Differences in proton-proton coupling constants of N+-CH2-CH2 protons of some betaines, N+-(CH2)2-3-COO-, and their complexes in aqueous solution

Article abstract of DOI:10.1016/S0022-2860(01)00497-5

Synthesis and ¹H NMR spectra in D₂O of 4 betaines and 19 betaine complexes with mineral acids containing 2 or 3 CH₂ groups in the tether, N⁺-(CH₂)_n-COO^-, n=2,3, and diverse volume of the positively charged groups are reported. In compounds containing three CH₂ groups in the tether and three substituents at the nitrogen atom or α, α′-disubstituted pyridine ring, a characteristic multiplet for an AA′MM′X₂ spin system is observed. This is consistent with preference for trans conformation (68-85%). In the spectra of compounds with two CH₂ groups in the tether or three CH₂ groups and unsubstituted pyridine ring, the multiplet changes to a triplet and gives apparent A₂X₂ and A₂M₂X₂ spectra, respectively, consistent with no significant conformational preference. Both the number of CH₂ groups in tether and the bulkiness of the charged groups are responsible for the observed differences of N⁺CH₂ multiplicity and reflect changes in conformational preferences. According to the PM3 calculations, in the gas phase a gauche-like conformer is more stable than the trans, but in aqueous solution it is reverse.

Full text of DOI:10.1016/S0022-2860(01)00497-5

Journal of Molecular Structure 563±564 ꢀ2001) 555±564
www.elsevier.nl/locate/molstruc
Differences in proton±proton coupling constants of N1±CH2±CH2
protons of some betaines, N1±ꢀCH2)2-3±COO2, and their
complexes in aqueous solution
a,
Mirosøaw Szafran , Zo®a Dega-Szafran , Barbara Nowak-Wydra , Mariusz Pietrzak
a
a
b
*
a
Â
Faculty of Chemistry, Adam Mickiewcz University, ul Grunwaldzka 6, 60-780 Poznan, Poland
bInstitute of Organic Chemistry, Polish Academy of Sciences, 01224 Warsaw, Poland
Received 31 August 2000; revised 18 December 2000; accepted 18 December 2000
Abstract
1
Synthesis and H NMR spectra in D2O of 4 betaines and 19 betaine complexes with mineral acids containing 2 or 3 CH2
groups in the tether, N1±ꢀCH2)n±COO2, n ˆ 2; 3; and diverse volume of the positively charged groups are reported. In
compounds containing three CH2 groups in the tether and three substituents at the nitrogen atom or a, a0-disubstituted pyridine
ring, a characteristic multiplet for an AA0MM0X2 spin system is observed. This is consistent with preference for trans
conformation ꢀ68±85%). In the spectra of compounds with two CH2 groups in the tether or three CH2 groups and unsubstituted
pyridine ring, the multiplet changes to a triplet and gives apparent A2X2 and A2M2X2 spectra, respectively, consistent with no
signi®cant conformational preference. Both the number of CH2 groups in tether and the bulkiness of the charged groups are
responsible for the observed differences of N1CH2 multiplicity and re¯ect changes in conformational preferences. According to
the PM3 calculations, in the gas phase a gauche-like conformer is more stable than the trans, but in aqueous solution it is
reverse. q 2001 Elsevier Science B.V. All rights reserved.
Keywords: Betaines; H NMR spectra; AA0MM0X2; A2M2X2; A2X2 spin systems; Conformation
1
1. Introduction
electronegativity of X and Y ꢀthe charges on X and
Y) and their volumes are most important [1±4]. In
betaines and sulfobetaines ꢀR3N1±CH2±CH2COO2/
SO32), the attractive electrostatic ꢀCoulombic) interac-
tions between the charged substituents are essentially
important.
Recently, Gregoire et al. [5] applied the Karplus
relationship in the form developed by Altona et al.
[3] to determine conformations of zwitterionic forms
of b-alanine ꢀH3N1±CH2±CH2±COO2), taurine
ꢀH3N1±CH2±CH2±SO32) and their N,N,N-trimethyl
derivatives. A statistical equilibrium of the gauche
and trans conformations were found in most
compounds, except N,N,N,-trimethyl-taurine, where
The relation between the magnitude of vicinal
coupling constants ꢀJvic), which vary from 0 to
16 Hz ꢀgenerally 5±8 Hz in freely rotating systems),
and dihedral angles in X±CH2±CH2±Y systems,
usually referred to as a Karplus-type equation, is
widely used in structural and conformational analysis
[1,2]. Spin system depends not only on the torsion
angle, but also on several other factors, which the
* Corresponding author. Tel.: 148-61-829-1320; fax: 148-61-
865-8008.
E-mail address: szafran@main.amu.edu.pl ꢀM. Szafran).
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-2860ꢀ01)00497-5

556
M. Szafran et al. / Journal of Molecular Structure 563±564 52001) 555±564
Scheme 1.
the trans form predominates. This is probably caused
by a steric hindrance ꢀthe SO3 group is more bulkier
than the CO2 one).
Intercharge distance in the homologous series of
carboxybetaines, R3N1ꢀCH2)nCOO2 with n ˆ 1±7
ꢀR ˆ CH3 [6]or long alkyl chain of C12 or C16 [7]),
and of sulfobetaines ꢀR3N1ꢀCH2)nSO32 with n ˆ 1±4
[8], R-PyN1ꢀCH2)3SO32 [8], PyN Ð pyridine) have
also been derived from NMR spectra or molecular and
quantum mechanics.
In the present work we investigate 1H NMR
spectra in D2O of alkylammonio- and pyridininio
betaines and their complexes with mineral acids,
containing two or three CH2 groups in tether
ꢀN1ꢀCH2)2-3COO2), different size of the positively
charged groups ꢀScheme 1) and focus our attention

M. Szafran et al. / Journal of Molecular Structure 563±564 52001) 555±564
557
Table 1
Melting points and H chemical shifts, d ꢀppm/DCC) at 300.07 MHz for the investigated compounds in D2O
1
Compound Mp ꢀ8C)
Ring
H-2
Tether
Substituent
H-3
H-4
H-5
H-6
N±CH2
CH2
CH2COO
1-2
1-3
127±128 8.89
107±198 8.88
140±142 8.94
160±161 8.89
8.07 8.56 8.07
8.09 8.57 8.09
8.08 8.57 8.08
8.09 8.57 8.09
8.09 8.57 8.09
8.09 8.56 8.09
8.09 8.57 8.09
8.09 8.57 8.09
8.10 8.58 8.10
8.09 8.61 8.09
8.39 8.39 8.04
8.04 8.46 7.91
8.89 4.83t ꢀ6.57)a
8.88 4.65t ꢀ7.25)
8.94 4.90t ꢀ6.31)
8.89 4.69t ꢀ7.29)
8.89 4.69t ꢀ7.42)
8.88 4.66t ꢀ7.14)
8.88 4.68t ꢀ7.42)
8.88 4.68t ꢀ7.42)
8.89 4.69t ꢀ7.28)
8.85 4.82s
±
2.93t
2.28
3.19t
2.51t
2.52t
2.40t
2.49t
2.51t
2.52t
±
2.28
±
2-2Br
2-3Cl
2-3Br
2-3I
2.33p
2.33p
2.32p
2.33p
2.32p
2.33p
±
161
115±117 8.88
77±78 8.88
8.89
2-3NO3
2-3ClO4
2-3SO4
3-2Cl
4-2Br
5-2Br
6-2Br
7-3Br
100±101 8.88
136±137 8.89
179±180 8.85
1.31s ꢀCH3)
181±183
133
±
±
±
±
9.11 5.08t ꢀ6.59)
8.87 4.94t ꢀ6.87)
8.59 4.76t ꢀ6.93)
±
3.19t
3.12t
3.03t
±
3.44t ꢀCH3), 4.08t ꢀCH2O)
189±190
171±172
7.73
±
7.67
±
2.57s ꢀ4-CH3), 2.81s ꢀ2-CH3)
1.30t ꢀCH3), 2.90s ꢀCH3),
4.22qꢀOCH 2)
8.21 7.76 8.21
±
4.56m
2.20m 2.69t
8-3Cl
9-2Br
205
±
±
±
±
±
7.53 7.99 7.53
±
±
±
±
±
4.35m
3.62t ꢀ7.69)
3.36m
1.95m 2.45t
±
2.67s ꢀCH3)
3.14s ꢀCH3)
173±175
220dec
188±189
149
±
±
±
±
±
±
±
±
±
±
±
±
2.81t
9-3Cl
9-3H2PO4
10-2I
2.09m 2.47t
2.09m 2.46t
3.14s ꢀCH3)
3.36m
3.13s ꢀCH3)
3.50t ꢀ7.55)
±
2.80t
1.06t ꢀCH3), 2.95s ꢀCH3)3,
4.01qꢀOCH 2)
11-3
11-1
103±108
155
±
±
±
±
±
±
±
±
±
±
±
3.27m
3.83s
1.95m 2.23t
0.87t ꢀCH3), 1.28 ꢀCH2)7, 1.34
ꢀCH2)2 1.73 ꢀCH2), 3.07s
ꢀCH3)2, 3.27m ꢀNCH2)b
0.87t ꢀCH3), 1.29 ꢀCH2)7, 1.34
ꢀCH2)2, 1.71 ꢀCH2), 3.20s
ꢀCH3)2, 3.57m ꢀNCH2)b
3.07s ꢀCH3)
±
3.83s
12-3Br
13-3Cl
162±164 3.53
116±121 3.67eq2.12 2.12 3.67eq±
3.09ax 3.09ax
2.21 2.21 3.53
3.37m
2.09m 2.48t
2.00m 2.49t
3.23m
a
Coupling constant.
Data for C11±CH2±N1, s Ð singlet, m Ð multiplet, p Ð pentet ꢀquintet), q Ð quartet, t Ð triplet, eq Ð equatorial, ax Ð axial
b
mainly on multiplicity of the N1CH2 protons. In most
compounds, as expected, the chemical shifts of the
N1±CH2 protons are equivalent by rapid rotational
interchange between the gauche and trans rotamers
and give apparent an A2X2 or A2M2X2 spectra [9]. In
some spectra, however, a multiplet is present, which
can be described by typical AA0MM0X2 spin system.
addition of respective v-halogenocarboxylic acids or
their esters ꢀ0.1 mol) to pyridines, N-methylpyrroli-
dine, pyrrolidine, trimethylamine ꢀ40% aqueous solu-
tion) or N-dodecyl-N,N-dimethylamine on cooling.
Then the mixture was stirred up to precipitation and
®nally left to stand from 2 d to 2 weeks to increase the
yield of the reaction [10,11]. In the case of 2,6-lutidine
or 3-chloropivalic acid the reaction was carried out at
1008C. The crude product was ®ltered off and washed
with diethyl ether to remove unreacted materials and
recrystallized from acetonitrile or acetonitrile±
methanol mixture ꢀ10:1). Yields were usually higher
than 60%.
2. Experimental
2.1. Synthesis
In the case of reaction between trimethylamine and
Most of the compounds were prepared by slow

558
M. Szafran et al. / Journal of Molecular Structure 563±564 52001) 555±564
1
Fig. 1. H NMR spectra of 2-2Br, 2-3Cl and 8-3Cl in D2O solution.

M. Szafran et al. / Journal of Molecular Structure 563±564 52001) 555±564
559
3-bromopropionic acid, we obtained 11-1, as
was described by Chen and Mak [12] and we did
not observe the Hoffmann elimination reaction as
was reported previously [6,7]. Indeed, the
Hoffmann elimination reaction occurs in the case of
2,6-lutidine, which is probably caused by temperature
ꢀ1008C).
estimated from the geometries of all trans and gauche
conformers.
3. Results and discussion
1
All the investigated H NMR spectra vary with
The obtained hydrochlorides or hydrobromides
ꢀ5 g) were then dissolved in 1:1 water±methanol
mixture ꢀ50 cm3) and treated with wet anion-
exchange resin, Amberlite 420 ꢀ60 g) in its basic
form. The mixture was stirred ca. 2 h until the organic
layer gave a negative Beilstein test for halide. The
residue was separated, washed with methanol and
the eluate was evaporated under vacuum below
708C. Each crude betaine was dried over P2O5 in
vacuum to get a solid product and recrystallized
from acetonitrile±methanol mixture ꢀ10:1).
The remaining complexes were prepared by
mixing equivalent of betaines with acid in
methanol. The precipitates were recrystallized from
acetonitrile±methanol mixture ꢀ5:1) or methanol.
Melting points of investigated compounds are listed
in Table 1.
concentration. When the solute concentration of 2-
2Br decreases progressively from 0.5 to 0.015 M,
the entire spectrum is shifted towards higher ®eld
ꢀDd is 0.08 ppm for 2,6-H, 0.06 ppm for N1CH2
and 3,5-H, 0.15 ppm for 4-H and CH2COO protons).
Correlations of the chemical shifts and concentration
is linear with a slope between 0.12 and 0.26 and r ˆ
0:86 2 0:99 and re¯ects the process of aggregation,
which is a typical process of zwitterions in water. The
effect of counterion on chemical shifts is small
ꢀcompare data for 2-3X in Table 1). FTIR spectra of
complexes in D2O show an intense band in the 1720±
1710 cm21 region attributed to the nCvO mode in
COOH group.
As follows from Fig. 1, the most interesting feature
is the multiplicity of the N1CH2 protons. The spectra
of compounds containing three CH2 groups in the
tether ꢀn ˆ 3† and three substituents at the nitrogen
atom ꢀsp3 hybridization) ꢀ9-3Cl, 9-3H2PO4, 11-3, 12-
3Br and 13-3Cl) or 2,6-disubstituted pyridine ring
ꢀsp2 hybridization) ꢀ7-3Br and 8-3Cl)0 are 0 charac-
terised by a multiplet typical of the AA MM X2 spin
system. In the spectrum of 11-3 the signals of both
methylene groups connected with the charged
nitrogen atom ꢀ±CH2±N1±CH2 2 ) show similar
multiplets but their chemical shifts are so close that
we were not able to separate them. The multiplet is
also present in the spectrum of 11-1 for C±CH2N1. In
the spectra of remaining compounds containing two
CH2 ꢀ1-2, 2-2Br, 3-2Cl, 4-2Br, 5-2Br, 9-2Br and 10-
2I) or three CH2 groups in tether and unsubstituted
pyridine ring ꢀ1-3, 2-3Cl, 2-3Br, 2-3I, 2-3NO2, 2-
3ClO4 and 2-3SO4) the N1CH2 protons display a
triplet with the coupling constant of 6.94 ^ 0.50 Hz
ꢀn ˆ 2† and 7.32 ^ 0.11 Hz ꢀn ˆ 3†: These molecules
also belong to AA0XX0 or AA0MM0X2 systems,
respectively, but they give apparent A2X2 and
A2M2X2 spectra.
2.2. Spectra
1
The H NMR spectra were recorded on a Varian
Gemini 300 VT spectrometer operating at
300.07 MHz. The spectra were measured in D2O
ꢀ20 mg in 0.7 cm3) relative to internal standard 3-
ꢀtrimethylsilyl)propionic-d4 acid sodium salt.
The FTIR spectra were measured on a Bruker IFS
113v instrument in D2O solution in cell with CaF2
windows, 0.025 mm thick.
Proton±proton coupling constants were analyzed
by a computer simulation with the Varian LAOCOON
software.
2.3. PM3 calculations
Structures and energies were calculated by
means of the semiempirical PM3 method [13] as
implemented in ampac 6.55 [14] and mopac 5.0
[15] program packages. The solvent effect was
taken into account by using the self-consistent
reaction ®eld ꢀSCRF) method [16±19]. The following
If the spectrum is representative of an AA0MM0
ꢀM ˆ X in AA0XX0) spin system, then JAM, JAM
,
0
Ê
Ê
0 0
JAA , JMM , nA and nM must have values such as to
cavity radius 3.3 A ꢀn ˆ 2† and 4.2 A ꢀn ˆ 3† were

560
M. Szafran et al. / Journal of Molecular Structure 563±564 52001) 555±564
Table 2
Coupling constants and chemical shifts for N1CH2±CH2 protons as part of AA0MM0 spin system and average percentage of trans conformers at
300.07 MHz and 218C
7-3Br
8-3Cl
9-3Cl
9-3H2PO4
12-3Br
13-3Cl
9-2Br
11-1
0
JAA
15.6
12.3
16.2
12.3
10.2
12.1
11.7
12.4
11.1
12.4
4.9
13.2
10.7
5.5
16.8
9.0
18.5
JAM
JAM
12.1
4.5
0
5.0
5.1
5.1
4.7
6.4
0
JMM
nA
15.6
16.2
10.2
11.7
11.1
944.0
559.6
83.5
13.2
970.0
602.0
68
13.1
1087.2
885.0
50
18.5
1370.1
656.6
81.5
1304.4
584.8
80.5
1014.0
628.7
85.5
1007.9
623.3
85.5
1071.6
517.3
82
nM
% trans
give line shapes that are consistent with the observed
line widths [20±23]. To solve this problem the Varian
LAOCOON software was used to simulate spectra.
The coupling constants obtained are listed in Table 2
and the experimental and simulated spectra are
compared in Figs. 2 and 3. The percentage of trans
conformers listed in Table 2 was estimated using the
procedure recently published by Gregoire et al.[5].
Our data for 9-2Br slightly differ from those given
in Ref. [5]. Note that the signal of the N1CH2 protons
in 9-2Br slightly differs from typical triplet ꢀFig. 2).
The long-range COSY experiment was used to esti-
mate the long-range couplings [23]. As Fig. 4 shows,
in 9-2Br and 9-3Cl there is a coupling through four
bonds between the CH3 and N1CH2 protons. In 9-3Cl
an additional coupling occurs between the N1CH2 and
CH2COO protons. The lack of coupling between CH3
and CH2COO protons implies that the distance
between them is much longer and this con®rmed an
extended structure ꢀ$80% of trans conformation).
When n ˆ 2 ꢀTable 3), the PM3 calculations
suggest that in the gas-phase, the trans conformer of
betaine and b-alanine should be less stable than the
gauche one. It can be expected that while water would
diminish the electrostatic effect by virtue of its high
dielectric constant, the strong gas-phase preference
for gauche conformation would not be maintained in
water solution. Indeed, the PM3 data suggest that in
aqueous solution the trans conformers are more stable
than the gauche one by 20.4 and 14.0 kcal/mol,
respectively. A similar situation is observed for
compound with n ˆ 3 ꢀTable 4); in the gas-phase
the most stable is gauche±gauche conformation,
while in aqueous solution the trans±trans conforma-
tion. For such differences of energy, rotation is suf®-
ciently fast such that the resulting isomers cannot be
1
separated in H NMR spectra at room temperature
[24].
Gregoire et al. [5] carried out ab initio calculations
for zwitterionic b-alanine. They found that in aqueous
solution the gauche-like conformer is more stable
than the trans-like by 2±3 kcal/mol, while in the gas
phase the difference is more than 20 kcal/mol. Gener-
ally, the PM3 method reproduces relative energies and
geometries of organic molecules comparable to those
of ab initio data. The discrepancy between the ab
initio data from Ref. [5] and our PM3 data are prob-
ably caused by the cavity radius. In our opinion,
ꢀ
values of r0 ˆ 1:2 A [5] used in the ab initio calcula-
tions is too small.
The accuracy of determining the conformational
preference is at least somewhat compromised by the
fact that we have no assurance that the rotational
Fig. 2. Observed ꢀupper) and calculated ꢀlower) resonance due to the
N1±CH2 group of 9-2Br.

M. Szafran et al. / Journal of Molecular Structure 563±564 52001) 555±564
561
angles for gauche and trans conformations are exactly
60 and 1808, respectively. Another problem is the size
of the cavity radius ꢀa0) ꢀsee Ref. [18]). Finally, zwit-
terions form micelles [25,26] and our calculations
were carried out for simple ꢀindividual) molecules.
In conclusion, we can state that the values obtained
are probably overestimated, however the predicted
changes on going from the gas-phase to the aqueous
solution seems reasonable. Because of the electro-
static attraction between the two charged end groups,
the conformation and dipole moment in solution,
depend strongly on tether ¯exibility ꢀenergy barriers
between rotational trans and gauche isomers), bulki-
ness and solvation of charged groups preventing their
close approach. If the charged groups are hydrated,
their volume is enlarged by hydration water volume,
and their closer approach would require water
removal from them, which is allowed if the electro-
static energy is larger than the dehydration energy.
Consequently, electrostatic attractions are controlled
by dielectric properties of the medium; if water is the
solvent, its high dielectric constant should reduce
intramolecular electrostatic attractions in zwitterions.
Lastly, zwitterionic molecules cause solvent polarisa-
tion and disturb dielectric discontinuity, which is a
source of an additional electric ®eld called reaction
®eld [27]. In consequence an extended conformer
ꢀclose to trans) in D2O solution should be more stable
than a folded one ꢀclose to gauche). Formation of
dimers or larger aggregates favour intermolecular
electrostatic interactions and extended conformations
[6,8,28,29].
The question arising now is why a triplet is
observed in the spectra of all compounds with two
CH2 groups in the tether or with three in the case of
non-substituted pyridine ring. Electrostatic attrac-
tions in zwitterions between the two charged
groups varies with the length of the tether, its bulki-
ness and phase, and consequently, their rotation
barriers are ca. three times higher relative to those
for the uncharged compounds. In betaine,
Me3N1CH2COO2, the calculated rotational barrier
of the carboxylic group is 9.7 kcal/mol [30]. In
micelles tumbling of individual molecules is
probably slowed down, which can cause an accelera-
tion of proton relaxation with diversi®ed relaxation
behaviour of individual lines because the domin-
ating relaxation mechanism is via dipole±dipole
Fig. 3. Observed ꢀupper) and calculated ꢀlower) spectrum of 9-
3H2PO4.

562
M. Szafran et al. / Journal of Molecular Structure 563±564 52001) 555±564
Fig. 4. Long-range COSY spectra of 9-2Br ꢀupper) and 9-3H2PO4 ꢀlower).

M. Szafran et al. / Journal of Molecular Structure 563±564 52001) 555±564
563
Table 3
PM3 relative energies and N´´´O distances for R3N1CH2CH2COO2
trans
gauche
R ˆ Me
R ˆ H
28.64
R ˆ Me
R ˆ H
Gas phase
Erel ꢀkcal/mol)
DHf ꢀkcal/mol)
16.5
237.08
4.431
0
0
238.61
4.193
253.57
2.948
4.247
267.25
2.560
4.146
Ê
N´´´O ꢀA)
Ê
N´´´O ꢀA)
4.647
4.889
Aqueous phase
Erel ꢀkcal/mol)
DHf ꢀkcal/mol)
0
2123.51
4.623
5.130
3.5
0
2112.10
4.511
5.039
3.5
20.4
2103.09
3.509
14.0
298.09
3.062
4.510
3.5
Ê
N´´´O ꢀA)
Ê
N´´´O ꢀA)
Ê
roꢀA)
4.773
3.5
Table 4
PM3 relative energies and N´´´O distances for Me3N1CH2CH2CH2COO2
trans±gauche
trans±trans
gauche±trans
gauche±gauche
Gas phase
Erel ꢀkcal/mol)
DHf ꢀkcal/mol)
Ê
N´´´O ꢀA)
1.8
246.21
3.876
17.2
230.75
5.182
14.6
233.43
4.866
0
247.99
3.370
4.879
Ê
N´´´O ꢀA)
5.591
5.972
5.488
Aqueous phase
Erel ꢀkcal/mol)
DHf ꢀkcal/mol)
Ê
N´´´O ꢀA)
Ê
N´´´O ꢀA)
14.3
289.70
4.086
5.760
4.2
0
2104.04
5.182
5.972
4.2
9.5
294.54
5.277
5.829
4.2
23.9
280.12
3.504
5.055
4.2
Ê
ro ꢀA)

564
M. Szafran et al. / Journal of Molecular Structure 563±564 52001) 555±564
[8] M. Galin, A. Chepoton, J.-C. Galin, J. Chem. Soc. Perkin
Trans. 2 ꢀ1993) 545.
interactions. Although replacement of pyridine a-
protons by CH3 group should not affect much the
free rotation of the tether [24], but it change its solva-
tion, some bonds, torsion angles and distances [31].
All these effects can be responsible for the observed
multiplicity differences.
The overall conclusion that we draw is that there
is still a need for more experiments for better
understanding of why compounds studied here with
n ˆ 3 differ from those with n ˆ 2 in spin multiplicity
and hence in conformational preferences.
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Acknowledgements
We thank Mr J. Sitkowski for measurements of
some spectra on 500.13 MHz spectrometer and valu-
able discussion. This work was supported by the
Polish Committee for Scienti®c Research ꢀKBN),
grant 3 TO9A 094 14.
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