Solid-state reaction mechanisms in monomer-dimer interconversions of p-bromonitrosobenzene. Single-crystal-to-single-crystal photodissociation and formation of new non-van der Waals close contacts

Article abstract of DOI:10.1021/jo051236u

Thermal and photochemical reactions and the phase transition mechanisms of solid-state monomer-dimer interconversions of p-bromonitrosobenzene were studied on the basis of kinetics data and single-crystal-to-single-crystal transformations. From the crystal structure and packing of p- bromobenzeneazodioxide and the previously determined structure of the freshly sublimed monomer, we have explained both consecutive steps in thermal dimerization. While the first reaction (formation of the metastable dimer) with first-order kinetics affords diminishing of the (2 2 0) critical crystal plane that intersects atoms of the nitroso groups, the second phase transformation step includes four critical planes, which show sigmoid kinetics. In the new phase growth, these crystal planes developed in two (Cartesian) dimensions as vectors perpendicular to ab and ac planes, which is in agreement with the dimensionality previously determined on the basis of the Avrami-Erofeyev analysis (with m = 2.01). Photochromic dissociation of the azodioxide at 100 K was followed by structure determination of the single-crystal-to-single-crystal transformation. A new metastable monomer was discovered, in which, despite bond breaking, the nitrogen atoms of the neighboring monomers remained very close to each other (2.30 A), i.e., 23.3% closer than is the sum of two N-atom van der Waals radii. Such an extraordinary close contact was also observed between N and O atoms. This tight packing can explain why the return to dimerization after the low temperature photodissociation occurs so rapidly at a temperature as low as 170 K.

Full text of DOI:10.1021/jo051236u

Solid-State Reaction Mechanisms in Monomer-Dimer
Interconversions of p-Bromonitrosobenzene.
Single-Crystal-to-Single-Crystal Photodissociation and Formation
of New Non-van der Waals Close Contacts
Ivan Halasz, Ernest Mestrovic, Helena Cicak, Zlatko Mihalic, and Hrvoj Vancik*
Department of Chemistry, Faculty of Science and Mathematics, University of Zagreb, Horvatovac 102A,
10000 Zagreb, Croatia
vancik@irb.hr
Received June 16, 2005
Thermal and photochemical reactions and the phase transition mechanisms of solid-state monomer-
dimer interconversions of p-bromonitrosobenzene were studied on the basis of kinetics data and
single-crystal-to-single-crystal transformations. From the crystal structure and packing of p-
bromobenzeneazodioxide and the previously determined structure of the freshly sublimed monomer,
we have explained both consecutive steps in thermal dimerization. While the first reaction (formation
of the metastable dimer) with first-order kinetics affords diminishing of the (2 2 0) critical crystal
plane that intersects atoms of the nitroso groups, the second phase transformation step includes
four critical planes, which show sigmoid kinetics. In the new phase growth, these crystal planes
developed in two (Cartesian) dimensions as vectors perpendicular to ab and ac planes, which is in
agreement with the dimensionality previously determined on the basis of the Avrami-Erofeyev
analysis (with m ) 2.01). Photochromic dissociation of the azodioxide at 100 K was followed by
structure determination of the single-crystal-to-single-crystal transformation. A new metastable
monomer was discovered, in which, despite bond breaking, the nitrogen atoms of the neighboring
monomers remained very close to each other (2.30 Å), i.e., 23.3% closer than is the sum of two
N-atom van der Waals radii. Such an extraordinary close contact was also observed between N
and O atoms. This tight packing can explain why the return to dimerization after the low
temperature photodissociation occurs so rapidly at a temperature as low as 170 K.
Introduction
Experimental kinetic curves were explained on the basis
of their fitting different theoretical models, such as the
Avrami-Erofeyev or Prout-Tomkins and Skrdla models,
etc.3 The obtained TA results were mostly used to explain
the phase transformation mechanisms, for instance, in
terms of the dimensionality (Cartesian) of the new phase
growth.
The disadvantage of methods based only on the TA
kinetics is their being based more on pure phenomenology
than on the molecularity of the mechanism.4 It is much
Chemical reactions in the solid state are in different
ways more or less accompanied and coupled with phase
changes in the crystal lattice.1 Since both processes, i.e.,
change in covalent topology (chemical reaction) and
conversion of supramolecular self-organization (e.g., phase
transformation), occur in more or less rigid condensed
systems, our concept of the reaction mechanism must be
extended in such a way that it includes both processes.
The most developed methods for kinetic studies of
solid-state reactions are based on thermal analysis (TA).2
(2) Brown, M. E.; Dollimore, D.; Galwey, A. K. Comprehensive
Chemical Kinetics; Elsevier: Amsterdam, 1980; Vol. 22.
(3) Skrdla, P. J. J. Phys. Chem. A 2004, 108, 6709-6712.
(4) Galwey, A. K. Thermochim. Acta 2004, 413, 139-183.
(1) Schmidt, G. M. J. et al. Solid State Photochemistry; Ginsburg,
D., Ed.; Verlag Chemie: Weinheim, New York, 1976; p 80.
10.1021/jo051236u CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/17/2005
J. Org. Chem. 2005, 70, 8461-8467
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Halasz et al.
SCHEME 1
were not able to determine to which crystal plane (or
ensemble of planes) belongs the X-ray diffraction peak
at 2Θ ) 25.8 ° showing this sigmoid kinetics.
In one of our previous papers7 we also demonstrated
that azodioxides such as D show photochromic dissocia-
tion to nitroso monomers in the solid state under cryo-
genic conditions (10-100 K). However, the obtained
nitroso monomers cannot survive a temperature much
higher than 170 K and dimerize so quickly that it was
not possible to measure this reaction rate. Evidently, such
a low-temperature photoproduct turns up as a metastable
polymorph of the monomer.
In summary, the overall photo- and thermochemical
monomer-dimer behavior of the solid p-bromonitrosoben-
zene can be represented by Scheme 1.
Sublimation of the crude polycrystalline dimer Dsp on
the coldfinger of the sublimator precooled to 8 °C yields
metastable single crystals of monomer Mms1, whose
structure was determined by X-ray diffraction on the
single crystal. Two processes, the 30 min long dimeriza-
tion with first-order nucleation kinetics that yields the
dimer in the metastable phase Dmp, followed by a 400
min long phase transformation with sigmoid kinetics, led
to the stable dimeric phase Dss.
In this work, we will try to answer two questions: (i)
why the solid state M f D dimerization is so slow if we
start with the freshly sublimed monomer Mms1 as
compared to the fast dimerization of monomer Mms2 that
was obtained by the low-temperature photolysis of dimer
Dss, and (ii) which is the critical crystal plane (or
ensemble of planes) that affords sigmoid kinetics in the
Dmp f Dss phase transition.
more informative when the solid-state reaction mecha-
nism and its relation to the reaction and/or phase change
kinetics is based on a combination of the TR-XRPD
methods and spectroscopy. To find out how a solid-state
chemical reaction and phase conversion are coupled, it
is appropriate to measure the chemical process and the
corresponding phase changes by different and indepen-
dent experimental methods. Whereas the rate of chemical
reaction can be best determined by spectroscopy by
following the change in concentration of the product or
the reactant with time, the solid-state phase transition
kinetics can be well studied by time-resolved X-ray
powder diffraction methods (TR-XRPD).5 If the crystal
structures of reactants, products, and perhaps intermedi-
ates (metastable polymorphs) are known, then we can
get a deeper insight into the molecularity of the chemical
change and its reflection on the kinetic behavior.
Results and Discussion
Single-Crystal-to-Single-Crystal Photolysis. To
answer these questions, we first determined the crystal
structure of the stable polymorph Dss and then analyzed
the structural changes that appear in the single-crystal-
to-single-crystal transformation during the low-temper-
ature photodissociation of azodioxide Dss to the nitroso
monomer, i.e., its polymorph Mms2.
Independently of how Dss is obtained, by sublimation
and phase transformation (through the metastable spe-
cies Mms1 and Dmp) or after these processes crystallize
from methylene dichloride, it belongs to the same crystal
phase. The powder diffractogram calculated on the basis
of the crystal structure of the single crystal Dss matches
the powder diffractogram recorded before crystallization
for crude Dss.
Applying crystallization from solution, we have pre-
pared the stable single crystal of dimer Dss and photo-
lyzed it by UV light in an X-ray diffractometer at 100 K.
After 9 h of irradiation, the color of the crystal changed
from colorless to blue. As we have shown previously by
FT-IR spectroscopy (see ref 7 and S1 Appendix in its
Supporting Information), since the 1665 cm-1 signal of
the azodioxide (ONdNO) group of the dimer disappears
completely after 1 h of irradiation under the same
photolytical conditions, we believe that after 9 h of
irradiation there is complete or almost complete conver-
In our recent paper6 we applied such an approach and
found that the thermal solid-state dimerization of sub-
stituted nitrosobenzenes can be a good model for such
mechanistic studies.
We found that freshly sublimed p-bromonitrosobenzene
M undergoes dimerization to the corresponding azodi-
oxide D (Scheme 1). This solid-state transformation
consists of two consecutive processes, the chemical reac-
tion coupled with nucleation, expressing a first-order
kinetics, and a consecutive sigmoid phase transition from
the metastable to the stable product phase.6a The likeli-
hood of the first process has been explained on the basis
of the disordered crystal structure of monomer M.6b Also
it has been found that this first-order kinetics curve
corresponds to the disappearance of the powder X-ray
diffraction peak at 2Θ ) 27.5 °, which was assigned to
the (2 0 0) crystal plane (we called it critical crystal plane)
that intersects N and O atoms of the nitroso groups. The
second consecutive process with the sigmoid kinetic curve
was measured for the diffraction peak at 2Θ ) 25.8 °.
However, since the crystal structure of the dimeric
product (D in Scheme 1) was not known at the time, we
(5) Kim, J. H.; Hubig, S. M.; Lindeman, S. U.; Kochi, J. K. J. Am.
Chem. Soc. 2001, 123, 87-95.
(6) (a) Vancik, H.; Simunic-Meznaric, V.; Mestrovic, E.; Halasz, I.
J. Org. Chem. 2004, 69, 4829-4834. (b) In a previous paper6a we
explained that only the first step (producing Mms1) of the reaction could
convert to 50%. However, the complete thermal reaction that includes
also a long period of the phase transformation led to the complete
dimerization.
(7) Vancik, H.; Simunic-Meznaric, V.; Caleta, I.; Mestrovic, E.;
Milovac, S.; Mlinaric Majerski, K.; Veljkovic, J. J. Phys. Chem. B 2002,
106, 1576-1580.
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Monomer-Dimer Interconversions of p-Bromonitrosobenzene
satisfactory for structure determination necessary for this
reaction mechanism study. In general, analysis of single-
crystal-to-single-crystal transformation is limited and
difficult, especially in this case when thickness of the
crystal was only 0.013 mm.
Both dimer Dss and monomer Mms2 crystallize in the
monoclinic crystal system. Unit cells of the compounds
differ only in one parameter, i.e., the a axis, which in
the monomer phase is half the length of that in the dimer
phase (Figures 2 and 3). Differences in other cell param-
eters are so small that they could be attributed to the
loss of the crystal quality due to the transformation.
The small decrease in crystal quality can be explained
by the fact that, using the same conditions of indexing,
almost all (∼98%) of the dimer phase diffraction peaks
could be indexed, whereas only approximately half of
them could be indexed for the monomer phase. Reflec-
tions of the monomer phase started to spread over a
significant number of diffraction images, but it was also
noted that all of the unindexed reflections were lying in
the (a*,b*) plane of the reciprocal space. This indicates
that structural changes occur only in the (a,b) plane of
the direct space with no movement of the molecules
parallel to the c axis. Also, as can be seen in the crystal
structure of the dimer phase, the two-dimensional net-
works of the hydrogen bond are parallel to this plane.
In the molecular structure of the Dss dimer molecule,
the benzene rings are parallel but do not lie in the same
plane (Figure 4). Atoms of the azodioxy group lie in the
same plane (the molecule lies on the inversion center)
with the torsional angle (C2-C1-N-O) is 59.1(7)°
(Figure 4, green structures). A possible reason for such
conformation could be the stabilization of the crystal
structure by the two-dimensional framework of C-H‚‚‚
O hydrogen bonds. Assuming that the atoms involved in
these interactions do not move significantly during the
transformation, then it could be said that these interac-
FIGURE 1. Superposition of the dimer molecules of Dss (red)
and the product monomers Mms2 (green) after the single-
crystal-to-single-crystal photodissociation.
sion of dimer to monomer. The quality of the reactant
and the photoproduct single crystals at 100 K was good
enough for resolving both structures. We found that the
single crystal of the photoproduct was the nitroso mono-
mer in the second metastable phase, Mms2, which by
warming above 170 K quickly converts to the stable
polycrystalline dimer Dsp. In this way, the complete
reaction cycle of the solid-state monomer dimer conver-
sions of the p-bromonitrosobenzene was closed (Scheme
1). Additionally, it must be mentioned that, given the
disorder in Mms1, the cyclic mechanism of Scheme 1
should lead to ordered and disordered phases of Mms2
depending on whether one starts with sublimed Mms1 or
Dss.
Photodissociation of the crystallized Dss, for which the
single-crystal-to-single-crystal conversion was observed,
led to the formation of the new metastable polymorph of
the monomer, Mms2. Although molecular orientations
changed markedly during the reaction (Figure 1), the
crystal retained its quality, which was not perfect, as
required for typical crystallography research, but quite
FIGURE 2. Crystal structure and packing of the stable dimer, Dss. The hydrogen bonding network is marked by yellow lines,
and bromine-bromine close connections are shown by red lines.
J. Org. Chem, Vol. 70, No. 21, 2005 8463

Halasz et al.
FIGURE 3. Crystal structure and packing of the metastable monomer, Mms2, obtained after photolytic single-crystal-to-single-
crystal conversion at 100 K. Hydrogen bonding network is shown by yellow lines, and bromine-bromine close connections are
marked by red lines.
FIGURE 4. Superposition of crystal packing of the sublimed monomer Mms1 (gray) and the stable dimer Dss (green).
tions govern the route of the transformation (Figures 2
and 3). Analogous crystal packing was observed in all
known crystal structures of p- and m-halogen-substituted
azodioxybenzenes. In the monomer Mms2 phase, this
(C2-C1-N-O) torsional angle is 46.2(2.4)°. It might be
expected that the energetically most favorable conforma-
tion of the monomer molecule would be the one in which
the nitroso group is in plane with the benzene ring.
However, this would leave hydrogen bonds weaker. This
assumption is supported by the crystal structure of
p-iodonitrosobenzene,8 which exists as a stable monomer
and in which the corresponding torsion angle is close to
zero.
conserved during the chemical reaction. The geometrical
parameters of these interactions do not change signifi-
cantly; they are almost within the estimated standard
deviations.
On the other hand, the N‚‚‚N distance greatly increases
upon the Dss f Mms2 transformation from 1.30 Å in the
monomer phase to 2.30 Å in the dimer phase. Since this
NdN bond is broken during the photodissociation, such
a change, as commented bellow, could be expected.
Mechanism of the Dmp to Dss Transition. Since we
know the crystal structures of the crucial points of the
reaction cycle in Scheme 1, i.e., structures of Mms1, Dss,
and Mms2, we can discuss the solid-state reaction mech-
anisms of monomer-dimer interconversions in more
detail.
As can be seen from Figures 2 and 3, not only hydrogen
bonds but also the Br‚‚‚Br interactions network remain
Let us first discuss the mechanisms of the Mms1 f Dmp
f Dss transformation. Two important geometrical changes
(8) (a) Webster, M. S. J. Chem. Soc. 1956, 2841. (b) Talberg, H. J.
Acta Chem. Scand. A 1979, 33, 289.
8464 J. Org. Chem., Vol. 70, No. 21, 2005

Monomer-Dimer Interconversions of p-Bromonitrosobenzene
FIGURE 5. Superposition of crystal structures of the sub-
limed monomer Mms1 (gray) and the stable dimer Dss (green).
Monomer molecules are 50% disordered (see ref 6).
can be recognized from the superposition of the crystal
packing of Mms1 and Dss (Figure 4). While the first
change includes the declination of the nitroso groups
plane out of the benzene rings plane, the second change
is the inclination of every second dimeric molecule and
the formation of a new packing plane. Marked differences
in packing between Mms1 and Dss could be a satisfactory
explanation for the relatively slow two-step process of the
stable dimer Dss formation. At the molecular level,
dimerization requires two spatial shifts, the monomer
molecule Mms1 (gray structure in Figure 5) must be
shifted in two directions, horizontally in the benzene ring
plane, but also vertically to this plane (Figure 5).
Perhaps one could speculate that these two changes
correspond to the “trajectories” associated with two
consecutive processes, the chemical reaction (Mms1 f
D
mp) that afforded first-order kinetics and the phase
transfer from Dmp to Dss with sigmoid kinetics. We found
previously that in the Mms1 f Dmp process the first-order
kinetics mostly shows a disappearance of the diffraction
peak of the (2 0 0) crystal plane (critical crystal plane)
where the nitroso group is positioned. To find such a
critical plane for the second process, the 400 min long
Dmp f Dss phase transformation characterized by sig-
moid kinetics, we must look at the crystal packing of
product Dss. The strongest sigmoid growth recorded by
the TR-XRPD shows the diffraction peak at 2Θ ) 25.8 °.
On the basis of the crystal structure of Dss, we assigned
this peak to a superposition of four diffraction peaks
corresponding to crystal planes (0 1 2), (1 1 2), (2 1 0),
and (10-4). The position of these planes relative to the
crystal packing of Dss is shown in Figure 6.
FIGURE 6. Crystal planes of the Dss single-crystal assigned
to X-ray diffraction peaks, which afford sigmoid kinetics,
observed for the Dmp f Dss phase transition.
extra, in this moment we do not know to which extent
particular planes contribute to the observed kinetics.
Non-van der Waals Close Contacts. Since we have
succeeded in observing the single-crystal-to-single-crystal
Dss f Mms2 transformation, we were also interested in
studying close contacts in more detail by looking at
changes in the charge distribution and electron (charge)
densities in close contact regions. Starting from the
experimentally determined geometry, we have calculated
the electron (charge) density surfaces for fragments of
three crystal-packed molecules of dimer Dss (Figure 7a
and c) and six crystal-packed molecules of monomer Mms2
(Figure 7b and d).
The non-zero electron density can be observed in the
positions of hydrogen bonds between m-CH groups of the
benzene ring and oxygens of nitroso groups (or azodioxide
groups in dimers). It is, however, more interesting to note
that a relatively high charge density remains between
photochemically disconnected nitrogen atoms in mono-
mers (Figure 7d). The NN distance of 2.30 Å is much
shorter than the sum of two nitrogen van der Waals radii
(3.0 Å). This shortening amounts to 23.3%, i.e., evidently
larger than corresponding shortening observed in the
formation of hydrogen bonds (10-15%).9a Until now, the
shortest NN close contact known from literature is 2.62
Å.9b Accordingly, the non-van der Waals NN close con-
One can observe that three of the planes [(0 1 2), (1 1
2), (2 1 0)] are parallel or nearly parallel with the ac plane
of the unit cell, whereas the (10-4) plane is parallel with
the ab plane. There is no plane parallel with bc of the
unit cell. If the directions of the new phase development
is described as vectors perpendicular to these four planes
(nearly parallel to two planes, ab and ac, respectively),
this phase growth is defined by four vectors distributed
more or less in two (Cartesian) dimensions. The previ-
ously measured Avrami-Erofeyev parameter m was
2.01,6 which indicates an almost two-dimensional growth.
This finding can now be confirmed on the basis of the
crystal structure determination. Perhaps, it would be
more convenient to describe a new phase growth multi-
dimensionally, i.e., in terms of the number of critical
crystal planes and the orientation of their growth vectors
(perpendicular to these planes) rather than in terms of
Cartesian three-dimensionality. Thus, our example be-
comes a four-dimensional problem because the phase
growth includes four critical planes. It must be added
that because we were not able to deconvolute these four
critical planes and to record kinetic curves of each plane
J. Org. Chem, Vol. 70, No. 21, 2005 8465

Halasz et al.
inside a close crystal packing. The appearance of such
structures after photolysis of the solid azodioxide D can
easily explain why the Mms2 polymorph returns so
quickly to the dimer at a temperature as low as 170 K.
On the suggestion of a referee, we have to be very
careful when observing an extraordinary shortening and/
or lengthening of interatomic distances in crystals. For
the so-called bond-stretch isomers,9c it was proposed
rather to be an epiphenomenon caused by compositional
disorder.9d However, since we have shown by spectros-
copy7 (vide supra) that photoconversion already is com-
plete after 45 min, it is difficult to believe in the posibility
of compositional disorder in the 9 h irradiated single
crystal. In his criticism of the bond-stretch isomer
phenomenon, Parkin9d found that while a small impurity
that possesses a molecule with a long interatomic dis-
tance drastically “stretches” the observed bond length of
the main constituent in the crystal, a small impurity of
the constituent with short interatomic distance has
almost no influence on the possible observation of short-
ening of long interatomic distances. In our case we
observed not stretched but squeezed N,N distance, and
in accordance with Parkin’s observations, even if the
crystal after photolysis contains small amounts of the
unreacted dimer (which has short N,N distances), it
cannot be cause of our observation of short contacts.
Conclusions
From the crystal structure and packing of p-bromoben-
zeneazodioxide D and the previously determined struc-
ture of the freshly sublimed monomer Mms1, we can
explain the solid-state reaction mechanism of both con-
secutive steps in their thermal conversion. Whereas in
the first paper we explained the first reaction (formation
of the metastable dimer Dmp) as first-order kinetics that
affords diminishing of the (2 2 0) critical crystal plane
intersecting atoms of the nitroso groups, here we found
four critical planes that show sigmoid kinetics of the
second phase transformation step. In the new phase
growth these crystal planes develop in two (Cartesian)
dimensions as vectors perpendicular to the ab and ac
planes, respectively. This finding is in agreement with
the dimensionality previously determined on the basis
of the Avrami-Erofeyev analysis (with m ) 2.01).
Photochromic dissociation of the azodioxide Dss at 100
K was followed by structure determination of the single-
crystal-to-single-crystal transformation. A new meta-
stable monomer Mms2 was discovered in which, despite
bond breaking, the nitrogen atoms of the neighboring
monomers remained very close to each other (2.30 Å),
i.e., 23.3% closer than the sum of two N-atom van der
Waals radii. Such an extraordinary close contact is also
observed between N and O atoms. Such tight packing
can explain why the dimerization after the low temper-
ature photodissociation occurs so quickly at a tempera-
ture as low as 170 K.
FIGURE 7. Electrostatic potential mapped on the total
electron density (isosurface value ) 0.0080 au) of (a) sequence
of three p-bromonitrosobenzene dimeres; (b) sequence of six
p-bromonitrosobenzenes; (c and d) details in the region of Nd
N bond, before (c) and after (d) photolysis. Calculations were
based on the crystal structure geometry.
SCHEME 2
tacts exist in the Mms2 crystal packing. A similar
extraordinary close contact, 2.35 Å, is also found between
the same nitrogen atom of one monomer and the nitroso
oxygen atom of the neighboring molecule. In this way, a
three-centric close contact is formed (Scheme 2). Ab initio
calculations indicate that the two-molecule complex as
the one in Scheme 2 is not persistent in the gas phase;
two monomer molecules just dislocate from each other.
This means that such close contacts are possible only
Experimental Section
p-Bromonitrosobenzene was prepared by standard proce-
dure described in previous literature (ref 6 and references
therein).
Crystal Preparation. Numerous experiments were per-
formed with different starting concentrations of the dimer. All
(9) (a) Desiraju, G. R. Crystal Engineering. The Design of Organic
Solids; Elsevier: Amsterdam, 1989; pp 92, 144. (b) Capiomont, A.;
Chion, B.; Lajzerowicz, J. Acta Crystallogr., Sect. B 1971, 27, 322. (c)
Yoon, K.; Parkin, G.; Rheingold, A. L. J. Am. Chem. Soc. 1991, 113,
1437-1438. (d) Parkin, G. Acc. Chem. Res. 1992, 25, 455-460.
8466 J. Org. Chem., Vol. 70, No. 21, 2005

Monomer-Dimer Interconversions of p-Bromonitrosobenzene
experiments gave similar results. From these experiments it
appears that the rate of evaporation of CH2Cl2 is a factor that
has the strongest influence on the crystal quality. The crystal
used in the diffraction experiment was selected from the
experiment in which 24 mg of the dimer was dissolved in 1
cm3 of the solvent.
Single-Crystal X-ray Diffraction. Single crystals of the
dimer were obtained by slow evaporation of a CH2Cl2 solution
as colorless elongated plates when examined separately and
brownish in bulk of the material. Dimer solutions were green
because, as with the majority of nitrosobenzenes, the monomer
is the dominant constituent of the solution.
crystallographic software package. Molecular structure dia-
grams and packing diagrams were prepared by Mercury1.3.12
Diagrams with overlapped structures were prepared by Mole-
kel4.3.13
Calculation Methods. Density functional theory (DFT)14
calculations were carried out using the B3LYP exchange-
correlation functional with the standard basis set of triple-ú
quality and with diffuse and polarization functions, namely,
the 6-311+G(d,p) basis set.15 This basis set is appropriate for
systems rich in lone pairs and involved in hydrogen-bonding
interactions. Population analysis of the experimentally ob-
tained X-ray structures was performed by the natural bond
orbital (NBO) method.16 All calculations were carried out with
the Gaussian 03 suite of programs,17 and surfaces were
constructed using the GaussView 3.09 visual program.
All diffraction experiments were performed at 100 K because
the monomer phase is unstable at temperatures higher than
170 K.7
The single crystal of the monomer phase was obtained by
irradiation of the single crystal of the dimer with a 250-W high-
pressure Hg lamp for 9 h in a stream of nitrogen at 100 K
while it was still mounted on the diffractometer. During
irradiation, the crystal was rotated at angular speed of
approximately 5°/s. The color of the crystal changed from
colorless to blue/green upon irradiation.
Single-crystal data were collected on an Oxford diffraction
Xcalibur 3 CCD diffractometer with graphite monochromated
Mo KR radiation. The data sets for both the dimer and the
monomer phase were collected by a series of ω scans, the scan
width being 1° for the dimer crystal and 2° for the monomer
crystal. Scan width for the monomer crystal data collection
was increased because preliminary diffraction images, taken
with a 1° scan width, suggested that the quality of the crystal
deteriorated as a result of the phase change. This could be
seen in the spreading of reflections over a larger number of
diffraction images. The data were collected up to 2θ ) 60° for
the dimer and up to 2θ ) 50° for the monomer, since it
diffracted very poorly at higher angles.
Acknowledgment. We thank Professor Emeritus D.
E. Sunko for helpful discussions and comments. The
financial support of the Ministry of Science and Tech-
nology, Republic of Croatia, through Grant 0119611 is
gratefully acknowledged.
Supporting Information Available: S1: Crystal deter-
mination tables for structures Dss and Mms2. S2: Single crystal
determination data. S3: ORTEP diagrams of Dss dimer and
Mms2 metastable monomer. Complete crystal structure data
in CIF format. S4: Powder diffraction diagram calculated on
the basis of the single-crystal structure of Dss. This material
is available free of charge via the Internet at http://pubs.acs.org.
JO051236U
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Both structures were solved by the Patterson method
implemented in the SHELXS10 program and refined by the
least-squares method on F2 using the SHELXL10 program. In
the dimer phase, all atoms were refined anisotropically,
without any restraints, whereas for the monomer phase,
anisotropic refinement of all atoms other than the bromine
atom gave meaningless parameters. Anisotropic refinement
with restraints on ADP resulted in no improvement of the
structural data so that all the atoms, except the bromine atom,
were refined isotropically. Because of rather high deviations
from the expected values of the geometrical parameters of the
benzene ring for the monomer phase, it was refined with some
restraints on C-C bond distances. In both structures, hydro-
gen atoms were placed in their geometrically calculated
position with the C-H bond distance of 0.93 Å and with Uiso
-
(H) ) 1.2Ueq(C) for the dimer crystal and Uiso(H) ) 1.2Uiso(C)
for the monomer crystal of the C atom to which they were
bonded. All calculations were performed using the WinGX11
(10) Sheldrick, G. M. SHELXL93-Program for Crystal Structure
Refinement; Institu¨t fu¨r Anorganische Chemie der Universita¨t: Go¨t-
tingen, Germany, 1993.
J. Org. Chem, Vol. 70, No. 21, 2005 8467