tion as well as ionization) in the ionizer. If the base line signal were eliminated, the minimum detectable signal should be on the order of 0.5 pV, and the maximum sensitivity could be increased by a factor of four. Stronger beams than those used here could be generated by multichannel sources with higher peaking factors than the single tube used in these experiments. However, the stronger beam would produce larger base line signals, and the detection limit would not decrease as rapidly as the beam strength increased.
mass spectrometer rather than the vacuum system, may be as low as 0.1 parts-per-million. The ratio of the trace and primary constituent signals is proportional to the concentration of the trace component. The major disadvantage is the time required to perform a complete mass analysis. A “scan” of the spectrum of a modulated molecular beam requires approximately one hour. Because the gas is continually flowing into the vacuum system during the analysis, larger quantities of gas are consumed with the modulated beam method than with conventional inlet systems.
CONCLUSIONS
The primary advantage of a modulated molecular beam inlet system is the ability to detect accurately parts-per-million concentrations of trace constituents without the ultra high vacuum conditions which conventional mass spectrometric gas analysis requires. The results of this study suggest that the detection limit, which is set by the performance of the
ACKNOWLEDGMENTS
The authors thank R. A. Krakowski for his helpful review of the manuscript. RECEIVED for review March 29, 1968. Accepted June 20, 1968. This work was supported by the United States Atomic Energy Commission.
Chelation Studies by Conformationa I Ana lysis Proton Magnetic Resonance of dl-Propylenediaminetetraacetic Acid James L. Sudmeier and Alan J. Senzel Department of Clieniistr),, Unicersitj OJ California, Los Angeles, Gal$ 90024 A method for studying the formation of .weak metal chelates by nuclear magnetic resonance (nmr) is presented and illustrated. It is shown that the rotational conformation of a diaminocarboxylate ligand is extremely sensitive to coordination of both ends to a metal ion. The formation of weak 1:l K+, Rb+, and (CH3)4N chelates of dl-propylenediaminetetraacetate (PDTA) is established for the first time, and their formation constants (1.3, 0.14, and 0.12, respectively) at 100 O C are determined. The alkali metal ions, like alkaline earth and transition metal ions, must coordinate to both ends of the ligand.
IT is often difficult to determine the structure of a metal complex from mere knowledge of its stability constant. For example, the K , values for alkali metal ethylenediaminetetraacetate (EDTA) complexes are much smaller than those for alkaline earth-EDTA complexes. That is, for 1 :1 N u , Li’, Ba2-, Sr2+,and Ca2+ complexes of EDTA, the log K , values are 1.66, 2.79, 7.76, 8.63, and 10.59, respectively ( I , 2). While it is known that alkaline earths form chelates involving both ends of the EDTA molecule, it is uncertain whether alkali metals form monodentate complexes or chelates of the alkaline earth variety. In the absence of chelation, it is expected that the bulky iminodiacetate groups of EDTA will be located as far apart as possible. Thus, the anti conformation with respect to the central carbon-carbon bond in free EDTA would be expected, although only the gauche conformation can exist in a chelate involving both ends of the molecule-e.g., both nitrogen atoms. Such reasoning has been used, for example, in explaining the greater stability of cyclohexanediaminetetra(1) G. Schwarzenbach and H. Ackerman, Helc. Chim. Acta., 30, 1798 (1 947). (2) V. Palatp. CurzJ. Chem., 41,18 (1962).
acetate (CyDTA) complexes over their EDTA counterparts (3, 4 ) . The iminodiacetate (IDA) groups in CyDTA are known to be gauche ( 5 ) and are, therefore, “pre-oriented” for chelate formation-in contrast, presumably, to the IDA groups in EDTA. The rotational conformation of EDTA also has important consequences with regard to kinetics of exchange (6, 7) of EDTA complexes and intramolecular hydrogen bonding (8). If monoprotonated EDTA (HY 3-) exists primarily in the conformation suggested by Chapman (8), where the NH proton is hydrogen bonded to the second nitrogen, then it must have the gauche conformation. Preorientation (gauche conformation) has been proposed (6) as the explanation for enhanced reaction rates of HY 3- over those of Y 4-. The application of nuclear magnetic resonance (nmr) to the determination of rotational conformations in ethane derivatives is well known (9). The nmr spectrum of the N-CH2CH,-N protons in EDTA consists of a singlet, and does not readily yield conformational information because of excessive degeneracy. Substitution of a methyl group for one of the N-CH2-CH2-N protons, however, as in the dl-propylenediaminetetraacetate (PDTA) anion, introduces sufficient asym~
(3) H. Kroll and M. Gordon, Ann. N . Y . Acud. Sei., 88, 341 (1960). (4) D. L. Wright, J. H. Holloway, and C. N. Reilley, ANAL.CHEM., 37, 884 (1965). (5) J. L. Sudmeier and C. N. Reilley, ibid.. 36, 1707 (1964). (6) D. W. Rogers, D. A. Aikens, and C. N. Reilley,J. Phys. C/zem., 66,1582 (1962). (7) J. L. Sudmeier and C . N. Reilley, Znorg. Cliem., 5, 1047 (1966). (8) D. Chapman, D. R. Lloyd, and R. H. Prince, J . Chem. SOC., 1963, 3645. (9) J. W. Emsley, J. Feeney, and L. H. Sutcliffe, “High Resolution Nuclear Magnetic Resonance Spectroscopy,” Vol. 1, Pergamon Press, New York, 1965, pp 560-72. VOL. 40, NO. 1 1 , SEPTEMBER 1968
1693
60 MHZ pH=13.0
A
Figure 1. Observed and calculated 60 MHz nmr spectra of K4PDTA at 36 "C metry to produce a complex splitting pattern (ABCXd whose analysis yields conformational data. The nmr spectrum of PDTA at various pH values has been reported (5), although a complete analysis of the ABCXa pattern arising from the N-CH(CH3)-CH2-N protons above pH 10 was not attempted. PDTA can exist as the following rotamers (and their mirror images) :
I
I1
I11 X
=
-CHBC02-
The determination of the relative amounts of I, 11, and I11 is based upon the Karplus equation (10,11) which relates vicinal coupling constants Jlzand JI3to their corresponding dihedral angles. The coupling constants are obtained by analysis of (10) M. Karplus, J . Chem. Phys., 30, 11 (1959). (11) M. Karplus, J. Amer. Chem. SOC.,85, 2870 (1963). 1694
ANALYTICAL CHEMISTRY
the ABCX3 pattern at 60 and 100 MHz with the aid of computers. EXPERIMENTAL
Chemicals. HIPDTA was synthesized by the method of Dwyer and Garvan (12). Ten per cent aqueous tetramethylammonium hydroxide was obtained from Eastman Organic Chemicals. Reagent grade RbOH and CsOH were obtained from K & K Rare and Fine Chemicals. All samples were approximately 0.5M in PDTA, and pH values were measured with a Radiometer pH Meter Model 26 Nmr Spectrometers. Spectra were recorded at 60 and 100 MHz using Varian A-60 and HA-100 nmr spectrometers. The spectia were run at a sweep width of 250 Hz (1 cm = 5 Hz) and a scan rate of 0.5 Hz/spc*. The line positions are estimated to be accurate within 1 0 . 5 Hz. Chemical shifts were measured relative to internal t-butanol, which is 1.233 ppm downfield of sodium 2,2-dirnethyl-2-silapentane5-sulfonate (DSS). The correct sweep width linearity for the Varian A-60 was established as that yielding the CHC13 proton resonance at 435.5 Hz downfield of TMS as determined using a standardized offset procedure. The sweep width linearity of the HA-100 was checked by the usual audio sideband method. (12) F. P. Dwyer and F. L. Garvan, J. Amer. Chem. SOC., 81,2955 (1959).
I
100 MHz pH=l3.0
\ uu
I . . . . , . . . .I . _ . . . . . . I . I
d ’ - - . .
.
.
I
. . . .
3.0
-
I ....
25
k
. . .
I...
I
....
I.i/.I
2.0
. . . .I
. * . &
1.0
Figure 2. Observed and calculated 100 MHz nmr spectra of KaPDTA at 36 “C Computations. Spectra were computed on an IBM 360-75 using LAOCOON I1 (13), equipped with a printed plot of the output. In the present work, only Part I of LAOCOON I1 was used for calculation of trial spectra and no use was made of the optimization capabilities of the program (Part 11). The parameters were refined until matching of experimental and calculated spectra was within experimental error in determination of line positions ( 1 0 . 5 Hz). Computer analyses of ABCXs spectra have previously been carried out (14, 15) at two different frequencies. The spectra reported herein tend to exhibit small chemical shifts and little fine structure, and might have led to ambiguous solutions. However, because the spectra are solved at two different frequencies, the chances for obtaining ambiguous solutions are very small. RESULTS AND DISCUSSION
Potassium PDTA. Figures l a and 2a give the observed nmr spectra at 60 and 100 MHz (probe temperature) of a 0.5M solution of tetrapotassium PDTA (K4PDTA). The (13) S . Castellano and A . A. Bothner-By, J. Chem. Phys., 41, 8363 (1964). (14) H. Finegold, Proc. Chern. SOC.(Londori), 1962, 213. (15 ) J. L. Sudmeier, J . Phys. Chern., 72, 2344 (1968).
sample was prepared by adjusting the pH of a PDTA solution to 13 through dropwise addition or concentrated KOH solution. Figures l b and 26 give plots of the calculated ABCX3 patterns of the N-CH(CH3)-CH2-N protons at 60 and 100 MHz, using the chemical shifts and coupling constants given in Table I, and line widths of 1.5 Hz. The AB patterns at low field, attributed to the acetate protons, have previously been assigned (9, and are not further considered in the present work. The intensities of methyl doublets are arbitrarily reduced in scale in Figures l b and 2b. In Figures l a and 2u the labels “b” and “c” indicate the chemical shifts of the -C&protons on the “backbone” of PDTA. The methine proton, which we shall call “a” (labeled HI in I, 11, and III), is known to appear at low field in similar compounds (14,15), and its intensity is buried beneath the AB patterns. The chemical shift value for proton a was varied over a range of i 5 Hz in the computations, with negligible effect on the line positions and intensities being produced. The resonance of proton a is not shown on the computed spectrum in Figure l b . Protons b and c are split with a geminal coupling constant Jbc = - 13.2 Hz. Both protons b and c are further split by proton n with vicinal coupling constants Jab = 9.3 (approximately equal to the spacing of the left-hand bars in Figure l a ) and J,, = 3.4 Hz (approximately equal to the spacing of the rightVOL. 40, NO. 11, SEPTEMBER 1968
* 1695
100 MHz pH=12.0
h
J
Figure 3. Observed and calculated 100 MHz nmr spectra of CaPDTA at 36 “C
hand bars in Figure l a ) . According to the Karplus equation (lo,ll), the dihedral angle between protons a and 6 is approximately 180” and that between protons a and c is approximately 60”. There are two possible assignments of protons b and c which could yield this result: HP = b and H3 = C, and the predominant rotamer is 111 (gauche); or Hz= c and Ha = b, and the predominant rotamer is I (anti). In the absence of independent evidence regarding the relative chemical shifts of HS and Ha, the correct rotamer cannot be unambiguously determined. In order to determine the chemical shifts of HPand H 3 in a compound which is known to be a metal chelate., and in which the ligand must exist predominantly as rotamer 111,the analysis
of the ABCX3pattern in CaPDTA was undertaken.
Rotamer
I1 is negligible in metal chelates because of the unfavorable steric interactions of the “axial” methyl group (16). Calcium PDTA. Figure 3a gives the observed nmr spectrum of CaPDTA at 100 MHz (probe temperature), and Figure 36 gives the calculated spectrum of the N-CH(CH3)CHz-N protons, using the parameters given in Table I. Labels “b” and “c” give the chemical shifts of the methylene protons and methine proton a is buried in the AB patterns as in the KIPDTA spectra. Because the CaPDTA exists primarily as rotamer 111, it must be concluded that H3is upfield of (16) R. J. Day and C. N. Reilley, ANAL.CHEM.,37, 1326 (1965).
Table 1. Chemical Shifts and Coupling Constants of the ABCX, Patterns in Various PDTA Solutions at 36 “C KiPDTA 6, 3.09 3.11 3.14
ab
6,
6,
Jab
pH 13.0 pH 12.4 pH 11.9
2.62 2.64 2.70
2.14 2.21 2.36
0.890 0.912 0.955
9.3 9.3 9.4
3.4 3.4 3.4
-13.2 -13.3 -13.4
6.4 6.4 6.4
pH 12.0
3.14
2.45
2.28
0.908
11.o
3.2
-14.0
6.3
J,,
Jbc
J,,
CaPDTA
1696
ANALYTICAL CHEMISTRY
Hz in this compound. If this order of chemical shifts prevails in the case of K4PDTAas well, then the latter compound must exist predominantly as rotamer 111. An experiment was carried out in which the chemical shifts of Hz and H a were monitored as CaPDTA was formed from K4PDTA by gradual addition of CaClz (at 100 “C to increase the ligand exchange rate). Although the b and c resonances are highly complex, it is quite clear that no crossover in their chemical shifts occurs in going from K4PDTA to CaPDTA. This leads to the somewhat surprising result that the ligand in K4PDTA exists predominantly as rotamer 111,constituting the first evidence of a potassium aminocarboxylate chelate. Tetramethylammonium PDTA. Figure 4a shows the portion of the 60 MHz nmr spectrum of [(CHs)4W4PDTA (pH 13) arising from protons b and c. This was the first case where Jac was greater than .Jab, which is apparent by inspection (note relative lengths of right-hand and left-hand bars). This spectrum and all succeeding spectra in the present work were run at 100 “C in order to obtain higher resolution-a result of decreased viscosity. Computer analysis of this spectrum yielded the parameters given in Table 11. Stepwise addition of KCl proves that there is no crossover in the chemical shifts of protons b and c as the potassium chelate is formed. In Figure 46, the K+/PDTA ratio is 1 :1, and in Figure 4c the K+/PDTA ratio is 6 :1. Analysis of these spectra leads to the parameters given in Table 11. The change in the relative magnitudes of Jab and Jac is conclusive proof of the formation of a potassium chelate. We are now able to assign Hz = proton b and Ha = proton c as shown in Ia, IIa, and IIIa. Addition of K+ to [(CH3)4Nl4PDTA serves to increase the relative proportion of IIIa to Ia. A quantitative calculation of the relative amounts o f la, IIa, and IIIa can be carried out if values of the coupling constants between gauche protons and trans protons are known. Because CaPDTA exists virtually completely as rotamer IIIa, protons a and b are trans and protons a and c
WI-!~~
:@ H :
I
N@ X,;i
Hb NX2
Ia
cH3
Ha
IIa
X
=
IIIa --CH2C02-
are gauche. Analysis of the CaPDTA spectrum yielded Jab = 11.0 and Juc = 3.2 Hz. With the assumption that Jtrana= 11.O Hz and JBavche= 3.2 Hz in all rotamers of PDTA, whether free or complexed, rotamer populations (estimated accurate to i 5 % ) are calculated (Table 11). Because PDTA complexes are usually more stable than their Corresponding EDTA complexes by a factor of 50-100 (3,4), these results do not imply that KaEDTA forms a chelate of appreciable stability. Rubidium PDTA and Cesium PDTA. Table I1 presents data obtained from nmr spectra of solutions of 0.5M Rb4PDTA and 0.5M Cs4PDTAprepared by dropwise addition of the corresponding hydroxide solution until pH 13 was obtained. Rotamer populations for the Cs+ complex remain unchanged as [Cs+] is increased through addition of excess CsOH, while IIIa for the Rb+ complex increases slightly at the expense of Ia as excess RbOH is added. We conclude from these results that Rb+ forms a PDTA chelate of measurable stability and Cs+ does not. The distribution of rotamers in Cs4PDTA presumably reflects the “natural” conformational preference of PDTA-
C Figure 4. Partial nmr spectra of PDTA solutions at 100 “C for various [(CH3)aN+]/[K+]ratios Asterisk (*) denotes impurity in (CHa)rNOH
free from the distorting effects of metal chelate formation. The fact that IIa and IIIa are equally populated (19% each) strongly implies that methyl groups in PDTA have negligible butane-gauche interaction with IDA groups [in contrast to 0.8 kcal/mole for the butane-gauche methyl-methyl interVOL 40, NO. 11, SEPTEMBER 1968
* 1697
Table 11. Spectral Parameters, Rotamer Populations, and Formation Constants of Various PDTA Species at 100 “C 8, 8b 8, 8, Jab Jac Jbc Jaz Z J.Q Z IIu Z IIIu Kj Pure[(CH8)4NldPDTA 3.07 2.67 2.40 1.04 5.7 7.5 -12.5 6.4 55 13 32 0.12 ... ... ... ... ’ / a K+ 6.5 6.5 -12.6 6.4 43 15 42 1.9 ... ... ... ... 1 K+ 7.0 5.9 -12.8 6.4 34 17 49 1.6 ... ... ... ... 4 K+ 7.9 4.2 -12.8 6.4 20 17 63 1.1 6 K+ 3.05 2.61 2.26 0.903 8.6 4.2 -12.7 6.3 13 18 69 1 .o 10 K+ 3.05 2.58 2.26 0.895 8.9 4.2 -13.0 6.1 13 14 73 1.0 Kjav = 1 . 3 Pure Rb4PDTA ... ... ... ... 5.9 6.4 -12.2 ... 41 24 35 0.15 3 Rb+ ... ... ... ... 6.2 6.4 -12.6 ... 41 20 39 0.13 6 Rb+ ... ... *.. ... 6.3 5.7 -12.4 ... 32 28 40 0.10 10 Rb+ ... ... ... ... 7.2 5.6 -12.6 ... 31 18 51 0.18 K , av = 0 . 1 4 ... ... ... ... 4.7 8.0 ... -12.6 Pure CsaPDTA 62 19 19 0 ... ... ... ... 4.7 8.1 ... -12.8 6 Cs+ 62 19 19 0 ... ... ... ... ... 4.6 7.8 -12.5 10 c s + 62 19 19 0
action (In]. If this is indeed the case, then the natural conformational preference of EDTA must be close to that of PDTA reported here, in which the free energy difference between the anti and gauche conformations is 0.9 kcal/mole at 100 “C. Formation Constants of PDTA Chelates. Table I1 lists K, values for the 1 :1 (CH&N+, K+, Rb+, and Cs+ chelates of PDTA. These K, values (estimated accurate to A 30%) were calculated from rotamer populations on the basis of the following assumptions: (1) K, = 0 for the Cs+ chelate, and (2) rotamer IIa is of negligible concentration in all chelates (16). Monoprotonated PDTA (HPDTA). In connection with in a species which might be intramolecular H-bonding termed a “proton chelate,” it is of interest to determine whether the conformation of the ligand in the 1:1 K+ chelate undergoes any change as the monoprotonated species (HPDTA) is formed. As previously reported (9),the chemical shifts of protons b and c are almost identical in HPDTA. This leads to degeneracy in the nmr spectrum which prevents the determination of Jaaand Jac. When the pH of a solution of K4 PDTA is lowered, the nmr spectrum of the solution is a
(a,
(17) E. L. Eliel, N. L. Allinger, S. J. Angayl, and G . A. Morrison, “Conformational Analysis,” Interscience, New York 1965, p 9.
1698
ANALYTICAL CHEMISTRY
weighted average of the spectra of the 1 :1 K+ chelate and HPDTA. Computer analysis of these 60 MHz spectra was possible until the proportion of HPDTA exceeded 20%. Satisfactory matching of observed and computed spectra at 60 MHz was obtained using the parameters given in Table I (pH 13.0, 12.4, 11.9). No change in J a b or JUc occurred over the 20% extrapolation, and thus it appears that the conformation of the ligand in the 1 :1 K+ chelate and HPDTA are the same. Any conclusion regarding the existence of a “proton chelate” in HPDTA, however, must await investigations at higher spectrometer frequencies, where a more extensive extrapolation can be performed. ACKNOWLEDGMENT
The assistance of D. T. Sawyer and Blair Baker, of the University of California at Riverside, in obtaining the 100MHz spectra is gratefully acknowledged. RECEIVED for review March 18,1968. Accepted June 24,1968. Research supported by US. Public Health Service Grant No. 1-R01-AM10889. One of the authors (A.J.S.) thanks the U.S. Office of Education for financial support. Contribution No. 2238 from the Department of Chemistry.
