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Isotope effects in structural and stereochemical elucidations of organic compounds by NMR
Journal of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 321 (1994) 89-96
Isotope effects in structural and stereochemical elucidations of organic compounds by NMR’ Lech Kozerski Instituteof Organic Chemistry, Polish Academy of Sciences, 00-961 Warszawa, Kasprzaka 44, Poland
(Received 24 June 1993)
Abstract Generalizations on the nature of the intrinsic isotope effect, leading to several spectroscopic and chemical applications of the deuterium isotope effect, are outlined. The equilibrium chemical shift isotope effect, which can be observed further from the substitution center than the intrinsic isotope effect, and its applications are also discussed.
The effect of isotopic substitution on NMR parameters has a vibrational origin [l]. Its influence on chemical shifts, S, depends on the shape of a potential energy curve characteristic of the molecular fragment experiencing the isotope change. The intrinsic Dotope e&t is found in rigid molecules [2] whereas a double minimum creates, in addition, the equilibrium chemical-shift isotope efict [2a]. Studies of the nature of intrinsic isotope effects [3,4] have revealed some useful generalizations.
+0.03 20.5
1. Both primary and secondary isotope effects are usually diamagnetic, i.e. a heavier isotope substitution results in increased shielding defined on the 6 scale: A6 = S”X - S”‘X
However, high frequency shifts can be observed as welI [5,6j and, in the case of a primary effect, are linked to the shape of a potential energy curve [5j (see Scheme 1).
-0.06
ppm ppm
m
11.0
Scheme 1 [5].
’ Presented at the Summer School “Isotopic Effects - Karpacz ‘93”, Karpacz, Poland, June 20-25, 1993. 0022-2860/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZ 0022-2860(93)08208-L
ppm ppm
90
L. Kozerski/J. Mol. Strut. 321 (1994) 89-96
In Scheme 1 the primary isotope effects of deuterium in hydrogen bonded systems are shown (CH2C12 solution). In the maleate ion the deuterium isotope effect is shifted to higher frequencies; the ion is described by a single minimum potential energy curve. The hydrogen bond is very strong, hence the system is described as a “proton sponge”. In the salicylaldehyde the deuterium isotope effect is shifted to lower frequencies; the hydrogen bond is much weaker as judged from the S value of the OH proton as compared with the S value of the OH proton in the maleate ion. 2. The value of an effect is proportional to the number of replaced isotopes [7] and fades along the chain away from the substitution center. This is illustrated below in the aliphatic chain. The effects depend on the character of the substituent and the hybridization of the carbon atoms. C,-H,
+ Q-D, - 1.O to approximately
CB-C-H,
+ Co-C-D, -0.5 to approximately
C,-C-C-H,
- 0.3 ppm
- 0.1 ppm
+ C,-C-C-D, -0.1 to approximately
- 0.05 ppm
In rigid molecules, within the last decade [2b-2f], the concept of a “dihedral angle dependence” of a three bond isotope effect in 13C NMR spectroscopy has been recognized and elaborated quantitatively
[2fl. 3. The larger the spread of the chemical shift scale for a given nucleus of observation, the larger is the isotope effect [7]. 4. The larger the fractional mass change of the isotopes the larger is the isotope effect [8]. Hence, the deuterium isotope effect is the most often studied as the one giving large spectral changes. These properties pave a way to several spectroscopic and chemical applications of the intrinsic deuterium isotope effect described in 5-8 below. 5. Spectral line assignments in 13C NMR based on changes of the S, T, and ‘J(i3C,H) values at a substitution center and structural sites displaced three bonds from the deuterium incorporation. The effects on the 6 values in a chain are shown
above (see point 2). They allow for assignment of three consecutive carbon atoms in a chain. The effects on the “J (i3C,H) coupling constants are also diagnostic in assignments since they obey the relation “J (13C,H)/“J(‘3C,D) = 6.51 [6]. The (Y carbon atoms are characteristic triplets (1:l: 1 for one deuterium atom) due to large, resolvable splitting with deuterium. The fl carbon atoms are usually broadened and y carbon atoms often show resolvable triplets since the vicinal J(13C,H) couplings can be larger than geminal coupling constants. The intensity of the cr carbon atom to deuterium is drastically reduced for two reasons: the Tl relaxation time of this carbon atom is much longer in the C-D unit than in the C-H unit due to a lack of dipole-dipole relaxation in the latter; this is also a reason for the diminished nuclear Overhauser enhancement (NOE) which primarily depends on this relaxation path. 6. Different vibrational properties of deuterium led to the observation of a steric isotope e&t [2,9,10] used for fine stereochemical assignments (see Scheme 2). The substitution pattern in compound I and I-d6 in Scheme 2 satisfies the condition that both compounds are described by a single potential energy minimum. Since the steric compression (shown in Scheme 2 by broken lines) results, in ‘H NMR, in a high frequency shift [2], the observed low frequency shift for hydrogens in I-d6givesevidence of a smaller steric compression of the deuterium compared with the protium. This phenomenon has been recognized as an intrinsic steric isotope effect [2]. Its chemical significance is interpreted as if the deuterium had smaller dimensions in space; this gives rise to subtle stereochemical applications.
1 I R=H I-d, , R=D A6(1-d,,I) = -0.0052ppm Scheme 2 [2a].
(lH NMR)
L. KozerskilJ.
91
Mol. Struct. 321 (1994) 89-96
Scheme 3 [13].
7. The influence of the deuterium on the chemical shifts, S, has tempted the definition of it as a substituent having weaker hyperconjugative effects than the protium [11,12]. However, a question has been posed of whether the term “hyperconjugation”, as an electronic criterion, should be used in interpretation of isotopic effects which have a vibrational origin [lld,e]. Nevertheless, it is often interpreted that the deuterium has worse electron-releasing properties and is a worse electron donating substituent than the protium. 8. The electronic properties of deuterium are reflected in the phenomenon of isotopic perturbation of resonance [ 13- 151used in studies of valence isomerism. The introduction of a deuterium atom into a cation, A, shown in Scheme 3 [13] results in a lifting of the degeneracy of chemical shifts of carbon atoms with hydrogens. Of the two resonance structures, B and C, the right hand form is more abundant due to a better hyperconjugative stabilization of the positive charge by protium than deuterium.
H 160 CH,CH,CH,SH
9. A regiospecific short-range influence of the isotope change on the chemical shifts is successfully used for tracing the fate of an atom in intermediates in reaction mechanism schemes [ 161 (see Scheme 4). The aqueous chlorination of 3-mercapto- lpropanol in enriched D:*O water shown in Scheme 4 [16] exemplifies the method of locating the oxygen labels and proposing the reaction intermediate. The isotope effects in parentheses (in p.p.b.) for specific carbon atoms in B and C were obtained for these products fully enriched with ‘*O isotope. The comparison of the isotope effects in a reaction product B with those in a reference compound confirms the hypothesis that endocyclic oxygen has been incorporated in the reaction because the isotope effects on the carbon a to that oxygen are very similar (-43 vs. -46p.p.b.). The isotopic composition in product C is also confirmed by comparison of the isotope effect with a reference compound. These facts confirm that species A is a common intermediate yielding
‘I2 + D, 180
1016
C-l A
’80D/
Scheme 4 [ 161.
L. Kozerski/J. Mol. Struct. 321 (1994) 89-96
92
B and C after C-O bond cleavage and subsequent attack by Cl- or ‘*OD- anions. 10. Biosynthetic studies are aided by the incorporation of isotopes [17]. In contrast to the short-range intrinsic isotope effect the equilibrium chemical shift isotope effect can be observed many bonds away from the substitution center. It can have either positive or negative sign. Equilibria of physical or chemical character in a molecule or in intermolecular processes are perturbed by isotopic substitution. These are governed by a relationship [ 181: K = (A + As)/(A
- As)
where K is the equilibrium chemical shift isotope effect constant, and A is the chemical shift difference characteristic for sites undergoing exchange. Isotopic perturbation of an equilibrium gives rise to long-range effects of both signs which compete with intrinsic effects within a distance of three bonds from the substitution center. Both intra- and intermolecular equilibria perturbed by isotopic substitution yield information not easily accessible by other means. Useful information has been gathered and applied to intramolecular processes as described below. 1. Lifting of degeneracy in conformational processes [19]. This creates a new method to distinguish between static and equilibrating structures. The isotopic perturbation of degeneracy gives rise to splitting As for otherwise equivalent nuclei and is dependent on the equilibrium isotope constant K and the difference A between chemical shifts averaged by the exchange. The effect applied to conformational processes is determined by the rules stating that [19]: (i) a nucleus positioned on a symmetry element will not change its chemical shift in any member of a set of conformational equivalents about that element and will not experience a net shift as a result of unequal weighting; (ii) the average shift of any set of nuclei interchanged by a symmetry operation on the average structure will not change as a result of weighting. The above mentioned rules are reflected in
D ?364 -77
Cs
290 2
10
-130
0 -40
5
7
0
28
29
-25
+
6 wb
Scheme 5.
isotope effects on carbon atoms shown in Scheme 5 for specifically monodeuterated cyclodecanone. Carbon atom 6, lying on a symmetry element, does not show any measurable isotope effect due to a change of the equilibrium between interconverting conformational species and is too far from the deuteration site (five bonds) for the intrinsic effect to occur. The otherwise equivalent nuclei, in the absence of deuteration, should show identical effects of opposite sign; this is best represented for the pair 5 and 7. This trend is also found for the pairs 4 and 8, and 2 and 10, although the effects are much larger than expected in positions 2 and 4 owing to an additional contribution of intrinsic effects. The violation of the rule for the pair 3 and 9 is accounted for by the apparent contribution of the steric deuterium effect in various pseudorotation partners [ 191. 2. Studies of fast conformational interconversions [20a]. This approach is based on another type of equilibrium isotope effect which occurs when the magnitude of an intrinsic NMR isotope shift is affected by isotopic perturbation of a rapid equilibrium [20]. Nonadditivity of the intrinsic isotope effect may occur when deuterium substitution changes the population of conformers which can have different intrinsic isotope effects owing to involvement of the C-H(D) bonds in electronic or steric interaction in a molecule. In the cyclopentyl cation shown in a Scheme 6 [12] the isotopic perturbation of the equilibria preferentially places the C-D bonds out of alignment with the p-orbital of the adjacent cationic center. This is the result of the worse hyperconjugative properties of the C-D bond compared
+ /Y
L. KozerskilJ.
93
Mol. Struct. 321 (1994) 89-96
I
\
++++fLq-~+ 19F
ENVELOPE
R
4
H H
D D
R
as
R
T’NIST
“F
Isotope Dl
shifts
=
D, Trans D, CIS
D4
(ppm)
0.138 = 0.258 = 0.299
D, GEM D3
TRANS
= 0.299 =
0.440
=
0.603
Scheme 6 [ 121.
with the C-H bonds (see above). The progressive substitution by deuterium in methylene groups reduces the electron supply to the catonic center which is reflected in the high frequency shift of the 19F resonance. The isotope effects are additive only in D2 cis and gem isotopomers with respect to the tetradeuterated compound (0.150 ppm/D). On that basis the hypothesis of a planar structure of the cation can be rejected because in a planar
structure the additivity should be observed in all isotopomers. Furthermore, it is obvious that it must be a conformation in which deuterium perturbs the equilibrium in the trans isotopomer but not in the cis. The cyclopentyl cation must then adopt a twist conformation, because it is the D2 trans isotopomer of this form in which the equilibrium is perturbed by isotopic substitution, resulting in nonadditivity of the observed intrinsic
L. Kozerski/J.
94
Mol. Slruct. 321 (1994) 89-96
isotope effect. Cis substitution in the twist conformation does not result in perturbation of the equilibrium and hence does not break the additivity of the isotope effect, in accord with experiment. 3. A description of the intramolecular tautomerit processes [la,21]. The deuterium isotope effect is especially useful for the detection of fast tautomeric equilibria in systems with protonchelated species as in Scheme 7 [21]. 4. Characterization of hydrogen bonds [22]. A recent paper [22a] (see also ref. 22b) reviews the studies concerning the characterization of intramolecular hydrogen bonds in relation to the values of the isotope effects. This topic is especially important in view of the interest in studies of the hydrogen bond in biological molecules [22d-e]. The quantitative description of the intramolecular hydrogen bond in terms of the relationship between the two-bond deuterium isotope effect and hydrogen bond enthalpy is also important [22f-i]. 5. Deuteration of dynamic protons, slowly exchangeable on the NMR time scale, also gives valuable structural information and serves as a tool for spectral assignments [23,24]. The isotope effect usually originates from the application of mixed solvents (H20/D20, 1 : 1) at an appropriate pH value [23f] securing the slow exchange. The method has been originally applied to the differentiation of primary and secondary carbonyl groups in peptides [23a,b], assignments of alcoholic and phenolic groups [23e], and assignments of OH resonances in cyclodextrins [23c] and saccharides [23d]. A simple dissolving of the acetamide in HzO/ D20 (1: 1) allows for observation of a triplet (1:2:1) of a carbonyl group with a separation of 0.06ppm. The splitting is due to the different -CONHI, isotopomers effect of isotope -CONH’D + -CONDHE and -COND,. The carbonyl group of N-methylacetamide, under appropriate conditions of pH, gives the doublet (1:l) of a carbonyl resonance due to isotopomers -CONHCHs and -CONDCHj.
Yy &II+ X
‘H .*’
Scheme 7 [21].
x.* y . H’
D,HO
D,Ho2!$Jo,
D,HOm0s03 OCH, Scheme 8 [25].
6. However, the isotope effects of dynamic protons undergoing fast exchange can be conveniently studied using coaxial sample tubes [25]. The method has been successfully applied to unambiguous assignment of the closely lying C-2, C-3 and C-5 resonances in sugars as shown in the case of methyl-p-o-glucoside [25] in Scheme 8. The assignment is based on the known information that the deuteration of the OH group in sugars creates the isotope effect of about 0.14ppm on the p carbon atom and 0.03 ppm on the y carbon atom. Taking into account the additivity of the isotope effects it can be calculated that the effects at positions C-2, C-3 and C-5 should be 0.17 (/I + r), 0.20 (p + 27) and 0.06ppm (2y), respectively. Good agreement with experiment is observed with the compound in Scheme 8. 7. Intermolecular interactions involving acidbase equilibria can be traced using coaxial sample techniques [26,27]. In principle, the method of coaxial sample tubes can be extended to studies of intermolecular interactions between various basic functional groups and acids of various strengths used as protio and deuterio isotopomers in coaxial samples, as for example: H20/DZO; C6HsOH/ C6HSOD; CH3COOH/CHsCOOD; CFsCOOH/ CFsCOOD. These interactions have a crucial importance in recognizing the reaction mechanisms as they represent the first step of acidic activation of substrates and can be used for monitoring the position of the equilibrium shown below in Eq. (1) and tracing the regioselectivity of interactions if multisite protonation can occur in a molecule. B+HA;--‘B...HA;-3BH+//A-zBH++AFree acid, base The
Hydrogen bond
interaction
Ion pair
between
the chalcone
Ions (1) and
L. KozerskilJ.
Mol. Struct. 321 (1994) 89-96
TFA-H/TFA-D acids (TFA = trifluoroacetic acid) is described below: km
B+TFA-H,B...TFA-H km
B+TFA-D.B...TFA-D K = KBHIG,D A preliminary study of these interactions in chloroform solution [26] has revealed that observed “isotope splittings” of the 13C resonances for a functional group and the carbon atoms in the side chain, Si, are described by a simple equation: Si=(K-l)A’+Asi
(2) In this equation Ai denotes the chemical shift difference for a given carbon atom Ci in TFA-D acid in chloroform solution: AZ = sTFA-D - kDC1, whereas A# for the same carbon atom is the true isotope shift characteristic for a definite species in Eq. (1): A8 = ST,,_, - SrrA.u The significance of Eq. (2) is that it shows that the observed splittings in such an experiment are a composite of the two terms: the first one describes the contribution of the equilibrium of Eq. (1) to the observed spectral splitting and the second term describes the intrinsic isotope effect of the species in Eq. (1) which is very different for a hydrogen bond interaction or in a free ion.
Acknowledgment
This work is part of a research program of the Institute of Organic Chemistry, Polish Academy of Sciences, sponsored by KBN grants.
References [l] (a) P.E. Hansen, Prog. NMR Spectrosc., 20 (1988) 207. (b) P.E. Hansen, Annu. Rep. NMR Spectrosc., 15 (1983) 105. [2] (a) F.A.L. Anet and A.M. Dekmezian, J. Am. Chem. sot., 101 (1979) 5449.
95
(b) R. Aydin and H. Gunther, Z. Naturforsch., Teil B, 34 (1979) 528. (c) J.L. Jurlina and J.B. Stothers, J. Am. Chem. Sot., 104 (1982) 4677. (d) R. Aydin, J.R. Wesener, H. Gunther, R.L. Santillan, M.E. Garibay and P. Joseph-Nathan, J. Org. Chem., 49 (1984) 3845. (e) Z. Majerski, M. Zuanic and B. Matelko, J. Am. Chem. Sot., 107 (1985) 1721. (f) R. Aydin, W. Frankmolle, D. Schmalz and H. Gunther, Magn. Reson. Chem., 26 (1988) 408. [3] C.J. Jameson, J. Chem. Phys., 66 (1977) 4983. [4] W.T. Raynes, Nucl. Magn. Reson., 8 (1979) 12. [5] G. Gunnarsson, H. Wennerstrom, W. Egan and S. For&n, Chem. Phys. Lett., 38 (1976) 96. [6] R. Aydin and H. Gunther, J. Am. Chem. Sot., 103 (1981) 1301. [7] M. Alei and W.F. Wageman, J. Chem. Phys., 68 (1978) 783. [8] L.J. Altman, D. Laungani, G. Gunnarsson, H. Wennerstrijm and S. For&n, J. Am. Chem. Sot., 100 (1978) 8264. [9] F.A.L. Anet, V.J. Basus, A.P.W. Hewett and M. Saunders, J. Am. Chem. Sot., 102 (1980) 3945. [lo] L. Ernst, S. Eltamany and H. Hopf, J. Am. Chem. Sot., 104 (1982) 299. [ll] (a) L. Ernst, H. Hopf and D. Wullbrandt, J. Am. Chem. Sot., 105 (1983) 4469. (b) R.A. Bell, C.L. Chan and B.G. Sayer, J. Chem. Sot., Chem. Commun., (1972) 67. (c) D.A. Forsyth and J.R. Yang, J. Am. Chem. Sot., 108 (1986) 2157. (d) D.A. Forsyth, J.S. Puckace and F.E. Shawcross, Tetrahedron Lett., 27 (1986) 3569. (e) C.J. Jameson and H.J. Osten, Annu. Rep. NMR Spectrosc., 17 (1986) 1. [12] D.A. Forsyth and J.H. Botkin, J. Am. Chem. Sot., 106 (1984) 4296. [13] M. Saunders and M.R. Kates, J. Am. Chem. Sot., 99 (1977) 8071. [14] M. Saunders, M.R. Kates, K.B. Wiberg and W. Pratt, J. Am. Chem. Sot., 99 (1977) 8072. [15] K.L. Servis and F.-F. Shue, J. Am. Chem. Sot., 102 (1980) 7233. [16] J.F. King, S. Skonieczny, K.C. Khemani and J.B. Stothers, J. Am. Chem. Sot., 105 (1983) 6514. [17] (a) G. Kunesch and C. Poupat, Biosynthetic Studies Using Carbon-13 Enriched Precursors, in E. Buncel and C.C. Lee, (Eds.), Isotopes in Organic Chemistry, Vol. 3, Elsevier, Amsterdam, 1977. (b) C.J. Jameson, in E. Buncel and J.R. Jones (Eds.), Istopes in the Physical and Biomedical Sciences, Isotopic Applications in NMR Studies, Elsevier, Amsterdam, 199 1.
96
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(c) S. Berger, in P. Diehl, E. Fluck, H. Gunther, R. Kosfeld and J. Seeling (Eds.), Isotope Effects in NMR Spectroscopy, NMR Basic Principles and Progress, Springer, New York, 1990. (181 M. Saunders, L. Telkowski and M.R. Kates, J. Am. Chem. Sot., 99 (1977) 8070. [19] G.C. Andrews, G.N. Chmurny and E.B. Whipple, Org. Magn. Reson., 15 (1981) 324. [20] (a) D.A. Forsyth, J. Am. Chem. Sot., 106 (1984) 4296. (b) D.A. Forsyth, P. Lucas and R.M. Burk, J. Am. Chem. Sot., 104 (1982) 240. [21] P.E. Hansen, F. Duus and P. Schmitt, Org. Magn. Reson., 18 (1982) 58. [22] (a) P.E. Hansen, Magn. Reson. Chem., 31 (1993) 23. (b) P.E. Hansen, J. Mol. Struct., 321 (1994) 79. (c) P.E. Hansen, Magn. Reson. Chem., 31 (1993) 71. (d) W.H. Gmeiner and J.D. Poulter, J. Am. Chem. Sot., 101 (1988) 7640. (e) E. Tiichsen and P.E. Hansen, Int. J. Biol. Macromol., 13 (1991) 1. (f) J. Schaefer, J. Phys. Chem., 79 (1975) 1888. (g) P.E. Hansen, Org. Magn. Reson., 24 (1986) 903.
[23]
[24] [25] [26] [27]
(h) J. Reuben, J. Am. Chem. Sot., 108 (1986) 1735. (i) J. Reuben, J. Am. Chem. Sot., 109 (1987) 316. (a) R.A. Newmark and J.R. Hill, J. Magn. Reson., 21 (1976) 1. (b) J. Feeney, P. Partington and G.C.K. Roberts, Org. Magn. Reson., 13 (1974) 268. (c) J.C. Christophides and D.B. Davies, J. Chem. Sot., Chem. Commun., (1982) 560. (d) K. Bock and R.U. Lemieux, Carbohydr. Res., 100 (1982) 63. (e) R.A. Newmark and J.R. Hill, Org. Magn. Reson., 13 (1980) 40. (f) J.J. Led and S.B. Petersen, J. Magn. Reson., 33 (1979) 603. P.E. Hansen, R. Kawecki, A. Krowczytiski and L. Kozerski, Acta Chem. Stand., 44 (1990) 826. P.E. Pfeffer, K.M. Valentine and F.W. Parrish, J. Am. Chem. Sot., 101 (1979) 1265. L. Kozerski, Org. Magn. Reson., 20 (1982) 194. L. Kozerski and W. von Philipsborn, Helv. Chim. Acta, 65 (1982) 2077.
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 321 (1994) 89-96
Isotope effects in structural and stereochemical elucidations of organic compounds by NMR’ Lech Kozerski Instituteof Organic Chemistry, Polish Academy of Sciences, 00-961 Warszawa, Kasprzaka 44, Poland
(Received 24 June 1993)
Abstract Generalizations on the nature of the intrinsic isotope effect, leading to several spectroscopic and chemical applications of the deuterium isotope effect, are outlined. The equilibrium chemical shift isotope effect, which can be observed further from the substitution center than the intrinsic isotope effect, and its applications are also discussed.
The effect of isotopic substitution on NMR parameters has a vibrational origin [l]. Its influence on chemical shifts, S, depends on the shape of a potential energy curve characteristic of the molecular fragment experiencing the isotope change. The intrinsic Dotope e&t is found in rigid molecules [2] whereas a double minimum creates, in addition, the equilibrium chemical-shift isotope efict [2a]. Studies of the nature of intrinsic isotope effects [3,4] have revealed some useful generalizations.
+0.03 20.5
1. Both primary and secondary isotope effects are usually diamagnetic, i.e. a heavier isotope substitution results in increased shielding defined on the 6 scale: A6 = S”X - S”‘X
However, high frequency shifts can be observed as welI [5,6j and, in the case of a primary effect, are linked to the shape of a potential energy curve [5j (see Scheme 1).
-0.06
ppm ppm
m
11.0
Scheme 1 [5].
’ Presented at the Summer School “Isotopic Effects - Karpacz ‘93”, Karpacz, Poland, June 20-25, 1993. 0022-2860/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZ 0022-2860(93)08208-L
ppm ppm
90
L. Kozerski/J. Mol. Strut. 321 (1994) 89-96
In Scheme 1 the primary isotope effects of deuterium in hydrogen bonded systems are shown (CH2C12 solution). In the maleate ion the deuterium isotope effect is shifted to higher frequencies; the ion is described by a single minimum potential energy curve. The hydrogen bond is very strong, hence the system is described as a “proton sponge”. In the salicylaldehyde the deuterium isotope effect is shifted to lower frequencies; the hydrogen bond is much weaker as judged from the S value of the OH proton as compared with the S value of the OH proton in the maleate ion. 2. The value of an effect is proportional to the number of replaced isotopes [7] and fades along the chain away from the substitution center. This is illustrated below in the aliphatic chain. The effects depend on the character of the substituent and the hybridization of the carbon atoms. C,-H,
+ Q-D, - 1.O to approximately
CB-C-H,
+ Co-C-D, -0.5 to approximately
C,-C-C-H,
- 0.3 ppm
- 0.1 ppm
+ C,-C-C-D, -0.1 to approximately
- 0.05 ppm
In rigid molecules, within the last decade [2b-2f], the concept of a “dihedral angle dependence” of a three bond isotope effect in 13C NMR spectroscopy has been recognized and elaborated quantitatively
[2fl. 3. The larger the spread of the chemical shift scale for a given nucleus of observation, the larger is the isotope effect [7]. 4. The larger the fractional mass change of the isotopes the larger is the isotope effect [8]. Hence, the deuterium isotope effect is the most often studied as the one giving large spectral changes. These properties pave a way to several spectroscopic and chemical applications of the intrinsic deuterium isotope effect described in 5-8 below. 5. Spectral line assignments in 13C NMR based on changes of the S, T, and ‘J(i3C,H) values at a substitution center and structural sites displaced three bonds from the deuterium incorporation. The effects on the 6 values in a chain are shown
above (see point 2). They allow for assignment of three consecutive carbon atoms in a chain. The effects on the “J (i3C,H) coupling constants are also diagnostic in assignments since they obey the relation “J (13C,H)/“J(‘3C,D) = 6.51 [6]. The (Y carbon atoms are characteristic triplets (1:l: 1 for one deuterium atom) due to large, resolvable splitting with deuterium. The fl carbon atoms are usually broadened and y carbon atoms often show resolvable triplets since the vicinal J(13C,H) couplings can be larger than geminal coupling constants. The intensity of the cr carbon atom to deuterium is drastically reduced for two reasons: the Tl relaxation time of this carbon atom is much longer in the C-D unit than in the C-H unit due to a lack of dipole-dipole relaxation in the latter; this is also a reason for the diminished nuclear Overhauser enhancement (NOE) which primarily depends on this relaxation path. 6. Different vibrational properties of deuterium led to the observation of a steric isotope e&t [2,9,10] used for fine stereochemical assignments (see Scheme 2). The substitution pattern in compound I and I-d6 in Scheme 2 satisfies the condition that both compounds are described by a single potential energy minimum. Since the steric compression (shown in Scheme 2 by broken lines) results, in ‘H NMR, in a high frequency shift [2], the observed low frequency shift for hydrogens in I-d6givesevidence of a smaller steric compression of the deuterium compared with the protium. This phenomenon has been recognized as an intrinsic steric isotope effect [2]. Its chemical significance is interpreted as if the deuterium had smaller dimensions in space; this gives rise to subtle stereochemical applications.
1 I R=H I-d, , R=D A6(1-d,,I) = -0.0052ppm Scheme 2 [2a].
(lH NMR)
L. KozerskilJ.
91
Mol. Struct. 321 (1994) 89-96
Scheme 3 [13].
7. The influence of the deuterium on the chemical shifts, S, has tempted the definition of it as a substituent having weaker hyperconjugative effects than the protium [11,12]. However, a question has been posed of whether the term “hyperconjugation”, as an electronic criterion, should be used in interpretation of isotopic effects which have a vibrational origin [lld,e]. Nevertheless, it is often interpreted that the deuterium has worse electron-releasing properties and is a worse electron donating substituent than the protium. 8. The electronic properties of deuterium are reflected in the phenomenon of isotopic perturbation of resonance [ 13- 151used in studies of valence isomerism. The introduction of a deuterium atom into a cation, A, shown in Scheme 3 [13] results in a lifting of the degeneracy of chemical shifts of carbon atoms with hydrogens. Of the two resonance structures, B and C, the right hand form is more abundant due to a better hyperconjugative stabilization of the positive charge by protium than deuterium.
H 160 CH,CH,CH,SH
9. A regiospecific short-range influence of the isotope change on the chemical shifts is successfully used for tracing the fate of an atom in intermediates in reaction mechanism schemes [ 161 (see Scheme 4). The aqueous chlorination of 3-mercapto- lpropanol in enriched D:*O water shown in Scheme 4 [16] exemplifies the method of locating the oxygen labels and proposing the reaction intermediate. The isotope effects in parentheses (in p.p.b.) for specific carbon atoms in B and C were obtained for these products fully enriched with ‘*O isotope. The comparison of the isotope effects in a reaction product B with those in a reference compound confirms the hypothesis that endocyclic oxygen has been incorporated in the reaction because the isotope effects on the carbon a to that oxygen are very similar (-43 vs. -46p.p.b.). The isotopic composition in product C is also confirmed by comparison of the isotope effect with a reference compound. These facts confirm that species A is a common intermediate yielding
‘I2 + D, 180
1016
C-l A
’80D/
Scheme 4 [ 161.
L. Kozerski/J. Mol. Struct. 321 (1994) 89-96
92
B and C after C-O bond cleavage and subsequent attack by Cl- or ‘*OD- anions. 10. Biosynthetic studies are aided by the incorporation of isotopes [17]. In contrast to the short-range intrinsic isotope effect the equilibrium chemical shift isotope effect can be observed many bonds away from the substitution center. It can have either positive or negative sign. Equilibria of physical or chemical character in a molecule or in intermolecular processes are perturbed by isotopic substitution. These are governed by a relationship [ 181: K = (A + As)/(A
- As)
where K is the equilibrium chemical shift isotope effect constant, and A is the chemical shift difference characteristic for sites undergoing exchange. Isotopic perturbation of an equilibrium gives rise to long-range effects of both signs which compete with intrinsic effects within a distance of three bonds from the substitution center. Both intra- and intermolecular equilibria perturbed by isotopic substitution yield information not easily accessible by other means. Useful information has been gathered and applied to intramolecular processes as described below. 1. Lifting of degeneracy in conformational processes [19]. This creates a new method to distinguish between static and equilibrating structures. The isotopic perturbation of degeneracy gives rise to splitting As for otherwise equivalent nuclei and is dependent on the equilibrium isotope constant K and the difference A between chemical shifts averaged by the exchange. The effect applied to conformational processes is determined by the rules stating that [19]: (i) a nucleus positioned on a symmetry element will not change its chemical shift in any member of a set of conformational equivalents about that element and will not experience a net shift as a result of unequal weighting; (ii) the average shift of any set of nuclei interchanged by a symmetry operation on the average structure will not change as a result of weighting. The above mentioned rules are reflected in
D ?364 -77
Cs
290 2
10
-130
0 -40
5
7
0
28
29
-25
+
6 wb
Scheme 5.
isotope effects on carbon atoms shown in Scheme 5 for specifically monodeuterated cyclodecanone. Carbon atom 6, lying on a symmetry element, does not show any measurable isotope effect due to a change of the equilibrium between interconverting conformational species and is too far from the deuteration site (five bonds) for the intrinsic effect to occur. The otherwise equivalent nuclei, in the absence of deuteration, should show identical effects of opposite sign; this is best represented for the pair 5 and 7. This trend is also found for the pairs 4 and 8, and 2 and 10, although the effects are much larger than expected in positions 2 and 4 owing to an additional contribution of intrinsic effects. The violation of the rule for the pair 3 and 9 is accounted for by the apparent contribution of the steric deuterium effect in various pseudorotation partners [ 191. 2. Studies of fast conformational interconversions [20a]. This approach is based on another type of equilibrium isotope effect which occurs when the magnitude of an intrinsic NMR isotope shift is affected by isotopic perturbation of a rapid equilibrium [20]. Nonadditivity of the intrinsic isotope effect may occur when deuterium substitution changes the population of conformers which can have different intrinsic isotope effects owing to involvement of the C-H(D) bonds in electronic or steric interaction in a molecule. In the cyclopentyl cation shown in a Scheme 6 [12] the isotopic perturbation of the equilibria preferentially places the C-D bonds out of alignment with the p-orbital of the adjacent cationic center. This is the result of the worse hyperconjugative properties of the C-D bond compared
+ /Y
L. KozerskilJ.
93
Mol. Struct. 321 (1994) 89-96
I
\
++++fLq-~+ 19F
ENVELOPE
R
4
H H
D D
R
as
R
T’NIST
“F
Isotope Dl
shifts
=
D, Trans D, CIS
D4
(ppm)
0.138 = 0.258 = 0.299
D, GEM D3
TRANS
= 0.299 =
0.440
=
0.603
Scheme 6 [ 121.
with the C-H bonds (see above). The progressive substitution by deuterium in methylene groups reduces the electron supply to the catonic center which is reflected in the high frequency shift of the 19F resonance. The isotope effects are additive only in D2 cis and gem isotopomers with respect to the tetradeuterated compound (0.150 ppm/D). On that basis the hypothesis of a planar structure of the cation can be rejected because in a planar
structure the additivity should be observed in all isotopomers. Furthermore, it is obvious that it must be a conformation in which deuterium perturbs the equilibrium in the trans isotopomer but not in the cis. The cyclopentyl cation must then adopt a twist conformation, because it is the D2 trans isotopomer of this form in which the equilibrium is perturbed by isotopic substitution, resulting in nonadditivity of the observed intrinsic
L. Kozerski/J.
94
Mol. Slruct. 321 (1994) 89-96
isotope effect. Cis substitution in the twist conformation does not result in perturbation of the equilibrium and hence does not break the additivity of the isotope effect, in accord with experiment. 3. A description of the intramolecular tautomerit processes [la,21]. The deuterium isotope effect is especially useful for the detection of fast tautomeric equilibria in systems with protonchelated species as in Scheme 7 [21]. 4. Characterization of hydrogen bonds [22]. A recent paper [22a] (see also ref. 22b) reviews the studies concerning the characterization of intramolecular hydrogen bonds in relation to the values of the isotope effects. This topic is especially important in view of the interest in studies of the hydrogen bond in biological molecules [22d-e]. The quantitative description of the intramolecular hydrogen bond in terms of the relationship between the two-bond deuterium isotope effect and hydrogen bond enthalpy is also important [22f-i]. 5. Deuteration of dynamic protons, slowly exchangeable on the NMR time scale, also gives valuable structural information and serves as a tool for spectral assignments [23,24]. The isotope effect usually originates from the application of mixed solvents (H20/D20, 1 : 1) at an appropriate pH value [23f] securing the slow exchange. The method has been originally applied to the differentiation of primary and secondary carbonyl groups in peptides [23a,b], assignments of alcoholic and phenolic groups [23e], and assignments of OH resonances in cyclodextrins [23c] and saccharides [23d]. A simple dissolving of the acetamide in HzO/ D20 (1: 1) allows for observation of a triplet (1:2:1) of a carbonyl group with a separation of 0.06ppm. The splitting is due to the different -CONHI, isotopomers effect of isotope -CONH’D + -CONDHE and -COND,. The carbonyl group of N-methylacetamide, under appropriate conditions of pH, gives the doublet (1:l) of a carbonyl resonance due to isotopomers -CONHCHs and -CONDCHj.
Yy &II+ X
‘H .*’
Scheme 7 [21].
x.* y . H’
D,HO
D,Ho2!$Jo,
D,HOm0s03 OCH, Scheme 8 [25].
6. However, the isotope effects of dynamic protons undergoing fast exchange can be conveniently studied using coaxial sample tubes [25]. The method has been successfully applied to unambiguous assignment of the closely lying C-2, C-3 and C-5 resonances in sugars as shown in the case of methyl-p-o-glucoside [25] in Scheme 8. The assignment is based on the known information that the deuteration of the OH group in sugars creates the isotope effect of about 0.14ppm on the p carbon atom and 0.03 ppm on the y carbon atom. Taking into account the additivity of the isotope effects it can be calculated that the effects at positions C-2, C-3 and C-5 should be 0.17 (/I + r), 0.20 (p + 27) and 0.06ppm (2y), respectively. Good agreement with experiment is observed with the compound in Scheme 8. 7. Intermolecular interactions involving acidbase equilibria can be traced using coaxial sample techniques [26,27]. In principle, the method of coaxial sample tubes can be extended to studies of intermolecular interactions between various basic functional groups and acids of various strengths used as protio and deuterio isotopomers in coaxial samples, as for example: H20/DZO; C6HsOH/ C6HSOD; CH3COOH/CHsCOOD; CFsCOOH/ CFsCOOD. These interactions have a crucial importance in recognizing the reaction mechanisms as they represent the first step of acidic activation of substrates and can be used for monitoring the position of the equilibrium shown below in Eq. (1) and tracing the regioselectivity of interactions if multisite protonation can occur in a molecule. B+HA;--‘B...HA;-3BH+//A-zBH++AFree acid, base The
Hydrogen bond
interaction
Ion pair
between
the chalcone
Ions (1) and
L. KozerskilJ.
Mol. Struct. 321 (1994) 89-96
TFA-H/TFA-D acids (TFA = trifluoroacetic acid) is described below: km
B+TFA-H,B...TFA-H km
B+TFA-D.B...TFA-D K = KBHIG,D A preliminary study of these interactions in chloroform solution [26] has revealed that observed “isotope splittings” of the 13C resonances for a functional group and the carbon atoms in the side chain, Si, are described by a simple equation: Si=(K-l)A’+Asi
(2) In this equation Ai denotes the chemical shift difference for a given carbon atom Ci in TFA-D acid in chloroform solution: AZ = sTFA-D - kDC1, whereas A# for the same carbon atom is the true isotope shift characteristic for a definite species in Eq. (1): A8 = ST,,_, - SrrA.u The significance of Eq. (2) is that it shows that the observed splittings in such an experiment are a composite of the two terms: the first one describes the contribution of the equilibrium of Eq. (1) to the observed spectral splitting and the second term describes the intrinsic isotope effect of the species in Eq. (1) which is very different for a hydrogen bond interaction or in a free ion.
Acknowledgment
This work is part of a research program of the Institute of Organic Chemistry, Polish Academy of Sciences, sponsored by KBN grants.
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