Gradients in Metallic Films. The ion microprobe was
used to study compositional variations in the metallic film Ag-Cu and the amorphous semimetallic alloy Al-Ge-Nb. Anderson (19) used a spectrophotometric method to monitor the vapor species produced by dc sputtering of the Ag-Cu eutectic alloy. He observed Ag/Cu ratios which varied with time and target temperature. At a target temperature of 80 "C,approximately 40 minutes of sputtering were required to obtain a constant Ag/Cu ratio. Longer presputter times were required at higher target temperatures. Several films were prepared by sputtering an Ag-Cu eutectic onto a cooled target after long presputter times to achieve steady-state. These films were then examined with the ion microprobe for variations in the Ag/Cu ratio as a function of depth. No substantial variations were found in several samples prepared in this manner. Thin film deposits of Al-Ge-Nb alloy were prepared by dc sputtering of an AI-Ge alloy button surrounded by an annulus of Nb. This configuration can be expected t o lead t o inhomogeneities in the ternary alloy film. The intensity versus sputtering time for 2 7 A l + and g*Nb+in the Al-Ge-Nb 19) G.
S. Anderson, J. Appl. Phys., 40,2884 (1969).
film is shown in Figure 4. Variations in concentration within the fdm are apparent. CONCLUSIONS
The ion microprobe has been shown to be a powerful tool for exploring isotopic and composition gradients in thin films of both metals and insulators. Suitable modifications of the instrument have provided a uniform sputtering rate and crater profile, so that reliable analyses can be obtained as a function of depth into the sample. Continuogs recording of intensities, and sputter rates as low as 0.5 A/sec enable depth resolutions of the order of 20 A. ACKNOWLEDGMENT
The authors thank J. P. S . Pringle for providing the duplex anodized T a 2 0 6 ; R. E. Pawel for the Ta2O5 anodized in HaPOa; and K. L. Chopra and M. R. Randlett for the AgCu and AI-Ge-Nb thin films. The comments and suggestions of R. F. K. Herzog and F. W. Satkiewicz are gratefully acknowledged. RECEIVEDfor review March 30, 1970. Accepted May 11, 1970.
Application of Photoelectron Spectrometry to Pesticide Analysis Photoelectron Spectra of Five-Membered Heterocycles and Related Molecules A. D. Baker, D. Betteridge,l N. R. Kemp, a n d R. E. Kirby2 Chemistry Department, University College of Swansea, Singleton Park, Swansea, U.K.
The photoelectron spectra of 15 five-membered aromatic heterocyclic and related compounds have been measured. It is shown that they are all sufficiently different to allow identification, and correlation diagrams have been prepared. The spectra are sensitive to changes in the substitution patterns of related compounds-e.g., 3-bromo- and 2-bromothiophene. The spectra are interpreted in terms of molecular orbital theory and it is shown that the electronegativity of the heterocyclic atom can be correlated with shifts of ionization potential.
WE ARE ENGAGED in examining the applicability of photoelectron spectrometry (PES) to the analysis of pesticides, herbicides, and other compounds of agricultural interest. This technique which measures the binding energies of electrons in molecules has considerable analytical potential (1) since the spectrum of a molecule reflects its molecular orbital energy diagram (2, 3). It has already been shown that the spectra of 1 To whom communications concerning this paper should be addressed. 2 Professor R. E. Kirby is on sabbatical leave from Queens College of the City University of New York.
(1) D. Betteridge and A. D. Baker, ANAL.CHEM., 42, (1) 43A (1 970). (2) W. G. Richards, Int. J. Mass Spectrom. Ion Phys., 2, 419 (1969). (3) J. H. D. Eland, ibid., p 471.
1064
relatively simple molecules, especially those containing atoms with nonbonding electrons, e.g., halogen, can be interpreted ( I ) , and that the spectra of substituted benzene compounds can be correlated with the inductive or mesomeric effect of the substituent. Most pesticides are too complex to have attracted theoretical chemists and pose technical difficulties to photoelectron spectroscopists. Nevertheless, rapid developments are taking place in theoretical chemistry and in photoelectron spectrometry so that we should be preparing for dealing with complex molecules by thoroughly investigating simpler pesticides, and examining cIosely related relatively complex molecules, which may serve as model compounds. This paper follows the last of these approaches by measuring and examining the photoelectron spectra of 15 five-membered aromatic heterocyclic and related molecules. The question of whether the heterocyclic atom in the aromatic system can be identified is obviously of analytical interest as also are possibilities of identification of substituent groups and substitution patterns. There are theoretical treatments of some of these molecules which have helped to interpret spectra and to answer these questions. It has also proved possible to prepare correlation diagrams which show that qualitative identification cia photoelectron spectrometry is possible and which may be of value in empirically interpreting the spectra of more complex molecules.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
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Table I. Ionization Potentials Measured from Photoelectron Spectra of 15 Compounds Studied AI and VI indicate adiabatic and vertical ionization potentials, respectively. Nondesignated values are taken from band maxima. Values are measured to rt0.05 eV, except for the cases marked with an asterisk, which may be less accurate
1st Band Compounds AI VI Thiophene 8.80 8.80 2-Bromothiophene* 8.50 8.50 8.90 9.00 3-Bromothiophene 2-Chlorothiophene 8.70 8.87 8.80 2,s-Dichlorothiophene 8.60 8.55 8.55 2-Iodothiophene 2-Methylthiophene* 8.14 8.32 3-Methylthiophene* 8.40 8.54 9.27 9.39 Pyrazole Pyrrole 8.22 8.22 N-Methyl Pyrrole 7.95 7.95 Furan 8.89 8.89 8.61 8.61 Cyclopentadiene Dicyclopentadiene 8.79 8.93 9.99 10.17 Isoxazole
2nd Band AI VI 9.44 9.44 9.41 9.41 9.51 9.51 9.62 9.62 9.71 9.78 9.47 9.47 8.96 8.96 9.11 9.11 10.00 10.00 9.22 9.22 8.80 8.80 10.30 10.42 10.60 10.70 11.82 11.13 11.29
3rd Band AI VI 11.46 11.86 10.74 10.74 10.67 10.67 11.55 11.55 11.58 10.00 10.00 11.8 11.94 10.53 10.73 12.70 11.92 12.67 12.63 12.87 11.78 12.59 15.90 13.31 13.85
4th 12.4 11.28 11.47 12.02 12.8 10.65 12.4 12.5 13.60 12.95 13.8 13.70 14.75 17.3 14.61
5th 13.1 12.18 12.3 12.36 13.1 12.17 13.0 13.1 14.62 13.6 17.1 14.35 16.35
6th 14.2 13.25 13.43 13.88 14.1 12.59 13.7 13.8 17.50 14.4
15.64
17.81
15.02
Bands 7th 16.4 13.85 14.1 14.37 14.4 13.5 16.1 15.9
8th 17.5 16.4 14.4 16.64 16.7 16.0
14.95
17.8
9th
10th
16.4 17.6 17.9
17.75
17.2
17.32
EXPERIMENTAL
The compounds studied were mainly commercially available samples. Cyclopentadiene, however, was prepared by cracking the dimer ( 4 ) . The sample of cyclopentadiene so obtained was cooled t o 0 “C while its spectrum was being taken t o prevent it reverting t o the dimer. Spectra were obtained by u$ng a slightly modified PerkinElmer PS 15 Helium 584-A photoelectron spectrometer. The PS 15 is similar in construction to the apparatus described previously by Turner (5), except that the effects of the Earth’s magnetic field is cancelled by a mu-metal shield rather than Helmholtz coils in the PS 15. The analyzer is also smaller in the commercial apparatus (5-cm mean plate radii as opposed t o 10-cm), and the slits used are not cuspoidal. The principal modification was t o the light source. A subsidiary pumping line was incorporated (Figure 1) t o pump away the majority of the helium gas emerging from the discharge tube. Previously all the helium entered the main chamber and was pumped away uia the instrument’s diffusion pump. The modification therefore reduced the main chamber background pressure, and $also reduced the amount of self-absorption of the H e 584 A line occurring in the collimating capillary tube. The net result was a n increase in counting rate compared with an unpumped lamp varying from 100 to 400% depending on the sample used and the pressure in the various parts of the instrument. Brundle (6) has previously used a lamp of this sort. Calibration of spectra was effected by using suitable compounds, e . g . , nitrogen, oxygen, methyl iodide, benzene, water, to provide “marker” peaks. The sample pressure and controls were then adjusted t o give the best possible spectrum. RESULTS
Spectra and Ionization Potentials. The spectra of the compounds studied are shown in Figures 2 to 12, in which bands containing distinct vibrational fine structure are usually shown o n a n expanded energy scale. The ionization potentials measured from the spectra are given in Table I. Comparisons of our results with existing experimental data
(4) E. H. Rodd, “Chemistry of Carbon Compounds, Volume IIa,” p 179, Elsevier, Amsterdam, 1967. ( 5 ) D. W. Turner, Proc. Roy. SOC.(London) 307A, 15 (1968). (6) C. R. Brundle, Bell Telephone Co., private communication, Murry Hill, N. J., Feb. 1969.
ll=’O’Rings
I
I
‘1 I
I
instrument body
f- Collimating
c apiIlary Figure 1. Incorporation of subsidiary pumping line into discharge lamp housing (see text)
are also given where they are of immediate relevance. Fuller consideration of the significance of the results is given in the Discussion section. Bands in the spectra are numbered in order of increasing ionization potential, which, in our spectra increases from left to right, Le., in the opposite way to those reported previously by Turner and his colleagues. (5). THIOPHENE (Figure 2). The spectrum closely resembles that reported previously by Eland (3), which was obtained on a parallel-plate type spectrometer. Better resolving power allows the more certain definition of fine structure in the first band, which indicates the excitation of vibrational modes having frequencies of about 600 and 1330 cm-l(75 meV and 165 mev), respectively. SUBSTITUTED THIOPHENES (Figures 3 t o 6). To our knowledge, these compounds have not been examined previously by photoelectron spectroscopy. PYRROLE AND N-METHYLPYRROLE (Figure 7). The spectrum of pyrrole is similar t o that reported previously by Eland (3). The principal difference is the improvement in the resolution of the fine structure associated with the first
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
1065
. . .
........
c-rr)
I
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Figure 2. (a) The 584-hi photoelectron spectrum of thiophene, and (b) the region between 8.8 eV and 9.9 eV shown on an expanded energy scale
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-I P(ew+
Figure 3. The 584-A photoelectron spectra of Z-chlorothiophene and 2,5-dichlorothiophene
band, which reveals that vibrational modes having frequencies 130 and 170 meV are excited by the ionization process. CYCLOPENTADIENE (Figure 8) and DICYCLOPENTADIENE (Figure 9). The high resolution photoelectron spectra of these compounds are reported here for the first time. The first band of the cyclopentadiene spectrum contains an especially complex fine structure, but progressions in 100 meV and 180 meV (ca. 810 and 1460 cm-1) can be picked out. A further progression intermediate between these also has to be invoked to account for the detailed structure in some of the peaks. 1066
10
11
12
13 -I
14
15
16
17
18
19
20
P (ev)
+
Figure 4. The 584-A photoelectron spectra of 2-bromothiophene and 3-bromothiophene
The spectrum of dicyclopentadiene is, as expected, completely different from that of cyclopentadiene, revealing only three distinct bands. FURAN (Figure 10). The spectrum of this compound has been reported previously by Baker and Turner (7) and by
(7) A. D. Baker and D. W. Turner, paper presented at the Royal
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9,AUGUST 1970
Society Symposium on Photoelectron Spectroscopy, London, February 1968.
-lJ?+
I
16
-73
-1
-
17
18
19
W
PleW-c
Figure 7. The 584-A photoelectron spectra of pyrrole and Nmethylpyrrole
l,f+,+
Figure 5. The 584-di photoelectron spectra of 2-methylthiophene and 3-methylthiophene
2nd WND-Exponded
--IPfeW+
Figure 6. The 584-A photoelectron spectrum of 2-iodothiophene
Eland (3). Eland has suggested that the seventh band of the spectrum consists of overlapping progressions in 110 and 118 meV. The overall shape of the band is however very similar to that of the 16.8 eV band of the benzene spectrum (8). By analogy with this, the 7th furan band can also be interpreted in terms of progressions in 100 meV and 354 meV, corresponding to the excitation of totally symmetric C-C and C-H stretching vibrations. ISOXAZOLE (Figure ll), and PYRAZOLE (Figure 12). These spectra are reported for the first time. DISCUSSION
Common Features in Spectra. From the point of view of molecular orbital theory, furan, pyrrole, cyclopentadiene, and thiophene are very similar. They can be regarded as being (8) A. D. Baker, C . R. Brundle, and D. W. Turner, Znt. J. Mass Spectrom. Zon Phys., 1,443 (1968).
1
-
1
x160
1
1
I;d
--1PW+ --IPff)--*
Figure 8. The 584-A photoelectron spectrum of cyclopentadiene
derived from benzene in that one of the -CH=CHgroupings of benzene is replaced in the former group of compounds by -0-, -NH-, -CH2-, and -S-, respectively. The heteroatoms 0, N, and S are all able to provide two a electrons, which in addition to the four a electrons of the ring makes a total of six ir electrons, which gives rise to an aromatic system. Classically combination of the five pz atomic orbitals produces five molecular orbitals, of which three are occupied in the neutral ground state molecules. The highest occupied orbital, a3,has nodes at the heteroatom and between
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
1067
-1
P(eV)+
-1
Figure 9. The 584-A photoelectron spectrum of dicyclopentadiene
I
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l
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1
,
l
l
I
l
l
9
70
11
72
13
74
15
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P (eVI-
-I P (eVI --* Figure 11. The 584-bi photoelectron spectrum of isoxazole
-I*
7th BAND Expanded
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..
:
: . . . /iI
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1
1
1
*: 1
.
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776 778 780
Figure 10. The 584-A photoelectron spectrum of furan
carbon atoms 3 and 4 ; the next highest, az,has nodes between the heteroatom and its adjacent carbon atoms, and the next, al,has no node. Thus the photoelectron spectra, which reflect the molecular orbital energy diagrams (1, 9, 10) should be, and are, similar. There are two bands in the region 8-12 eV which show some resolved fine structure. These can be ascribed to electron ejection from the two highest occupied a-orbitals. There are also present broad and usually featureless bands between 12 and 20 eV which are mainly due to the ejection of electrons from u-bonding orbitals. The spectra of the substituted compounds are similar to those of the parent molecules, but with additional bands due to ionization from the substituent group. The halogen substituted compounds exhibit sharp distinctive bands in the region 9-12 eV, between the upper two a and the u bands of the parent molecule, and the ionization potentials decrease in the order chlorine, bromine, iodine. The methyl derivatives have a broad hump, reminiscent of the methane spectrum ( 9 , 11), (9) A. D. Baker, Accounts Chem. Res., 3, (1970). (10) D. W. Turner, Plzys. Methods Adcan. Inorg. Chem., 74 (1968). (11) A. D. Baker, C. Baker, C. R. Brundle, and D. W. Turner, Intern. J. Mass Specfrom.Ion Plzys., 1, 285 (1968).
1068
0
-1 P lev)+
Figure 12. The 584-A photoelectron spectrum of pyrazole
through which peaks show. Substitution also results in changes in this region of the spectrum, both in the appearance of new peaks and the shift of others. Clearly, from the differences between all the spectra, they could be useful as "fingerprints." More quantitative information could be obtained if differences in the spectra can be ascribed to particular electronic or steric configurations. At the present time, there is little that can usefully be said about the a-region, but variations in the a-bands and lone pairs can be related to chemical changes. These correlations are dealt with in detail below. The highest occupied a orbital (a3)in the molecules containing one heteroatom has a node through the heteroatom. According to the simple LCAO approximation therefore, identical energies would be expected for the a3 orbital for these compounds. N-Methylpyrrole and pyrrole would also be expected to have identical a3energies on this basis. Early electron impact studies did appear to indicate a constancy in the lowest ionization potentials of pyrrole, furan, cyclopenta-
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
Table 111. Halogen Lone Pair Ionization Potentials in Halobenzenes and Halothiophenes Halogen lone pair Compound IPS, eV Chlorobenzene 11.3, 11.7 a-Chlorothiophene 11.6, 12.0 p-Dichlorobenzene 11.6 a,a’-Dichlorothiophene 11.6, 11.9 Bromobenzene 10.6, 11.2 a-Bromothiophene 10.8, 11.3 0-Bromothiophene 10.7, 11.5 Iodobenzene 9.6, 10.4 a-Iodothiophene 10.0, 10.7
Table 11. Eigenvalues for Thiophene and Furan Calculated by the CNDO MO Method Compound Eigenvalues, eV
Thiophene“
10.21 ( n ) 10.44 ( n ) 10.8 ( u ) 12.35 ( u ) 12.96 ( u ) 14.06 ( u ) 18.88 ( n ) 20.51 ( u )
Furan*
10.18 ( n ) 12.07 ( n ) 12.18 ( u ) 12.48 (a) 14.10 ( u ) 19.61 ( n )
Ref (27).
* Ref. (28).
diene (and butadiene) (12). However, it is clear from the evidence of the present, and earlier, studies that there is no such constancy. Eland (3) has successfully expressed some of the experimental observations using H M O theory by allowing for effect on the molecules of all substituents other than aromatic carbon. An appreciable number of experimental observations could nevertheless not be explained o n the basis of this extended Hiickel approach. For thiophene, the participation of S 3d orbitals in the bonding was mentioned as a possible contributing factor. Also such a participation would account for the similarities in many of the properties of thiophene and benzene (13). The heteroatom in the n2 orbital is a center of charge density; therefore, its energy should be dependent upon the nature of the heteroatom. Consequently, one would expect there to be a difference between a3 and nz as the heteroatom alters, and this is observed in the spectra. Evidence for S 3d Participation in Thiophene Bonding. The earliest references postulating S 3d orbital participation in the formation of compounds such as thiophene were due t o Schomaker and Pauling (14) and Longuet-Higgins, ( 1 3 , who and directed attention to the analogy between -CH=CH-S :- with 3p/3d hybrids. Consideration of this possibility has been incorporated into M O calculations. Eland (3) was unable t o predict n ionization potentials of thiophene by simple M O theory, although such a n approach was suitable for related compounds. He therefore concluded that S 3d orbital participation was probably responsible. Clark (16) however has recently carried out a series of CNDO M O calculations on the thiophene molecule, and concluded that as far as electron distribution is concerned, the inclusion of S 3d, 4s and 4p orbitals in his calculations had a larger effect on the u than the n electrons. His eigenvalues (17) are given in Table 11, together with those for furan calculated by a similar method (18). Dewar and Liicken (19) have proposed that the higher 35Cl nuclear quadrupole frequencies of chlorothiophenes compared with chlorobenzene provide good evidence for S 3d involvement. Their argument is that the increased electronegativity of S in thiophene compared with other S compounds brought about in thiophene by promotion of a 3p electron into a 3d (12) References given in A. Streitweiser, “Molecular Orbital Theory for Organic Chemists,” John Wiley & Sons, Inc., New York, N. Y., 1961, p 195. (13) J. Ridd, in “Physical Methods in Heterocyclic Chemistry,” A. R. Katrizky, Ed., Vol. I Academic Press, N. Y.,1963, Chapter 2. (14) V. Schomaker and L. Pauling, J. Amer. Chem. SOC.,61, 1769 (1939). (15) H. C. Longuet-Higgins, Trans. Faraday SOC.,45, 174 (1959). (16) P. Clark, Tetrahedron, 24, 2663 (1968). (17) P. Clark, University of Durham, U.K., private communication, 1970. (18) P. Clark, Tetrahedron, 24, 3285 (1968). (19) M. J. S. Dewar and E. A. Lucken, J. Chem. Soc., 1958, 2653.
Table IV. #3 and #2-Ionization Potentials for Butadiene and 5-Membered Heterocycles $3 n IP $2 n IP Compound (vertical values given) (vertical values given) 9.09
11.55
8.89
10.30
8.20
9.20
8.80
9.44
8.61
10.70
10.17
11.29
9.39
10.73
H
orbital thus forming the appropriate valence state is responsible. According to the Pauling and Allred-Rochow scales of electronegativities, S and C have the same values, and thus if there were no hybridization, Dewar and Liicken argue that the W l quadrupole frequencies would be expected t o be identical in chlorobenzene and chlorothiophene. Photoelectron spectroscopy reveals that the ionization potentials of halogen “p lone-pair” electrons in halothiophenes are also somewhat higher than the corresponding potentials for halobenzenes (20) (Table 111). The binding energies of core electrons of particular atoms have been found to mirror the effective charge of the atom concerned, calculated from the Pauling electronegativity (21). A similar approach for valence shell “lone pair” binding energies can be used. A straightline graph results from a plot of the F, C1, Br, and I px lone pairs ionization potentials of C6H$, CsHsCl, C6HaBr, and C6H61against the Pauling Electronegativity of the halogen substituents. The corresponding plot using Sanderson’s “relative-compactness” electronegativities (22) is slightly curvilinear. Analogous graphs are obtained for the halothiophenes and they suggest that the F 2p(x) lone pair ionization potential of 2-fluorothiophene is 13.8 eV. The C1 3p, ionization potentials in chlorobenzene, chloropyridine, and chlorofuran (23) predictably reflect the elec(20) A. D. Baker and D. W. Turner, Oxford University, unpublished
work. (21) K. Hamrin, C. Johannson, A. Fahlman, C. Nordling, K. Siegbahn, and B. Lindberg, Chem. Phys. Letters, 1, 557 (1968). (22) R. T. Sanderson, “Inorganic Chemistry,” Reinhold, New York, N. Y. 1967, p 77. (23) A. D. Baker, Ph.D. Thesis, London University, 1968.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
1069
9
8 l
1
l
10 1
Q
l
12
11 l
l
1
1
1
15
14
13 1
1
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13
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14
15
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17
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-I
Figure 13. Schematic representation of the spectra of all the compounds studied (see text) Each horizontal line represents a line drawn across each spectral band at half its height. Where bands in the spectrum overlay, an extrapolative procedure has been used to estimate the half-widths. Thus overlapping horizontal lines indicate a broad band with clearly defined overlapping components. Vertical lines indicate the positions of peak or band maxima.
tronegativity of the ring heteroatom. The electronegativities of the heteroatoms decrease in the order 0 > N > C while the C1 3p, ionization potential decreases through the series chtorofuran > chloropyridine > chlorobenzene. The C1 3p, q u a t i o n potential of 2-chlorothiophene is therefore anomal&s in that it is greater than that of chlorobenzene, which again iqplies involvement of S 3d electrons. On the Sanderson $ a l e of electronegativities that of S is greater than of C so that further work is necessary t o establish conclusively that the above anomaly is significant evidence for the participation of the S 3d orbitals in the binding of thiophene. Further work ~)n the applicability of electronegativities t o the determination ofvalence shell ionization potentials is at present in hand. Effects of Substituents on the r8- and arOrbitals. The effects of the heteroatom or "heterogroup" on the r3-and a?arbitals are of rather different types as can be seen from the ionization potentials of the unsubstituted compounds studied '$970
(Table IV). The biggest reduction in both AZ and a3ionization potentials in comparison with those of butadiene is for pyrrole. This is consistent with the larger + M effect associated with the -NH? group, than for example -OH, in compounds such as aniline and phenol (24). Conversely, the introduction of a nitrogen atom into a 5membered ring already containing one heteroatom greatly increases the r 3and r2ionization potentials (compare furan with isoxazole, and pyrrole with pyrazole), owing t o the electronegativity of N being greater than C. Since the lone pair associated with the additional nitrogen atom has the wrong symmetry to enter the aromatic r-system, its mesomeric effect is of no consequence, Separate bands due to this nitrogen lone pair should be visible in the spectra of isoxazole, (24) A. D. Baker, D. P. May, and D. W. Turner, J. Chem. SOC.,B, 22 (1968).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
~
pyrazole (and thiazole), but they occur in the same region as the a3- and 72-bands, and thus overlap with them. The bearing of these results on the interpretation of the spectra of pyridine and related molecules will be discussed elsewhere (25). When substitutents are introduced into thiophene, the ?r3 and TZ orbital ionization potential difference is changed significantly compared with unsubstituted compounds. a-Substitution is found to split the 7r3, TZbands more than P-substitution, an observation which may prove important in diagnostic analysis. Figure 13 shows in schematic form the spectra of all the compounds studied, so that shifts and correlations can be rationalized more rapidly. Each horizontal line in the figure has been constructed to be equivalent to a line drawn across a photoelectron spectral band at half its maximum height; each vertical line indicates the peak maximum and approximate relative intensity. We have found this sort of diagram to be more useful for correlation purposes than the conventional energy level diagrams used previously ( I I ) , in which a straight line was drawn on an energy scale at a position equivalent to an adiabatic (or vertical) ionization potential. The proposed new system immediately shows the presence of overlapping bands, whether a spectral band is sharp or broad, the presence of fine structure, and also indicates whether the peak is asymmetric. This last is especially useful since it is probable that the shape of peaks is important in identification work. Halogen Lone Pair Peaks. All the monohalothiophenes contain in their photoelectron spectra two bands attributable to orbitals approximating to C1 3p, Br 4p, or I 5p lone pairs [pz and py components: cf., halobenzenes (9,20, 24)]. The shapes and associated ionization potentials of the “lone-pair” peaks in a-and P-bromothiophene are rather different, as are other features in the spectra of these isomers. These could, of course, serve to distinguish the isomers. The shapes of the chlorine lone pair peaks in 2, 5-dichlorothiophene are also changed in comparison with the corresponding peaks in a-chlorothiophene. In principle, since each chlorine atom has associated with it two 3p lone pairs of electrons, there could be four “chlorine lone pair” peaks in the spectra of dichlorothiophenes if each lone pair were in a different electronic environment. However, because of the symmetrical arrangement of the chlorine atoms in 2,5dichlorothiophene, it might be expected that there were only two nonequivalent environments, and that these were equivalent to those in 2-chlorothiophene. This is confirmed in the spectrum of 2,5-dichlorothiophene by the appearance of only two bands which are obviously “lone pair” in origin. (The two chlorine lone-pair bands in the spectra of the mono- and dichloro derivatives are not exactly equivalent since the sharpest lone pair peak has the higher ionization potential in the dichlorothiophene spectrum, and the lower in monochlorothiophene.) Since there are four chlorine “lone pairs” in the dichloro compound and only two in the monochloro compound, the intensity of the chlorine peaks in the spectrum of the former should be twice that for the latter. The intensities of the chlorine peaks can be compared with those of the Tbands which are due to the same number of electrons in each molecule. The measurement of the peak areas under the chlorine and T peaks reveals that the “Cl peaks” area is about 170% greater in the di- than the mono- substituted compound. An exact 2 :1 ratio would not be expected since a number of factors .~
-
(25) A. D. Baker, D. Betteridge, N. R. Kemp, and R. E. Kirby, Chem. Commurz., 286 (1970).
a e.g. pyrrole thiophene
b eg. furan cyclopentadiene
Figure 14. Schematic diagram showing a predissociative pathway for (A)ions of pyrrole and thiophene ~
Table V. Vibrational Progressions Observed in x 3 Bands of Compounds Studied ProgresProgressions, sions, Compound cm-l Compound cm-’ Thiophene 565, 1290 Furan 1050, 1370 2-Chlorothiophene 435, 1290 Pyrrole 1050, 1450 2,5-Dichlorothiophene 325, 1615 Isoxazole 565, 1370 3-Bromothiophene 970 Pyrazole 870 2-Iodot hiophene 970 Cyclopentadiene 685
determine the relative intensities of bands (26), and we have not corrected for these. Nevertheless, that the observed ratio is significantly closer to 2: 1 than 1 : 1 indicates that PES might be used to estimate the number of halogen substituents in compounds. Fine Structure in the x 3 and x 2 Bands. The photoelectron spectra of furan and cyclopentadiene reveal well-defined fine structure in both the r3and in bands, but the spectra of pyrrole and thiophene reveal clear structure only in the first band. This is puzzling in view of the similarities of the ionization processes occurring in each case. The explanation is possibly that the ions resulting from the ionization of an electron from the r 2orbitals of pyrrole and thiophene are predissociated according to the scheme shown in Figure 14, whereas the corresponding ions formed from furan and cyclopentadiene are not. Such an explanation would be in accord with the greater 7ra-rZ ionization potential difference for furan and cyclopentadiene compared with pyrrole and thiophene. The fine structure observed in the first bands of the spectra of furan, thiophene, pyrrole, and cyclopentadiene is of a similar type in each case. The excitation of at least two vibrational modes is indicated, with frequencies summarized in Table V. On the basis of the nodal properties of the highest (26) D. P. May and D. W. Turner, J. Chem. Phys., 45, 471 (1966).
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Compound Progressions
2-Chlorothiophene
(cm-l)
Table VI. Vibrational Progressions in the 2,5-Dichloro2-Iodothiophene thiophene Furan
970
970
~2
Bands of Compounds Studied
Pyrrole
Isoxazole
Pyrazole
Cyclopentadiene
970
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1535
805
970
805
ger 50.
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.
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-
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,
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-
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Figure 16. Infrared spectra of a- and fl-bromothiophene 1072
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
-
in L e
700
6W
500
400
300
ZW
occupied a-orbital (high electron density over the chain of four carbon atoms) one of the vibrations excited is almost certainly v 3 (27), and it seems reasonable to assign this to the highest frequency vibration observed in each case. The frequencies of the vibrational progressions observed in the second bands of the spectra are summarized in Table VI. Comparison with Other Spectroscopic Methods (Figures 15 and 16). The mass spectra and infrared spectra of 2-bromo- and 3-bromothiophene are shown for comparison (28, 29). The former does not allow ready differentiation between the two compounds; the latter, over the range scanned, does not directly reflect the presence of bromine in the molecules. Both, of course, convey information that is not contained in the photoelectron spectra; thus all the techniques are complementary. It is not possible to say what information might ultimately be gleaned a priori from the photoelectron spectra by an expert because so far there has been no systematic attempt to (27) Notation of G. Herzberg, “Molecular Spectra and Molecular Structure, Part 111,” Van Nostrand, New York, N. Y., 1966. (28) B. Akesson and S . Gronowitz, Ark. Kemi, 28, 155 (1967). (29) S. Gronowitz, Ark. Kemi. Mineral. Geol., 7,267 (1954).
correlate the a-bonding region from 13-21 eV, and so few spectra have been measured. However, even at the present time it would be possible at the very least to deduce the presence of a halogen substituent adjacent to a a-system, to identify the individual halogen, to infer that the a-system was associated with an aromatic system other than benzene and, with the aid of correlation diagrams and comparative spectra, to identify the compound. Insofar as the basic ionization process is similar to that in the argon-ionization detector used in gas chromatography, it is reasonable to assume that great improvement in sensitivity is possible and that photoelectron spectrometry should become a useful tool of the analyst.
ACKNOWLEDGMENT We are grateful to the Agricultural Research Council for providing the photoelectron spectrometer. RECEIVED for review February 2, 1970. Accepted March 23, 1970. A research fellowship to A.D.B. from the Agricultural Research Council and a maintenance grant to N.R.K. from the Science Research Council are gratefully acknowledged.
Determination of Actinides in Biological Samples with Bidentate Organophosphorus Extractant F. E. Butler’ and R. M. Hall E . I . du Pont de Nemours & Company, Savannah River Laboratory, Aiken, S. C. 29801 A procedure for the determination of actinides was developed using the bidentate extractant dibutyl N,Ndiethylcarbamylphosphonate. Nine actinides were extracted from 12N “Os, back-extracted to 2N “Os, and counted in a low-background alpha counter. A procedure was developed for sequential extraction of plutonium, neptunium, and uranium with tri-isooctylamine (TIOA), followed by extraction of thorium, americium, curium, berkelium, californium, and einsteinium with bidentate. Compared with previous methods, the new procedure is simpler, requires less analysis time, and gives better recovery. The recovery of Am-Cm-Cf from 250 ml of urine or 20 grams of feces was 90%. Sensitivity of analysis is 0.02 f 0.01 d/min/sample. An alternative method of exchange of trivalent actinides as oxalate anion complexes with TIOA is also described.
PRODUCTION OF TRANSPLUTONIUM elements has increased in the past five years, thus increasing the risk of personnel exposure ( I ) . Programs for the large-scale production of 244cm (2) and 252Cf (3, 4) have been reported. A dependable, simple bioassay method was needed therefore to determine the 24aAm,244cm, and 262Cf produced in these programs. Present address, Southeastern Radiological Health Laboratory, Montgomery, Ala. 36101
The previous method for determining the trivalent actinides in this laboratory employed the liquid ion exchanger di-2ethylhexylphosphoric acid (HDEHP) (5). Although americium, curium, and californium exchanged to HDEHP from acid solution adjusted to pH 4 to 5, calcium also exchanged in sufficient amount to interfere with the determination. An additional extraction step was required to extract the actinides to thenoyl trifluoroacetone. This extraction was also from a solution adjusted to p H 4 to 5. These extractions are time consuming and tedious. The method was used for analysis of urine and blood, but it was not suitable for analyzing feces. Siddall reported on bidentate chelating compounds with the unique property of extracting trivalent lanthanide and actinide elements from highly acidic waste concentrates from the reprocessing of nuclear fuels (6). These compounds contain two C==O or P = O complexing groups. Dibutyl N,N-diethylcarbamylphosphonate (DDCP), the most promising of the bidentates, was used in development of the procedure reported here. However, because this bidentate was not initially available in sufficient quantity for routine analyses, another procedure for oxalate anion complexing of trivalent actinides with tri-isooctylamine (TIOA) was developed. This procedure is also reported. EXPERIMENTAL
(1) D. H. Denham, HealthPhys., 16,475 (1969). (2) H. J. Groh, R. T. Huntoon, C . S. Schlea, J. A. Smith, and F. H. Springer, Nucl. Appl., 1, 327-36 (1965). (3) W. C. Reinig, ibid., 5, 24 (1968). (4) D. E. Ferguson and J. E. Bigelow, Actinides R e a , 1, 213 (1969).
Bidentate Method. REAGENT. Dibutyl N,N-diethylcarbamylphosphonate (DDCP), was shown by Siddall (6) to be ( 5 ) F. E. Butler, ANAL.CHEM., 37, 340 (1965). (6) T. H. Siddall 111, U. S. Patent 3,243,254 (1966).
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