42 research outputs found
Charge-Transfer-Induced <i>para</i>-Selective sp<sup>2</sup> C–H Bond Activation of Arenes by Use of a Hypervalent Iodine Compound: A Theoretical Study
The reaction mechanism
of the C–H bond activation of toluene
promoted by the hypervalent iodine compound TIPP-IÂ(OH)ÂOTs was investigated
in detail by density functional theory calculations. Our calculations
show that a plausible reaction pathway of the C–H bond activation
of toluene contains two stages: (1) the ligand exchange process on
TIPP-IÂ(OH)ÂOTs, involving the substitution of the hydroxyl group and
tosyloxyl group with TfOH, and (2) the C–H bond activation
of toluene promoted by the hypervalent iodine center with the assistance
of the triflate anion. The second step is the rate-limiting step with
a relatively low free energy barrier of 19.6 kcal mol<sup>–1</sup> in acetonitrile, which is in accord with the experimental fact that
the reaction takes place at room temperature. Frontier molecular orbits
and natural population analysis show that partial electron transfer
from the toluene to the hypervalent iodine moiety occurs in the charge-transfer
complex, resulting in the activation of the C–H bond at the
para position of toluene. Further calculations show that this hypervalent
iodine compound promoted C–H bond activation reaction will
be effective if the substrate is electron-rich and a strong Brønsted
acid is used
Generalized Energy-Based Fragmentation Approach and Its Applications to Macromolecules and Molecular Aggregates
ConspectusThe generalized
energy-based fragmentation (GEBF) approach provides
a very simple way of approximately evaluating the ground-state energy
or properties of a large system in terms of ground-state energies
of various small “electrostatically embedded” subsystems,
which can be calculated with any traditional <i>ab initio</i> quantum chemistry (X) method (X = Hartree–Fock, density functional
theory, and so on). Due to its excellent parallel efficiency, the
GEBF approach at the X theory level (GEBF-X) allows full quantum mechanical
(QM) calculations to be accessible for systems with hundreds and even
thousands of atoms on ordinary workstations. The implementation of
the GEBF approach at various theoretical levels can be easily done
with existing quantum chemistry programs.This Account reviews
the methodology, implementation, and applications
of the GEBF-X approach. This method has been successfully applied
to optimize the structures of various large systems including molecular
clusters, polypeptides, proteins, and foldamers. Such investigations
could allow us to elucidate the origin and nature of the cooperative
interaction in secondary structures of long peptides or the driving
force of the self-assembly processes of aromatic oligoamides. These
GEBF-based QM calculations reveal that the structures and stability
of various complex systems result from a subtle balance of many types
of noncovalent interactions such as hydrogen bonding and van der Waals
interactions. The GEBF-based <i>ab initio</i> molecular
dynamics (AIMD) method also allows the investigation of dynamic behaviors
of large systems on the order of tens of picoseconds. It was demonstrated
that the conformational dynamics of two model peptides predicted by
GEBF-based AIMD are noticeably different from those predicted by the
classical force field MD method.With the target of extending
QM calculations to molecular aggregates
in the condensed phase, we have implemented the GEBF-based multilayer
hybrid models, which could provide satisfactory descriptions of the
binding energies between a solute molecule and its surrounding waters
and the chain-length dependence of the conformational changes of oligomers
in aqueous solutions. A coarse-grained polarizable molecular mechanics
model, furnished with GEBF-X dipole moments of subsystems, exhibits
some advantages of treating the electrostatic polarization with reduced
computational costs. We anticipate that the GEBF approach will continue
to develop with the ultimate goal of studying complicated phenomena
at mesoscopic scales and serve as a practical tool to elucidate the
structure and dynamics of chemical and biological systems
Generalized Energy-Based Fragmentation CCSD(T)-F12a Method and Application to the Relative Energies of Water Clusters (H<sub>2</sub>O)<sub>20</sub>
The generalized energy-based fragmentation
(GEBF) approach has
been implemented for the explicitly correlated F12a of coupled-cluster
with the noniterative triples corrections [CCSDÂ(T)-F12a] method for
medium- and large-sized systems. By combining the canonical Hartree–Fock
(HF) total energies and the GEBF-X correlation energies, the GEBF-X/HF
method is illustrated to be more accurate than the origin GEBF-X method,
where X could be any electron correlation method, such as second-order
Møller–Plesset perturbation theory (MP2), MP2-F12, CCSDÂ(T),
and CCSDÂ(T)-F12a. By combining the GEBF-X/HF results at the MP2-F12
and CCSDÂ(T)-F12a levels, we can approximately achieve the CCSDÂ(T)
complete basis set (CBS) limit. Our test calculations for 10 low-energy
isomers of water 20-mers show that for the relative energies of large
water clusters, both the basis set and high-level electron correlation
effects should be taken into account, in which the former is even
more important. In addition, the GEBF-CCSDÂ(T)/HF method at the CBS
limit is used to evaluate 32 levels of density functional theory (DFT)
methods. The results show that the DFT methods are difficult to predict
the relative energies between the isomers of water 20-mers. The GEBF-CCSDÂ(T)/HF
method at the CBS limit is expected to be a benchmark for DFT and
other electron correlation methods for medium- and large-sized systems
with complex structures, in which both the basis set and electron
correlation effects are important
Improved Cluster-in-Molecule Local Correlation Approach for Electron Correlation Calculation of Large Systems
An improved cluster-in-molecule (CIM)
local correlation approach
is developed to allow electron correlation calculations of large systems
more accurate and faster. We have proposed a refined strategy of constructing
virtual LMOs of various clusters, which is suitable for basis sets
of various types. To recover medium-range electron correlation, which
is important for quantitative descriptions of large systems, we find
that a larger distance threshold (Îľ) is necessary for highly
accurate results. Our illustrative calculations show that the present
CIM-MP2 (second-order Møller-Plesser perturbation theory, MP2)
or CIM-CCSD (coupled cluster singles and doubles, CCSD) scheme with
a suitable Îľ value is capable of recovering more than 99.8%
correlation energies for a wide range of systems at different basis
sets. Furthermore, the present CIM-MP2 scheme can provide reliable
relative energy differences as the conventional MP2 method for secondary
structures of polypeptides
Generalized Energy-Based Fragmentation Approach for Localized Excited States of Large Systems
We have extended
the generalized energy-based fragmentation (GEBF)
approach to localized excited states of large systems. In this approach,
the excited-state energy of a large system could be expressed as the
combination of the excited-state energies of “active subsystems”,
which contains the chromophore center, and the ground-state energies
of “inactive subsystems”. The GEBF approach has been
implemented at the levels of time-dependent density functional theory
(TDDFT) and approximate coupled cluster singles and doubles (CC2)
method. Our results show that GEBF-TDDFT can reproduce the TDDFT excitation
energies and solvatochromic shifts for large systems and that GEBF-CC2
could be used to validate GEBF-TDDFT result (with different functionals).
The GEBF-TDDFT method is found to be able to provide satisfactory
or reasonable descriptions on the experimental solvatochromic shifts
for the <i>n</i> → π* transitions of acetone
in various solutions, and the lowest π → π* transitions
of pyridine and uracil in aqueous solutions
Accurate Relative Energies and Binding Energies of Large Ice–Liquid Water Clusters and Periodic Structures
Relative energies and binding energies
are crucial quantities that
determine various molecular properties of ice and water. We developed
a new effective method to compute those energies of bulk ice–liquid
water systems. In this work, ten ice–liquid 144-mers and ten
periodic ice–liquid (H<sub>2</sub>O)<sub>64</sub> systems are
taken from the molecular dynamics simulations in the melting process
of ice-Ih crystals. They are investigated at the levels of density
functional theory (DFT), explicitly correlated second-order Møller–Plesset
perturbation theory (MP2-F12), and coupled-cluster singles and doubles
with noniterative triples corrections [CCSDÂ(T)-F12b] in the framework
of generalized energy-based fragmentation approach. Our results show
that the changing of noncovalent interactions significantly influences
the performances of DFT and electron correlation methods for those
systems in the melting process of ice. Various DFT methods predict
quite different results for ice and mixed ice–liquid structures
but give similar results for pure liquid ones. It also explains why
many DFT-based simulations lead to inaccurate densities of ice and
liquid water. The CCSDÂ(T)-F12b results suggest that the MP2-F12 method
provides satisfactory results and is expected to be employed to simulate
the phase transitions of ice crystal
Accurate Prediction of Lattice Energies and Structures of Molecular Crystals with Molecular Quantum Chemistry Methods
We extend the generalized energy-based
fragmentation (GEBF) approach
to molecular crystals under periodic boundary conditions (PBC), and
we demonstrate the performance of the method for a variety of molecular
crystals. With this approach, the lattice energy of a molecular crystal
can be obtained from the energies of a series of embedded subsystems,
which can be computed with existing advanced molecular quantum chemistry
methods. The use of the field compensation method allows the method
to take long-range electrostatic interaction of the infinite crystal
environment into account and make the method almost translationally
invariant. The computational cost of the present method scales linearly
with the number of molecules in the unit cell. Illustrative applications
demonstrate that the PBC-GEBF method with explicitly correlated quantum
chemistry methods is capable of providing accurate descriptions on
the lattice energies and structures for various types of molecular
crystals. In addition, this approach can be employed to quantify the
contributions of various intermolecular interactions to the theoretical
lattice energy. Such qualitative understanding is very useful for
rational design of molecular crystals
Generalized Energy-Based Fragmentation Approach for the Electronic Emission Spectra of Large Systems
The excited-state (ES) geometry optimization and electronic
emission
(fluorescence and phosphorescence) spectra and the ES vibrational
spectra of large systems are great challenges in quantum chemistry.
In this work, we develop a generalized energy-based fragmentation
(GEBF) approach to compute the localized ES structures and vibrational
frequencies of large systems. In this approach, the ES energy derivatives
(gradients or Hessians) for a localized ES of a large system can be
obtained by combining the ES energy derivatives of the corresponding
active subsystems (including local excitation center) and the ground-state
energy derivatives of inactive subsystems. Two strategies are adopted
to overcome two difficulties from state-classification and state-tracking
for treating specific ESs. First, for state-classification, we develop
an improved density-based spatial clustering applied with noise algorithm
with a modified transition orbital projection (TOP) algorithm, which
allow a certain ES energy and energy derivatives of the whole system
to be calculated with different ES energies and energy derivatives
of active subsystems. Furthermore, we also employ the TOP algorithm
for tracking the ESs in their geometry optimizations at the time-dependent
density functional theory (TDDFT) level. Then, the GEBF approach is
applied to investigate the optimized ES geometries or ES vibrational
frequencies for two typical systems. Our results show that the cost-effective
GEBF approach can accurately reproduce the TDDFT fluorescence spectra
of the cytosine derivative and the experimental phosphorescence spectra
of the β-cyclodextrin derivative. The GEBF approach is expected
to be routinely applied to investigate the electronic emission spectra
of very large systems with local chromophores
Block-Correlated Coupled Cluster Theory with up to Four-Pair Correlation for Accurate Static Correlation of Strongly Correlated Systems
A block-correlated coupled cluster
method with up to
four-pair
correlation based on the generalized valence bond wave function (GVB-BCCC4)
is first implemented, which offers an alternative method for electronic
structure calculations of strongly correlated systems. We developed
some techniques to derive a set of compact and cost-effective equations
for GVB-BCCC4, which include the definition of n-block
(n = 1–4) Hamiltonian matrices, the combination
of excitation operators, and the definition of independent amplitudes.
We then applied the GVB-BCCC4 method to investigate several potential
energy surfaces of strongly correlated systems with singlet ground
states. Our calculations demonstrate that the GVB-BCCC4 method can
provide nearly exact static correlation energies as the density matrix
renormalization group method (on the basis of the same GVB orbitals).
This work highlights the significance of four-pair correlation in
quantitative descriptions of static correlation energy for strongly
correlated systems
Automatic Reaction Pathway Search via Combined Molecular Dynamics and Coordinate Driving Method
We proposed and implemented
a combined molecular dynamics and coordinate
driving (MD/CD) method for automatically searching multistep reaction
pathways of chemical reactions. In this approach, the molecular dynamic
(MD) method at the molecular mechanics (MM) or semiempirical quantum
mechanical (QM) level is employed to explore the conformational space
of the minimum structures, and the modified coordinate driving (CD)
method is used to build reaction pathways for representative conformers.
The MD/CD method is first applied to two model reactions (the Claisen
rearrangement and the intermolecular aldol reaction). By comparing
the obtained results with those of the existing methods, we found
that the MD/CD method has a comparable performance in searching low-energy
reaction pathways. Then, the MD/CD method is further applied to investigate
two reactions: the electrocyclic reaction of benzocyclobutene-7-carboxaldehyde
and the intramolecular Diels–Alder reaction of ketothioester
with 11 effectively rotatable single bonds. For the first reaction,
our results can correctly account for its torquoselectivity. For the
second one, our method predicts eight reaction channels, leading to
eight different stereo- and regioselective products. The MD/CD method
is expected to become an efficient and cost-effective theoretical
tool for automatically searching low-energy reaction pathways for
relatively complex chemical reactions