Abstract
Guided by ab initio calculations, Fourier transform microwave rotation spectra in the 6.5–22 GHz region are obtained for the complex formed between trans-1,2-difluoroethylene and hydrogen fluoride, including the normal isotopomer and two singly substituted species in natural abundance. Spectra are also obtained for the analogous three species formed using deuterium fluoride. Analysis of the spectra provides rotational and hyperfine constants that are used to determine a structure for trans-CHFCHF–HF. This structure is similar to that obtained for 1,1-difluoroethylene-HF [H. O. Leung et al., J. Chem. Phys.131, 204301 (2009)] in that a primary, hydrogen bonding interaction exists between the HF donor and a F atom acceptor on the 1,2-difluoroethylene moiety, while a secondary interaction occurs between the F atom on the HF molecule and the H atom cis to the hydrogen-bonded F atom on the substituted ethylene and causes the hydrogen bond to deviate from linearity. Because the two F atoms and the two H atoms in trans-1,2-difluoroethylene form electrostatically equivalent pairs, the structure of the complex with HF provides insight into the contribution of steric effects to the observed geometries of fluoroethylene-protic acid complexes. A comparison of the observed hydrogen bond lengths and deviations from linearity in 1,1-difluoroethylene-HF and trans-1,2-difluoroethylene-HF suggests that the F atoms in trans-1,2-difluoroethylene are more nucleophilic than those in 1,1-difluoroethylene and that the H atoms are similarly more acidic. Ab initio calculations of electrostatic potentials mapped onto total electron density surfaces for these two molecules support these conclusions.
We would like to thank Professor Norman Craig for generously providing us with a sample of trans-1,2-difluoroethylene. B.K.A. thanks the Arnold and Mabel Beckman Foundation and the Frank Fowler Dow 1874 Fund at Amherst College for support. This material is based on work supported by the National Science Foundation under Grant No. CHE-0809542.
I. INTRODUCTION
II. AB INITIO CALCULATIONS
III. EXPERIMENT
IV. RESULTS
A. Spectral analysis
B. Structure determination
V. DISCUSSION
VI. CONCLUSION
Key Topics
- Chemical bonds
- 32.0
- Hydrogen bonding
- 28.0
- Acids
- 18.0
- Electrostatics
- 12.0
- Hyperfine structure
- 8.0
Figures
The structures of the (a) vinyl fluoride- and (b) 1,1,2-trifluoroethylene- complexes (, HCl, HCCH).
The structures of the (a) vinyl fluoride- and (b) 1,1,2-trifluoroethylene- complexes (, HCl, HCCH).
The geometrical parameters used in constructing the one-dimensional relaxed potential scan for trans-CHFCHF–HF. is the distance between the center of the bond and the H atom of HF. The angle between and the bond is , and is the angle between the HF molecular axis and .
The geometrical parameters used in constructing the one-dimensional relaxed potential scan for trans-CHFCHF–HF. is the distance between the center of the bond and the H atom of HF. The angle between and the bond is , and is the angle between the HF molecular axis and .
A one-dimensional relaxed potential scan for trans-CHFCHF–HF in which and are optimized while is varied in 10° steps from 0° to 180° (see Fig. 2). Two additional calculations are done to locate the two minima apparent in the figure. The points are joined by a cubic spline curve.
A one-dimensional relaxed potential scan for trans-CHFCHF–HF in which and are optimized while is varied in 10° steps from 0° to 180° (see Fig. 2). Two additional calculations are done to locate the two minima apparent in the figure. The points are joined by a cubic spline curve.
(a) The transition for the trans-CHFCHF–HF isotopomer. Four Doppler doublets are observed, and they are due to HF nuclear spin-spin interactions (one Doppler component, the strongest line in the spectrum, is common to two hyperfine components); (b) The transition for the trans-CHFCHF–DF isotopomer. The three Doppler doublets represent hyperfine components due to deuterium nuclear quadrupole coupling interactions.
(a) The transition for the trans-CHFCHF–HF isotopomer. Four Doppler doublets are observed, and they are due to HF nuclear spin-spin interactions (one Doppler component, the strongest line in the spectrum, is common to two hyperfine components); (b) The transition for the trans-CHFCHF–DF isotopomer. The three Doppler doublets represent hyperfine components due to deuterium nuclear quadrupole coupling interactions.
A comparison among the complexes between fluorine substituted ethylenes and HF: These figures are taken from, respectively, Refs. 1 and 9, this work, and Ref. 6.
A comparison among the complexes between fluorine substituted ethylenes and HF: These figures are taken from, respectively, Refs. 1 and 9, this work, and Ref. 6.
Electrostatic potential mapped onto electron density for (a) 1,1-difluoroethylene and (b) trans-1,2-difluoroethylene. The red areas of the surfaces represent negative electrostatic potential and the blue areas positive electrostatic potential.
Electrostatic potential mapped onto electron density for (a) 1,1-difluoroethylene and (b) trans-1,2-difluoroethylene. The red areas of the surfaces represent negative electrostatic potential and the blue areas positive electrostatic potential.
Tables
Structural parameters and rotational constants of two trans-1,2-difluoroethylene-HF isomers obtained from ab initio calculations.
Structural parameters and rotational constants of two trans-1,2-difluoroethylene-HF isomers obtained from ab initio calculations.
Spectroscopic constants (in megahertz, except as otherwise noted) for six isotopomers of trans-1,2-difluoroethylene-HF. standard deviations in the parameters are given in parentheses.
Spectroscopic constants (in megahertz, except as otherwise noted) for six isotopomers of trans-1,2-difluoroethylene-HF. standard deviations in the parameters are given in parentheses.
Coordinates for the two C atoms in the CHFCHF subunit and the H and F atoms in the HF subunit of trans-1,2-difluoroethylene-HF.
Coordinates for the two C atoms in the CHFCHF subunit and the H and F atoms in the HF subunit of trans-1,2-difluoroethylene-HF.
The values of determined using HF nuclear spin-spin coupling constants and deuterium quadrupole coupling constants. standard deviations in the parameters are given in parentheses.
The values of determined using HF nuclear spin-spin coupling constants and deuterium quadrupole coupling constants. standard deviations in the parameters are given in parentheses.
The values of the structural parameters of trans-1,2-difluoroethylene-HF determined by fits to the values of and of the HF- and DF-containing isotopomers. standard deviations in the parameters are given in parentheses. (These standard deviations reflect only the statistical uncertainty in the least squares fit to the geometry and do not include the effects of model errors or zero-point averaging.)
The values of the structural parameters of trans-1,2-difluoroethylene-HF determined by fits to the values of and of the HF- and DF-containing isotopomers. standard deviations in the parameters are given in parentheses. (These standard deviations reflect only the statistical uncertainty in the least squares fit to the geometry and do not include the effects of model errors or zero-point averaging.)
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