5
14 /   1.0 / –0.5 / 0 / 1.5
15 /   1.0 / –1.5 / 0 / 1.5

The partition function refers to sufficiently high rotational states of the ground vibrational state. Information on the vibrational partition function is available.
The dipole moment was taken from a quatum chemical calculation by
(5) C. F. Jackels, 1982, J. Chem. Phys. 76, 505.