Content uploaded by Alexander Ivanov
Author content
All content in this area was uploaded by Alexander Ivanov
Content may be subject to copyright.
This journal is cthe Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 8805–8810 8805
Cite this:
Phys. Chem. Chem. Phys
., 2011, 13, 8805–8810
Molecular wheel to monocyclic ring transition in boron–carbon mixed
clusters C
2
B
6
and C
3
B
5
w
Timur R. Galeev,
a
Alexander S. Ivanov,
b
Constantin Romanescu,
c
Wei-Li Li,
c
Konstantin V. Bozhenko,*
b
Lai-Sheng Wang*
c
and Alexander I. Boldyrev*
a
Received 9th February 2011, Accepted 22nd March 2011
DOI: 10.1039/c1cp20359b
In this joint experimental and theoretical work we present a
novel type of structural transition occurring in the series of
C
x
B
8x
(x= 1–8) mixed clusters upon increase of the carbon
content from x=2tox= 3. The wheel to ring transition is
surprising because it is rather planar-to-linear type of transition
to be expected in the series since B
8
,B
8
,B
82
and CB
7
are
known to possess wheel-type global minimum structures while C
8
is linear.
Carbon and boron being neighbours in the Periodic Table
have very different geometric structures of their small clusters.
Small carbon clusters are either linear or cyclic,
1
whereas,
those of boron are either planar or quasi-planar.
2
Thus, one
can expect peculiar transitions from planar to linear structures
upon increasing the number of carbon atoms in mixed
boron–carbon clusters. An example of such planar-to-linear
structural transition as a function of the number of carbon
atoms has been found to occur in the mixed boron–carbon
clusters, C
x
B
5x
(x= 1–5) between x= 2 and 3.
3
Larger
boron–carbon mixed clusters have been computationally
proposed to exemplify unusual hexa-, hepta and octa-
coordinated planar carbon species (CB
62
,
4
CB
7
,
5
and CB
86
).
However, it was shown later in joint experimental and
theoretical works
7–9
that carbon avoids the central position
in those wheel-type global minimum geometries and occupies
the peripheral position instead. It is known that the B
8
,B
8
and B
82
clusters
10–14
and the CB
7
cluster
8
have wheel-type
heptagon structures with one boron atom in the center. The C
8
cluster has a linear global minimum structure with the cyclic
isomer being about 10 kcal mol
1
higher.
1,15,16
Therefore, a
planar-to-linear transition could be expected in the C
x
B
8x
(x= 1–8) series upon increasing the carbon content in the
clusters. Surprisingly, we found a novel structural transition
that has never been observed before—wheel-to-ring transition
between C
2
B
6
and C
3
B
5
structures of the series. We would
like to point out that ring-like structures have been previously
reported in a theoretical study by Shao et al.
17
for the neutral
C
n
B
3
(n= 4–8) clusters.
The C
2
B
6
and C
3
B
5
clusters were investigated by photo-
electron spectroscopy (PES) and ab initio calculations. The
experiment was performed with a laser-vaporization cluster
source and a magnetic-bottle photoelectron spectrometer
(see Experimental section).
18
The photoelectron spectra of the C
2
B
6
cluster are
presented in Fig. 1 and the photoelectron spectra of C
3
B
5
are shown in Fig. 2. The experimentally observed vertical
detachment energies (VDEs) for the clusters are given in
Tables 1 and 2 and are compared to the theoretically calcu-
lated data.
Fig. 1 shows the PES spectra of C
2
B
6
at three photon
energies. The 193 nm spectrum of C
2
B
6
reveals six well-
resolved features labelled A–F (Fig. 1b). At the low binding
energy side, we observe a very broad feature (X, X0) corres-
ponding to photodetachment from the two lowest lying
isomers of C
2
B
6
. The ninth band (G) can be tentatively
identified, but the signal-to-noise ratios are poor at the high
binding energy side. The spectra recorded at 266 nm (Fig. 1a)
reveal fine vibrational features for the A and B electronic
bands. For the A band we measured a vibrational spacing of
330 30 cm
1
using the PES measured at 355 nm (see the inset
of Fig. 1a). The B band shows a fine structure both at 193 nm
and 266 nm, however, the measurement of the vibrational
spacing is complicated by the existence of two nearly isoenergetic
photodetachment channels.
The spectra of C
3
B
5
are shown in Fig. 2. The VDE of the X
band was measured to be 3.94 0.03 eV. The intensity change
between the X and the A bands confirms that there are two
features rather than a vibrational progression. The X band
shows a short vibrational progression with a spacing of
380 30 cm
1
.
Computationally, we first performed the global minimum
structure search for the C
2
B
6
ion using the Coalescence
Kick (CK)
19–21
program written by Averkiev. The CK method
a
Department of Chemistry and Biochemistry, Utah State University,
Old Main Hill 0300, Logan, UT 84322-0300, USA.
E-mail: a.i.boldyrev@usu.edu; Fax: +1 435-797-3390;
Tel: +1 435 7971630
b
Department of Physical and Colloid Chemistry, Peoples’ Friendship
University of Russia, 6 Miklukho-Maklaya St., Moscow 117198,
Russian Federation. E-mail: kbogenko@mail.ru
c
Department of Chemistry, Brown University, 324 Brook Street,
Providence, Rhode Island 02912, USA.
E-mail: Lai-Sheng_Wang@brown.edu; Tel: +1 401-863-3389
wElectronic supplementary information (ESI) available: Adiabatic
detachment energies of the C
2
B
6
and C
3
B
5
clusters, valence
canonical molecular orbitals and AdNDP revealed chemical bonding
patterns for the wheel-type and ring isomers of both clusters. See DOI:
10.1039/c1cp20359b
PCCP Dynamic Article Links
www.rsc.org/pccp COMMUNICATION
Published on 12 April 2011. Downloaded by Utah State University on 17/08/2013 00:10:23.
View Article Online
/ Journal Homepage
/ Table of Contents for this issue
8806 Phys. Chem. Chem. Phys., 2011, 13, 8805–8810 This journal is cthe Owner Societies 2011
subjects large populations of randomly generated structures to
a coalescence procedure in which all atoms are pushed gradu-
ally to the molecular center of mass to avoid generation of
fragmented structures and then optimized to the nearest local
minima. The CK calculations were performed at the B3LYP/
3-21G level of theory. All the low-lying (DEo25 kcal mol
1
)
isomers revealed were reoptimized with follow up frequency
calculations at the B3LYP/6-311+G* level of theory. Single
point calculations for the lowest energy structures were
performed at the RCCSD(T)/6-311+G(2df) level of theory
using the B3LYP/6-311+G* optimized geometries. The rela-
tive energies of a series of representative isomers are given in
Fig. 3.
The CK search revealed that the cyclic structure I.6 is the
lowest isomer with the structures I.1–I.5 being 12–25 kcal mol
1
higher at the B3LYP/3-21G level of theory. However, when we
reoptimized all the low-lying structures at the B3LYP/
6-311+G* level of theory we found a significantly different
order of the isomers with the wheel-type structure I.1 being
the lowest isomer and the cyclic structure I.6 lying
9.1 kcal mol
1
higher (Fig. 3). Therefore, the set of isomers
for subsequent investigation should be formed of all the
structures lying in the range of about 20 kcal mol
1
relative
energies at this level of theory as it was done in the current
work. Moreover, the B3LYP/6-311+G* calculated results
were further corrected by the single point calculations at the
RCCSD(T)/6-311+G(2df)//B3LYP/6-311+G* level of theory
(Fig. 3). Thus, according to our most accurate calculations the
wheel-type structure I.1 is the global minimum for the C
2
B
6
cluster. In order to verify this theoretical prediction we calcu-
lated theoretical VDEs for the global minimum structure I.1 at
three levels of theory: TD-B3LYP/6-311+G(2df), UOVGF/
6-311+G(2df) and R(U)CCSD(T)/6-311+G(2df) all at the
B3LYP/6-311+G* optimized geometries. We also calculated
VDEs for the second lowest structure I.2 at the same three
levels of theory and found out that it also contributes to the
experimental PES of C
2
B
6
. Results of the VDEs calculations
are summarized in Table 1.
The broad feature X(X0) in the experimental spectrum of
C
2
B
6
(Fig. 1) can be assigned to the electron detachment
from the singly-occupied HOMO 7a
1
of the global minimum
structure I.1. The broad shape of the peak is an indication of a
large geometry change upon the electron detachment, which
was confirmed by geometry optimization of the neutral C
2
B
6
cluster (see Fig. S1, ESIw). The electron detachment from the
singly occupied HOMO 11a0of the I.2 isomer can also
contribute to this peak since the first VDE is very close to
that of I.1. None of the calculated VDEs of I.1 could be
assigned to the experimental feature at B3.2 eV. This peak
confirms the presence of the second-lowest isomer I.2 of C
2
B
6
in the molecular beam since it can be clearly explained by the
electron detachment from 3a00 of I.2 leading to the final
3
A00
state. Electron detachment processes with final triplet states
are expected to be more prominent in the experimental
Fig. 1 Photoelectron spectra of C
2
B
6
at (a) 266 nm (4.661 eV) and
(b) 193 nm (6.424 eV). The inset shows a partial PES at 355 nm
(3.496 eV). The short vertical lines represent the TD-B3LYP values of
VDE for structures I.1 (bottom) and I.2 (top).
Fig. 2 Photoelectron spectra of C
3
B
5
at (a) 266 nm (4.661 eV) and
(b) 193 nm (6.424 eV).
Published on 12 April 2011. Downloaded by Utah State University on 17/08/2013 00:10:23.
View Article Online
This journal is cthe Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 8805–8810 8807
spectra, therefore, we discuss only the transitions leading to
the final triplet states. The feature at 3.54 eV in the
experimental PES is due to the electron detachment from
HOMO 12b
1
with the final state
3
B
1
. The next feature
(4.35 eV) corresponds to the electron detachment from
HOMO 21a
2
to the final
3
A
2
state. Both of these detach-
ment channels are of isomer I.1. The features at 4.68 and
4.77 eV can be explained only by the electron detachment from
2a00 and 10a0of I.2 corresponding to two final states:
3
A0and
3
A00. The sharp peak at 4.96 eV is due to the detachment from
HOMO 36a
1
leading to the
3
A
1
state of I.1. The broad
feature at about 6 eV can be assigned to two detachments of
electrons from HOMO 44b
2
of I.1 and from HOMO 49a
0
of I.2 with the final states
3
B
2
and
3
A0, respectively. The
excellent agreement between the experimentally observed
and the theoretically calculated VDEs confirms the predicted
structures of the two lowest-lying isomers I.1 and I.2 contri-
buting to the experimental PES of C
2
B
6
.
According to the CK search the global minimum structure
of the C
3
B
5
cluster is a cyclic isomer II.1 with the lowest
wheel-type structure II.5 being 62.1 kcal mol
1
higher at the
B3LYP/3-21G level of theory (Fig. 4).
Geometry optimization for the low-lying isomers
(DEo25 kcal mol
1
) and the lowest-found wheel-type
structure II.5 at the B3LYP/6-311+G* level of theory with
subsequent single point calculations at the RCCSD(T)/
6-311+G(2df)//B3LYP/6-311+G* level revealed the presented
(Fig. 4) order. Thus, according to our calculations the global
minimum is the cyclic isomer II.1 and the structural transition
from the wheel-type structure to the monocyclic ring occurs
between C
2
B
6
and C
3
B
5
.The lowest wheel-type structure
II.5 is 28.8 kcal mol
1
higher than the global minimum
(RCCSD(T)/6-311+G(2df)//B3LYP/6-311+G*).
Again, we calculated VDEs of the proposed structure II.1 to
compare those with the experimental PES. Only the isomer II.1
is expected to contribute to the experimental PES of the C
3
B
5
cluster, since the lowest alternative isomer II.2 is 13 kcal mol
1
higher than II.1. The VDE calculations were performed at
the same three levels of theory: TD-B3LYP/6-311+G(2df),
ROVGF/6-311+G(2df) and RCCSD(T)/6-311+G(2df) all at
Table 1 Comparison of the experimental VDEs with calculated values for the structures I.1 C
2v
(
2
A
1
) and I.2 C
s
(
2
A0) of the C
2
B
6
cluster. All
energies are in eV
Feature VDE (exp)
a
Final state and electronic configuration
VDE (theor.)
TD-B3LYP
b
UOVGF
c
R(U)CCSD(T)
d
I.1 C
2v
(
2
A
1
)
X02.3(1)
1
A
1
,{...5a
1(2)
4b
2(2)
6a
1(2)
1a
2(2)
2b
1(2)
7a
1(0)
} 2.26 2.59 (0.89) 2.17
B 3.54(3)
3
B
1
,{...5a
1(2)
4b
2(2)
6a
1(2)
1a
2(2)
2b
1(1)
7a
1(1)
} 3.51 3.54 (0.89) 3.64
1
B
1
,{...5a
1(2)
4b
2(2)
6a
1(2)
1a
2(2)
2b
1(1)
7a
1(1)
} 3.91 —
e
—
e
C 4.35(5)
3
A
2
,{...5a
1(2)
4b
2(2)
6a
1(2)
1a
2(1)
2b
1(2)
7a
1(1)
} 4.25 4.25 (0.88) 4.41
1
A
2
,{...5a
1(2)
4b
2(2)
6a
1(2)
1a
2(1)
2b
1(2)
7a
1(1)
} 4.46 —
e
—
e
F 4.96(5)
3
A
1
,{...5a
1(2)
4b
2(2)
6a
1(1)
1a
2(2)
2b
1(2)
7a
1(1)
} 4.93 4.88 (0.89) 5.06
G 5.7(2)
3
B
2
,{...5a
1(2)
4b
2(1)
6a
1(2)
1a
2(2)
2b
1(2)
7a
1(1)
} 5.58 5.72 (0.89) 5.79
1
A
1
,{...5a
1(2)
4b
2(2)
6a
1(1)
1a
2(2)
2b
1(2)
7a
1(1)
} 5.66 —
e
—
e
1
B
2
,{...5a
1(2)
4b
2(1)
6a
1(2)
1a
2(2)
2b
1(2)
7a
1(1)
} 5.93 —
e
—
e
I.2 C
s
(
2
A0)
X 2.2(1)
1
A0,{...9a0
(2)
10a0
(2)
2a00
(2)
3a00
(2)
11a0
(0)
} 2.19 —
f
2.09
A 3.23(2)
3
A00,{...9a0
(2)
10a0
(2)
2a00
(2)
3a00
(1)
11a0
(1)
} 3.12 —
f
3.28
1
A00,{...9a0
(2)
10a0
(2)
2a00
(2)
3a00
(1)
11a0
(1)
} 3.52 —
f
—
e
D 4.68(5)
3
A0,{...9a0
(2)
10a0
(1)
2a00
(2)
3a00
(2)
11a0
(1)
} 4.62 —
f
4.81
E 4.77(5)
3
A00,{...9a0
(2)
10a0
(2)
2a00
(1)
3a00
(2)
11a0
(1)
} 4.69 —
f
—
e
1
A00,{...9a0
(2)
10a0
(2)
2a00
(1)
3a00
(2)
11a0
(1)
} 4.91 —
f
—
e
1
A0,{...9a0
(2)
10a0
(1)
2a00
(2)
3a00
(2)
11a0
(1)
} 5.31 —
f
—
e
G 5.7(2)
3
A0,{...9a0
(1)
10a0
(2)
2a00
(2)
3a00
(2)
11a0
(1)
} 5.90 —
f
—
e
a
Numbers in parentheses represent the uncertainty in the last digit.
b
VDEs were calculated at the TD-B3LYP/6-311+G(2df)//B3LYP/6-311+G *
level of theory.
c
VDEs were calculated at the UOVGF/6-311+G(2df)//B3LYP/6-311+G* level of theory. Values in parentheses represent the pole
strength of the OVGF calculation.
d
VDEs were calculated at the R(U)CCSD(T)/6-311+G(2df)//B3LYP/6-311+G* level of theory.
e
VDE value
cannot be calculated at this level of theory.
f
These VDEs are not presented because of large spin contamination at the UHF/6-311+G(2df) level of
theory.
Table 2 Comparison of the experimental VDEs with calculated values for the structure II.1 C
2v
(
1
A
1
) of the C
3
B
5
cluster. All energies are in eV
Feature VDE (exp)
a
Final state and electronic configuration
VDE (theor.)
TD-B3LYP
b
ROVGF
c
RCCSD(T)
d
X 3.94(3)
2
B
2
{...5a
1(2)
1b
1(2)
6a
1(2)
2b
1(2)
1a
2(2)
5b
2(1)
} 3.82 3.99 (0.87) 3.94
A 4.04(3)
2
A
2
{...5a
1(2)
1b
1(2)
6a
1(2)
2b
1(2)
1a
2(1)
5b
2(2)
} 4.03 4.09 (0.88) 4.11
B 5.26(5)
2
B
1
{...5a
1(2)
1b
1(2)
6a
1(2)
2b
1(1)
1a
2(2)
5b
2(2)
} 5.06 5.26 (0.87) 5.38
C 5.47(5)
2
A
1
{...5a
1(2)
1b
1(2)
6a
1(1)
2b
1(2)
1a
2(2)
5b
2(2)
} 5.41 5.55 (0.85) 5.53
a
Numbers in parentheses represent the uncertainty in the last digit.
b
VDEs were calculated at the TD-B3LYP/6-311+G(2df)//B3LYP/6-311+G *
level of theory.
c
VDEs were calculated at the ROVGF/6-311+G(2df)//B3LYP/6-311+G* level of theory. Values in parentheses represent the pole
strength of the OVGF calculation.
d
VDEs were calculated at the RCCSD(T)/6-311+G(2df)//B3LYP/6-311+G* level of theory.
Published on 12 April 2011. Downloaded by Utah State University on 17/08/2013 00:10:23.
View Article Online
8808 Phys. Chem. Chem. Phys., 2011, 13, 8805–8810 This journal is cthe Owner Societies 2011
the B3LYP/6-311+G* optimized geometry. The VDEs calcu-
lated are summarized in Table 2. Our calculations revealed
two very close transitions corresponding to the electron
detachments from HOMO (5b
2
) and HOMO 1 (1a
2
).
The first VDE is 0.1–0.2 eV lower than that assigned to the
detachment from HOMO 1 according to all the three theory
levels. These two transitions are responsible for the features X
and A in the experimental spectra of the C
3
B
5
cluster (Fig. 2).
There is a big gap between these two transitions and the
transition corresponding to electron detachment from
HOMO 2 (2b
1
) which varies from 1.0 eV (TD-DFT) to
1.3 eV (RCCSD(T)). This computational prediction is
confirmed by the experimental spectra, showing the gap of
1.2 eV between feature A and feature B. The fourth electron
detachment occurs from HOMO 3 (6a
1
) and the calculated
VDE agrees well with the experimental value (5.47 eV). The
sharp shape of the first peak in the PES spectra of the C
3
B
5
cluster is consistent with the calculated small geometry change
upon the electron detachment (see Fig. S1, ESIw). The perfect
agreement between the experimental and the theoretical VDEs
confirms the global minimum structure II.1 for the C
3
B
5
cluster.
In order to trace structural change from the wheel-type
structure to the monocyclic ring structure we calculated the
monocyclic structure for the cluster CB
7
. The calculated
relative energy of the wheel-type global minimum structure
with respect to the monocyclic ring structure is presented in
Fig. 5 as well as the corresponding relative energies of the
monocyclic ring and wheel-type structures for the C
2
B
6
and
C
3
B
5
clusters.
One can see that the energy difference between the wheel-
type and monocyclic ring isomers dropped from 79.0 to
20.5 kcal mol
1
(at RCCSD(T)/6-311+G(2df)//B3LYP/
6-311+G*) upon transition from CB
7
to C
2
B
6
. The sub-
stitution of another boron by a carbon atom in C
3
B
5
leads to
the inversion of the wheel-type and monocyclic structures with
the monocyclic structure being now the global minimum. It
was proposed by Zubarev and Boldyrev
22
that the wheel-type
structures appear in boron clusters beginning from B
8
since the
dangling electron density at the center of the monocyclic
cluster cannot be supported by the valence charge of the boron
atoms. Migration of one of the boron atoms into the center of
the ring provides the necessary electrostatic field stabilization
in the wheel-type structures and that is the reason why those
structures are the global minima. The substitution of boron
atoms in the peripheral ring by carbon atoms provided addi-
tional electrostatic field stabilization at the center of the ring
due to the higher valence charge of carbon, which eventually
leads to the higher stability of the monocyclic structures over
the wheel-type structures in the mixed carbon–boron clusters.
It was previously shown
7
that the global minimum wheel-
type structure of CB
7
is doubly aromatic. The CB
7
mono-
cyclic structure has a conflicting aromaticity. The chemical
bonding is consistent with the higher stability of the doubly
aromatic wheel-type structure relative to the monocyclic
structure with the conflicting aromaticity.
We performed chemical analysis in the studied clusters
using the Adaptive Natural Partitioning method (AdNDP)
developed by Zubarev and Boldyrev.
23
Results of the AdNDP
analysis are summarized in the ESI.wAccording to our
AdNDP analysis, chemical bonding in the wheel-type structure
of the C
2
B
6
cluster (Fig. S2, ESIw) can be described as a
combination of three 2c–2e sB–B bonds, four 2c–2e sC–B
bonds, three delocalized p-bonds (responsible for p-aromaticity),
Fig. 3 Representative optimized isomers of the C
2
B
6
cluster, their
point group symmetries, spectroscopic states and relative energies. The
ZPE corrected energies are given at the RCCSD(T)/6-311+G(2df)//
B3LYP/6-311+G*, B3LYP/6-311+G* (in square brackets), and
B3LYP/3-21G (in curly brackets) levels of theory.
Fig. 4 Optimized isomers of the C
3
B
5
cluster, their point group
symmetries, spectroscopic states and relative energies. The ZPE
corrected energies are given at the RCCSD(T)/6-311+G(2df)//
B3LYP/6-311+G*, B3LYP/6-311+G* (in square brackets), and
B3LYP/3-21G (in curly brackets) levels of theory.
Published on 12 April 2011. Downloaded by Utah State University on 17/08/2013 00:10:23.
View Article Online
This journal is cthe Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 8805–8810 8809
three delocalized doubly-occupied s-bonds and one deloca-
lized singly-occupied s-bond. Here and elsewhere the terms
delocalized s-bonds and delocalized p-bonds mean that those
bonds cannot be reduced to 2c–2e bonds by the AdNDP
method. It was previously proposed
24
to name cases like the
one we have here with an odd number of electrons for
s-delocalized bonds as 1
2-s-antiaromatic. ‘‘1
2’’ is used as a label
and means that the s-system with singly occupied delocalized
bond is half way down to being s-antiaromatic in the
wheel-type structure of the doubly-charged C
2
B
62
ion. The
1
2-s-antiaromatic nature of the global minimum structure I.1
of the C
2
B
6
cluster is consistent with relatively low first VDE
of the cluster. When the extra electron in the C
2
B
6
cluster is
removed from the singly-occupied orbital the resulting neutral
C
2
B
6
species becomes a doubly-aromatic system which is
consistent with the round structure of the neutral species
(Fig. S1, ESIw).
The chemical bonding in the monocyclic structure I.6 of the
C
2
B
6
cluster can be described as follows. There are four 2c–2e
sB–B bonds, four 2c–2e sC–B bonds, three delocalized
p-bonds (responsible for p-aromaticity), two delocalized
doubly-occupied s-bonds and one delocalized singly-occupied
s-bond. Thus, the structure I.6 is 1
2-s-aromatic since the singly
occupied delocalized bond is half way down to being
s-aromatic in the monocyclic ring-type structure of the
doubly-charged C
2
B
62
ion (see Fig. S2, ESIw). The presence
of 1
2-s-antiaromaticity in the wheel-type structure and of
the 1
2-s-aromaticity in the monocyclic structure explains the
relatively low energy difference compared to that of the CB
7
structures (Fig. 5).
The global minimum monocyclic structure II.1 of the C
3
B
5
cluster (Fig. S3, ESIw) is doubly-aromatic with two 2c–2e s
B–B bonds, six 2c–2e sC–B bonds, three delocalized p-bonds
(responsible for p-aromaticity), three delocalized s-bonds
(responsible for s-aromaticity). The doubly-aromatic nature
of the global minimum structure C
3
B
5
is consistent with the
rather high first VDE of this cluster.
Chemical bonding analysis of the lowest-lying wheel-type
isomer II.5 (Fig. S3, ESIw) revealed one 2c–2e sB–B bond, six
2c–2e sC–B bonds, three delocalized p-bonds (responsible for
p-aromaticity) and four delocalized s-bonds (responsible for
s-antiaromaticity). The s-antiaromaticity leads to deformation
of the heptagon structure into the hexagon structure with one
carbon atom coordinated to the edge of the hexagon. As a
result of that we have three s-bonds delocalized over
the hexagon and one 3c–2e s-bond delocalized over the
external carbon atom and the two edge boron atoms
(see Fig. S3, ESIw). The structure II.5 possessing conflicting
aromaticity is higher in energy than the doubly-aromatic
global minimum structure II.1.
In the above discussion we presented chemical bonding
explanation for different stabilities of the wheel-type and
monocyclic ring-type structures. With the chemical bonding
analysis we can explain why the C
2
B
6
cluster has relatively
low first VDE compared to that of the C
3
B
5
cluster. However
we would like to stress that we believe that the transition from
the wheel-type to the ring-type structures in the series occurs
due to the increase of the stabilizing electrostatic field at the
center of the cluster as a result of the increased number of
carbon atoms in C
3
B
5
, which makes the presence of the
central boron atom unnecessary.
Experimental section
The experiment was performed using a magnetic-bottle PES
apparatus equipped with a laser vaporization cluster source,
details of which have been published elsewhere.
25,26
Briefly,
the carbon-doped boron clusters were produced by laser
vaporization of a disk target made of isotopically enriched
10
B(B6% wt), C (B0.6% wt), and Bi. The clusters were
entrained by the helium carrier gas supplied by two pulsed
Jordan valves and underwent a supersonic expansion to form
a collimated molecular beam. The cluster composition and the
cooling were controlled by the time delay between the pulsed
beam valves and the vaporization laser. The negatively
charged clusters were extracted from the cluster beam and
analyzed with a time-of-flight mass spectrometer. The clusters
of interest were mass selected and decelerated before being
intercepted by the probe photodetachment laser beam: 193 nm
(6.424 eV) from an ArF excimer laser and 355 nm (3.496 eV)
or 266 nm (4.661 eV) from a Nd:YAG laser. Photoelectrons
were collected at nearly 100% efficiency by a magnetic bottle
and analyzed in a 3.5 m long electron flight tube. The cluster
PE spectra were calibrated using the known spectra of Bi
.
The kinetic energy resolution of the magnetic bottle apparatus,
DE/E, was typically better than 2.5%, i.e. B25 meV for 1 eV
electrons.
Theoretical section
We searched for the global minimum of the C
2
B
6
and C
3
B
5
clusters using the Coalescence Kick (CK) program
19–21
with
the B3LYP/3-21G method for energy and gradient calculations.
Then we reoptimized the geometries and performed frequency
calculations for the lowest isomers (Eo25 kcal mol
1
)at
the B3LYP/6-311+G* level of theory and recalculated total
Fig. 5 Wheel-type to monocyclic ring structural transition in the
series of the C
x
B
8x
(x= 1–3) clusters. Relative energies are given at
RCCSD(T)/6-311+G(2df)//B3LYP/6-311+G*.
Published on 12 April 2011. Downloaded by Utah State University on 17/08/2013 00:10:23.
View Article Online
8810 Phys. Chem. Chem. Phys., 2011, 13, 8805–8810 This journal is cthe Owner Societies 2011
energies of the isomers at the RCCSD(T)/6-311+G(2df)//
B3LYP/6-311+G* level of theory. The VDEs for the global
minima I.1 and II.1 and the low-lying isomer I.2 were calcu-
lated using the R(U)CCSD(T)/6-311+G(2df) method, the
outer-valence Green Function method (R(U)OVGF/
6-311+G(2df)) and the time-dependent DFT method
(TD B3LYP/6-311+G(2df) at the B3LYP/6-311+G * g e o m e t r i e s .
The calculations were performed with the Gaussian 03
27
and
Molpro
28
software. Molecular orbitals were visualized with
the MOLDEN 3.4
29
and Molekel 5.4.0.8
30
programs.
Acknowledgements
The experimental work at Brown University was supported by
the National Science Foundation (DMR-0904034).
The theoretical work at Utah State University was supported
by the National Science Foundation (CHE-1057746). An
allocation of computer time from the Center for High
Performance Computing at the University of Utah is
gratefully acknowledged. Computer time from the Center for
High Performance Computing at Utah State University is also
gratefully acknowledged.
Notes and references
1 A. V. Orden and R. J. Saykally, Chem. Rev., 1998, 98, 2313.
2 A. N. Alexandrova, A. I. Boldyrev, H.-J. Zhai and L. S. Wang,
Coord. Chem. Rev., 2006, 250, 2811.
3 L. M. Wang, B. B. Averkiev, J. A. Ramilowski, W. Huang,
L. S. Wang and A. I. Boldyrev, J. Am. Chem. Soc., 2010, 132,
14104.
4 K. Exner and P. v. R. Schleyer, Science, 2000, 290, 1937.
5 Z. X. Wang and P. v. R. Schleyer, Science, 2001, 292, 2456.
6 R. M. Minyaev, T. N. Gribanova, A. G. Starikov and
V. I. Minkin, Mendeleev Commun., 2001, 11, 213.
7 B. B. Averkiev, D. Yu. Zubarev, L. M. Wang, W. Huang,
L. S. Wang and A. I. Boldyrev, J. Am. Chem. Soc., 2008, 120, 9248.
8 L. M. Wang, W. Huang, B. B. Averkiev, A. I. Boldyrev and
L. S. Wang, Angew. Chem., Int. Ed., 2007, 46, 4550.
9 B. B. Averkiev, L. M. Wang, W. Huang, L. S. Wang and
A. I. Boldyrev, Phys. Chem. Chem. Phys., 2009, 11, 9840.
10 I. Boustani, Phys. Rev. B: Condens. Matter, 1997, 55, 233.
11 V. Bonacic-Koutecky, P. Fantucci and J. Koutecky, Chem. Rev.,
1991, 91, 1035.
12 H.-J. Zhai, A. N. Alexandrova, K. A. Birch, A. I. Boldyrev and
L. S. Wang, Angew. Chem., Int. Ed., 2003, 42, 6004.
13 A. N. Alexandrova, H.-J. Zhai, L. S. Wang and A. I. Boldyrev,
Inorg. Chem., 2004, 43, 3552.
14 Q. S. Li, Y. Zhao, W. Xu and N. Li, Int. J. Quantum Chem., 2005,
101, 219.
15 J. M. L. Martin and P. R. J. Taylor, J. Phys. Chem., 1996, 100,
6047.
16 Yu. Shlyakhter and S. Sokolova, J. Chem. Phys., 1999, 110, 10725.
17 J. Shao, C. He, R. Shi, C. Wang, X. Zhu and X. Lu, THEOCHEM,
2010, 961,17.
18 L. S. Wang, H. S. Cheng and J. Fan, J. Chem. Phys., 1995, 102,
9480.
19 B. B. Averkiev, PhD thesis, Utah State University, USA, 2009.
20 W. Huang, A. P. Sergeeva, H.-J. Zhai, B. B. Averkiev, L. S. Wang
and A. I. Boldyrev, Nat. Chem., 2010, 2, 202.
21 M. Saunders, J. Comput. Chem., 2004, 25, 621.
22 D. Yu. Zubarev and A. I. Boldyrev, J. Comput. Chem., 2007, 28,
251.
23 D. Yu. Zubarev and A. I. Boldyrev, Phys. Chem. Chem. Phys.,
2008, 10, 5207.
24 D. Yu. Zubarev, A. P. Sergeeva and A. I. Boldyrev, in Chemical
Reactivity Theory. A Density Functional View, ed. P. K. Chattaraj,
CRC Press. Taylor & Francis Group, New York, 2009,
pp. 439–452.
25 J. W. Fan and L. S. Wang, J. Chem. Phys., 1995, 102, 8714–8717.
26 L. S. Wang and X. Li, in Proc. Int. Symp. on Clusters and
Nanostructure Interfaces, Oct. 25–28, 1999, Richmond, VA, ed.
P. Jena, S. N. Khanna and B. K. Rao, World Scientific, River
Edge, New Jersey, 2000, pp. 293–300.
27 M. J. Frisch, et al.,Gaussian 03, Revision D.01, see ESIw.
28 H.-J. Werner, et al.,MOLPRO, version 2006.1, see ESIw.
29 G. Schaftenaar, MOLDEN 4.3, CAOS/CAMM Center, The
Netherlands, 1998.
30 U. Varetto, Molekel 5.4.0.8, Swiss National Supercomputing
Centre, Manno, Switzerland, 2009.
Published on 12 April 2011. Downloaded by Utah State University on 17/08/2013 00:10:23.
View Article Online
