NMR studies of solid state - Solvent and H/D isotope effects on hydrogen bond geometries of 1:1 complexes of collidine with carboxylic acids

Article abstract of DOI:10.1016/j.molstruc.2004.02.023

¹H and ¹⁵N NMR spectra of 10 complexes exhibiting strong OHN hydrogen bonds formed by ¹⁵N-labeled collidine and different proton donors, partially deuterated in mobile proton sites, have been observed by low-temperature NM

Full text of DOI:10.1016/j.molstruc.2004.02.023

Journal of Molecular Structure 700 (2004) 19–27
www.elsevier.com/locate/molstruc
NMR studies of solid state—solvent and H/D isotope effects on hydrogen
bond geometries of 1:1 complexes of collidine with carboxylic acids
Peter M. Tolstoya,b, Sergei N. Smirnovb, Ilya G. Shenderovicha,b, Nikolai S. Golubevb,
Gleb S. Denisovb, Hans-Heinrich Limbacha,
*
a
¨
¨
Institut fur Chemie, Freie Universitat Berlin, Takustrasse 3, D-14195 Berlin, Germany
bV.A. Fock Institute of Physics, St Petersburg State University, 198504 St Petersburg, Russian Federation
Received 11 December 2003; revised 12 February 2004; accepted 13 February 2004
Abstract
1H and 15N NMR spectra of 10 complexes exhibiting strong OHN hydrogen bonds formed by 15N-labeled collidine and different proton
donors, partially deuterated in mobile proton sites, have been observed by low-temperature NMR spectroscopy using a low-freezing
CDF3/CDF2Cl mixture as polar aprotic solvent. The following proton donors have been used: HCl, formic acid, acetic acid, various
substituted benzoic acids and HBF4. The slow hydrogen bond exchange regime could be reached below 140 K, which allowed us to resolve
15N signal splittings due to H/D isotopic substitution. The valence bond order model is used to link the observed NMR parameters to
hydrogen bond geometries. The results are compared to those obtained previously [Magn. Reson. Chem. 39 (2001) S18] for the same
complexes in the organic solids. The increase of the dielectric constant from the organic solids to the solution (30 at 130 K) leads to a change
of the hydrogen bond geometries along the geometric correlation line towards the zwitterionic structures, where the proton is partially
transferred from oxygen to nitrogen. Whereas the changes of spectroscopic and, hence, geometric parameters are small for the systems which
are already zwitterionic in the solid state, large changes are observed for molecular complexes which exhibit almost a full proton transfer
from oxygen to nitrogen in the polar liquid solvent.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Low-temperature NMR; Hydrogen bond; Isotope effect; Solvent effect; Collidine–carboxylic acid complex
1. Introduction
the proton from a molecular complex A–H· · ·B to a quasi-
symmetric complex Ad2· · ·H· · ·Bdþ [7] and eventually to a
zwitterionic complex A2· · ·H–Bþ [8,9] through the hydro-
gen bond center. In other cases also changes of equilibria
between tautomeric forms have been observed [10,11]. In
ab initio calculations, these effects can be accounted for by
including static electric fields [1,2] or by polarized
continuum model calculations, obtained within the Onsager
or Tomasi models [12,13].
The average proton and the heavy atom positions of
hydrogen bonded systems are very sensitive to electric fields
[1–3]. Even small static electric field existing in solid inert
gas matrixes can produce qualitative changes in the
structure of hydrogen bonded complexes [4–6] or crystals
by surrounding atoms or molecules. In the liquid state, large
fluctuating fields arise from the dipole moments of the
mobile solvent molecules. The resulting local solvation then
influences strongly the average hydrogen bond complex
geometry. In the case of neutral acid–base complexes, in
the absence of steric hindrance between both complex
constituents, in some cases a continuous change of the
solvent polarity can lead to a gradual displacement of
The influence of the temperature dependence of the
solvent polarity on hydrogen bond geometry for small
intermolecular complexes in liquid state has been studied
experimentally by low-temperature NMR methods [14].
This technique has been shown to be very useful because
several NMR spectral parameters, such as chemical shifts
and coupling constants can be observed in the slow
1
hydrogen bond exchange regime. Especially, H, 19F and
*
Corresponding author. Tel.: þ49-30-8385-5375; fax: þ49-30-8385-
15N NMR chemical shifts as well as homonuclear and
heteronuclear scalar couplings between these spins across
5310.
E-mail address: limbach@chemie.fu-berlin.de (H.-H. Limbach).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.02.023

20
P.M. Tolstoy et al. / Journal of Molecular Structure 700 (2004) 19–27
the hydrogen bond can be observed, which are quite
sensitive to the complex geometries [15–17]. Using dipolar
interactions observed by solid state NMR, partially
combined with data obtained from neutron diffraction or
using quantum-mechanical calculations, NMR parameters
were correlated with hydrogen bond distances [18–26].
These correlations were built on geometric hydrogen bond
correlations observed previously by neutron crystallogra-
phy, which show that a shift of the proton towards the
hydrogen bond center is associated with a compression of
the hydrogen bond [27]. All correlations can now be used in
order to estimate hydrogen bond geometries in the liquid
state by NMR [14–16,21,28,29]. However, these corre-
lations have to be established for each system separately.
In our laboratories, in particular, hydrogen bonded
complexes of pyridine-15N with various acids have been
studied [2,14,28,30]. In order to link the hydrogen bond
geometries to the NMR parameters by dipolar solid state
NMR, we tried for a long time to crystallize molecular
complexes of pyridine with carboxylic acids, but were
unsuccessful. However, a series of solid state 1:1 complexes
between 2,4,6-trimethylpyridine (collidine) and various
Scheme 1. Structure of 2,4,6-trimethylpyridine (collidine) and the acids
studied in this paper.
Ref. [33], was added to the samples by vacuum transfer. The
approximate overall concentration of the samples, estimated
by weighing acids and measuring the volume of the solution
at low temperatures (around 120 K), was about 0.02 M.
1
acids could be crystallized [31] for which solid state H
and 15N NMR spectra could be obtained [23].
2.2. NMR spectra
Therefore, in this study, we have measured the
corresponding spectra of the same systems, but now
dissolved in a liquid CDF3/CDF2Cl mixture, exhibiting a
dielectric constant of between 20 at 170 K and 38 at 103 K
[14]. Our goal was to establish the changes of the hydrogen
bond geometries of these complexes induced by the polar
solvent, as compared to the fairly non-movable solid
The Bruker AMX-500 NMR spectrometer used was
equipped with a low-temperature probe-head which allowed
us to perform experiments down to 100 K. 1H NMR
chemical shifts were measured using residual CHF3 as
internal standard, and were converted to the conventional
TMS scale. The 15N NMR spectra were referenced to free
collidine dissolved in CDF3/CDF2Cl. The total deuterium
fraction was determined by comparison of the integrated
signal intensity of the residual mobile proton site with those
of the immobile CH protons.
1
environment, by elucidation of the H and 15N chemical
shifts as well as H/D isotope effects.
2. Experimental
2.1. Materials
3. Results
Commercially available substituted benzoic acids, as
well as formic acid, hydrochloric acid, acetic acid and HBF4
water solution were purchased from Aldrich. 15N-enriched
2,4,6-trimethylpyridine (collidine) was synthesized from
15NH4Cl according to Ref. [32]. Deuteration of substituted
benzoic acids was performed by repeatedly dissolving them
in methanol-OD and removing the solvent under vacuum.
HCl was deuterated according to the same procedure but
already in complex with collidine. Formic acid, acetic acid
and HBF4 were not deuterated. Chemicals were weighed
and placed into thick-walled sample NMR tubes equipped
with PTFE valves (Wilmad, Buena). Collidine was added by
vacuum transfer from the capillar tube connected to vacuum
system. In case of HCl and HBF4 first the complexes were
synthesized, then they were weighed and added to the
sample tube. The solvent, CDF3/CDF2Cl (freezing point
below 100 K), prepared by modified method described in
The chemical structures of the acids 1–10 used in this
study are depicted in Scheme 1. These acids can either form
1:1 or 1:2 complexes with collidine-15N as illustrated in
Scheme 2 in a similar way as found previously for the case
of pyridine-15N [30]. The low-temperature 1H and 15N
Scheme 2. Schematic representation of the structure of the hydrogen
bonded 1:1 and 1:2 collidine–acid complexes in polar solution.

P.M. Tolstoy et al. / Journal of Molecular Structure 700 (2004) 19–27
21
Fig. 1. Low-temperature 1H and 15N NMR signals of partially deuterated complexes of collidine with different proton donors in CDF3/CDF2Cl solution. HBF4
(1), HCl (2), 2-nitrobenzoic acid (3), 2-chloro-4-nitrobenzoic acid (4), 3,5-dinitro-4-methylbenzoic acid (5), 3,5-dichlorobenzoic acid (6), 3-nitro-4-
chlorobenzoic acid (7), formic acid (8), benzoic acid (9) and acetic acid (10). L ¼ H, D.
signals of the solutions of collidine-15N with the partially
deuterated acids are depicted in Fig. 1, which also indicates
the signal assignments. The NHO signals of 1:1 complexes
are split into doublets, arising from scalar coupling with
15N, characterized by the coupling constants 1JðNHOÞ: The
corresponding N HO signals in the 15N NMR spectra are
singlets as {1H} decoupling was employed. Because of
partial deuteration, additional NDO signals are observed.
As mentioned above, the spectra are referenced to free
collidine dissolved in CDF3/CDF2Cl. As the deuterium
signal intensities in 15N spectra allowed us to assign the
signals to the H or D isotopologs (see Section 2 for details).
We note that in the case of 2 the collidine added was
enriched with 15N only to about 70%, leading to a line trio,
where the central line arises from the complexes with
collidine-14N.
In the cases of 6 and 9, additional signals were observed
arising from the corresponding 1:2 complexes. This assign-
ment was confirmed by adding increasing amounts of
acid, leading to an increase of the line intensities of the
1:2 complexes. By 1H NMR the signals NHOCOHO,
1
fraction xD was determined from H spectra, the relative
Table 1
Experimental NMR parameters of 1:1 hydrogen bonded complexes formed between collidine and various proton donors in CDF3/CDF2Cl solution at low
temperature
pKa
CAH
CB
T (K)
xD
dðNHOÞ
dðNDOÞ
dðNDOÞ–dðNHOÞ
dðNHOÞ
1JðNHOÞ
HBF4 (1)
HCl (2)
,210
27
0.012
0.022
0.017
0.013
0.015
0.013
0.020
0.013
0.088
0.018
0.012
0.022
0.009
0.014
0.015
0.009
0.024
0.018
0.070
0.060
120
130
200
130
130
130
120
120
120
110
0
2104.67
296.31
292.63
290.28
287.77
287.27
287.13
286.55
283.58
274.85
12.62
15.72
16.82
18.06
18.86
18.91
19.06
19.21
19.80
20.18
291.9
289.1
287.0
284.6
281.7
281.1
280.8
279.1
276.9
265.4
0.80
0.20
0.25
0.15
0.10
0.20
0
297.52
294.57
292.65
290.66
290.57
290.42
21.21
21.94
22.37
22.89
23.30
23.29
2-Nitrobenzoic acid (3)
2-Chloro-4-nitrobenzoic acid (4)
3,5-Dinitro-4-methylbenzoic acid (5)
3,5-Dichlorobenzoic acid (6)
3-Nitro-4-chlorobenzoic acid (7)
Formic acid (8)
2.16
1.96
2.97
3.46
3.66
3.75
4.19
4.75
Benzoic acid (9)
Acetic acid (10)
0.30
0
288.44
24.86
C
AH and CB; concentrations of the acid and of collidine in mol l21; xD; deuterium fraction in the mobile proton sites; d; chemical shifts in ppm referenced to
free collidine or TMS dissolved in CDF3/CDF2Cl; 1JðNHOÞ; coupling constants in Hz, the sign is assumed to be negative as usual.

22
P.M. Tolstoy et al. / Journal of Molecular Structure 700 (2004) 19–27
NHOCODO, NHOCOHO, and NDOCOHO are observed,
where the first two signals exhibited again a scalar coupling
with 15N, characterized by the coupling constants
1JðNHOCOLOÞ; L ¼ H, D. Isotopic substitution in a
neighbouring H-bond influences the chemical shift of a
given H-bond proton, an effect which was called ‘vicinal
isotope effect’ [34].
Deuterium fractions, chemical shifts and coupling
constants for all systems studied are collected in Tables 1
and 2. The NMR data of the solid collidine-acid 1:1
complexes reported previously [23] are listed in Table 3
together with hydrogen bond distances calculated as
described in the Section 4.2.
d
d
d
d
4. Discussion
4.1. NMR parameters and structure of collidine–acid
complexes in CDF3/CDF2Cl
The spectral phenomena observed for collidine–acid
complexes in CDF3/CDF2Cl are similar to those described
previously for a series of pyridine–acid complexes [30].
When the acid is very strong, e.g. HBF4 (1), typical features
of a weakly or non-hydrogen bonded ion pair A2· · ·H–Bþ
are observed. They include
a
a proton resonance
high-field 15N signal at
d(NHO) ¼ 12.62 ppm,
d(N HO) ¼ 2104.67 ppm with respect to free collidine
dissolved in CDF3/CDF2Cl, and a large coupling constant
1JðNHOÞ ¼ 291:9 Hz which is typical for this type of
protonated heterocyclic systems [30,35].
d
d
d
d
When the acid becomes weaker, the proton is gradually
shifted towards the anion, leading to a decrease of 1JðNHOÞ
and a low-field shift of the d(N HO). During this process, the
hydrogen bond contracts as indicated by the increase of the
1H chemical shift of d(NHO). Eventually, with acetic acid
(10), a strongly hydrogen bonded quasi-symmetric complex
of the type Ad2· · ·H· · ·Bdþ is obtained, characterized by a
low-field proton signal d(NHO) ¼ 20.18 ppm, a consider-
ably reduced coupling constant 1JðNHOÞ ¼ 265:4 Hz; and
a value of d(NHO) ¼ 274.85 ppm with respect to free
collidine. At this point we cannot say whether the proton or
deuteron moves in a double or a single well potential. This
problem could be solved in the future by measuring the
chemical shift of the hydrogen bond deuteron, which will be
downfield for single well and upfield for double well
potentials [36]. Here, we note an important difference with
pyridine, where a molecular complex of the type A–H· · ·B
1
with acetic acid was observed, with l JðNHOÞl , 5 Hz;
dðNHOÞ ¼ 17:5 ppm and dðNHOÞ ¼ 227 ppm with respect
to free pyridine [30]. This difference indicates the stronger
basicity of collidine, which exhibits in water a pKa value of
7.43 at 298 K, whereas the corresponding value of pyridine
is 5.32 [37].

P.M. Tolstoy et al. / Journal of Molecular Structure 700 (2004) 19–27
23
Table 3
Experimental NMR parameters and geometric parameters of 1:1 complexes formed between collidine and various proton donors in the solid state
d15N(H)
(ppm)
d15N(D)
(ppm)
d15N(D)–d15N(H)
(ppm)
d1H
(ppm)
˚
rOH (A)
˚
rHN (A)
˚
q1 (A)
˚
q2 (A)
Proton donors
(2)
(3)
HCl
2105
295
281
275
234
247
289
2120
16.2
18.3
1.55
1.43
1.30
1.26
1.05
1.11
1.37
1.89
1.08
1.12
1.19
1.22
1.56
1.42
1.15
1.03
0.24
0.15
2.64
2.55
2.50
2.48
2.61
2.53
2.52
2.92
2-Nitrobenzoic acid
2-Chloro-4-nitrobenzoic acid
3-Nitro-4-chlorobenzoic acid
Benzoic acid
298
22.8
(4)
(7)
0.06
281
231
26.5
3.2
18.9
14.0
18.2
19.7
0.02
(9)
20.25
20.16
0.11
(11)
(12)
(13)
4-Nitrobenzoic acid
3,5-Dinitrobenzoic acid
HB(C6H5)4
244
293
3.1
24.0
22.4
2122
0.43
NMR parameters from Ref. [23]. The distances are described in Section 4.2. The 15N chemical shifts are referenced to neat frozen collidine.
4.2. NMR parameters and hydrogen bond geometries
of collidine–acid complexes in CDF3/CDF2Cl
established the parameters in Eq. (1). These parameters
depend slightly on the heavy atoms of the hydrogen bond as
well as on the actual data set used. In a previous paper on
solid collidine–acid complexes [23], some of us have used
the following values for NHO hydrogen bonds
In order to compare the results obtained for liquid state
with those obtained for the solid state, we have to link the
ensemble of NMR parameters such as chemical shifts,
coupling constants and H/D isotope effects on these
parameters with the hydrogen bond geometries. As a direct
determination of such geometries is not possible for the
liquid state, we used for this purpose hydrogen bond
correlations which have been well established [28,29]. The
basis of these correlations is the experimental finding that
the two distances rAH and rHB of a hydrogen bond AHB are
not independent of each other. Thus, both distances change
when the acidity or basicity of the interacting molecules are
varied. This correlation is predicted by the valence bond
order concept of Pauling [38]. For a hydrogen bonded
system A–H· · ·B it is assumed, that to both bonds so-called
valence bond orders pAH and pHB can be associated whose
sum is unity, i.e.
ꢀ
ꢀ
rOH8 ¼ 0:942 A;
bOH ¼ 0:371 A;
ð3Þ
ꢀ
ꢀ
rHN8 ¼ 0:992 A;
bHN ¼ 0:420 A:
By combination of Eqs. (1)–(3) the geometric hydrogen
bond correlation line of Fig. 2(a) is obtained. When H is
pAH þ pHB ¼ 1;
pAH ¼ expð2ðrAH 2 rAH8Þ=bAHÞ;
ð1Þ
pHB ¼ expð2ðrHB 2 rHB8Þ=bHBÞ:
In this equation, rAH8 and rHB8 represent the distance in the
free molecules AH and HB. The parameters bAH and bHB
describe the decay of the bond orders with increasing
distance. Note that the hydrogen bond angle does not enter
Eq. (1), which is, therefore, independent of this angle. In the
case of NHO hydrogen bonds studied here, we set A;O and
B;N. For convenience, instead of the rOH and rHN we will
use variables q1 and q2; defined as
q1 ¼ 1=2ðrOH 2 rHNÞ;
q2 ¼ ðrOH þ rHNÞ:
ð2Þ
For a linear hydrogen bond q2 corresponds to the heavy
atom distance N· · ·O and q1 to the displacement of the
proton from the hydrogen bond center. We would like to
point out that for heteronuclear hydrogen bonds the
geometric center does not have a particular physical
meaning, as in case of homonuclear hydrogen bonded
systems. An analysis of published low-temperature neutron
structural crystallographic data by several authors [27]
Fig. 2. Hydrogen bond correlations of collidine–acid complexes. (a)
Geometric hydrogen bond correlation q2 vs. q1: (b) 15N chemical shift
1
correlation (Eq. (4)). (c) H chemical shift correlation (Eq. (5)). The solid
state data points are taken from Ref. [23], the liquid state data points were
obtained in this study.

24
P.M. Tolstoy et al. / Journal of Molecular Structure 700 (2004) 19–27
23
ND
distance r
some of us [23] have recently established
the following experimental relation between the 15N
chemical shifts of polycrystalline collidine–acid complexes
and rND which was transformed using Eqs. (1)–(3) into the
following expression
dðNHOÞ ¼ dðNÞ1 2 ½dðNÞ1 2 dðHNÞ8ꢀpHN
dðNÞ1 2 dðHNÞ8 ¼ 130 ppm:
;
ð4Þ
dðHNÞ8 and dðNÞ1 are the 15N chemical shifts of the
fictive isolated collidinium and of free collidine. The
value of full protonation shift dðNÞ1 2 dðHNÞ8 ¼ 130 ppm
was obtained in Ref. [23] by fitting the experimental 15N
chemical shifts of collidine–acid complexes vs the dipolar
distances rND: For a comparison of the solid and liquid
state 15N chemical shift data it is convenient to set the
value of the neat frozen solid dðNÞ1ðsolidÞ ¼ 0: This
value corresponds to 268 ppm with respect to solid
15NH4Cl [23], which resonates at 2341.168 ppm with
respect to liquid nitromethane [39]. Because of a weak
hydrogen bond interaction of collidine with CDF3/CDF2Cl,
its 15N nucleus experiences a high field shift of about
2 8 ppm as compared to the neat frozen state. This solvent
shift could not be directly measured. Therefore, it was
treated as a parameter which was varied in such a way that
the liquid state 15N chemical shifts could be described as
well as the solid state data by Eq. (4) as using Eq. (4), the
valence bond orders and hence the values of rOH; rHN; q1 and
q2 were obtained for the collidine–acid 1:1 complexes in
the solid state and in CDF3/CDF2Cl from the experimental
15N chemical shifts d(NHO). The results are listed in Tables
3 and 4, and the data points visualized in Fig. 2(a) and 2(b).
Because of this procedure, the data points in these two
graphs are located exactly on the calculated dotted
correlation line.
Fig. 3. Hydrogen bond correlations of collidine–acid complexes. (a)
1JðNHOÞ coupling correlation (Eq. (6)). (b) Secondary H/D isotope effect
on the 15N chemical shifts. (c) dðNHOÞ vs dðNHOÞ correlation. For
further explanation see text.
shifted from O towards N, q2 decreases to a minimum. Once
the proton has crossed the hydrogen bond center where q1 <
0; q2 increases again.
By measuring the dipolar 2H–15N coupling constant
which is proportional to the average inverse cubic N· · ·D
For the 1H chemical shifts correlation the following
relation holds, which had been proposed previously for
Table 4
Valence bond orders and hydrogen bond geometries for acid–collidine complexes in CDF3/CDF2Cl
˚
rOH (A)
˚
rHN (A)
˚
q1 (A)
˚
q2 (A)
Type
pOH
pHN
HBF4 (1)
HCl (2)
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:2
1:2
0.133
0.198
0.226
0.244
0.263
0.267
0.268
0.273
0.296
0.363
0.225
0.225
0.867
0.802
0.774
0.756
0.737
0.733
0.732
0.727
0.704
0.637
0.775
0.775
1.69
1.54
1.49
1.47
1.44
1.43
1.43
1.42
1.39
1.32
1.50
1.50
1.05
1.08
1.10
1.11
1.12
1.12
1.12
1.13
1.14
1.18
1.10
1.10
0.32
0.23
0.20
0.18
0.16
0.16
0.15
0.15
0.13
0.07
0.20
0.20
2.74
2.63
2.59
2.56
2.58
2.55
2.55
2.55
2.53
2.50
2.59
2.60
2-Nitrobenzoic acid (3)
2-Chloro-4-nitro-benzoic acid (4)
3,5-Dinitro-4-methyl-benzoic acid (5)
3,5-Dichloro-benzoic acid (6)
3-Nitro-4-chloro-benzoic acid (7)
Formic acid (8)
Benzoic acid (9)
Acetic acid (10)
3,5-Dichloro-benzoic acid (6)
Benzoic acid (9)
p
OH; valence bond order of the OH unit; pHN; valence bond order of the HN unit; rOH; O· · ·H distance; rHN; N· · ·H distance; q1 ¼ 1=2ðrOH 2 rHNÞ;
q2 ¼ rOH þ rHN calculated according to Eqs. (1)–(4) as described in the text.

P.M. Tolstoy et al. / Journal of Molecular Structure 700 (2004) 19–27
25
pyridine–acid complexes in Ref. [28] and supported
experimentally by solid state 1H NMR in the case of
the solid collidine–acid complexes [23]
where A ¼ 104 and B ¼ 7 are empirical parameters. Eq.
(7) allows one to associate to each value of rOH and rHN
i.e. of qH1 and q2H corresponding values of rOD and rDN
;
;
i.e. of qD1 and qD2 ; and hence of the H/D isotope effects
on the nitrogen chemical shifts using Eq. (4). The liquid
state data are in good agreement with the calculated
dotted line in Fig. 3(b). Included are also the experi-
mental values obtained for the solid state [23]. Here, the
data are less precise, although the distances were derived
directly from dipolar interactions, without making several
assumptions, as we did during the processing of the
liquid state data.
dðNHOÞ ¼ 4DdðNHOÞpOHpHN þ dðOHÞ8pOH þ dðHNÞ8pHN
:
ð5Þ
dðOHÞ8 and dðHNÞ8 represent the fictitious limiting
chemical shift values of the monomeric acids and of
the free pyridinium cation, and DdðNHOÞ the excess
deshielding of the proton in the shortest hydrogen bond
configuration. The experimental results indicated values
of dðOHÞ8 ¼ 1 ppm; dðHNÞ8 ¼ 7 ppm; and DdðNHOÞ ¼
16:7 ppm leading to the dotted line in Fig. 2(c). The
agreement with the experimental data is satisfactory. Of
Finally, we have included in Fig. 3(c) the d(NHO) vs
d(N HO) correlation, where the dotted line is easily obtained
by the combination of Eqs. (4) and (5). We had obtained a
similar curve for the complexes of pyridine with various
acids in CDF3/CDF2Cl previously [28]. This correlation
curve represents the NMR analog of the geometric
correlation q1 vs. q2 curve of Fig. 2(a). It exhibits, however,
a principal difference because the distance between
the separate residues can be varied in an infinite number
of ways during which their chemical shifts remain constant.
Therefore, the dotted correlation line in Fig. 3(c) ends at the
left and the right sides. In Fig. 3(c) we have labeled
the environment of certain data points, where S stands for
the solid state.
1
course, H chemical shift of the OH group of free acid
should be slightly different for each given acid molecule,
but in Eq. (5) the value dðOHÞ8 ¼ 1 ppm is merely a
fitting parameter, which depends on the actual data set
being fit. The deviation of the point near q1 ¼ 0 from the
correlation curve probably indicates a larger N· · ·O
distance for a shared proton. Probably, this deviation
arises from a low barrier for the proton motion. We note
already now from Fig. 2, that the complexes in the liquid
state are shifted on average towards the zwitterionic
region as compared to the solid state. This effect will be
discussed below in more detail.
We observe for the collidine–acid complexes, that both
the solid state as well as the liquid state data are located on
the predicted dotted correlation curve in a satisfactory way.
Again, we observe that the polar medium shifts the data
points on the correlation line from the molecular complex
towards the zwitterionic species. Note, however, that the
relative order of the data points in the solid and the solution
data series is preserved.
For the dependence of 1JðNHÞ coupling constants on
valence bond orders in NHF bonds, an expression has been
obtained [15], which is here applied for NHO bonds in the
following way
1JðNHOÞ ¼ 1JðNHÞ8pHN 2 8DJðNHOÞp2OHpHN
:
ð6Þ
1JðNHÞ8 is the limiting coupling constant of pyridinium
cation, and DJðNHOÞ an excess term which has to be
determined experimentally. 1JðNHÞ8 coupling constants for
protonated pyridines can be as large as 90 Hz [30b] and
unusually high value of 115 Hz in Eq. (6) is the result of the
fitting procedure, similar to one done in Ref. [15], where
the value of 100 Hz was obtained. Eq. (6) corresponds to
4.3. Solid state—solvent effects on the hydrogen bond
geometries of collidine–acid complexes
Let us now discuss these results in a more quantitative
way. Unfortunately, the two series of complexes did not
overlap completely, because some of complexes studied in
the solid state were not soluble in the freon mixture used. On
the other hand, for certain complexes observed in the liquid
state no polycrystalline powdered materials could be
obtained. In order to facilitate the discussion we have
plotted in Fig. 4(a) the solvent shift dðNHOÞl –dðNHOÞs on
the 15N NMR chemical shifts—where the subscripts refer to
l ¼ liquid and s ¼ solid—as a function of the hydrogen
bond coordinate q1s in the solid state. Fig. 4(b) depicts the
corresponding solvent shift on the hydrogen bond coordi-
nate q1l –q1s:
1
the dotted line in Fig. 3(a), with the parameters JðNHÞ8 ¼
115 Hz; DJðNHOÞ ¼ 14:4 Hz: The agreement between the
experimental data and the dotted line is satisfactory, but
more data are needed in order to support the parameters of
Eq. (6).
For the H/D isotope effects on the nitrogen chemical
shifts, the following relations have been proposed [23]. It
was assumed that the valence bond order difference Dp=2 of
the NHO and NDO is given by
The origin of this transformation is the stabilization of
more polar structures by the polar medium, having at
130 K a dielectric constant of about 30 [14]. The dielectric
constant of the freonic mixture is subject to some changes
with temperature and solvent composition, but for
Dp=2 ¼ ðpDN 2 pHNÞ 2 ðpOD 2 pOH
¼ ðpOH 2 pODÞ 2 ðpHN 2 pDN
Þ
Þ
B
< AðpOHpHNÞ ðpHN 2 pOHÞ;
ð7Þ

26
P.M. Tolstoy et al. / Journal of Molecular Structure 700 (2004) 19–27
Acknowledgements
This research has been supported by the Deutsche
Forschungsgemeinschaft, Bonn, the Fonds der Chemischen
Industrie, Frankfurt, and the Russian Foundation of Basic
Research, Grants 03-03-04009, 03-03-06504 and 03-03-
´ ´
32272. P.T. thanks the Fonds for a Kekule scholarship.
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