Solvent and pH Effects on the Sign of the ORD Spectrum of N-Methyl-4-phenyl-7-methoxy-1,2,3,4-tetrahydroisoquinoline in the Visible Spectral Range

It has been described in the literature¹ that the protonation of the nitrogen in 1-benzyl-1,2,3,4-tetraisoquinoline derivatives does not affect the sign of the ORD spectra. We have recently observed² that in the case of several (R) and (S) N-methyl-4-phenyl-1,2,3,4-tetrahydroisoquinol ine derivatives this is true only for the three Cotton effects below 300 nm (the ¹L_b, ¹L_a and ¹B electronic transitions³ of the aromatic moiety), although appreciable variations in intensity and slight shifts in the positions of the extrema were also noticed. On the other hand, the sign of the rotation in the visible spectral range was reversed upon protonation in 0.1N methanolic HCl (or hydrochloride of 1 in methanol).