29 research outputs found
Toward Predicting Full Catalytic Cycle Using Automatic Reaction Path Search Method: A Case Study on HCo(CO)<sub>3</sub>-Catalyzed Hydroformylation
Toward systematic prediction of reaction pathways in complex chemical reaction systems by quantum chemical calculations, a new automatic reaction path search approach has been proposed on the basis of the artificial force induced reaction (AFIR) method [<i>J. Chem. Theory Comput.</i> <b>2011</b>, <i>7</i>, 2335–2345.]. We demonstrate in this Letter that this approach enabled semiautomatic determination of the full catalytic cycle of the HCo(CO)<sub>3</sub>-catalyzed hydroformylation. The search was fully systematic; no initial guess was required concerning the entire reaction mechanism as well as each transition-state structure. This approach opens the door to nonempirical prediction of complex reaction mechanisms involving multiple steps in multiple pathways, such as full cycles of catalytic reactions
CASPT2 Study of Photodissociation Pathways of Ketene
The mechanism of various photodissociation
channels of ketene involving
the three lowest singlet states (S<sub>0</sub>, S<sub>1</sub>, and
S<sub>2</sub>) and the three lowest triplet states (T<sub>1</sub>,
T<sub>2</sub>, and T<sub>3</sub>) was investigated by means of the
(MS-)ÂCASÂ(8e,8o)ÂPT2/6-31+G* method. Stationary structures, i.e., global
minima (GMs), local minima (LMs), transition states (TSs), minimum
energy conical intersections (MECIs), and minima on seam of crossing
(MSXs), were explored systematically by the global reaction route
mapping (GRRM) strategy. On the basis of these structures, we discussed
related dissociation channels starting from S<sub>2</sub> that have
been studied experimentally with 193–215 nm excitation wavelength.
Five working pathways were found for relaxation to the low-lying S<sub>0</sub>, S<sub>1</sub>, and T<sub>1</sub> potential energy surfaces
(PESs) from the Franck–Condon region of S<sub>2</sub>, and
the relaxation is expected to occur very quickly. On these low-lying
states, five dissociation channels are open: three CH<sub>2</sub> +
CO channels for different CH<sub>2</sub> electronic states, H + HCCO,
and H<sub>2</sub> + C<sub>2</sub>O. Pathways for all of these five
channels were identified and discussed, including new minor paths
leading to H<sub>2</sub> + C<sub>2</sub>O
Sampling of Transition States for Predicting Diastereoselectivity Using Automated Search Methodî—¸Aqueous Lanthanide-Catalyzed Mukaiyama Aldol Reaction
To
predict the stereoselectivity of large and flexible reaction
systems, structural sampling of many transition states (TSs) is required.
We used an automated search method, the artificial force induced reaction
(AFIR) method, for TS sampling and found 91 <i>syn</i>-
and 73 <i>anti</i>-TSs for the diastereoselective C–C
bond formation step of the aqueous lanthanide-catalyzed Mukaiyama
aldol reaction. Among them 11 <i>syn</i>- and six <i>anti</i>-TSs are found to contribute significantly to the diastereomeric
ratio at room temperature
A Theoretical Study on the Photodissociation of Acetone: Insight into the Slow Intersystem Crossing and Exploration of Nonadiabatic Pathways to the Ground State
Structures of transition states (TSs) and minima on seam of crossing (MSXs) for potential energy surfaces (PESs) of acetone of the S<sub>0</sub>, S<sub>1</sub>, and T<sub>1</sub> states were explored. On the basis of the results, we propose a new mechanism, slow intersystem crossing from S<sub>1</sub> to T<sub>1</sub> without seam of crossing, followed by CH<sub>3</sub> dissociation via a TS on T<sub>1</sub>; this slow pathway will be overtaken by a more efficient S<sub>1</sub> pathway for higher energy. This is consistent with the observed long lifetime of the S<sub>1</sub> species. Moreover, four channels, including three new ones, were found to regenerate the ground state acetone from the S<sub>1</sub> PES, and they all may be involved in the roaming channel that has been proposed recently as a new route of CO generation in a 230 nm photolysis. There are significant differences in MSX structures and energies between the present CASPT2 results and previous CASSCF results
Global ab Initio Potential Energy Surfaces for Low-Lying Doublet States of NO<sub>3</sub>
We report analytical global potential energy surfaces
(PESs) for
three low-lying doublet states (D<sub>0</sub>, D<sub>1</sub>, and
D<sub>2</sub>) of NO<sub>3</sub>. The fits are made on roughly 74000
MS-CASÂ(17e,13o)ÂPT2/aug-cc-pVTZ calculations of electronic energies,
where these PESs are invariant of permutations of oxygen atoms. The
surfaces describe two roaming pathways for NO<sub>3</sub> →
NO<sub>2</sub>-----O → NO + O<sub>2</sub> involving different
electronic states discovered in the photolysis of NO<sub>3</sub> [Xiao, H. Y. et al. <i>J. Phys. Chem. Lett.</i> <b>2011</b>, <i>2</i>, 934]. These pathways become accessible
at excess energy of ∼210 kJ/mol above the ground-state global
minimum of NO<sub>3</sub>. The ab initio data below 360 kJ/mol are
reproduced very well by the fitted PESs with the fitting rms errors
of less than 5.5 kJ/mol for all the three states. Moreover, key local
minima and energy profiles along the roaming pathways on the fitted
PESs are compared with those on the ab initio PESs. In addition, potential contour maps in the roaming
region are also compared. These careful evaluations of the fitted
PESs suggest that the present fitted PESs are well suited for future
dynamics calculations of this system
Far-Field Emission Patterns of Nanowire Light-Emitting Diodes
We investigated far-field (FF) emission
patterns of nanowire light-emitting
diodes (NW-LEDs). NW-LEDs were fabricated using vertical InP-NW arrays
with axial pn-junctions grown on InP (111)ÂA substrates, and the emission
intensity of NW-LEDs was measured as a function of view angle θ,
where θ = 0° indicates the direction normal to the substrate
or that along the NWs. For NW arrays with pitch <i>a</i> of around 1 μm, we found a clear dip in the emission intensity
at θ = 0°, which was explained by an analogy with dipole
antenna, or a smaller contribution of the lowest order guided modes
for emission as compared with higher order guided and free-space radiation
modes. Results of the simulation of radiation patterns by the finite-difference
time-domain method and near-field to far-field transformation are
also described. They also confirm that the dip at θ = 0°
is specific to light emission from NWs. We also investigated the dependence
of the FF pattern on the pitch of the NW array, and the observation
was qualitatively explained by the relative contribution of the guided
and free-space radiation modes
Theoretical Study on the Photodissociation of Methylamine Involving S<sub>1</sub>, T<sub>1</sub>, and S<sub>0</sub> States
Various photodissociation pathways
of methylamine involving the
three lowest electronic states, namely, singlet ground S<sub>0</sub> state, singlet first excited S<sub>1</sub> state, and triplet ground
T<sub>1</sub> state, were studied by the (MS-)ÂCASÂ(8e,8o)ÂPT2/6-31++G**
method. All critical points, i.e., minima, transition states, minimum
energy conical intersections, and minima on the seam of crossing,
were explored systematically by the global reaction route mapping
(GRRM) strategy utilizing the anharmonic downward distortion following
(ADDF) and artificial force induced reaction (AFIR) methods. On the
basis of obtained structures, we discuss the photodissociation mechanism
of methylamine in the experimental excitation wavelength range 222–240
nm in detail. Especially, the T<sub>1</sub> potential energy surface
was explored systematically for the first time. The N–H bond
rupture is a primary channel on the S<sub>1</sub> state. Along the
N–H dissociation path on S<sub>1</sub>, there is a low-energy
conical intersection (CI), and through this CI the system can go back
to the S<sub>0</sub> state; from the CI the system can directly dissociate
to CH<sub>3</sub>NH + H or reproduce the original CH<sub>3</sub>NH<sub>2</sub> on S<sub>0</sub>. There is a seam of crossing between S<sub>0</sub> and T<sub>1</sub> in a partially dissociated CH<sub>3</sub>---NH<sub>2</sub> geometry, and through this seam the system may
go up to the T<sub>1</sub>. On the T<sub>1</sub> state, a roaming-like
pathway giving CH<sub>4</sub> + NH (X<sup>3</sup>Σ<sup>–</sup>) products was found, which would explain the recently proposed intersystem
crossing mediated roaming dynamics
Excited-State Roaming Dynamics in Photolysis of a Nitrate Radical
Roaming dynamics has been recognized as one of the key mechanisms in atmospheric and combustion processes. In this Letter, we report the first example of roaming dynamics that occurs on an excited-state potential energy surface. In the photodissociation of a nitrate radical, systematic CASPT2 reaction path search and DFT dynamics calculations show that the roaming dynamics occurs on the first excited doublet state (D<sub>1</sub>). The direct dissociation on D<sub>1</sub> gives the minor vibrationally cold O<sub>2</sub>, while the indirect dissociation after the nonadiabatic transition to the ground doublet state (D<sub>0</sub>) produces the major vibrationally hot O<sub>2</sub>; this proposal explains the recent experimental results
Automated Exploration of Photolytic Channels of HCOOH: Conformational Memory via Excited-State Roaming
To elucidate the photodissociation mechanism of HCOOH,
we systematically
explored reaction pathways starting from the first excited singlet
state (S<sub>1</sub>) by using automated reaction path search methods.
All critical points, that is, minima, transition states, minimum energy
conical intersections, and minima on seam of crossing, for the S<sub>0</sub>, T<sub>1</sub>, and S<sub>1</sub> potential energy surfaces
(PESs) obtained in the present search were optimized at the CASPT2
level. The structure list obtained by the search explained all experimentally
reported photolytic channels. A new mechanism for the previously suggested
but unexplained conformational memory in the 193 nm photolysis is
proposed, which involves two steps: partial dissociation and succeeding
roaming of one of H atoms on the S<sub>1</sub> PES, followed by intramolecular
recombination on the S<sub>0</sub> PES after radiationless transition
through a conical intersection at a partially dissociated geometry.
This is partially similar to the excited-state roaming recently discovered
for the NO<sub>3</sub> radical
Theoretical Study of Hydrogenation Catalysis of Phosphorus Compound and Prediction of Catalyst with High Activity and Wide Application Scope
Catalysis
of phosphorus compound 10-P-3<sup>2</sup>3,7-diÂ(<i>tert</i>-butyl)-5-aza-2,8-dioxa-l-phosphabicycloÂ[3.3.0]Âocta-2,4,6-triene <b>1P</b> and its analogues in transfer hydrogenation of azobenzene
with ammonia-borane (NH<sub>3</sub>BH<sub>3</sub>) was investigated
using DFT, CCSDÂ(T), and ONIOMÂ(CCSDÂ(T):MP2) methods. Theoretical calculations
clearly reveal that this transfer hydrogenation reaction occurs through
the phosphorus–ligand cooperative catalysis function with a
change in the phosphorus oxidation state between +I and +III. It is
noteworthy that the change in phosphorus oxidation state is a characteristic
feature of the cooperative catalysis by <b>1P</b>: it is not
easy for a main-group element to change its oxidation state. This
characteristic feature differs from that of metal–ligand cooperative
catalysis in which the metal oxidation state does not change. Theoretical
calculations also disclose that the catalytic activity of <b>1P</b>-like compound is improved by introducing electron-withdrawing substituents
to the ligand and using the conjugated planar framework of the ligand.
The best catalyst here is predicted to be active for transfer hydrogenations
of various ketones and acetaldehyde with NH<sub>3</sub>BH<sub>3</sub>, indicating that <b>1P</b>-like compound exhibit a wide application
scope as a catalyst