Recently we published [ Liu et al. J. Chem. Phys. 2013 , 139 , 154312 ] an analysis of the rotational structure of the B̃-X̃ origin band spectrum of isopropoxy, which confirmed that the double methyl substitution of methoxy to yield the isopropoxy radical only slightly lifted the degeneracy of the former's X̃(2)E state. Additionally the spectral results provided considerable insight into the relativistic and nonrelativistic contributions to the experimental splitting between the components of the (2)E state. However, left unexplained was how the Jahn-Teller (JT) vibronic coupling terms within methoxy's (2)E state manifest themselves as pseudo-Jahn-Teller (pJT) vibronic coupling between the Ã(2)A″ and X̃(2)A' levels of isopropoxy. To cast additional light on this subject we have obtained new isopropoxy spectra and assigned a number of weak, "forbidden" vibronic transitions in the B̃-X̃ spectrum using new electronic structure calculations and rotational contour analyses. The mechanisms that provide the nonzero probability for these transitions shed considerable information on pJT, spin-orbit, and Coriolis coupling between the à and X̃ states. We also report a novel mechanism caused by pJT coupling that yields excitation probability to the B̃ state dependent upon the permanent dipole moments in the B̃ and à or X̃ states. By combining a new B̃-à and the earlier B̃-X̃ rotational analyses we determine a much improved value for the experimental Ã-X̃ separation.