Intramolecular London Dispersion Interactions Do Not Cancel in Solution

Article abstract of DOI:10.1021/jacs.0c09597

We present a comprehensive experimental study of a di-t-butyl-substituted cyclooctatetraene-based molecular balance to measure the effect of 16 different solvents on the equilibrium of folded versus unfolded isomers. In the folded 1,6-isomer, the two t-bu

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Intramolecular London Dispersion Interactions Do Not Cancel in
Solution
́
̈
Jan M. Schumann, J. Philipp Wagner, Andre K. Eckhardt, Henrik Quanz, and Peter R. Schreiner*
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ABSTRACT: We present a comprehensive experimental study of a di-t-butyl-substituted cyclooctatetraene-based molecular balance
to measure the effect of 16 different solvents on the equilibrium of folded versus unfolded isomers. In the folded 1,6-isomer, the two
t-butyl groups are in close proximity (H···H distance ≈ 2.5 Å), but they are far apart in the unfolded 1,4-isomer (H···H distance ≈
7 Å). We determined the relative strengths of these noncovalent intramolecular σ−σ interactions via temperature-dependent nuclear
magnetic resonance measurements. The origins of the interactions were elucidated with energy decomposition analysis at the density
functional and ab initio levels of theory, pinpointing the predominance of London dispersion interactions enthalpically favoring the
folded state in any solvent measured.
ondon dispersion (LD), the attractive part of the van der
Waals potential,1 is ubiquitously present but often
zene in fluorobenzene.16 Subsequent investigations suggested
based on a comparison of perfluoroalkane versus alkane chain
folding that the LD contributions are largely attenuated in
solution in reference to computed gas-phase values. Whereas
LD was deemed non-negligible, solvophobic effects were found
to be dominant in driving the association of apolar chains in
aqueous solution.7
In choosing an optimal balance to determine solvent effects
on LD interactions, we followed the suggested guidelines.19
Such a balance should display high symmetry, show
distinguishable NMR signals at variable temperatures, and
have a reasonably low barrier for interconversion of the folded
L
underestimated.2 This is particularly true for molecules in
solution, where it is often assumed that LD is overridden by
solvent effects.3 Recently, this view has been carefully
refined,4−9 but an analysis of a nonpolar molecular balance
that avoids additional interactions arising from (local) dipoles
is still lacking.10 Hence, by utilizing a pure hydrocarbon
balance, we show here that solvents do affect LD interactions,
mostly through changes in the solvent reorganization entropy,
but that LD enthalpically remains constant and comparable to
gas-phase values. We employ the sterically very different bond
shift isomers of 1,4- and 1,6-di-t-butyl cycloocta-1,3,5,7-
tetraene (1,4- and 1,6-COT, Figure 1) and demonstrate that
the thermodynamic preference for LD-stabilized but visually
more crowded 1,6-COT isomer is preserved across 16 solvents
ranging from polar to apolar as well as from protic to aprotic.
Highly functionalized molecular balances bearing heter-
oatoms “fold” (having a preference for the more crowded
structure) due to a multitude of interactions (polar, induced
dipolar, hydrogen bonding, etc.) and therefore make it difficult
to discern these effects from unperturbed LD interactions. A
̈
case in point is the often employed “Wilcox−Troger base-
torsion balance” that was initially developed to evaluate
nonbonding intramolecular interactions between aromatic
rings.11 Rigid and polar N-arylamide balances led to the
conclusion that LD is only a small component of the aromatic
stacking interaction in solution, in contrast with its dominant
role in vacuo;12 the theoretically predicted distance depend-
ence (R−6) of the observed LD stabilization was also
confirmed.13
Strikingly, the often invoked aryl π−π interactions are also
mimicked by nonaromatic, highly polarizable groups such as
cyclohexyl and t-butyl (“σ−σ interactions”14) that show the
same conformational preference for the chemically less
intuitive “folded” state.15 A pertinent example is the favored
all-cis conformation of 1,3,5-tribromo-2,4,6-trineopentylben-
Figure 1. Simplified equilibrium of 1,4- and 1,6-COT. The
noncovalent interaction (NCI) surfaces (CCSD(T)/CBS//MP2/cc-
pVTZ) are colored on a blue−green−red scale according to an
isovalue s(ρ) of 0.35, ranging from ρ(r) −0.025 to 0.025 Å. Blue
indicates strong attractive interactions, green corresponds to weak
NICs, and red indicates strong repulsion.17,18 (See the SI.)
Received: September 7, 2020
https://dx.doi.org/10.1021/jacs.0c09597
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© XXXX American Chemical Society
A

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Communication
and unfolded states. We found that 1,4- and 1,6-COT
perfectly fulfill these guidelines and therefore chose this system
to determine the effects of LD on its folding preference in a
large variety of solvents.20
that formally violates Hund’s rule.31 Both di-t-butyl isomers
individually undergo conformational ring isomerization
through TSRI and TSRI′, which are connected via rate-limiting
TSDBS, whose description requires multiconfigurational wave
functions (such as CASSCF), which do not, however, treat LD
in a balanced fashion. The energy of TSDBS implies that this
equilibration will require significant time at ambient temper-
atures (vide infra). The 11.4 kcal mol−1 higher energy of the di-
t-butyl-substituted COT versus the unsubstituted system at
TSDBS may be attributed to twice the t-butyl strain of
∼6 kcal mol−1 reported for the axial t-butyl ring strain on
cyclohexane.32 Notwithstanding the complexity of the
equilibration, only the 1,4- and 1,6-valence bond isomers are
observable and can readily be distinguished via NMR
spectroscopy (Figure S19). Gratifyingly, the agreement
between experimental and computed (enso/xTB) splitting
patterns is also in good agreement (Figure S14).
The determination of pure LD interactions in solution is
challenging because electrostatic and inductive interactions as
well as solvophobic effects favor folding.21 The energy
differences in hydrocarbon configurational isomers are often
rather low, and the barriers for their interconversion are high.
In most cases, only Gibbs free energies (ΔG) are discussed
because they can be determined directly from temperature-
dependent equilibrium measurements. Free energies, however,
are rather difficult to interpret due to the often encountered
enthalpy (ΔH) and entropy (ΔS) compensation.22 Second, in
a first approximation, LD is temperature-independent,23 so it is
instructive to analyze ΔH and ΔS separately. This is important
because LD contributes to ΔH, whereas ΔS reflects the
disorder of the solute and predominantly solvent reorganiza-
tion.24 Our experiments provide the standard isomerization
enthalpies (ΔrH⊖) and entropies (ΔrS⊖) assuming that both
are temperature-independent within the chosen experimental
window (20−80 °C).
We synthesized 1,4- and 1,6-COT utilizing a modification of
Paquette’s route starting with an ortho-alkylation of 1 using the
method of Ershov et al. (Scheme 2).33 This was followed by a
a
Scheme 2. Synthesis of the Target Structures
The COT isomers considered here were first synthesized by
Streitwieser et al.,20 and a molecular mechanics study helped
rationalize the unexpected preference for the apparently more
crowded isomer in terms of “intramolecular van der Waals
attraction”25 as early as 1982.26 Shortly thereafter, Paquette et
al. synthesized 1,4- and 1,6-COT along a different route,
confirming the preference of the more crowded diastereomer
in C6D6 and CDCl3.25 These results were confirmed and
27
refined in 1992 by Anderson and Kirsch, who demonstrated
that with increasing substituent size the preference for the
more crowded structure increases on the basis of attractive
steric interactions.28 Early semiempirical29 and force-field
studies26 on a series of disubstituted COT isomers also
suggested that LD might be solely responsible for the observed
preference. Remarkably, these pivotal studies have largely been
overlooked. The advent of modern computational and
improved spectroscopic techniques as well as an improved
synthetic approach (vide infra) prompted us to study this
fascinating system in more detail.
a
PCC, pyridinium chlorochromate; CAN, ceric(IV) ammonium
nitrate; LED, light-emitting diode.
Corey−Suggs oxidation34 of 3,6-di-t-butyl-pyrocatechol (2) to
3,6-di-t-butyl-o-benzochinone (3). The Diels−Alder reaction
of 3 with cyclobutadiene iron(II) tricarbonyl (4), synthesized
via Pettit’s method,35 gave 1,6-di-t-butyltricyclo[4.2.2.00.25]-
deca-3,7-diene-9,10-dione (5) as a storable COT precursor
from which the target compounds 1,4- and 1,6-COT can be
prepared cleanly right before the NMR measurements via
Strating−Zwanenburg photodecarbonylation.36 For the syn-
thesis, see the SI.
Scheme 1 displays an idealized computed version of the
complex valence shift isomerization (VSI) in cyclooctate-
traenes;30 there is spectroscopic evidence of the planar singlet
double-bond shift transition state TSDBS of the parent COT
We decided to take 16 of the most common organic solvents
covering most parameters of the empirical solvent polar-
Scheme 1. Computed Double-Bond Valence Shift (DBS)
and Ring Inversion (RI) of Cyclooctatetraene Enthalpies
(ΔH0) at the Level Given
a
izability (SP) and solvent polarity−polarizability (SPP) scale
37
́
established by Catalan and Hopf to estimate their influence
on the COT equilibrium. Because 1,4- and 1,6-COT are barely
soluble in water and hexafluorobenzene, the equilibrium
constant (Keq) could not be determined in these.38 We
allowed the isomeric mixtures to equilibrate for at least 1 h at
each temperature inside the NMR spectrometer (see details in
the SI) by initially warming the sample to the highest possible
temperature for a given solvent and decreasing it in 5−10 °C
steps immediately thereafter. The resulting experimental data
are summarized in Figure 2. Whereas the values of neither
Streitwieser et al.25 nor Paquette et al.39 (both are different)
were reproduced, our results agree well with the most recent
measurements of Anderson and Kirsch in the same temper-
ature regime (Figure S1).28
a
See the SI for details.
B
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Figure 3. Correlation of thermodynamic quantities at 298 K in kcal
Figure 2. Temperature-dependent Gibbs free energies and equili-
brium constants of the 1,4- and 1,6-COT equilibrium in various
solvents.
mol−1 with clockwise increasing relative solvent polarizabilities (min =
37
́
0.00 vacuum, max = 1.00 CS2) (Catalan−Hopf SP scale ).
In looking at Keq always being larger than unity, it is clear
that 1,6-COT is always favored, just to different degrees
depending on the solvent. Whereas solvents of low polarity
show the smallest preference for the folded isomer, it is highly
favored in polar and chlorinated solvents, which shows that the
inductive effect of the dipole moment also is present.
Remarkably, the ΔrH⊖ values cover a range of only −0.4 to
−0.9 kcal mol−1 over this wide range of solvents, indicating
that the preference for 1,6-COT is an intrinsic structural
property. We therefore conclude that the isomer structures are
comparable in different solvents and that the enthalpic
intramolecular LD stabilization of 1,6-COT must be rather
similar. The ΔrS⊖ values extrapolated from the intercepts with
the ordinate vary considerably and thereby have a much larger
effect on the ΔrG⊖ values. Entropy consistently favors
1,4-COT, which is at odds with simple symmetry consid-
erations that would favor the Cs over the C2 symmetric
structure due to its reduced rotational symmetry number.40
Thus we attribute the entropy changes to solvent reorganiza-
tion. Because of the small changes in enthalpy and the large
entropic contributions, we estimate an enthalpy−entropy
compensation temperature range of 291 57 K over all 16
solvents (Figure S4). This indicates that only at elevated
temperatures, for all solvents above their boiling point, is LD
overcome by entropy. We choose 298 K as the temperature for
comparing our results for ΔrG⊖ (Figure 3).
showed that the best results for dipole computations are
obtained by the ωB97xD/cc-pVQZ level of theory.42 The
computed gas-phase dipole moments of 1,4- and 1,6-COT are
rather small (1,6-COT, 0.6 D; 1,4-COT, 0.1 D) but might
favor 1,4-COT in the gas phase and nonpolar solvents. For
comparison, the difference in dipole moments between toluene
(0.4 D)43 and benzene (0.0 D)43 is similar. Taking solvent
polarity into account, we find that highly polar solvents indeed
favor the population of the higher dipole moment 1,6-COT
isomer. However, this cannot necessarily be traced back to an
increased enthalpic interaction but to a favorable entropic
origin. One might envision that competition of the solute for
dipolar interactions with the solvent disrupts the organized
internal structure of the solvent, resulting in a favorable solvent
entropy contribution. Because the computed solvent-accessible
area of 1,4-COT (260 Å2) is even larger than that of 1,6-COT
(256 Å2), solvent interactions should favor 1,4-COT.
Having established the given preference for the more
crowded isomer irrespective of the environment, we returned
to a more detailed molecular analysis and took the B3LYP-
D3(BJ)/def2-QZVPP optimized geometries, removed the
COT moiety, and saturated the resulting radicals with
hydrogen (Figure S18). This should bring the “isolated”
interactions of the alkyl groups depicted in the NCI plot in
Figure 1 as a green surface to the fore by excluding the COT
moiety. To separate electrostatic and inductive effects from the
LD, we used symmetry-adapted perturbation theory (DF-
SAPT2/aug-cc-pVTZ),44 which revealed that LD is the largest
component of all evaluated interactions, in particular, in 1,6-
COT. Apart from Pauli exchange repulsion, all other
contributors (electrostatics and induction together) favor
1,6-COT by ∼0.5 kcal mol−1, in good agreement with our
other energy evaluations (summarized in the SI). In line with
this finding, the 1,4- and 1,6-COT isomers are almost
isoenergetic at the Hartree−Fock level, and the preference
for the more crowded structure only comes from the electron
correlation, which is the quantum-mechanical origin of LD.
Counterintuitively, intramolecular LD interactions favor the
visually sterically more demanding 1,6-COT isomer in all
solvents measured. This preference is assisted by induction and
electrostatics, which, in sum, counterbalance the Pauli
exchange repulsion. LD is thereby the largest single energy
The ΔrH⊖ values increase steadily with increasing solvent
polarizabilities, with the exception of a few outliers like DMSO
and acetonitrile (Figure 3). This intriguing finding is hard to
rationalize and strongly contradicts the importance of
competitive dispersive solute−solvent interactions that are
thought to diminish LD interactions in solution.41 Accordingly,
comparing the results within solvent groups, we do see trends:
In hydrocarbons, the 1,6-COT preference is proportional to
increasing polarizability, hexane < cyclohexane < toluene <
benzene. The same is true for chlorinated solvents, CH2Cl2 <
CCl4 < CHCl3, and for alcohols with methanol < ethanol. This
contradicts the expected simple correlation between solvent
polarizability and LD interactions with and within the solute.
Because only LD interactions play a role in solute−solvent
interactions, the polar effects of permanent dipole moments
have to be taken into account as well. A benchmark study
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contributor. The equilibrium ΔrH⊖ values vary from −0.4 to
−0.9 kcal mol−1, whereas ΔrG⊖ values at 298 K (−0.2 to −0.4
kcal mol−1) attenuate these by a positive entropy contribution
arising from solvent reorganization. Even though this partial
enthalpy−entropy compensation is apparent, it does not
qualitatively change the preference for the more crowed
structure that is stabilized through intramolecular LD
interactions.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
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Comparison with previous measurements as well as
synthetic and computational details (PDF)
AUTHOR INFORMATION
Corresponding Author
Peter R. Schreiner − Institute of Organic Chemistry, Justus
Liebig University, 35392 Giessen, Germany; orcid.org/
0000-0002-3608-5515; Email: prs@uni-giessen.de
■
(13) Hwang, J.; Li, P.; Smith, M. D.; Shimizu, K. D. Distance-
Dependent Attractive and Repulsive Interactions of Bulky Alkyl
Groups. Angew. Chem., Int. Ed. 2016, 55 (28), 8086−8089.
(14) Fokin, A. A.; Gerbig, D.; Schreiner, P. R. σ/σ- and π/π-
Interactions Are Equally Important: Multilayered Graphanes. J. Am.
Chem. Soc. 2011, 133 (50), 20036−20039.
Authors
Jan M. Schumann − Institute of Organic Chemistry, Justus
̈
Liebig University, 35392 Giessen, Germany
J. Philipp Wagner − Institute of Organic Chemistry, Justus
Liebig University, 35392 Giessen, Germany; orcid.org/
0000-0002-1433-0292
(15) Nakamura, K.; Houk, K. N. Theoretical Studies of the Wilcox
Molecular Torsion Balance. Is the Edge-to-Face Aromatic Interaction
Important? Org. Lett. 1999, 1 (13), 2049−2051.
́
Andre K. Eckhardt − Institute of Organic Chemistry, Justus
Liebig University, 35392 Giessen, Germany; orcid.org/
0000-0003-1029-9272
Henrik Quanz − Institute of Organic Chemistry, Justus Liebig
University, 35392 Giessen, Germany
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J.; Cohen, A. J.; Yang, W. Revealing Noncovalent Interactions. J. Am.
Complete contact information is available at:
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Chem. Soc. 2010, 132 (18), 6498−6506.
(18) Contreras-García, J.; Johnson, E. R.; Keinan, S.; Chaudret, R.;
Piquemal, J.-P.; Beratan, D. N.; Yang, W. NCIPLOT: A Program for
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Notes
The authors declare no competing financial interest.
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non-covalent interactions. Chem. Soc. Rev. 2010, 39 (11), 4195−4205.
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ACKNOWLEDGMENTS
■
We dedicate this work to Henning Hopf for his seminal
contributions to hydrocarbon chemistry and on the occasion of
his 80th birthday. This work was supported by the priority
program “Control of London Dispersion in Molecular
Chemistry” (SPP1807) of the Deutsche Forschungsgemein-
schaft. A.K.E. and J.P.W. thank the Fonds der Chemischen
Industrie for fellowships. We thank Heike Hausmann for
performing the NMR measurements. J.M.S. thanks Marcel A.
Strauss and Robert Pollice for fruitful discussions and
Sebastian Ahles for synthetic advice.
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