Ion pair pKs of some amines: Extension of the computed lithium pK scale

Article abstract of DOI:10.1021/jo202253q

The pK of p-(methylamino)biphenyl, 1, on our Li scale, pK(Li) = 22.09, compared to the cesium scale, pK(Cs) = 28.60. For hexamethyldisilazane, HMDS, pK(Li) = 23.05, pK(Cs) = 29.26. These results are those for the monomers in THF; corrections were made for dimers present in some cases. The pK(Li) of these two amines fit well the previously found correlation with Hartree-Fock calculations at 6-31+g(d) using RLi coordinated with three dimethyl ethers as a computational model for RLi in THF. The results are also compared with earlier pK(Li)s reported from equilibria with lithium amides in which aggregation was not considered.

Full text of DOI:10.1021/jo202253q

Article
pubs.acs.org/joc
Ion Pair pKs of Some Amines: Extension of the Computed Lithium pK
Scale‡
Andrew Streitwieser,* Antonio Facchetti, Linfeng Xie, Xingyue Zhang, and Eric C. Wu
Department of Chemistry, University of CaliforniaBerkeley, Berkeley, California 94720-1460, United States
S
* Supporting Information
ABSTRACT: The pK of p-(methylamino)biphenyl, 1, on our Li scale, pK(Li) =
22.09, compared to the cesium scale, pK(Cs) = 28.60. For hexamethyldisilazane,
HMDS, pK(Li) = 23.05, pK(Cs) = 29.26. These results are those for the monomers
in THF; corrections were made for dimers present in some cases. The pK(Li) of
these two amines fit well the previously found correlation with Hartree−Fock
calculations at 6-31+g(d) using RLi coordinated with three dimethyl ethers as a
computational model for RLi in THF. The results are also compared with earlier
pK(Li)s reported from equilibria with lithium amides in which aggregation was not considered.
Acidity Measurements. The extinction coefficient of the
lithium salt, 1-Li, was determined by deprotonating a known
amount with the lithium salt of 9,9,10-trimethyldihydroan-
thracene (PDDALi, λmax 460.5 nm),3 with the presence of
excess nonequilibrating base indicating complete reaction.
Three individual experiments over the formal concentration
range 5.9 × 10−4 to 3.1 × 10−5 M yielded 34 spectra from which
the contributions of the base were removed to yield the
deconvoluted spectra having λmax = 392 nm independent of
concentration. A plot of absorbance versus path length times 1-
Li concentration (Figure S1, Supporting Information) yields
the slope as the averaged extinction coefficient of 32 000 200
cm−1 M−1.
The pK(Li) was determined using the general double-
indicator technique described previously.4−6 A known amount
of 1 was deprotonated with PDDALi, and 9-methylfluorene (9-
MeFl) was added as an indicator. Equilibration was allowed to
occur over 15 m, and spectra were obtained by successive
dilutions with THF. This experiment was repeated to give six
series totaling 26 spectra covering a formal concentration range
of 1-Li of 5.4 × 10−4 to 3.0 × 10−5 M. Each spectrum was
computationally deconvoluted to give the spectra of the two
lithium salts, 9-MeFlLi and 1-Li. The concentration of 9-
MeFlLi was corrected for dissociation to free ions,5,6 and
stoichiometry gives the corrected acidity equilibrium constant,
Kcorr, eq 1, where the curly brackets6 indicate the formal
concentration.
INTRODUCTION
■
We have recently shown how computed lithium exchange
energies that include specific coordination of solvent to lithium
correlate well with experimental ion pair pK values in THF for
contact ion pair (CIP) monomeric organolithium compounds.1
That correlation, however, contained only two amines from our
own previous work and did not include the results of several
other studies of organolithium pKs in THF in which the
possible role of aggregation was not considered. In the present
paper, we report experimental results for two additional amines
with explicit consideration of the possible role of aggregation
and extend the results to other work in the literature.
p-(Methylamino)biphenyl. The method used successfully
in the past for determining ion pair pKs and aggregation
constants requires substrates having distinctive UV−vis
absorptions, i.e., conjugated systems. Thus, one amine chosen
for this study is p-(methylamino)biphenyl, 1. p-Aminobiphenyl
was converted to the tosylate, mp 156−7 °C (lit.2 160 °C) and
methylated by treating a mixture of 4.69 g (14.5 mmol) and
0.58 g. (14.5 mmol) of NaOH in 100 mL of 50% aq THF with
1.27 mL (14.5 mmol) of dimethyl sulfate. The resulting
precipitate was dissolved with an additional 50 mL of THF.
After stirring for 24 h, 5 mL of 10% NaOH was added and the
mixture was refluxed for 30 m. The cooled solution was
extracted with ether. After drying, the ether was removed and
the cream solid was recrystallized from 100 mL of ether and 25
mL of THF to give yellow needles, mp 127−8 °C (lit.2 128
°C). This tosylate was hydrolyzed by refluxing 2.23 g (6.6
mmol) with 6.6 mmol of perchloric acid and 11 g of acetic acid
for 3 h. The cooled solution was neutralized with aq sodium
carbonate and extracted with ether. The organic layer was
washed with aq NaOH and dried over sodium sulfate, and the
solvent was removed in vacuo. The product was purified by
alumina column chromatography and sublimed twice using
ice−water coolant to give 1.09 g (90% yield), mp 38 °C (lit.2 38
°C).
{1‐Li}[9‐MeFl]
[1][9‐MeFl]
K
=
corr
(1)
The results are detailed in Table S1 (Supporting
Information) and give an average pK(Li) of 22.09 0.04 on
our pK(Li) scale.7 The constancy of this pK with concentration
indicates that 1-Li is monomeric; more exactly the amount of
Received: November 1, 2011
Published: December 25, 2011
© 2011 American Chemical Society
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dimer present is too small to be detected, and therefore K1,2
<
were recorded. The contribution of the cesium salt of the
indicator was determined by the spectra in a high wavelength
region where 1-Cs has no absorbance. This component was
subtracted from the total spectrum to give the spectrum of 1-
Cs. These deconvoluted spectra showed λmax varying over the
range 406−413 nm. From the spectra and stoichiometry, the
concentrations of all four components were determined and
give the Kobs at each point (Table S3, Supporting Information).
Plots of {1-Cs}/Kobs give straight lines indicative of monomer−
dimer equilibria. The intercept and slope give K0 and the
dimerization constant, K1,2, for each experiment (eq 3).10,13 K0
together with the pK of the indicator gives the pK(Cs) of 1.
The indicator 9-PX gives pK(Cs) = 28.58, Kd = 1260. Three
experiments with 9-PSX give pK(Cs) = 28.63, 28.61, 28.61 and
Kd = 897, 881, and 1039 (Figures S3−S6, Supporting
Information). These values are in agreement with the SVD
results. We take the average values as pK(Cs) = 28.60 0.02,
Kd = 1010 100.
102 M−1. This result agrees with the value of K1,2 = ca. 70 M−1
found for the lithium salt of N-methylaniline in THF at 17 °C
by Jackman and Scarmoutzos.8 For K1,2 = 100 M−1, at a
nominal concentration of 0.01 M, the concentrations of
monomer and dimer are equal, but 2/3 of the amide units
are present in the dimer. At a nominal concentration of 10−4 M,
typical of UV−vis spectroscopy, the amide is almost all
monomer, whereas at 10−1 − 1 M, typical of NMR or
preparative methods, the compound is present mostly as dimer.
The extinction coefficient of 1-Cs was determined by
deprotonating a known quantity of 1 with cumylcesium9 in
the presence of a nonequilibrating indicator of higher acidity,
the spectral appearance of whose anion signifies full conversion
of 1 to 1-Cs. Over the formal concentration range studied (6.3
× 10−4 to 5.4 × 10−5 M) a spectral shift was noted for the
deconvoluted 1-Cs (λmax 403−407.5 nm), indicating the
presence of more than one component, although the range
was too small to distinguish a clear isosbestic point. The
extinction coefficient found at λmax is 31200 200 (Figure S2
and Table S2, Supporting Information). The spectral data for
the first two runs (mj010 and mj030) were subjected to
singular value decomposition (SVD) analysis.10 Assuming a
monomer−dimer (M-D) equilibrium, the first two vectors gave
the M-D spectra in Figure 1. Results from the two runs show
2
−
+
K
= K + 2K K {R Cs }/K
0 d 0 obs
(3)
obs
HMDS. The second amine in this study is hexamethyldisi-
lazane (HMDS), an important amine in synthetic chemistry
whose lithium salt (LiHMDS) has been extensively studied
with respect to solvation, aggregation, and mixed aggrega-
tion.14−18 Its pK(Li), however, has not been carefully measured.
In this work, both the lithium and cesium (CsHMDS) salts
were studied. These salts have no usable UV spectra and their
pKs were determined by the single indicator technique.3,19
Three indicators were used with triply sublimed LiHMDS:20
2,3-benzofluorene (2,3-BF, pK(Li) 22.95, λmax 430 nm (25500
cm−1 M−1), 9-isopropylidenefulvene (IPF pK(Li) 22.33, λmax
379 nm (27700 cm−1 M−1), and 9-methylfluorene (9-MeFl
pK(Li) 22.46, λmax 387 nm (16200 cm−1 M−1).7 To a known
amount of LiHMDS in THF was added a known amount of
indicator solution, and the mixture was allowed to come to
equilibrium. From the indicator absorbance at equilibrium, the
equilibrium constant could be calculated. THF was added to
dilute the mixture which was then allowed to reach the new
equilibrium. Such dilutions were repeated to give a series of
about six measurements in each experiment. Unfortunately,
equilibration was so slow (about 8−10 h for 2,3-BF, 45−48 h
for IPF and >50 h for 9-MeFl) that adventitious quenching
occurred. With 2,3-BF the absorbance reached a maximum after
8−10 h and then decreased linearly for 10−35 h until it reached
a plateau. Extrapolation of the linear decrease to zero time
showed quenching of about 1% for the absorbance maximum of
2,3-BF and about 6−7% after 35−40 h. Since IPF and 9-MeFl
required these long periods to reach equilibrium, the observed
absorbances were corrected for these quenching factors.
Nevertheless, the results for 9-MeFl differed so greatly from
the other two indicators that its experiments were discarded.
The results with 2,3-BF and IPF are detailed in Table S4
(Supporting Information) and Figure 2. Each individual
experiment shows a variation of less than 0.1 pK units. The
relative constancy of pK with concentration indicates that
LiHMDS is monomeric in these solutions. Kimura and
Brown14 have reported the monomer−dimer equilibrium
constants for LiHMDS in THF at low temperatures.
Extrapolating their results to room temperature indicates that
at our concentrations (∼10−3 M), less than 1% of the LiHMDS
is present as the dimer, an amount too small for us to observe.
Figure 1. Monomer and dimer spectra of 1-Cs derived from SVD
analysis of runs mj010 (green, λmax 399 and 410 nm) and mj030 (red,
λmax 397 and 409 nm.
good consistency. These spectra were used to determine the M
and D concentrations and the equilibrium constant for
dimerization (eq 2), K1,2 = 987 46 (Table S2, Supporting
Information).
[dimer]
[monomer]2
K1,2
=
(2)
Acidity measurements were made with two indicators, 9-
phenylxanthene (9-PX, λmax 491 nm (17600 cm−1 M−1),
pK(Cs) 28.73)11 and 9-phenylthioxanthene (9-PSX, λmax 445
nm (11 700 cm−1 M−1), pK(Cs) 27.88).12 A known amount of a
mixture of amine and indicator was partially deprotonated by
successive additions of cumylcesium in THF, and the spectra
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Figure 3. Correlation of pK(Cs) and pK(Li) for four amines. The
regression line is 4.08 1.83 + (1.000 0.093)x; R2 = 0.986. This
correlation is not expected to be general for other cesium amides as
discussed in the text.
Figure 2. Equilibration experiments of LiHMDS with two indicators,
2,3-BF (circles) and IPF (squares). The variations found with respect
to concentration are a measure of experimental error. If aggregation
were significant, the lines would have opposite slope.
excellent correlation in Figure 3 is deceptive and would not be
expected to apply generally to other amines.
The pK(Li) found for HMDS is 23.05
0.13 (Table S4,
Ab Initio Modeling. We next inquire as to how well these
new lithium pK data fit the previously established correlation of
a variety of pK(Li) results (fluorinated benzenes, dithianes,
carboxamides, ketones, etc.) with ab initio computations.1 That
correlation was based on eq 4, an isodesmic-type equation with
solvation considered by explicit coordination of solvent with
lithium. THF was modeled by dimethyl ether (E).
Supporting Information).
For CsHMDS two indicators were used, 9-PX and 9-phenyl-
10,10-dimethyldihydroanthracene (PDDA, pK(Cs) = 28.11).11
The results are summarized in Table S5 (Supporting
Information). Note that the RCs equilibria are rapid and
quenching is not a problem. The pK results are independent of
concentration indicative of a monomer at these concentrations.
The average pK(Cs) from four experiments is 29.26 0.09.
ΔE
(4)
RH + PhLi·3E = RLi·3E + PhH
RESULTS AND DISCUSSION
The pK(Li) and pK(Cs) determined in the present work are
summarized in Table 1 and compared to the two amines from
The theory used was the Hartree−Fock energy plus the zero
point energy (ZPE) with the 6-31+g(d) basis set. Note that this
was the best of several other theory levels tested.1 The
regression correlation of 23 pKs of lithium contact ion pairs of
fluorinated benzenes, carboxamides, sulfur compounds, ke-
tones, and two amines is given by eq 5.
■
Table 1. pKs of Amines Determined in THF
amine
pK Li
pK Cs
diphenylamine10
19.05
13.48
22.09
23.05
24.20
19.24
28.60
29.26
pK(Li) = 39.88 0.62 + (0.654 0.019)ΔE
carbazole21
R2 = 0.983
(5)
p-(methylamino)biphenyl, 1
HMDS
The new computation results are summarized in Table 2
with full details in the Supporting Information. With the two
new amine pKs established in the present work, the correlation
becomes that shown in Figure 4. The new points are the two
blue circles indicated. The new points clearly fit well such that
no significant change in the regression correlation is required.
Extension to Other Amines. There are relatively few
publications concerned with the effective pKs of organolithium
compounds in THF despite the widespread use of these
reagents. Moreover, most of these few papers date to the 1980s,
a period when there was less concern with the complications
introduced by aggregation. Moreover, much of this work was
done by NMR methods at relatively high concentrations in
which aggregation would be exacerbated. With our present
our previous studies. As expected, the Li CIP are tighter than
the corresponding Cs CIP and lead to lower effective pK values.
The two sets of pKs are linearly related as shown in Figure 3.
Such correlations are not uncommon; for example, the Cs and
Li pKs of enolates are also linearly related.6 The present case,
however, is an instructive example of a false correlation. The
slope of almost unity is determined primarily by the extreme
point of carbazole. We would expect a slope similar to the N-M
bond ratios6 and, indeed, the three points other than carbazole
give a steeper slope. Our study of the cesium and lithium salts
of carbazole showed that for the cesium salt π-bonding of Cs+
to the ring is competitive with σ-bonding to nitrogen.21 We
would not expect, therefore, that cesium carbazide would fit a
correlation given by normal cesium amides. The apparently
ability to compute such pKs with a precision of better than
1
pK unit, we are now in a position to reevaluate some of this
literature. For example, in 1986, Ahlbrecht and Schneider22
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a
Table 2. Computation Results at HF 6-31+G* for Amines and Coordinated Lithium Amides
amine
RH, E+ZPE, au
RLi.3E, E+ZPE, au
ΔE, eq 4 kcal mol−1
pK(Li), exptl
pK(Li), eq 5
1
−554.080958
−870.430163
−134.143025
−173.150914
−212.156645
−251.160375
−290.168822
−285.859436
−250.022586
−328.040082
−406.040599
−521.917079
−329.170680
−1022.979887
−1339.322225
−603.018469
−642.022981
−681.025226
−720.030576
−759.033998
−754.737285
−718.896682
−796.906875
−28.092
−29.877
−14.236
−13.912
−10.830
−12.614
−6.834
−16.078
−13.809
−8.563
22.1
23.1
21.55
24.37
31.19
32.58
34.01
33.35
35.41
30.21
31.75
34.75
35.53
35.46
34.99
HMDS
b
dimethylamine
29.7
b
methylethylamine
methylisopropylamine
methyl-tert-butylamine
diisopropylamine
30.9
b
31.7
b
32.4
b
34.4
b
morpholine
28.8
b
piperidine
30.7
b
2,6-dimethylpiperidine
2,2,6,6-tetramethylpiperidine
dicyclohexylamine
isopropyl-tert-butylamine
33.6
c
b
−874.905474
−990.782134
−11.258
−12.524
−12.094
37.9
b
34.7
c
b
−798.036867
36.3
a
b
c
Only the first two pK(Li) values were determined in the present work. Reference 22. Optimization of RLi.3E gave a structure with one long Li−O
bond (4.9 Å). This amide probably has no more than two coordinated ethers.
measured a number of pK(Li) values for amines. They used an
NMR method at concentrations of 0.4 M with a scale based on
our value for the pK of the cesium salt of diphenylmethane. This
reference choice is probably satisfactory. The relative pKs of
solvent separated ion pair (SSIP) lithium salts are similar to
those of the CIP cesium salts,7 and diphenylmethylithium is
known to be a mixture of comparable amounts of CIP and SSIP
in THF at room temperature.23 Thus, the pK(Li) that would be
an appropriate reference is probably numerically similar to the
corresponding pK(Cs). A closer comparison is with triphenyl-
methane, Ph3CH. Ahlbrecht and Schneider derived pK = 30.4
for the lithium salt which they compared to the pK(Cs) = 31.0
that we had reported in 1983.24 Later,25 we were able to obtain
a direct measurement of pK(Li) = 31.0 for Ph3CH on the
present scale, a value sufficiently close to theirs.
A more important problem is their assumption that the
lithium amides are monomeric since it is now well established
that lithium diisopropylamide26 (LDA), lithium isopropylcy-
clohexylamide,27 and lithium diethylamide28 are mostly dimeric
in typical THF solutions. Even the highly hindered lithium
tetramethylpiperidide29−31 is substantially dimeric in THF
solution. It seems likely that all of the lithium amides measured
by Ahlbrecht and Schneider are extensively or wholly
aggregated and that therefore their experimental pKs cannot
be directly compared to our theoretical values, which are
applicable only to monomers.
Figure 4. Plot of experimental pK(Li) for various monomeric RLi vs
the HF energy for eq 4. This figure adds the two amines of the present
study to the correlation reported previously in ref 1. The regression
line shown excludes tetralones. The line shown is pK(Li) = 39.76
0.61 + (0.651
insignificantly from eq 5.
0.019)x; R2 = 0.981. This correlation differs
Table 3. Computation Results at HF 6-31+G* for Some Fraser Compounds
compound
methyldithiane
RH, E+ZPE au
RLi.3E, E+ZPE au
ΔE, eq 4kcal mol−1
pK(Li), Fraser
pK(Li), eq 5
ab
,
−990.034949
−551.224531
−344.450248
−667.114353
−951.028449
−228.557241
−247.728872
−263.747731
−341.701147
−381.172526
−1458.903216
−1020.102244
−813.311361
−1135.980474
−1419.906442
−697.431381
−716.591297
−732.6276082
−810.566037
−850.052610
−8.773
−14.672
−4.284
35.7
33.3
39.0
38.6
34.8
35.6
39.5
33.7
37.0
33.2
35.9
34.14
30.28
37.08
35.02
30.15
31.73
36.54
29.37
35.53
29.29
32.14
c
thiophene
c
anisole
c
thioanisole
−7.426
a
d
d
d
d
d
dithiane
−14.876
−12.458
−5.107
−16.059
−6.654
furan
N-methylpyrrole
N-methylimidazole
N-(dimethylamino)pyrrole
benzofuran
−16.188
−11.830
de
,
N-methylpyrazole
−263.729205
−732.602344
a
b
c
d
e
Reference 34. Assumed reference value. Reference 36. Reference 38. Value for N-(n-propyl)pyrazine.
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A comparison is made in Table 2 between the reported
experimental and computed pKs. Computations were made at
the HF 6-31+g(d) level for the amines studied by Albrecht and
Schneider22 and the corresponding lithium amides coordinated
to three dimethyl ethers for application of eqs 4 and 5. For
amines such as diisopropylamine that can exist in several
conformations, only the most stable conformation was used.
Computational results are summarized in Table 2, and
complete details are given in the Supporting Information.
The effective pK of an aggregated lithium salt is always lower
than that of the monomer. In Table 2 it is shown that most of
the “experimental” pKs determined by Ahlbrecht and Schneider
are indeed lower than the computed values, but not by much.
This comparison suggests that the aggregation equilibrium
constants are not largeno more than a few powers of ten.
The several amines whose experimental pKs are higher than the
computed values are all highly hindered amines. It seems likely
in these cases that transmetalaton is slow and equilibrium was
not reached. Such behavior was recorded in some of our
experimental results discussed above.
In the 1980s, Fraser et al. published a number of pK(Li)
values for a variety of compounds in a series of generally short
communications with a paucity of experimental details.32−39
They generally used NMR measurements with 1 M solutions in
THF at room temperature, but Ahlbrecht and Schneider
noted22 that in several cases where both groups examined the
same compounds the results disagreed. For example, the Fraser
group32 reported a pK(Li) difference between diisopropyl-
amine and tetramethylpiperidine of 1.6 compared to Ahlbrecht
and Schneider’s 2.6. One important difference is the choice of
reference system. The Fraser group chose our cesium pK = 37.8
of methyldithiane in cyclohexylamine40 as the reference. This
salt is undoubtedly a CIP, and the lithium salt is expected to be
a tighter ion pair corresponding to a lower pK. The calculated
value using eq 5 is 34.14 (Table 3). One direct comparison is
provided by triphenylmethane where Fraser’s pK of 32.9 is two
units higher than our 31.0 (vide supra). A further indication
that Fraser’s scale is about 2−3 units too high compared to our
present scale is their reported value for HMDS, 25.8,37
compared to our value of 23.1 reported in this paper. They
reported pK(Li)s for several other amines whose probable
aggregation affects limit their generality. These amides were
then used to determine lithiation equilibria with other
compounds whose lithium salts are probably monomeric.
Because the lithium amides were used in approximately the
same concentrations throughout, any aggregation effects
probably largely cancel in these applications and the relative
lithium acidities obtained are probably not unreasonable.
In Table 3 we compare some of the Fraser nonamine pKs
with those calculated using the correlation in eq 5. Many of
their values are within 2−4 pK units of the calculated pKs, with
an average difference of 3.2, indicative that the major difference
is the choice of reference. The comparison is shown more
clearly in Figure 5. The result shows a respectable correlation
with two outlying points (N-(dimethylamino)pyrrole and
thiophene), but the slope of 0.76 suggests that the Fraser
scale (which covers only 6 pK units) is somewhat compressed
relative to ours, perhaps because of some systematic error
stemming from use of lithium amide aggregates.
Figure 5. pK(Li) reported by Fraser for compounds in Table 3
compared to calculated values from eq 5. The regression line without
the two outlying points (blue) is (0.763 0.048)x + 11.37 1.57; R2
= 0.973.
coincident with those obtained from Fraser...”42, but unfortu-
nately no experimental data are reported to permit a
quantitative evaluation of this generalization. They are primarily
interested in α-oxy carbanions, and none of the compounds
studied are on our pK(Li) scale. Thus, extension of our
theoretical treatment to their results would be a separate study.
CONCLUSIONS
■
Earlier measurements of basicities of lithium amides often have
only qualitative significance because account was not taken of
the now known aggregation of many such amides. Under
appropriate conditions, such lithium amides can still be used in
metalation equilibria to give useful information. Organolithium
structures with coordinated dimethyl ethers are useful models
for ab initio computations of relative stabilities in THF
solutions.
ASSOCIATED CONTENT
* Supporting Information
■
S
Tables of experimental and computational results. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: astreit@berkeley.edu.
■
ACKNOWLEDGMENTS
■
This work was supported in part by a Dreyfus Senior Mentor
grant. We also thank Dr. Kathleen Durkin and Dr. Jamin
Krinsky of the Molecular Graphics and Computational Facility
(supported in part by NSF grants CHE-0233882 and CHE-
0840505), College of Chemistry, UC Berkeley, for their
assistance. We also thank Michael J. Ross for important
experiments, Terry Singhapricha, Steven Vu, Kamesh Shah, and
More recently, Sardina et al.41−43 have used organotin−
lithium exchange equilibria as quantitative measures of
organolithium stabilization. They used Fraser’s pK(Li) as
reference and report that with one exception, “our data were
Julius Reyes for preliminary computations and Thorsten Muh
̈
for experimental assistance.
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(34) Fraser, R. R.; Bresse, M.; Mansour, T. S. J. Chem. Soc., Chem.
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