Nuclear Magnetic Resonance Studies of Protonation of So me PoIy a mino ca r boxy1a te Co mpo unds Co ntu ining Asymmetric Carbon Atoms JAMES L. SUDMEIER and CHARLES N. REILLEY Department of Chemistry, University o f North Carolina, Chapel Hill, N.
b Proton magnetic resonance spectra of trans - (1,2 - cyclohexylenedinitri1o)tetraacetic ac:id (CyDTA), trans - 1,2 - diaminocyclohexane, dl( 1,2 -prop y Ien edinit rilo),tet ra acetic acid (PDTA), and meso [2,3 - butylenedinitri1o)tetraacetic acid (BDTA) at various pH values are presented and interpreted. The functional groups in the former two compounds occupy predominantly diequatorial positions on the cyclohexane ring at pH = 5 to 14. The theoretical basis ,for AB splitting patterns exhibited b y certain acetate protons i s discussed in terms of ligand conformation in solution, as affected b y the rate of inversion of nitrogen atoms and relative population of various rotational states.
-
I
EARLIER PAPER ,,"9)> studies of proton magnetic resonance spectra of a number of polyaininocarbosylate compounds were reported in which t8he pair of methylenic protons attached to alpha carbon atoms of acetate groups exhibits a single sharp resonance peak a t all pH values studied. The present paper reports the results of siiiiilar studies of thc following polyaminocarboxylate com; iounds in which the two protons on the alpha carbon atom of an acetate grouii do not have the same values of chemical shift (6) and, b h g spin-coupled , split each other into .1B patterns: trans-(1,2-cyclohexylenedinitri1o)tetraacetic acid (CyDT.i), dl-( 1,2-propyleriedinitrilo)tetraacetic acid (PDT.I), arid meso-(2,3butylenedinit,rilo)trtraacetic acid (EDT-1). .Iswill be shon-n, the important characteristic of these compounds for purposes of this paper is the fact that they contain asymmetiic carbon atoms in proximity to acetail? protons. Result.: of S l I R studies of trans-1.2-diamirioc~yclohrxanea t various pH value. art' givcn and are .shown to support thc conclusion that the iminodiacetate groui).: in C>-I)T.I occul)y diequatorial e Imitions on the c y ~ l o h ~ x a nring. Tht. throwtiral tiasis for the observpd .I13 s~ilitting IiattcJins is discussed in w r i m of ligand c-oiiforin:rtion in qolution, a s :rffrctrtl liy the rat(, of inversion of
C.
nitrogen atom\ and the relative population of various rotational qtates. Some of the concluqions reached in S l I R studieq of these analytically important compounds Ehould 1)roi.e useful in the interpretation of metal chelate .pectra which are often characterized by .1B splitting pattern, (6, 7 ) . EXPERIMENTAL
The proton magnctic resoiiance spectra were recorded using a S'arian 1-60 high resolution S l I R spectrometer. The experimental procedures and conditions are identical to those described previously (f 9). Chemical shifts
are reported relative to sodium 3-(trimethylsilyl) propane sulfonate (tms*). I n order t,o minimize the tendency to form alkali metal complexes, pH was varied by addition of COu-freepotassium hydroxide. The pIL's \yere determined a t 26-27" C., using the same solutions employed in the SlIR mea;.-urernents. Concentrations ranged from 0.4 t o 0.5M. and no added electrolyte wa3 present. CyI>T.I was obtained from Gri,rry Chemical Co. as the uncharged acid (H4Y) arid was used without further purification. trans-1.2-Diaminocyclohexane was obtained from ;ildrich Chemical Co. as the slightly soluble sulfate and was converted to the di-
x AX
Figure 1 .
Chemical shift data of CyDTA a t various pH values VOL. 36, NO. 9, AUGUST 1964
1707
hydrochloride by treatment with slightly less than one equivalent of barium chloride (to minimize contamination of the amine with barium), followed by aging and filtration of the resulting precipitate. dl-PDTA was prepared by the method of Dwyer and Garvan (9). meso-BDTA was prepared by the following steps: 2,3-diaminobutane was synthesized by reduction of dimethylglyoxime with Raney nickel and the meso and dl components were separated by fractional crystallization of the dihydrochloride, mesa being eight times more insoluble in methanol (8). Purity of the compounds thus obtained was estimated by examination of the KMR spectra of the various dihydrochloride fractions in aqueous solution, dl and meso being distinguishable owing to a slight displacement in chemical shifts-Le., the center of the methyl resonances occurs a t 1.50 p.p.m. in mesa and 1.45 in dl, and the center of methine resonances occurs a t 3.77 p.p.m. in mesa and 3.87 in dl. After two or three fractional crystallizations the meso compound obtained is approximately 99% pure. The free diamine was allowed to react with monochloracetic acid according to a procedure adapted directly from Dwyer and Garvan (9). The reaction is complete after four days a t room temperature with a yield of approximately 75%. Several reprecipitations of the uncharged compound (H4Y) from acidic solution to remove foreign salts were carried out. The copper(I1) complex of meso-BDTA exhibits maximum absorption a t X = 720 mp.
The spacing between lines 1 and 2 (or 3 and 4) of the A B quartet yields a value of the spin coupling constant Jab = 16.5-17.0 c.p.s. a t all p H values studied. Chemical shift values of a and b computed from various spacings in the AB pattern (15) are plotted us. pH in Figure 1. The labeling of a and b in Figure 1 which in reality may be reversed, is not intended to imply knowledge of the absolute configuration. The observed chemical shift values of a and b protons when n (the number of equivalents of acid added to the free base) is equal to 0, 1, and 2, are given in Table I. Comparable values are calculated from substituent shielding constants under the assumption that each nitrogen atom is protonated 50% of the time a t n = 1, and lOOyoat n = 2 , and are also given in Table I. These values agree well with values of a protons in CyDTA but are -0.2 to 0.4 p.p.m. higher than values of b protons. The observed values of the center of the highly split band (half-width approximately 14 c.P.s.) which appear immediately upfield of the d B pattern are given for n = 0, 1, and 2,in Table I. Because of difficulties inherent in the use of substituent shielding constants to calculate chemical shift values of methine protons, no data are yet available for comparison. However, the band is assigned to methine c protons because its relative area corresponds to two protons, it occurs a t the next lowest RESULTS AND DISCUSSION field after the A B quartet, and it is most shifted by protonation of nitrogen CyDTA. Figure 1 shows the strucatoms (A8 = 0.90p.p.m.), tural formula of C y D T A tetraanion (Y-"), a typical proton resonance From inspection of the remaining four spectrum, and a plot of chemical shift highly split and overlapping bands it is values of the various protons as a apparent that a single conformation of thecyclohexane ring is preferred ( I S ) . As function of pH. The structural formula exhibits eight acetate protons (four p H is changed, the four bands exhibit labeled a and four labeled b ) , two a coherence in movement on the chemidentical methine protons in the cycloical shift scale which is invaluable in dishexane ring labeled c, and eight methyltinguishing one from another. The halfwidth of each band is estimated to be enic protons of four types labeled approximately 14 c.p.s. Chemical shift d (axial), e (equatorial), f (equatorial), and g (axial). values plotted in Figure 1, and given for The observed spectrum in order of n = 0, 1, and 2 in Table I were taken as increasing field strength consists of band centers, determination of which an intense A B quartet, an isolated involves a rather'large error (estimated highly split resonance band, and a 10.05 p.p.m.) for such overlapping bands. group of overlapping highly split resonance bands in which four individual Changes in chemical shift caused by bands containing approximately equal addition of two equivalents of acid are areas are discernable. The ratio of 0.24,0.19,0.42,and 0.32 p.p.m. for the areas cont:tined in the pattern and bands four bands in order of increasing field in the order mentioned is 4:l:l:l:l:l. strength which are expected to indicate The latter five bands are displayed in relative proximity to nitrogen atoms of Figure 1 at five times the amplitude of protons giving rise to the particular bands. the A B quartet. The -4B splitting pattern can unAnother guide to assignment of protons d, e, f , and g is the finding equivocably be assigned to a and b of Muller and Tosch ( I S ) from low protons for three reasons: relative areas; comparison of the chemical temperature studies of cyclohexane shift values of a and b with acetate that the chemical shift values of normal protons in analogous compounds; and equatorial and axial protons are 1.65 comparison with values calculated using and 1.17 1xp.m. us. tms, respectively. substituent shielding constants (19). This suggests the assignment of e, f,d,
170 8 *
ANALYTICAL CHEMISTRY
Table I.
Chemical Shift Values, p.p.m. vs. tms* n = O n = l n = 2
CyDTA a b a, b (calcd.) C
e
fd s
3.35 3.08 3.30 2.58 1.98 1.78 1.20 1.10
3.tii 3.42 3.68 3.07 2.15 1.90 1.49 1.28
3.95 3.65 4.05 3.48 2.22 1.97 1.62 1.42
2.35 1.82 1.68 1.27 1.17
2.87 2.05 1.80 1.47 1.37
3.50 2.22 1.92 1.67 1.50
3.26 3.33 3.02 2.93 3.30 (2.47)
3.70 3.62 3.57 3.48 3.68 3.02
4.02 3.92 3.68 3.92 4.05 3.53
trans-1 ,?-Diarnino-
cyclohexane C
e
fd9 dl-PDTA a a'
b b' a,a',b,b' (calcd.) h,i h,i (calcd. for
EDTA)
j
2.55 3 . 1 0 3 . 6 5 0.93 1 . 2 2 1.40
meso-BDTA a b a,b (calcd.) C
j
3.30 3.13 3.30 2.75 1.08
3.65 3.65 3.68 3.30 1.38
4.01 3.88
4.0; 3.77 1.67
and g protons to the remaining four bands in order of increasing field strength. Since protons attached to the same carbon atom are expected to undergo nearly equal changes in chemical shift upon protonation, the fact that As, # Aad (0.24 # 0.42 p.p.m.) and A8, # As, (0.19 # 0.32 p.p.m.) represents a discrepancy which may be explained in part by the uncertainty of the determination of band centers. However, the absence of such a discrepancy in the case of trans-l,2-diaminocyclohexane, which appears to have the same conformation of CyDTA, supports the given assignment. Whether the iminodiacetate groups occupy diequatorial or diaxial positions on the cyclohexane ring is important in regard to both acid-base and metal chelating properties. For example, values of heats and entropies of formation of metal-CyDTh chelates found by Anderegg ( I ) , Xoeller and Hseu(Ik?), and Reilley and Wright (21) suggest a preorientation of the free ligand (Y-4) such as is found in the diequatorial conformer. Kroll and Gordon ( I I ) , however, from the comparison of various acid dissociation constants propose that the iminodiacetate groups are diaxial in the tetraanion (Y-4) because of mutual electrostatic repulsion of the negatively charged carboxylate groups but are diequatorial in the uncharged acid (H4Y). The conformational problem may be stated as whether the energy of electrostatic and steric re-
pulsion of iminodiacetate groups in diequatorial positions is greater or less than the energy of electrostatic repulsion of iminodiacetate groups in diaxial positions plus the energy of the four butane-gauche type interactions. Because of the small distance between nitrogen atoms in adjacent iminodiacetate groups in CyDTA (2.2 A, diequatorial; 3.4 A, diaxial), mutual electrostatic repulsion of these positively charged atoms a t n == 2 may be important. 4s will be shown, the energy of electrostatic repulsion in trans-1,2diaminocyclohexane is not great enough to alter the conformation as n is changed from 0 to 2. I n fact, the half-width of the methine proton bands indicates that in both trans- 1,2-diaminocyclohexane and CyDTA, the functional groups possess the diequatorial conformation a t all pH's studied. The half-width of the methine proton band is an important feature which has previously been employed in conformational analysis of 1,2-disubstituted cyclohexanes (4, 16) The primary factor determining this half-width is the magnitude of the trans spin coupling constant-Le., J c d between methine
proton c and the adjacent trans methylenic proton d-which in turn depends upon #, the dihedral angle between them. As shown in Figure 1, when the iminodiacetate groups are diequatorial, then $cc = 60' and dcd = 180'. On the other hand, if the iminodiacetate groups are diaxial, then # c e = 60' and qicd = 60'. J is generally 2 to 4 c.p.s. when = 60°, and J = 10 to 12 c.p.s. = 180'. Therefore, the when observed methine proton half-width of 14 c.p.s. is best explained by the existence of predominantly diequatorial conformer. trans - 1,2 - Diaminocyclohexane. Figure 2 shows the structural formula, a typical proton resonance spectrum, and a plot of chemical shift values of the various protons as a function of p H . The structural formula exhibits two identical methine protons labeled c and eight methylenic protons of four types labeled d (axial), e (equatorial), f (equatorial), and g (axial). At low field strength, an isolated highly split resonance band of halfwidth approximately 14 c.p.s. occurs. Observed values of the band centers are given for n = 0, 1, and 2, in Table I.
+
8, p,p.m.vs. trns" Figure 2. values
Chemical shift data of frans-l,2-diaminocyclohexane at various pH
The band is assigned to methine c protons because its relative area corresponds to two protons, it occurs a t low field, and it is most shifted by protonation of nitrogen atoms (A6 = 0.85 p.p.m.). ;It higher field strength, a group of overlapping highly-split bands occurs containing a total area which is four times that of the methine peak. Inspection shows the similarity of the spectrum of trans-1,2-diaminocyclohexane to that of CyDTA and the fact that both of these compounds possess the diequatorial conformation. With assistance from the chemical shift dependency upon pH, four components having a half-width approximately 14 C.P.S. and containing an area approximately equal to that of the methine band are distinguished. Chemical shift values plotted in Figure 2 are taken as band centers, determination of which involves an estimated error of 10.05 p.p.m. for such overlapping bands. Changes in chemical shift caused by addition of two equivalents of acid are 0.40, 0.24, 0.40, and 0.33 p.p.m. for the four bands in order of increasing field strength, which are expected to indicate relative proximity to nitrogen atoms of protons giving rise to the particular band. By reasoning identical to that employed in the case of CyDTA, the assignments e , f , d, and g to the above bands in order of increasing field strength are made. In this case, better agreement is obtained in protonation shifts of different protons attached to the same carbon atom ( A b d = 0.40 and A& = 0.40 p.p.m.; A6/ = 0.24 and A6* = 0.33 p.p.m.). The half-width of the methine proton band is a constant 14 c.p.s. a t all p H values studied. Therefore, by reasoning identical to that employed in the discussion of CyDTA] the conformation of the functional groups in trans-1,2diaminocyclohexane is predominantly diequatorial and is not affected appreciably by protonation. dl-PDTA. Figure 3 shows the structural formula of L-PDTA tetraanion (YP4),a typical proton resonance spectrum, and a plot of chemical shift values of the various proton resonances as a function of pH. The structural formula exhibits four protons on the acetate groups situated most closely to the optically active carbon atom (two labeled a and two labeled b ) , and four protons on the acetate groups located at greatest distance from the optically active carbon atom (two labeled a' and two labeled b'). The methine proton, attached to the optically active carbon, is labeled c, the methylenic protonq, being nonequivalent in general, are labeled h and i, and the methyl protons are labeled 1. Tmo separate A B splitting patterns exist over most of the p H range studied. VOL. 36, NO. 9, AUGUST 1964
1709
The spin coupling constants J a b and are a constant 16.5-17.0 c.p.s. This fact is invaluable in distinguishing the two patterns which repeatedly overlap and cross one another as p H is varied. The expected smooth transition of the pattern center in shifting from higher to lower field strengths with decreasing pH is the basis of maintaining their identities through several crossover points. Because of their relative areas and the good agreement of the chemical shift values with those of acetate protons in analogous compounds, the A B patterns are assigned to a combination of a , b, a‘, and b’. The difference in chemical shift of one a and b pair decreases as n is changed from 0 to 1, and increases as n is changed from 1 to 2. The difference in chemical shift of the other a and b pair decreases as n is changed from 0 to 1 and decreases to zero, causing the A B quartet to collapse into a singkt as n is changed from 1 to 2. Although the present data do not permit a certain choice t o be made, the former splitting pattern is assigned to the a and b pair of acetate protons and the latter pattern to the a’ and b’ pair. The reason for this choice is that the chemical shift difference between the protons nearest to the asymmetric atom which causes the difference is likely to persist over the greater pH range. I n addition, the p H dependency of the chemical shifts of h and i protons appears to correlate mith this assignment, being different a t n = 0 and identical a t n = 1 and 2. Chemical shift values of a , b, a’, and b‘ computed from various spacings in the A B patterns are plotted us. p H in Figure 3. The labeling of the various a and b pairs in Figure 3, which in reality may be reversed, is not intended to imply knowledge of their absolute configuration. The observed chemical shift values of a, b, a’, and b’ protons are given for n = 0, 1, and 2 in Table I along with values calculated from substituent shielding constants on the assumption that each nitrogen atom in dl-PDTA is protonated 50y0 of the time a t n = 1 and 100% a t n = 2. As was true in the case of CyDTh, these calculated values agree well with values of the more deshielded proton in each acetate pair (a and a’) but are -0.2 to 0.4 p.p.m. higher than values of the more shielded protons ( b and b’), The resonance bands of methine proton c, being broad and lorn in intensity are presumed to be obscured by A B splitting patterns. Immediately upfield of the A B patterns lies a combination of four or more resonance peaks containing a total area equal approximately to one fourth the total area of both -4B patterns, which is assigned to methylenic protons h and i. This combination of Jo’b’
17 10
ANALYTICAL CHEMISTRY
8, pp.m. vs. tms“ Figure 3.
Chemical shift data of dl-PDTA at various pH values
peaks, in spectra obtained at p H 4 through pH 10 (shown in Figure 3), simulates the B1 part of an ABP splitting pattern (bhe .4 part being methine proton c) showing that h and i have identical chemical shift values a t pH 4 through 10. -4s indicated in Figure 3 the larger peak includes lines 5 and 6 of the .4B2 pattern and the smaller peak slightly upfield is part,ly separated into lines 7 and 8 ( 1 5 ) . The chemical shift values of h and i which are obtained as the midpoint between lines 5 and 7 a t p H 4 through 10, are plott,ed as a solid line in Figure 3. Above pH 10, at least two additional peaks appear on the upfield side and the patt’ern generally becomes spread out,. This suggests that the chemical shift values of h and i are no longer identical, and so produce complex splitting patterns. Exact solutions of these patterns have not been effected and in Figure 3 the chemical shifts of both h and i protons above pH = 10 are roughly
indicated by the dotted line, which bisects the total area of the pat,tern. ;Is shown in Table I, the observed values of the chemical shift, of h and i protons a t n = 1 and 2 are in good agreement with the corresponding chemical shift values of ethylenic protons in EDTA, calculated with substituent shielding constants, assuming protonation of nitrogen atoms only. The well defined doublet a t high field strength, containing three fourths the combined area of .4B patterns, is assigned to methyl protons j . Peak separation is a constant 6.5 c.p.s., representing the approximate value of J,,, the spin coupling const,ant between met,hine c and methyl j protons. The midpoints of the doublet, taken as chemical shift of j , are give’i for n = 0, 1, and 2 , in Table I. The form of the doublet peaks shown in Figure 3 remains constant over the entire pH range studied indicating a nearly constant
rotational distribution about the ethylenic carbon-carbon bond. meso-BDTA. Figure 4 shows t h e structural formula of meso-TjDTA tetraanion (Y-"), a typical proton resonance spectrum, and a plot of chemical shift values of the various protons as a function of pH. The structural formula exhibits eight acetate protons (four labeled a , and four labeled b ) , two identical methine protons labeled c, and six identical methyl protons labeled j. The observed spectrum in order of increasing field strength generally consists of an A B quartet, a highly split band (half-width approximately 12-13 c.p.s.), and a somewhat broadened doublet. The ratio of areas contained in the resonances in the order given is 4: 1 :3. Spin-coupling constant J o bis found to have a value of 16.5-17.0 c.p.s. a t all pH values studied. Chemical shift values of a and b are computed and are plotted us. pH in Figure 4. For n = I ,
the difference in chemical shifts of a and b protons is equal to zero, and the A B quartet collapses into a sharp singlet. For n = 2, the difference becomes appreciable and the A B pattern reappears. It is impossible from these data to determine whether the more deshielded proton a t n = 0, labeled a, remains the more deshielded proton a t n = 2 or whether a crossover takes place. The former interpretation is favored because it leads to better agreement with the observed chemical shifts of a and b protons in dl-PDTA The labeling of a and b protons in Figure 4, which in reality may be reversed, is not intended to imply knowledge of their absolute configuration. The observed chemical shift values of a and b protons for n = 0, 1, and 2 are given in Table I along with values calculated on the assumption that each nitrogen atom in meso-BDTh is protonated 50y0 of the time a t n = 1 and
100% a t n = 2. As was true in the cases of C y D T d and dl-PDTh, these calculated values agree well with values of a protons but are consistently higher than values of b protons. Observed values of the center of the highly split band for n = 0, 1, and 2 are given in Table I. The band is assigned to methine c protons because its relative area corresponds to two protons, it occurs at low field, and it is greatly shifted by protonation of nitrogen atoms (A6 = 1.02 p.p.m.). The broadened doublet peaks a t high field are assigned to methyl protons and are separated a t all pH values by 6.5 c.p.s., an amount which is approximately equal to J c j . The midpoints, taken as the chemical shift values of j, are given for n = 0, 1, and 2 in Table I. Methyl protons in meso-BDT-1 are -0.2 p.p.m. less shielded and undergo a somewhat larger shift upon protonation from n = 0 to n = 2 than do methyl protons in dl-PDTA (A6 = 0.59 compared to A6 = 0.47 p.p.m.). The shape of the methyl doublet shown in Figure 4 remains constant over the pH range studied and, from the calculations of Anet ( 2 ) and Bothner-By and S a a r Colin (3) for the AX3A'X3' system, indicate. appreciable spin coupling between adjacent methine protons. This indicates a significant predominance of the rotomer in which the iminodiacetate groups are anti to each other. GENERAL DISCUSSION
b) U
Figure 4.
The protons a and b attached to the same carbon atom in any compound X are generally nonequivalent unless a in compound X is superimposable with b either in compound X or its mirror image. Differences in population of rotational states and a n intrinsic asymmetry effect (10, 14, ZO) may then give rise to differences in the chemical shift values of such nonequivalent protons a and b. When a and b are strongly spin coupled only to each other and are sufficiently diffrrent in chemical shift values, an .ZB splitting pattern results in tlicir XMK qpectrum.
Chemical shift data of meso-BDTA a t various pH values VOL. 36, NO. 9, AUGUST 1964
171 1
I1
value which is a t least 3 to 4 units below the pK, value is probably required for observation of the effects of slow nitrogen inversion by NMR. Thus, the AB splitting patterns exhibited by C y D T h , dl-PDTA, and meso-BDTA a t p H values as high as 14 cannot be explained on the basis of slow nitrogen inversion without postulating radically low rates. Based upon A u . ~ = 25 c.P.s., the greatest difference in chemical shift of acetate protons which has been observed, the lifetime of a particular nitrogen atom configuration must be less than -0.005 second in order to cause mutual splitting of acetate protons-Le.,
Compound I represents (ethylenedinitri1o)tetraacetic acid (EDTA), d-CyDTA, D-PDTA, meso-BDTA, and compound I1 represents their respective mirror image compounds EDTA, 1CyDTA, b P D T A , and meso-BDTX. Preferably with the aid of ball-andstick models, one may test these compounds by the superposition criterion to determine what equivalences are predicted for the various acetate protons. I n EDTA, for example, where R1 = Rz = R3 = R,, it is possible t o show that a1 = b2‘, b1 = q’,az = bl’, and b2 = al’ merely by superimposing I upon itself. Then, by superposition of I upon 11, it is shown that al = b2 = a’’ = b2‘ and az = bl = a‘ = bl’. No further equivalences are possible without permitting umbrella inversion of nitrogen atoms. The experimental observation that all eight acetate protons in EDTA are equivalent at p H 4 to 14 (19) is indicative of the fact that the rate of inversion of nitrogen atoms is rapid with respect to the inherent speed of the N M R measurement. Allowing both nitrogen atoms in I to undergo a single inversion, it becomes possible to demonstrate the equivalence of all acetate protons in the molecule. The nitrogen atom inversion rate of 2 X lo6 second-‘ found by Saunders and Yamada (17) for dibenzylmethyl ammonium chloride indicates that a p H
= 0.005 second. Therefore, 2?rAu,a the lifetimes of the intramolecularly Hbonded structure of monoprotonated E D T A (HY-3) proposed by Schwarzenbach and Ackermann (18 and the trifurcated H-bond structure of Chapman, Lloyd, and Prince (6) are probably less than -0.005 second. In view of the demonstrated chair conformation of CyDTA, the equivalences R1 = Rc and R2 = R3 may be stated. Superposition of either dCyDTA (I) upon itself or Z-CyDTd (11) upon itself proves the following equivalences; al = b2’, bl = az’, q = bl’, and bp = al’. hllowing each nitrogen atom to perform a single inversion proves that al = q = b,‘ = b2’ and bl = b2 = al’ = a2’. Therefore, each acetate group contains a pair of nonequivalent protons, and a single A B pattern, in agreement with experimental results, is predicted. In the case of D-PDTA (I) and G PDTA (11) groups R I , Ra, and R4 are equivalent. No superposition of I upon I, I1 upon 11, or I upon I1 is possible. However, rapid inversion of nitrogen atoms allows the following equivalences: al = q, bl = b2, al’ = q’,and bl’ = b2’. The four types of nonequivalent protons are expected to give rise to two A B patterns, in agreement with experimental findings. meso-BDTA contains two sets of
R~-+--R~ I
.’N‘.‘. I
/
(bz’)H-CLH(uzr)
I
(b,’)H%-H(ar’)
I
coz-
1712
co;
ANALYTICAL CHEMISTRY
7%-
equivalent groups, R1 = RI and R2 = R4 Superposition of I and I1 demonstrates the equivalences ai = a’’, a,L = az’,bl = bl’, and b2 = b2‘. Rapid nitrogen inversion permits the further statements, al = = al’ = az’and bl = b2 = b,’ = b2‘. Therefore, one A B splitting pattern is predicted, in agreement with experimental results. LITERATURE CITED
(1) Anderegg, G., Helv. Chim. Acta. 46, 1833 119631. (2) Anet, F. A. L., J . Am. Chem. SOC.84, 747 (1962). (3) Bothner-By, A. A., Saar-Colin, C., Ibid., p. 743. (4) Brownstein, S., Miller, R., J . Org. Chem. 24, 1886 (1959). (5) Chan, S. I., Kula, R. J., Sawyer, D. T., J . Am. Chem. SOC.86, 377 (1964). (6) Chapman, D., Lloyd, D. R., Prince, R. H., J . Chem. SOC.1963, 3645. (7) Day, R. J., Reilley, C. X., ANAL. CHEM.36, 1073 (1964). (8) Dickey, F. H., Fickett, W., Luqas H. J . , J . Am.Chem.Soc. 74,944(1932). (9) Dwyer, F. P., Garvan, F. L., Ibid., 81, 2955 (1959). (10) Gutowsky, H. S., J . Chem. Phys. 37, 2196 (1962). (11) Kroll, H., Gordon, XI., Ann. N . Y . B a d . Sci. 88, 341 (1960). (12) Moeller, T., Hseu, T. At., J . Inorg. iNucl. Chem. 24, 1635 (1962). (13) XIuller, N., Tosch, W. C., J . Chem. Phys. 37, 1167 (1962). (14) Yair, P. M.! Roberts, J. D., J . Am. Chem. SOC.79, 4566 (1957). (15) Pople, J. A;: Schneider, W. G., Bernstein, H. J., High Resolution Xuclear Magnetic Resonance.” McGraw-Hill. Ned; York, 1959. (16) Reeves, L. W., Str@mrne, K. O., Trans. Faraday SOC.57, 390 (1961). (17) Saunders, M., Yamada, F., J . Am. Chem. SOC.85, 1882 (1963). (18) Schwarzenbach, G., Ackermann, H., He1v:Chim. Acta. 30, 1798 (1947). (19) Sudmeier, J. L., Reilley, C. N., ANAL.CHEM.36, 1698 (1964). (20) Whitesides, G. M., Kaplan, F., Sagarajan, K., Roberts, J. D., Proc. Natl. A c u ~S. C ~c’. . S . 48, 1112 (1962). (21) Wright, D. L., ReillPy, C. S . , 1)ivision of Analytical Chemistry, 15th S. E. Regional Xleeting, ACd. Charlotte, X. C., Xovember, 1963. RECEIVED for review January 17, 1964. Accepted April 1, 1964. Research supported in part by Sational Institutes of Health Grant RG--8349.
