STUDY OP PHOSPHORUS-SULFUR COMPOUNDS
3975
solvent molecule contains ?r electrons as in acetone and dimethyl sulfoxide. To explain the graphical observations we contend that this dipole-dipole interaction must be of the following form. ud(i,a)
=
ud(i)ud(a)
(10)
Concerning Figure 2 the vertical deviations from the nonpolar line represent the contributions due to the dipole-dipole interactions. For a given class of solutes (ie., mono-dipolar, di-dipolar or iri-dipolar) this dipole-dipole effect must be proportional to the solvent part of the van der Waals term since linear lines can be passed thru each class of solutes, the lines intersecting at a common point where S*(i,CC&) equals zero. The differences in slopes are accounted for by ud(i) which must be a function of the number of dipoles in the solute molecule as well as the geometrical shape. The fact that all solute points lie on the same linear line in the acetone-dimethyl sulfoxide plot (Figure 3) also suggests that the solvent dipole term is proportional to the solvent van der Waals term. That is ud(a) = P d ( a )
UT(@!)
(11)
where Pd((Y) takes into consideration the shape of the solvent molecule, its dipole moment, the ?r-electron configuration, and other factors. For acetone and dimethyl sulfoxide the probability term Pd(a) must be nearly identical because of the similarities in geometrical structure, in their ?r-electron distribution, and in the location of the dipoles. I n conclusion, then, we propose that the solvent effect on proton magnetic resonance of the molecules studied here can be accounted for the by expression S(i,a)
=
6,(i)
+ uda) +
Ub((Y)
+
uw(i,a)
+ u & a ) + ud(i,a)
Here u&a) is a modified reaction-field term, having zero value for nonpolar solvents. The last term, Ud(i,a)) accounts for the dipole-dipole interaction between the solute and solvent molecules. This term is found to be proportional to the van der Waals term. The exact nature of this term is not fully understood.
Acknowledyment. This investigation was supported in part by the U. S. Army Research Office (Durham) under Contract No. DA-31-124-ARO-D-90.
A Study of Phosphorus-Sulfur Compounds by Inner-Orbital
Photoelectron Spectroscopy: Thiono-Thiolo Sulfur by Wojciech J. Stec,’ William E. Moddeman,2Royal G. Albridge, and John R. Van Wazer*a Departments of Chemistry and Physics, Vanderbilt University, Nashville, Tennessee
37,905
(Received ApriE 7 , 1071)
Publication costs borne completely by The Journal of Physical Chemistry
The binding energies of the “2s” and “2p” orbitals of sulfur and the “2p” orbitals of phosphorus have been measured for a series of covalent phosphorus cornpounds in which a sulfur atom substitutes for an oxygen either as an electron-pairacceptor for the phosphorus (Le., in the “isolated” position) or as a bridge between two phosphorus atoms. For this series of compounds, the measured sulfur inner-orbital binding energies are higher by about 1 eV for the P-S-P linkage than for the P=S arrangement. The position of the sulfur in the molecule has a smaller effect on the phosphorus “2p” binding energy. Comparison of these results with similar data on related compounds shows that the findings cannot be simply related from one series of compounds t o another. Some new nmr data are also presented.
I. Introduction Because the chemistry of compounds involving a sulfur bonded to phosphorus still poses many unresolved problems concerning whether or not the sulfur is in a bridging position between the phosphorus and another atom (thiolo) or is in an isolated position (thiono) on the
phosphorus, we thought that the relatively new techniWe of inner-orbital Photoelectron sPeCtrosCoPY5 (1) On leave of absence for 1970 from the Polish Academy of Sciences,Eods. (2) PoStdo~toralFellow 1970-1971. (3) TOwhom reprint,requests should be addressed.
T h e Journal of Physical Chemistry, Vol. 76, N o . 86,1971
W. STEC,W. MODDEMAN, R. ALBRIDUE, AND J. VANWAZER
3976
might be a fruitful one for study of this topic. Although a considerable amount of data has resulted from sulfur inner-orbital photoelectron spectroscopy of sulfur compounds616and there is also some information on the binding energies of phosphorus inner orbitals of phosphorus-sulfur compound^,^^^ the possibility of characterizing a sulfur atom as thiono or thiolo has not previously been investigated by this technique. Emphmis in this study has been on the derivatives of 5,5-dimethyl-1, 3-dioxapho~phorinanyl~ since these compounds are high-melting solids having low vapor pressures, and their structures seem to be reasonably well demonstrated by the chemical reactions whereby they were obtained (see the pertinent references in the Experimental Section), as well as by infrared and proton nuclear magnetic resonance (nmr) dah9-12 These compounds have the following structures
a,
x=Y
I =
z= 0
b,X=S;Y=Z=O c,X=Z=S;Y=O d,X=Z=O;Y=S e,X=Y=S; Z = O
I1 a,X b,X
=
=
Z =0 S; 2 = 0
11. Experimental Section Photoelectron Spectroscopy. The photoelectron spectrometer used in this study with magnesium and aluminum X-ray sources has been previously described.13 The spectra were calibrated using the carbon “1s” binding energy and the experimental energy resolution was 0.17%. All of the carbon “1s” lines for compounds I a to Ie, IIa, and I I b of Table I showed partially resolved line structures in which there was a main peak about twice as high as an overlapped secondary peak, the latter exhibiting a binding energy about 2.1 eV higher than the main peak. Since this main peak was the only one to grow with increasing on the sample), time (due to buildup of “pump it was assumed that this peak would exhibit a binding energy of 285.0 eV, which is the accepted value for the carbon “1s” line of aromatic and aliphatic hydrocarbons.16 For compounds V and VI of Table I, the carbon reference peak was arbitrarily assigned the same binding energy of 285.0 eV, and this was also done for the compounds VI1 through IX, where the carbon was presumed t o be attributable t o residual carbon vapor. Peak widths at half-height were measured for the observed lines to adjudge whether or not a given line might consist of two coalesced peaks. Unfortunately the entire set of measurements were made over a 6month period so that the settings of the slit defining the electron beam were, because of mechanical diffieulties, not always identical. Therefore the observed line widths have only qualitative meaning. Both The Journal of Physical Chemistry, Vol. 76,N o . 96,1971
magnesium and aluminum were employed as anodes in the X-ray source and each anode was used for every compound to assure that the peak-width measurements were not affected by the presence of a foreign line, such as an Auger peak. Note that the P p used for the aluminum anode was 116.780 and for the magnesium anode 104.500 G-cm.l6 Nuclear Magnetic Resonance Spectroscopy. The alp nmr studies were carried out on a Varian XL-100-15 spectrometer, with the sample being contained in a 5-mm tube centered in a 12-mm tube. The annular space between the tubes was filled with deuteriochloroform to maintain a heteronuclear lock on the deuterium nucleus, plus a small amount of triphenyl phosphite as an “external” slPreference for the sample. All measurements were converted into 85% phosphoric acid as the reference standard (with positive shifts being upfield), on the basis that the shift of triphenyl phosphite under the conditions of measurement is 180.1 ppm downfield from H3P04. (4) For example, see D. E. C, Corbridge, “Topics in Phosphorus Chemistry,” Vol. 3, M. Grayson and E. J. Griffith, Ed., Interscience, New York, N. Y., 1966, p 309. Also see E . Fluck. “Topics in Phosphorus Chemistry,” Vol. 4, M. Grayson and E. J. Griffith, Ed., Interscience, New York, N. Y., 1967, p 370. ( 5 ) K. Siegbahn, C. Nordling, A. Fahlman, R. Nordberg, K. Hamrin, J. Hedman, G. Johansson, T.Bergmark, S.E. Karlsson, I. Lindgren, and B. Lindberg, Nova Acta Regiae SOC.Sci. Upsal., [4] 20, 1 (1967). (6) J. Hedman, P. F. Heden, R. Nordberg, C. Nordling, and B. J. Lindberg, Spectrochim. Acta, Part A , 26,761 (1970). (7) W. E. Morgan, W. J . Stec, R . G. Albridge, and J. R. Van Wazer, Inorg. Chem., 10, 926 (1971). (8) M. Pelavin, D. N. Hendricltson, J. M. Hollander, and W. L. Jolly, J . Phys. Chem., 74, 1116 (1970). (9) For compounds Ia, Ib, IC,and Ie, see K. D. Bartle, R. 8. Edmundson, and D. W. Jones, Tetrahedron, 23, 1701 (1967). (10) For compounds I b and Id, see A. R. Katritsky, M. R. Nesbit, J. Michalski, A. Zwierzak, and 2. Tulimowski, J. Chem. Soo. B , 140 (1970). (11) For compound IIb, see R. K. Harris, J. R. Wolpin, and W. J . Stec, Chem. Commun., 1391 (1970). (12) Additionally, the structures of compounds I I a and I I b have been fully proven by X-ray analysis, see 2. Gatdecki and J. Wojciechowska, paper submitted for publication. (13) A, Fahlman, R. G. Albridge, R. Nordberg, and W. M. LaCasse, Rev. Sci. Instrum., 41, 596 (1970); R. Nordberg, H. Brecht, R. G. Albridge, A. Fahlman, and J. R. Van Waaer, Inorg. Chem., 9, 2469 (1970), (14) Since our spectrometer uses a turbo-molecular pump instead of an oil pump, this commonly used phrase is a misnomer. Also, silicone O-rings and silicone greases are used throughout the spectrometer so that one might imagine that the “pump oil” line might be attributable to the carbon of dimethylsiloxanes, which does not exhibit a “1s” binding energy of 285.0 eV. However, a study using magnesium fluoride as the substrate for the “pump oil” showed the usual buildup of the carbon ”1s” line but no evidence for the concomitant appearance of any silicon lines even after 12 hr. Thus, we believe that the residual carbon appearing in our spectrometer is due to stray organic vapor (perhaps volatiles coming from the adhesives on the Scotch tape) which has a sufficient number of methyl and methylene groups (as well as aromatic groups having only one or two noncarbon substituents per ring) so that the value of 285.0 eV is applicable for its “1s” binding energy. (15) U. Gelius, P. F. H e d h , J. Hedman, B. J. Lindberg, R. Mannep R. Nordberg, C. Nordling, and K. Siegbahn, Phys. Scripta, 2, 70 (1970). (16) K. Siegbahn in “Beta and Gamma Ray Spectroscopy,” Vol. I, K . Siegbahn, Ed., North-Holland Publishing Co., Amsterdam, 1965, Chapter 111.
STUDY OF PHOSPHORUS-SULFUR COMPOUNDS
3977
Table I : Observed Inner-Orbital Electronic Binding Energies of Phosphorus and Sulfur
Structure
Compd
"25"
"2P"
...
I&
134.3
Ib
134.2 + 0 - 2
227.2
162.7 31 0 . 2
IC
134.3
0.2
227.1
162.6 i: 0 . 2
Id
133.9 i:0 . 2
228,O
163.8 i.0 . 2
Ie
133.9 f 0 . 2
227.3
163.2 f 0.3"
11%
133.4
IIb
133,5 f 0 , l
I11 IV
x->p-
(H3NC6HI,)+
(CzHsO)zPO-Na+
I , .
162,7 I 0 . 1
133.4 =t0 . 2
...
162.0 f 0 . 2
134.3 =k 0 . 1
...
163.2 f 0 . 1
S S
v
CHsC'
162.5 f 0 . 4
Na+ '0 0
CHaC
VI1 VI11
p4s10
IX a
//
VI
P4Ss
... 134.9 i: 0 . 1 134.5 f 0 . 3
SS
...
161.8 f 0 . 3
*..
163.1 i 0 . 4 163.4 f 0 . 1 163.9 f 0.1
..,
227.9 =k 0.1
Graphical resolution of this wide peak led to two peaks exhibiting binding energies of 163.6 and 162.3 eV
Sample Preparation. All of the compounds used in this study were fully crystalline materials which (except for the salts) had been recrystallized at least once. Each sample was prepared by grinding a batch of crystals in a small vibrating ball-mill. The resulting powder was dusted onto a piece of Scotch-brand permanent-mending tape which was then tapped several times; more powder was sprinkled on, followed by a final tapping to affix the powder so that none of the underlying Scotch tape would be exposed to the X-ray beam. At least three replicates from start to finish were run for each sample in Table I, where the observed standard deviations between repeated measurements are shown. The 5,5-dimethyl-1,3-dioxaphosphorinany1 derivatives were synthesized according to the methods given in the literature. These include a preparation" for Ia, IC,and 111; another for Ib, and one for Id.19 Compounds I I a and b were made according to another reference.20 Compounds V and VI were prepared by appropriate neutralization of commercially
available thioacetic acid. The P& was a commercial sample; whereas the P4Sl0and Ss were recrystallized from commercial samples.
111. Results and Discussion Inner-Orbital Binding Energies. The experimentally determined inner-orbital-electron binding energies of phosphorus and sulfur are reported in Table I for the compounds studied herein. It should first be noted that the difference between the highest and the lowest reported "2p" binding energy is 1,5 eV for phosphorus and 2.1 eV for sulfur. Thus, we are dealing with small changes that lie within the width at half-height (ranging from 2.5 to 4.0 eV) of any of the peaks. The presence of two different kinds of phosphorus or two different (17) R. S. Edmundson, Tetrahedron, 21,2379 (1965). (18) R. S. Edmundson, Chem. I n d . (London),784 (1963). (19) J. Michalski, M. Mikolajczyk, B. Mlotkowska, and A . Zwierzak, Angew. Chem., I n t . Ed., 6 , 1079 (1967). (20) W. Stec and A. Zwierzak, Can. J . Chem., 45,2513 (1967). T h e Journal of Physical Chemistru, Vol. '76,
86, 1973
3978
W. STEC,W. MODDEMAN, R. ALBRIDGE, AND J. VANWAZER
kinds of sulfur atoms in a molecule generally resulted in a widening of the respective spectral peaks, with no evidence of separation into two peaks. The phosphorus “2p” data on compounds Ia through e demonstrate that substitution of a bridging oxygen atom by a sulfur reduces the “2p” binding energy of the phosphorus by ea. 0.5 eV, whereas, for a similar substitution in the isolated position on the phosphorus, the change in binding energy is inappreciable (50.1 eV). Likewise the thiolo sulfur exhibits a binding energy which is ea. 1 eV higher than the thiono. Clearly, the difference between thiono and thiolo shows up more distinctly in the sulfur rather than the phosphorus “2p” binding energy. Note that, in conformity with previous experience, the changes in the sulfur “2s” binding energies reasonably parallel those in the sulfur “2p” binding energies. Because compounds Ib, Ie, and IIb contain phosphorus in two chemically different positions and compound Ie has two distinguishable sulfur atoms, it should be expected that the respective “2p” peaks would be unusually wide. This was not found to be the case for the phosphorus in compounds Ib, Ie, and I I b but was observed for the sulfur in compound Ie. Further, an equimolar mixture of compounds I b and d was run several times, and the results were compared with the binding-energy spectrum of compound Ie. Both the phosphorus and sulfur “2p” peaks of the mixture were unusually wide (with the sulfur peak being about as broad as that observed for compound Ie), and the sulfur “2p” binding energy was 163.2 i 0.3 eV and the phosphorus 133.9 i 0.2 eV. Graphical resolution of the broad sulfur “2p” peaks of the mixture and of compound Ie indicated that the thiolo and thiono binding energies differ by ea. 1 eV--the same value which was obtained from separate peak measurements on compounds I b or ICvs. Id. For compounds IIa and b, it is again seen that the substitution of sulfur for oxygen in the isolated position has no appreciable effect on the phosphorus “2p” binding energy. Further, the sulfur “2p” binding energy of the thiono sulfur is the same mhether the adjacent phosphorus is bonded to another phosphorus atom (compound IIb) or is connected to that phosphorus through an oxygen bridge (compounds I b and e). Compound I11 exhibits a phosphorus “2p” binding energy which is ca. 1 eV smaller than that of compound IC,indicating that converting from a bridging to a negatively charged oxygen in the structure leads to reduction in the positive charge5 on the phosphorus atom. This is also accompanied by a change in the same direction, although smaller, in the charge borne by the thiono sulfur. The observed diff ercnces in measured binding energies between compounds I11 and IV are difficult to interpret since they may be attributable in some part to small differences in the carbon “1s” reference peak.*I This The Journal of Physical Chemistry, Vol. 76, No. 16,2071
also applies to a direct comparison of the data of compounds V and VI with the other binding energies shown in Table I. However, it is important to note that the sulfur “2p” energy of compound J7 is larger than that of compound VI; whereas that of compound I b is smaller than that of compound Id. If we accept the common view that the silver atoms of silver thioacetate will be bonded to the sulfur atoms, whereas the sodium atoms of sodium thioacetate will be ionized to leave a preponderance of negative charge on the oxygens, the data of Table I suggest that a sulfur atom bridging between a carbon and a silver atom has a different binding energy than a sulfur bridging between two phosphorus atoms. This corresponds to a greater electron-withdrawing effect on the sulfur by the phosphorus atoms in compound Id or e than by the silver and carbon atoms in compound VI. Compounds VI1 through IX are included in Table I for reference purposes. Since the carbon peak observed for these structures is surely attributable to residual carbon, the reported binding energies may not be directly comparable to those of the other structures, owing to small differences in the absolute value of the binding energy corresponding to the center of the carbon “1s” peak. However, it is interesting to note that the reported “2p” binding energies for the sulfur bridging between two phosphorus atoms in compound Id and for the sulfur bridging between two sulfur atoms in Ss (compound IX) are close to each ohher, although these values are 0.8 eV higher than the reported sulfur ‘ ( 2 ~binding ” energy for P4S3(compound VII) in which each sulfur atom bridges between two phosphorus atoms. The nuclear magnetic reso3lP Chemical Shifts. nance spectroscopy results on compounds I a through e and for I I a and b are given in Table I1 from which it can be seen that the alp chemical shift of the phosphorus bearing the phosphoryl oxygen lies in an entirely different region of the spectrum (-10 to -25 ppm) than that of the phosphorus bearing the thiono sulfur (-65 to -40 ppm). It is obvious from the data of Table I1 that nuclear magnetic resonance spectroscopy is a much more powerful tool for differentiating between these various structures than is inner-orbital photoelectron spectroscopy. By analogy to known compounds, the values of the chemical shifts shown in Table I1 can be roughly predicted. Thus, from the shifts of 41,-26, -68, -95, and f9 ppm of OP(OR)z, OP(OR)2(SR), SP(OR)s, SP(OR)dSR), and OP(OIQ\”-\{, -/ where R = C211s,obtained from the 0literature,22we would estimate the following shifts for (21) The carbon bonded t o an oxygen will exhibit a different binding energy (ca. 287 eV) than the other carbon atoms in a purely hydrocarbon environment (285 eV). Thus, the average peak position may vary depending on the ratio of carbon to oxygen atoms in the alkoxy1groupings.
3979
STUDY OF PHOSPHORUS-SULFUR COMPOUNDS the compounds shown in Table 11: Ia, + l o ; Ib, +lo, -59; Id, -17; and Ie, -17, -86 ppm, Note that these estimated values (which are systematically low by about 10 ppm) are based on the assumption that the effect of the C-0-P linkage on the chemical shift of the phosphorus would not be much different than that of a P-0-P linkage.
such as phosphorus depends mainly on the p and d contributions (including cross terms) to this matrix,22 these properties exhibit quite different dependencies on the molecular wave function. Therefore, although we have interrelated these properties in another paper7 (via a calculation of atomic charges), it does not seem profitable to do this again for the data given in Tables I and 11.
IV. Conclusions Table 11:
Nmr D a t a Taken on Saturated Solutions in Deuteriochloroform Chemical shifts,bppm
Compd‘
60P
It3
21.5
Ibc
21.9 -6.0 -6.0 1.7 5.5
Id
Ie IIa
IIb
asp
Coupling constants,
JHCOP
Hz
JPP
16
-43.6 26 -64.5
-58.9
12 20 (0)
491
18 (SI a Because of very low solubilities in CHC13, [ ( C H S ) ~ N ] ~ Cand O , CF&OOH, no measurements compound IC. Referenced to 85% H3POa,with being upfield. ‘ Edmundsonls has reported shifts to +22.5 and -45.5 ppm, with JHCOP = 28 €12.
CDC13, CeH8, were made on positive shifts corresponding
Since the variations in the inner-orbital binding energies are primarily sensitive5 to the diagonal terms corresponding to the chosen atom in the charge-bond order matrix22while the nmr chemical shift of a nucleus
The results reported herein are disappointing in that the differences in inner-orbital binding energies from one compound to another are small relative to the precision of the measurements. However, the reported data do indicate experimentally that the switching of a sulfur atom from the thiono to the thiolo position has a small electron-donating effect on the phosphorus atom, coupled with electron removal from the sulfur. The major value of inner orbital photoelectron spectroscopy lies in the fact that there is a reasonably good correlation between the measured binding energy and the atomic ~ h a r g e , Thus ~ the method, although crude and inherently not capable of a large increase in precision, furnishes unique data of chemical interest and hence serves as a qualitative check on theoretical predictions as to charge assignments in molecules.
Acknowledgment. We wish to thank the Kational Science Foundation for partial support of this work. (22) M. M. Crutchfield, C. H. Dungan, J. H. Letcher, V. Mark, and
J. R. Van Waner, “Pgl Nuclear Magnetic Resonance,” Wiley-Interscience, New York, N. Y., 1967.
The Journal of Physical Chemistry, Vol. 76, N o . 26,1971
