ABSTRACT
Trifluoroethanol (TFE) and its aggregates are studied via supersonic jet FTIR and Raman spectroscopy as well as by quantum chemistry and simple force field approaches. A multi-slit nozzle is introduced to study collisionally excited clusters. Efforts are made to extract harmonic frequencies from experiment for better comparison to theory. Based on deuteration, the OH stretching anharmonicity changes weakly upon dimerization, but increases for trimers. Among the possible dimer conformations, only an all-gauche, homoconfigurational, compact, OH-F connected structure is observed in an extreme case of chiral discrimination. Quantum tunneling assisted pathways for this surprising helicity synchronization are postulated. The oscillator coupling in hydrogen-bonded trimers is analyzed. Trans conformations of TFE start to become important for trimers and probably persist in the liquid state. Simple force fields can be refined to capture some molecular recognition features of TFE dimer, but their limitations are emphasized.
Subject(s)
Peptides/chemistry , Quantum Theory , Trifluoroethanol/chemistry , Dimerization , Hydrogen Bonding , Protein Conformation , Spectroscopy, Fourier Transform Infrared , Spectrum Analysis, RamanABSTRACT
The Cl- -C6H5CH3*Ar, Cl- -C6H5NH2*Ar, and Cl- -C6H5OH*Ar anion complexes are investigated using infrared photodissociation spectroscopy and ab initio calculations at the MP2/aug-cc-pVDZ level. The results indicate that for Cl- -C6H5NH2 and Cl- -C6H5OH, the Cl- anion is attached to the substituent group by a single near-linear hydrogen bond. For Cl--C6H5CH3, the Cl- is attached to an ortho-hydrogen atom on the aromatic ring and to a hydrogen atom on the methyl group by a weaker hydrogen bond. The principal spectroscopic consequence of the hydrogen-bonding interaction in the three complexes is a red-shift and intensity increase for the CH, NH, and OH stretching modes. Complexities in the infrared spectra in the region of the hydrogen-bonded XH stretch band are associated with Fermi resonances between the hydrogen-stretching vibrational modes and bending overtone and combination levels. There are notable correlations between the vibrational red-shift, the elongation of the H-bonded XH group, and the proton affinity of the aromatic molecule's conjugate base.
ABSTRACT
Hydrogen bonded complexes between two different aliphatic alcohols exhibit donor/acceptor isomerism. In a supersonic jet expansion, the less stable isomer can isomerize to the more stable isomer if the energy difference is sufficiently large and the barrier sufficiently low. We show by FTIR jet spectroscopy that this is progressively the case for methanol/methanol-d1, methanol/ethanol, and methanol/tert-butyl alcohol, until no metastable donor/acceptor isomer persists in the expansion. Collisional relaxation experiments, 18O labeling and quantum chemical calculations are used to assign the spectra. Differences between energetical and spectroscopic acceptor strengths are discussed.
ABSTRACT
The subtle trans-gauche equilibrium in the ethanol molecule is affected by hydrogen bonding. The resulting conformational complexity in ethanol dimer manifests itself in three hydrogen-bonded OH stretching bands of comparable infrared intensity in supersonic helium expansions. Admixture of argon or nitrogen promotes collisional relaxation and is shown to enhance the lowest frequency transition. Global and local harmonic frequency shift calculations at MP2 level indicate that this transition is due to a gauche-gauche dimer, but the predictions are sensitive to basis set and correlation level. Energetically, the homochiral gauche-gauche dimer is predicted to be the most stable ethanol dimer conformation. The harmonic MP2 predictions are corroborated by perturbative anharmonicity contributions and CCSD(T) energies. Thus, a consistent picture of the subtle hydrogen bond energetics and vibrational dynamics of the ethanol dimer is starting to emerge for the first time.
