Central-atom size effects on the methyl torsions of group XIV tetratolyls

Article abstract of DOI:10.1002/chem.201202105

The Group XIV tetratolyl series X(C₆H₄-CH ₃)₄ (X=C, Si, Ge, Sn, Pb) were studied by using inelastic neutron scattering to measure the low-energy phonon spectra to directly access the methyl-group torsional modes. The effect of increased molecular radius as a function of the size of the central atom was shown to have direct influence on the methyl dynamics, reinforced with the findings of molecular dynamics and contact surface calculations, based upon the solid-state structures. The torsional modes in the lightest analogue were found to be predominantly intramolecular: the Si and Ge analogues have a high degree of intermolecular methyl-methyl group interactions, whilst the heaviest analogues (Sn and Pb) showed pronounced intermolecular methyl interactions with the whole phonon bath of the lattice modes. Size matters! The size of the central atom in the Group XIV tetratolyls was shown to determine the solid-state structures courtesy of the effect that it has on the overall size of the molecules (see figure). Because the outer-most methyl groups enter into close intermolecular interactions (for the Si and Ge analogues), the structure is driven to lower symmetry; as a consequence, the smallest (C) and largest (Sn and Pb) analogues are isostructural. Copyright

Full text of DOI:10.1002/chem.201202105

FULL PAPER
DOI: 10.1002/chem.201202105
Central-Atom Size Effects on the Methyl Torsions of Group XIV Tetratolyls
Maggie C. C. Ng,[a] Jason B. Harper,[a] Anton P. J. Stampfl,[b] Gordon J. Kearley,[b]
Stꢀphane Rols,[c] and John A. Stride*[a, b]
Abstract: The Group XIV tetratolyl
series X(C6H4-CH3)4 (X=C, Si, Ge, Sn,
ics, reinforced with the findings of mo-
lecular dynamics and contact surface
calculations, based upon the solid-state
structures. The torsional modes in the
lightest analogue were found to be pre-
dominantly intramolecular: the Si and
Ge analogues have a high degree of in-
termolecular methyl–methyl group in-
teractions, whilst the heaviest ana-
logues (Sn and Pb) showed pronounced
intermolecular methyl interactions with
the whole phonon bath of the lattice
modes.
ACHTUNGTRENNUNG
Pb) were studied by using inelastic
neutron scattering to measure the low-
energy phonon spectra to directly
access the methyl-group torsional
modes. The effect of increased molecu-
lar radius as a function of the size of
the central atom was shown to have
direct influence on the methyl dynam-
Keywords: crystal engineering
·
density functional calculations · in-
termolecular interactions · molecu-
lar dynamics · neutron scattering
Introduction
symmetry space group Pc[6] (Figure 1; CCDC references for
all species are: CTol4: 712743, SiTol4: SUCZIZ, GeTol4:
SUCZOF, SnTol4: PTOLSN, PbTol4: SOMMEM). The ra-
tionale behind this rather bizarre behavior is believed to lie
in the contact interactions of the methyl groups on neigh-
boring molecules. Thus, in CTol4, the relatively small molec-
ular radius results in methyl group dynamics independent of
those on neighboring molecules. Meanwhile, in SnTol4 and
PbTol4, the methyl groups interdigitate with the phenyl
groups of nearest neighbor molecules effectively coupling
them to the lattice modes. In SiTol4 and GeTol4, the molecu-
lar radii are such that the methyl groups on neighboring
molecules are potentially in direct contact, resulting in the
lowering of symmetry. Whilst the tunneling spectra gave a
direct indication of this behavior, we have now sought to di-
rectly observe the methyl dynamics in these materials to
confirm this simple, yet effective rationale. Because pure
torsional modes are inactive in both infrared and Raman
spectroscopies due to the absence of a modulation in the
dipole moment or polarizability tensor, this requires inelas-
tic scattering of thermally moderated neutrons, for which
there are no vibrational selection rules.[7]
The tetraphenyl derivatives of Group XIV elements have
great promise as supramolecular constructs in extended
porous networks.[1–3] The tetratolyl Group XIV derivatives
are important precursors toward readily usable constructs
courtesy of the ease of functionalization at the site of the
para-methyl group, yet they make a fascinating molecular
series in their own right. The methyl groups constitute the
very outer contact sphere of individual molecules and so in-
termolecular interactions potentially dominate the methyl-
group dynamics. We have previously shown that the methyl-
tunneling spectra of this series, recorded at high-resolution
inelastic neutron scattering, vary dramatically across the
series.[4] Because the tolyl groups are chemically similar
throughout the series, the dominant effect is simply that of
molecular size, which is dictated by the central atom. The
ꢀ
smallest analogue, tetratolylmethane, CACHTUNRTGNE(UNG C6H4 CH3)4 (CTol4)
is, in fact, isostructural with the largest two of the series,[4,5]
the stannane and plumbane analogues (all in space group
¯
I4), with the silane and germane species lying in the lower-
[a] Dr. M. C. C. Ng, Dr. J. B. Harper, Prof. J. A. Stride
School of Chemistry, University of New South Wales
Sydney 2052 (Australia)
Experimental Section
Fax : (+61)2-9385-6141
E-mail: j.stride@unsw.edu.au
[b] Dr. A. P. J. Stampfl, Prof. G. J. Kearley, Prof. J. A. Stride
Bragg Institute, Australian Nuclear Science and
Technology Organisation, PMB 1
General: The silane, germane, and stannane compounds were synthesized
by using a general reaction Scheme involving the nucleophilic addition of
an organometallic reagent to the appropriate tetrachloride (Scheme 1 in
the Supporting Information). The synthesis of the carbon analogue pro-
ceeded through the corresponding tetraphenyl derivative (Scheme 2 in
the Supporting Information) and the plumbane through lead(II) chloride.
Single-crystal X-ray datasets were collected for all of the compounds and
were found to be consistent with published data, with no phase transi-
tions observed down to T=150 K; X-ray powder-diffraction profiles were
Menai, NSW 2234 (Australia)
[c] Dr. S. Rols
Institute Laue-Langevin, 6 rue Jules Horowitz
BP156, 38042 Grenoble Cedex 9 (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202105.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
1
&
ÞÞ
These are not the final page numbers!

In the case of the Taipan instrument, the samples were loaded into alumi-
num-foil sachets and mounted onto a cadmium plate, in such manner
that data were obtained in reflection, and loaded into a bottom-loading
closed-cycle refrigerator. Taipan was configured to a final energy of Ef =
13.7 meV at scattering angles 2q of 40.78ꢁ2qꢁ20.08 with a range of inci-
dent energies 15ꢁEi ꢁ60 meV, giving a near-linear range of Q between
2.4 and 4.4 ꢁꢀ1. Data were collected in a stepwise manner and normal-
ized to incident monitor counts.
DFT calculations: A single unit cell was used for the silicon analogue,
whilst to obtain a large enough unit to prevent interaction wrap around
from the periodic boundaries, a 112 supercell (in unit cell units) was used
for the carbon and tin compounds. All DFT calculations were carried out
with VASP[12–15] by using the project augmented wave potential[16] (PAW)
and the Perdew–Burke–Ernzerhof[17] exchange-correlation functional
with the unit cell parameters being constrained to the experimental
values. The atomic positions for all cells were optimized by minimizing
the forces acting on the atoms by using VASP with high precision:
energy cut-off of 500 eV, a convergence condition of 10ꢀ4 eV, and a 3ꢂ3ꢂ
6 Monkhorst–Pack k point mesh.[18] These minimizations are in excellent
agreement with the experimentally determined crystal structures other
than slight changes to the methyl-group orientations. These minimized
structures were used as the starting point for the MD simulations that
were carried out at lower precision with an energy cut-off of 300 eV and
a single k point. All MD simulations used a time step of 1 fs with a
target temperature of 40 K. An initial isokinetic run with velocity scaling
of 4 ps was made followed by a thermalization run in the microcanonical
ensemble of 8 ps and a production run of 17 ps. Trajectories from the pro-
duction run were stored for analysis.
Results and Discussion
The datasets from the two instruments are complementary,
because they cover different regions of reciprocal space:
IN4 accessing a large region of S
a function of both momentum transfer, Q and energy trans-
fer, w), whilst Taipan accessed a cut through S(Q,w), but to
ACHTUNGTNER(NUNG Q,w) (the scattering law as
AHCTUNGTRENNUNG
a higher w. Both datasets are in full accord with each other
and provided vibrational data up to 47 meV (380 cmꢀ1) cov-
ering the low-energy lattice modes and large-body reorienta-
tions. No significant change in the spectra recorded on IN4
was observed as a function of increased temperature, other
than a moderate degree of mode softening and an increased
thermal background. The low-temperature spectra for each
of the analogues, as was measured on IN4, is shown in
Figure 2 and the complementary Taipan data shown in
Figure 3.
Figure 1. Structure of CTol4 viewed along the c axis,[2] isostructural to the
Sn (top), and Pb analogues and SiTol4 viewed along the a axis,[4] isostruc-
tural to GeTol4 (bottom). Carbon atoms: grey, hydrogen atoms: orange,
silicon atoms: yellow; note the different methyl group environments.
All of the samples displayed vibrational excitations with a
Q dependence characteristic of such modes. The low-energy
spectrum of CTol4 is dominated by a single-methyl torsional
band, only marginally broader than the instrument resolu-
tion, with acoustic phonon branches around 5–10 meV and
low-lying molecular deformations between 20 and 30 meV.
also collected for all bulk samples and were found to correspond to the
calculated profiled based upon the structural data (Scheme 3 in the Sup-
porting Information). Inelastic neutron scattering (INS) data were re-
corded on two spectrometers, the IN4 time-of-flight spectrometer at ILL,
Grenoble, France over a range of temperatures, 2ꢁTꢁ65 K[8] (all five
analogues) and at T=10 K on the triple-axis spectrometer Taipan at the
Bragg Institute, ANSTO, Australia (CTol4, SiTol4 and SnTol4).[9]
¯
Despite possessing the same I4 symmetry, the spectra of the
Sn and Pb analogues displayed less pronounced features,
with spectral intensity grouped into a broad feature up to
approximately 15 meV and a second feature around 20–
25 meV. In contrast, the lower symmetry (Pc) of the Si and
Ge analogues relates to an apparent splitting of the methyl
torsions, consistent with the presence of either nonequiva-
lent, or interacting methyl sites. The Taipan data follows a
In the case of the IN4 instrument, the samples were loaded into flat alu-
minum cans, in such manner that the sample transmission was about
90% (0.1 mm thickness), and loaded into a cryostat prior to irradiation.
The instrument was configured for an incident wavelength of 1.52 ꢁ
(Ei =35.3 meV). Data were corrected for that of the empty can and were
normalized with a vanadium sample to correct for different detector effi-
ciencies and the number of scattering centers in the samples. All data
manipulation was performed by using the LAMP program at ILL.[10]
&
2
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Central-Atom Size Effects on the Methyl Torsions
FULL PAPER
Figure 2. INS spectra of XTol4 (X=C, Si, Ge, Sn, Pb) displayed as re-
corded on IN4 and displayed as a surface in SCAHTUNGTNER(NUNG Q,w). The false color plots
have greater intensity moving from blue to red. The trace superimposed
on the C, Si, and Sn spectra represent the trajectory through SACHTNUTGRNEUNG(Q,w) of
the Taipan data.
Figure 4. Low-energy INS spectra across the XTol4 series from CTol4
(top) to PbTol4 (bottom) obtained at T=1.5 K. The methyl torsions lie in
the region of 10–20 meV; dotted lines represent the symmetry splitting of
the methyl torsions. Spectral resolution is indicated by solid black line su-
perimposed onto the CTol4 spectrum.
from CTol4 down to PbTol4, despite the symmetry associated
considerations. The rational explanation for this behavior is
that the symmetry allowed phonon spectrum becomes more
extensive, when the central atom increases in size when
¯
moving through the I4 isostructural compounds CTol4,
SnTol4, and PbTol4. In the case of the C analogue, the
methyl groups undergo largely unimolecular methyl torsions,
which are then increasingly mixed into the phonon spec-
trum, when the central atom effectively exerts intermolecu-
lar pressure, resulting in a dispersive behavior. This results
in the systematic decrease of the spectral intensity of the
peak assigned to be predominantly due to pure-methyl tor-
sions (an approximation, which is evidently less valid upon
descending Group XIV) to give relative intensities of
7.67:1.31:1 for the C, Sn, and Pb analogues, respectively. For
the particular case of incoherent neutron scattering in the
one-phonon approximation, the intensity I can be represent-
ed as follows:[11]
Figure 3. INS spectra obtained on Taipan at 10 K. Spectra correspond to
a single, essentially linear path through S
sponding to 1.3ꢁwꢁ26.3 meV. Data points correspond to recorded spec-
tra and solid lines correspond to calculated spectral functions scaled only
in intensity, with no least-squares fitting to the data.
AHCTUNGTRENNUNG
given trajectory through SACTHNUTRGNEUGN(Q,w), which was found to be
fully consistent with that obtained on IN4. This general pic-
ture of the methyl-torsional dynamics is in full accord with
both the crystalline structures and the high-resolution
methyl-tunneling spectra reported previously.[4]
By summing across the whole of the measured SACTHNUGTRNEUNG(Q,w)
surface, a vibrational density of states across the accessible
regions of the Brillouin zone was obtained (Figure 4).
Again, this representation highlights the fact that discrete
molecular excitations become less pronounced in moving
ꢀ
ꢁ
ꢂ
ꢃ
Q2 u2
nðwÞ þ 1
I / SincðQ; wÞ / sQ2 exp ꢀ
gðwÞ
3
w
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
3
&
ÞÞ
These are not the final page numbers!

J. A. Stride et al.
in which s is the total neutron-scattering cross-section of the
molecule, n(w) is the Bose occupation factor, and <u(T)2 >
is the mean-square displacement of the atoms, and g(w) is
the density of states. The incoherent cross-section of protons
is 80.26ꢂ10ꢀ24 cm2, which is around an order of magnitude
larger than any other cross-section in the samples studied.
Therefore, the recorded spectra are heavily proton weight-
ed, and so only proton motions need be considered in g(w)
to a first approximation. When all of the IN4 spectra were
normalized for the molar concentration (the effective
number of scattering centers), the partial cross-sections of
the methyl groups were equal in each spectrum, causing the
final intensity of the methyl torsions to be proportional to
the mean-square displacement <u(T)2 > of the protons un-
dergoing the methyl torsion. Therefore, the relative ampli-
tudes of motion in CTol4, SnTol4, and PbTol4 are
2.76:1.15:1.0, respectively. This picture of increasingly re-
stricted motion is consistent with the methyl groups in CTol4
being relatively sterically unhindered, whilst in SnTol4 and
PbTol4, the methyl groups are increasingly hindered due to
the interdigitation into the pockets between the aromatic
groups of neighboring molecules, resulting in greater mixing
with the general-phonon spectrum. In contrast, the methyl-
torsional amplitudes in SiTol4 and GeTol4 are essentially the
same, and both are similar to that in CTol4; however, this in-
tensity is distributed over the symmetry-split modes.
To evaluate the notion of increasingly hindered Me
groups upon descending Group XIV beyond the single-
phonon approximation, detailed molecular dynamics were
performed for the C, Si and Sn analogues. The interactions
of interest in the present study are comparatively short
range, so that to avoid the uncertainties of empirical force-
field methods, DFT methods were used to calculate the en-
ergies (see the Experimental Section).
The main validation of the MD simulations is their ability
to reproduce the recorded INS spectra, which were obtained
by using the program nMoldyn.[19] The measured dynamical
structure factor SACHTUNGTRENNUNG(Q,w) is related to the simulated trajecto-
ries through a Fourier transform of the incoherent inter-
mediate scattering function:
X
FðQ; tÞ ¼
bihexp½ꢀiQ ꢂ Rið0Þꢃexp½iQ ꢂ RiðtÞꢃi
Figure 5. Comparison of recorded and calculated spectra for XTol4 (X=
C, Si, Sn). Calculated spectra are shown for all atoms (complete spec-
trum) and partial spectra— only for Me group and only aromatic-group
hydrogen atoms.
in which the momentum transfer Q and time t are on a regu-
lar grid, R(0) is the atomic position at t=0, and b is the
total scattering cross-section (in the incoherent approxima-
tion). The summation was done over all atoms (i), but it ap-
proximates to a sum over only the hydrogen atoms due to
the very large incoherent cross section of H. A comparison
of the calculated spectra of the analogues with the spectra
recorded on Taipan and IN4 (Figures 3 and 5, respectively)
shows reasonable agreement, and is certainly adequate for
an assignment of the regions of spectral intensity.
quency spectra of the methyl and aromatic protons. Whilst
the motion of the Me protons will be principally a convolu-
tion of the methyl-group torsions with torsions of the aro-
matic ring, the ring protons will only have the latter motion.
These frequency spectra (intensity was transformed to be
INS spectra) are illustrated in Figure 5. In the case of the C
and Si analogues, the total spectrum is dominated by the
motion of the Me groups, with only marginal contribution
from the ring hydrogen atoms. However, this distinction is
not as apparent in the case of the Sn compound, in which
Our major interest is in the dynamics of the methyl
groups and the aromatic rings to which they are attached.
They are easily separated by calculating the separate fre-
&
4
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Central-Atom Size Effects on the Methyl Torsions
FULL PAPER
the Me and ring hydrogen atoms both make considerable
contributions to the total spectrum, consistent with the pic-
ture of increased mode mixing in the larger analogues. As a
final verification of this model, the low-energy phonon spec-
trum can be approximated to the Fourier transform of the
autocorrelation function of the whole body motions of the
molecules within the boundary confines of the molecular dy-
namics simulation (Figure 6). Whilst the molecular motions
Figure 6. Fast Fourier transform of the autocorrelation function of whole-
body motions for XTol4 (X=C, Si, Sn).
of the C and Si analogues are largely separated from the re-
spective Me group dynamics, this is not the case for the Sn
system, representing a considerable mode mixing between
the Me motions and the molecular-phonon spectrum in the
latter.
The degree of mode mixing induced by nearest-neighbor
interactions can be assessed by investigating the Hirshfeld
molecular surfaces calculated from the solid-state structures,
effectively the contact surfaces of discrete molecules.[20] By
using the freely available CrystalExplorer package,[21] the
Hirshfeld surfaces of the XTol4 molecules were investigated
based upon their published X-ray structures with hydrogen
atoms added in riding positions. Figure 7 shows the value
dnorm mapped onto the surface for each analogue: dnorm nor-
malizes the interatomic separation for the corresponding
van der Waals interactions, and as such, negative values
(shown in red) have interatomic separations below the van
der Waals contact distance, whilst blue regions on the sur-
face have positive dnorm values, or separations greater than
the corresponding van der Waals radii. As such, white re-
gions mapped onto the Hirshfeld surface represent near van
der Waals interactions, which can be seen to generally in-
crease moving down the group from the C to Pb analogues.
In CTol4, the closest interactions are intermolecular H···C
interactions and H···H contacts at the ring (Ar) sites.
Moving to the lower symmetry Si and Ge analogues, the
prominent (below van der Waals) interactions are dominat-
ed by intermolecular Me interactions, with additional inter-
Figure 7. Hirshfeld surfaces mapped with effective van der Waals interac-
tions (dnorm; red hvan der Waals; bluei van der Waals) for XTol4 (X=C,
Si, Ge, Sn, Pb). Inset: dependence of nearest-neighbor contact types with
~
&
*
=
the size of the central atom.
=H(i)···H(e);
=C(i)···H(e);
H(i)···C(e).
molecular H···C contacts. Back to the high-symmetry Sn and
Pb analogues and the Me interactions are more coupled to
the general phonon bath, with hot spots corresponding to in-
ꢀ
termolecular Me C and H(Ar)···C contacts in SnTol4 and
only the H(Ar)···C contacts in PbTol4, although the regions
of near van der Waals interactions are more pronounced
throughout the whole of the surfaces. Further analysis of
these surfaces provides insights to the different types of in-
teractions according a % contribution to the complete sur-
face: in this way, atoms inside the Hirshfeld surface, X(i),
can be correlated to atoms external to the surface, X(e),
providing
a greater impression of the overall contact
terms.[22] Generally, H···H interactions decrease as the cen-
tral atom increases in size, with a corresponding increase in
C···H interactions, when the whole structure becomes more
interdigitated. Indeed, a plot of the relative contributions of
H(i)···H(e), C(i)···H(e), and H(i)···C(e) display a linear re-
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
5
&
ÞÞ
These are not the final page numbers!

J. A. Stride et al.
sponse to the central-atom size, underlining the size effect
of X in these materials.
the tunneling spectrum of SiTol4 with a mode at
2.82(1) meV can only be rationalized with the observed tor-
sional spectrum in a sixfold potential having a barrier height
of 24.5 meV, giving torsions at 20.5 and 23 meV. This pic-
ture, purely based on the observed spectral modes, only ob-
tainable in a combination of neutron-scattering experiments,
is fully consistent with the crystallographic data and calcu-
lated Hirshfeld surfaces; isolated rotors in CTol4, interacting
rotors in SiTol4 and GeTol4, and rotors more generally cou-
pled into the phonon bath in SnTol4 and PbTol4 as a result
of interdigitation between neighboring molecules.
Finally, the present work based upon the direct observa-
tion of the methyl torsions, MD simulations, and near-neigh-
bor contacts across the whole series is fully consistent with
our previously published work into the methyl-tunneling
spectra.[4] The tunneling spectra are uniquely sensitive to the
methyl-group environment,[23] and the tunneling spectra of
the XTol4 compounds highlighted that the methyl groups
became increasingly hindered moving down the Group.[4]
However, now we have the complete picture with both the
tunneling and torsional frequencies, which allows to accu-
rately evaluate the local rotational potential and to compare
that to the time-averaged contact surfaces based upon the
crystallographic data. The rotational potential of a rigid
methyl-group rotor hindered by a potential barrier V, when
it precesses through an angle f, and can be expressed by the
Hamiltonian:[24]
Conclusion
The Me motions in the series of compounds XTol4 (X=C,
Si, Ge, Sn, Pb) are hindered by the effects of molecular
packing when the central atom size increases, resulted in an
initial increase in the mean vibrational frequency and finally
resulted in strongly perturbed methyls by virtue the restrict-
ed amplitude of vibration, making the series an ideal-case
study for the confinement of diffusive motions. The charac-
teristic behavior of the Me groups upon central-atom substi-
tution are fully in accord with both the crystallographic and
spectroscopic data and is clearly rationalized with the con-
cept of increased molecular crowding or chemical pressure;
reinforced with detailed molecular dynamics simulations
and numerical modeling of the molecular contact surfaces,
which highlight nearest-neighbor contacts.
ꢀꢀh2 d2
H ¼
þ V
dꢀ2
2I
in which the potential I is the moment of inertia of the
rotor, and V can be represented as a Fourier expansion:
X
1
2
V ¼
V3nf1 þ cos½3nðꢀ þ f3nÞꢃg
nꢄ1
in which only the first two terms, n=1 and 2 (V3 and V6),
are considered (with a relative phase of f=f6ꢀf3), allowing
the definition of the potential-energy trajectory, based on
tunneling frequencies, which can then be directly related to
the torsional modes (Table 1). The CTol4 tunneling spectra
gave three distinct frequencies at 4.99(3), 6.87(31), and
8.52(24) meV, which correspond to calculated first torsional
modes at 12.8, 12.1, and 11.8 meV, respectively (and a
second torsion at 22 meV), with threefold potential energy
barriers of 33, 35.5, and 31.0 meV. Likewise, the estimated
tunneling frequency of SnTol4 of 0.6 meV suggests a first tor-
sional frequency of 16 meV and a second at 30 meV, with a
threefold potential with a barrier height of 55 meV. Finally,
Acknowledgements
This research was supported by UNSW ECR and ARC (DP0880199)
grants and an AMFRP access grant (No. 07/08-N-16).
[1] a) D. Lalibertꢃ, T. Maris, J. D. Wuest, Can. J. Chem. 2004, 82, 386–
398; b) H. M. El-Kaderi, J. R. Hunt, J. L. Mendoza-Cotꢃs, A. P.
Cꢄtꢃ, R. E. Taylor, M. OꢅKeeffe, O. M. Yaghi, Science 2007, 316,
268–272.
[2] W. Lu, D. Yuan, D. Zhao, C. I. Schilling, O. Plietzsch, T. Muller, S.
Brꢆse, J. Guenther, J. Blꢇmel, R. Krishna, Z. Li, H.-C. Zhou, Chem.
Mater. 2010, 22, 5964–5972.
Table 1. Calculated torsional frequencies based on the measured tunnel-
ing modes Ref [4] and are directly comparable to the inelastic neutron-
scattering spectra.[a]
[3] A. Trewin, A. I. Cooper, Angew. Chem. 2010, 122, 1575–1577;
Angew. Chem. Int. Ed. 2010, 49, 1533–1535.
[4] M. C. C. Ng, D. J. Craig, J. B. Harper, L. van-Eijck, J. A. Stride,
Chem. Eur. J. 2009, 15, 6569–6572.
[5] M. Charissꢃ, S. Roller, M. Drꢆger, J. Organomet. Chem. 1992, 427,
23–31.
Tunneling Calculated torsional fre-
frequency quency [meV]
Barrier Rotational
height potential
A
ACHTUNGTNER[NUNG meV]
[6] C. Schneider, M. Drꢆger, J. Organomet. Chem. 1991, 415, 349–362.
[7] P. C. H. Mitchell, S. F. Parker, A. J. Ramirez-Cuesta, Vibrational
Spectroscopy with Neutrons: With applications in Chemistry, Biology,
Materials Science and Catalysis, World Scientific Publishing,
London, 2005.
[8] a) http://www.ill.fr/YellowBook/IN4/; b) G. Cicognani, H. Mutka, F.
Sacchetti, Physica B 2000, 276–278, 83–84.
[9] S. Danilkin, G. Horton, R. Moore, G. Braoudakis, M. Hagen, J. of
Neutron Research 2007, 15, 55–60.
CTol4 4.99
6.57
8.53
SiTol4 2.82
12.8
12.1
11.8
13.7
20.5
16
22
22
22
23
33
threefold
35.5
31.0
40.5
24.5
55
threefold
sixfold
threefold
SnTol4 0.6
30
¯
[a] XTol4 species within the I4 space group are best described with three-
fold potentials, whilst those in Pc have sixfold potentials consistent with
interacting rotors (shaded threefold potential for SiTol4 highlights a poor
correlation to observation).
[10] LAMP, the Large Array Manipulation Program, http://wwwold.ill.fr/
data treat/lamp/lamp.html.
&
6
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Central-Atom Size Effects on the Methyl Torsions
FULL PAPER
[11] S. W. Lovesey, Theory of Neutron Scattering from Condensed Matter,
Clarendon, Oxford, 1984.
[20] M. A. Spackman, D. Jayatilaka, CrystEngComm 2009, 11, 19–32.
[21] http://ra.bcs.uwa.edu.au/CrystalExplorer/wiki/index.php/Main_Page.
[22] J. J. McKinnon, D. Jayatilaka, M. A. Spackman, Chem. Commun.
2007, 3814–3816.
[23] B. Nicolaꢈ, G. J. Kearley, M. R. Johnson, F. Fillaux, E. Suard, J.
Chem. Phys. 1998, 109, 9062–9074.
[12] G. Kresse, J. Furthmuller, Phys. Rev. B 1996, 54, 11169–11186.
[13] G. Kresse, J. Furthmuller, Comput. Mater. Sci. 1996, 6, 15–50.
[14] G. Kresse, J. Hafner, Phys. Rev. B 1993, 47, 558–561.
[15] G. Kresse, D. Joubert, Phys. Rev. B 1999, 59, 1758–1775.
[16] P. E. Blçchl, Phys. Rev. B 1994, 50, 17953–17979.
[17] J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77,
3865–3868.
[24] M. R. Johnson, B. Frick, H. P. Trommsdorff, Chem. Phys. Lett. 1996,
258, 187–193.
[18] H. J. Monkhorst, J. D. Pack, Phys. Rev. B 1976, 13, 5188–5192.
[19] G. R. Kneller, V. Keiner, M. Kneller, M. Schiller, Comp. Phys.
Commun. 1995, 91, 191–214.
Received: June 14, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
7
&
ÞÞ
These are not the final page numbers!

J. A. Stride et al.
Size matters! The size of the central
atom in the Group XIV tetratolyls was
shown to determine the solid-state
structures courtesy of the effect that it
has on the overall size of the molecules
(see figure). Because the outer-most
methyl groups enter into close inter-
molecular interactions (for the Si and
Ge analogues), the structure is driven
to lower symmetry; as a consequence,
the smallest (C) and largest (Sn and
Pb) analogues are isostructural.
Crystal Engineering
M. C. C. Ng, J. B. Harper,
A. P. J. Stampfl, G. J. Kearley, S. Rols,
J. A. Stride* ................... &&&& — &&&&
Central-Atom Size Effects on the
Methyl Torsions of Group XIV Tetra-
tolyls
&
8
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!