Brugia Malayi - Clark Science Center - Smith College

Effects of Hydrogen-Bonding on O-H Stretch Overtone Excitation for
Fluorinated Hydroperoxides
Mojdeh Mostafavi and Julie Vallejo
Hydrofluorocarbons, also known as HFCs, are molecules that currently serve as refrigerants and as chlorofluorocarbon (CFC)
replacements. Hydrogen containing HFCs have shorter atmospheric lifetimes than CFCs, since H atom abstraction by OH radicals
leads to their oxidation in the troposphere before the molecules can find their way upwards to the stratosphere. 1,2 Laboratory
studies predict that O 2
can convert the alkyl radicals, formed from the reaction between HFCs and OH radicals, into peroxy
radicals that can then react with HO 2
radicals to form hydroperoxides in the atmosphere. 2 Here we focus on the effect of increased
fluorination on the absorption spectra in the visible wavelength region for these atmospheric hydroperoxides, in comparison to
their non-fluorinated analogs.
As a first step in predicting absorption spectra, Gaussian09 calculations were used to find the most stable conformers of
the molecules of interest: CF 3
OOH, CHF 2
OOH, CH 2
FOOH, and CH 3
OOH. For each conformer, we performed calculations
of energies, dipole moment components and O-H stretch frequencies. Because previous studies on hydroperoxides have shown
absorption in the visible wavelength region to correspond to excitation of O-H stretch vibration and torsion about the O-O bond,
calculations were extended to C-O-O-H dihedral angles every 10°, from 0° to 360°. 3 At each torsional angle, calculations were
gathered with six compressed and six extended O-H bond lengths in 0.05 Å steps around the equilibrium value. The information
gathered with Gaussian09 was then used to predict absorption spectra using methods developed previously. 3
In comparing the series, we found that intramolecular hydrogen bonding between the O-H and fluorines causes the
fluorinated organic hydroperoxides to behave differently from the non-halogenated species. The presence of hydrogen bonding
was evident in the relative stability of conformers, anti and gauche. In some molecules, the gauche conformer was more stable
than the anti. The ones with stable gauche conformations had the hydroperoxide hydrogen in proximity to a fluorine, indicating
that hydrogen bonding was responsible for the increased stability of what is typically thought as the unstable conformer. As more
fluorines were added, the hydroperoxides also showed a higher O-O bond dissociation energy and lower O-H frequency, which
both work against overtone in the atmosphere. 4 (Supported by a Henry Dreyfus Teacher-Scholar Award and the National Science
Foundation).
Advisor: Shizuka Hsieh
References:
1
McCulloch, A.; Journal of Fluorine Chemistry. 1999. 100. 163-173.
2
Sehested, J.; MøGelberg, T.; Fagerström, K.; Mahmoud, G.; Wallington, T. J. International Journal of Chemical Kinetics. 1997. 29. 9. 673-682.
3
Haynes, L. M.; Vogelhuber, K. M.; Pippen, J. L.; Hsieh, S. J. Chem. Phys. 2005. 123. 234306.
4
Donaldson, D. J.; Tuck, A. F.; Vaida, V. Chem. Rev. 2003, 103, 4717-4729.
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