7 research outputs found
Comprehensive Benchmark of Association (Free) Energies of Realistic HostâGuest Complexes
The
S12L test set for supramolecular Gibbs free energies of association
Î<i>G</i><sub><i>a</i></sub> (Grimme, S. Chem. Eur. J. 2012, 18, 9955â9964) is extended
to 30 complexes (S30L), featuring more diverse interaction motifs,
anions, and higher charges (â1 up to +4) as well as larger
systems with up to 200 atoms. Various typical noncovalent interactions
like hydrogen and halogen bonding, ÏâÏ stacking,
nonpolar dispersion, and CHâÏ and cationâdipolar
interactions are represented by ârealâ complexes. The
experimental Gibbs free energies of association (Î<i>G</i><sub><i>a</i></sub><sup><i>exp</i></sup>) cover a wide range from â0.7 to
â24.7 kcal mol<sup>â1</sup>. In order to obtain a theoretical
best estimate for Î<i>G</i><sub><i>a</i></sub>, we test various dispersion corrected density functionals
in combination with quadruple-ζ basis sets for calculating the
association energies in the gas phase. Further, modern semiempirical
methods are employed to obtain the thermostatistical corrections from
energy to Gibbs free energy, and the COSMO-RS model with several parametrizations
as well as the SMD model are used to include solvation contributions.
We investigate the effect of including counterions for the charged
systems (S30L-CI), which is found to overall improve the results.
Our best method combination consists of PW6B95-D3 (for neutral and
charged systems) or ÏB97X-D3 (for systems with counterions)
energies, HF-3c thermostatistical corrections, and Gibbs free energies
of solvation obtained with the COSMO-RS 2012 parameters for nonpolar
solvents and 2013-fine for water. This combination gives a mean absolute
deviation for Î<i>G</i><sub><i>a</i></sub> of only 2.4 kcal mol<sup>â1</sup> (S30L) and 2.1 kcal mol<sup>â1</sup> (S30L-CI), with a mean deviation of almost zero compared
to experiment. Regarding the relative Gibbs free energies of association
for the 13 pairs of complexes which share the same host, the correct
trend in binding affinities could be reproduced except for two cases.
The MAD compared to experiment amounts to 1.2 kcal mol<sup>â1</sup>, and the MD is almost zero. The best-estimate theoretical corrections
are used to back-correct the experimental Î<i>G</i><sub><i>a</i></sub> values in order to get an empirical
estimate for the âexperimentalâ, zero-point vibrational
energy exclusive, gas phase binding energies. These are then utilized
to benchmark the performance of various âlow-costâ quantum
chemical methods for noncovalent interactions in large systems. The
performance of other common DFT methods as well as the use of semiempirical
methods for structure optimizations is discussed
Comprehensive Benchmark of Association (Free) Energies of Realistic HostâGuest Complexes
The
S12L test set for supramolecular Gibbs free energies of association
Î<i>G</i><sub><i>a</i></sub> (Grimme, S. Chem. Eur. J. 2012, 18, 9955â9964) is extended
to 30 complexes (S30L), featuring more diverse interaction motifs,
anions, and higher charges (â1 up to +4) as well as larger
systems with up to 200 atoms. Various typical noncovalent interactions
like hydrogen and halogen bonding, ÏâÏ stacking,
nonpolar dispersion, and CHâÏ and cationâdipolar
interactions are represented by ârealâ complexes. The
experimental Gibbs free energies of association (Î<i>G</i><sub><i>a</i></sub><sup><i>exp</i></sup>) cover a wide range from â0.7 to
â24.7 kcal mol<sup>â1</sup>. In order to obtain a theoretical
best estimate for Î<i>G</i><sub><i>a</i></sub>, we test various dispersion corrected density functionals
in combination with quadruple-ζ basis sets for calculating the
association energies in the gas phase. Further, modern semiempirical
methods are employed to obtain the thermostatistical corrections from
energy to Gibbs free energy, and the COSMO-RS model with several parametrizations
as well as the SMD model are used to include solvation contributions.
We investigate the effect of including counterions for the charged
systems (S30L-CI), which is found to overall improve the results.
Our best method combination consists of PW6B95-D3 (for neutral and
charged systems) or ÏB97X-D3 (for systems with counterions)
energies, HF-3c thermostatistical corrections, and Gibbs free energies
of solvation obtained with the COSMO-RS 2012 parameters for nonpolar
solvents and 2013-fine for water. This combination gives a mean absolute
deviation for Î<i>G</i><sub><i>a</i></sub> of only 2.4 kcal mol<sup>â1</sup> (S30L) and 2.1 kcal mol<sup>â1</sup> (S30L-CI), with a mean deviation of almost zero compared
to experiment. Regarding the relative Gibbs free energies of association
for the 13 pairs of complexes which share the same host, the correct
trend in binding affinities could be reproduced except for two cases.
The MAD compared to experiment amounts to 1.2 kcal mol<sup>â1</sup>, and the MD is almost zero. The best-estimate theoretical corrections
are used to back-correct the experimental Î<i>G</i><sub><i>a</i></sub> values in order to get an empirical
estimate for the âexperimentalâ, zero-point vibrational
energy exclusive, gas phase binding energies. These are then utilized
to benchmark the performance of various âlow-costâ quantum
chemical methods for noncovalent interactions in large systems. The
performance of other common DFT methods as well as the use of semiempirical
methods for structure optimizations is discussed
Reaction of OH with Aliphatic and Aromatic Isocyanates
Isocyanates are highly relevant industrial intermediates
for polyurethane
production. In this work, we used quantum chemistry and transition
state theory (TST) to investigate the gas-phase reaction of isocyanates
with the OH radical, which is likely one of the most significant chemical
sinks for these compounds in the troposphere. para-Tolyl-isocyanate (p-tolyl-NCO) was chosen as a proxy substance for
the large-volume aromatic diisocyanate species toluene diisocyanate
and methylene diphenyl diisocyanate. Besides p-tolyl-NCO + OH, the
model reactions CH3NCO + OH, H2CCHNCO
+ OH, C6H5-NCO + OH, C6H5-CH3 + OH, and C6H6 + OH have been
studied as well to analyze various substituent effects and to allow
for comparison with literature. Quantum chemical computations at the
CCSD(T)/cc-pV(T,Q â â)Z//M06-2X/def2-TZVP level were
used as the basis for tunneling-corrected canonical TST calculations.
For CH3NCO + OH, H abstraction by OH at the methyl group
is the main reaction channel according to our calculations and predicted
to be four orders of magnitude faster than OH addition at the NCO
group. The calculated rate coefficient (8.8 Ă 10â14 cm3 moleculeâ1 sâ1) at 298 K is in good agreement with experimental data from the literature.
Likewise, for aromatic isocyanates, OH attack at the isocyanate group
was found to be only a minor pathway compared to addition to the aromatic
ring. In the OH + p-tolyl-NCO reaction, OH addition at the ortho-position relative to the NCO group has been identified
as the main initial reaction channel (branching fraction: 53.2%),
with smaller but significant branching fractions for the H abstraction
at the methyl group (9.6%) as well as the other ring addition reactions
(ipso: 2.3%, meta: 24.5%, para: 10.5%, all relative to the NCO group).
By comparing OH addition to the aromatic ring in p-tolyl-NCO with
the respective ring addition reactions of phenyl isocyanate and toluene,
the site-selective reactivity trends observed for ring addition in
the OH + p-tolyl-NCO could be rationalized by a dominating positive
mesomeric effect of the NCO group and a positive electron-donating
(inductive) effect of the CH3 group. Except for the OH
ring adduct formed from OH addition in ipso-position to the NCO group,
we estimate the first-generation radical intermediates in the OH +
p-tolyl-NCO reaction to have sufficiently long lifetimes to react
with O2 under atmospheric conditions and undergo typical
oxidative reaction cascades like those known for benzene or toluene
Synthesis, Chiral Resolution, and Absolute Configuration of Dissymmetric 4,15-Difunctionalized [2.2]Paracyclophanes
Despite the fact that functionalized
planar chiral [2.2]Âparacyclophanes
have received a lot of attention, the chemistry of pseudo-<i>meta</i> 4,15-distubstituted [2.2]Âparacyclophanes is largely
unexplored. This is mainly due to the fact that the 4,5-dibromo-functionalized
[2.2]Âparacyclophane is much less prone to halogen-metal exchange reactions
than its constitutional pseudo-<i>ortho</i> or pseudo-<i>para</i> isomers. Here, we give an account of an efficient protocol
to achieve this, which allows the synthesis of a broad variety of
4,15-disubstituted [2.2]Âparacyclophanes. Furthermore, we were able
to resolve several of the racemic compounds via chiral HPLC and assign
the absolute configurations of the isolated enantiomers by X-ray diffraction
and/or by the comparison of calculated and measured CD-spectra
Synthesis, Chiral Resolution, and Absolute Configuration of Dissymmetric 4,15-Difunctionalized [2.2]Paracyclophanes
Despite the fact that functionalized
planar chiral [2.2]Âparacyclophanes
have received a lot of attention, the chemistry of pseudo-<i>meta</i> 4,15-distubstituted [2.2]Âparacyclophanes is largely
unexplored. This is mainly due to the fact that the 4,5-dibromo-functionalized
[2.2]Âparacyclophane is much less prone to halogen-metal exchange reactions
than its constitutional pseudo-<i>ortho</i> or pseudo-<i>para</i> isomers. Here, we give an account of an efficient protocol
to achieve this, which allows the synthesis of a broad variety of
4,15-disubstituted [2.2]Âparacyclophanes. Furthermore, we were able
to resolve several of the racemic compounds via chiral HPLC and assign
the absolute configurations of the isolated enantiomers by X-ray diffraction
and/or by the comparison of calculated and measured CD-spectra
Substituent Effects and Supramolecular Interactions of Titanocene(III) Chloride: Implications for Catalysis in Single Electron Steps
The electrochemical properties of
titanoceneÂ(III) complexes and
their stability in THF in the presence and absence of chloride additives
were studied by cyclic voltammetry (CV) and computational methods.
The anodic peak potentials of the titanocenes can be decreased by
as much as 0.47 V through the addition of an electron-withdrawing
substituent (CO<sub>2</sub>Me or CN) to the cyclopentadienyl ring
when compared with Cp<sub>2</sub>TiCl. For the first time, it is demonstrated
that under the conditions of catalytic applications low-valent titanocenes
can decompose by loss of the substituted ligand. The recently discovered
effect of stabilizing titanoceneÂ(III) catalysts by chloride additives
was analyzed by CV, kinetic, and computational studies. An unprecedented
supramolecular interaction between [(C<sub>5</sub>H<sub>4</sub>R)<sub>2</sub>TiCl<sub>2</sub>]<sup>â</sup> and hydrochloride cations
through reversible hydrogen bonding is proposed as a mechanism for
the action of the additives. This study provides the critical information
required for the rational design of titanocene-catalyzed reactions
in single electron steps
HYDROPHOBE Challenge: A Joint Experimental and Computational Study on the HostâGuest Binding of Hydrocarbons to Cucurbiturils, Allowing Explicit Evaluation of Guest Hydration Free-Energy Contributions
The
hostâguest complexation of hydrocarbons (22 guest molecules)
with cucurbit[7]Âuril was investigated in aqueous solution using the
indicator displacement strategy. The binding constants (10<sup>3</sup>â10<sup>9</sup> M<sup>â1</sup>) increased with guest
size, pointing to the hydrophobic effect and dispersion interactions
as driving forces. The measured affinities provide unique benchmark
data for the binding of neutral guest molecules. Consequently, a computational
blind challenge, the HYDROPHOBE challenge, was conducted to allow
a comparison with state-of-the-art computational methods for predicting
hostâguest affinity constants. In total, three quantum-chemical
(QM) data sets and two explicit-solvent molecular dynamics (MD) submissions
were received. When searching for sources of uncertainty in predicting
the hostâguest affinities, the experimentally known hydration
energies of the investigated hydrocarbons were used to test the employed
solvation models (explicit solvent for MD and COSMO-RS for QM). Good
correlations were obtained for both solvation models, but a rather
constant offset was observed for the COSMO data, by ca. +2 kcal mol<sup>â1</sup>, which was traced back to a required reference-state
correction in the QM submissions (2.38 kcal mol<sup>â1</sup>). Introduction of the reference-state correction improved the predictive
power of the QM methods, particularly for small hydrocarbons up to
C5