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 (−NEt₂), 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^–1) 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_tot) and its nuclear contribution (Δμ_geom) using the ab initio method, then the electronic contribution (Δμ_elec) was simply obtained by the difference, Δμ_tot – Δμ_geom. In all cases, the nuclear contribution has dominant contribution to VSE. The hydroxyl stretching mode with neighboring hydrogen acceptor showed larger Δμ_geom than that of dangling bonds. Furthermore, the calculated Δμ_geom 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_geom) and hydroxyl bond length and also between the Stark tuning rate (Δμ_geom) and the change of hydroxyl bond length. Similarly, the electronic contribution of calculated frequencies and Stark tuning rate (Δμ_elec) showed a good linear relationship with atomic charge derived by electronic perturbation (Δq_elec) and change of that (Δ­(Δ<i>q</i>_elec)), 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 × 10⁷ to 2.3 × 10⁸ V·m^–1 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 D₂O and H₂O–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^–1/(MV·cm^–1) at a field strength from 1.8 × 10⁸ to 6.4 × 10⁷ V·m^–1, and the Stark tuning rate of ν­(O–H) is 10–16 cm^–1/(MV·cm^–1) at a field strength from 2.3 × 10⁸ to 9.2 × 10⁷ V·m^–1. 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