Benzhydroximic acids - NMR study of trimethylsilyl derivatives

Article abstract of DOI:10.1016/S0022-328X(99)00708-1

NMR spectra (¹H ¹³C, ¹⁵N, and ²⁹Si) of trimethylsilylated para and meta substituted benzhydroxamic acids were studied in chloroform solutions. The silylation products have the structure of Z-O,O′-bis(trimethylsilyl) derivatives of benzhydroximic acid, independently of the ring substituent. According to aromatic proton chemical shifts, the geometry of the hydroximic group and its torsion angle with the ring plane are not affected by the para substituent. The chemical shifts of the nuclei in the hydroximic part of the molecule show surprisingly strong dependence on the remote ring substituent. The two ²⁹Si chemical shifts exhibit essentially the same sensitivity to substitution despite the fact that the Si(O¹) is one bond closer to the substituent than the Si(O⁴) silicon. It is suggested that while electron donor substituents increase the shielding of the silicon atoms they also increase the basicity of the oxygen in the Si-O moiety, thus leading to the stronger hydrogen bonding with the solvent. Association with chloroform partially compensates the direct substituent effect on the shielding in the case of Si(O¹) silicon. The influence of other factors not covered by substituent constants is demonstrated by excellent correlations with the chemical shifts in analogous tert-butyldimethylsilyl derivatives.

Full text of DOI:10.1016/S0022-328X(99)00708-1

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 597 (2000) 200–205
Benzhydroximic acids — NMR study of trimethylsilyl derivatives
a
a
Jan Schraml a,*, Magdalena Kv´ıcˇalova´ , Ludmila Soukupova´ , Vratislav Blechta a,
Otto Exner b
a Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, 16502 Prague 6, Czech Republic
b Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 16610 Prague 6, Czech Republic
Received 7 October 1999; received in revised form 4 November 1999
Dedicated to Professor Stanis*law Pasynkiewicz on the occasion of his 70th birthday in recognition of his contribution to organometallic
chemistry.
Abstract
NMR spectra (1H 13C, 15N, and 29Si) of trimethylsilylated para and meta substituted benzhydroxamic acids were studied in
chloroform solutions. The silylation products have the structure of Z-O,O%-bis(trimethylsilyl) derivatives of benzhydroximic acid,
independently of the ring substituent. According to aromatic proton chemical shifts, the geometry of the hydroximic group and
its torsion angle with the ring plane are not affected by the para substituent. The chemical shifts of the nuclei in the hydroximic
part of the molecule show surprisingly strong dependence on the remote ring substituent. The two 29Si chemical shifts exhibit
essentially the same sensitivity to substitution despite the fact that the Si(O1) is one bond closer to the substituent than the Si(O4)
silicon. It is suggested that while electron donor substituents increase the shielding of the silicon atoms they also increase the
basicity of the oxygen in the SiꢀO moiety, thus leading to the stronger hydrogen bonding with the solvent. Association with
chloroform partially compensates the direct substituent effect on the shielding in the case of Si(O1) silicon. The influence of other
factors not covered by substituent constants is demonstrated by excellent correlations with the chemical shifts in analogous
tert-butyldimethylsilyl derivatives. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Trimethylsilyl derivatives; Substituted benzhydroximic acids; NMR chemical shifts; Hammett correlations
sponding benzhydroximic acids, as proven by their 29Si,
1. Introduction
1
15N, 13CꢁN chemical shifts and J(13C–13CN) coupling
constants.
We have recently shown that silylation (either
trimethysilylation or tert-butyldimethylsilylation) of
benzhydroxamic acid solely yields bis-silyl derivatives
of the tautomeric benzhydroximic acid with the Z
configuration [1]. A subsequent study [2] of 12 para and
meta substituted benzhydroxamic acids has shown that
the substituent, which in other respects profoundly
affects the structure of the acids [3–6], in the crystal,
does not influence the structure of the product of
tert-butyldimethylsilylation in solution to the extent
noticeable by NMR spectroscopy. Upon tert-
butyldimethylsilylation all the substituted benzhydrox-
amic acids yielded Z-O1,O4-bis(tert-butyldimethylsilyl)
derivatives (1, Scheme 1, R=C(CH3)3) of the corre-
While the stable tert-butyldimethylsilyl (TBDMS)
derivatives 1 permit time-consuming measurements nec-
essary for their structure determination, data on the
less-stable trimethylsilyl (TMS) derivatives (2, Scheme
1, R=CH3) are needed for a comparison with other
series of compounds. Also interesting is a quantitative
comparison of the corresponding atoms in the two
Scheme 1. Structure and atom numbering of fully silylated Z-benzhy-
droximic acids (in TMS derivatives, 2, R=CH3; in TBDMS deriva-
tives, 1, R=C(CH3)3).
* Corresponding author. Fax: +42-02-20920661.
E-mail address: schraml@icpf.cas.cz (J. Schraml)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00708-1

J. Schraml et al. / Journal of Organometallic Chemistry 597 (2000) 200–205
201
series 1 and 2; for series so similar one expects close
correlations with the slopes near unity. In the present
paper, we report on the results of a limited study of
TMS derivatives 2.
16 000 Hz. WALTZ decoupling was applied both dur-
ing acquisition (1 s) and relaxation delay (2–5 s). Zero
filling to 64 K and 1–3 Hz line broadening were used in
data processing.
Aromatic carbon chemical shifts were assigned by
combining additive increments (using the values from
Ref. [9]) and the shifts assigned for the parent Z-O-
trimethylsilyl trimethylsilylbenzhydroximate through a
2D INADEQUATE experiment [10].
When a fluorine-containing substituent was present
(4-F, 4-CF3), the 13C–19F couplings were used to check
the assignment of aromatic carbon lines.
2. Experimental
The substituted benzhydroxamic acids were prepared
by standard procedures as described elsewhere [3,7].
TMS derivatives were prepared from the parent acids
by silylation using (trimethylsilyl)dimethylamine (TMS-
DMA) in 1.2 molar excess. The reaction mixture was
heated (60°C) and stirred for 2–5 h. After completion,
the remaining reagent was removed by distillation un-
der reduced pressure. The isolated compounds were
identified and checked by 1H- and 13C-NMR spec-
troscopy. (1H-NMR spectra were analyzed in terms of
coupling constant and chemical shift values of aromatic
protons only in the case of para substituted com-
pounds; in the other cases the splitting pattern was used
to check the position of the substituent.)
In contrast to 19F–15N coupling across six bonds
observed earlier in the 15N-NMR spectrum of Z-O1,O4-
bis(tert-butyldimethylsilyl) 4-fluorobenzhydroximic acid
[2], no such coupling was detected (JB0.5 Hz) in its
TMS analogue. Apparently, crowding at the hydrox-
imic end of the molecule plays some role; such effects
were noticed in other through-space 15N–19F couplings
[11].
Solvent accessible surface (A) [12] was calculated as
described elsewhere [13].
The NMR spectra were measured in dry CDCl3
solutions containing 1% (v/v) of hexamethyldisilane
(HMDSS) as a secondary reference. The reported 13C
and 29Si chemical shifts were obtained from dilute
solutions. The sample concentration was reduced until
the 13C chemical shift of HMDSS was l= −2.489
0.02, relative to the central line of the solvent at 76.99
ppm (see Ref. [8] for details of this standard proce-
dure). High sample concentrations (ca. 33% v/v) were
used in 15N-NMR and INADEQUATE measurements.
15N-NMR chemical shifts are referenced externally to
CH3NO2 (50%, w/w) in CDCl3.
3. Results and discussion
The assigned chemical shifts and coupling constants
of the studied compounds are summarized in Tables 1
and 2. Since the chemical shifts (13C, 15N, and 29Si)
correlate well with substituent constants, we will briefly
compare these correlations with those found in related
classes of compounds also bearing TMS substituents.
In all the studied ring-substituted benzhydroxamic
acids, trimethylsilylation yields only one product (using
29Si INEPT spectra of concentrated solutions with ex-
cellent S/N it can be estimated that if any other
trimethylsilylated product were formed, it would consti-
tute only less than 2% of the main product). This is in
agreement with the earlier findings on trimethylsilyl-
ation [16] or tert-butyldimethylsilylation of benzhy-
droxamic acid [2]. Using the values of one-bond 13C–
13C couplings (13C–13CN), we have shown [1] that the
sole product of trimethylsilylation of the parent ben-
zhydroxamic acid (R=H) is not an exchange-averaged
mixture of E- and Z-isomers, as suggested by Rigaudy
et al. [16], but only the Z-trimethylsilyl ester of N-
(trimethylsiloxy)benzoimidic acid (2). Since the chemi-
cal shifts in the studied series of TMS derivatives of the
ring-substituted benzhydroxamic acids follow some ob-
vious and regular dependences on Hammett constants
of the ring substituents, and the parent compound fits
these dependences well, one can safely assume that all
the studied compounds have the same structure 2.
Similarly, we extend the assignment of 29Si lines in 1 (as
derived from model compounds in Ref. [1]) to all
compounds 2.
1H-, 13C-, and 29Si-NMR spectral measurements were
performed on a Varian UNITY-200 spectrometer (op-
erating at 200.04 MHz for 1H, at 50.3 MHz for 13C and
at 39.7 MHz for 29Si-NMR measurements) and 15N
spectra were measured on a Varian UNITY 500 spec-
trometer (at 50.667 MHz). In all cases the standard
software (APT, INADEQUATE, and INEPT pulse
sequences) was used. The spectra were recorded in the
temperature range 22–24°C. The 29Si-NMR spectra
were measured by INEPT with the pulse sequence
optimized [8] for TMS derivatives, i.e. for coupling to
nine protons and the coupling constant of 6.5 Hz.
Acquisition (2.0 s) was followed by a relaxation delay
of 5 s. During the acquisition period, WALTZ decou-
pling was used and FID data (16 K) were sampled for
the spectral width of 4000 Hz. Zero filling to 32 K and
a mild exponential broadening were used in data pro-
cessing. The 29Si p/2 pulses were at maximum 17 ms
long, whereas 1H p/2 pulses were 12 ms in a 5 mm
switchable probe. The 29Si spectra were referenced to
the line of HMDSS at l= −19.79. The 13C-NMR
spectra were measured using the spectral width of

Table 1
29Si-, 15N- and 13C-NMR chemical shifts (ppm) in Z-RꢀC6H4ꢀC(O4SiMe3)ꢁNꢀO1ꢀSiMe3 derivatives a
b
R
|
l(29Si)
l(15N) c
l(13C)
CꢁN
Si-1
Si-4
C-1
C-2
C-3
C-4
C-5
C-6
CH3Si
Other
Parasubstituted
N(CH3)2
NHSiMe3
OCH3
CF3
NO2
H
CH3
F
Cl
−0.83
−0.66 e
−0.27
0.54
0.78
0.00
−0.17
0.06
0.23
24.48
24.62
25.24
26.84
27.66
25.76
25.44
25.95
26.24
20.3
−84.9 d
−84.7 g
−82.4
−74.80
−71.3 k
−78.6
−80.0
−80.3
−78.1
154.38
154.21
153.74
152.76
152.26
153.82
153.96
153.07
153.05
120.47
121.87
125.35
136.32
139.03
132.80
130.01
128.90 l
131.36
127.37
127.59
127.72
126.45
126.92
126.22
126.17
128.17 m
127.52
111.43
115.42
113.40
124.97 h
123.33
128.02
128.74
114.99 n
128.25
151.41
149.04
160.82
131.06 i
148.44
129.58
139.63
163.82 c,o
135.55
111.43
115.42
113.40
124.97 h
123.33
128.02
128.74
114.99 n
128.25
127.37
127.59
127.72
126.45
126.92
126.22
126.17
128.17 m
127.52
1.47/−0.55
1.51/−0.56
1.52/−0.57
1.55/−0.61
1.61/−0.63
1.55/−0.57
1.50/−0.57
1.57/−0.59
1.56/−0.59
40.31
−0.03
55.28
20.41 f
21.00
22.51
23.26
21.38
21.10
21.72
21.97
124.18 j
/
21.35
Metasubstituted
NO2
Cl
0.71
0.37
0.12
27.39
26.51
25.86
23.33
22.25
21.49
−75.3 p
−77.0
−77.9
152.00
152.73
153.69
134.81
134.05 q
134.22
121.19
126.26
111.65
148.25
134.70 q
159.29
124.14
129.57 q
115.26
129.04
129.31 q
129.06
131.90
124.34
118.78
1.61/−0.61
1.57/−0.60
1.51/−0.58
OCH3
55.20
Orthosubstituted
OSiMe3
24.73
19.94 r
−77.2
154.48 q
125.64
153.57 q
120.20 q
130.80 s
120.96 q
130.24 s
1.91/0.51
−0.41
a Unless otherwise indicated the values are from diluted solutions.
b Values of Hammett | constants taken from Ref. [14].
c Value(s) from concentrated solution(s).
d N(CH3)2 at −330.
597
(
e Value for NH2 group.
f SiNH at 3.57.
g NH at −308.7 (J(NH)=76.5 Hz).
h J(FC)=3.9 Hz.
200
205
i J(FC)=32.7 Hz.
j J(FC)=272.0 Hz.
k NO2 at −10.1.
l J(FC)=2.8 Hz.
m J(FC)=8.3 Hz.
n J(FC)=22.0 Hz.
o J(FC)=249.1 Hz.
p NO2 at −9.9.
q Assignments in the row can be interchanged.
r Si at 19.86, the two lines cannot be assigned.
s Assignments in the row can be interchanged.

J. Schraml et al. / Journal of Organometallic Chemistry 597 (2000) 200–205
203
Table 2
chemical shifts. The protons ortho to the hydroximic
group (H-2% in Table 2) exhibit a significant linear
correlation with the substituent chemical shifts (SCS)
induced by the substituents in monosubstituted ben-
zenes [15], l(H-2%)=7.81+1.15*SCSmeta, R=0.947,
n=8; the slope does not differ significantly (on 90%
significance level) from unity. Following the reasoning
of Hol´ık and Mate˘jkova´ [20], we can conclude that the
geometry of hydroximic group and its torsion angle
with a benzene ring do not change under the influence
of the substituent.
The substituents exercise very ‘normal’ effects on the
electronic structure, as manifested by Hammett-type
correlations of NMR chemical shifts, and do not have
such dramatic effects on the structure of Z-O,O%-
bis(trimethylsilyl) hydroximic acids, as reported for the
parent hydroxamic acids [6] and their alkali metal salts
[4]. Thus, in para substituted compounds 2 the 13C
chemical shifts of C-1 carbons exhibit Hammett-type
correlation with the same slope (b=11.55, R=0.9684,
SE=1.66, n=9), as found in monosubstituted ben-
zenes for the para carbons (b=11.5 [9]). The negative
slope in the correlation for the CꢁN carbon (meta and
para derivatives combined b= −1.46; para only b=
−1.31) was not unexpected. The negative slope was
already discussed [2] in connection with compounds 1
where it was somewhat smaller in absolute value (b=
−1.30 and 1.14, respectively). In both series 1 and 2
the slopes are smaller than in methyl benzoates
(−1.907 and −2.408, respectively) [21], benzonitriles
(−2.66 and –2.39, respectively) [22], and N,N-
dimethylbenzamides (−2.33 para only) [23]. Consider-
ing the polarizability of the involved groups and the
properties of OꢀSi bonds, the low values of the slopes
in 1 and 2 may be accounted for by the proposed model
in which the double CꢁN bond behaves as an isolated
unit polarized by the distant substituent [2].
The two silicon-29 chemical shifts in 2 correlate
linearly with Hammett substituent constants; surpris-
ingly, the slopes have the same value (1.9890.11 and
1.9590.15 for Si-1 and Si-4, respectively). As follows
from the above-mentioned correlations between TMS
and TBDMS derivatives (with the slopes close to
unity), the slopes do not differ significantly from those
found in Hammett correlations for bulkier TBDMS
derivatives (1.8290.09 and 1.9690.13 for Si-1 and
Si-4, respectively) [2]. This sensitivity of silicon shield-
ing to substituent effects is lower than that reported for
ring-substituted phenoxytrimethylsilanes (4.9390.85 in
CCl4 and 4.7790.94 in neat liquids) [24]. While this
might seem to be an obvious example of the role of
substituent distance, a part of this reduced sensitivity is
due to different experimental conditions. The com-
pounds studied here were measured in dilute chloro-
form solutions where hydrogen bonding reduces the
sensitivity of the silicon to substituent effects (as elec-
1H-NMR data and substituent induced chemical shifts (ppm) of
aromatic protons in para substituted Z-4RꢀC6H4ꢀC(O4SiMe3)ꢁ
NꢀO1ꢀSiMe3 derivatives a
b
R
l(1H)
H-2%
J(2,3)
SCS c
H-3%
Ortho
Meta
N(CH3)2
NHSiMe3
OCH3
CF3
NO2
H
CH3
F
Cl
7.6625
7.577
7.7275
7.9005
8.1925
7.798
7.678
7.7735
7.721
6.6555
6.597
6.860
7.593
7.9485
7.345
7.1415
7.018
7.304
9.0
8.8
8.9
8.3
9.0
8.2
8.4
8.5
8.7
−0.66
−0.18
d
d
−0.48
0.32
0.95
−0.09
0.14
0.26
0.00
−0.12
0.00
0.00
−0.20
−0.26
0.03
−0.02
a The values are from diluted solutions, 1H chemical shifts in l
scale, coupling constants in Hz units; H-2% and H-3% denote protons
ortho and meta to the hydroximic group, respectively.
b Values of coupling constants between protons H-2% and H-3%.
c Values of substituent induced chemical shifts, SCS, taken from
Ref. [15].
d Not available.
These ‘empirical’ assignments are supported by the
correlations of chemical shifts in 2 with those in the
TBDMS analogues 1 in which the Z configuration was
confirmed by the measurements of J(13C–13CN) cou-
1
pling constants in all the compounds of the series [2].
These correlations of chemical shifts in 1 against those
in 2 are extremely good, having slopes (b) close to unity
and the standard errors (SD) of the predicted chemical
shift in 2 do not appreciably exceed the experimental
errors (12 data points in all cases; Si-1: R=0.9999,
b=0.93290.004, SD=0.013; Si-4: R=0.9996, b=
1.02390.009, SD=0.030 ppm; 15N: R=0.9953, b=
1.0190.03, SD=0.4 ppm; 13C(CꢁN): R=0.9988,
b=0.90590.014, SD=0.035 ppm). The slope values
in 29Si correlations are similar to those found in other
pairs of TBDMS and TMS derivatives with SiꢀO bonds
(e.g. in silylated alcohols 0.990 [17] or amino acids
0.860 [18]). The quality of these correlations allowed us
to identify the deviating points as those of silicon atoms
in different moieties [19]; no such significant deviations
have been found in the series studied here. The quality
of these chemical shift–chemical shift correlations indi-
cates that the ring substituents control the shifts in
question by the same mechanisms in these two closely
related series of compounds, and that if the bulky
TBDMS group exercises some steric effects they are
constant within the series of 1. In contrast, the some-
what worse Hammett-type correlations discussed below
indicate that Hammett constants do not cover all the
factors involved.
Further support for the suggested structure and de-
tailed information can be drawn from aromatic proton

204
J. Schraml et al. / Journal of Organometallic Chemistry 597 (2000) 200–205
Table 3
Statistics of the correlations of substituent-induced l(15N) shifts in different series of compounds
e
f
Compound series a
Explanatory variables b
Regression coefficients c
SD d
R
N
1
2
3
4
5
6
RꢀCOꢀNHꢀOH g
|
3.22(26)
3.79(39)
7.89(63)
7.73(65)
15.14(1.11)
10.46(55)
0.43
0.54
1.05
1.09
1.34
0.53
0.969
0.951
0.969
0.966
0.987
0.993
12
12
12
12
7
m,p
h
RꢀCOꢀNH2
|
m,p
RꢀC(OT)ꢁNꢀOT i
RꢀC(OT)ꢁNꢀOT j
RꢀCHꢁNꢀOH k
RꢀCꢁN l
|
m,p
|
m,p
|
m,p
|
7
m,p
a R=XꢀC6H4.
b Values of substituent constants taken from Ref. [14].
c Standard deviation in parentheses.
d Standard deviation from the regression.
e Correlation coefficient.
f Number of observations.
g Benzhydroxamic acids, Ref. [7].
h Benzamides, Ref. [25].
i Z-O1,O4-bis(tert-butyldimethysilyl) benzhydroximic acids, T=Si(CH3)2C(CH3)3, Ref. [2].
j Z-O1,O4-bis(trimethysilyl) benzhydroximic acids, T=Si(CH3)3, this work.
k Benzaldoximes, Ref. [26].
l Benzonitriles, Ref. [25].
tron donors increase the shielding of the silicon they
also increase the basicity of the SiꢀO oxygen, and hence
enhance H-bonding which in turn decreases the shield-
ing [13]). Some combinations of these two mechanisms
also contribute to the observed equal sensitivity of Si-1
and Si-4 to substitution. However, the interplay of the
above mechanisms is not obvious as the calculated
solvent accessible surface (A) of the O1 oxygen is
smaller than that of O4 both in TMS and in the bulkier
TBDMS derivatives (the actual numerical values
strongly depend on the conformation).
4. Conclusions
Trimethylsilylation of ring-substituted benzhydrox-
amic acids produces solely syn (Z) O,O%-disilylated
derivatives of benzhydroximic acid. The hydroximic
structure follows directly from 29Si and 15N chemical
shifts; the Z configuration follows from one-bond 13C–
13C couplings of 13CꢁN carbon in the parent compound
and from comparison with analogous TBDMS deriva-
tives. The geometry of the hydroximic group does not
change with the ring substituent.
The 15N chemical shifts exhibit similarly good linear
correlations with substituent constants. Since such cor-
relations have not been recently reviewed, some 15N
correlations for closely related series of compounds are
presented in Table 3. Again, correlations with Hammett
constants have the same slopes in TMS and TBDMS [2]
derivatives of benzhydroximic acids; their slopes are
intermediate between those in benzhydroxamic acids [7]
or benzamides [25] and those in benzonitriles [26]. This
observation is certainly in accord with the bond order
of the CꢁN bond in hydroximic derivatives as interme-
diate between those in amides and nitriles. Neverthe-
Chemical shifts of various nuclei in the hydroximic
moiety of TMS and TBDMS derivatives exhibit excel-
lent mutual correlations and also good Hammett-type
correlations. Notably, the dependences of S(O4) and
Si(O1) show the same sensitivity to substituent effects
despite the latter being one bond closer to the sub-
stituent; the sensitivity is lower than that in phe-
noxytrimethylsilanes. It is suggested that hydrogen
bonding with the solvent plays a certain role in this
observation. In accord with the hydroximic structure,
the sensitivity of 15N chemical shifts falls between that
in amides and nitriles.
less, such
a
picture would be oversimplifying.
Comparison with benzaldoximes [26] and benzamides
[25] indicates that the value of the slope is not simply
related to the CꢀN bond order; polarizability of the
other substituents obviously plays a significant role here
also. Attempts to analyze the results of Table 2 in terms
of the dual substituent parameter (DSP) treatment us-
ing |F and |R were successful only in some cases
(RCONHOH, RCONH2). The correlations were better
than with |m,p, but the improvement was of little
significance.
Acknowledgements
The authors acknowledge the financial support pro-
vided by the Granting Agency of the Academy of
Sciences of the Czech Republic (grant. no. A4072605).
This project was also supplied with subvention by the
Ministry of Education of the Czech Republic (Project
LB98233). We thank Dr J. Mindl (University of Pardu-
bice) for a gift of some samples of benzhydroxamic acids.

J. Schraml et al. / Journal of Organometallic Chemistry 597 (2000) 200–205
205
[13] J. Schraml, M. Jakoubkova´, M. Kv´ıcalova´, A. Kasal, J. Chem.
Soc. Perkin Trans. 2 (1994) 1.
References
[14] C. Hansch, A. Leo, R.W. Taft, Chem. Rev. 91 (1991) 165.
[15] J. Beeby, S. Sternhell, T. Hoffmann-Ostenhof, E. Pretsch, W.
Simon, Anal. Chem. 45 (1973) 1571.
[16] J. Rigaudy, E. Lytwyn, P. Wallach, K.C. Nguyen, Tetrahedron
Lett. 21 (1980) 3367.
[1] J. Schraml, M. Kv´ıcˇalova´, L. Soukupova´, V. Blechta, Magn.
Reson. Chem. 37 (1999) 427.
[2] J. Schraml, M. Kv´ıcˇalova´, L. Soukupova´, V. Blechta, O. Exner,
J. Phys. Org. Chem. 12 (1999) 1.
[3] J. Podlaha, I. C´ısarˇova´, L. Soukupova´, J. Schraml, to be pub-
lished.
[4] J. Podlaha, I. C´ısarˇova´, L. Soukupova´, J. Schraml, O. Exner, J.
Chem. Research (S) (1999) 520.
[5] J. Podlaha, I. C´ısarˇova´, M. Kv´ıcˇalova´, J. Schraml, Supramol.
Chem., in press.
[6] J. Podlaha, I. C´ısarˇova´, L. Soukupova´, J. Schraml, J. Mol.
Struct. (1999).
[7] J. Schraml, M. Tkadlecova´, S. Pataridis, K. Volka, L. Souku-
pova´, V. Blechta, O. Exner, Magn. Reson. Chem. (1999), in
press.
[8] J. Schraml, Progr. NMR Spectrosc. 22 (1990) 289.
[9] H.-O. Kalinowski, S. Berger, S. Braun, 13C-NMR Spek-
troskopie, Georg Thieme Verlag, Stuttgart, 1984, pp. 284–285.
[10] R. Freeman, M.H. Levitt, J. Magn. Reson. 43 (1980) 478.
[11] F.B. Mallory, E.D. Luzik, C.W. Mallory, P.J. Carroll, J. Org.
Chem. 57 (1992) 366.
&
[17] J. Schraml, M. Kv´ıcˇalova´, V. Blechta, J. Cerma´k, Magn. Reson.
Chem. 35 (1997) 659.
&
[18] R. Kubec, J. Vel´ısˇek, M. Kv´ıcˇalova´, J. Cerma´k, J. Schraml,
Magn. Reson. Chem. 33 (1995) 458.
&
[19] J. Schraml, M. Kv´ıcˇalova´, V. Blechta, R. Rerˇicha, J. Rozenski,
P. Herdewijn, Magn. Reson. Chem. 36 (1998) 55.
[20] M. Hol´ık, B. Mateˇjkova´, Collect. Czech. Chem. Commun. 55
(1990) 261.
[21] M. Budeˇsˇ´ınsky´, O. Exner, Magn. Reson. Chem. 27 (1989) 585.
[22] O. Exner, M. Budeˇsˇ´ınsky´, Magn. Reson. Chem. 27 (1989) 27.
[23] R.G. Jones, J.M. Wilkins, Org. Magn. Reson. 11 (1978) 20.
[24] J. Schraml, R. Ponec, V. Chvalovsky´, G. Engelhardt, H. Jancke,
H. Kriegsman, M.F. Larin, V.A. Pestunovich, M.G. Voronkov,
J. Organomet. Chem. 178 (1979) 55.
[25] P.S. Pregosin, E.W. Randall, A.I. White, J. Chem. Soc. Perkin
Trans. 2 (1972) 513.
[26] P.W. Westerman, R.E. Botto, J.D. Roberts, J. Org. Chem. 43
(1978) 2590.
[12] F.M. Richards, Ann. Rev. Biophys. Bioeng. 6 (1977) 151.
.