4434 J. Am. Chem. Soc., Vol. 123, No. 19, 2001
Thaimattam et al.
Table 2. Packing Potential Energies (PPE in kcal/mol) and
the MACH3 diffractometer. Structure solution was carried out with
SHELXS-86 and SHELXS-97 while the refinements were performed
with either SHELXL-93 or SHELXL-97 program packages.13 Details
of data collection, structure solution, and refinement are summarized
in Table 1. In the case of the disordered structures, the refinement of
the site occupancy factors and atomic displacement factors were carried
out in stages and in no case were both parameters allowed to vary
simultaneously.
Packing Coefficients (PC) in Solvates 2-15
PPE
PC
structure
n
Ea
Eb
Ec
with guest without guest
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1
-55.85 -39.55 -16.30
77.4
76.2
76.2
78.4
76.2
78.1
77.5
77.0
76.4
76.5
77.7
74.4
77.2
75.6
66.2
67.8
65.8
56.3
67.6
67.2
67.4
54.6
71.9
72.0
70.3
44.9
45.0
60.4
0.67 -45.49 -37.31 -12.28
0.67 -55.90 -40.05 -23.67
0.5 -49.99 -36.23 -26.98
0.5 -47.90 -36.14 -23.52
0.5 -48.35 -36.74 -23.21
0.5 -48.53 -36.33 -24.41
0.5 -46.10 -35.93 -21.02
0.33 -44.23 -38.95 -15.86
0.33 -44.75 -38.98 -17.35
0.33 -48.32 -39.13 -27.61
Desolvation and Resolvation Experiments. Crystalline 2 (4 mM)
was heated at 65, 78, 83, 97, and 110 °C for 2 h under a mild vacuum
(20 mmHg). Sufficient care was taken to ensure that material was not
lost upon application of vacuum. The weight loss of the sample at these
temperatures (except at 65 °C where the loss is minimal) was measured
on an electronic balance (Mettler AJ100) at regular intervals. All weight
loss was assigned to solvent only. All experiments were repeated with
powdered samples and similar weight losses were obtained. The single
crystal samples became opaque upon losing solvent. In the resolvation
experiments, the samples that were heated at the above temperatures
for 2 h were soaked for 3 days at 0 °C in a minimum amount of THF
just enough to wet the crystals. Similar procedures were also followed
in the solvent exchange experiments. The samples were initially dried
on filter paper and then left in open air for ∼3 days. The solvent
2
1
1
-73.13 -24.41 -24.36
-43.59 -18.16 -25.43
-56.78 -28.33 -28.45
n ) number of guest molecules per molecule of host. Ea is the PPE/
molecule of the host for the host-guest system ) PPE per unit cell/
Zhost. Eb is the stabilization in PPE/molecule of the host without solvent
molecules (host alone) ) PPE(without guest)/unit cell]/Z(host). Ec is the
stabilization in the PPE/molecule of host due to one molecule of the
guest ) Ea - Eb/n.
1
exchange was monitored by H NMR and X-ray powder spectra.
Powder X-ray Analysis and Simulations. Powder spectra of some
solvated, desolvated, resolvated, and solvent exchanged 2 were recorded
on a Siemens 5000 diffractometer with Cu KR radiation. Quantitative
phase analysis was carried out on the X-ray powder spectrum of 2
heated at 97 °C with the Rietveld method in the Cerius2 program.14
Two phases, THF solvate 2 and CHCl3 solvate 10, were considered
for the refinements. The cell parameters of solvate 10 were also allowed
to vary along with percentage composition (the initial composition taken
was 1:1) while the cell parameters of 2 and the atomic and thermal
parameters of both phases were fixed. The Rietveld refinement
converged to a final R factor of 32.29 (R - WP ) 48.34, goodness of
fit, S ) 1.79). The two phases 2 and 10 converged to 9.03 and 90.97%,
respectively. In another set of Rietveld refinements, the THF-II
(rhombohedral phase) was used instead of the CHCl3 solvate 10.
However, neither the percentage compositions nor the cell parameters
varied significantly.
Energy Calculations. The nitrobenzene dimer energy was calculated
with the Hartree-Fock ab initio method at the 6-31G** level.15 Due
to the constraints involved in considering two molecules of 1
independently, two preoptimized nitrobenzene molecules were taken.
The initial energy was calculated based on the dimer geometry as seen
in 2. The geometry was then optimized to produce a highly overlapped
inverted dimer. The energy difference between the propeller and
V-shaped conformations of 1 was calculated with AM1 methods.16 The
donor strengths of mesitylene and collidine were correlated from the
HOMO energies as obtained from AM1 calculations.
axis and only one of the possible two orientations was considered. The
orientations that arise due to non-2-fold disorder of guests in 5 and 9
were, however, taken into account. Similarly, in 8, only three H atoms
of each methyl group of the guest were included. In 12, the 3-fold
disordered DMF guest molecule was considered in one of the three
possible ways while in 15, only one collidine was considered among
the possible four. Packing coefficients of the solvates and desolvates
are based on free-volume calculations (Cerius2) and the results are given
in Table 2.
Results and Discussion
Diamondoid Group. (a) Solvate 2, (1)‚(THF). As shown
in Figure 1, the stacking of host molecules 1 in 2 lying on 2-fold
axes along [001] (space group Fddd) resembles the tetraphenyl
embrace of Dance, but the arrangement does not involve edge-
to-face phenyl-phenyl interactions. The separation of the central
C-atoms is at a distance of ∼7.7 Å as observed in other
tetraphenylmethanes.1 Of the 16 tetraphenylmethanes in the CSD
(October 1999 release, Version 5.18),17 14 have the propeller
conformation and only two are V-shaped. Indeed, in compound
1 the propeller conformation is more stable than the V-shaped
one by around 4.5 kcal/mol (MOPAC, AM1), and this is in
agreement with the CSD statistics. Yet it is not the observed
conformation in solvate 2, or for that matter in any of the other
solvates in this study: all of them have the V-shaped conforma-
tion. Now, the propeller conformation, which is already the more
stable one, is further stabilized in crystal structures of tetra-
phenylmethanes by a herringbone or C-H‚‚‚π arrangement.2b,3
So its absence in the present family of structures needs some
explanation. It would appear that the V-shaped conformation
is stabilized by several intermolecular C-H‚‚‚O hydrogen
bonds18 (d, 2.53-2.74 Å) between the activated C-H groups
(adjacent to the nitro group) and the nitro O-atoms. Each
molecule of 1 is connected to four others from adjacent stacks
by two distinct sets of centrosymmetric C-H‚‚‚O hydrogen
bonds. These patterns are only possible with the V-shaped
conformation and are shown as synthons I and II (Figure 2).
The phenyl rings involved in this mode of association form a
ring over bond overlap optimizing the local dipole-dipole/π‚‚‚π
The Burchart-DREIDING force field with Gasteiger charges was
used for the packing energy calculations (Cerius2). The Ewald sum-
mation method was used. Due to the uncertainties involved in the
positions of the guest molecules (owing to disorder), they were
optimized with AM1 without varying their relative position and
orientation in the unit cell. In 5-9, the guest molecules lie on the 2-fold
(12) Sheldrick, G. M. SADABS; Program for Empirical Absorption
Correction of Area Detector Data; University of Go¨ttingen: Go¨ttingen,
Germany, 1996.
(13) (a) Sheldrick, G. M. SHELXS-97; A Program for the Solution of
Crystal Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(b) Sheldrick, G. M. SHELXL-97; Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (c)
Sheldrick, G. M. SHELXS-86. Acta Crystallogr. 1990, A46, 467-473. (d)
Sheldrick, G. M. SHELXL-93; A Program for the Refinement of Single-
Crystal Diffraction Data; University of Go¨ttingen: Go¨ttingen, Germany,
1993.
(14) Cerius2 Program; Molecular Simulations: 9685 Scranton Road, San
Diego, CA 92121-3752, USA, and 240/250 The Quorum, Barnwell Road,
Cambridge CB5 8RE, UK.
(15) Ahlrichs, R.; Ba¨r, M.; Ha¨ser, M.; Horn, H.; Ko¨lmel, C. Chem. Phys.
Lett. 1989, 162, 165-169.
(17) Allen, F. H. Acta Crystallogr. 1998, A54, 758-771.
(18) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural
Chemistry and Biology; Oxford University Press: Oxford, 1999.
(16) Dewar, M. J. S.; Zoebisch, E. G.; Healey, E. F.; Stewart, J. J. P. J.
Am. Chem. Soc. 1985, 107, 3902-3909.