Higher-nuclearity group 14 metalloid clusters: [Sn 9{Sn(NRR′)}6]

Article abstract of DOI:10.1002/anie.200600292

(Figure Presented) A perfect tin! Two 15-nuclear tin metalloid clusters [Sn₁₅Z₆] (Z = N (2,6-iPr₂C₆H ₃)-(SiMe₂X); X = Me, Ph), having an unprecedented body-centered metal core (see picture; only N atoms of the amide ligands shown), were prepared by reduction of the appropriate amidotin(II) chloride with KC₈ or Li[BHsBu₃]. Moessbauer spectra showed two distinct quadrupole-splitting sites in the ratio of 1.5:1 for the Sn₉ atoms and the six amido-bound tin atoms.

Full text of DOI:10.1002/anie.200600292

Angewandte
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exceeds the number of metal–ligand contacts and by the
presence of metal atoms which participate exclusively in
metal–metal interactions”.[1] Reviews of such Al and Ga clus-
ters have appeared[2] and were extended to those of the
heavier Group 14 elements.[3] Previously reported tin clusters
[SnxRy] (x > y) include [Sn8R4] (I),[4] [Sn8R6]2À (II),[5] [Sn9R3]
(III),[6] [Sn10R3]À (IV),[6] in each of which R represents an aryl
or silyl ligand.In the spirit of Schnöckelꢀs definition, we now
present data on the highest-nuclearity Group 14 “metalloid”
clusters to be reported thus far, the isoleptic compounds
[Sn15Z6] (Z = N(2,6-iPr2C6H3)(SiMe2X); X = Me (1), Ph (2)).
Compounds 1 and 2 are the first Group 14 metal clusters
bearing amido ligands and are also unprecedented within this
compound class for being body-centered and have as locus a
tin atom with relatively close contacts (ca. 3.1 to 3.4 Š) to
each of the remaining 14 tin atoms.
Cluster Compounds
DOI: 10.1002/anie.200600292
Higher-Nuclearity Group 14 Metalloid Clusters:
[Sn9{Sn(NRR’)}6]**
Marcin Brynda, Rolfe Herber,* Peter B. Hitchcock,
Michael F. Lappert,* Israel Nowik, Philip P. Power,*
ˇ
˚ ˇ ˇ
Andrey V. Protchenko, AlesRuzicka, and Jochen Steiner
Dedicated to Professor Hansgeorg Schnöckel
on the occasion of his 65th birthday
The term “metalloid cluster” was coined by Schnöckel and co-
workers in the context of Al chemistry to designate those
metal clusters in which “the number of metal–metal contacts
[*] Dr. P. B. Hitchcock, Prof. M. F. Lappert, Dr. A. V. Protchenko,
The best-known stable polyhedral clusters of the heavier
Group 14 elements are the ligand-free Zintl anions, such as
M52À or M9nÀ (M = Ge, Sn, or Pb; n = 2, 3, 4).[7] An intriguing
aspect of progressively higher-nuclearity metalloid clusters
[MxRy] (where x is large and x @ y) is that their morphology
should increasingly resemble sections of the bulk metal.
Detailed structural data for such species may provide insight
into changes that occur at or near the surface of the metal
compared with those within its bulk.For main-group chemis-
try, relevant information was previously available only for
Group 13 metalloid clusters, as in [Al77Z20]2À [8a] or
[Ga84Z20]3À [8b] (Z = N(SiMe3)2).The characterization of
[Sn15Z6] (1, 2) is now a step along the same path for clusters
of Group 14 elements.A recent contribution in this area
relates to the synthesis and X-ray structure of [Si8(SitBu3)6].[9]
The work herein was begun at the University of Sussex in
2003.[10] The crystalline compound 1 was isolated as small
black crystals of space group Pnnm (1(Pnnm))by treatment
of the colorless compound [{Sn[N(2,6-iPr2C6H3)(SiMe3)](m-
Cl)}2] (3)[11] with KC8 in Et2O at about À308C and successive
filtering, concentration of the filtrate, addition of PhMe, and
prolonged storage at À278C.By this procedure compound
ˇ
˚ ˇ
Dr. A. Ruzicka
Department of Chemistry
University of Sussex
Brighton, BN19QJ (UK)
Fax: (+44)1273-677-196
E-mail: m.f.lappert@sussex.ac.uk
Dr. M. Brynda, Prof. P. P. Power, Dr. J. Steiner
Department of Chemistry
University of California
One Shields Avenue, Davis, CA 95616 (USA)
Fax: (+1)530-752-8995
E-mail: pppower@ucdavis.edu
Prof. R. Herber, Dr. I. Nowik
Racah Institute of Physics
The Hebrew University of Jerusalem
91904 Jerusalem (Israel)
Fax: (+972)2-658-6347
E-mail: herber@vms.huji.ac.il
[**] We thank the EPSRC (A.V.P.) and the Royal Society (A.R.) for
postdoctoral fellowships and the CRC Program of the U.S. National
Science Foundation for financial support. R=2,6-iPr2C6H3;
R’=SiMe3 or SiMe2Ph.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
Table 1: Comparison of average bond lengths (Š) in clusters 1 and 2.[a]
1(Pnnm) was obtained in very low yield together with
unreacted 3.[10] A subsequent experiment, in which the
À
À
À
À
Compound
Sn1 SnA
Sn1 SnB
SnA SnB
SnA SnA
crystallization was carried out at ambient temperature,
1(Pnnm)
3.18Æ0.03
3.15Æ0.03
3.16Æ0.03
3.18Æ0.02
3.15Æ0.03
3.36Æ0.02
3.40Æ0.01
3.40Æ0.01
3.33Æ0.04
3.41Æ0.02
3.01Æ0.03
3.02Æ0.02
3.02Æ0.02
2.99Æ0.02
3.02Æ0.04
3.67Æ0.07
3.64Æ0.02
3.64Æ0.03
3.67Æ0.06
3.64Æ0.04
¯
furnished 1(R3) in very low yield as black (or dark red in
¯
1(R3)
thin layers) hexagonal prisms.The new crystalline amidoti-
n(II) chloride 3[11] was prepared in Et2O from SnCl2 and one
equivalent of Li[N(2,6-iPr2C6H3)(SiMe3)][12a] (76%) or
[Sn{N(2,6-iPr2C6H3)(SiMe3)}2][12b] (93%).Attempted reduc-
tion of three other amidotin(II) compounds failed to yield
isolable clusters.Reaction of [Sn{N(SiMe 3)2}2] or [{Sn[N-
(SiMe3)2](m-Cl)}2] with KC8 or NaC10H8 gave elemental tin,
and reaction of [{Sn(tmp)(m-Cl)}2] (tmp = 2,2,6,6-tetrame-
thylpiperidinato) with KC8 yielded a tar of unidentified
content.
¯
1(R3c)
1(P21/n)
2(P21/n)
[a] Sn1: central Sn atom; SnA: Sn atoms at cube apices; SnB: ligand-
bound Sn atoms.
Complementary experiments were subsequently carried
out at the University of California, Davis, where a new
synthetic strategy was introduced, namely the use of a
reductive elimination from a precursor metal chloride with
Li[BHsBu3] (L-selectride).Thus, the cluster 1 (ca.5%) was
obtained by treatment of the in situ generated amidotin(II)
chloride 3 with a less-than-stoichiometric amount of Li-
[BHsBu3] at À788C.Two forms of 1 were obtained by this
route; crystallization from PhMe or hexane yielded X-ray-
¯
quality crystals 1(R3c) or 1(P21/n), respectively.Compound 2
(7%) was obtained similarly from an analogue of 3 (prepared
in situ from [Sn{N(2,6-iPr2C6H3)(SiMe2Ph)}2] and SnCl2) and
Li[BHsBu3].It is likely that a transient amidotin(II) hydride
is an intermediate in these syntheses.In support of this
postulate, we note that i) treatment of 3 with an equivalent of
[{Cu(m-H)(PPh3)}6] in C6D6 resulted in evolution of H2 (NMR
experiment) and ii) thermolysis of the sterically hindered
Sn(II) hydride [{Sn[2,6-(2,4,6-iPr3C6H2)2C6H3](m-H)}2][13] in
toluene afforded the [Sn9R3] cluster III.[6]
Although single crystals of 1 were obtained in four
[14]
¯
¯
different space groups (Pnnm, R3, R3c, P21/n),
the
molecular structures that were deduced from each are
À
essentially identical, and the various Sn Sn distances are
also closely similar to those of 2 (Table 1).[15] The structures of
1 and 2 are illustrated in Figure 1.The most striking feature of
each structure is the body-centered arrangement of the 15 tin
atoms, an architecture previously unknown for metalloid
clusters of the Group 14 elements.The ligand-free tin atoms
in 1 and 2 form a body-centered distorted cube, and each of
the six faces is capped by an {Sn(NRR’)} moiety, as shown in
Figure 2.
Figure 1. Thermal-ellipsoid plot (ellipsoids set at 50% probability) of
1(R3c) (upper) and 2 (lower) with isotropic representation of the
C atoms and the central Sn atoms (H atoms are omitted for clarity).
¯
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Chemie
molecule, thus making NMR relaxation very slow, and the
sample is thus quickly saturated.
Temperature-dependent 119Sn Mössbauer spectroscopic
experiments on a sample of 1 were carried out[18] to comple-
ment the above X-ray structural data.A representative
spectrum is shown in Figure 3.Two distinct quadrupole-
Figure 2. Polyhedral representation of the cluster core in 1 and 2
(unsubstituted SnA atoms black, N atoms and ligand-bound SnB atoms
gray; only the N atoms of the amido ligands shown).
À
The metalloid character of 1 and 2 is revealed in the Sn
Sn interactions between the central tin atom (Sn1) and the
eight tin atoms at the cube apices (SnA; Figure 2) of an Sn8
Figure 3. 119Sn Mössbauer spectrum of 1 at 89 K (TR=relative trans-
mission). The velocity scale is with respect to the centroid of a room-
temperature BaSnO3-absorber spectrum. The A site is ascribed to the
core Sn9 atoms, and the B site to the six ligand-bound Sn atoms.
À
cube.The Sn1 SnA distances have an average value close to
3.15 Š,[15] which is considerably longer than the average value
of 2.81 Š in white tin (b-Sn), but closer to the average value of
À
3.10 Š in gray tin (a-Sn, diamond lattice).The Sn A SnB bonds
between the amido-substituted tin atoms (SnB; Figure 2) and
those at the apices of the cube (SnA) have an average length of
splitting (QS) sites were revealed in a ratio of 1.5:1 at 90 K.
These are assigned to the nine “naked” tin atoms Sn1 and SnA
(site A) and the six ligand-bound atoms SnB (site B), respec-
tively.Thus, the QS values of the A and B sites were
determined to be 1.341 Æ 0.012 (at 90 K) and 0.13 Æ
0.01 mmsÀ1 (essentially temperature-independent), respec-
tively.
The isomer shifts (IS) were found at 2.733 Æ 0.011 and
2.63 Æ 0.01 mmsÀ1 at 90 K for the A and B sites, respectively.
These values are close to that of gray tin (a-Sn, 2.56 Æ
0.02 mmsÀ1 at room temperature) or white tin (b-Sn,
2.68 mmsÀ1 [19]), and are within the range for bivalent tin
compounds.For example, [{Sn[N(SiMe 3)2](m-Cl)}2] has IS and
QS values of 3.28 and 3.10 mmsÀ1, respectively.[20] The IS
values for quadrivalent organic tin compounds are substan-
tially lower; for example, SnCl{CH(SiMe3)2}3 has IS and QS
values of 1.27 Æ 0.03 and 2.18 mm Æ 0.05 mmsÀ1, respec-
tively.[21]
Detailed analysis of the temperature dependence of the
Mössbauer spectra is provided in the Supporting Information,
which shows that the ratio of the two tin sites B and A is 0.66
(ca.6/9) in the temperature range 120–230 K.It has not been
possible to analyze the Mössbauer data in terms of three
distinct tin atom types, and the resonance associated with the
larger quadrupole-splitting site includes that from the central
tin atom, which is located in a distorted cubic environment.
The geometry optimization of the model tin cluster
[Sn9{Sn(NH2)}6] (M1) was performed in the gaseous phase
by using DFT calculations.The geometry optimization was
performed at the B3LYP/LanL2DZ level of theory by using
the Gaussian03 package.[22] The calculations reproduced the
À
3.02 Š. The SnB N bonds have an average length of 2.09 Š
(for 1) and 2.11 Š (for 2), and are unexceptional (for
comparison, Sn N bond length in 3: 2.068(2) Š).
[11]
À
The
À
slightly larger average SnB N bond length in 2 is probably a
result of the greater steric demand of the silyl group of the
ligand.The only main-group-metal cluster that possesses a
structure similar to that of 1 and 2 is the neutral-atom cluster
[SiAl8(AlCp*)6] (Cp* = h5-C5Me5), which has a bcc arrange-
ment of Al atoms in which the central aluminum atom is
replaced by a silicon atom and the faces are capped by
{AlCp*} units.[16]
The structural motif of the Sn-centered core in 1 or 2 is
similar to that of typical metallic structures.Thus, the
bcc arrangement of the nine central tin atoms in 1 (black
Sn atoms in Figure 2) corresponds to that in the a-iron
structure, but not to those of white or gray tin.The
bcc structure is observed for elemental tin only at high
pressure, above 45 Æ 5 GPa.[17] This relationship between the
number of associated metal atoms in a molecular structure
and high-pressure modifications of the corresponding bulk
metal has also been observed in large clusters of Group 13
elements.[2]
Attempts to record solution 119Sn NMR spectra of 1 and 2
failed possibly because of their very low solubility at ambient
temperature as a result of their high symmetry and molecular
weight.Heating the complex 1(Pnnm) in [D8]toluene led to
precipitation of metallic tin.Moreover, the tin nuclei are in a
rigid cage structure that forms part of a relatively large
Angew. Chem. Int. Ed. 2006, 45, 4333 –4337
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Communications
2: [Sn{N(2,6-iPr2C6H3)(SiMe2Ph)}2] (1.25 g, 1.75 mmol) and SnCl2
(0.332 g, 1.75 mmol) were mixed in freshly degassed THF (40 mL) at
room temperature and stirred for 6 h.The reaction mixture was
cooled to about À788C, and Li[BHsBu3] (2.8 mL, 1.0m solution in
THF, 2.8 mmol) was added quickly. After slowly warming to room
temperature, the solution was stirred for 24 h.The solvent was
removed in vacuo, and the black residue was extracted into hexanes
(30 mL).The deep-black extract was filtered, and the filtrate was
concentrated (ca.20 mL). Storage of the solution at room temper-
ature for 2 days afforded crystalline 2 (0.057 g, 7%).
main features of the experimentally observed Sn15 cluster with
the optimized bond lengths being about 3% larger than those
observed in the crystal structures of complexes 1 and 2.In the
polyhedral representation of the cluster core, the symmetri-
cally inequivalent tin atoms became fully symmetric after
À
optimization.The calculated Sn
SnA distances are 3.78 Š,
A
and the corresponding distances in the crystal structures of 1
and 2 vary over the range 3.60–3.74 Š (see Table 1). The
SnB atoms are symmetrically distant from the central cube
vertices by 3.15 Š (in contrast to the slightly asymmetric
distribution of distances in the X-ray structures, 2.97 to
3.06 Š).
Received: January 23, 2006
Revised: April 25, 2006
In conclusion, we report the crystalline metalloid clusters
1 and 2, the first examples of a body-centered cluster of
Group 14 elements.[23] Compounds 1 and 2 are also note-
worthy for being i) the highest-nuclearity Group 14 clusters to
be described, ii) unprecedented in this compound class for
being body-centered, and iii) the first Group 14 metal clusters
to have amido ligands.The inner cube of eight tin atoms (Sn A)
have the closer contacts to Sn1, and the distances between Sn1
and the remaining six tin atoms (SnB) are longer.The
preferred method of synthesis of 1 and 2 involved a novel
route, namely the reduction of the appropriate amidotin(II)
chloride by the reducing agent Li[BHsBu3].DFT calculations
on the model cluster [Sn9{Sn(NH2)}6] reasonably reproduced
Keywords: amide ligands · cluster compounds ·
Mössbauer spectroscopy · tin · X-ray diffraction
.
[1] A.Purath, R.Köppe, H.Schnöckel,
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[2] a) A.Schnepf, H.Schnöckel, Angew. Chem. 2002, 114, 3682;
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W.Uhl, N.Wiberg,
Molecular Clusters of the Main Group
Elements (Eds.: M. Driess, H. Nöth), Wiley-VCH, Weinheim,
2004, chap.2.3, p.126; c) H. Schnöckel,
Dalton Trans. 2005,
3131.
[3] a) N.Wiberg, P.P.Power, Molecular Clusters of the Main Group
Elements (Eds.: M. Driess, H. Nöth), Wiley-VCH, Weinheim,
2004, chap.22. , p.188; b) A.Schnepf, Angew. Chem. 2004, 116,
680; Angew. Chem. Int. Ed. 2004, 43, 664.
[4] B.E.Eichler, P.P.Power, Angew. Chem. 2001, 113, 818; Angew.
Chem. Int. Ed. 2001, 40, 796.
[5] N.Wiberg, H-.W.Lerner, S.Wagner, H.Nöth, T.Seifert,
Naturforsch. B 1999, 54, 877.
[6] A.F.Richards, B.E.Eichler, M.Brynda, M.M.Olmstead, P.P.
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2005, 44, 2546.
[7] J.D.Corbett, Angew. Chem. 2000, 112, 682; Angew. Chem. Int.
Ed. 2000, 39, 670.
À
À
À
À
the experimental Sn1 SnA, Sn1 SnB, SnA SnB, and SnA SnA
distances.Mössbauer spectra of 1 are consistent with the X-
ray structure in showing that there are two distinct quadru-
pole-splitting sites in the ratio 1.5:1, which correspond to the
Sn9 core and the six {Sn(NRR’)} moieties, but the Mössbauer
data do not distinguish the Sn1 atom from the SnA atoms.The
isomer shift of the atoms in the Sn9 core is almost identical to
that for b-tin.
Z.
[8] a) A.Ecker, E.Weckert, H.Schnöckel, Nature 1997, 387, 379;
Experimental Section
b) A.Schnepf, H.Schnöckel,
Angew. Chem. Int. Ed. 2001, 40, 712.
[9] G.Fischer, V.Huch, P.Mayer, S.K.Vasisht, M.Veith, N.Wiberg,
Angew. Chem. 2005, 117, 8096; Angew. Chem. Int. Ed. 2005, 44,
7884.
Angew. Chem. 2001, 113, 733;
All manipulations were carried out under anaerobic and anhydrous
conditions.
¯
1(Pnnm, R3): KC8 (0.11 g, 0.81 mmol) was added with stirring to
a solution of 3 (0.47 g, 0.55 mmol) in Et2O (40 mL) at about À308C.
The mixture was stirred for 5 h and filtered.The dark red-brown
filtrate was concentrated to about 3 mL, toluene (10 mL) was added,
and the mixture was stored at À278C for 3 months (initially, a black
powder precipitated, which was filtered off and discarded) to yield
several small black crystals of compound 1(Pnnm) along with large
pale crystals of unreacted 3.In another experiment, when the crystals
of unreacted 3 had precipitated, the remaining solution was decanted,
concentrated again, and stored at room temperature to produce black
[10] H. Cox, D.J. Doyle, P.B. Hitchcock, M.F. Lappert, L.J-.M.
˚
ˇ
ˇ
Pierssens, A.V.Protchenko, A.Ru zicka, J.R.Severn, XIth Int.
Conf.on the Coordination and Organometallic Chemistry of
Germanium, Tin, and Lead, Santa Fe, July 2004, Abstracts Paper
O-43.
[11] The synthesis and X-ray structure of 3 are described in the
Supporting Information.
[12] a) B. Luo, V.G. Young, Jr., W.L. Gladfelter,
J. Organomet.
¯
(or dark red in thin layers) hexagonal prisms of compound 1(R3).
Chem. 2002, 649, 268; b) J.R.Babcock, L.Liable-Sands, A.L.
Rheingold, L.R.Sita, Organometallics 1999, 18, 4437.
¯
1(R3c, P21/n): [Sn{N(2,6-iPr2C6H3)(SiMe3)}2] (1.23 g, 2 mmol)
and SnCl2 (0.38 g, 2 mmol) were mixed in freshly degassed THF
(50 mL) at room temperature and stirred for 6 h.The reaction
mixture was cooled to about À788C, and Li[BHsBu3] (L-selectride)
(3.0 mL, 1.0m solution in THF, 3.0 mmol) was added quickly. After
slowly warming to room temperature, the solution was stirred for
48 h.The solvent was removed in vacuo, and the black residue was
extracted into toluene (50 mL).The deep-black extract was filtered,
and the filtrate was concentrated (ca.30 mL).Storage of the solution
[13] B.E. Eichler, P.P. Power, J. Am. Chem. Soc. 2000, 122, 8785.
[14] Crystal data for 1(Pnnm) with MoKa radiation (l = 0.7107 Š) at
173(2) K: black block, orthorhombic, space group Pnnm, a =
17.1182(2), b = 17.1312(2), c = 22.8882(2) Š, V= 6712.09(13) Š3,
Z = 2, R1(obs.data) = 0.025, wR2(all data) = 0.092. Crystal data
¯
for 1(R3) with MoKa radiation (l = 0.7107 Š) at 173(2) K: black
¯
prism, trigonal, space group R3, a = 15.1565(2), b = 15.1565(2),
c = 44.6776(10) Š, g = 1208, V= 8888.3(3) Š3, Z = 3, R1(obs.
¯
at room temperature for 5 days afforded crystalline 1(R3c) (0.045 g,
data) = 0.036, wR2(all data) = 0.073. Crystal data for 1
¯
¯
5%).The synthesis of 1(P21/n) was identical to that of 1(R3c), except
(R3c)·PhMe with MoKa radiation (l = 0.7107 Š) at 90(2) K:
¯
that the residue was extracted with hexane.Crystals of compound 1
were only sparingly soluble in hydrocarbons or ethers.
black block, hexagonal, space group R3c, a = 15.0335(2), b =
15.0335(2), c = 103.042(3) Š, b = 1208, V= 20168.2(7) Š3, Z =
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Angewandte
Chemie
6, R1(obs.data) = 0.0262, wR2(all data) = 0.0560. Crystal data
for 1(P21/n) with MoKa radiation (l = 0.7107 Š) at 90(2) K:
black plate, monoclinic, space group P21/n, a = 16.2048(6), b =
24.0255(9), c = 16.6026(6) Š, b = 113.159(1)8, V= 5943.0(4) Š3,
Z = 2, R1(obs.data) = 0.0224, wR2(all data) = 0.0549. Crystal
data for 2·hexane with MoKa radiation (l = 0.7107 Š) at 90(2) K:
black block, monoclinic, space group P21/n, a = 17.5916(6), b =
18.8745(6), c = 21.7499(7) Š, b = 90.904(1)8, V= 7220.8(4) Š3,
Z = 2, R1(obs.data) = 0.0249, wR2(all data) = 0.0693. CCDC-
295619–295624 contain the supplementary crystallographic data
for this paper.These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.ca-
m.ac.uk/data_request/cif.
¯
¯
À
[15] The Sn Sn distances in each of 1(Pnnm, R3, R3c, P21/n), 2, and
III are given in the Supporting Information.[6]
[16] A.Purath, C.Dohmeier, A.Ecker, R.Köppe, H.Krautscheid,
H.Schnöckel, R.Ahlrichs, C.Stoermer, J.Friedrich, P.Jutzi,
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Am. Chem. Soc. 2000, 122, 6955.
[17] S.Desgreniers, Y.K.Vohra, A.L.Ruoff,
10360.
Phys. Rev. B 1989, 39,
[18] Variable-temperature 119Sn Mössbauer experiments were car-
ried out by using a room-temperature Ba119mSnO3 source in
transmission geometry as described previously.[19] Spectrometer
calibration was effected by using an a-Fe absorber (10.52 mg Fe
per cm2).All isomer shifts are reported with respect to the
centroid of a BaSnO3 absorber spectrum at room temperature.
Temperature control over the data acquisition periods (typically
8 to 24 h) was better than Æ 0.2 K.
[19] R.H.Herber, G.I.Parisi, Inorg. Chem. 1966, 5, 769.
[20] H.Cox, PB. .Hitchcock, MF..Lappert, L-J.M. .Pierssens,
Angew. Chem. 2004, 116, 4600; Angew. Chem. Int. Ed. 2004,
43, 4500.
[21] J.D.Cotton, P.J.Davidson, M.F.Lappert, J.D.Donaldson, J.
Silver, J. Chem. Soc. Dalton Trans. 1976, 2286.
[22] Gaussian98 (RevisionA.7), M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G.
Zakrzewski, J.A.Montgomery, R.E.Stratmann, J.C.Burant, S.
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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