Use of Paramagnetic Shift Reagents for Simplication of Nuclear Magnetic Resonance Spectra of Organic Nitriles Judy A. Young a n d J e a n e t t e G. Grasselli The Standard Oil Company, (Ohio), Research Department, Cleveland. Ohio 4 4 128
William M. Ritchey Case Western Reserve University, Cieveland, Ohio 4 4 106
The use of paramagnetic shift reagents to simplify the proton NMR spectra of several classes of organonitriles has been demonstrated. Because organonitriles are weak donors, the fluorinated shift reagents Eu(FOD)3 and Pr(FOD)3, which are stronger Lewis acids than the DPM chelates, were far more effective in generating spectral shifts for the compounds studied. For the unsaturated nitriles, dramatic simplifications of complex second-order spectra were obtained. Chemical shifts for the complexes were confirmed by spin-decoupling experiments. and accurate coupling constants were measured. For heterofunctional molecules, steric effects around the complex site and of the lanthanide-shift reagent were both found to be significant. The oxynitriles showed that the ether oxygen is the preferred site of complexing over the nitriles, but participation of both sides was demonstrated. It is suggested that when two sites are involved they act independently, essentially as if they were present on different molecules. Equilibrium constants for a representative group of the compounds studied were determined and found to be of the expected magnitude.
Reagents. The lanthanide shift reagents were purchased from Norell Chemical Company in New Jersey. The reagents were stored in a desiccator. The shift reagents used were tris(2,2,6,6,tetramethyl-3,5-heptanedionato)europium(III)also known as tris(dipiva1omethanato)europium [Eu(DPM)s] and t r i s ( l , l , l , 2,2,3.3-heptafluoro-7,7-dimethyl-4,6-octanedione)europium(III) or -praseodymium(III). Abbreviated forms for the latter two shift reagents are Eu(FOD)3 and Pr(FOD)3. Procedure. Spectra of the nitriles, all reagent-grade chemicals, were obtained in deuterated chloroform and/or in carbon tetrachloride. Chloroform-d was initially passed over 4A molecular sieves to remove acid impurities which may decompose the complex. Concentrations, in moles per liter of ligand, were in the range 0.2 to 2.3. Shift reagent was added incrementally to a solution in some series. and in others each sample was prepared individually. The mole ratio refers to moles of shift reagent/mole of ligand. Straight line concentration graphs were obtained by least-squares fitting of the data. Slopes (gradients) and the y intercepts (extrapolated chemical shift of the free ligand) were also obtained from this least-squares program. The shift difference between the pure complex and the free ligand, defined as A' and approximately equal to the slopes (gradients) of the lines, was calculated for most of the compounds using the method described by Goldberg and Ritchey ( 2 ) . All the compounds were examined over a wide range of mole ratios, and the total ligand concentration was also varied.
RESULTS AND DISCUSSION Recent work has been reported to demonstrate the effective simplification of nuclear magnetic resonance spectra of alcohols, ketones, ethers, esters, and amines with the use of paramagnetic shift reagents ( I ) . The effect of the pseudo-contact interaction between the lanthanide chelate and the coordinated ligand is an induced shift in the NMR spectrum for various protons in the ligand. The magnitude and direction of the shift is a function of the metal complex used, the mole ratio of complex to ligand, the concentration of the solution, and the distance and geometry of the protons from the coordination site. This study reports the use of various shift reagents with organonitriles. In addition to alkyl and unsaturated nitriles, a group of compounds containing difunctional coordination sites was studied with special interest in the effect of a homodifunctional us. a heterodifunctional group on site and strength of complexation.
EXPERIMENTAL Apparatus. The compounds studied and some of the conditions used are outlined in Table I. NMR spectra were obtained on a Varian XL-100-12 spectrometer operating a t 30 "C and using tetramethylsilane (TMS) as an internal standard. Spectra were recorded at various sweep widths and times in order to facilitate accurate measurement of chemical shifts and to better observe splitting patterns for interpretation. Chemical shifts and coupling constants are accurate to 0.1 Hz for data measured at a sweep width of 100 Hz. Homonuclear spin decoupling experiments were performed using the Varian gyrocode. ( 1 ) J. K.
Sanders and D. H. Williams, J.
(1971).
1410
Arner. Chern. SOC..93, 3, 641
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
Alkyl Nitriles. Although organonitriles are weak Lewis bases, it has been reported that complexes do form involving the nitrile group and various transition metals (3).N o NMR studies using, shift reagents have previously been reported on this class of compounds, but they represent a particularly attractive group because of the fixed geometry of the nitrogen lone pair, directing an end-on planar configuration of the resulting complex. This allows some conclusions regarding the effect of geometry and distance on the proton shifts to be more readily drawn. Because organonitriles are poor electron donors, the fluorinated shift reagents Eu(FOD)3 and Pr(FOD)3, which are stronger Lewis acids than the DPM chelates, were far more effective in generating spectral shifts for the compounds studied ( 4 ) . This is demonstrated in Table I1 where results are given for some of the alkyl nitriles using both shift reagents. For acetonitrile, propionitrile, and isobutyronitrile, it required twice the concentration of the DPM chelate to produce a shift comparable to that obtained with the FOD chelate. When the same amounts of FOD and DPM chelate were added to valeronitrile, the induced shift in the FOD complex was about twice the magnitude of that in the DPM complex. The Eu(FOD)3 reagent was actually more effective in every case. Of the simple alkyl nitriles, valeronitrile, 4-methylval(2) L. Goldberg and W . M. Ritchey. Spectrosc. L e t t . . 5 . 6, and 7 , 201 (1972). (3) B. L. Ross, J. G . Grasselli, W . M . Ritchey. and H . Kaesz, Inorg. Chern.. 2, 1023 (1963). (4) R. E. Rondeau and R. E. Sievers, J. Arner. Chern. SOC.,93,6 . 1522 (1971).
Table I. Organonitriles Studied and Representative Conditions Compd
Mole ratio chelate/ ligand
Concn of ligand, mol/l.
Solvent
0.09 0.1 7 0.05 0.1 1 0.16 0.42 0.39 0.50 0.74 0.51
0.509 0.413 1.128 0.980 0.452 0.677 0.936 0.664 0.514 1.334 0.51 5
CDC13 CDC13 CDC13 CDC13 CDC13 CDC13 CDC13,CC14 CCl4 CDC13 CCl4 CDC13
0.07 0.28 0.28 0.23 0.19 0.18 0.16
1.030 0.51 7 0.622 0.340 0.137 1.110 0.928
CDC13 CDC13 CDC13 CDC13 CDC13 CDC13 CDC13
0.20 0.35 0.51 0.20 0.16 0.24 0.24 0.23 0.1 1 0.20 0.21
2.280 0.670 0.384 0.192 0.144 0.312 0.231 0.252 0.278 0.707 0.490
CDC13 CDC13 CDC13 CDC13 CDC13 CDC13 CDC13 CDC13 CDC13 CDC13 CDC13
0.31 0.34
0.206 0.238
CDC13 CDC13
Alkyl nitriles Acetonitrile Propionitrile
0.05
lsobutyronitrile Valeronitrile 4-Met h ylvaleron itri le Hexanenitrile Hepanenitrile Unsaturated nitriles Allyl cyanide Phenylacetonitrile Acrylonitrile Methacrylonitrile Difunctional compounds Adiponitrile Suberonitrile 7-Hydroxyheptanenitrile 4-Phenoxybutyronitrile 3-Methoxypropionitrile 3-Ethoxypropionitrile P,B'-Oxydipropionitrile 3,3-(Tetramethylene dioxy)dipropionitrile
Table II. Effectiveness of Shift Reagents Compd Acetonitrile Propionitrile lsobutyronitrile Valeronitrile
Mole ratio chelate/ ligand 0.09 0.17 0.05 0.11 0.05 0.16 0.42 0.39
Shift reagent
EU( FOD)3 Eu(DPM)~ Eu(FOD)3 EU(DPM)3 EU(FOD)3 Eu(DPM)3 EU(FOD)3 Eu(DPM)3
Metal chelate Eu(FOD)~ EU(DPM) 3 Eu(FOD)~ ELI (DPM)3 Eu(FOD)~ EU(DPM) 3 EU(FOD)3 EU(DPM)3 EU(FOD)3 EU(FOD)3 EU(FOD)3 Pr (FOD)3 EU(FOD)3 EU(FOD) 3 Pr(FOD)3 Pr ( F 0 D ) s EU(FOD) 3 EU(FOD)3 EU(FOD)3
EU(FOD) 3 EU(FOD)3 Eu(FOD)~ EU(DPM) 3 EU(FOD)3 EU(DPM)3 EU(FOD)3 EU(DPM)3 Eu( FOD)3 EU(DPM)3 EU(FOD)3 EU(DPM)3
Table III . Incremental Chemical Shifts for Acrylonitrile
2.00 2.00 2.35 2.35 2.70 2.70 2.32 2.32
(6c -
\
6L)b
0.26 0.29 0.57 0.49 0.50 0.70 3.14 1.37
6~ = chemical shift in ppm for CH3, CH1, or CH cy to CN for free ligand. 6c = chemical shift in ppm for CH3, CH2, or CH cy to CN for ligand plus shift reagent. e r o n i t r i l e , h e x a n e n i t r i l e , and h e p t a n e n i t r i l e e x h i b i t spect r a w i t h o v e r l a p p i n g resonances w h i c h s i m p l i f y a l m o s t c o m p l e t e l y t o f i r s t o r d e r w i t h use o f t h e s h i f t reagent. C o u p l i n g c o n s t a n t s a r e easily m e a s u r e d f r o m t h e s p e c t r a of t h e c o m p l e x e s a n d , t o g e t h e r w i t h s p l i t t i n g p a t t e r n s and i n t e g r a t i o n o f peaks, p r o t o n a s s i g n m e n t s a r e r e a d i l y m a d e . H e x a n e n i t r i l e was s t u d i e d in b o t h d e u t e r o c h l o r o f o r m and c a r b o n t e t r a c h l o r i d e t o d e t e r m i n e i f t h e r e was any s o l v e n t e f f e c t o n r e s u l t s . N o n e was f o u n d but p r e c a u t i o n s in u s i n g o n l y p u r i f i e d d e u t e r o c h l o r o f o r m and f r e s h s h i f t reagents were t a k e n t h r o u g h o u t . U n s a t u r a t e d N i t r i l e s . T h e unsaturated nitriles show d r a m a t i c s i m p l i f i c a t i o n o f t h e i r c o m p l e x second-order
,"
2H,
A=
6LO
/
jH/C=c\cN Mole ratio
0.414 m o l
PI.
OQ
5.65
Ob
5.50
0.030 0.160 0.247 0.283
5.44 5.08 4.93 4.82
0.06 0.42 0.57 0.68
6.04 6.00 5.97 5.82 5.74 5.70
AN/1
0.03 0.18 0.26 0.30
6.1 7 6.14 6.09 5.83 5.71 5.63
0.05 0.31 0.43 0.51
6. L. Ross, J. G. Grasselii, W . M . Ritchey, and H. D. Kaesz, Inorg. Chem., 2, 1023 (1963). Extrapolated from experimental data. C Differ-
ence between chemical shifts of complex and free ligand extrapolated from exDerimental data. s p e c t r a w h e n s h i f t reagents a r e used. A c r y l o n i t r i l e ( A N ) i s a classic e x a m p l e o f a t h r e e s p i n ABC s y s t e m g i v i n g a 12-15 l i n e s p e c t r u m w h i c h i s n o t r e a d i l y a n a l y z e d f o r c h e m i c a l s h i f t o r c o u p l i n g c o n s t a n t i n f o r m a t i o n even a t 100 MHz. I t s s p e c t r a l p a r a m e t e r s h a v e b e e n r e p o r t e d ( 3 ) , b a s e d o n c a l c u l a t i o n s u s i n g a c o m p u t e r p r o g r a m , E x a n 11. T h e c h e m i c a l s h i f t s f o r p u r e a c r y l o n i t r i l e and w i t h s h i f t r e a g e n t a d d e d a r e g i v e n in T a b l e 111. T h e Pr(FOD)3 s h i f t r e a g e n t was a d d e d i n c r e m e n t a l l y t o a s o l u t i o n o f 0.414 ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
1411
i
CHz=CH-CN
in CDC13
0 . 2 8 3 mole r a t i o Pr(FOD)3/AN in CDC13
Figure 1. 100 MHz proton NMR spectrum of acrylonitrile
Table I V . Incremental Chemical Shifts for Methacrylonitrile
'H,
,CH3'
,c=c 3H
C 'N
0.7-0.9 mol MAN/l Mole ratio
Chemical shift, ppm
EU(FOD)3/
MAN
H'
OQ
2.00 2.02 2.12 2.19 2.29
Ob 0.06 0.09 0.16
Ac
H2
0.10 0.17 0.27
5.73 5.73 5.80 5.86 5.94
Ac
H3
0.07 0.13 0.21
5.82 5.84 5.96 6.04 6.18
A C
0.12 0.20 0.34
a N. S. Bhacca, L. Johnson, and J. N. Shoolery, High Resolution NMR Spectra Catalogue, 1, 97 (1962). Extrapolated from experimental data. Difference between chemical shifts of complex and free ligand extrapolated from experimental data.
*
mol AN/l. to give the mole ratios shown. Spectra were also taken using the E u chelate but, as has been reported (51, the ligand lines shift downfield using Eu and upfield using Pr. For the Eu(FOD)3 complex the downfield shifts for the various protons resulted in more overlap of resonances rather than in spectral simplification. For example, H1 is the farthest upfield resonance in the free ligand and it experiences the greatest shift in the complex due to its proximity to the complex site. Using Pr(FOD)3, this (5) D. R. Crump, J. K. Sanders, and D. H Williams, Tetrahedron Left., 57, 4949 (1970).
1412
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
shift is even farther upfield and therefore away from the H2 and H3 resonance region. I t is of interest to note the relative positions of H2 and H3. In the free ligand, H* is upfield from H3, but a t a mole ratio of about 0.16 chelate/ AN a cross-over occurs and H2 appears a t lower field than H3. This will be discussed more thoroughly after reviewing the data on the other unsaturated nitriles. However, the chemical shifts for the free ligand, obtained by extrapolation to zero of the least squares fit line from the measured shifts of the complexes us. mole ratio, are in good agreement with the quoted literature values. The 100 MHz NMR spectra of acrylonitrile and of a 0.28 mole ratio complex are given in Figure 1. The insert a t a sweep width of 250 Hz in the spectrum of the complex allowed even the very small geminal coupling constant for acrylonitrile to be measured. The effective simplification of the spectrum for the complex shown is equivalent to a 240 MHz spectrum of the free ligand! Table IV presents data for methacrylonitrile (MAN) a t a ligand concentration of 0.7-0.9 mol/l. The methyl protons, labeled HI, are well displaced from H* and H3, so the choice of shift reagent was not critical and Eu(FOD)3 was used. The assignments for protons 2 and 3 were based on the magnitude of their relative shifts, as compared with the other unsaturated nitriles in the study, and were consistent with the assignments in acrylonitrile, which are well accepted. They were supported by the excellent agreement obtained between the extrapolated values calculated for the free ligand from the graph of the induced chemical shift us. mole ratio and the assignments reported
pling constants which were computer-generated for acrylonitrile were 11.64 for J 1 2 , 17.94 for 513, and 0.91 for J 2 3 . For methacrylonitrile the long-range coupling cis into the methyl group is slightly larger than the trans, as expected. The signs of these coupling constants are undoubtedly negative, but were not measured. The effectiveness of the spectral simplification is also shown by the excellent agreement between these measured constants and those given in Table V from spin decoupling experiments. Table VI1 shows the data for allyl cyanide. The spectrum of the allyl cyanide complex, taken a t 0.517 mol/l. ligand concentration, is shifted and somewhat simplified, but cannot be reduced to first order. Even so, separation of the lines is sufficient to allow integration and assignment of protons. Spin decoupling experiments, given in Table VIII, confirmed the assignments made from the spectra of the shift reagent complexes and also allowed
Table V. Data from Decoupling Experiments for 0.248 Mole Ratio Eu (FOD)s/ Methacrylonitrile
2H\
/CHS1
3H
C 'N
/c=c
0.481 mol MAN/l Proton irradiated
Protons observed
Chemical shift
H'
H2 doublet H3 doublet H1 doublet H3 quartet H1 doublet H2 quartet
6.18 6.56 2.60 6.56 2.60 6.18
H2
H3
Coupling constants J23
= 0.7 H Z
J13
= = = =
J13
J72 J12
1.1 HZ 1.1 H Z 1.6 H Z 1.6 HZ
Table VI. Chemical Shifts and Coupling Constants
Acrylonitrile
Methacrylonitrile Chemical shift. oom
Nitrile
Complex (measured)=
H' 4.82 2.60
Acrylonitrile Methacrylonitrile
Free ligandb
H'
H3 5.63 6.56
H2 5.70 6.18
5.50 2.02
H2 6.00 5.73
Coupling constants
H3 6.14 5.84
0.283 mol ratio Pr(FOD)3/acrylonitrile in CDC13. 0.25 mol ratio Eu(FOD)3/methacrylonitrile in CDCI3.
Jiz
J13
J23
11.5 1.6
17.5 1.2
0.9 0.8
Extrapolated from experimental data.
Table V I I . Incremental Chemical Shifts for Allyl Cyanide
0.517 m o l allyl cyanide/l Mole ratio EU(FOD)?/ allyl cyanide
a
Chemical shift, m m
ti'
OQ 5.80 0.06 5.92 0.10 6.06 6.27 0.16 0.23 6.39 0.28 6.48 Extrapolated from experimental data.
Ab
H2
0.12 0.26 0.47 0.59 0.68
5.34 5.42 5.48 5.60 5.68 5.75
Ab
0.08 0.14 0.26 0.34 0 41
H3
5.43 5.66 5.78 5.98 6.27 6.42
Ab
H4
Ab
0.23 0.35 0.55 0.84 0.99
3.1 7 3.50 3.74 4.12 4.48 4.72
0.47 0.57 0.95 1.31 1.55
Difference between chemicai shifts of complex and free ligand extrapolated from experimental data
in the Varian catalog. Subsequent spin decoupling experiments on a complex of 0.25 mole ratio Eu(FOD)3/MAN confirmed all assignments and allowed an accurate measure of the coupling constants to be made. The data are presented in Table V. The chemical shifts and coupling constants for acrylonitrile and methacrylonitrile are summarized in Table VI. The agreement between the chemical shifts as calculated for the free ligands from these experimental data and the values given in the literature (6, 7) is very good. The cou(E) W. Brugel, Nuci. Magn. Resonance Spectra Chem. Struct..
1, 144 (1967). (7) L. M. Jackman and S. Sternhall. "Applications of Nuclear Magnetic
Resonance Spectroscopy in Organic Chemistry." Pergamon Press, Germany, 1959, 171 pp,
some of the coupling constants to be measured. In allyl cyanide H1 is the lowest field band, while in acrylonitrile H1was the highest field band. The ordering of these olefinic resonances determines which shift reagent to use for maximum clarification of the spectrum. With allyl cyanide, Eu(FOD)3 was most effective; however, with acrylonitrile, as previously mentioned, Pr(FOD)3 was preferred. The methylene a to the nitrile in allyl cyanide, labeled H4, experiences the largest shift, as would be expected because it is the closest to the site of complexing. The interesting effects of geometry and distance on the chemical shifts of the various protons in this series of unsaturated nitriles are summarized in Table IX. In every case, H2 experiences the least shift because it is the farthest from the coordination site and the effect varies with ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
1413
the inverse cube of the distance. For acrylonitrile, H1 which is geminal to the coordination site is shifted more than H3, and this is consistent with observations in other systems, such as the cyclohexanols and borneols studied by Demarco e t al. (8). In methacrylonitrile, H1 becomes the hydrogen of a methyl group, one carbon removed in distance but also probably less deshielded now than H3 due to the spatial relationships between the coordination site and the protons under consideration, and consequently it is less shifted than H3. The difference in gradients between H1 and H3 for acrylonitrile was 0.61 while for methacrylonitrile it was only 0.43. Although the relative positions of H1 and H3 are reversed in AN and MAN, the effect on HI and H3 from the shift reagent in MAN is approximately the same ( A = 0.27 for H1 and 0.34 for H3, Table IV) while in acrylonitrile HI has a significantly greater shift than H3 ( A = 0.68 for H1 and 0.51 for H3, Table ID) because of both distance and geometry. The angular dependence of the shift is clearly shown in allyl cyanide, for now the difference in gradients between H3 and
Table V I I I . Data from Decoupling Experiments for 0.28 Mole Ratio Eu(FOD)3/Allyl Cyanide
2H ‘C=C’ 3H’
H’
‘CHz4CN
0.517 mol allyl cyanide/l Proton irradiated
Protons observed
Chemical shift
H1 -t H3
H2 singlet H4 singlet H1 triplet H3 singlet H4 doublet H1 second order H2 doublet of doublets H3 second order
5.72 4.65 6.44 6.40 4.65 6.44 5.71
J24
6.40
J13
H2
H4
___
Coupling constants not observed
= 3.3 HZ not observed J 1 4 = 3.0 HZ J1*= 7.5 Hz J 2 3 = 3.2 HZ J14
J13
not observed
~~
Table IX. Comparison of Proton Gradients for Unsaturated Nitriles
‘H\
/H1 or CH,’
/c=c \R R
=
CEN or CHzCN
Proton gradientsa H’
Acrylonitrile Pr(FOD)3 Methacrylonitrile Eu (FOD) 3 Allyl cyanide Eu(FOD)3 a
2.39 1.71 2.54 f 0.2
H*
H3
Gradient differences
1.06 1.34 1.50
1.78 2.14 3.55
H’ - H3 = 0.61 H3 - H’ = 0.43 H3 - H’ = 1.01
.
Standard deviations are * O , l except where noted.
0 . 3 8 4 MOLES/LITER CDC13
HO-CH2-CH2-CH2-CH2-CH2-
.I
.2 .3
MOLE RATIO
CH2- CN
.4 . 5 .6 Eu (Fool3
7-Hydroxyheptanenitri le
Figure 2. Chemical shifts for the various protons in 7-hydroxyheptanenitrile 1414
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
HI is 1.01 and H3 is more shifted. Even though HI is closer, the deshielding effect on H3 is much greater due to the fixed axial geometry of the europium and the nitrile group and the planar configuration around the double bond. Difunctional Compounds. Among the difunctional compounds, the dinitriles, adiponitrile, and suberonitrile showed complexing a t both nitrile groups of the molecules. For all the polyfunctional molecules, correspondingly more shift reagent is required to form the complex and induce effective spectral shifts. Of the heterodifunctional compounds studied, 7-hydroxyheptanenitrile was the most striking example of simplification to a first-order spectrum. Figure 2 shows chemical shifts for the various protons in this molecule plotted as a function of increasing mole ratio of Eu(FOD)3. The largest shifts are observed for the CH2 a to the hydroxy group as expected because of the very large difference in donor properties of the hydroxy group as compared with the nitrile ( I ) . Attenuation down the chain is in qualitative agreement with the inverse 13 relationship. Protons labeled F do experience more shift than can be explained on the basis of distance from the hydroxy group; therefore, it is probable that both donor sites are involved in complexing with the shift reagent. However, the magnitudes of the shifts are such as to suggest that the sites act almost independently and do not complex a t the same time. In Table X the interesting results obtained with shift reagents are given for a group of heterofunctional compounds, all of which are oxynitriles. They were each examined using both the FOD and DPM chelates of Eu. The oxygen of the ether group should be favored over the ni(8)p. V. Demarco, T. K . Elzey. B. R. Lewis, and E. Wenker, J , Arner. Chern. SOC., 92, 19, 5734, (1970).
Table X. Effect of Shift Reagents on Heterofunctional Molecules Concn Compound 4-Phenoxybutyronitrile
A
B
C
3-Methoxypropionitrile CH30CHzCHzCN A B C
Shift reagent
of ligand, moljl.
Eu(FOD)~
0.192
ELI (DPM)3
0.144
Protons
Gradients
A B C A
1.75 2.30 4.04 0.72 0.69 1.04 5.65 6.37 6.16 13.15 12.40 7.16 1.40 2.00 3.42 4.68 2.44 4.49 4.25 2.83 4.26 3.22 1.27 1.47 1.76 1.23 1.05 1.12 1.35 1.80 1.96 1.68
B C EL(I FOD) 3
0.312
A
0.835
C A
B EU(DPM)3
B 3-Ethoxypropionitrile CH3CH20CH2CH2CN A B C D
C A B C D A B C
Eu(FOD)~
0.252
EIJ(PDM)~
0.278
EU(FOD)3
0.707
A
EU(DPM)3
0.490
A
Eu(FOD)~
0.206
EIJ(DPM) 3
0.238
A B C D A B C D
D /3,/3'-Oxydipropionitrile NCCH2CH20CH2CH2CN A B
B B
3,3-(Tetrarnethylene dioxy) dipropionitrile
NCCH~CH~OCHZCH~CH~CH~OCH~CHZCN A
B
C
D
trile as a site of bonding, but steric effects around the oxygen might be important. It is also likely that steric factors are significant due to the shape of the FOD chelating agent as compared with the DPM, and this, as well as size, might influence the choice of complex site. For 4-phenoxybutyronitrile the importance of steric factors in the molecule and in the shift reagent are clear. The gradients for the protons of the Eu(FOD)3 complex indicate that the nitrile is the only site of bonding, for the shift experienced by proton A is of about the proper magnitude for an effect from distance alone. Even using the Eu(DPM)3, the preferred site is still the nitrile group although the effect on proton A indicates some complexing a t the oxygen might occur. The steric effect of the phenyl ring is certainly predominant, however, and even the absolute magnitudes of the proton gradients indicate that complexing is primarily with the nitrile for this molecule. The complex of 3-methoxypropionitrile and Eu(DPM)s shows gradients an order of magnitude larger than the 4phenoxybutyronitrile Eu(DPM)3 complex which illustrates that the ether oxygen is the preferred site over the nitrile, if a t all possible If there were any complexing a t the nitrile for this complex, it would be expected that proton C should show a larger gradient. Using the Eu(FOD)3 chelating agent, however, the data support the effect of the size of the FOD, for they suggest that both sites are involved in bonding, but they are acting so independently. Changing from a methoxy to an ethoxy group makes the ether oxygen somewhat less accessible, and even in the Eu(DPM)3 complex there is evidence that both sites participate in bonding. For the oxydinitrile and the dioxydinitrile compounds, the magnitude of the proton gradients indicate that although complexes do form with the shift
reagents, they are weak and mostly with the nitrile. The ether oxygen does participate but the steric effect of substituents around it is relatively more important than the difference in size of the chelating reagent. The effect of angular geometry must also be considered in interpreting the above results, and it is clear that although the nitrile group directs an end-on configuration for the complex, the lone pairs on the ether oxygen are available to the lanthanide chelate both above and below the axis of the molecule, so protons attached by freely-rotating carbon bonds of a planar zigzag skeleton can be very influenced by the angle of the deshielding cone. The preceding discussion of difunctional molecules is based largely on interpretation of the observed results considering steric effects for both the chelate and the ligands, and this is consistent with the observed data. HOWever, there is also some evidence (9) in the case of difunctional compounds such as dicarbonyls and dialcohols that both functional groups could complex simultaneously with a given Eu atom. This would probably be most favorable when a six-membered ring could be formed. There is little chance of chelate formation, if the resulting complex structure would be a seven-member or larger ring. In such cases, the two functional groups would act independently. Equilibrium Constants. I t has been shown ( 2 ) that A', the differential shift of the pure complex, and the equilibrium constant, K C L , for the 1:l complex is given by Equ'ation 1
(9) W . M. Ritchey, Case Western Reserve University, Cleveland, Ohio,
unpublished results, 1972. ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
1415
-~ ~
~
Table XI. Calculated Values for
A’ and Equilibrium Constants
Compound
Structure
Proton
A0
KCL~
Valeronitrile
CH3CH2CH2CH2‘CN
1
4
1
4-Methylvaleronitrile
CH3CH-CH2CH2’CN
1
4
11
1
5
4
1 2
2 1
17
3
1
1 2
2
3
2
1
20
3
4 1 4
1
2
26
I
CH3
Hexanenitrile
CH3CH2CH2CH2CH2’CN
Acrylonitrile
Methacrylonitrile
Allyl cyanide
2
3,3-(Tetramethylene dioxy) dipropionitrile a
NCCH2’CH2OCH2CH2CH2CH2OCH2CH2’CN
17
1
Significant to one figure.
where [L]” = concentration of ligand in mol/l., A = differential shift of equilibrium mixture, and x = mole ratio of initial concentrations of chelate/ligand. Utilizing the above expression, values of K C L were obtained and are given in Table XI. These appear to be in agreement in magnitude with the value of 12 reported for the acetone/ FOD system (2) and for amine and alcohols (20). The values of KCL are only significant to one digit since the calculation of KCLinvolves small differences between sets of experimental data, i.e., differential shifts and mole ratios. Constant values for the equilibrium constants were (10) I. Armitage, G. Dunsmore, L. D. Hall, and A. G. Marshall, Chem. Cornmun., 1971, 1281.
not obtained in several cases, suggesting that those systems were not simply 1:l complexes in the concentration range studied or that experimental accuracy was insufficient. Equilibrium constants for mono and difunctional groups in general are still under investigation. Work is under way to extend the study of these organonitrile-shift reagent complexes to l3C NMR observations in order to further elucidate bonding in the structures and other factors involved in their formation. Received for review October 31, 1972. Accepted February 1, 1973. Presented a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 6-10, 1972.
External Standards in X-Ray Photoelectron Spectroscopy A Comparison of Gold, Carbon, and Molybdenum Trioxide William P. Dianis and Joseph E. Lester’ Department of Chemistry, Northwestern University, Evanston, 111. 60207
The binding energies of the 4fs/2 and 4f7/2 peaks of gold and lead in Pb3O4 were measured and found to differ from the previous literature values. We suggest the following values: for gold, 4f5/2 = 87.7 eV, 4f7/2 = 84.0 eV; for Pb304, Pb 4f512 = 142.7 eV, Pb 4f712 = 137.8 eV. When making binding energy measurements in nonconductors, a surface charge may build up, causing an error in the observed binding energies. The abilities of Moo3, 1416
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 8, J U L Y 1973
vapor-deposited gold, and carbon impurity to compensate for surface charge errors were evaluated. I t was concluded that the use of gold and carbon, but not Mo03, as an external standard would allow one to correct for surface charge errors. However, uncertainty about the carbon 1 s binding energy makes it an unsatisfactory standard. Criteria for appropriate binding energy references are discussed.
