Raman spectra of 4-methyl isochroman and its aromatic ring substituted derivatives indicate the presence of two conformers in the liquid state and only one in the solid, while the 1- and 3-methyl analog assume one conformation only in both states. The presence of a methyl group adjacent to the ring oxygen atom sterically prevents adoption of one of the two possible conformations due to 1,3-interaction. No such restriction is imposed on the 4-methyl compounds.
Stanley K. Freeman, "Application of Laser-Excited Raman Spectroscopy to Organic Chemistry. II. Conformers of some Isochromans," Appl. Spectrosc. 24, 42-43 (1970)