Structural Analysis of Perfluoropropanoyl Fluoride in the Gas, Liquid, and Solid Phases

Article abstract of DOI:10.1021/acs.jpca.6b00424

The coexistence of two conformers in perfluoropropanoyl fluoride, CF₃CF₂C(O)F, differing in the CC-CF dihedral angle (gauche 85(10)% and anti 15(10)%), has been determined by means of gas-phase electron diffraction (GED). Quantum-chemical calculations performed at the MP2 and B3LYP approximations and cc-pVTZ basis sets reproduce the experimental values with confidence. By contrast, FTIR spectra give no clear evidence for the anti-conformer in the gas phase. Information on this less abundant but stable rotamer is obtained from matrix-isolation/FTIR spectroscopy and liquid Raman spectroscopy. In situ crystallization and single-crystal X-ray diffraction (XRD) data reveal the presence of solely the gauche-conformation in the solid state. A set of intermolecular interactions including C=O?C=O, C-F?F-C, and F?C=O is detected. The nature of bonding and the relative stabilities of gauche- and anti-conformers are explored using natural bond orbitals.

Full text of DOI:10.1021/acs.jpca.6b00424

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Article
Structural Analysis of Perfluoropropanoyl
Fluoride in the Gas, Liquid and Solid Phases
Yanina Berrueta Martínez, Christian G. Reuter, Yury V. Vishnevskiy, Yanina
Belén Bava, Andrea Lorena Picone, Rosana Mariel Romano, Hans-Georg
Stammler, Beate Neumann, Norbert W. Mitzel, and Carlos Omar Della Védova
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b00424 • Publication Date (Web): 29 Mar 2016
Downloaded from http://pubs.acs.org on March 31, 2016
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Structural Analysis of Perfluoropropanoyl Fluoride in the Gas,
Liquid and Solid Phases
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†
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⊥
†
Yanina Berrueta Martínez, Christian G. Reuter, Yury V. Vishnevskiy, Yanina B. Bava, Lorena A.
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⊥
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Picone, Rosana M. Romano, HansꢀGeorg Stammler, Beate Neumann, Norbert W. Mitzel, and
†
,*
Carlos O. Della Védova
†CEQUINOR (UNLP-CONICET), Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata,
7 esq. 115, 1900 La Plata, República Argentina.
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⊥
Lehrstuhl für Anorganische Chemie und Strukturchemie, Centre for Molecular Materials CM , Bielefeld University,
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Universitätsstraße 25, 33615 Bielefeld, Germany.
*
carlosdv@quimica.unlp.edu.ar
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ABSTRACT
The coexistence of two conformers in perfluoropropanoyl fluoride, CF CF C(O)F, with the CC−CF
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dihedral angle (gauche 85(10) % and anti 15(10) %) has been determined by means of gas phase
electron diffraction (GED). Quantumꢀchemical calculations performed at the MP2 and B3LYP
approximations and ccꢀpVTZ basis sets reproduce the experimental values with confidence. By
contrast, FTIR spectra give no clear evidence for the antiꢀconformer in the gas phase. Information on
this less abundant but stable rotamer was obtained from matrixꢀisolation/FTIR spectroscopy and liquid
Raman spectroscopy. In situ crystallization and single crystal Xꢀray diffraction (XRD) data reveal the
presence of solely the gaucheꢀconformation in the solid state. A set of intermolecular interactions
including C=O···C=O, C–F···F–C and F···C=O were detected. The nature of bonding and the relative
stabilities of gaucheꢀ and antiꢀconformers were explored using natural bond orbitals.
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KEY WORDS: Gas electron diffraction, Xꢀray diffraction, perfluoropropanoyl fluoride, gas infrared
spectrum, hydrolysis, matrix isolation, liquid Raman spectrum.
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INTRODUCTION
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A wide variety of scientific and technological applications require perfluorinated compounds due to
their singular and particular characteristics. Their high chemical and biological stability is conferred by
the nature of the carbonꢀfluorine bond. Thus, applications are related to their properties as water and
grease repelling coatings, standing textiles, adhesives, batteries, fuel cells, agrochemicals, and
pharmaceutical products. This long list which includes different areas confirms the strengths and
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versatility of these compounds. On the other hand, their release to the environment and the
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subsequently implied pollution transformed their study and use into a global concern issue. Some of
these compounds were dangerously found in human blood and many of them are spread all over the
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world due to their chemical persistence. They have been identified in Antartic as well as in Artic
regions.
Perfluoropronanoyl fluoride (PFPF), CF CF C(O)F, contains a short perfluorinated alkylꢀchain and a
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concomitant high vapor pressure. For this reason it has also been found in the atmosphere, and
consequently is there a demand for a detailed knowledge of these species being the title molecule a
good candidate. The high vapor pressure of the compound invites studying it in the gas phase by
spectroscopic and diffraction techniques. On the other side, the crystal structure of PFPF can reveal
information on the diversity of intermolecular interactions, which are of importance for crystal
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engineering and predominantly in the production of different drugs. Here we offer a detailed
characterization of this model molecule.
EXPERIMENTAL SECTION AND THEORETICAL METHODS
Synthesis
PFPF was synthesized by heating perfluoropropanoyl chloride with sodium fluoride in the presence of
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acetonitrile for 2 h at 70°C. In agreement with Vilenchik et al. an almost quantitative yield was
achieved. In an alternative method a mixture of SbCl (0.15 ml), SbF (3 g) and CF CF C(O)Cl (2.0 ml)
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in sulfolane (6.0 ml) as solvent was reacted at 100º C. In this case only 50 % yield was attained.
CF CF C(O)F is very difficult to separate from the CF CF C(O)Cl precursor since both have high
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vapor pressure and small amounts of the starting materials are always found in the products. In
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conclusion, the method reported by Vilenchik is simpler and more efficient in terms of purity and
yield.
Quantumꢀchemical calculations
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The Gaussian 03 program was used to perform quantumꢀchemical calculations, among with the
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calculation of the potential energy function about a specific dihedral angle is included, followed by the
geometry optimization of the corresponding minima and the calculation of their harmonic frequencies.
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For these purposes B3LYP and MP2 methods were chosen in conjunction with the ccꢀpVTZ basis
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set. NBO calculations were run with the NBO 5.G package incorporated in Gaussian 03. The
dimeric structure was also computed using Gaussian 03. In this case a potential energy curve and the
corresponding optimization of the minimum obtained were performed with the B3LYP/6ꢀ311+G(d)
level of approximation.
Analytical harmonic and the numeric cubic force fields for the gas electron diffraction (GED) analysis
have been computed with the B3LYP/ccꢀpVTZ approximation. Using these data meanꢀsquare
interatomic vibrational amplitudes and vibrational corrections to the equilibrium structure were
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calculated with the SHRINK program.
Infrared spectra
Infrared spectra (IR) were recorded with a Nicolet™ 6700 spectrometer using a double wall cell with
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00 mm optical path length and Si windows of 0.5 mm thickness. The resolution and the number of
–1
scans were 0.5 cm and 64, respectively.
Matrix isolation
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The gas mixture was deposited on a 15 K CsI window using the pulse deposition technique.
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low temperature was achieved by means of a Displex closedꢀcycle refrigerator (SHIꢀAPD Cryogenics,
model DEꢀ202). The corresponding FTIR spectra were acquired using the instrument described earlier.
A SpectraꢀPhysics HgꢀXe arc lamp operating at 1000 W was used to irradiate the matrix in the 200 –
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00 nm broad range. In order to avoid the matrix heating, a water filter was place between the lamp and
the matrix. Several spectra were recorded at different irradiation time.
Annealing experiments were performed by warming up to 25 K and then freezing down to the original
temperature of the matrix.
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Raman spectra
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The dispersive Raman spectra of the liquid were obtained by means of a HoribaꢀJobinꢀYvon T64000
Raman spectrometer, with a confocal microscope and CCD detection. A 514.5 nm excitation laser was
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used (Ar). The calibration was performed with the 459 cm band of CCl .
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UV/Vis spectra
A double beam UV−2600 Shimazdu spectrometer with a LoꢀRayꢀLight grade blazed holographic
grating monochromator and a Rꢀ928 photomultiplier detector were used to record the UV/Vis spectra.
The PFPF gas phase UV/Vis spectrum was recorded between 800 and 185 nm with 0.01 nm resolution.
X−Ray diffraction
A single crystal of PFPF was grown in situ in a capillary. This was achieved by first establishing a
solidꢀliquid equilibrium close to the melting point at 146 K, then melting all solid but a tiny crystal
seed (using a thin copper wire as external heat source) followed by slowly lowering the temperature
until the whole capillary was filled with a single crystalline specimen and afterward chilling to 100K
with 44 K per hour. The crystal was kept at 100.0(1) K during data collection on a Agilent SuperNova
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diffractometer at Bielefeld University. Using Olex2, the structure was solved by direct methods and
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refined using the SHELXꢀ97 package.
Gas electron diffraction
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The Balzers Eldigraph KD−G2 gasꢀphase electron diffractometer at Bielefeld University was used to
measure diffraction patterns on Fuji BAS MP 2025 imaging plates. The plates have been scanned on a
calibrated Fuji BASꢀ1800II scanner. Table S1 (S denotes material in the Supporting Information)
describes the experimental set up. After scanning the imaging plates, intensity curves (Figures S1 and
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S2) were obtained according to the method described by Vishnevskiy. In the same experiments
diffraction patterns of benzene have been measured and used for calibration of the sector function and
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electron wavelengths as usually. For refinements in UNEX v1.6b program molecular geometries
were defined in form of Zꢀmatrices and the independent geometrical parameters were refined in groups
(see Tables S2 – S3). Differences between parameters in groups have been taken from MP2/ccꢀpVTZ
calculations and were kept fixed during refinements. Experimental amplitudes were also refined in
groups (Tables S4 – S5), keeping ratios between different amplitudes in each group constant at the
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theoretical values. In the refinement of the title compound, only one GED model corresponding to the
mixture of both (gauche and anti, with respect to the CCꢀCF dihedral angle, respectively) conformers
was used. This model described experimental data very well (R = 4.65 %, which is close to the
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estimated experimental error) and the refined parameters were physically sound and consistent.
Therefore, there was no need to use a dynamic model in the present study. All correlations between
refined parameters were below 0.7.
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RESULTS AND DISCUSSION
Computational calculations
Conformational study of the monomer
In order to find out which conformations are expected to coexist in equilibrium in the gas phase at
room temperature, the potential energy function about the dihedral angle Φ(CC−CF) of
perfluoropropanoyl fluoride (PFPF), CF CF C(O)F, has been calculated using the approximations
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B3LYP/ccꢀpVTZ and MP2/ccꢀpVTZ (Figure 1). The scan calculated with the B3LYP level computes a
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lower energy barrier than the MP2, however, both methods exhibit very similar results.
Figure 1. Potential energy curve for PFPF with respect to the Φ(C4C3−C2F1) dihedral angle.
According to Figure 1, the absolute minimum corresponds to a gaucheꢀconformation with a dihedral
angle Φ(C4C3−C2F1) of 76.5° (MP2) and 73.1° (B3LYP). At 180° (MP2 and B3LYP) another
minimum appears, corresponding to an antiꢀconformation.
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These minima were then fully optimized and their harmonic vibrational frequencies were computed
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using the same levels of theory (Figure 2). The difference of the calculated thermal energies (ꢁE/kcal
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mol ) and Gibbs free energies (ꢁG/kcal mol ) of the second conformation (anti) against the gaucheꢀ
conformer are detailed in Table 1. The populations ( ) of each conformation were estimated by using
χ
the Boltzmann distribution at 298 K considering that the gaucheꢀconformer is doubly degenerated by
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symmetry considerations.
Figure 2. Optimized conformations of gauche (left) and anti (right) PFPF.
Table 1. Calculated anti–gauche thermal energy difference ꢁE and Gibbs free energy ꢁG. The fraction of population χ for each
conformer was calculated at 298 K
MP2/ccꢀpVTZ B3LYP/ccꢀpVTZ
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ꢁ
E(kcal mol )
0.23
0.49
18.0
0.28
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18.6
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ꢁG(kcal mol )
χ(anti) (%)
The used approximations reproduced the gaucheꢀform as the most stable conformer. This is
remarkable, especially taking into account that for 3,3,3ꢀtrifluoropropionyl fluoride, CF CH C(O)F, the
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antiꢀconformation has been reported to be the most stable form. In order to rationalize this singular
behavior quantumꢀchemical calculations at the B3LYP/6ꢀ31G level of approximation have been also
performed for both CF CF C(O)F (PFPF) and CF CH C(O)F computing the results obtained by
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removal of electron delocalization, i.e., by deleting all nonꢀlocalized NBOs (anti bonding molecular
orbitals) from the basis set in the NBO calculations. The full, localized (L), and nonꢀlocalized (NL)
electronic energetic contribution differences (antiꢀgauche) are listed in Table 2 for these compounds.
The data show that the delocalized contributions are more important than the localized ones for PFPF.
This can be understood in terms of a stabilization of the gaucheꢀconformation due to electron
delocalization from the bonding to the antibonding molecular orbitals. For CF CH C(O)F, although the
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NL contribution favors the gaucheꢀconformer, the localized factor exceeds the delocalized
contribution, stabilizing the antiꢀconformation.
Table 2. Antiꢀgauche full localized (L) and delocalized (NL) energy contribution differences for both CF CF C(O)F and CF CH C(O)F
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molecules (kcal mol )
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CF CF C(O)F CF CH C(O)F
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–0.59
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–0.15
As can be seen from the Figure 2, the bonds C3−F6 and C3−F7 are oriented in a different way with
respect to both the C2=O5 and C2−F1 groups in both the most stable gauche and antiꢀconformations.
In the antiꢀconformation both C3−F6 and C3−F7 bonds are in different planes from the C2=O5 and
C2−F1 groups, while in the gaucheꢀconformer C3−F6 and C3−F7 are almost coplanar with the C2=O5
and C2−F1 groups, respectively. This relative arrangement causes the stabilization of the gaucheꢀ over
the antiꢀconformation. Thus, a stabilizing interaction between the π(C2=O5) and the σ*(C3−F6)
molecular orbital is only allowed in the gaucheꢀconformation.
In order to quantify these electron delocalization energies in the PFPF molecule, quantumꢀchemical
calculations (B3LYP/6ꢀ311+G(d)) have been performed where specific donorꢀacceptor interactions
were removed. The energy of a concrete donorꢀacceptor interaction was computed by the subtraction of
the energy obtained with specific removed interactions from the full electronic energy. These energies
are listed in Table 3. As can be seen from this table, the contribution from the π(C2=O5)→σ*(C3−F6)
interaction for the gauche form is one order of magnitude higher than the rest of the interactions and
responsible for the stabilization of the gaucheꢀ over the antiꢀconformation.
Table 3. Specific electron delocalization energies of PFPF
–
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Molecular Orbitals
Donor Acceptor
ꢁE (kcal mol )
gauche
anti
σ(C2−F1) σ*(C3−F6) –0.061
σ(C2−F1) σ*(C3−F7) –0.049
–0.067
–0.047
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π(C2=O5) σ*(C3−F6) –0.302
π(C2=O5) σ*(C3−F7)
–0.068
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The experimental gas UV/vis spectrum of PFPF presents only one maximum located at 214.0 nm.
Quantumꢀchemical calculations (TD − B3LYP/6ꢀ311+G(d)) reproduce the maxima at 222 and 221 nm
fairly well for the antiꢀ and gaucheꢀconformers, respectively. According to these calculations, the
transition σ(C=O)→π*(C=O) causes these bands. Moreover, another transition (LP(O)→π*(C=O))
occurs simultaneously at 221 nm for the gaucheꢀconformer. In the case of the antiꢀform the former
transition takes place at 165 nm. Figure 3 compares the experimental and computed gas UV/vis
spectrum for PFPF.
Figure 3. Gas and computed (TD − B3LYP/6ꢀ311+G(d)) UV/vis spectra for PFPF. The theoretical spectrum was
calculated using a composition of 81 % gauche and 19 % anti.
Infrared spectrum
The optimized structures of both gaucheꢀ and antiꢀconformations were used to calculate the IR
harmonic frequencies. These frequencies were corrected by the reported scale factors: 0.949 (MP2/ccꢀ
2
8
pVTZ) and 0.965 (B3LYP/ccꢀpVTZ). The computed spectra calculated for a mixture of both
conformations show a very good agreement with the experimental one (Figure S3). This comparison
allowed us proposing an assignment of all normal modes of vibration. Figure 4 illustrates the
experimental gas FTIR and its comparison with the computed spectra for both antiꢀ and gaucheꢀ
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conformers. These Figures, in particular Figure 4 b, show only few features – the broadening of the
–
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bands at 1345, 1236 and 1013 cm – that possibly indicate of the presence of an antiꢀconformer.
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vibration mode due to its characteristic intensity and location.
On the other hand, the second most
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intense band in this spectrum corresponds to the carbonyl stretching normal mode of vibration (1889
–
1
cm ). A complete assignment of the infrared spectrum is presented in Table S6.
Figure 4. Gas FTIR spectrum of PFPF (top) and computed IR spectra for the antiꢀconformer (middle) and gaucheꢀ
conformer (down). The relative computed weighted composition of these two forms is taken into account in the
–1
–1
absorbances: complete spectra (a), sections between 1400 and 900 cm (b) and between 800 and 500 cm (c).
–
1
The signal at 694 cm in the gas FTIR spectrum of PFPF presents a defined “A” band contour (see
Figure 4 c). The oscillation of the FCO scissorꢀdeformation vibration of the most abundant conformer
with respect to the A principal axis of inertia gives rise to this band with the corresponding contour
(Figure S4).
Matrix isolated CF CF C(O)F in Ar (1:300)
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A gas mixture of PFPF in Ar (1:300) was trapped onto a CsI window at a temperature of ca. 15 K and
the FTIR spectrum of this solidified mixture was obtained (Figure 5). The band assignments of both
antiꢀ and gaucheꢀconformers could be straightforwardly performed from the FTIR matrixꢀisolation
spectrum shown in Figure 5 where the small population of the antiꢀconformer is clearly observed.
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Figure 5. Gas and Arꢀmatrix (1:300) FTIR of PFPF. Complete spectra (a) and 1390 – 1325 cm (b) and 1320 – 1250 cm
1
regions (c). Black circles indicate the contribution of the antiꢀPFPF conformer.
Annealing
After performing one annealing step of 10 K the concentrations of the antiꢀ and the most populated
gaucheꢀconformers were found to be smaller, while simultaneously the formation of a dimer species is
observed.
Structural study of dimeric PFPF
In order to obtain the dimeric structure, the length of the ROꢂꢂꢂC(O)R intermolecular interaction
obtained from XRD data (see XRD section below) was first optimized in an energy curve at B3LYP/6ꢀ
3
11+G(d) level of approximation. The minimum was subsequently optimized using the same
theoretical method and its harmonic vibrational frequencies were then computed. The structure
corresponds to a doubleꢀgauche dimer with a O···C=O length and angle of 3.04 Å and 90.3°,
respectively (see Figure 6).
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Figure 6. Left: Quantumꢀchemically optimized dimer of PFPF. Right: Relative energy of the PFPF dimer (kcal mol )
with respect to the O(1)ꢂꢂꢂC=O(2) length.
The small contribution of the antiꢀconformer under these experimental conditions precludes further
analysis related to this species. FTIR spectra of the diluted PFPF in Ar at cryogenic temperatures
before and after an annealing of 25 K are depicted in Figure 7. The corresponding assignment was
performed with the assistance of computed values. Table S6 shows the assignment of all bands
observed in the matrix spectra.
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Figure 7. Comparison of the matrix FTIR spectra of PFPF diluted in Ar 1:300 before (15 K) and after annealing at 25 K.
Matrix isolated CF CF C(O)F in Ar (1:1000)
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UVꢀVis broad band irradiation
A sample of PFPF diluted 1:1000 with argon at cryogenic temperatures was exposed to UVꢀvis broad
band irradiation (200 ≤ λ ≤ 800 nm). FTIR spectra were taken at different irradiation times (t), i.e.
without irradiation and at 0.25, 1, 4, 8, 16, 32 and 64 min. The effect of the irradiation is to originate
the rupture of the dimer structures linked by van der Waals forces with a subsequent increase in the
concentration of the monomeric units. The evolution of the relative areas of selected bands belonging
to both, dimer and gaucheꢀspecies, is shown in Figure 8. No further photoevolution was observed in the
matrix.
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The situation is completely different when PFPF is irradiated in the gaseous state. Cariati et al. reported
that PFPF irradiated in the 232 − 240 nm region undergoes a photoevolution generating CO, C F and
4
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COF according to Scheme 1. This process is not allowed in matrix conditions since it needs the
2
coincidence of two molecules of monomeric PFPF.
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Scheme 1: Reported photochemical evolution of gaseous PFPF.
2
CF CF C(O)F + hυ → CO + C F + OCF
3 2 4 10 2
Figure 8. Relative areas (%) of selected FTIR bands of the dilution of PFPF in Ar (1:1000) at cryogenic temperatures. On the left and right side
of the figure the photoevolution of bands corresponding to the gaucheꢀconformers and the dimer are shown, respectively.
Annealing after photolysis
The FTIR spectra of the diluted PFPF in Ar (1:1000) at cryogenic temperatures taken before and after
an annealing at 25 K are depicted in Figure 9. The assignment was again performed with the assistance
of computed values. Table S6 lists the assignment of all bands found in the matrix spectrum. This
diluted matrix shows a relative diminution of the dimer species in comparison with the conformational
equilibrium of the free forms.
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Figure 9. Different sections of Arꢀmatrix (1:1000) FTIR spectra of PFPF before and after annealing. a) Between
ꢀ
1
ꢀ1
ꢀ1
1910 and 1850 cm , b) between 1380 and 1330 cm and c) between 1315 and 1260 cm .
The diffusion of the species near the boiling point of the Ar allows the regeneration of the dimer even
at higher concentrations than the originally deposited from the mixture at room temperature.
Liquid phase Raman spectrum
Figure 10 presents the comparison between the liquid phase Raman spectrum of PFPF and its
computed spectrum (MP2/ccꢀpVTZ) of a 18 % anti and 82 % gauche weighted conformer composition.
This figure provides another clear evidence of the contribution of the antiꢀrotamer through the band at
–1
3
29 cm in agreement with the proposed assignment on Table S6.
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Figure 10. Left: Comparison between the liquid Raman spectrum of PFPF and the computed (MP2/ccꢀPVTZ) spectrum.
Right: Comparison between the computed spectra for the antiꢀ and gaucheꢀconformers in the region between 700 and
–1
1
00 cm with the experimental liquid phase Raman spectrum. The arrow in the experimental spectrum indicates the
contributions of the antiꢀrotamer.
Gas Phase Structure
Electron scattering intensities for gaseous PFPF have been recorded using the electron diffractometer at
Bielefeld University. Computed geometries (MP2/ccꢀpVTZ) of the two stable conformations, gauche
and anti were used as starting geometries for the leastꢀsquare refinements of the structural parameters
of PFPF by GED. The radial distribution functions are presented in Figure 11. The quality of fit of the
twoꢀconformer model can be assessed by the flatness of the difference curve and by the final Rꢀfactor
of 4.65 %. The GED experiment reveals a conformational composition of 85(10) % gauche and
1
5(10) % anti (three times standard deviations are in parentheses). According to the Hamilton
3
2
criterion the relative amount of antiꢀconformer is in the range between 10 and 20% at a significance
level of 99% (a plot is provided in the supplementary information: Figure S5). The previously
described calculations are qualitatively consistent with the GED experiment. Relevant geometric
parameters obtained by XRD, GED and theoretical calculations are listed and compared in Table 4.
The refined in GED Cartesian coordinates of both gaucheꢀ and antiꢀconformers are listed in Tables S8
and S9, respectively. Refined interatomic distances and vibrational amplitudes are presented in Tables
S4 and S5, respectively.
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Figure 11. Experimental (dots) and model (continuous line) radial distribution functions for PFPF. Vertical lines
correspond to interatomic distances in both conformers; most important are assigned here; for the exact numeration and
the full list of terms see Tables S4 and S5.
Crystal Structure
The crystal structure of PFPF (Figure 12) contains only molecules in the gaucheꢀconformation. Their
structure resembles that described for the most stable form in the gas phase.
Crystals of PFPF belong to the space group Pbca (no. 61) with metrics a = 9.0597(7) Å, b = 6.1421(5)
3
Å, c = 18.7284(13) Å. V = 1042.16(13) Å , Z = 8, ꢁ(MoꢀKα) = 0.289, 28654 reflections measured,
1
028 unique (Rint = 0.0390) which were used in all calculations. The final wR was 0.1014 for all data
2
and R was 0.0390 for 994 reflections with I >3σ(I). Structural parameters are shown in Table 4. All
1
crystallographic data obtained from the structural analysis and refinement of PFPF are provided in
3
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Table S7.
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Figure 12. PFPF crystal structure (gaucheꢀconformation).
In general, the structural parameters obtained with the different methods are comparable. As expected
in view of the different phases involved, values of the dihedral angles present a noticeable dependence
on the technique, especially those regarding to the gaucheꢀconformation, where differences are found
for the solid and gas structure. This can be explained in terms of the intermolecular interactions present
in the solid.
a
Table 4. Experimental GED, XRD and theoretical structural parameters of gauche, and antiꢀPFPF
Parameter
gauche
anti
GED
XRD
MP2
B3LYP
GED
MP2
B3LYP
r
g
re, ∠
rα, ∠
re, ∠
re, ∠e
1.175
1.338
1.339
1.546
1.555
113.2
123.8
126.1
110.1
109.2
109.0
73.1
r
g
re, ∠
re, ∠e
re, ∠e
1.176
1.338
1.340
1.542
1.552
113.7
123.5
127.4
109.1
109.1
109.0
180.0
e
α
e
e
C=O
C(O)ꢀF
CꢀF)av
1.183(5)
1.332(1)
1.331(1)
1.548(3)
1.556(3)
1.178(5)
1.324(1)
1.324(1)
1.536(3)
1.542(3)
112.4(9)
124.9(8)
124.0(21)
111.1(22)
109.1(1)
109.3(4)
76.2(62)
1.166(2)
1.324(2)
1.328(3)
1.535(3)
1.531(3)
111.8(2)
123.1(2)
125.8(2)
111.1(2)
109.7(2)
108.9(2)
86.8(2)
1.183
1.333
1.333
1.536
1.542
112.4
124.0
125.9
110.0
109.2
109.2
75.6
1.184(5)
1.332(1)
1.332(1)
1.546(3)
1.553(3)
1.180(5)
1.325(1)
1.325(1)
1.533(3)
1.540(3)
113.2(9)
124.7(8)
125.4(21)
110.0(22)
109.1(1)
109.2(4)
1.185
1.333
1.334
1.533
1.539
113.2
123.8
127.3
108.9
109.1
109.1
180.0
(
C(O)ꢀC
CꢀC
C(O)ꢀCꢀC
FꢀC=O
CꢀC=O
CꢀC(O)ꢀF
(
CꢀCꢀF)av
FꢀCꢀF)av
(
b
FꢀC(O)ꢀCꢀC
180.0
a
Bond length are
Å, angles in degrees. In parentheses are three times standard deviations obtained from least squares analysis. See
Supporting Information for details about refinement of structures in GED. MP2 and B3LYP calculations were done with ccꢀpVTZ basis
set to compute both the analytical harmonic and the numeric cubic force fields.
b
Assumed parameter.
Intermolecular assembly in solid state
Six intermolecular contacts with lengths shorter than the sum of the van der Waals radii (vdW) of the
involved atoms were detected from the XRD experimental data. All of them are listed in Table 5 along
with their corresponding length, the difference between their length and the sum of the involved van
3
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der Waals radii, rvdW
.
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Table 5. List of intermolecular contacts
a
Atoms involved Length
Length−rvdW
–0.417
a
O5ꢂꢂꢂC2
2.803(2)
2.848(2)
2.886(2)
3.171(3)
3.129(2)
2.970(2)
b
F8ꢂꢂꢂF6
F7ꢂꢂꢂF9
–0.092
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c
–0.054
a
O5ꢂꢂꢂC3
–0.049
d
F1ꢂꢂꢂC2
–0.041
a
O5ꢂꢂꢂF7
–0.020
a 3/2–X,ꢀ1/2+Y,+Z; b 1/2+X,1/2–Y,1–Z; c +X,–1+Y,+Z; d 2–X,–1/2+Y,3/2–Z
The secondꢀorder perturbation stabilization energy E(2) which relates the charge transfer within donor
and acceptor orbitals were computed for adjacent molecules at the NBO B3LYP/6ꢀ311+G(d) level of
approximation. In order to perform this calculation a system centered in one molecule coordinated by
35
eight molecules as in the crystal phase was used.
2
Table 6 lists these interactions together with the second order perturbation energy (ꢁE ) and the
donor/acceptor character of the corresponding orbitals that participate in these contacts among other
parameters.
Table 6. List of intermolecular contacts in the crystal structure of PFPF. The direction of the arrow is defined by the donor/acceptor
character of the orbital involved
2
–1
Atoms involved ꢁE /kcal mol
Donor orbital Acceptor orbital
O5 →
F8 ↔
F7 →
O5 →
F1 →
C2
F6
F9
C3
C2
0.88
0.15
0.08
0.05
0.07
LP(1)O
LP(1)F8/6
LP(3)F
π*(O=C)
RY*F6/8
RY*F
LP(2)O
LP(3)F
σ*(C3−C4)
π*(O=C)
O···C interactions
Two different OꢂꢂꢂC interactions were reported from the XRD analysis of PFPF crystal structure. The
NBO analysis demonstrates a high contribution of electron delocalization between one oxygen lone
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pair (LP) and the π*(O5=C2) molecular orbitals, and between the second oxygen lone pair and the
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antibonding σ*(C3−C4) molecular orbital. The second order perturbation energies (ꢁE ) involved in
–1
both cases are 0.88 and 0.05 kcal mol , respectively.
This OꢂꢂꢂC=O intermolecular interaction is the basis for the dimer formation of the substance evidenced
in the matrix experiment and can be further described in terms of geometric parameters. For PFPF the
perpendicular interaction within the carbonyl groups is preferred over the parallel, where the CꢂꢂꢂO=C
angle is 163.3(2)°, a value that perfectly fits in the 150−180° range found in the literature for these
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orthogonal interactions. Defining a mean plane by C3, O5 and F1 the vector pointing from O5 of the
neighbor molecule to C2 shows a nearly perpendicular angle of 83.7(1)° to this plane (Figure 13). In
addition, the PFPF carbonyl carbon C2 gets 0.013(2) Å out of this plane due to its interaction with the
nucleophilic oxygen atom O5 from the neighbor molecule.
Figure 13. Plane formed by F1–C3 and O5.
C–F···F–C interactions
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Attractive FꢂꢂꢂF interactions have been the subject of controversial discussions. In 1989 Desiraju et al.
questioned their nature in terms of halogenꢂꢂꢂhalogen interactions, concluding that no stabilization was
generated by them in close packing. Later on, Bianchi, Forni and Pilati, determined an experimental
charge density demonstrating that FꢂꢂꢂF intermolecular interactions clearly contribute to the stabilization
of crystal packing in the donor acceptor complex of (E)ꢀ1,2ꢀbis(4ꢀpyridyl)ethylene with 1,4ꢀ
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diiodotetrafluorobenzene. Furthermore, Matta et al. proved that some intramolecular FꢂꢂꢂF contacts
present in crystals of aromatic compounds behave as weak closedꢀshell interactions with stabilization
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energies up to 14 kcal mol . Intramolecular FꢂꢂꢂF contacts were also described in a charge density
study of 2,3,5,6ꢀtetrafluoroꢀ and pentafluoroꢀpyridine; in both molecules there are weak F⋅⋅⋅F and F⋅⋅⋅C
type interactions, but they become less important for determining the intermolecular arrangement,
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when the stronger F⋅⋅⋅H type interactions are also present, like in 2,3,5,6ꢀtetrafluoroꢀpyridine.
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remarkable fact about FꢂꢂꢂF interactions became evident in the crystal structure of haloperidol, an
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antipsychotic drug, where these interactions are preferred over the more common ClꢂꢂꢂCl halogen
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contacts. Chopra and coworkers have recently expressed that, despite the fact that there is evidence for
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many FꢂꢂꢂF intermolecular interactions, the exact nature of these interactions remains still unclear.
In the present case of PFPF, two FꢂꢂꢂF contacts have been detected in its crystal structure. According to
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Reichenbächer a total of 788 compounds with FꢂꢂꢂF distances below 3 Å could be found in the
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Cambridge Structural Database (CSD), by the year 2005. As can be seen from Table 5 and Figure 14,
both FꢂꢂꢂF lengths present in the PFPF crystal resemble the reported values. An angle analysis of these
intermolecular interactions allows complementing the structural characterization. In this regard,
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Desiraju et al. classified halogen interactions into type I when θ ≈ θ and type II if θ ≈ 180° and θ ≈
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0°, where θ are the angles involving the FꢂꢂꢂF interaction. Table 7 presents these interactions in
conjunction with the respective angles values (Figure 14) and allows the conclusion that both FꢂꢂꢂF
interactions in solid PFPF belong to type I.
Table 7. θ and θ values of PFPF interꢀfluorine intermolecular contacts
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Involved atoms
F6ꢂꢂꢂF8
Angle
Interaction type
C3−F6ꢂꢂꢂF8 θ1 167.2(1)°
F6ꢂꢂꢂF8−C4 θ2 138.2(1)°
C3−F7ꢂꢂꢂF9 θ1 157.5(1)°
F7ꢂꢂꢂF9−C4 θ2 155.3(1)°
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F7ꢂꢂꢂF9
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Figure 14. Schemes of F6ꢂꢂꢂF8 (left) and F7ꢂꢂꢂF9 (right) intermolecular interactions.
In most cases, type I interactions involve a crystallographic inversion center. The type I CFꢂꢂꢂFC
interaction in PFPF is an exception to this generalization since the involved fluorine atoms are not
equivalent. The stabilizing delocalization energy obtained by NBO calculations (see Table 6),
corroborates that both fluorine atoms (F6 and F8) involved in a FꢂꢂꢂF interaction act as electron donor
and acceptor at the same time, being a lone pair orbital and a Rydberg orbital the donor and the
acceptor orbitals, respectively. On the other hand, F7 and F9 participate in a polarized FꢂꢂꢂF contact,
being F7 an electron donor to F9 an electron acceptor.
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C−F···C=O interactions
The contact between the fluorine and carbon atoms is, according to the NBO analysis, caused by the
electron delocalization from one fluorine lone pair and the πꢀcarbonyl antibonding molecular orbital
(C−F···C=O). The first study of intermolecular interactions involving organic fluorine and carbonyl
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groups has been reported by Olsen and coꢀworkers. They performed an exhaustive search of the
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Cambridge Structural Database (CSD), finding 43 close C−F···C=O contacts shorter than the sum of
the C and F van der Waals radii out of a total of 573 interactions of this kind. According to these
statistics ran over the 573 contacts, the intermolecular C−F···C=O length (d ) covers a range from 2.5 to
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.0 Å, a C···F−C angle (α1) a range between 100 and 160º and the fluorine atom involved in the
mentioned intermolecular contact tends to be in a position nearby the pseudotrigonal axis of the C=O
unit. Our results for PFPF are included in the precedent reported ranges (see Figure 15).
Figure 15. C−FꢂꢂꢂC=O intermolecular contact between two PFPF molecules. The bond length d (C−FꢂꢂꢂC=O) = 3.129(2) Å, the dihedral
1
angle Φ(C−FꢂꢂꢂC=O) = 114.8(2)º, the angles α1(CꢂꢂꢂF−C) = 146.5(1)º and α2(O=CꢂꢂꢂF) = 75.8(1)º are highlighted
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O···F interactions
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According to the XRD data (Table 5), the OꢂꢂꢂF interaction appears to be negligible when it is
compared with the previously described types of contact. Moreover, such interactions do not seem to
be relevant in the NBO calculation, that’s why they are not listed in Table 6.
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In 1983 MurrayꢀRust et al. studied the crystallographic environment of different fluorinated carboxylic
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acids, dividing the intermolecular interactions involving C−F groups into three categories. The
second of those categories, C−FꢂꢂꢂO and C−FꢂꢂꢂN, is of particular interest in this section. The PFPF
C−FꢂꢂꢂO intermolecular contact (2.97 Å) fits in the 2.85 Å < d (C−FꢂꢂꢂO) < 3.2 Å interval found by
running a statistics over 71 C−FꢂꢂꢂO interactions.
Conclusion
We prepared and characterized perfluoropropanoyl fluoride (PFPF) using vibrational spectroscopy in
different phases. The assignment of the vibrational normal modes was based on quantumꢀchemical
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calculations. Matrix isolation coupled to IR spectroscopy allowed resolving the contributions of the
antiꢀ and gaucheꢀconformations, and in addition of a dimeric species formed by gaucheꢀrotamers. The
effect of increasing the concentration of PFPF under matrix conditions results in an increased
concentration of dimer, while heating up to 10 ºC (by means of an annealing) induced cleavage of the
dimers into gaucheꢀmonomers.
The structure of the title compound has been also explored in both, crystal and gas phase, by XRD and
GED, respectively. The crystal structure manifests different types of intermolecular interactions; they
were analyzed in terms of geometry and type and strength of overlap by supporting NBO calculations.
The structure of CF F C(O)F in the gas phase allowed studying the geometry without any
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intermolecular distortions; the conclusion is that the intermolecular interactions in the crystal are too
weak to cause substantial distortion of the molecular structure in this case.
While the gaucheꢀconformation is the sole form present in the crystalline state, the GED data indicate
that both conformations, gauche and anti, are present in equilibrium in the gaseous phase. This is
reproduced by quantumꢀchemical calculations for single molecules. The gaucheꢀconformer represents
the absolute minimum in energy, and due to symmetry degeneration, is far more abundant in the gas.
This is not the case reported for the less fluorinated CF CH C(O)F for which the antiꢀ was determined
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to be more stable than the gaucheꢀconformer. As a result, a comparison of these two compounds and
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their possible conformations was undertaken. NBO calculations indicate that in the case of
CF CH C(O)F steric effects predominate, that stabilize the antiꢀconformer, but in CF CF C(O)F with
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dominating gaucheꢀconformation, delocalization effects, especially the interaction π(C2=O5)→σ*(C3–
F6), are more important.
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ACKNOWLEDGEMENT
The authors thank Deutsche Forschungsgemeinschaft (scholarship for Y. B. M. in Germany and core
facility GED@BI), Alexander von Humoldt Siftung (stipend for Yu. V. V.), Agencia Nacional de
Promoción Científica y Técnica (ANPCyT), Consejo Nacional de Investigaciones Científicas y
Técnicas (CONICET), Comisión de Investigaciones de la Provincia de Buenos Aires (CIC), Facultad
de Ciencias Exactas, Universidad Nacional de La Plata (UNLP) and Bielefeld University for financial
support.
Supporting information available:
Total intensity curves and additive background lines of PFPF for short and long nozzleꢀtoꢀplate
distances (Figure S1), experimental and model molecular intensity curves of PFPF for short and long
nozzleꢀtoꢀplate distances (Figure S2), experimental and theoretical B3LYP/ccꢀpVTZ and MP2/ccꢀ
pVTZ IR spectra of PFPF (Figure S3), principal axes of inertia of gaucheꢀPFPF and the derivative
dipole unit vector corresponding to the δsciss(COF) vibrational normal mode (Figure S4) and
dependence of the GED total Rꢀfactor on the relative amount of anti conformer (Figure S5). Set up of
the GED experiments (Table S1), Zꢀmatrices (Tables S2–S3), best interatomic distances, mean square
amplitudes and vibrational corrections (Tables S4–S5), comparison between experimental and
theoretical vibrational bands corrected with the corresponding scale factors and the corresponding
proposed assignment (Table S6), crystallographic data (Table S7), refined Cartesian coordinates of
gauche and antiꢀCF CF C(O)F in GED (Table S8 and S9).
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