11800
J. Phys. Chem. B 2006, 110, 11800-11808
EPR and Solid-State NMR Studies of Poly(dicarbon monofluoride) (C2F)n Marc Dubois,*,† Je´ roˆ me Giraudet,†,‡ Katia Gue´ rin,† Andre´ Hamwi,† Ziad Fawal,§ Pascal Pirotte,‡ and Francis Masin‡ Laboratoire des Mate´ riaux Inorganiques, UMR CNRS 6002-UniVersite´ Blaise Pascal, 24 AVenue des Landais, 63177 Aubie` re Cedex, France, Matie` re Condense´ e et Re´ sonance Magne´ tique, UniVersite´ Libre de Bruxelles, CP 232, BouleVard du Triomphe, B-1050 Bruxelles, Belgium, and Faculte´ des Sciences III, UniVersite´ Libanaise, Tripoli, Lebanon ReceiVed: March 1, 2006; In Final Form: April 27, 2006
Poly(dicarbon monofluoride) (C2F)n was studied by electron paramagnetic resonance (EPR) and solid-state nuclear magnetic resonance (NMR). The effects of physisorbed oxygen on the EPR and NMR relaxation were underlined and extrapolated to poly(carbon monofluoride) (CF)n and semi-covalent graphite fluoride prepared at room temperature. Physisorbed oxygen molecules are shown to be an important mechanism of both electronic and nuclear relaxations, resulting in apparent spin-lattice relaxation time and line width during NMR and EPR measurements, respectively. The effect of paramagnetic centers on the 19F spin-lattice relaxation was underlined in accordance with the high electron spin density determined by EPR. 19F magic angle spinning (MAS) NMR, 13C MAS NMR, and 13C MAS NMR with 19F to 13C cross polarization (CP) underline the presence of two types of carbon atoms, both sp3 hybridized: some covalently bonded to fluorine and the others linked exclusively to carbon atoms. Finally, a C-F bond length of 0.138 ( 0.002 nm has been determined thanks to the re-introduction of dipolar coupling using cross polarization.
1. Introduction Direct fluorination of graphite at high temperature results in the formation of poly(carbon monofluoride) (CF)n and poly(dicarbon monofluoride) (C2F)n.1 The latter substance was first synthesized in 1979 by Kita et al.2 Because of their many industrial applications such as electrode material for primary lithium batteries,3,4 solid lubricants,5,6 or as reservoir for very active molecular oxidizers such as BrF3 and ClF3,7 these material have been extensively studied. They are insulators and exhibit purely covalent C-F bonds. As the carbon hybridization is sp3, the fluorocarbon layers are corrugated, consisting in trans-linked cyclohexane chairs. Contrary to (CF)n in which each carbon atom is covalently bonded to a fluorine atom, in (C2F)n, pairs of buckled sheets are present, and only half of the carbon atoms are fluorinated. Every pair of the adjacent carbon sheets are bonded to each other by covalent C-C bonds. From a neutron diffraction study, Sato et al.8 proposed for (C2F)n an AB rather than an AA′-type structural model. From these data, the C-F bond length in both (C2F)n and (CF)n was determined to be 0.136 nm; this value slightly differs from the previous values determined for (CF)n: 0.136-0.137 nm,9 0.135 nm,10 and 0.141-0.145 nm11 from theoretical data and 0.140 nm,12 which was experimentally determined. This parameter can also be estimated by NMR because the C-F bond length is included in the expression of the dipolar interaction, which can be determined using techniques such as rotational echo double resonance (REDOR)13 and transferred echo double resonance (TEDOR).14,15 This information is lost when Hartmann-Hahn cross polarization (CP) associated to MAS is used. In this study, * Corresponding author. Tel: +33 4 73 40 71 05. Fax: +33 4 73 40 71 08. E-mail:
[email protected]. † UMR CNRS 6002-Universite ´ Blaise Pascal. ‡ Universite ´ Libre de Bruxelles. § Universite ´ Libanaise.
we re-introduce the dipolar couplings into the spectrum thanks to the Inverse Cross Polarization (ICP) sequence allowing the C-F bond length to be determined. As a matter of fact, with particular conditions, which will be detailed in the text, the amplitude of the CP signal at short contact times is expected to be oscillatory with a frequency linked to the C-F bond length.16 In our previous study17 the analysis of the T1 19F spin-lattice relaxation time of poly(carbon monofluoride) (CF)n has shown that the presence of atmospheric paramagnetic oxygen, physisorbed on the surface, is much more important for nuclei relaxation than the paramagnetic centers (PC) present within the sample. As a matter of fact, the amount of these PC is not sufficient to significantly govern the relaxation mechanisms. In this work, various NMR characterizations (19F MAS, 13C MAS, and 13C CP MAS) of (C2F)n are described. Spin-lattice relaxation mechanisms are studied in order to investigate the effects of atmospheric oxygen and of paramagnetic centers (PC). The density of such species has been determined by electron paramagnetic resonance (EPR) giving supplementary information about the nature and density of PC. 2. Experimental Section Poly(dicarbon monofluoride) (C2F)n was synthesized using the conventional method. A monel boat containing carbon was placed in a nickel reactor and heated to 350 and 380 °C under pure F2 gas flow for (C2F)n-350 and (C2F)n-380, respectively. The starting carbon material was Madagascar natural graphite powder (grain size of 7.5 µm). The fluorination level was calculated by weight uptake and the formula of our samples checked to be CF0.51 for (C2F)n-350 and CF0.60 for (C2F)n-380. The (CF)n sample was prepared using petroleum coke heated at 600 °C under pure fluorine gas. X-ray diffraction powder (XRD) patterns were obtained using a diffractometer Siemens D501 with a Cu(KR) radiation. EPR
10.1021/jp061291m CCC: $33.50 © 2006 American Chemical Society Published on Web 05/26/2006
EPR and NMR Studies of (C2F)n
J. Phys. Chem. B, Vol. 110, No. 24, 2006 11801 TABLE 1: Parameters of the EPR Spectra ∆Hpp (G)
(C2F)n-350 (C2F)n-380 (C2F)n-380V (C2F)n-380Vair (CF)n (CF)n-V
line 1 (5G
line 2 ( 0.2 G
S1/S2
Ds (spins‚g-1)
60 80 50 50 unresolved SHFS 80 + SHFSa unresolved SHFS 80 + SHFSa
11.1 13.5 12.4 16.5 20.9
1.66 1.65 0.23 1.15 11.6b
2.1 1020 1.7 1020 1.3 1020 2.1 1020 0.1 1020
18.8
6.2b
0.1 1020
a Superhyperfine structure (SHFS) with (2nI + 1) ) 7 lines where n ) 6 is the number of neighboring fluorine nuclei (nuclear spin number I ) 1/2) (coupling constant A ) 45 ( 2 G, line width ∆Hpp ) 36 G ( 2 G).17 b S1 ) SSHFS + Sunresolved SHFS.
Figure 1. X-ray diffractograms of (C2F)n synthesized at 350 and 380 °C and of (CF)n.
spectra were performed with a Bruker EMX digital X band (ν ) 9.653 GHz) spectrometer. Diphenylpicrylhydrazyl (DPPH) was used as a calibration reference to determine both the resonance frequency and the densities of spin carriers. NMR experiments were performed on a Tecmag Discovery spectrometer (working frequency for 1H, 13C, and 19F: 300.1, 75.4, and 282.2 MHz, respectively). Two NMR Bruker probes were used: a static and a special cross polarization/magic angle spinning probe with fluorine decoupling on a 4 mm rotor. The 19F-13C match was optimized on poly(tetrafluoroethylene) (PTFE) by adjusting the 13C power level to coincide with the previously determined 19F π/2 pulse width (4 µs). For the other spectra, a simple sequence (τ-acquisition) was used with a single π/2 pulse length of 3.5, 4, and 3.5 µs for 1H, 19F, and 13C, respectively. Spin-lattice relaxation time T1 was measured using a saturation recovery sequence. 1H and 13C chemical shifts were externally referenced to tetramethylsilane (TMS). 19F chemical shifts were referenced with respect to CFCl3. 3. Results and Discussion 3.1. X-ray Diffraction and EPR Characterization. Figure 1 shows X-ray diffraction patterns of the two (C2F)n samples synthesized at 350 and 380 °C. A diagram of (CF)n is added for comparison. The interlayer distance d is close to 0.86 nm and corresponds to the (001) diffraction line (2θ ≈ 10.3°) of (C2F)n. This value is in agreement with the structural model proposed by Watanabe et al.3 The second strong peak at 2θ ≈ 41.9° is assigned to the (100) reflection (d ≈ 0.21 nm). For both samples no intercalated phases are present as indicated by the absence of the expected line at 2θ ≈ 18.9°. Such phases have been observed during the first stage of formation of hightemperature graphite fluoride.18 In the case of (C2F)n-350, residual graphite is still present as shown by the presence of the (002) diffraction peak of the graphitic structure (2θ ≈ 26.6°). An increase of the fluorination temperature up to 380 °C is necessary to remove this residual phase; this treatment does not result in an improvement of crystalline order since the coherence lengths along the c- and a-axis, Lc and La, are not changed. These values, estimated using Scherrer’s equation from (001) and (100) reflections,19 are close to 7 and 11 nm for Lc and La, respectively. When the fluorination temperature is equal to 600 °C, the formation of (CF)n occurs resulting in a decrease of both the interlayer distance d001 ) 0.623 nm (2θ ≈ 14.2°) and of Lc (5 nm). The EPR spectra of (C2F)n-350, (C2F)n-380, and (CF)n are given in Figure 2. In these samples, in comparison with other
fluorinated carbons obtained in F2 atmosphere (starting from graphite treated at 600 °C,17,20 amorphous carbon thin film,21,22 or nanosized graphite fluorides23), carbon dangling bonds having a localized spin are the only possible spin carriers. As observed by NMR, the concentration of intercalated species is very low. So, the hypothesis of the presence of spin carriers formed through the charge transfer of the intercalated species toward the fluorographite layers in a classical intercalation model24-26 is not retained for the compounds studied here. Whatever the sample, the g-factor, which is typical of free radicals and localized structural defects, is close to 2.003 ( 0.002. Simulation of the spectra reveals two contributions for (C2F)n (Figure 2a and Table 1) and three for (CF)n (Figure 2b).17,20 These simulations are performed using WinSimfonia (Bruker software). For (C2F)n, by analogy with (CF)n, the broad signal (line 1) could be first assigned to the unresolved hyperfine interaction between dangling electron bonds and the neighboring fluorine nuclei, thus yielding a superhyperfine structure (SHFS) (Table. 1). Contrary to (CF)n, for (C2F)n the resolved SHFS is not visible; this is explained by the structural disorder: the environment of the dangling bonds formed by surrounding fluorine atoms is certainly not well organized. On the (CF)n spectrum, unresolved and resolved SHFS with seven lines coexist resulting in a complicated signal (Figure 2b). For both (CF)n and (C2F)n, the broadness of signals is interpreted by the joint effect of dipole-dipole and exchange interactions between paramagnetic centers. An additional line (denoted line 2) is also present in the EPR spectrum (11.1 < ∆Hpp < 16.5 G and g ) 2.003 ( 0.002) indicating a second type of dangling bonds (defects with different environments). As (C2F)n is formed at lower temperature (380 °C) than (CF)n (600 °C), the presence of such structural defects is expected. Moreover, its structure consisting in fluorographite layers connected in pairs by interlayer covalent C-C bonds seems to favor the formation of these defects. So, the spin density (i.e., the number of electronic spin per unit mass of sample) is larger for the two (C2F)n (2.1 × 1020 and 1.7 × 1020 spins‚g-1 for (C2F)n-350 and (C2F)n-380, respectively) than for (CF)n (0.1 × 1020 spins‚g-1). An increase of the fluorination temperature then results in a partial elimination of structural defects (i.e., dangling bonds). To clarify the origin of the broad line (line 1), EPR spectra of (C2F)n-380 were recorded after outgassing for 2 h under vacuum (Figure 2a). The quartz EPR tube containing the sample is pumped down and then sealed off. The outgassed sample is denoted (C2F)n-380V, and when reexposed to atmospheric air, it is denoted as (C2F)n-380Vair. Outgassing results in a removal of physisorbed molecules from the sample surface, such as
11802 J. Phys. Chem. B, Vol. 110, No. 24, 2006
Dubois et al.
Figure 2. EPR spectra recorded at room temperature and their simulation for the various samples (raw and outgassed materials): (C2F)n (a) and (CF)n types (b). To facilitate comparison, the intensities are divided by the sample mass.
Figure 3. Static NMR spectra of (C2F)n-380 and (C2F)n-380V: 1H (a) and 19F (b).
atmospheric paramagnetic O2 and HF molecules, which are formed by reaction of water traces with the fluorocarbon matrix. The amount of 1H nuclei observed by NMR (line at +9 ppm/ TMS) (Figure 3a) drastically decreases upon outgassing. Nevertheless, even in the raw material, this amount is low as revealed by the low signal/noise ratio. This HF removal occurs without rearrangement of the fluorocarbon matrix as shown by 19F NMR (Figure 3b). Indeed, the two spectra are similar, both exhibiting a broad line with a rectangular envelope, which is typical of strong fluorine homonuclear coupling.20,27 This line centered near -190 ppm/CFCl3 is assigned to CF groups in the rigid fluorographite matrix. The absence of a narrow line due to mobile species involving HF molecule and mobile F(i.e., FHF-) confirms the EPR results and then excludes the formation of such species and consequently their contribution to the EPR spectra via a charge transfer toward the host fluorocarbon matrix resulting in additional spin carriers. In addition, such intercalated species should give rise to a narrow NMR resonance line at -156.5 ppm/CFCl3.28
A simulation of the EPR spectra of all samples was carried out. This revealed the presence of various contributions. Two Lorentzian contributions consisting of a broad (line 1) and a narrow line (line 2) are necessary to perfectly fit the spectra of the various (C2F)n, whether outgassed or not. Taking into account the S1/S2 ratio of the area for the two EPR lines, 1 and 2 (area under the derivative of absorption), the outgassing of (C2F)n-380 leads to a decrease of this ratio from 1.65 to 0.23 (Figure 2 and Table 1). The area of the broad line decreases more than that of the narrow one. On the contrary, this ratio remains nearly constant for the two (C2F)n recorded under atmospheric conditions (1.66 for (C2F)n-350). After exposure to air, the S1/S2 value instantaneously increases to 1.15. This ratio and more particularly the broad line are related to the presence of paramagnetic O2 or/and H2O molecules. Atmospheric moisture can react with the fluorocarbon matrix to form HF molecules and FHF- both species that can affect the S1/S2 ratio. This is however not the case because the evolution after exposure to air of the EPR signal is very fast
EPR and NMR Studies of (C2F)n whereas the supposed reaction with moisture must certainly be longer. Moreover, as revealed by NMR the amount of HF and FHF- species present in the raw materials is low. The second hypothesis involves paramagnetic dioxygen molecules. It is wellknown that the presence of these species on the surface of conjugated polymers significantly changes the profile of the EPR spectra. This phenomenon was observed in the case of protonated emeraldine form of polyaniline,29 of polyaniline sulfonatebased hybrid nanocomposite,30 and of polypyrrole31 and results in each case in significant broadening of the EPR signal. This was explained by the effect of physisorbed oxygen, which induces a spin flip scattering with the paramagnetic O2 molecules. Moreover, in these cases, the initial EPR signal is recovered after vacuum treatment showing that the reaction with oxygen at room temperature is reversible. Similar reversible processes involving physisorbed paramagnetic oxygen also occur for poly(dicarbon monofluoride). When the sample is outgassed, these oxygen species are removed, and the intensity of the broad line decreases. It re-increases after exposure to air, which demonstrates the reversibility of the process. Outgassing desorbs O2 but also leads to a modification of the material by the removal of HF molecules. It is therefore difficult to compare the spin densities Ds of the various samples of (C2F)n. The outgassing of (C2F)n-380 leads to a decrease of Ds from 1.7 × 1020 to 1.3 × 1020 spins‚g-1. As this phenomenon is reversible (Ds increases to 2.1 × 1020 spins‚g-1 after exposure to air) and as Ds follows the concentration of paramagnetic O2, physisorbed oxygen seems to participate to the spin density. The line width of the signal also changes with outgassing; however, the observed variations are low (Table 1). This therefore demonstrates that the EPR parameters (line width and spin density) are very sensitive to the presence of physisorbed paramagnetic oxygen. Previously reported values found in the literature are all generally obtained under normal atmospheric conditions. They are thus all apparent and dependent on both the amount of physisorbed O2 and surface properties of the examined samples. To investigate if these findings can be extrapolated to other graphite fluorides, a similar EPR study has been performed on poly(carbon monofluoride) (CF)n. The samples are denoted (CF)n and (CF)n-V for respectively the raw and outgassed samples. Moreover, these materials contain no HF molecules. After outgassing, two of the three contributions obtained by simulation of the (CF)n spectra are changed. The two that are affected are the broad line initially attributed to the unresolved SHFS and the narrow line related to dangling bonds due to structural defects. On the contrary, the intensity of the resolved SHFS remains unchanged. As in the case of (C2F)n, oxygen desorbtion results in a decrease of the intensity of the broad line and consequently in a large decrease of the S1/S2 ratio: for (CF)n, S1 corresponds to the sum of the integrals for SHFS and unresolved SHFS. S1/S2 are equal to 11.6 and 6.2 for (CF)n and (CF)n-V, respectively. The effect of the removal of physisorbed oxygen is lower for (CF)n than for (C2F)n (Table 1). As a matter of fact, the amount of physisorbed O2 depends on the specific surfaces of the two types of graphite fluoride; these surfaces certainly differ due to their different fluorination temperature (i.e., 350 or 380 and 600 °C for (C2F)n and (CF)n, respectively). Nevertheless, in both cases, O2 species is seen to act on the paramagnetic relaxation of the dangling bonds by lowering the two parameters, T1 the spin-lattice relaxation time and/or T2 the spin-spin relaxation time, the latter being inversely proportional to the line width ∆Hpp. The observed broad line is in fact due to the superimposi-
J. Phys. Chem. B, Vol. 110, No. 24, 2006 11803
Figure 4. (a) 13C MAS NMR spectra of (C2F)n-350, (C2F)n-380, and (CF)n. (b) comparison for (C2F)n-380 of the 13C MAS NMR spectra obtained with and without 19F to 13C cross polarization (the spinning rate is 10 kHz).
tion of the contribution of dangling bonds in interaction with oxygen and the unresolved superhyperfine structure, which exhibits nearly the same line width. For (CF)n (Figure 2b), upon outgassing, the area of the broad line decreases by 44% whereas the integral of line 2, corresponding to dangling bonds with no interaction with O2, increases by 24% showing that a conversion between the two signals occurs when oxygen molecules are desorbed. We then considered a third kind of graphite fluorides, which are prepared at room temperature thanks to the use of a catalytic gaseous mixture F2(g)-HF(g)-MFn(g), where MFn is IF5, BF3, or ClFx.28,32,33 In this case also the effect of paramagnetic oxygen explains the particular behavior observed when the samples are heated at temperature close to 150 °C. It is observed that the EPR line width decreases from 82 to 72 G after a treatment at 150 °C under vacuum and that this evolution is not related to the spin density, which remains unchanged (i.e., spin-spin interaction was not the main factor of line broadening).33 On the basis of what is shown in the present work, such a decrease of ∆Hpp in the case of room temperature graphite fluoride can be related to the desorbtion of paramagnetic oxygen. This effect can then be generalized to various graphite fluorides with different properties (covalence of the C-F bonds, hybridization of the carbon atoms, etc.). 3.2. 13C and 19F Solid-State NMR. The room temperature 13C MAS NMR spectra of (C F) and (CF) (Figure 4a) show 2 n n the presence of several lines due to three types of carbon atoms. First, whatever the sample, the isotropic line at 84/TMS ppm is assigned to carbon atoms in strong interaction with fluorine.17,27,33,34 These atoms covalently bonded to fluorine, denoted SC-F in Figure 4, form the “rigid” fluorocarbon matrix. The second type of carbon atoms, present only for (C2F)n samples, exhibits a broad NMR peak with a chemical shift (d ≈ 130 ppm) and a profile expected for graphitic carbons (the resonance of pure graphite is at ≈120 ppm35). As the intensity of this line is higher for (C2F)n-350 than for (C2F)n-380 and, taking into account the XRD study that reveals the presence of residual graphite in (C2F)n-350 contrary to (C2F)n-380 (Figure 1), this resonance is unambiguously attributed to residual graphite and is denoted SG in Figure 4. This residual phase in (C2F)n-380 can be revealed thanks to the higher sensitivity of NMR as compared to XRD concerning diluted phases. The third reso-
11804 J. Phys. Chem. B, Vol. 110, No. 24, 2006 nance is observed at 42 ppm: this chemical shift corresponds to sp3 aliphatic carbon atoms as proposed by Wilkie et al.,36 who obtained with (CxF)n (x > 1) a similar chemical shift (δ ) 35 ppm). Hamwi32 also found in room temperature graphite fluoride a weak resonance at 35 ppm, which was assigned to sp3 carbon atoms; however, these sp3 carbons atoms were in fact due to the presence of a secondary phase. To our knowledge this is the first time that this type of carbon atoms is observed by NMR in graphite fluorides. Considering the high intensity of the NMR line for both studied (C2F)n, the content of these carbon atoms is high. According to the structural model given in ref 8, sp3 carbon atoms (which ensure the connection between the fluorocarbon layers associated in pairs) correspond approximately to half the total carbon content; however, no NMR study has, to date, reported their presence. In comparison with pure carbonaceous materials, fluorinecarbon interaction induces a shift in the NMR line (e.g., resonance of graphitic carbons in weak interaction with fluorine is observed in the range 128-138 ppm instead 120 ppm for pure graphite). As the resonance line of pure diamond carbon is expected at 35 ppm, a weak fluorine-carbon interaction can lead to a chemical shift value similar to the one measured in the case of (C2F)n (δ ) 42 ppm). Therefore, in accordance with the structural model, we shall attribute this line (denoted SC-C) to sp3 carbon atoms. Some calculations and experiments were performed in order to confirm this attribution. First, by simulation of the (C2F)n spectra using three Lorentzian, the proportion of each type of carbon can be evaluated. This reveals that (i) the area ratio SC-F/SC-C does not change for the two samples, (C2F)n-350 and (C2F)n-380, but differs significantly from the theoretical value of 1 (SC-F/SC-C ) 1.5); (ii) taking into account the sum of the three contributions SC-C + SC-F + SG ) Stot related to the total content of carbon atoms, the ratio SC-F/Stot ) 0.55 is then equal to the molar ratio F/C and in good agreement with the composition obtained by mass uptake (F/C ) 0.60, for (C2F)n-380). The difference could be explained by the presence of >CF2 and -CF3 groups (as shown below). Surprisingly, the line at 42 ppm is also observed for (CF)n; this can be explained by the presence of very low amounts of residual (C2F)n due to uncomplete fluorination for small parts of the fluorographite layers. NMR measurements performed using MAS and 19F to 13C cross-polarization may differentiate the various carbon atoms present. A comparison of the spectra acquired with MAS and CP-MAS for (C2F)n-380 (Figure 4b) confirms our attributions for the three kinds of carbon atoms. As C-F groups are favored with CP-MAS in comparison with second neighbors (i.e., sp3 carbon atoms), only the line corresponding to C-F bonds from the fluorocarbon matrix is increased contrary to both sp3 hybridized carbons exclusively bonded to carbon atoms (SC-C) and graphitic carbons (SG), the latter completely disappearing. Moreover, resonance of >CF2 groups is also favored with CPMAS, and a small line is observed at 110 ppm as a shoulder of the SC-F line (Figure 4b). Such groups have already been observed elsewhere in various (CF)n.17,27,34,36 The room temperature 19F MAS NMR spectra of (C2F)n-380 recorded with spinning rates of 10.0 and 14.5 kHz are displayed in Figure 5. One intense isotropic line at -189 ppm /CFCl3 is present with its spinning sidebands. This line corresponds to fluorine atoms involved in covalent C-F bonds. This covalent character was confirmed by FT-IR spectroscopy (not shown here) since the FT-IR spectra of (C2F)n show a vibration band at 1215 cm-1, which is assigned to covalent C-F bond2. Moreover, in addition to the signal of covalent C-F at 1215
Dubois et al.
Figure 5. 19F MAS NMR spectra of (C2F)n-380 with two spinning rate; * and ∆ markers denote spinning sidebands related to the isotropic lines at -189 and -122 ppm, respectively.
cm-1 an additional peak at 1380 cm-1 is related to CF2 groups.2 These groups are assigned either to fluorine atoms located at the edge of graphite layers or to structural defects. In agreement with FT-IR results, a second resonance, less intense, at -119 ppm confirms the presence of CF2 groups. As a matter of fact, the spectra exhibit at -119 ppm a shoulder on one of the spinning sideband of the C-F line. Nevertheless, their content is small but sufficient to be detected by 19F MAS NMR. The sidebands of the >CF2 resonance are also present, in particular the one superimposed with the left side of the isotropic line of C-F groups and resulting in a shoulder. These 19F MAS experiment allow others groups (-CF3) to be detected despite their very low amount. As a matter of fact, several narrow lines are present in the -60/-90 ppm range, superimposed with spinning sidebands of C-F and >CF2 lines. Such CF3 groups result in a shoulder of the main broad line in the static 19F spectra of the two (C2F)n (Figure 3b). These groups are localized on the fluorocarbon sheet edges and probably possess a spinning motion around the C-C bonds, explaining the narrowness of the resonance. 3.3. Determination of the C-F Bond Length. The C-F distance can be estimated using the inverse cross polarization (ICP) sequence that re-introduces the dipolar couplings and therefore C-F distance into the spectrum. At short contact times, the amplitude of the CP signal is found to be oscillatory with a frequency related to the C-F bond length.16 Considering a rotating sample with both abundant spins I having a high gyromagnetic ratio (γI) and rare spins S with a low gyromagnetic ratio (γS), when the spinning frequency (ωr) exceeds both the I-I and S-S dipolar interactions, then the Hartmann-Hahn matching condition is split into a series of new sidebands with ∆ ) ω1I - ω1S ) nωr (ω1I and ω1S are the amplitudes of the applied radio frequency fields); an efficient CP is obtained for n ) ( 1, ( 2.37,38 Dipolar couplings measurements by ICP sequence require two necessary conditions: (i) the offsets of 19F and 13C are exactly on resonance and (ii) the HartmannHahn condition is matched with n ) ( 1, ( 2. The n ( 1 sidebands matching conditions is preferred because it generally
EPR and NMR Studies of (C2F)n
J. Phys. Chem. B, Vol. 110, No. 24, 2006 11805 the rotor axis, and DIS is the heteronuclear dipolar coupling, DIS ) (µ0γIγSp/4πr3). At the n ) ( 1 sidebands matching conditions, the maximum splitting in the powder pattern is found for β ) π/4 and is smaller by a factor x2 than the splitting of the intense singularities of the Pake pattern.16 So, φ ) (DIS/2x2), and the following expression for rCF can be extracted from the previous equations:
(nm) x 10.0463 φ 3
rCF )
Figure 6. (a) Time evolution of the 13C magnetization for carbon atoms covalently bonded to fluorine (SC-F) and exclusively bonded to carbon (SC-C) of (C2F)n-380 with a spinning rate of 14.5 kHz at the n ) +1 Hartmann-Hahn condition. Solid line represents the fit performed using eq 1. (b) Fourier transform of the resulting oscillation.
allows larger efficiency of the heteronuclear interaction recoupling. So, in practice we used n ) +1. Using the ICP sequence with these particular conditions, the integrated peak intensity of the carbon spectra was calculated as a function of contact time thus revealing the CP dynamics (Figure 6a displays at a spinning rate of 14.5 kHz the evolution of the 13C magnetization of both the carbon atoms of C-F group and of the sp3 carbon atoms). For powdered samples with fast spinning rates, the transferred polarization Ms(t) involving abundant spins I and rare spins S evolves as
Ms(t) )
γI γS
[
M0S* 1 -
( )
1 t exp m+1 τd
( )
]
t m cos(φt) (1) exp m+1 2τc where τd is the characteristic diffusional time of interaction between the SIm group and the spin reservoir; τc is the correlation time of the dipolar fluctuation, m is the number of fluorine atoms per carbon that are involved in the dipolar interaction (m ) 1 in our case according to the structural model8); φ is the angular dependence of the dipolar interaction, for n ) ( 1: φ ) (DIS/ 4)x2 sin2β where β is the angle between the SI direction and
According to Bertani et al.,16 this model is available for powder averaged systems. As expected, the signal amplitude (Figure 6a) exhibits an oscillatory behavior only for carbon covalently bonded to fluorine. The low variation of the magnetization for sp3 carbon atoms bonded exclusively to carbon atoms indicates that these nuclei are only slightly affected by the heteronuclear dipolar coupling, confirming our initial attribution. τc, τd, and φ obtained by fitting the experimental data, using eq 1, for the carbon atoms of C-F groups are collected in Table 2. The corresponding experimental C-F bond is then rC-F ) 0.135 ( 0.001 nm. Fourier transform of the 13C magnetization evolution for C-F groups gives the Pake-like structure (Figure 6b). The line shape can be interpreted by an other way: it can result from the presence of three fluorine nuclei at the corners of a rigid equilateral triangle, neighboring groups broaden the line and smooth out the discontinuities observed for isolated rigid triangle.39 As such configuration does not exist in the (C2F)n structure, this hypothesis was not retained. The main peak at 0 Hz of the Pake structure is related to spin diffusion and corresponds to the Fourier transform of the nonoscillating term of eq 1. A part of this peak broadening could also be due to the fact that the magnetization does not go to 0 for long times. For similar experiments performed with (CF)n the spin diffusion peak was narrower than in the case of (C2F)n: 1.8 and 3.6 kHz for (CF)n and (C2F)n, respectively. The correlation time of the dipolar fluctuation, obtained using eq 1, is then τc ) 0.13424 × 10-3 and 0.86586 × 10-4 s for (CF)n and (C2F)n, respectively. This broadening cannot be due to a lower structural order since the coherence Lc is higher for (C2F)n (7 nm) than for (CF)n (5 nm). Another hypothesis concerns molecular motional averaging which results in the reduction of dipole-dipole interactions; this can be estimated by the full width at halfmaximum (fwhm) for the two graphite fluoride types: the values of 19F fwhm at room temperature are 41 and 55 kHz for (CF)n and (C2F)n, respectively. This divergence cannot fully explain the difference between the profile of the spin diffusion peak. The C-F bond length can be deduced from the Pake structure using the wings that are linked to dipolar fluctuation. The heteronuclear interaction was calculated by the line spitting S1 between the maximum of the wings because DIS ) (2)1/2S1,16 allowing the C-F bond length rCF to be measured:
(nm) x 20.0926 S1 3
rCF )
From Figure 6b, the experimental value of S1 is equal to 7700
TABLE 2: Parameters Obtained by Fitting the 13C Magnetization Evolution of the C-F Group for (C2F)n-380 τc (s)
τd (s)
φ (Hz)
0.86586 × 10-4 ( 7.192 × 10-6
0.13198 × 10-3 ( 4.467 × 10-5
4061.2 ( 66
11806 J. Phys. Chem. B, Vol. 110, No. 24, 2006 Hz, and the C-F bond distance is 0.138 ( 0.002 nm. In this case the error of the distance is estimated by comparison with theoretical pake structure obtained with different bond length (0.136 < rCF < 0.140 nm). This value slightly differs from the one obtained by using the fitting procedure. As the magnetization evolution is short, this procedure introduces a more important error, so the value of 0.138 nm obtained from the Pake structure seems to be more exact. The length of 0.138 ( 0.002 nm is in accordance with the value recently obtained from neutron diffraction data (0.136 nm).8 Our value estimated by NMR could be overestimated due to a possible molecular motion since this latter results in a lowering of the second moment, inversely proportional to the distance C-F. The C-F bond length in (C2F)n is close to the values obtained by the same NMR procedure for (CF)n (0.138 nm),17 indicating that the nature of the C-F bonding is similar in these two compounds. 3.4. 19F T1 Spin-Lattice Relaxation Mechanism. At 44 MHz, Panich et al.20 found a shorter T1 for (C2F)n (53 ms) than for (CF)n (400 and 660 ms for samples prepared starting from petroleum coke and natural graphite, respectively) and attributed it to the presence of paramagnetic centers (PC). The effect of paramagnetic centers on nuclear spin relaxation has been the subject of many studies.40-42 As a matter of fact, coupling between nuclear spin and unpaired electron spin (even in small concentration, because the electron has a much larger gyromagnetic ratio) produces a very effective channel for nuclear spin-lattice relaxation. The relaxation behavior depends on the spin diffusion constant. First, in the case of diffusion-limited relaxation the magnetization Mz(t) possess two different regions: for very short time Mz(t) is proportional to t1/2 and proceeds asymptotically to an exponential function of time.40 For a diffusionless process, the growth of magnetization has been shown to be in t1/2 for very short time,40 for longer time the magnetization evolves as exp[-(t/T1)1/2].41,42 In absence of relaxation by PC or in the case of rapid spin diffusion, Mz(t) follows an exponential function of time for all values of t without the transient region in t1/2 .40 To investigate the effect of PC on the relaxation, we have compared the growth of the nuclear magnetization following an excitation pulse sequence (saturation-recovery) for the various (C2F)n and (CF)n samples. Figure 7 shows the magnetization as a function of the recovery time; the abscissa of curves for successive compounds has been shifted to prevent overlapping (the shift values are indicated in the figure caption). A second sample of (CF)n is also studied: it consists of poly(carbon monofluoride) prepared by direct fluorination at 600 °C starting from natural graphite. This new sample is denoted (CF)n-NG to be differentiated from the sample obtained with graphitizated petroleum coke, (CF)n-coke. (CF)n-NG sample is interesting because it contains a large density (close to 15.6 × 1020 spins‚g-1) of paramagnetic centers (dangling bonds resulting from structural disorder). To emphasize the nature of the magnetization growth, we have plotted the magnetization as a function of t and t1/2 (Figure 7, panels a and b, respectively). The ordinate is 1 - Mz(t)/Mz0 plotted on a logarithmic scale for Figure 7a and on a linear scale for Figure 7b. These figures show a different magnetization recovery for the different samples; first for (CF)n-coke, the magnetization evolves exponentially (Figure 7a). The absence of t1/2 evolution for short t as seen in Figure 7b indicates that paramagnetic centers do not play a major role in the relaxation. But we cannot a priori completely exclude a relaxation mechanism governed by paramagnetic centers but with rapid
Dubois et al.
Figure 7. Nuclear magnetization recovery for the various graphite fluorides; logarithm of magnetization vs t (a) and t1/2 (c) and magnetization vs t1/2 (b). The abscissa has been shifted to separate the curves: 1 s, 0.1 s1/2, and 0.4 s1/2 for panels a-c, respectively.
spin diffusion.40 For the other samples, (C2F)n and (CF)n-NG, the initial magnetization starts linearly when the recovery time is in the form t1/2 and clearly does not proceed as an exponential function of t. The total magnetization recovers in fact as exp[(t/T1)1/2] (Figure 7c). This is typical of relaxation in the presence of PC and without spin diffusion.41,42 The T1 19F spin-lattice relaxation times values extracted from the corresponding evolution function are found to be equal to 220, 240, and 220 ms for (C2F)n-380, (C2F)n-380V, and (C2F)n-380Vair, respectively (Table 3). For (CF)n sample the T1 values are 450 and 745 ms for (CF)n-coke and (CF)n-NG, respectively.
EPR and NMR Studies of (C2F)n TABLE 3:
19F
J. Phys. Chem. B, Vol. 110, No. 24, 2006 11807
T1 Values for the Various (C2F)n-380
(C2F)n-380
(C2F)n-380V
(C2F)n-380Vair
220
240
220
T1 (ms)
Moreover, in the case of solids with arbitrary space dimension D, Furman et al.43 have shown that the magnetization growth is proportional to exp[-AtR]. In an homogeneous distribution of the paramagnetic centers and nuclei, R is equal to D/6. On the other hand, for an inhomogeneous distribution, the sample must be divided into subsystems, each of them including only one PC surrounded by nuclei. These subsystems are packed in a d-dimension space. In this case, R is equal to (D + d)/6. Moreover, using a saturation recovery sequence the magnetic moments of the subsystems are aligned along the magnetic field and then d ) 1. From our data, R is equal to 1/2 corresponding to D ) 3 or 2 for homogeneous and inhomogeneous distribution, respectively. As graphite fluorides possess a lamellar structure and as EPR shows that the PC are not within a well-organized environment, we favor the value D ) 2 for an inhomogeneous distribution. Our previous study on (CF)n samples17 revealed that paramagnetic oxygen is involved in the nuclear relaxation mechanism. The effect of oxygen was also investigated in the case of (C2F)n. Contrary to (CF)n for which the removal of physisorbed oxygen by outgassing results in an increase of T1 from 0.45 to 10.1 s, in the case of (C2F)n-380 a similar treatment is less efficient (T1 increases from 220 to 240 ms). However, 1H, 19F NMR, and EPR revealed a modification of the sample upon outgassing, with a removal of HF molecules in addition to O2 desorbtion. To investigate the effect of paramagnetic oxygen, (C2F)n-380V and (C2F)n-380Vair must be compared because they contain nearly the same amount of HF molecules. A low decrease of T1 (220 ms) is then observed after exposition to air indicating that physisorbed oxygen is involved in the nuclear relaxation as in the case of (CF)n but with less efficiency. This fact could be easily explained by a different ratio of physisorbed oxygen due to different surface properties. 4. Conclusion Using complementary characterization techniques, EPR and NMR, poly(dicarbon monofluoride) (C2F)n was investigated and compared to (CF)n. As expected, 19F MAS and 13C MAS obtained with cross polarization show the presence of covalent CF groups in both cases. For the first time, NMR studies have shown the presence of sp3 hybridized carbon atoms, which are exclusively bonded to other carbon atoms and ensure the cohesion between fluorocarbon sheet pairs in accordance with the structural model of (C2F)n. By reintroducing the dipolar coupling through CP techniques, C-F bond length of 0.138 ( 0.002 nm has been determined by NMR and compared to previous values determined elsewhere using other techniques and the one found for (CF)n. Physisorbed paramagnetic oxygen acts both in nuclear resonance and in electron resonance; this results in changes of the 19F nuclear spin-lattice relaxation time and in the line width of the EPR signal of the dangling bonds. These phenomena can be generalized both to poly(carbon monofluoride) (CF)n and to semi-covalent room temperature graphite fluoride. These parameters, when investigated without precaution in an ambient atmosphere, are apparent and depend on the solid surface properties (i.e. the amount of physisorbed oxygen). In addition to these external paramagnetic centers, other intrinsic ones, identified as dangling bonds, are involved in the
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