Record low CH stretch frequencies detected

In a collaboration between the University of California at Riverside and the FELIX Laboratory at Radboud University, C-H stretch vibrations at unexpectedly low frequencies have been uncovered in the infrared spectra of deprotonated alcohols. The scientists experimentally verified that these bands do not occur around their usual position near 3000 cm^-1, but instead fall between 2200 and 2600 cm^-1. Hyperconjugation can qualitatively explain the low frequency of the a-CH-stretch oscillation in the negatively charged ions, but quantum-chemical calculations fail to accurately predict these values. Results have appeared in the journal Angewandte Chemie.

In an infrared (IR) spectrum, the absorption bands are due to the oscillatory motion of atoms within a molecule, which occur at resonance frequencies dependent on the type of atoms and their bond strength.  Every spectroscopist knows that the stretching vibrations of C-H bonds in organic compounds occur in an isolated and diagnostic range of the infrared spectrum around wavelengths of 3.3 mm (i.e. frequencies of 3000 cm^-1 ± about 10%). This position is mainly determined by the C-H bond strength which is reasonably similar for different organic molecules, so that the force constant k of the CH stretch oscillation has a reasonably constant value.

The current study addresses the CH stretch vibrations in negatively charged ions, in particular the a-CH stretch vibration in primary and secondary alkoxide ions (deprotonated alcohols with one or two H-atoms attached to the a-carbon atom, see Figure A).  IR spectra were recorded for these anions in the gas phase where the ions are in full isolation. In solutions or in salts, the effects reported here would not have been observable because the negative charge would disperse over the environment.

IR spectra were recorded for several fluorinated and non-fluorinated alkoxide ions (Figure B). Take 1,1,1-trifluoro-propoxide in Figure C as an example. The CH stretch vibrations of the methyl group are found at their normal position near 2900 cm^-1 and the quantum-chemical calculation, indicated by the stick spectrum, has no problem predicting the right value for their frequencies. However, the a-CH vibration is observed near 2400 cm^-1 and the calculation misses this position by nearly 200 cm^-1! To verify that the observed band indeed corresponds to the a -CH vibration, the spectrum was also recorded for the anion with a deuterium atom in the a -position (spectra in red); the band indeed shows its characteristic red shift upon H/D isotope substitution.

Although quantum-chemical calculations fail to correctly predict the absorption frequency, the large shift in measured absorption frequency can be qualitatively explained by an inspection of the valence bond structure of the anion. Hyperconjugation between the orbitals that form the C-O and the C-H bond suggests that there exists a resonant structure that can be characterized as a “double-bond/no-bond” structure (Figure A). The weakened “no-bond” C-H linkage must induce a sizable reduction in force constant and hence lead to a red shift of the CH stretch vibration as observed. The experiments therefore suggest that this resonant structure, and hence hyperconjugation, plays an important role in these negative ions. Counterintuitively, the H-atom acts as an electron sink in the presence of the much more electronegative oxygen atom.

Experimental method

Gaseous ions are formed and mass-selected in a mass spectrometer. The fluorinated alcohols are sufficiently acidic so that they undergo facile deprotonation in electrospray ionization (ESI) and form the alkoxide ions of interest. This is not the case for the unfluorinated alcohols. The propoxide ions (5 and 6 in Figure B) are therefore generated by gas-phase proton abstraction from vapor-phase propanol by the CF₃^- ion, which is again generated by ESI.

The density of ions in a mass spectrometer is many orders of magnitude too low to enable a transmission spectrum to be recorded. The ions stored in an ion trap mass spectrometer are therefore irradiated by a powerful and wavelength-tunable IR laser. Whenever the laser frequency is in resonance with an absorption band of the ion, the absorbed energy activates the ion and induces dissociation (or electron detachment) of the ion, which is observed in the mass spectrum. From a series of mass spectra taken with the laser at different IR frequencies, an IR spectrum can thus be reconstructed. Both the free electron laser FELIX as well as an optical parametric oscillator/amplifier (OPO/OPA) system have been used to cover the entire IR frequency range shown in the Figures.

Publication

Low-frequency CH stretch vibrations of free alkoxide ions
Jos Oomens, Giel Berden and Thomas Hellman Morton

http://dx.doi.org/10.1002/anie.201609437

More information?

Jos Oomens: j.oomens@science.ru.nl