Structural Preferences, Argon Nanocoating, and Dimerization of n-Alkanols As Revealed by OH Stretching Spectroscopy in Supersonic Jets

Abstract

n-Alkanols can occur in a multitude of energetically competitive conformational states. Using the OH stretching vibration as an infrared and Raman spectroscopic sensor in supersonic jet expansions, the torsional preferences around the Calpha-O and Cbeta-Calpha bonds are probed for n-propanol through n-hexanol. Raman detection is more powerful for isolated monomers, whereas IR spectroscopy is more sensitive for molecular complexes. The subtle IR vibrational shift induced by the nanocoating of n-alcohols with Ar atoms is shown to alternate with chain length. A large number of alcohol dimer absorptions is observed and subjected to collisional relaxation and nanocoating conditions. Essential features of the dimer spectra are modeled successfully by a simple force field approach. Exploratory quantum chemical calculations up to the MP2/aug-cc-pvqz level encourage a rigorous theoretical study of the subtle conformational aspects in monomers and possibly also in dimers of linear alcohols.

FTIR/PCA study of propanol in argon matrix: The initial stage of clustering and conformational transitions

“…The general trend in the assignment of the H-bond bands (3000-3600 cm -1 ) in simple alcohols, most frequently met in the literature, looks like -"the lower band frequency the higher aggregate". In many cases this is confirmed by proper quantum chemistry calculations [4,5,8,12]. Following this scheme, the sharp peaks of the highest frequencies, i.e., at 3660 and 3643 cm -1 (Figs.…”

“…These bands are absent in many other works on 1-propanol studies [4,5,8]. We like to suppose that they might be originated from the conformers of mixed propanol-water dimers.…”

“…We have considered only the so-called Gt conformer of 1-propanol which was found to be the lowest energy conformer at both DFT and coupled cluster levels [4,8,20]. We did not consider any vibrational frequency scaling.…”

“…The most significant results on this field have been obtained using the vibrational spectroscopy (FTIR absorption and Raman) together with trapping/isolation in various cryogenic matrices [4][5][6][7][8][9][10][11][12][13][14]. This is mainly because the spectral acquisition carried out on isolated clusters at very low temperatures causes the appearance of narrow peaks with minimal band overlapping.…”

“…This is mainly because the spectral acquisition carried out on isolated clusters at very low temperatures causes the appearance of narrow peaks with minimal band overlapping. Hence, it provides the possibility to study the spectral properties of individual clusters of different sizes and even various molecular conformers within those clusters [4,5,8]. Thus the studies of small H-bond clusters of different size and conformations having the possibility to manipulate them and to resolve these effects in spectra experimentally can provide extremely valuable information for further progress in this field.…”