Isomerization around C–C and C–O bonds in 1-propanol: Collisional relaxation in supersonic jets and selective IR photo-isomerization in cryogenic matrices

The conformational equilibria and isomerization processes of 1-propanol are analyzed by vibrational spectroscopy at low temperatures. Supersonic jet experiments provide constraints on the energy sequence. All five distinguishable monomer conformers are assigned in the OH stretching region. Collisional relaxation affects the OH group conformation (gauche or trans, g/t), but the barriers to alkyl chain isomerization (G/T) prevent relaxation of the latter. Alkyl chain isomerization can be induced in rare gas and nitrogen cryogenic matrices by suitable monochromatic irradiation. Selective excitation of the OH stretch of the Gt isomer transfers the alkyl chain from G to T. In N2 matrices, reversible OH group isomerization between Tt and Tg can also be induced by selective irradiation, whereas in solid CCl4 matrices, only Gt ⇌ Gg OH group isomerization is observed. The competition between intramolecular and intermolecular energy transfer and the matrix-dopant interaction determines the torsional subspace dynamics of the vibrationally excited propanol molecules.