13 research outputs found
Doping Effect on Edge-Terminated Ferromagnetic Graphene Nanoribbons
The
doping effect on intramolecular magnetic exchange coupling
of an edge-terminated zigzag graphene nanoribbon (ZGNR) with organic
radicals was studied with density functional theory calculation. We
investigated magnetic behaviors of boron (B)- and nitrogen (N)-doped
ZGNRs, terminated with trimethylenemethane (TMM) and 6-oxoverdazyl
(OVER) radicals, that is, TMM-ZGNR-TMM, OVER-ZGNR-OVER, and TMM-ZGNR-OVER.
A doping with B or N on the spin-coupling pathway of radical-ZGNR-radical
changed the spin distribution pattern of each system and hence its
magnetic ground configuration, magnetic coupling strength, and magnetic
moment. The first doping switched the magnetic ground configuration
of a system from antiferromagnetic (AFM) to ferromagnetic (FM) and
vice versa. An additional doping switched it back to its original
magnetic ground configuration. Moreover, N doping on a radical-terminated
edge increased the magnetic coupling strength as compared with the
undoped system, while B doping decreased it. Furthermore, B or N doping
on a TMM-terminated edge increased the magnetic moment of the system,
while the same doping on an OVER-terminated edge decreased it. Our
results demonstrate a possibility of reversible spin control of organic
magnetic materials from AFM to FM and vice versa by chemical doping
and the enhancement of the magnetic coupling strength of edge-terminated
ZGNRs
Systematic Approach To Design Organic Magnetic Molecules: Strongly Coupled Diradicals with Ethylene Coupler
The intramolecular magnetic coupling constant (<i>J</i>) values of diradical systems linked with two monoradicals
through
a coupler (para-substituted phenyl acetylene (Model I), meta-substituted
phenyl acetylene (Model II), ethylene (Model III)) were investigated
by unrestricted density functional theory calculations. We divided
eight monoradicals into α-group and β-group according
to Mulliken spin density values of the connected atoms. The overall
trends in the strength of magnetic interactions of diradicals were
found to be identical in three different model systems. The diradicals
with para-substituted phenyl acetylene coupler resulted in almost
twice stronger intramolecular magnetic coupling interactions of the
corresponding diradicals as compared to the meta-substituted one with
opposite magnetism. <b>NN</b>-Ethylene-<b>PO</b> (nitronyl
nitroxide radical coupled to phenoxyl radical via ethylene coupler)
was calculated to have the strongest magnetic coupling constant with
ferromagnetism, and to be even stronger (more than twice) than <b>NN</b>-ethylene-<b>NN</b> (nitronyl nitroxide diradical
with ethylene coupler), which was reported to have strong antiferromagnetic
interactions in a previous experiment. It was found that the spin
density values of the connected atoms are closely related to the determination
of magnetic interactions and <i>J</i> values. The spin states
of the ground state in diradical systems were explained by means of
the spin alternation rule
Catalytic Mechanism for the Ruthenium-Complex-Catalyzed Synthesis of Amides from Alcohols and Amines: A DFT Study
Details
of the reaction mechanism for the Ru–PNN pincer complex catalyzed
amidation from an alcohol and an amine proposed by Milstein et al.
was elucidated using M06 density functional theory calculations. In
addition, the bifunctional double hydrogen transfer (BDHT) mechanism
for the dehydrogenative oxidation step was investigated for comparison.
Finally, the BDHT mechanism was found to be preferred over the β-H
elimination pathway that was proposed by Milstein et al. On the basis
of the analysis of NBO charges and orbital interactions of intermediates
and transition states, we designed a new catalyst with the addition
of an electron-donating substituent (−NEt<sub>2</sub>), which
provided much reduced energy barriers and a lower potential energy
surface along both mechanisms
Scaling Approach for Intramolecular Magnetic Coupling Constants of Organic Diradicals
The
intramolecular magnetic coupling constants (<i>J</i>) of
9 diradicals (<b>i</b>–<b>ix</b>) coupled with
an aromatic ring were investigated by means of unrestricted density
functional theory (DFT) calculations [UB3LYP/6-311++G(d,p)]. For these
diradicals, a remarkable linear relationship between the calculated
and experimental <i>J</i> values was found. In this study,
we suggest that the slope (0.380) of the linear relationship can be
utilized as a scaling factor for estimating <i>J</i> values.
By applying this scaling factor and calculating <i>J</i> values, we could predict the reliable <i>J</i> values
of four dithiadiazolyl (<b>DTDA</b>) diradicals coupled with
an aromatic ring. It was also found that this scaling scheme shows
a dependence on the length of a coupler. Nevertheless, this scaling
approach could be used to estimate <i>J</i> values for diverse
diradical systems coupled with a particular coupler by DFT calculations
Organic Magnetic Diradicals (Radical–Coupler–Radical): Standardization of Couplers for Strong Ferromagnetism
The intramolecular magnetic coupling
constant (<i>J</i>) values of sets of diradicals linked
to bis-DTDA, OVER, and NN radicals
(DTDA, OVER, and NN groups) through an aromatic coupler were studied
by unrestricted density functional theory calculations (UB3LYP/6-311++G(d,p)).
Among 15 aromatic couplers, 9 compounds with an odd number of carbon
atoms along its spin coupling path were found to interact ferromagnetically
upon coupling with bisradicals while the other 6 couplers with an
even number of carbon atoms along its spin coupling path give rise
to antiferromagnetic coupling. The overall trends in the strength
of magnetic interactions of aromatic couplers were preserved for DTDA,
OVER, and NN groups so that the trend can be utilized as an index
for the magnetic strength of a given coupler. It was found that the
differences in the nucleus-independent chemical shift (NICS), bond
order of connecting bonds, and Mulliken atomic spin density at connected
atoms between triplet and BS states are closely related to the intramolecular
magnetic behavior. 2,4- and 2,5-phosphole couplers exhibit the strongest
intramolecular ferromagnetic and antiferromagnetic interactions among
15 aromatic couplers when linked to diverse bisradicals
Ferromagnetic Graphene Nanoribbons: Edge Termination with Organic Radicals
The intramolecular magnetic exchange
coupling of edge terminated
zigzag graphene nanoribbon (ZGNR) was studied with density functional
theory calculations. In order to examine the applicability of the
spin alternation rule and a classification scheme for radicals and
couplers on functionalized graphene nanoribbons, we investigated the
magnetic behaviors of pristine zigzag graphene nanoribbon with eight
zigzag chains (8-ZGNR) and 8-ZGNRs terminated with trimethylenemethane
(TMM) and 6-oxoverdazyl (OVER) radicals,that is, TMM-ZGNR-TMM (TZT),
OVER-ZGNR-OVER (OZO), and TMM-ZGNR-OVER (TZO). As expected, only ZGNR
terminated with different group radicals on each edge (TZO) had a
ferromagnetic (high-spin) ground state with an energy gap of 39 meV/supercell
(321.57 cm<sup>–1</sup>) relative to the low-spin state. This
strongly supports the validity of the spin alternation rule and the
classification scheme for radicals and couplers on extensively conjugated
large graphene nanoribbons. TZT and OZO were found to have an antiferromagnetic
(low-spin) ground state with magnetic coupling weaker than that of
interedge antiferromagnetic superexchange of pristine 8-ZGNR. Based
on the spin distribution pattern on magnetic ground states, GNR prefers
to have each edge in antiferromagnetic order, which satisfies Lieb’s
theorem on the Hubbard model and spin alternation rule. All of the
terminated ZGNRs exhibited semiconducting properties with an energy
gap of 0.06–0.21 eV
Electronic and Nuclear Contributions to Vibrational Stark Shifts of Hydroxyl Stretching Frequencies of Water Clusters
In spite of the importance
of vibrational Stark effect (VSE) and
many attempts, origin of VSE is still unclear in molecular level.
Here, we studied on origin of VSE of hydroxyl stretching vibration
in small water clusters (monomer, dimer, and tetramer) assuming that
VSE can be separated by nuclear and electronic contribution. We calculated
total Stark tuning rate (Δμ<sub>tot</sub>) and its nuclear
contribution (Δμ<sub>geom</sub>) using the ab initio method,
then the electronic contribution (Δμ<sub>elec</sub>) was
simply obtained by the difference, Δμ<sub>tot</sub> –
Δμ<sub>geom</sub>. In all cases, the nuclear contribution
has dominant contribution to VSE. The hydroxyl stretching mode with
neighboring hydrogen acceptor showed larger Δμ<sub>geom</sub> than that of dangling bonds. Furthermore, the calculated Δμ<sub>geom</sub> became larger in larger cluster due to the hydrogen bond
network. The comparison between Stark tuning rates including and excluding
anharmonicity supports the importance of potential anharmonicity in
VSE, as previously reported. Interestingly, a good linear relationship
is observed between the hydroxyl stretch frequency (ν<sub>geom</sub>) and hydroxyl bond length and also between the Stark tuning rate
(Δμ<sub>geom</sub>) and the change of hydroxyl bond length.
Similarly, the electronic contribution of calculated frequencies and
Stark tuning rate (Δμ<sub>elec</sub>) showed a good linear
relationship with atomic charge derived by electronic perturbation
(Δq<sub>elec</sub>) and change of that (Δ(Δ<i>q</i><sub>elec</sub>)), respectively
Simple but Useful Scheme toward Understanding of Intramolecular Magnetic Interactions: Benzene-Bridged Oxoverdazyl Diradicals
It
has recently been shown that the types of intramolecular magnetic
interactions of diradical systems can be changed by the types of radical
group: syn-group (or α-group) and anti-group (or β-group).
The aim of this study is to establish a useful scheme to understand
and explain the intramolecular magnetic interactions in diradical
systems regardless of radical groups and the topology of a coupler.
We investigated the intramolecular magnetic coupling constant (<i>J</i>) of six oxoverdazyl diradicals (<b>i</b>–<b>vi</b>) coupled with a benzene ring based on the unrestricted
DFT calculations. On the basis of our results, we devised a simple
but useful scheme by combining the spin alternation rule and the concept
of radical group classification. Consequently, it was found that the
calculated <i>J</i> values and plots of spin density distributions
were consistent with our proposed scheme. In addition, we discussed
the closed-shell singlet (<b>CS</b>) state and the dihedral
angle effect on <i>J</i> values in detail to comprehensively
understand the magnetic interactions of diradical systems. Our scheme
can provide the basic framework to design future organic high-spin
molecules and organic magnetic materials
Effect of Electric Field on Condensed-Phase Molecular Systems. II. Stark Effect on the Hydroxyl Stretch Vibration of Ice
We studied the Stark effect on the
hydroxyl stretching vibration
of water molecules in ice under the influence of an external electric
field. Electric fields with strengths in the range from 6.4 ×
10<sup>7</sup> to 2.3 × 10<sup>8</sup> V·m<sup>–1</sup> were applied to an ice sample using the ice film capacitor method.
Reflection absorption infrared spectroscopy was used to monitor the
field-induced spectral changes of vibrationally decoupled O–H
and O–D bands of dilute HOD in D<sub>2</sub>O and H<sub>2</sub>O–ice, respectively. The spectral changes of the hydroxyl
bands under applied field were analyzed using a model that simulates
the absorption of a collection of Stark-shifted oscillators. The analysis
shows that the Stark tuning rate of ν(O–D) is 6.4–12
cm<sup>–1</sup>/(MV·cm<sup>–1</sup>) at a field
strength from 1.8 × 10<sup>8</sup> to 6.4 × 10<sup>7</sup> V·m<sup>–1</sup>, and the Stark tuning rate of ν(O–H)
is 10–16 cm<sup>–1</sup>/(MV·cm<sup>–1</sup>) at a field strength from 2.3 × 10<sup>8</sup> to 9.2 ×
10<sup>7</sup> V·m<sup>–1</sup>. These values are uniquely
large compared to the Stark tuning rates of carbonyl or nitrile vibrations
in other frozen molecular solids. Quantum mechanical calculations
for the vibrations of isolated water and water clusters show that
the vibrational Stark effect increases with the formation of intermolecular
hydrogen bonds. This suggests that that the large Stark tuning rate
of ice is due to its hydrogen-bonding network, which increases anharmonicity
of the potential curve along the O–H bond and the ability to
shift the electron density under applied electric field
Phase Cycling RT-TDDFT Simulation Protocol for Nonlinear XUV and X‑ray Molecular Spectroscopy
Real-time
time-dependent density functional theory (RT-TDDFT) provides
a practical algorithm for propagating a many-electron system driven
by external laser fields. The fields are included nonperturbatively
in the propagation, and the molecular reduced single-electron density
operator and various spectroscopic and diffraction signals can be
computed directly, avoiding the expensive calculation of many-body
states. Nonlinear optical signals contain contributions of multiple
pathways. A phase cycling protocol is implemented in order to separate
these pathways. Simulations of XUV four-wave mixing signals in the
CO molecule are compared with ab initio sum-over-states calculations