On the existence of the elusive monomethyl ester of carbonic acid [CH 3OC(O)OH] at 300 K: 1H- and 13C NMR measurements and DFT calculations

Article abstract of DOI:10.1002/ejic.200500622

The elusive monomethyl ester of carbonic acid [CH₃OC(O)-OH] has been prepared at 300 K by protonation of the sodium salt NaOC(O)OCH₃ with anhydrous HCl or water and characterized by ¹H- and ¹³C NMR spectroscopy. The stability of the acid and its reactivity towards hydroxo ions and methylating agents under ambient conditions are discussed. The energetics and the mechanism of the investigated reactions are examined on the basis of density functional calculations. For kinetic and thermodynamic reasons CH₃OC(O)OH is unlikely to be formed by insertion of CO₂ into the O-H bond of methanol. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200500622

FULL PAPER
DOI: 10.1002/ejic.200500622
On the Existence of the Elusive Monomethyl Ester of Carbonic Acid
1
13
[
CH OC(O)OH] at 300 K: H- and C NMR Measurements and DFT
3
Calculations
[
a]
[a]
[a]
[a]
Angela Dibenedetto,* Michele Aresta, Potenzo Giannoccaro, Carlo Pastore,
[b]
[b]
Imre Pápai,* and Gábor Schubert
Keywords: Carbon dioxide / Density functional calculations / Methylation / NMR spectroscopy
The elusive monomethyl ester of carbonic acid [CH
OH] has been prepared at 300 K by protonation of the so-
dium salt NaOC(O)OCH with anhydrous HCl or water and
characterized by 1H- and 13C NMR spectroscopy. The sta-
bility of the acid and its reactivity towards hydroxo ions and
methylating agents under ambient conditions are discussed.
The energetics and the mechanism of the investigated reac-
3
OC(O)-
tions are examined on the basis of density functional calcula-
tions. For kinetic and thermodynamic reasons CH OC(O)OH
into the O–H
3
3
is unlikely to be formed by insertion of CO
bond of methanol.
2
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
Introduction
about its stability/reactivity are available. Conversely, the
salts of 1 [MHCO , M CO , MЈCO ; M = group 1 metal
3
2
3
3
Carbonic acid (H CO , 1) and its amino [RRЈNC(O)OH,
2
3
or ammonia; MЈ = group 2 metal], 2 [RRЈNC(O)ORЈЈ; RЈЈ
ammonium or metal cation], and 3 [ROC(O)OM] or the
2] and monoalkyl derivatives [ROC(O)OH, 3] are elusive
=
species. Their isolation as pure compounds is not trivial as
they easily decompose to afford CO plus water [Equa-
tion (1)], amine [Equation (2)], or alcohol [Equation (3)].
organic esters of 2 and 3 (R, RЈ, RЈЈ = alkyl or aryl group)
have been known for a long time to be stable compounds
at room temperature.
2
Despite their instability, compounds 1–3 are often pro-
posed to be intermediates in reactions in which carbon di-
oxide is implied. Thus, 2 has been suggested to exist in solu-
[
4]
tions containing amines and carbon dioxide and 3 has
been proposed to be formed by direct interaction of
[
5]
alcohols and CO2, a reaction relevant to the synthesis of
organic carbonates by direct carboxylation of alcohols that
occurs at temperatures ranging from 330 to 450 K and at a
[
7]
CO pressure of 0.1–20 MPa.
2
In the course of our research on carbon dioxide chemis-
try, we have synthesized and characterized the first exam-
In particular, 1 has never been isolated pure (only its
water solutions are known), and 2 was only very recently
isolated and characterized as a dimer in the solid state. The
X-ray structure shows the existence of H-bonding through
the carboxylic moieties [R = C H CH , RЈ = H; or R,RЈ =
[
1,2]
ples of the elusive species 2.
We wish to report here the
first evidence of the existence of CH OC(O)OH at room
3
temperature and describe its reactivity. Density functional
calculations have also been carried out to provide a ration-
ale for our experimental findings.
6
5
2
[
1,2]
PhP(OCH CH ) ].
For 3 only a report of its low-tem-
2
2 2
[
3]
perature IR spectrum can be found in the literature; no
report of its existence at room temperature or information
[
a] Department of Chemistry, University of Bari and CIRCC,
Via Celso Ulpiani 27, 70126 Bari, Italy
Fax: +39-080-544-2430
Results and Discussion
E-mail: a.dibenedetto@chimica.uniba.it
b] Chemical Research Center, Hungarian Academy of Sciences,
P.O.B. 17, 1525 Budapest, Hungary
Although it has been suggested that 3 could be formed
[
[
5]
1
13
by direct reaction of ROH with CO
, our H- and
C
2
Fax: +36-1-325-7554
NMR measurements run on pressurized (5 MPa) solutions
E-mail: papai@chemres.hu
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
of CO in pure, freshly distilled, and absolutely anhydrous
2
methanol did not give any evidence of CO insertion into
2
908
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 908–913

The Monomethyl Ester of Carbonic Acid
FULL PAPER
the O–H bond of methanol to afford CH OC(O)OH [4;
Equation (4)].
It is worthwhile mentioning that previous theoretical
3
[
8–11]
studies
from H O and CO , a reaction that is analogous to Equa-
on the mechanism of carbonic acid formation
2
2
tion (4), have excluded the possibility of a direct bimolecu-
lar insertion because the activation energy predicted for this
–
1 [8–10]
step is about 50 kcalmol ,
while the energy barrier de-
–1 [8]
creases substantially (to about 15 kcalmol ) if an ad-
One can argue that reaction 4 may either occur through ditional water molecule is assumed to participate in the hy-
a direct bimolecular insertion [Equation (4)] or through the dration process. Our calculations carried out at the B3LYP/
–
generation of a CH O anion by autoprotolysis of methanol 6-311++G** level of density functional theory for
3
[
Equation (5a)], which may attack the electrophilic carbon CH OC(O)OH formation involving two CH OH molecules
3
3
–
of CO with formation of CH OC(O)O [Equation (5b)]. (see Figure 1) predict a relatively high energy barrier for
2
3
The protonation of this latter species would afford 4 [Equa- this process (∆G‡ = 28.9 and ∆G‡
= 22.7 kcalmol if
–1
gas
solv
tion (5c)].
solvent effects due to the methanol environment are in-
cluded), indicating that reaction 4 is likely to be kinetically
hindered. Furthermore, CH OC(O)OH is found to be ther-
3
modynamically unstable with respect to its dissociation
products, since the calculations give ∆Ggas = +9.9 kcalmol–1
for the gas-phase reaction shown in Figure 1, which de-
creases to ∆Gsolv = +4.4 kcalmol–1 in the solvated model
[
for the isolated CH OC(O)OH molecule the calculated
3
dissociation free-energy values are ∆G
= +15.1 and
gas
–1
∆
Gsolv = +11.6 kcalmol ].
Accordingly, 4 has been reported to be a labile species
and has only been characterized so far by IR spectroscopy
[
3]
–
at low temperature. Therefore, it is not likely that it will
exist for a long time at temperatures above 273 K. These
The abundance of CH O is very low in pure anhydrous
3
methanol as it is generated by the autoprotolytic dissoci-
ation of methanol [Equation (5)], which has an equilibrium
results suggest that the direct insertion of CO into the O–H
2
–
16 [6]
bond of methanol is quite unlikely to occur under reaction
conditions such as those used for the synthesis of organic
constant, K , of 10
.
The lack of formation of 4 under
dis
these conditions can be explained by taking into consider-
ation the thermodynamics of reaction 4 and its kinetic as-
pects, as discussed below.
[
7]
carbonates, and CH OC(O)OH is unlikely to be an inter-
3
mediate in such a reaction.
3 2
Figure 1. Formation of CH OC(O)OH from CO and two molecules of methanol. Selected bond lengths obtained from B3LYP/6-
311++G** calculations are given in Ångstroms.
Eur. J. Inorg. Chem. 2006, 908–913
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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909

A. Dibenedetto, M. Aresta, P. Giannoccaro, C. Pastore, I. Pápai, G. Schubert
FULL PAPER
13
We generated 4 by treating the sodium salt NaO C(O)
OCH (5) with either anhydrous HCl/CD Cl or water in a
3
2
2
1
3
biphasic system. NaO C(O)OCH is easily formed by reac-
tion of sodium methoxide with CO [Equation (6)].
3
1
3
2
Compound 5 is a stable, white solid that has already been
[
12]
described.
Its IR spectrum shows strong bands at 1631
–
1
and 1381 cm for the νC=O absorption of the carbonate
moiety. The reaction of 5 with anhydrous HCl/CD Cl or
2
2
1
water was monitored by carrying out a detailed NMR ( H
1
3
13
and C) study. CO was used in the synthesis of 5 in
2
1
3
order to reduce the acquisition time of the C NMR spec-
trum, thereby minimizing the risk of decomposition of 4.
When 5 was suspended in CD Cl under 0.1 MPa of
2
2
1
3
1
13
CO , the H- and C NMR spectra of the solution did
2
not show any signal that could be attributed to 5 due to its
very poor solubility in the solvent. The addition of a sub-
1
Figure 2. Evolution of the H NMR spectrum of CH
formed upon addition of (A) HCl/CD Cl and (B) H
OC(O)ONa suspended in CD Cl . i) soon after the addition;
ii) after 10 min; iii) after 60 min. ᭿ CH OC(O)OH; ᮀ CH OC(O)
OH; ᭹ CH OH; ᭺ CH OH.
3
OC(O)OH
2
2
2
O to
stoichiometric amount of either anhydrous HCl/CD Cl or CH
3
2
2
2
2
water to the suspension at 295 K caused the immediate ap-
3
3
1
13
3
3
pearance of new signals in the H- and C NMR spectra.
1
The H NMR spectrum shows a singlet (1 H) at δ = 4.76
and a second signal at δ = 3.38 ppm (3 H; Table 1), which
the signal of free CO generated by conversion of 4 is found
2
1
3
are attributed to the CD Cl -soluble HO C(O)OCH spe-
2
2
3
at δ = 121.4 ppm. Conversely, when water is used a second
cies formed according to reaction 7.
13
signal appears at δ = 159.62 ppm in the C NMR spectrum
besides the signal at δ = 159.92 ppm that is attributed to
3
Table 1. Spectroscopic properties of labile CH OC(O)OH.
13
NaH CO . In the latter case, the formation of methanol
3
Compound
CH OC(O)OH 1779
730
νCO
νOH 1H NMR (δ, ppm) 13C NMR (δ, ppm)
3484 4.76 (1 H, OH) 50.58 (CH
3.38 (3 H, CH 159.92 (C=O)
13
and NaH CO can occur according to two mechanisms:
3
13
3
3
)
i) decomposition of CH O C(O)OH into methanol and
3
1
3
)
13
CO2 [Equation (8a)] and subsequent reaction [Equa-
tion (8b)] of 13CO with the NaOH formed in reaction 7.
2
1
3
OH + 13CO
CH
3
O C(O)OH Ǟ CH
3
2
(8a)
(8b)
13CO
+ NaOH Ǟ NaH CO
3
13
2
–
ii) direct interaction of the hydroxy moiety OH with
1
3
CH O C(O)OH [Equation (9a)] by attack at the methyl
3
The 13C NMR spectrum confirms the formation of 4, as group with formation of methanol and concurrent forma-
demonstrated by the appearance of a signal at δ = tion of NaH CO [Equation (9b)].
13
3
1
59.92 ppm due to the carbon atom of the hemicarbonate
1
3
–
13
–
CH
3
O C(O)OH + OH Ǟ CH
3
OH + H CO
3
(9a)
(9b)
moiety. Continuing the addition of anhydrous HCl up to a
stoichiometric amount produced an increase of the inten-
sity of the signals mentioned above (Figure 2a). The ad-
dition of water produces broader peaks (Figure 2b), al-
though they are located at the same positions.
13
–
+ Na+ Ǟ NaH CO
13
H CO
3
3
The energy barrier obtained for the decomposition of
CH OC(O)OH via the reverse reaction shown in Figure 1
3
If the NMR tube is left under the same conditions after is ∆G‡solv = 18.3 kcalmol (∆G‡gas = 19.0 kcalmol ),
–1
–1
the addition of the HCl/CD Cl solution to NaO(O) whereas the calculations carried out for the methyl-transfer
2
2
1
13
–
COCH at 295 K, the H- and C NMR spectra change: reaction between CH OC(O)OH and OH (see Figure 3)
the signals due to 4 slowly disappear while the signals of give a much smaller barrier (∆G
CH OH [δ = 4.81 (1 H) and 3.42 ppm (3 H)] appear (Fig- 4.4 kcalmol ). Moreover, the S 2-type nucleophile substi-
ure 2). When CH O C(O)OH is generated using sub-stoi- tution depicted in Figure 3 is predicted to be highly exer-
3
3
‡
= 11.3 and ∆G‡
=
solv
gas
–1
3
N
1
3
3
–1
chiometric amounts of anhydrous HCl/CD Cl it has a gonic (∆Gsolv = –39.4 and ∆Ggas = –49.5 kcalmol ). These
2
2
longer lifetime than when it is generated with water. There- data indicate that reaction 9a is both kinetically and ther-
fore aged solutions of 4 show a different behavior according modynamically more feasible than the direct decomposition
to their preparation. In fact, when anhydrous HCl is used of CH OC(O)OH. This agrees with the experimental obser-
3
910
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 908–913

The Monomethyl Ester of Carbonic Acid
FULL PAPER
vation of the stability of 4 in the absence and presence of
hydroxy ions.
3
Scheme 1. Conformations of CH OC(O)OH.
agent. The interest of reaction 10 lies in the fact that the
product is an organic carbonate.
CH
3
OC(O)OH + SubOR Ǟ CH
3
OC(O)OR + SubOH
(10)
Figure 3. Stationary points located for the methyl-transfer reaction
If CH OC(O)OH could be formed from methanol and
3
and CH OH. Se- CO , reaction 10 would be an effective way to produce
3
2
OC(O)OH to OH– forming HCO
–
from CH
3
3
lected bond lengths obtained from B3LYP/6-311++G** calcula-
tions are given in Ångstroms.
DMC, a chemical that has several industrial applications as
[
15]
[16]
an alkylating
or methoxycarbonylating
agent, a pre-
[
17]
cursor of pharmaceuticals and agrochemicals,
vent, and as an additive to gasoline.
a sol-
[
18]
[19]
The rate of conversion of 4 into methanol and CO in
In particular, we investigated the reaction of 4 with the
2
–
[20,21]
the presence of OH is affected by the solvent: an apolar methylating agent O-methylisourea (6)
[Equation (11)],
solvent slows the reaction as it does not favor the formation which is obtained by treating dicyclohexylcarbodiimide
of hydroxy ions in solution from insoluble NaOH. Con- (DCC) with methanol [Equation (12)]. This reagent was
versely, when a polar organic solvent or a stoichiometric preferred to other classical methylating systems (e.g. BF3/
amount of water is added to CD Cl containing 5, 4 is CH OH) as the latter may carry Brønsted acids that decom-
2
2
3
transformed so rapidly that it cannot be detected and only pose 4.
the final products (CH OH and NaHCO ) are observed.
3
3
These experimental data support the direct interaction of 4
–
with OH more than the dissociation of 4 into CH OH and
3
CO and the subsequent reaction of the latter with NaOH.
2
However, this study gives thermodynamic and kinetic infor-
1
mation about 4 and allows us to complete, with the H- and
1
3
C NMR spectra, the characterization of the labile species
4
for which only the low-temperature IR spectrum had been
reported so far.
As noted in a previous theoretical study,[13] the global
energy-minimum of the CH OC(O)OH molecule corre-
3
sponds to the trans-trans orientation of the CH and OH
3
groups (see Scheme 1). Our present calculations predict two
When isourea 6 was added to a CH Cl solution contain-
2 2
other conformations that lie only 1.3 (cis-trans) and ing HOC(O)OCH3 generated from NaOC(O)OCH3 di-
–
1
3
.2 kcalmol (trans-cis) above the most stable isomer to be methyl carbonate was immediately formed according to re-
local minima on the gas-phase potential energy surface. We action 11. Both products (urea and DMC) were isolated
1
13
also estimated the acidity of CH OC(O)OH by computing and identified from their NMR spectra ( H and C) and
3
the aqueous pK value for the trans-trans isomer (see Com- GC-MS profiles compared with those of authentic samples.
a
putational Details) and found that CH OC(O)OH is When the reaction was carried out in an NMR tube it was
3
slightly less acidic than H CO . In fact, we calculated a pK
possible to follow the disappearance of the signals of 4 and
for CH OC(O)OH of 1.7 and for H CO of 1.1; the experi- the appearance of new signals due to DMC (δ = 3.69 ppm,
2
3
a
3
2
3
mental pK value for H CO is 3.6. The difference between CH ). This reaction both confirms the existence of 4 in
a
2
3
3
calculated and experimental pK values for H CO is within solution and underlines the role of 6 as a methylating agent,
a
2
3
[
20,21]
the range of typical errors obtained with similar calcula- as already reported in the literature.
If the isourea is
[
14]
tions
model.
and can be attributed to the simplified solvation treated with NaOC(O)OMe no methylation reaction occurs,
although the addition of a small amount of water allows
Besides the conversion of 4 into methanol and CO , we the reaction to proceed to form urea and DMC as
2
also investigated the reactivity of 4 towards a methylating CH OC(O)OH is formed.
3
Eur. J. Inorg. Chem. 2006, 908–913
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
911

A. Dibenedetto, M. Aresta, P. Giannoccaro, C. Pastore, I. Pápai, G. Schubert
FULL PAPER
3 3 3 3 3 3 3 3
Figure 4. Stationary points located for the CH OC(O)OH + CH N=C(OCH )(NHCH ) Ǟ CH OC(O)OCH + CH HNC(O)NHCH
model reaction. Selected bond lengths obtained from B3LYP/6-311++G** calculations are given in Ångstroms.
In order to explain the mechanism of reaction 12 we car- stable at temperatures above 300 K and is not formed for
ried out DFT calculations on the reaction of 4 with a sim- kinetic and thermodynamic reasons at room temperature
plified model of 6 where the cyclohexyl groups have been from alcohols and CO [nor in alcohol under 30 MPa of
2
replaced by methyl moieties (see Figure 4).
CO nor in supercritical CO (30 MPa) with added alcohol].
2 2
The transition state located for the model reaction de- In order for 4 to play a role in the formation of DMC from
scribes a methyl transfer from isourea to 4, while the O–H methanol and CO , very high pressures of CO should most
2
2
proton of 4 is already transferred to the iminic nitrogen of probably be used (likely ϾϾ100 MPa).
the isourea. IRC calculations indicate that the reaction
takes place in a single step because the H-bonded
Experimental Section
4···isourea minimum is reached directly from the transition
state in the reverse direction. The reaction is thermodynam-
ically favored (∆Gsolv = –12.4, ∆Ggas = –11.8 kcalmol )
All solvents and starting reagents were RP Aldrich products.
–1
[22]
Alcohols and solvents were dried, distilled,
and stored under
‡
and the energy barrier is predicted to be ∆G
=
dinitrogen. Carbon dioxide was purchased from Rivoira IP
solv
9.9 kcalmol (∆G‡ = 25.5 kcalmol ), which is slightly (99.999% purity). NMR experiments were carried out with a 300-
–
1
–1
1
gas
higher than that found for reaction 13 (∆G‡
=
MHz Bruker apparatus using deuterated CIL solvents. IR spectra
were recorded with a FTIR Perkin–Elmer 1710 apparatus. GC-MS
analyses were carried out with a Shimadzu 17 A gas chromato-
solv
–
1
[20]
1
4.5 kcalmol ).
2
ROH + CO
2
+ DCC Ǟ (RO)
2
CO + CyNHC(O)NHCy
(13) graph (capillary column: 30 m; MDN-5S; Ø 0.25 mm, 0.25 µm
film) coupled to a Shimadzu QP5050A mass spectrometer. Quanti-
tative determinations on the reaction solutions were recorded using
system generated from 5 and HCl/CD Cl or water de- a Hewlett–Packard 6850 GC-FID (capillary column: 30 m; MDN-
It is worthwhile noting that the concentration of 4 in the
2
2
creases rapidly with an increase of temperature, and above 5S; Ø 0.25 mm, 0.25 µm film).
3
10 K we were not able to detect the formation of 4 at all.
Methylisourea was prepared as reported in the literature.[20,21]
This agrees with the lability of 4, which converts into meth-
anol and CO according to Equation (3), and makes it un-
likely that CH OC(O)OH is an intermediate in the synthesis
of carbonates from alcohols and CO2.
Synthesis of the Monomethyl Ester of Carbonic Acid [CH
3
OC(O)-
OH] and NMR Studies: NaOC(O)OMe [or NaO C(O)OMe]
35.0 mg, 0.36 mmol) freshly prepared from MeOH and Na under
2
13
3
(
a CO atmosphere was placed in an NMR tube in 0.7 mL of anhy-
2
1
2 2
drous CD Cl and the tube was closed with a silicone stopper. H-
1
3
and C NMR spectra were recorded for this sample: no signals
were evident besides that of the solvent. Anhydrous HCl in CD Cl
Conclusions
2
2
was injected through the stopper in different amounts until the stoi-
Our work has demonstrated for the first time the exis-
1
13
chiometric ratio was reached and the H- and C NMR spectra
were recorded after each addition. NMR spectroscopic data are
tence of CH OC(O)OH (4) at room temperature and its sta-
bility/lability under the same conditions. Although 4 is very presented and discussed in the main text. Similarly, the reaction
3
labile, it exists in solution at room temperature for the time was monitored when water was added instead of anhydrous HCl.
1
13
necessary to record its H- and C NMR spectra and to
study its conversion with characterization of the products.
Reaction of 4 with Methylisourea: HOC(O)OCH
described above from sodium methyl carbonate (78.0 mg,
3
was generated as
1
It can be easily methylated to afford DMC, which is a reac- 0.8 mmol) and anhydrous HCl/CD Cl (or water). Once the
H
2
2
tion of potential practical importance. Nevertheless, 4 is not NMR signals of 4 had reached their maximum intensity methyl-
9
12 Eur. J. Inorg. Chem. 2006, 908–913
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

The Monomethyl Ester of Carbonic Acid
FULL PAPER
isourea (200 mg, 0.8 mmol) was added to the solution. After one
hour at room temperature the H NMR signal (δ = 3.69 ppm) of
[5] K. N. West, C. Wheeler, J. P. McCarney, K. N. Griffith, D.
1
Bush, C. L. Liotta, C. A. Eckert, J. Phys. Chem. A 2001, 105,
3
947–3948.
dimethyl carbonate became evident. The solution was dried under
vacuum and the resulting solid suspended in diethyl ether. The
ether solution was filtered from the solid. The latter was shown to
be urea [CyHNC(O)NHCy]. DMC was isolated from the solution
by evaporating the solvent.
[
6] a) P. Ballinger, F. A. Long, J. Am. Chem. Soc. 1960, 82, 795–
7
98; b) J. Hine, M. Hine, J. Am. Chem. Soc. 1952, 74, 5266–
5271.
[
[
[
7] M. Aresta, A. Dibenedetto, C. Pastore, Inorg. Chem. 2003, 42,
256–3261.
8] M. T. Nguyen, T. K. Ha, J. Am. Chem. Soc. 1984, 106, 599–
02.
3
Computational Details: DFT calculations were carried out at the
B3LYP/6-311++G** level[ in order to obtain information about
the structures and relative energies of energy minima and transition
states relevant to the reactions investigated. The nature of the sta-
tionary points obtained from the geometry optimizations was veri-
fied by subsequent vibrational frequency analysis. Intrinsic Reac-
tion Coordinate (IRC) calculations were performed from the lo-
cated transition states in order to check whether these structures
indeed connect the appropriate minima on the proposed reaction
pathways. Most of the investigated reactions in the present work
involve charged species, therefore we estimated the solvation free-
6
23]
9] C. A. Wight, A. I. Boldyrev, J. Phys. Chem. 1995, 99, 12125–
12130.
[10] K. R. Liedl, S. Seku sˇ ak, E. Mayer, J. Am. Chem. Soc. 1997,
119, 3782–3784.
[
[
[
[
11] P. Ballone, B. Montanari, R. O. Jones, J. Chem. Phys. 2000,
12, 6571–6575.
1
12] M. Kunert, P. Wiegeleben, H. Gorls, E. Dinjus, Inorg. Chem.
Commun. 1998, 1, 131–133.
13] H. Sun, S. J. Mumby, J. R. Maple, A. T. Hagler, J. Phys. Chem.
1995, 99, 5873–5882.
14] Y. Takano, K. N. Houk, J. Chem. Theory Comput. 2005, 1, 70–
[24]
energies using a recent implementation of the Polarizable Con-
tinuum Model (PCM).[ In these calculations, the cavities of the
solute molecules were constructed using the atomic radii of the
77.
25]
[15] Y. Ono, Appl. Catal., A 1997, 155, 133–166.
[16] M. Aresta, A. Dibenedetto, E. Quaranta, Green Chem. 1999,
1, 237–242.
universal force field (UFF)[
26]
model with individual hydrogen
[
[
[
[
17] S. C. Stinson, Chem. Eng. News 2001, 79, 15–16.
spheres to be able to describe transition states for hydrogen-transfer
reactions. The dielectric constant was always chosen according to
the solvent used in the experiments (ε = 8.93 for CH Cl and 32.63
2 2
for methanol). The Gibbs free-energies calculated for the gas phase
and solvated models are denoted ∆Ggas and ∆Gsolv, respectively, in
the text, and include the thermal corrections as well. The aqueous
18] A. A. G. Shaikh, S. Sivaram, Chem. Rev. 1996, 96, 951–976.
19] M. A. Pacheco, C. L. Marshall, Energy Fuels 1997, 11, 2–29.
20] M. Aresta, A. Dibenedetto, E. Fracchiolla, P. Giannoccaro, I.
Pápai, C. Pastore, G. Schubert, J. Org. Chem. 2005, 70, 6177–
6186.
[21] E. Dabritz, Angew. Chem. Int. Ed. Engl. 1966, 5, 470–477.
[22] D. D. Perrin, W. L. F. Armarego, D. R. Perrin, Purification of
Laboratory Chemicals, Pergamon Press, Oxford, England,
pK
a 3
of CH OC(O)OH was estimated according to the procedure
[14]
described by Takano and Houk
and using the UAHF cavity
1986.
model in the PCM calculations to obtain the solvation free-ener-
gies. All these calculations were carried out using the Gaussian 03
[
23] a) A. D. Becke, J. Chem. Phys. 1993, 98, 5648–5652; b) C. Lee,
W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785–789; c) P. J.
Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys.
Chem. 1994, 98, 11623–11627.
[
27]
software package.
Supporting Information (see the footnote on the first page of this
article): Cartesian coordinates and total energies of located struc-
tures.
[
[
[
[
24] M. Cossi, G. Scalmani, N. Rega, V. Barone, J. Chem. Phys.
2
002, 117, 43–54.
25] S. Miertus, E. Scrocco, J. Tomasi, Chem. Phys. 1981, 55, 117–
29.
1
26] A. K. Rappi, C. J. Casewit, K. S. Colwell, W. A. Goddard III,
W. M. Skid, J. Am. Chem. Soc. 1992, 114, 10024–10035.
27] Gaussian 03, Revision C.02, M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A.
Montgomery Jr.,T. Vreven, K. N. Kudin, J. C. Burant, J. M.
Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M.
Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji,
M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M.
Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene,
X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, C. Adamo, J.
Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Aus-
tin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Mo-
rokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G.
Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O.
Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B.
Foresman, J. V. Ortiz, Q. Cui, A G. Baboul, S. Clifford, J. Cios-
lowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Ko-
maromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill,
B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A. Po-
ple, Gaussian, Inc., Wallingford CT, 2004.
Acknowledgments
The Italian authors wish to thank the Ministry of Education, Uni-
versity and Research (COFIN grants MM03027791 and
2003039774) and the EU Project TOPCOMBI for financial sup-
port, and Dr. Paride Papadia for experimental assistance in NMR
studies. The Hungarian authors acknowledge the support of the
Hungarian Research Foundation (OTKA grants T037345 and
T034547).
[
1] M. Aresta, D. Ballivet-Tkatchenko, M. C. Bonnet, R. Faure,
H. Loiseleur, J. Am. Chem. Soc. 1985, 107, 2994–2995.
2] M. Aresta, D. Ballivet-Tkatchenko, D. Belli Dell’Amico, M. C.
Bonnet, D. Boschi, F. Calderazzo, R. Faure, L. Labella, F.
Marchetti, Chem. Commun. 2000, 1099–1100.
[
[
3] a) G. Gattow, W. Von Behrendt, Angew. Chem. Int. Ed. Engl.
1
972, 11, 534–535; b) G. Gattow, W. Von Behrendt, Z. Anorg.
Allg. Chem. 1973, 398, 198–206.
[4] M. H. Jamroz, J. C. Dobrowolski, J. E. Rode, M. A. Borowiak,
J. Mol. Struct. (Theochem) 2002, 618, 101–108.
Received: July 13, 2005
Published Online: January 12, 2006
Eur. J. Inorg. Chem. 2006, 908–913
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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