J. L. SUDMEIER
2344
Analysis of Proton Magnetic Resonance Spectra of trans-2,5-Dimethylpiperazine and Its Hydrochlorides. Effect of Amine Substituents on Chemical Shifts and Coupling Constants' by J. L. Sudmeier Contribution No. 8109, Department of Chemistry, University of California, Los Angeles, California 90004 (Received September l$?1967)
Proton magnetic resonance spectra of trans-2,5-dimethylpiperazine, trans-2,5-dimethylpiperazine monodihydrochloride are recorded at 60 and 100 MHz in aqueous hydrochloride, and trans-2,5-dimethylpiperazine solution. All spectra are of the ABCXS type and are fully analyzed with the aid of computers. The axial and equatorial protons undergo nearly equal protonation shifts. Surprisingly, the vicinal-coupling constants exhibit positive shifts with increasing substituent electronegativity, and the geminal-coupling constants exhibit negative shifts. That is, the vicinal-coupling constant of the ring protons is larger' for the hydrochlorides, contrary to the usual trend. The geminal-coupling constant of the methylene protons is smaller (more negative) for the hydrochlorides, suggesting that the 0 substituent effect is greater than the opposing a substituent effect in these compounds.
It is well known that protonation of a basic site in a molecule leads to downfield chemical shifts for neighboring -CH protons.2 The magnitude of the protonation shift depends upon (1) the nature of the substituent, ( 2 ) the distance (number of bonds) from the substituent,a and (3) the spatial relationship (e.g., dihedral angle) of the substituent and the particular -CH proton. The primary cause of protonation shifts is generally the increase in electronegativity of the protonated substituent, and this satisfactorily accounts for factors 1 and 2 . Protonation shifts which depend upon the dihedral angle, however, are caused either by an angular dependence of the electron-withdrawing effect or by changes in the local anisotropic magnetic shielding resulting from protonation. The compounds studied in the present paper, trans-2,5-dimethylpiperazine (P), trans-2,5-dimethylpiperazine monohydrochloride (HP+), and trans-2,5dimethylpiperazine dihydrochloride (H2P2+)exist almost exclusively (greater than 98%)4in the diequatorial conformation I. I n P, the conformation of electron
I
pairs is ~ n c e r t a i nas , ~will be discussed later. A knowledge of protonation shifts is essential in studies of microscopic acid-base e q ~ i l i b r i awhose , ~ ~ ~ results depend heavily upon the assumed protonationshift values. T h e Journal of Physical Chemistry
Considerable interest has been shown in the effect of substitutents on coupling constants.' For a large number of compounds, the vicinal-coupling constants are inversely proportional to the sum of the substituent-atom electronegativities,8 in agreement with the valence-bond theory of vicinal coupling.9 I n studies of rotational isomerism of a-amino acids at various pH values, the effect of amine substituents on the coupling constants is of primary importance. Calculation of residence times depends upon assumed values of J , and J,, the trans- and gauche-vicinalcoupling constants. Pachler,lO for example, showed (1) This research was supported by National Institutes of Health Grant No. 1-R01-AM10889-01. (2) E. Grunwald, A. Loewenstein, and S. Meiboom, J . Chem. Phys., 27,630,641 (1957). (3) See, for example, J. L. Sudmeier and C. N. Reilley, A n a l . Chem., 36, 1698 (1964). (4) E. L. Eliel, N. L. Allinger, S. J. Angyal, and G. A. Morrison, "Conformational Analysis," Interscience Publishers, Inc., New York, N. Y., 1965. (5) (a) M. Aroney and R. J. W. LeFevre, J . Chem. Soc., 3002 (1958); N. L. Allinger and J. C. Tai, J . A m e r . Chem. SOC.,87, 1227 (1965); N. L. Allinger, J. G. D. Carpenter, and F. M. Karkowski, ibid., 87, 1232 (1965); J. B. Lambert and R. G. Keske, ibid., 88, 620 (1966); E. L. Eliel and M. C. Knoeber, ibid., 88, 5347 (1966); (b) J. B. Lambert, R. G. Keske, R. E. Carhart, and A. P. Jovanovich, ibid., 89,3761 (1967). (6) See, for example, (a) A. Loewenstein and J. D. Roberts, ibid., 82, 2705 (1960); (b) D.E.Leyden and D. B. Walters, 152nd National Meeting of the American Chemical Society, New York, N. Y., Sept 1966. (7) For a recent review, see A. A. Bothner-By, "Advances in Magnetic Resonance," Val. I, J. s. Waugh, Ed., Academic Press Inc., New York, N. Y., 1965. (8) R.J. Abraham and K. G. R. Pachler, Mol. Phys., 7, 165 (1964). (9) M.Karplus, J . Amer. Chem. SOC.,85,2870 (1963);H. 8. Gutowsky, M. Karplus, and D. M. Grant, J . Chem. Phys., 31, 1278 (1959). (10) K. G. R. Pachler, Spectrochim. Acta, 20, 581 (1964).
ANALYSIS OF PMRSPECTRA
2345
I .
I
3.0
I
I
2.8
I
I
I
2.6
6,
I
I
I
2.4 ppm E.
I
2.2
11 III in 111. The positive shift for a given substituent in 11, however, is approximately one-half as large as the negative shift in 111, so that in rotationally averaged compounds the net effect is negative. Protonation of a secondaxy amine group causes an appreciable increase12 in electronegativity, which is easily achieved by decreasing the solution pH.
I
2.0
1.0
I
DSS
Figure 1. Nuclear magnetic resonance spectra of trans-2,5-dimethylpiperasine(P) at 60 MHz:
that J t = 13.56 cps and J, = 2.60 cps are justifiable values for amino acids in alkaline solution, and that residence times could be calculated on this basis. Knowledge of these coupling constants at lower pH values, however, is lacking. The compounds employed in the present study permit the observation of amine substituent effects in the absence of changes in rotamer population. The molecular orbital treatment of Pople and Bothner-By" for geminal coupling predicts that the values of geminal-coupling constants, J,,, should be directly proportional to the electronegativity of an a substituent. For 9, substituents, a dependence on the dihedral angle is predicted, such that as the electronegativity of substituent X increases, J,, becomes larger (more positive) in 11,and smaller (more negative)
I
(a) observed;
(b) e d c u l a t d ,
Experimental Section trans-2,5-Dimethylpiperazinewas obtained from Aldrich Chemical Co. and was recrystallized from benzene. A solution of approximately 0.4 M was p r e pared using deionized water. This solution was passed through a column of Bio-Rad Chelex 100 chelate resin to remove any paramagnetic metals, which could broaden the spectral lines. Potentiometric titration with 4 M HC1 yielded values of pK2 = 10.20 and pK1 = 5.85. The pH values of solutions corresponding to P, HP+, and H1P2+are 12.8, 7.7, and 1.3, respectively. The spectra at 60 MHz were recorded using a Varian A-60 nmr spectrometer. All samples were run at probe temperature (-44"). A sweep width of 100 cps (1 em = 2 cps) and a scan rate of 0.2 cps/sec were employed. A radiofrequency field strength of 0.02 mG gave optimal signal-to-noise without saturation. Chemical shifts were measured relative to internal tbutyl alcohol but reported relative to 2,2-dimethyl-2-silapentane &sulfonic acid, sodium salt (DSS), whose methyl resonance lies 1.233 ppm upfield of tbutyl alco(11) J. A. Pople and A. A. Bothner-By. J . Chem. Phys., 42, 1339 (1964). (12) Values of 2.85 and 3.03, respectively. on the group-eleetr+ negativity soale of Cavanaugh and Dailey (J. R. Cavanaugh and B. P. Dailey. {bid., 34, 1099 (1961)) derived from data from squeous solutions given in ref 3. Primary and tertiary amine substituents yield similar values: 2.86; -+"I. 3.01; -NRz, 2.81; and -+NHR*. 3.09.
-"I.
Volume 7%Number 7 July 1968
J. L. SUDMEIER
2346 100 MHz pH 12.8
I
0
t
I
I
I
I
I
2.9
2.8
2.7
2.6
2.5
2A
2.3
2.2
\':I' I
i Figure 2. Nuclear magnetic resonance spectra of trans-2,5dimethylpiperazine (P) at 100 MHz:
hol. Sweepwidth linearity was checked after recording each spectrum, and any deviation from correct linearity (that producing the CHC13 proton resonance at 435.5 cps downfield of TMS (tetramethylsilane) on the 100-cps sweep width using offset) was corrected for. Spectralline positions are estimated accurate to *0.2 cps. Removal of dissolved oxygen produced negligible line sharpening and was omitted thereafter. The 100MHe spectra were recorded on a Varian HA-100 nmr spectrometer, and linearities were checked by the usual audio sideband method. Computation of spectra was performed on the IBM 7094,13 using LAOCOON 1114 with modifications by Maddox" (combining parts I and 11) and by the present author (plot subroutine).
Results
P exists in greater than 98% abundance as the diequatorial conformer shown in I for HQz+. This structure exhibits Ci symmetry, with pairs of equivalent methyl groups, methine protons, and two methylene protons related by reflection through the center of symmetry. Because of rapid exchange, the -NH proton resonance merges with that of water, and the The Journal of Phyakd Chemialiy
(a) observed; (b) calculated
-NH and -CH protons are effectively decoupled*under the conditions reported herein. The nmr spectrum of P a t 60 AlHz is shown in Figure la. ABCXa spectra for 1,2-disuhstituted propaneg have been reported.16 The high-field doublet, containins an area equal to that of the low-field portion, is readily assigned to the methyl protons in P, and the lowfield portion is assigned to the methine and methylene protons labeled A, B, and C in I. The spectral lines in Figure la, like those in all remaining figures, are somewhat broad, probably due to quadrupole relaxation by N14 and/or to unresolved long-range coupling through the nitrogen atoms. The ABC: portion of the nmr spectrum of P a t 100 MHz, shown in Figure 2a, is the easiest to interpret by inspection. The four-line pattern a t high fields consists roughly of doublets of doublets with repeated (13) We thank the UCLA Computing Facility for generously providing computer time and programming assistance. (14) S. Castellano and A. A. Bothner-By. J . Chem. Phys.. 41, 3863 (1964). (15) M. Maddox. Ph.D. Thesis. University of California, Los Angeles. Calif.. 1966. (16)
H. Finegold, Pwc. Chem. Soc.. 213 (1962).
ANALYSIS OF PMRSPECTRA spacings of -12 and -10 cps. One of these values must arise from geminal coupling, and the other from trans-vicinal coupling. The only proton which could be so coupled is proton C in structure I. This assignment is further supported by the high-field chemical shift of this proton, characteristic of axial protons such as C. The proton-B resonance is also expected to consist of doublets of doublets, split by the geminal coupling constant of -10 or -12 cps (absolute value) and by a smaller gauche-vicinal-coupling constant. The four lines at lowest field strengths fit this description, exhibiting repeated spacings of -12 and -3 cps, and are thus assigned to proton B. Both proton B and C are four bonds removed from the methyl protons and are thus not expected to couple strongly with them. The proton-A resonance is expected to be split into quartets by the three vicinal methyl protons, with a coupling constant of -6 cps, the separation of the methyl doublet peaks. I n addition, the proton-A resonance should be split into doublets by proton C with the trans-vicinal-coupling constant, now known to be -10 cps, and further split into doublets by proton B with the gauche-vicinal-coupling constant of -3 cps, and is thus expected to be quite complex. The first stage in the computation of spectral parameters with LAOCOON 11 is computation of a trial spectrum, using only part I (with plot output). The use of known spectral parameters from analogous compounds is invaluable in making the first approximations. For example, the signs of the geminal- and vicinalcoupling constants in similar compounds are opp0site.l’ Although this condition was assumed for the initial computations, the excellent fit of spectra at both 60 and 100 MHz confirms the relative signs of the coupling constants. The parameters were then refined by trial and error until 1 : l matching of computed and observed lines was possible. Typically, 110 calculated lines, many of which are triply degenerate, were matched out Of the total of 972 calculated lines-the unmatched lines generally being very low in intensity. The second stage of the computation utilizes both parts I and I1 of LAOCOON 11, in which the program optimizes all parameters so that a “best fit” of calculated and observed lines is obtained by least squares. Additional refinement at the second stage was generally needed, as unusually large deviations revealed the presence of improper assignments. Table I gives the best parameters obtained for P, HP+, and H2Pz++.The average and root-meansquare errors in the fit of observed and calculated lines and the probable errors in the individual parameters were always less than and hence limited by the error (estimated i0.2 cps) in determining the positions of these rather broad lines. Initially, the unresolved long-range couplings due to JBXand J C X were set equal to zero. At a later stage,
2347
Table I : Chemical Shifts and Coupling Constants of trans-2,5-Dimethylpiperazineand Its Hydrochlorides P (pH 1 2 . 8 )
2.644 2.859 2.293 0.960 2.9 10.8 6.4 -12.5 -0.1
-0.2
HP (pH 7 . 7 ) +
3.103 3.248 2.700 1,180 3.0 11.5 6.2
-13.5 -0.1 -0.3
HzP2+ (pH 1.3)
3.693 3.728 3.239 1.422 3.4 12.2 6.1
-14.2 -0.2 -0.3
these restrictions were removed, and small negative values resulted. Figures l b and 2b show computed spectra of P at 60 and 100 MHz obtained from part I with the parameters given in Table I, with plot output, using an interval of 0.25 cps and a peak half-width of 1.00 cps. The methyl resonance in Figure l b is arbitrarily reduced in scale relative to the remainder of the spectrum. Figures 3a and 4a give the observed spectra of HP+ and HzPz+ at 100 MHz. Similar procedures were employed in calculating the parameters shown in Table I. Figures 3b and 4b give the computed spectra of HP+ and HzPz+ at 100 MHz, using parameters in Table I. The observed and calculated spectra of HP+ and HzP2+ (not shown) also exhibit good agreement.
Discussion Chemical Shifk. The protonation shifts occur in two unequal steps, with the change produced in going from P to HP+ being -20% smaller than those produced in going from H P + to H2P2+. The magnitudes of these protonation shifts, in order of decreasing pH values, are: for proton A, 0.46 and 0.59 ppm; for proton B, 0.39 and 0.48; for proton C, 0.41 and 0.53; and for proton X, 0.22 and 0.24. Unsubstituted piperazine exhibits the same effect with corresponding values of 0.40 and 0.48 ppm,3 the reason for which is unclear. The total protonation shift of A, 1.05 ppm, is larger than that of protons B or C , in agreement with the larger substituent effects often found for methine protons. The total protonation shifts of B and C, 0.87 and 0.94 ppm, are almost identical, indicating negligible dependence of the dihedral angle of the substituents. The chemical-shift difference A ~ B Qis 0.57 ppm in P, 0.55 ppm in HP+, and 0.49 ppm in HzPz+. According to Lambert, et u Z . , ~ ~a difference in the chemical shifts of axial and equatorial protons, As,,, of 0.40-0.50 ppm in piperidine derivatives indicates that the lone (17) J. W. Emsley, J. Feeney, and L. H. Sutcliffe,“High Resolution Nuclear Magnetic Resonance Spectroscopy,” Vol. 1, Pergamon Press Ltd., New York, N. Y., 1965, pp 172-174. Volume 78, Number 7 July 1868
J. L. SUDMEIER
2348
100 MHr DH 7.7
I
3.3
.
3.2
I
3.1
3.0
.
2.9
.
2.8
2.7
2.6
.
Figure 3. Nuclear magnetic resonance spectra of trans-2,5dimethylpiperazinemonohydrochloride (HP+) at 100 MHz: (a) observed; (h) calculated.
pair is equatorial, and a difference of 0.80-0.94 ppm indicates that the lone pair is axial. However, the situation is more complex in piperazine derivatives, apparently because of deshielding of axial protons by axial lone pairs on the 0 nitrogen. This 1-3 deshielding interaction is deduced from the value of A L = 0.63 ppm for N,N'-dimethylpiperazine a t low temperatures.18 The lone pairs in this compound are predominantly axial, and the a substituent is expected to contribute as much as 0.94 to A L , the value found in N-methylpiperidine at low temperatures. The decrease of -0.3 pprn is attributed to the deshielding of axial protons by the axial p lone pairs. Because the shielding contributions of axial lone pairs on the 01 and p nitrogen atoms in piperazine derivatives are opposite, A6., tends to be independent of the lone pair conformation. By analogy with recent findings The Journal of Phydeol Chemislry
for ~iperidine,~b however, the lone pairs in P are probably largely equatorial. The anisotropic shielding of nitrogen lone pairs discussed here can be represented by the model, IV, where the plus sign indicates increased magnetic shielding and the minus sign indicates de~hielding.'~,'~a
-E.A+
Iv
(18) R. K. Harris and R. A. Spragg. Chem. Commun.. 314 (1966). (19) For a similar derivation of a carbonyl group shielding model, see G. J. Karebatsos, G. C. Sonnichsen, N. Hsi, and D. J. Fenoglio, J . Amw. Chem. Soc.. 89, 5067 (1967).
ANALYSIS OF PMRSPECTRA
2349
100 MHz pH 1.3
.
.
.
..
..
.
...
.
... ,
..'.....,
I
..,.
.,,,.........,. I .. .
Figure 4. Nuclear magnetic resonance spectra of lrana-2,5dimethylpiperazine dibydrochloride (H,P'+) at 100 MHz: (a) observed; (b) calculated.
VicinaGCoupling Cunstanls. The vicinal-coupling constants JABand Jac are almost certainly positive'J' and increase in going from P to H2P2+. This trend is contrary to that predicted by valence-bond theory,O which states that vicinal-coupling constants should be inversely proportional to the substituent electronegativities. A possible explanation for this anomalous substie uent effect may be found in ring deformations caused either by rehybridization of the nitrogen atoms with nrotonation or bv electrostatic renulsion between the r positive charges in HQ2+' The latter cause can p ' parently he ruled out, being inconsistent with the monotonic trend in coupling constants of the series p, Hp+, and H2P*+. ~
~~~
~
~
~
crease the ring N-GC bond angles, (3) slightly decrease the H-C-C bond angles (ring carbon atoms),*l and (4) decrease ring H-CC-H dihedral angles. Factor 3 argues in favor of the observed increase in the vicinal-coupling constants and factor 4 argues in favor of the increase in JABand against the increase in JAc. Thus if factor 3 is predominant, it would account for the anomalous substituent effect. Recent nmr studies,z0 however, indicate that the extent of nitrogen rehybridization is only a few per cent
~~~~
~~~
(1Qe) NOTE ADDED IN PROOF. This model agrees with recent chemi-
shift data from asiridines: H. Saitd. K. Nukada. T. Kobayashi. x.Moritar J . Chm. 8% 6605 (1967). (20) G . Binnch. J. B. Lambert. B. W. Roberts, and J. D. Roberts,
cal
iW.. 86. 6564 (1964).
J. L. SUDMEIER
2350 Table I1 : Spectral Parameters of Some a-Amino Acids Compound
n
JBC,
JAB,
JAG
CPS
CP8
CPS
SC,
8A ppm
8B v ppm
ppm
3.00 3.27
2.27 3.10
f
Ref
Phenylalanine" Phenylalanine
0 1
-13.45 -14.50
5.34 5.14
7.75 8.00
3.47 3.97
Phenylalanine Phenylalanine
0
-13.7 -14.7
5.5 5.6
7.8 7.7
0.000 0.000
-0.482 -1.014
-0.686 -1.137
B B
Cystine' Cystine
0 4
-13.7 -15.4
4.8 4.2
7.7 8.2
0.000 0.000
-0.448 -1.035
-0.662 -1.175
B
Aspartic acid" Aspartic acid
0 0.5d
-15.4 -17.0
3.8 3.5
9.9 8.3
3.56 3.78
2 63 2.73
2.28 2.53
h h
Aspartic acid' Aspartic acid
0
-15.5
...
4.1 5.4
9.7 5.4
0.000 0.000
-0.927 -1.258
-1.252 -1.258
B B
Cysteinee Cysteine Cysteine Cysteine
0 0.5 1.o
-12.8 -13.2 -13.9
3.3 3.6 3.9
9.5 8.7 8.2
5.1
5.1
-3.55 -3.52 -3.49 -3.47
-3.92 - 3 79 -3.64 -3.47
2
3
2.0
...
-3.26 -3.05 -2.83 -2.53
f
B
z
i
i i
Chemical shifts converted to ppm us. DSS by the present author (t-butyl alcohol set a t 1.233 ppm us. DSS). For all values above, Chemical shifts converted to ppm vs. 6~ by the present author. All parameters obtained by first-order analysis and thus are approximate. Chemical shifts obtained by interpolation of graphical data and reported us. DSS. Value of n approximated by the present author, using graphical interpolation. Chemical shifts (us. external benzene) converted to ppm by the present author. Bulk-susceptibility corrections were not made. J. R. Cavanaugh, J . Amer. Chem. Soc., 89, 1558 (1967). K. G. R. Pachler, Spectrochim. Acta., 19,2085 (1963). F. Taddei and L. Pratt, J. Chem. Soc., 1553 (1964). R. B. Martin and R. Mathur, J . Amer. Chem. Soc., 87, 1065 (1965). 6 increases with decreasing field strength.
'
in s character between NHa and NH4+ ion. The same result for secondary amines is evidenced by the C-N-C bond angles from the following electron and X-ray diffraction data (dimethylamine (g), 111 f 3°;22and piperidine hydrochloride (s), 112.3') which lies well within the range of internal angles (109.8 to 113.4') in this compound.2a Geminal-Coupling Constants. The geminal-coupling constants are almost certainly negative79l7 and decrease from -12.5 to -13.5 t o -14.2 cps in P, HP+, and H2P2+. The methylene groups are each situated between two amine substituents, one directly attached (a) and one a carbon-carbon single bond removed (p), Because the theory of geminal couplingll states that a substituent effects are positive and p effects in this case are negative, our results tend to indicate that the p effect is greater than the a: effect, as previously suggested for 1,2-disubstituted ethanes and propanes.s Ring deformations due to nitrogen rehybridization with protonation tend to decrease the internal N-C-C angle and increase the geminal H-C-H angle. The resultant increase in s character of the C-H bonds tends to increase the value of JBC rather than decrease its value as observed. It is quite possible that the a: substituent effect for amines is anomalous in the sense that back-donation of nonbonding electrons on nitrogen atoms may have a greater effect than the increased inductive effect of protonated nitrogen atoms. That is, withdrawal of u The Journal of Physical Chemistry
electrons from the symmetric bonding orbital as the nitrogen is protonated may be more than compensated by the loss of nonbonding electrons back-donated into the antisymmetric bonding orbital by the unprotonated nitrogen atom, with the result that nitrogen protonation could yield a negative a: substituent effect. At present, there is no literature data on this point. However, there is independent evidence (Table 11)to support the contention that the p inductive effect alone is sufficient to account for a decrease in J B Cof the magnitude (1.7 cps) observed in going from P to H2P2+. Data for phenylalanine and cystine in Table I1 show that J B Cin these compounds undergo decreases of 1.0 and 1.7 cps, respectively, on protonation of the amine group. The labeling of protons A, B, and C is given in V, which is the most abundant rotamer in both alkaline and neutral solutions.
H2Nwuc
. HB
y
HA
V
For some of the comparisons in Table 11,the value of
H+added to the fully unprotonated form of the compound, changes rather abruptly (e.g., from 0-4 in cystine), indicating pro-
n, the number of equivalents of
(22) P. w. Allen and L. E. Sutton, Acta Crystallogr., 3, 46 (1960). (23) P. C. Rerat, {bid., 13, 72 (1960).
SOLUTIONS OF N-SUBSTITUTED AMINOACIDS tonation of carboxylate and other groups in addition to amine groups. Nearly all the observed change in J B C , however, is caused by the amine protonation alone, shown, for example, in the data for phenylalanine. The absence of significant changes in rotamer populations, particularly in the case of phenylalanine, is shown by the constancy of the vicinal-coupling constants. (Note also the absence of positive changes in the amino acid vicinal-coupling constants with increased protonation, as were found in the present study.) When 1 equiv of H + is added to cysteine, a decrease of 1.1 cps in the geminal-coupling constant is observed. The first equivalent of H+ added to cysteine protonates not only amino groups but also mercaptide groups with roughly equal distribution^.^^^^^ Once again, the posi-
2351 tive contribution of increasing the electronegativity of the a substituent (-S- to -SH) must be negligibly small compared to the positive contribution of increasing the electronegativity of the /3 substituent (-NH2 to -+NHa). Some redistribution in rotamer populations may be indicated by the small changes in the vicinal-coupling constants in cysteine from n = 0 to n = 1, but rotamer V undoubtedly remains the predominant one. Acknowledgments. The author is grateful to Professor F. A. L. Anet for helpful discussions and to Professor D. T. Sawyer of the University of California a t Riverside for his kindness in providing the spectra at 100 MHz. (24) D. P. Wrathall, R. M. Izatt, and J. J. Christensen, J. Amer. Chem. SOC.,86, 4780 (1964).
Solutions of N-Substituted Amino Acids. 111. The Influence of Solvent on the Tautomeric Equilibrium by David A. Horsmal and Charles P. Nash Department of Chemistry, Universitu of California, Davis, California 95616 (Received September 15, 1967)
Dielectric constant measurements have been made on solutions of N,N-di-n-butyl-8-aminopropionic acid in seven nonaqueous solvents. The results lead to dipole moments for the monomeric acid which vary from 6.3 D in benzene to 12 D in methanol. Intermediate values are interpreted on the basis of shifts in the equilibrium between the zwitterion and the classical tautomers of the amino acid. The equilibrium constants thus obtained are compared with the predictions of continuum electrostatic theory and the deviations are rationalized on the basis of specific solute-solvent interactions.
Introduction It has been accepted for some 30 years that two tautomeric forms of an amino acid, neither of which carries a net charge, can exist in solution in equilibrium. In aqueous solutions of naturally occurring aliphatic amino acids, the dipolar ion (zwitterion) tautomer outnumbers the “chargeless” form by a factor of about 1 million. In alcohol-water mixtures, however, the ratio may be reduced to a few hundred.2 It is evident that a study of this simple tautomerization reaction in media having different macroscopic dielectric constants would be a worthwhile test of the extent to which essentially electrostatic processes contribute to the over-all free energy change for a model chemical reaction. Spectroscopic investigations by Barrow3 and Cook4 have shown, in a qualitative way, that N,N-dialkylated amino acids are soluble in a wide range of solvents and that the tautomeric equi-
librium can be made to shift within readily observable limits. It has also been shown that in nonpolar media, dimeric and higher aggregates, in which proton transfers also occur, form very r e a d i l ~ . ~ , ~ The present paper reports dielectric constant measurements on solutions of N,N-di-n-butyl-p-aminopropionic acid in seven solvents over concentration ranges in which unimolecular solute species dominate. The apparent dipole moments thus obtained are interpreted in terms of solvent effects on the tautomeric equilibrium. (1) NDEA Predoctoral Fellow 1962-1965.
(2) J. T.Edsall and M.H.Blanchard, J. Amer. Chem. SOC.,55, 2337 (1933). (3) G.M.Barrow, ibid., 80,86 (1968). (4) D. B. Cook, Ph.D. Thesis, University of California, Davis, Calif., 1965. (5) C.P. Nash, E. L. Pye, and D. B. Cook, J . Phys. Chem., 67, 1642 (l963). Volume 78, Number 7 July 1008
