X-Ray Photoelectron Spectroscopy of Quaternary Phosphonium Compounds William E. Swartz, Jr., and David M. Hercules Department of Chemistry, University of Georgia, Athens, Ga. 30601
X-ray photoelectron spectroscopy (ESCA) has been used to determine the phosphorus 2p binding energies for quaternary phosphonium salts, R(C6H&P+Y The observed binding energies do not correlate with the electronegativity of the R-substituent. However, a linear relationship has been established between the P(2p) binding energies and the *lP NMR chemical shifts for the salts. Significance can be placed on this relationship because the NMR data are obtained on the phosphonium salts in solution while the ESCA data were obtained on solids. This implies that lattice and sample charging effects are small or non-existent in ESCA studies of the quaternary phosphonium salts.
+.
X-RAYPHOTOELECTRON SPECTROSCOPY (ESCA) is a technique concerned with measuring core electron binding energies. ESCA has been developed such that it can now be used to correlate the chemical environment of an atom to the electron density about the atom ( I ) . As the electron density around an atom increases, the core-electron binding energy necessarily decreases since the effective nuclear charge decreases. These changes in binding energy as a function of the chemical environment yield “chemical shifts” which can be related to the structure of the compounds from which the core electrons have been ejected. Therefore, ESCA has great potential for becoming a powerful technique in structural chemistry. This potential has been realized in several investigations (1-5). Most recently, Jack and Hercules reported an extensive investigation of the nitrogen 1s binding energies in quaternary nitrogen compounds (6). The effect of counterion on binding energy in the tetra-alkyl ammonium salts was determined. Ring substituent effects were also investigated in the pyridinium salts. The data indicated that ESCA could distinguish between a true quaternary and the corresponding hydrohalide, substituted and unsubstituted ring quaternary compounds, and between systems containing two or more rings and tetra-alkyl or pyridinium compounds. We wish to report the phosphorus 2p binding energies for a series of quaternary phosphonium salts, R(CBH&P+Y-. The binding energies were studied as a function of the R substituent. A correlation between the phosphorus 2p binding energies and the *lP nuclear magnetic resonance chemical shifts has been established.
EXPERIME;LT.SL
The electron spectra were obtained with a 30-cm, double focusing, iron-free electron spectronetei of the split-coil solenoidal type. A det?.iled descrlprioa a:‘ thc apparatus has been previously reported (6). Reagents. The quaternary phosphonium salts werc i;Ltained from the Aldrich Chemical Co., Alfa ‘Incii~gm~c and the Research Organic,’Ioorganic Chemical C . x p N o further purifications were performed. Procedure. The spectra were all obtained ? t an mental resolution of 0.04 %. lnstrumental resolution igiiores the width of the exciting radiation and the inherent width of the P(2p) electron level. The aluminum Kal., line was used as the X-ray excitation source. The X-ray paver supply was run at 25 kV and 25 mA. The detector was operated at 3700 volts. All samples were run as powders dusted onto a backing of double-backed cellophane type. The samples were mounted on a brass plate which served as an electron and heat sink. The work function of the spectrometer we: not determined since only relative binding energies wero measured. In calculating the binding energies, the spectronwter work function was assumed to be 4.0 eV, a rezsona5le value for this type of spectrometer. Calibration was obtairled using the phosphorus 2p binding energy of 133.3 eV in sodium pyrophosphate, Na4PzOi, previously reported by Pclavin et al. (7). The calibration line was measured after each unknown P(2p) electron line. The NMR data were obtained on a Varian-HA-Ir30 nuclear magnetic resonance spectrometer. The spectrs were run on 2Oz solutions of the salts in a 5 : 1 acet0ne:wster mixture. The chemical shifts were measured relative to a standard of 85 phosphoric acid. Apparatus.
RESULTS
The quaternary phosphonium salts investigated are given in Table I, together with the experimentally dttermined phosphorus 2p binding energies and *IPNMR chemical shifts. Also given in Table I are the calculated group electronegativities for the R-substituent. The binding energy for each compound was determined using a minimum of three replicate samples with the average of all the determinations reported. Typical spectra of the phosphorus 2p line for sodium pyrophosphate, Na4P2O7,and chloromethyl-triphenylphosphonium chloride are shown in Figure 1 I
(1) K. Siegbahn et ai., “ESCA ‘Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy’,’’ Almqvist and Wiksells, Uppsala, 1967. (2) D. N. Hendrickson, J. M. Hollander, and W. L. Jolly, Znorg. Chem.,8, 2642 (1969). (3) U. Gelius, P. F. Heden, J. Hedman, B. J. Lindberg, R. Manne, R. Nordberg, C. Nordling, and K. Siegbahn, Institute of Physics, Uppsala University Report UUIP-714, July 1970. (4) J. Hedman, P. F. Heden, R. Nordberg, and C. Nordling, Specfrochim. Acta, 26A, 761 (1970). (5) G. Axelson, K. Hamrin, A. Fahlman, and C. Nordling, ibid., 23A, 2015 (1967). (6) J. J. Jack and D. M. Hercules, ANAL.CHEM.,43, 729 (1971). 1066
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
DISCIJSSIBN
Perhaps the most striking feature of the binding energy data on the quaternary phosphonium salts is that the total range of binding energies is small (2.6 eV relative to NaAO,). Jack and Hercules have reported a range of approxirnatclq 12 eV (relative to K N O J for the nitrogen I s electron binding energies in the quaternary nitrogen compounds (6). How__-____
.----
(7) M. Pelavin, D. N. Hendrickson, J. M. Hollander. and W L. Jolly, J . Phys. Chem., 74, 1116 (1970).
Table I. P(2p) Binding Energies, Group Electronegativity of R - , and 31PNMR Chemical Shifts for the Quaternary Phosphonium Salts P(2p) binding CPD No. Compound energy (eV). X Groupb 631P PPM 132.3f 0.30 2.08 -26.1 1 (CHaCHzCHzCHz)rP+Cl131.6f 0.30 2.08 -33.8 2 (CH3CH2CHzCH~)dPfBr131.1 f 0.10 2.24 +7.8 3 (C&6) dH)P+Br130.7 f 0.15 2.60 -22.2 4 (ClCHz)(CeHa)aP+Cl130.7 f 0.20 2.36 -22.5 5 (C &P+Br130.5 f 0.17 2.04 -17.8 6 (CeH&Hz)(CdHs)aP+Cl130.5f 0.14 2.28 -2n.5 7 (CHz==CHCHz)(CeH 6) 3P+Br130.4f 0.14 ... -22.5 8 (~-NO~C~HI)(C~H~)~P+B~130.2f 0.10 2.00 -16.2 9 (CH3)(CeHs)aPCCl130.1f 0.14 2.32 -19.5 10 (CzHsOCC(O)CHz)(CeH&P+BT129.7 f 0.16 2.25 -11.8 11 (CH30CHz)(CeH 6 ) 3P+BrError limits are standard deviations. b Reference 13, electronegativity in Pauling Units. c 31P NMR chemical shift relative to 85 phosphoric acid.
ever, the data reported by Pelavin et a/. (7), which covered a much wider range of compound types, showed a range of only 4.5 eV (relative to the carbon 1s line) in phosphorus 2p binding energies. Introduction of fluorine, the most electronegative element, into the system increases the range to 7.5 eV. However, fluorinated compounds can be considered somewhat extraordinary since the shifts observed are normally much larger than for nonfluorinated compounds. When the phosphonium salt phosphorus 2p binding energies are considered from this perspective, the range of shifts observed appears t o be reasonable. The binding energies determined for tetra-n-butylphosphonium bromide (No. 2) and tetra-n-butylphosphonium chloride (No. 1) may be indicative of counter ion effects commonly seen in ESCA studies (6,8). The two compounds are equivalent except for the counterions, therefore the difference in the phosphorus 2p binding energies between compounds No. 1 and No. 2 must be due to the counterion effect. The chloride ion is more electronegative than the bromide; therefore, the effective nuclear charge felt by the P(2p) electron would be larger in the chloride salt. The difference in effective nuclear charge would be evidenced by a higher phosphorus 2p binding energy in the chloride salt than in the bromide salt. This is observed, the chloride salt has a larger binding energy by 0.7 eV. Pelavin et al. (7) have also reported the phosphorus 2p binding energies for benzyl-triphenylphosphonium chloride (No. 6 ) and tetra-n-butylphosphonium chloride (No. 1). Our value of 132.3 f 0.3 eV for tetra-n-butylphosphonium chloride (No. 1) agrees well with the 132.3 eV reported by Pelavin et al. (7). However, our value of 130.5 f 0.17 eV for benzyl-triphenylphosphonium chloride is 2.0 eV smaller than reported by Pelavin et al. (7). Pelavin et al. have reported no error limits with their data. Since the standard deviation on replicate measurements in our data is less than 0.2 eV, we tend to place more faith in our value. This is supported by the fact that the 130.5 f 0.17 eV binding energy fits the NMR correlation to be discussed below, while a value of 132.5 eV does not. Deviations from Pelavin's data have also been reported by Blackburn et al. (9). Pelavin used the carbon 1s line from hydrocarbon contamination of (8) R. G. Albridge, K. Hamrin, G. Johansson, and A. Fahlman, 2.Physik, 209, 419 (1968). (9) J. Blackburn, R. Nordberg, F. Stevie, R. G. Albridge, and M. M.Jones, Inorg. Chem., 9, 2374 (1970).
IO00
I 7. n v
1333ev
I
I600
v)
8
1400
v)
I-
2 0
1200
1000
e
8W ,
2.6550
2.6600
I
#
#
2.6650
SPECTROMETER CURRENT (amps) Figure 1. Phosphorus 2p electron spectra for sodium pyrophosphate (0.0) and chloromethyltriphenylphosphonium chloride (000)
the sample for calibration. Nordberg et al. (10) have reported that erroneous results are possible when using this procedure because the carbon 1s energy has been found to vary from one contaninant to another. Several workers have reported that the observed binding energies can be correlated with the electronegativities of substituent groups. Thomas has reported a linear correlation between the carbon 1s binding energies and the electronegativities of the substituents for halogenated methanes (11). Baker et QI. have also reported a linear relationship between the ionization potentials for hydrogen and the electronega(10)R. Nordberg, H.Brecht, R. G. Albridge, A. Fahlman, and J. R. Van Wazer, ibid., p 2469. (11) T. D.Thomas, J. Amer. Chem. Soc., 92,4184 (1970). ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
1067
Figure 2. Correlation of phosphorus 2p binding energies with *lP NMR chemical shifts for quaternary phosphonium salts
I
I
129
I30
OS
I31
I
132
---
P(2p) BINDING ENERGY (eV)
tivity of the halide in the hydrogen halides (12). Siegbahn et al. have also discussed the correlation of binding energy with calculated group electronegativities for a series of organic molecules (1). An attempt was made to explain the ordering of the binding energies in the quaternary phosphonium salts on the basis of the electronegativity of the R-substituent. Davis (13) has calculated the group electronegativities of the Rgroups present in the phosphonium salts investigated. These group electronegativities are listed in Table I. It is apparent that group electronegativities fail to explain the observed order of binding energies. For example, since ClCH2-(X,, = 2.60) is more electronegative than CBHs-(X,, = 2.36), one would expect the binding energy for the C1CH2-salt (No. 4) to be somewhat larger than that for the CaH,-salt (No. 5). However, the binding energies are equal. A similar discrepency exists for the C G H C H ~ - ( N O 6). and CH-CHCH?(No. 7) salts as well as for the CH3-(No. 9) and CH30CHs(No. 11) salts. Plots of P(2p) binding energy us. X,, (group electronegativity) of R - yield only a scatter diagram. Even if corrections for the variation of the counter ion are made (based in the difference of No. 1 and No. 2 as discussed above), a scatter diagram results. Nuclear magnetic resonance and ESCA may be considered analogous in that NMR measures the magnetic shielding of the nucleus by the electrons from an external field, while ESCA measures the electrostatic shielding of a photoelectron as it is removed from the atom. Basch (14) has proposed that a direct link should exist between NMR and ESCA. He proposes that the diamagnetic contribution to the nuclear shielding in molecules can be directly related to the difference in potential, due to difference in charge distribution, felt by a n atom in chemically different environments. With Basch’s hypothesis in mind, we attempted to correlate the observed phosphorus 2p binding energies with observed 31Pchemical shifts for the same compounds. Figure 2 is a plot of P(2p) binding energy us. chemical shift. With two exceptions, compounds No. 1 and No. 3, a linear correlation is seen to exist. The line is drawn using a least squares fit that omitted points 1 and 3. The observed chemical shifts agree to within several ppm with those observed by other workers (15, 16). Since different solvents (12) A. D. Baker, D. Betteridge, N. K. Kemp, and R. E. Kirby, Int. J . Mass Specrrom. Ion Phys., 4,90 (1970). (13) M.A. Davis,J. Org. Chem.,32, 1161 (1967). (14) H.Basch, Chem. Phys. L e r r . , 5, 337 (1970). (15) S. 0.Grim. W. McFarlane. E. F. Davidoff. and T. J. Marks,
i.Phys. Chern:, 70,581 (1966).’ (16) V. Mark, C. H. Dungan, M. M. Crutchfield, and .I. R. Van ’
Wazer, “Topics In Phosphorus Chemistry,” Vol. V, Interscience, New York, N. Y., 1965, Chapter 4, p 227.
1068
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
and concentrations were used in our measurements, it is not surprising that some small discrepancies do exist. The slope of the line is in the direction one would theoretically predict. The larger negative chemical shifts in the NMR data imply shifts t o lower field where the diamagnetic shielding is smaller. A smaller diamagnetic shielding results from a smaller electron density. Smaller electron densities in turn yield higher binding energies. Therefore, the more negative the NMR chemical shift, the larger the P(2p) binding energy should be. This is seen to be the case in Figure 2. Sevier (17) has been able to establish a “moderate” correlation between the lacNMR chemical shifts and carbon 1s binding energies for a series of organic compounds. The fact that the NMR data obtained in solution, can be correlated to ESCA binding energy measurements, made on solids, implies that lattice or charging effects in the quaternary phosphonium salts are either small or non-existent. A great deal of concern exists about the charging of the sample created by photoelectron ejection during a n ESCA measurement. The fact that the NMR and ESCA data show a linear correlation suggests that this phenomenon may not be as serious as previously hypothesized. Although not entirely eliminating charging effects as a matter of concern, these data d o suggest that they are unimportant for measurements made on a n homologous series or similar compounds. Thus relative comparisons are valid. The deviation of tetra-n-butylphosphonium chloride (No. 1) from linearity in Figure 1 can perhaps be explained as follows. The observed NMR chemical shift between the tetra-nbutylphosphonium chloride, No. 1, and the tetra-n-butylphosphonium bromide, No. 2, is opposite to that observed in similar compounds as reported elsewhere (16). It is shown that a more electronegative counter-ion should produce a more negative NMR chemical shift. This is a reasonable observation in relation to the diamagnetic hypothesis since as electrons are removed from the resonant atom, a negative field shift between the bromide salt and the chloride salt is observed in this study. Since several other workers have observed the expected negative shift in going from a bromide to chloride salt (16), it is felt that the chloride salt in this investigation has yielded a n anomalous NMR shift. Indeed, if the NMR shift were more negative for No. 1 than for No. 2, the correlation of P(2p) binding energy and *lP NMR shift would be better. The large deviation from linearity of triphenylphosphonium bromide (No. 3) observed in Figure 2 can perhaps be ex(17) K. D. Sevier, Paris, France, private communication, 1970.
plained by crystal lattice effects. Fadley e t al. (18) have pointed out the importance of the lattice, or Madeling corrections, in binding energy measurements on solids. If the crystal structures of two compounds vary, the Madeling contribution to the binding energies would also vary, and thus affect relative binding energy measurements. The Madeling contributions to binding energy would be constant if the crystal structures were equivalent and thus not affect relative binding energy measurements. In the phosphonium salts studied here, one can best rationalize a difference in crystal structure for the triphenylphosphonium salt (No. 3). Since the proton (H) is so much smaller than any of the other R-groups in the series, it is likely that its crystal structure is different from the others. (18) C. S. Fadley, S. B. M. Hagstrom, M. P. Klein, and D. A. Shirley, J . Chem. Phys., 48,3779 (1968).
If this were the case, the phosphorus 2p binding energy observed for compound No. 3 would be expected to diviate from the observed linearity, since the Madeling contribution to the binding energy would be different than that for the other molecules. ACKNOWLEDGMEhT
We thank R. H. Cox of the Department of Chemistry at NMR data. the University of Georgia for obtaining the RECEIVED for review February 1, 1971. Accepted April 15, 1971. One of us (W. E. S.) thanks the National Institutes of Health for a pre-doctoral fellowship during the term of this research. This work was supported in part through funds provided by the U. S. Atomic Energy Commission under Contract AT-(38-1)-645.
Spectroele,ctrochemistry-Application of Optically Transparent Minigrid Electrodes under Semi-Infinite Diffusion Conditions Milica Petek, T.E. Neal, and Royce W. Murray1 Department of Chemistry, University of North Carolina, Chapel Hill, N.C.27514 The Au minigrid electrode is evaluated for both potential and current step spectroelectrochemical experiments under semi-infinite diffusion conditions. The test reactions of 0-tolidine oxidation and titanium(1V) reduction using 2000 Ipi minigrid yield results where the optical response corresponds to linear diffusion theory at times exceedin 10-20 milliseconds. This is the period necessary for diffusion profile-averaging and is predictable by semi-theoretical arguments.
THE SPECTROELECTROCHEMICAL EXPERIMENT in which the course of an electrochemical reaction is monitored by transmission spectrophotometry of the electrode reaction diffusion layer was conceived in 1964 by Kuwana, Darlington, and Leedy ( I ) . These authors employed tin-oxide coated glass as a transparent working electrode and followed the absorbance of the quinone-imine oxidation product of otolidine in acidic aqueous solution. Kuwana and coworkers have continued study of transmission spectroelectrochemical experiments and have described the electrochemical and optical transmission properties of the SnOz-coated electrode (2, 3), sensitive kinetic spectrometers for fixed ( 4 ) and rapid (5) wavelength scan measurements, theoretical relations for transient absorbance-time response under potential step control (2, 6 4 , and several applications (2-8) which dem(1) T. Kuwana, R. K. Darlington, and D. W. Leedy, ANAL. CHEM., 36, 2023 (1964). (2) J. W. Strojek and T. Kuwana, J . Electroanal. Chem., 16, 471 (1968). (3) T. Osa and T. Kuwana, ibid., 22, 389 (1969). (4) T. Kuwana and J. W. Strojek, Discuss. Faraday SOC.,45, 134 (1968). (5) J. W. Strojek, G. A. Gruver, and T. Kuwana, ANAL. CHEM., 41, 481 (1969). (6) J. W. Strojek, T. Kuwana, and S. W. Feldberg, J . Amer. Chem. Soc., 90, 1353 (1968). (7) N. Winograd, H. N. Blount, and T. Kuwana, J. Phys. Chem., 73, 3456 (1969). (8) G . C. Grant and T. Kuwana, J. Electroanal. Chem., 24, 11 ( 1 970).
onstrate the considerable utility of transmission experiments for mechanistic understanding and kinetic characterization of electrode processes involving coupled chemical reactions. Previous semi-infinite diffusion transmission spectroelectrochemical data have been acquired mainly with the Sn02coated electrode. The deposited thin metal film (Au or Pt) electrodes, developed for internal reflection (9-11) and thin layer (12, 13) spectroelectrochemistry, are also logically adaptable to the transmission experiment (3). The mirrorlike deposited metal films could alternatively be employed in a reflecting mode under semi-infinite diffusion conditions to acquire the same absorbance-time information. Another transparent working electrode, the Au minigrid electrode, has been introduced, to transmission spectroelectrochemistry for thin solution layer experiments (14-16). Certain advantages could accrue with this particular electrode in semi-infinite diffusion transmission experiments. Because the optical transparency of the minigrid is derived from its perforated character, the minigrid electrode transmission response should be less susceptible to artifacts caused by film deposition during electrolysis. The minigrid surface properties seem to be essentially those of bulk gold, which lacks many of the eccentricities exhibited by the SnOz electrode material. Lastly, the minigrid electrode, unlike the SnOzcoated and metal film electrodes, has a low internal ohmic (9) B. S. Pons, J. S. Mattson, L. 0. Winstrom, and H. B. Mark, Jr., ANAL. CHEM., 39, 685 (1967). (10) A. Prostak, H. B. Mark, Jr., and W. N. Hansen, J. Phys. Chem., 72, 2576 (1968). (11) W. von Benken andT. Kuwana, ANAL. C H E M . ,1114(1971). ~~, (12) A. Yildiz, P. T. Kissinger, and C. N. Reilley, ibid., 40, 1018 (1968). (13) C. N. Reilley, Reu. Pure Appl. Chem., 18, 137 (1968). (14) R. W. Murray, W. R. Heineman, and G. W. O'Dom, ANAL. CHEM.,39, 1666 (1967). (15) W. R . Heineman, J. N. Burnett, and R. W. Murray, ibid., 40, 1970 (1968). (16) Ibid., p 1974. ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
1069
