The effect of solvent accessible surface on Hammett-type dependencies of infinite dilution 29Si and 13C NMR shifts in ring substituted silylated phenols dissolved in chloroform and acetone

Article abstract of DOI:10.1002/mrc.2860

Infinite dilution ²⁹Si and ¹³C NMR chemical shifts were determined from concentration dependencies of the shifts in dilute chloroform and acetone solutions of para substituted O-silylated phenols, 4-R-C₆H₄-O-SiR′₂R″ (R = Me, MeO, H, F, Cl, NMe₂, NH₂, and CF₃), where the silyl part included groups of different sizes: dimethylsilyl (R′ = Me, R″ = H), trimethylsilyl (R′ = R″ = Me), tert-butyldimethylsilyl (R′ = Me, R″ = CMe₃), and tert-butyldiphenylsilyl (R′ = C₆H₅, R″ = CMe₃). Dependencies of silicon and C-1 carbon chemical shifts on Hammett substituent constants are discussed. It is shown that the substituent sensitivity of these chemical shifts is reduced by association with chloroform, the reduction being proportional to the solvent accessible surface of the oxygen atom in the Si-O-C link. Copyright

Full text of DOI:10.1002/mrc.2860

Research Article
Received: 6 October 2011
Revised: 3 November 2011
Accepted: 4 November 2011
Published online in Wiley Online Library: 15 February 2012
(wileyonlinelibrary.com) DOI 10.1002/mrc.2860
The effect of solvent accessible surface on
Hammett-type dependencies of infinite
29
13
dilution Si and C NMR shifts in ring
substituted silylated phenols dissolved in
chloroform and acetone
Vratislav Blechta,a Stanislav Šabata,a Jan Sýkora,a Jiří Hetflejš,a
Ludmila Soukupováa and Jan Schramla*
Infinite dilution 29Si and 13C NMR chemical shifts were determined from concentration dependencies of the shifts in dilute
chloroform and acetone solutions of para substituted O-silylated phenols, 4-R-C6H4-O-SiR′2R00 (R = Me, MeO, H, F, Cl, NMe2,
NH2, and CF3), where the silyl part included groups of different sizes: dimethylsilyl (R′ = Me, R00 = H), trimethylsilyl (R′ = R00 = Me),
tert-butyldimethylsilyl (R′ = Me, R00 = CMe3), and tert-butyldiphenylsilyl (R′ = C6H5, R00 = CMe3). Dependencies of silicon and C-1
carbon chemical shifts on Hammett substituent constants are discussed. It is shown that the substituent sensitivity of these
chemical shifts is reduced by association with chloroform, the reduction being proportional to the solvent accessible surface
of the oxygen atom in the Si-O-C link. Copyright © 2012 John Wiley & Sons, Ltd.
Keywords: 29Si NMR; 13C NMR; 1H NMR; solvent accessible surface; substituent effects; chemical shifts; Hammett-type dependence
Introduction
Experimental
The 29Si chemical shift becomes more sensitive to substituent
effects when these are transmitted through oxygen or nitrogen
atoms bonded to the silicon.[1–3] In trimethylsilylated phenols,
X-C6H4-O-SiMe3, the shift changes with Hammett constant of
substituent X twice as much than in compounds X-C6H4-SiMe3
in which the silicon atom is one bond closer to the substituent.[1]
The high sensitivity of 29Si chemical shifts to substituent effects
transmitted through oxygen atom has laid the basis for 29Si
NMR tag used for identification of functional groups.[4–7] The
Synthesis
Starting materials
Phenol and its 4-substituted derivatives (4-CH3, 4-CH3O, 4-Cl, 4-NO2,
4-NH2, 4-F, and 4-CF3), were supplied by Sigma-Aldrich and used as
obtained. 4-(Dimethylamino)phenol was prepared by the cleavage
of the corresponding N,N-dimethyl-p-anisidine with HBr in glacial
acetic acid according to Slotta and Benish.[15] 1,1,1,3,3,3-Hexam-
ethyldisilazane (99.9%), 1,1,3,3-tetramethyldisilazane (>97%), tert-
butyldimethylsilyl chloride (97%), and tert-butyldiphenylsilyl
chloride (>97%) were Sigma-Aldrich products and were used
without further purification.
29Si chemical shift shows also dependence on steric effects[8,9]
,
but only when sufficiently diluted solutions could be measured
(after the invention of INEPT polarization transfer[10] and its
application to these compounds[11,12]), the steric effects could
be interpreted.[9] The interpretation[13,14] assumes an association
of the molecule with solvent (chloroform), which is controlled by
the solvent accessible surface (SAS) and basicity of the oxygen
atom of the siloxy group. In the trimethylsilylated secondary
steroids used to model the effects,[13,14] the SAS was varied by
changing the steroid geometry while maintaining essentially
the same oxygen basicity.
Naturally, it is of interest to determine how the bulkiness of
the silyl group that affects the oxygen accessible surface reflects
in the sensitivity to substituent effects and how these are
influenced by the solvent used. The results of such a study of
four silyl groups [dimethylsilyl (R′ = Me, R00 = H), trimethylsilyl
(R′ = R00 = Me), tert-butyldimethylsilyl (R′ = Me, R00 = CMe3), and
tert-butyldiphenylsilyl (R′ = C6H5, R00 = CMe3)] are presented in
this study.
General procedure for trimethyl- and (dimethyl)silylation of phenols
A reaction flask provided with a reflux condenser, a thermometer,
an argon inlet, and a magnetic stirrer was charged with the
corresponding phenol (0.04 mol) and then a silylation agent
(hexamethyl- and tetramethyldisilazane, respectively) was gradu-
ally added (0.025 mol) at ambient temperature. After completion
of the addition, the reaction mixture was warmed up to a
maximum of 160 ꢀC during 2 h. The cooled mixture was then
*
Correspondence to: Jan Schraml, Institute of Chemical Process Fundamentals of
the ASCR, v. v. i., 16502 Prague 6, Czech Republic. E-mail: schraml@icpf.cas.cz
a Institute of Chemical Process Fundamentals of the ASCR, v. v. i., 16502 Prague 6,
Czech Republic
Magn. Reson. Chem. 2012, 50, 128–134
Copyright © 2012 John Wiley & Sons, Ltd.

Solvent accessible surface and 29Si and 13C chemical shifts
vacuum distilled to yield the corresponding silylated phenols in
95%–99% GC purity.
tube sealed by a septum plug. Molar fraction of the studied
compound in the solutions varied between 1 ꢁ 10-3 and 3 ꢁ 10-2.
The solvents used (chloroform-d and acetone-d6, 99.9% isotopic
purity; Aldrich) were dried over activated molecular sieve and
contained 1% (v/v) of hexamethyldisilane as a secondary internal
Preparation of tert-butyldimethylsilylated phenols
To tert-butyldimethylsilyl chloride (0.084 mol) dissolved in 12 ml
dimethylformamide, the corresponding phenol (0.070 mol) and
imidazole (0.175 mol) were added. The mixture was stirred under
argon at 85 ꢀ–90 ꢀC for 2–3 h. The cooled mixture was extracted
with dry diethyl ether (20 ml) and vacuum distilled to yield
tert-butyldimethylsilylated phenols in a high purity (97% by NMR).
1
reference for the H and 29Si spectra (d(1H) = 0.040 and d(29Si) =
ꢂ19.79, respectively). The 13C chemical shifts were referenced
to the solvent line at d = 76.99 for chloroform-d and d = 30.20 for
acetone-d6. The reported (29Si and 13C) chemical shifts were
obtained by linear extrapolation of the measured values to
infinite dilution.
1H, 13C, and 29Si NMR (one- and two-dimensional) spectral
measurements were performed on a Varian UNITY-200 spectrom-
Preparation of tert-butyldiphenylsilylated phenols following the described
procedure of Stern and Swenton[16]
1
eter (operating at 200.04 MHz for H, at 50.3 MHz for 13C, and at
39.7 MHz for 29Si NMR measurements and using NMR 6.1B
software version) at a temperature of 30 ꢀC. The 29Si NMR spectra
were measured by the INEPT[10] with the pulse sequence
optimized for trimethylsilyl derivatives,[11] that is, for coupling
to nine protons and the coupling constant of 6.5 Hz. Acquisition
(1.0, 2.0, or 4.0 s) was followed by a relaxation delay of 5 or 10 s.
During the acquisition period, WALTZ[17] decoupling was used
and FID data were sampled for the spectral width of 4000 or
8000 Hz. A mild exponential broadening and zero filling to at
least double of acquisition data points was used in data proces-
sing. The 29Si p/2 pulses were at maximum 20 ms long, whereas
tert-Butyldiphenylsilyl chloride (0.024 mol) was added to a solu-
tion of the corresponding phenol (0.093 mol) and imidazol
(0.015 mol) in dry acetonitrile (5 ml).The reaction mixture warmed
noticeably, and a white solid precipitated. The mixture was stirred
for 10 min, and the slurry was poured into diethyl ether (30 ml),
diluted with water (4 ꢁ 20 ml), dried over Na2SO4, and then
concentrated under reduced pressure. The crude product so
obtained was recrystallized from ethanol, yielding the white
crystals in purity sufficient for NMR measurements.
All the studied compounds are listed in Scheme 1. For brevity,
the following abbreviations are used throughout the text and
tables to denote various groups: Me for CH3, Ph for C6H5, and
tBu for C(CH3)3.
1
the H p/2 pulses were 11 ms in a 5-mm switchable probe. The
13C NMR spectra were measured using the spectral width of
16000 Hz. WALTZ[17] decoupling was applied both during acquisi-
tion (1 or 2 s) and relaxation delay (5 or 10 s). Mild line broaden-
ing was used in data processing.
NMR measurements
From each silylated phenol, four to eight solutions were prepared
by weighing the solution components directly into a dry NMR
Most of aromatic carbon chemical shift assignments were
derived from long-range 29Si-13C couplings over Si-O-C link as
SiMe2H series
R
O Si(CH3)2H
R = N(CH3)2, NH2, OCH3, CH3, H, F, Cl, CF3, NO2
R = N(CH3)2, OCH3, CH3, H, F, Cl, CF3, NO2
R = N(CH3)2, OCH3, CH3, H, F, Cl, CF3, NO2
R = N(CH3)2, OCH3, CH3, H, F, Cl, CF3, NO2
4
1
3
2
SiMe3 series
R
O Si(CH3)3
4
1
3
2
SiMe2tBu series
R
O Si(CH3)2C(CH3)3
4
1
3
2
SiPh2tBu series
R
O Si(C6H5)2C(CH3)3
4
1
3
2
Scheme 1. Structures of studied series of compounds
Magn. Reson. Chem. 2012, 50, 128–134
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc

V. Blechta et al.
described earlier,[18] some were assigned by a combination of
signal intensity and multiplicity and additive increments.[19]
When a fluorine-containing substituent was present, the 13C–19F
couplings were used to check the assignment of aromatic
carbon lines.
of C-1 chemical shifts was lower (estimated at ꢃ0.04 ppm)
because of the poor SNR for this quaternary carbon.
The results of 29Si NMR measurements are summarized in
Table 1, along with the values of Hammett s constants[24] used
in the correlations reported here. 13C chemical shifts of the
benzene ring carbons are given in Tables 2 and 3 for chloroform
and acetone solutions, respectively. Tables 4–7 list selected NMR
parameters of the four silyl groups studied.
Calculations of SAS
Using previously optimized geometries[18] (by DFT B3LYP method
and 6-31G** basis set) of compounds with X = H, we have calcu-
lated oxygen atom surface accessible to the solvent by Connolly
program.[20] The calculations used van der Waals atomic radii
recommended by Bondi,[21] and the solvent was represented by
a sphere of radius of 100 pm (for the discussion of the suitable
radius values see ref.[13]). The calculated SAS values are given in
104 pm2 units (Å2).
Hammett-type dependencies of 29Si chemical shifts
The data in Table 8 indicate linear correlations between 29Si
chemical shifts and Hammett substituent constants sp for each
studied silyl group in both solvents. All the correlations are signif-
icant (at 5% significance level) and have positive slopes; they
follow the simplistic and naive notion that when the more
electron-withdrawing substituents are in the para position, the
less shielded should be the nuclei in the terminal groups on the
opposite end of the molecule. Although different intercepts
reflect significantly different shielding values of the silicon nuclei
in the different silyl groups, the slopes differ only little (consider-
ing their statistical errors). In comparison with the data obtained
earlier for neat solutions[1] or concentrated tetrachloromethane
solutions[26] of the SiMe3 series, the slopes for infinite dilution
shifts are smaller. Although the sensitivity to substituent effects
(as measured by the slopes of these dependencies) in acetone
solutions is different for each series of compounds (increasing
in the order SiMe2H < SiPh2tBu < SiMe2tBu < SiMe3), in chloro-
form solutions the sensitivity of 29Si chemical shifts is in all cases
lower. Association with chloroform equalizes the sensitivities in
the series of SiMe3, SiMe2tBu, and SiPh2tBu compounds, leaving
that of the SiMe2H series lower. Association apparently balances
out electronic effects in the three former series of the compounds.
Quantum chemical calculation of substituent and association
effects on chemical shifts
The calculations for several selected compounds and their associ-
ation complexes with chloroform were performed using Gaussian
03 (revision D 02) program.[22] For geometry optimization, B3LYP
and MP2 methods were used with 6-31G** basis set. Atomic basis
set IGLO-III[23] was used for the NMR (B3LYP) calculation. Calcula-
tion of molecular associates started from the optimized struc-
tures of silylated phenols and chloroform put 5 A apart with a full
optimization allowed. The calculations were performed for the
isolated molecules as well as in the solvent, using polarized con-
tinuum model (PCM). Inclusion of the PCM has changed slightly
the calculated values but not affected the trends discussed.
Hammett-type dependencies of 13C chemical shifts of C-1 carbons
Results and Discussion
In this study, we exclusively dealt with infinite dilution shifts of
29Si and 13C nuclei, which were obtained by extrapolation from
the concentration dependencies of the chemical shifts measured
in the range of molar fractions of 1 ꢁ 10-3 to 3 ꢁ 10-2. We do not
intend to go into the details of these concentration dependen-
cies. For our purpose, it suffices to state that usually some four
to eight concentration points were sufficient for the extrapola-
tion. In all cases, the linear extrapolation yielded infinite dilution
shifts with the precision better than ꢃ 0.02 ppm. The precision
The C-1 carbon chemical shifts reported here fit the simplest
linear correlations with Hammett sp (see Table 9), similarly to
the 29Si chemical shifts. Considering the error in the determined
slopes (it is mainly due to poor S/N of C-1 signal in the spectra),
the slope does not depend on the nature of the silyl group, and
the slopes in chloroform solutions are consistently always
somewhat smaller than their counterparts in acetone solutions.
Comparison of the slopes shows that the C-1 chemical shifts
are more sensitive to the substituents R than the chemical shifts
Table 1. 29Si infinite dilution chemical shifts in para substituted silylated phenols 4-R-C6H4-O-SiR′2R00 in chloroform-d and acetone-d6 and Hammett
substituent constants used in correlationsa
SiR′2R00
SiMe2H
Acetone
SiMe3
SiMe2tBu
Acetone
SiPh2tBu
Acetone
R
sp
CDCl3
CDCl3
Acetone
CDCl3
CDCl3
4-NMe2
4-NH2
4-MeO
4-Me
H
ꢂ0.63
ꢂ0.57
ꢂ0.28
ꢂ0.14
0.00
5.087
5.198
5.486
5.008
5.011
6.036
6.076
6.276
7.319
4.436
4.209
5.119
4.764
4.967
6.151
6.367
6.866
7.975
18.834
—
17.893
—
20.060
—
19.587
—
ꢂ7.227
—
ꢂ7.155
—
19.352
19.122
19.363
20.111
20.484
21.121
22.666
18.644
18.546
18.932
19.830
20.383
21.364
22.863
20.567
20.286
20.564
21.286
21.659
22.269
23.827
20.226
20.136
20.470
21.305
21.821
22.666
24.082
ꢂ6.555
ꢂ6.800
ꢂ6.404
ꢂ5.683
ꢂ5.426
ꢂ4.885
ꢂ3.349
ꢂ6.428
ꢂ6.489
ꢂ6.142
ꢂ5.358
ꢂ5.008
ꢂ4.385
ꢂ3.208
4-F
0.06
4-Cl
0.22
4-CF3
4-NO2
0.53
0.81
aChemical shifts in d scale. Values of sp constants were taken from Exner.[24]
wileyonlinelibrary.com/journal/mrc
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Magn. Reson. Chem. 2012, 50, 128–134

Solvent accessible surface and 29Si and 13C chemical shifts
Table 2. 13C infinite dilution chemical shifts of aromatic carbons in para substituted silylated phenols 4-R-C6H4-O-SiR′2R00in chloroform-da
SiR′2R00
SiMe2H
Si Me3
R
C-1
C-2
C-3
C-4
C-1
C-2
C-3
C-4
4-NMe2
4-NH2
4-MeO
4-Me
H
147.08
148.00
149.18
153.13
155.46
151.42
154.09
158.27
161.23
119.74
119.91
119.95
119.06
119.33
120.18
120.57
119.34
119.28
114.61
116.36
114.67
130.00
129.54
115.89
129.48
127.00
125.92
146.23
140.64
154.38
130.84
121.59
157.77
126.59
123.80
142.19
146.75
—
120.46
—
114.47
—
146.09
—
148.89
152.85
155.21
151.13
153.85
158.12
161.18
120.66
119.82
120.10
120.90
121.34
120.10
120.03
114.55
129.89
129.42
115.77
129.37
126.89
125.85
154.27
130.64
121.42
157.73
126.44
123.64
142.10
4-F
4-Cl
4-CF3
4-NO2
SiR′2R00
SiMe2tBu
SiPh2tBu
R
C-1
C-2
C-3
C-4
C-1
C-2
C-3
C-4
4-NMe2
4-MeO
4-Me
H
147.33
149.38
153.33
155.66
151.60
154.33
158.57
161.65
120.38
120.60
119.81
120.11
120.85
121.32
120.12
120.09
114.53
114.49
129.82
129.36
115.69
129.31
126.82
125.82
145.96
154.12
130.42
121.26
157.60
126.23
123.49
142.00
147.51
149.45
153.29
155.58
151.57
154.23
158.35
161.34
119.86
120.13
119.35
119.74
120.43
120.93
119.78
119.95
114.42
114.31
129.63
129.17
115.50
130.02
126.65
125.60
145.61
153.81
130.11
121.01
157.34
125.95
123.23
141.86
4-F
4-Cl
4-CF3
4-NO2
aChemical shifts in d scale.
Table 3. 13C infinite dilution chemical shifts of aromatic carbons in para substituted silylated phenols 4-R-C6H4-O-SiR′2R00 in acetone-d6
a
SiR′2R00
SiMe2H
SiMe3
R
C-1
C-2
C-3
C-4
C-1
C-2
C-3
C-4
4-NMe2
4-NH2
4-MeO
4-Me
H
148.02
147.85
150.33
154.42
156.76
152.93
155.66
159.96
163.34
120.78
120.77
121.07
120.17
120.44
121.62
122.05
120.92
119.29
115.51
116.61
115.88
131.18
130.82
117.06
130.71
128.18
127.14
147.77
143.99
155.89
131.83
122.83
159.00
127.36
124.49
143.00
147.81
—
121.43
—
115.44
—
147.58
—
150.05
154.24
156.53
152.65
155.44
159.80
162.52
121.70
120.95
121.17
122.28
122.77
121.62
121.55
115.70
131.09
130.65
116.89
130.57
128.16
126.93
155.70
131.61
122.58
158.97
127.12
124.22
143.39
4-F
4-Cl
4-CF3
4-NO2
SiR′2R00
SiMe2tBu
SiPh2tBu
R
C-1
C-2
C-3
C-4
C-1
C-2
C-3
C-4
4-NMe2
4-MeO
4-Me
H
148.16
150.46
154.56
156.90
153.08
155.83
160.16
162.84
121.40
121.72
120.94
121.22
122.29
122.83
121.68
121.61
115.40
115.72
131.03
130.65
116.89
130.58
128.17
126.92
147.50
155.69
131.52
122.56
158.84
127.09
124.20
143.36
148.20
150.48
154.56
156.79
153.00
155.70
159.88
162.51
120.86
121.24
120.52
120.81
121.85
122.40
121.32
121.42
115.12
115.52
130.89
130.45
116.73
130.39
127.97
126.72
147.21
155.48
131.43
122.44
158.62
126.97
124.15
143.31
4-F
4-Cl
4-CF3
4-NO2
aChemical shifts in d scale.
of 29Si nuclei, which are two bonds further apart; the correlations
for both nuclei have positive slopes. Association with chloroform
lowers the slopes in 13C correlations relatively less than it does in
the case of 29Si correlations. In naive terms, association affects
transmission of the electronic effects through the oxygen more
than to this atom.
Magn. Reson. Chem. 2012, 50, 128–134
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V. Blechta et al.
a
Table 4. Selected infinite dilution NMR parameters in para substituted silylated phenols 4-R-C6H4-O-SiMe2H in chloroform-d and acetone-d6
R
d(13C-MeSi)
Acetone
d(1H-MeSi)
Acetone
d(1H-HSi)
Acetone
1J(29Si-1H)
Acetone
CDCl3
CDCl3
CDCl3
CDCl3
4-NMe2
4-NH2
4-MeO
4-Me
H
ꢂ1.38
ꢂ1.44
ꢂ1.45
ꢂ1.43
ꢂ1.43
ꢂ1.54
ꢂ1.55
ꢂ1.56
ꢂ1.64
ꢂ0.96
ꢂ0.98
ꢂ1.05
ꢂ1.04
ꢂ1.05
ꢂ1.15
ꢂ1.20
ꢂ1.22
ꢂ1.22
0.32
0.32
0.33
0.34
0.35
0.34
0.35
0.38
0.41
0.28
0.26
0.29
0.30
0.33
0.32
0.34
0.38
0.42
4.89
4.88
4.89
4.92
4.96
4.91
4.92
4.99
5.03
4.84
4.81
4.85
4.88
4.91
4.87
4.89
4.96
4.99
206.9
207.0
207.6
207.5
207.9
207.9
208.4
209.1
210.1
205.3
205.1
206.0
206.3
206.5
207.0
207.4
208.6
209.9
4-F
4-Cl
4-CF3
4-NO2
aChemical shifts in d scales, absolute values of one-bond spin–spin couplings 1J(29Si-1H) in hertz as determined here; all the 1J(29Si-1H) couplings are
assumed to be negative.[25]
reflected also different accessibility of the oxygen and not only
its inherent basicity in steroids.[14]) Obviously, such assumption
does not hold for the compounds studied here. The oxygen
(Lewis) basicity changes with the nature of para substituent R,
and a correlation exists between 29Si chemical shift and relative
basicity in the SiMe3 series of compounds.[2] The increase in
the 29Si shielding is paralleled by the increase of oxygen basicity,
as measured by IR association shift Δn(OH) of phenol.[27] Consid-
ering the symmetry of the SiMe3 group, the SAS of the oxygen
atom in this series remains constant (unless the geometry of
the Si-O-C grouping changes dramatically with substitution in
para position, which we consider rather unlikely) throughout
the series, making thus electronic effect responsible for the
observed changes in 29Si shifts and Δn(OH). Although the other
silyl groups studied have lower symmetry than SiMe3, one can
assume, at least to the first approximation, that the SAS of
oxygen remains the same within each series under study. Then,
comparison of the correlations in Table 8 demonstrates the role
of SAS under these more complex conditions. The slopes in
chloroform solutions are all systematically lower than their
counterparts in acetone solutions. This observation agrees with
the usual picture—hydrogen bond between chloroform and
oxygen atom of the siloxy group attenuates the substituent effect
as experienced by the silicon shielding in chloroform solutions.
The relationship is remarkably simple; the chloroform-induced
decrease in the silicon shift sensitivity to substituent effects
Table 5. Selected infinite dilution NMR parameters in para substituted
silylated phenols 4-R-C6H4-O-SiMe3 in chloroform-d and acetone-d6
a
R
d(13C-MeSi)
Acetone
d(1H-MeSi)
Acetone
CDCl3
CDCl3
4-NMe2
4-MeO
4-Me
H
0.16
0.11
0.18
0.21
0.08
0.10
0.17
0.19
0.58
0.52
0.56
0.57
0.43
0.44
0.47
0.44
0.23
0.24
0.25
0.26
0.25
0.25
0.29
0.33
0.18
0.20
0.21
0.23
0.22
0.24
0.29
0.33
4-F
4-Cl
4-CF3
4-NO2
aChemical shifts in d scales.
The effect of SAS
In the original study[13,14] of the steric effects on 29Si chemical
shifts mediated by the SAS of oxygen of the trimethylsiloxy
group, the effect of SAS was separated from the electronic effects
by restricting the studied compounds to the silylated secondary
steroid alcohols. It was reasonable to assume that the electronic
effects were the same throughout the series and that the oxygen
atoms in all the compounds had the same basicity. (The different
basicities indicated by IR measurements of the association shifts
a
Table 6. Selected infinite dilution chemical shifts in para substituted silylated phenols 4-R-C6H4-O-SiMe2tBu in chloroform-d and acetone-d6
R
d(13C-Me/tBu)
d(13C-C/tBu)
d(13C-MeSi)
d(1H-Me/tBu)
d(1H-MeSi)
CDCl3
Acetone
CDCl3
Acetone
CDCl3
Acetone
CDCl3
Acetone
CDCl3
Acetone
4-NMe2
4-MeO
4-Me
H
25.76
25.71
25.72
25.69
25.65
25.62
25.58
25.50
26.48
25.44
26.41
26.40
26.38
26.35
26.29
26.22
18.18
18.17
18.20
18.20
18.16
18.18
18.21
18.25
19.08
19.09
19.09
19.11
19.10
19.13
19.15
19.18
ꢂ4.46
ꢂ4.50
ꢂ4.46
ꢂ4.43
ꢂ4.54
ꢂ4.52
ꢂ4.44
ꢂ4.40
ꢂ3.93
3.99
0.97
0.98
0.98
0.99
0.98
0.97
0.99
1.00
0.96
0.96
0.97
0.98
0.97
0.97
0.99
1.00
0.16
0.16
0.18
0.20
0.17
0.18
0.22
0.26
0.14
0.16
0.17
0.19
0.19
0.20
0.25
0.29
ꢂ3.96
ꢂ3.95
ꢂ4.07
ꢂ4.05
ꢂ4.02
ꢂ4.04
4-F
4-Cl
4-CF3
4-NO2
aChemical shifts in d scales.
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Solvent accessible surface and 29Si and 13C chemical shifts
a
Table 7. Selected infinite dilution chemical shifts in para substituted silylated phenols 4-R-C6H4-O-SiPh2tBu in chloroform-d and acetone-d6
R
d(13C-Me/tBu)
d(13C-C/tBu)
d(1H-Me/tBu)
CDCl3
Acetone
CDCl3
Acetone
CDCl3
Acetone
4-NMe2
4-MeO
4-Me
H
26.61
26.57
26.55
26.53
26.50
26.46
26.42
26.33
27.28
27.30
27.28
27.25
27.23
27.19
27.13
27.07
19.44
19.43
19.45
19.46
19.42
19.42
19.44
19.43
20.24
20.27
20.28
20.27
20.27
20.27
20.28
20.30
1.09
0.96
1.09
1.10
1.10
1.09
1.11
1.12
1.06
1.07
1.07
1.08
1.08
1.08
1.10
1.11
4-F
4-Cl
4-CF3
4-NO2
aChemical shifts in d scales.
Table 8. Hammett-type correlations of 29Si chemical shifts with sp constants of substituents R in silylated phenols, 4-R-C6H4-O-SiR′2R00, in acetone
and chloroform solutionsa
SiR′2R00
Acetone
Intercept
Chloroform
Intercept
Slope
r (n)
Slope
r (n)
SiMe2H
2.53 ꢃ 0.30
3.56 ꢃ 0.37
3.22 ꢃ 0.35
2.78 ꢃ 0.25
5.65 ꢃ 0.14
19.55 ꢃ 0.16
21.06 ꢃ 0.20
ꢂ5.72 ꢃ 0.15
0.953 (9)
0.969 (8)
0.966 (8)
0.976 (8)
1.41 ꢃ 0.31
2.65 ꢃ 0.38
2.60 ꢃ 0.39
2.64 ꢃ 0.33
5.72 ꢃ 0.14
19.94 ꢃ 0.16
21.13 ꢃ 0.20
ꢂ5.98 ꢃ 0.14
0.862 (9)
0.943 (8)
0.939 (8)
0.956 (8)
b
SiMe3
SiMe2tBu
SiPh2tBu
aCorrelation coefficient, r, and number of data points, n. Slope and intercept in ppm units.bThe earlier published data[1] obtained in concentrated
tetrachloromethane solutions gave slopes 4.66 ꢃ 0.49 and 3.51 ꢃ 0.60 with r = 0.968 and r = 0.911 when NH2 derivative was excluded (n = 8) and
included (n = 9), respectively. sp values of substituents Rs recommended by Exner[24] were used in the calculations.
Table 9. Hammett-type correlations of 13C chemical shifts of C-1 carbon with sp constants of substituents R in silylated phenols, 4-R-C6H4-O-SiR′2R00,
in acetone and chloroform solutionsa
SiR′2R00
Acetone
Intercept
Chloroform
Intercept
Slope
r (n)
Slope
r (n)
SiMe2H
SiMe3
10.7 ꢃ 1.1
10.2 ꢃ 1.3
10.2 ꢃ 1.3
9.9 ꢃ 1.3
154.4 ꢃ 0.5
154.2 ꢃ 0.6
154.5 ꢃ 0.6
154.4 ꢃ 0.6
0.965 (9)
0.953 (8)
0.955 (8)
0.953 (8)
9.5 ꢃ 1.1
9.9 ꢃ 1.4
9.8 ꢃ 1.3
9.5 ꢃ 1.3
153.1 ꢃ 0.5
152.8 ꢃ 0.6
153.3 ꢃ 0.6
153.2 ꢃ 0.6
0.956 (9)
0.948 (8)
0.948 (8)
0.946 (8)
SiMe2tBu
SiPh2tBu
aCorrelation coefficient, r, and number of data points, n. Slope and intercept in ppm units.
(measured as a difference, Δ, of the slopes in acetone and chloro-
form solutions) is linearly related to SAS of the siloxy groups
oxygen (Fig. 1). The larger the SAS, the stronger attenuation of
the substituent effect is observed. It is worth mentioning that also
the correlation coefficients (r) are systematically larger in acetone
than in chloroform solutions, in line with the more complex nature
of substituent effects in the latter associative solvent.
X-C6H4-SiMe2H series (6.7 Hz/s), in agreement with the expected
attenuation of substituent effects by intervening atoms. The
values reported here fit linear dependencies on Hammett con-
stants with even smaller slopes (in CDCl3: J = 2.1069s + 208.04,
r = 0.9775; in acetone J = 3.2159s + 206.9, r = 0.9850), and associ-
ation with chloroform again lowers the sensitivity. In contrast, 1H
chemical shift follow similar trends as discussed previously for
29Si chemical shifts. 1H chemical shifts of SiH both in CDCl3
and acetone solutions depend about twice more strongly
on Hammett s constants compared with their counterparts in
X-C6H4-SiMe2H series (measured in CCl4).[29] Again, in chloroform
solutions, the sensitivity is lower than in acetone solutions. The
dependencies of 1H chemical shifts of SiCH3 groups measured
here both in SiMe2H and in SiMe3 series are stronger in acetone
and weaker in chloroform than in X-C6H4-SiMe2H and X-C6H4-
SiMe3 compounds, respectively, measured in CCl4.[29,30]
The substituent effects on other NMR parameters
It is worth noting that the 1J(29Si-1H) couplings reported here for
the compounds of SiMe2H series are reasonably close to the
values reported for the same compounds in carbon tetrachloride
solutions by Watanabe et al.[28] They found, however, that the
oxygen bridge in X-C6H4-O-SiMe2H lowers the sensitivity of
1J(29Si-1H) to substituent effects to half (3.3 Hz/s) of that in
Magn. Reson. Chem. 2012, 50, 128–134
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V. Blechta et al.
and the Grant Agency of the Academy of Sciences of the
Czech Republic (grant. no. IAA 400720706).
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Figure 1. Relationship between chloroform-induced decrease in the sen-
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Conclusion
If the sensitivity to para substitution is measured by the slopes of
the simplest Hammett-type dependence of the chemical shifts
on sp constants, the 13C chemical shifts of C-1 carbons are more
sensitive to substituent effects than the 29Si chemical shifts of
the Si atom separated by two bonds further away from the
substituent. The sensitivity of 29Si chemical shifts depends on
the nature of the silyl group. An analogous sensitivity of 13C
chemical shifts is obscured by large errors in the determined
slopes, which makes them virtually independent on the nature
of the attached siloxy group. The effect of association with
chloroform on this carbon chemical shift sensitivity is very small
except for SiMe2H-substituted phenols. The same effect on 29Si
chemical shift is linearly related to SAS of the oxygen atom in
the Si-O-C link, making thus the effect in the series of the
SiPh2tBu compounds negligible.
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The authors acknowledge the financial support provided by the
Science Foundation of the Czech Republic (grant no. 203/06/0738)
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Copyright © 2012 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2012, 50, 128–134