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The diazenylium ion (N2H+) is a ubiquitous ion in dense molecular clouds. This ion is often used as a dense gas tracer in outer space. Most of the previous works on diazenylium ion have focused on the shared-proton stretch band, νH+. In this work, we have performed reduced-dimensional calculations to investigate the vibrational structure of N2H+Ng, Ng = {He, Ne, Ar, Kr, Xe, and Rn}. We demonstrate a few interesting things about this system. First, the vibrational coupling in N2H+can be tuned to switch on interesting anharmonic effects such as Fermi resonance or combination bands by tagging it with different noble gases. Second, a comparison of the vibrational spectrum from N2H+He to N2H+Rn shows that the νH+can be swept from an “Eigen-like” to a “Zundel-like” limiting case. Anharmonic calculations were performed using a multilevel approach, which utilized the MP2 and CCSD(T) levels of theories. Binding energies for the elimination of Ng in N2H+Ng are also reported.

The structures of a pyrazine dimer (pyrazine)2and (pyrazine)(benzene) hetero-dimer cooled in a supersonic beam were investigated by the measurement of the infrared spectra in the C–H stretching region with infrared-vacuum ultraviolet (IR-VUV) spectroscopy and quantum-chemical calculations. The stabilization energy calculation at the CCSD(T)/aug-cc-pVTZ level of theory predicted three isomers for (pyrazine)2and three for (pyrazine)(benzene) with energy within 6 kJ/mol. Among them, the cross-displaced π–π stacked structure is the most stable in both dimers. In the observed IR spectra, both dimers exhibited two intense bands near 3065 cm–1, with intervals of 8 cm–1in (pyrazine)2and 11 cm–1in (pyrazine)(benzene), while only one band appeared in the monomer. For (pyrazine)(benzene), we also measured the IR spectrum of (pyrazine)(benzene-d6), where the interval of the two bands was unchanged. The analysis of the observed IR spectra with anharmonic calculations suggested the coexistence of three isomers of (pyrazine)2and (pyrazine)(benzene) in a supersonic jet. For (pyrazine)2, the two isomers which were previously assigned to the H-bonded planar and the π–π stacked structures respectively were reassigned to the cross-displaced π–π stacked and T-shaped structures, respectively. In addition, the quantum chemical calculation and IR-VUV spectral measurement suggested the coexistence of the H-bonded planar isomer in the jet. For (pyrazine)(benzene), the IR spectrum of the (pyrazine) site showed a similar spectral pattern to that of (pyrazine)2, especially the split at ∼3065 cm–1. However, the anharmonic analysis suggested that they are assigned to the different vibrational motions of (pyrazine). The anharmonic vibrational analysis is essential to associate the observed IR spectra with the correct structures of the dimer.

We use periodic DFT calculations at LDA and PBE level to investigate 3dtransition metal dihalide (TMDH) monolayers in H‐ and T‐phase. By analyzing the phonon dispersion, we have obtained a rough overview which combinations may form stable structures. We have focused on identifying and explaining trends in the predicted electronic properties. Although their geometric structures are simple, the associated electronic and magnetic properties are not as easy to understand. At first glance, it seems that there is no clear trend, as even isovalence‐electronic TMDH monolayers formed from the same metal but different halides can feature different magnetic moments. The identification of potential trends is further complicated by the fact that for a significant number of species, LDA results and PBE results predict different ground‐state electronic structures. By rigorously analyzing the potential energy surfaces associated with different magnetic moments, we could show that the apparent inconsistencies can be easily understood as a result of the differences in the relative energy between electronic states of different magnetic moments. We further show that the trends in the band gaps can be easily rationalized by an electron counting rule based on simple symmetry arguments. Trends in band gaps and magnetic moments of transition metal dihalides are rationalized using DFT calculations.

Recently, ether-based solvents and their fluorinated derivatives have shown extraordinary ability to facilitate the formation of a highly stable ultrathin solid-electrolyte interphase (SEI) layer in lithium metal batteries (LMBs). Herein, density functional theory based molecular dynamics (DFT-MD) simulations have been performed to provide a mechanistic insight on the stability of Li bis(fluoromethanesulfonyl)imide ([Li][FSI]) salt dissolved in dimethoxybutane (DMB) and fluorinated dimethoxybutane (FDMB) solvents near the surface of lithium metal which is employed as anode in LMBs. It is observed that because of the strong reducing nature of Li, the FSI anions in both of the electrolytes readily dissociate to form LiF, Li2O, and other species. Our analysis reveals that while the DMB molecules in the [Li][FSI]-DMB system are relatively stable against reductive dissociation, the FDMB molecules in the [Li][FSI]-FDMB system easily undergo reductive dissociation upon charge transfer from the Li surface. It is shown that the SF bonds of the FSI anions and CF bonds of FDMB molecules are the first ones to break upon their exposure to the Li metal surface. It is observed that the dissociated species of [Li][FSI]-FDMB electrolyte cover the Li surface completely, preventing further dissociation of other FSI anions and solvent molecules. It is found that the FDMB dissociation majorly contributes F atoms forming additional LiF which is an essential component for the formation of a uniform and robust SEI.

Vibrational spectra of the CH stretching vibration of diazine molecules, pyrimidine and pyrazine, were measured by infrared (IR)–vacuum ultraviolet (VUV) spectroscopy under the jet‐cooled gas‐phase condition. The observed IR spectra were analyzed by three anharmonic algorithms to account for the Fermi‐resonance (F‐R). The anharmonic analysis of the F‐R pattern was performed with second‐order vibrational perturbation theory (VPT2) with quartic potentials (QPs) at the DFT level of B2PLYP/6–311++G(d,p), followed by vibrational configuration interaction (VCI) method with the same QP. The VPT2 + QP method reasonably reproduced most of the bands in the observed spectra for all the species, especially for pyrimidine, a decent agreement is obtained with respect to the band positions and relative intensities. The analyses of the spectra show that all the observed spectra can be well interpreted by the F‐R between the CH stretching fundamentals and the first overtone and 1 + 1 combination bands involving the in‐plane CH bending vibrations and the contribution of the higher‐order anharmonic coupling to the observed spectra seems to lead to red‐shift of the F‐R patterns. Discrete variable representation based methods with potential energy surfaces at CCSD/aug‐cc‐pVDZ theory with 10 degrees of freedom were also carried out to assess the quality of QP at DFT level. Progress of recent anharmonic calculation enabled to analyze the Fermi‐resonance of the CHstretching vibrations of aromatic molecules

Vibrational spectra of the methyl groups in CH3SH, CH3SCH3, CH3OH, and CH3OCH3monomers were measured to analyze their complex features as a result of bend/umbrella‐stretch Fermi resonance. Multiple bands were recorded between 2800–3000 cm−1associated with the vibrations of the methyl groups in CH3OH, and CH3OCH3. On the other hand, the corresponding spectra of CH3SH and CH3SCH3are relatively simpler with main peaks between 2900 to 3000 cm−1. Theoretical ab initio anharmonic algorithm using six normal modes on a methyl group with potential energy surface at CCSD(T)‐F12/aug‐cc‐pVTZ quality is able to account for all the experimentally observed features across these four species. In CH3SCH3and CH3SH, all three CH stretch fundamentals are above 2900 cm−1, but the fundamental of umbrella mode is lower than 1350 cm−1, thus only the overtone states of two bending modes are able to borrow sufficient intensities. In CH3OCH3and CH3OH, umbrella modes have frequencies similar to the bending modes and the CH stretch fundamentals are red‐shifted to 2850–2950 cm−1leading to more complex vibrational features due to better energy‐matching condition. Fermi resonance in CH stretch region of the methyl groups on alcohol, thiol, and their ethers are analyzed by a joint experimental infrared–vacuum ultraviolet and theoretical ab initio anharmonic algorithms.

This paper distinguished hydrogen roles to improve electron mobility and carrier concentration in ZnO and Al doped ZnO sputtered films. By combining experimental evidences and theoretical results, we find out that hydrogen located at oxygen vacancy sites (HO) is the main factor gives rise to increase simultaneously mobility and carrier concentration which has not been mentioned before. Introducing appropriate hydrogen content during sputtering not only results in crystalline relaxation but also supports doping Al into ZnO, increasing carrier concentration and electron mobility in the film. First principles calculations confirmed hydrogen substitutional stability for oxygen vacancy, significantly reducing electron conductivity effective mass and hence increasing electron mobility. In particular, 0.8% hydrogen partial pressure ratio achieved 61 cm2V−1s−1maximum electron mobility, optical transmittance above 82% in visible and near-infrared regions, and 2 × 1020 cm−3carrier concentrations for HAl co-doped ZnO film. These values approach ideal electrical and optical properties for transparent conducting oxide films. The presence of one maximum electron mobility was attributed to competition between increasing mobility due to restoring effective electron mass and hydrogen passivation of native defects, and decreased electron mobility due to electron-phonon scattering.

Vibrational spectra of 1:1 clusters of 2-pyridone (2PY) with water, ammonia, and other hydrogen bond-forming molecules have been measured by several experimental groups over the past two decades. Complex vibrational signatures associated with the N–H stretching fundamental at 3 μm are often observed. Several anharmonic coupling schemes have been proposed; however, the origin of these commonly seen complex features remains unclear. In this work, we present our theoretical analysis on the structure and vibrational spectra of these clusters using ab initiorandom search and ab initioanharmonic algorithms, respectively. Low-energy conformers were found to be hydrogen-bonded clusters and their vibrational spectra at 3 μm were simulated with ab initioanharmonic algorithms. We demonstrate that simple anharmonic mechanisms of Fermi resonance (FR), coupling between NH stretching modes, and overtone/combinations of skeleton modes of 2PY can lead to the complex vibrational signatures observed experimentally. Since this vibrational coupling scheme is inherent to the cis-amides with adjacent N–H and C═O groups when a hydrogen bond is formed with 2PY as the donor and acceptor, we believe that such a phenomenon is general to other hydrogen-bonding systems with the same functional group.

When a solvent molecule is replaced by another molecule with larger proton affinity, the strength of all other hydrogen bonds decreases. This is the concept of anticooperativity by successive substitution in a mixed solvation system. In the present study, this concept is demonstrated in H+(CH3OH)m(H2O)n(m+ n= 5 and 6) mixed clusters by a joint theoretical and infrared (IR) spectroscopic approach. The observed IR spectra of the mixed clusters exhibit two high-frequency shifts of hydrogen-bonded OH stretch bands with increasing methanol content. These trends are well reproduced by first-principle IR spectra simulated by thermal averaging over a set of configurational isomers under the quantum harmonic superposition approach. Theoretical analysis on the magnitude of charge transfer from the protonated site to the solvent molecules is found to be in agreement with the spectroscopic measurement that the individual hydrogen bond in the mixed clusters is weakened with an increase of the mixing ratio of methanol to water.

The first wetting layer on the GaN (0001) surface has been investigated at the level of density functional theory. Many water adsorption models have been analyzed and it is observed that the number of water molecules that can be dissociated is limited to 0.375 ML of adsorption sites; further water dissociation will cost energy penalty. The coverage of hydroxyl groups on surface could be up to 0.75 ML instead. It is also observed that the additional charge on the surface will totally transfer to water adsorbates when the water dissociation number is 0.375 ML. Meanwhile, the surface states will disappear when all the adsorption sites are occupied by dissociated or intact water. All of these phenomena can be attributed to the electron counting rule of III–V semiconductor growth theory. We suggest that the electron counting rule could be generally applied to the water adsorption on polar III–V and II–VI semiconductor surfaces.

The hydrogen spillover mechanism on B-doped graphene was explicitly investigated by first-principles calculations. By the incorporation of boron into graphene, our theoretical investigation shows that B doping can substantially enhance the adsorption strength for both H atoms and the metal cluster on the substrate. The firmly bound catalytic metal on B-doped graphene can effectively dissociate H2molecules into H atoms, and the H atom is more likely to migrate from the bridge site of the H-saturated metal to the supporting graphene sheet. Further investigation on the BC3sheet gives sufficiently low activation barriers for both H migration and diffusion processes; thus, more H atoms are expected to adsorb on BC3substrate via H spillover under ambient conditions compared with the undoped graphene case. Our result is in good agreement with recent experimental findings that microporous carbon has an enhanced hydrogen uptake via boron substitution, implying that B doping with spillover is an effective approach in the modification of graphitic surface for hydrogen storage applications.

Intriguing vibrational features of solvated protonated methanol between 2400–3800 cm–1are recorded by infrared predissociation spectroscopy. Positions of absorption bands corresponding to OH stretching modes are sensitive to changes in solvation environments, thus leading to changes in these vibrational features. Two anharmonic coupling mechanisms, Fermi resonance (FR) contributed by bending overtones and combination band (CB) associated with intermolecular stretching modes, are known to lead to band splitting of OH stretching fundamentals in solvated hydronium and ammonium. Theoretical analyses based on the ab initioanharmonic algorithm not only well reproduce the experimentally observed features but also elucidate the magnitudes of such couplings and the resulting interplay between these two mechanisms, which provide convincing assignments of the spectral patterns. Moreover, while the hydroxyl group plays the leading role in all the above-mentioned features, the role of the methyl group is also analyzed. Through the H/D isotope substitution, we identify overtones of the methyl–hydroxyl rocking modes and their participation in FR.

Recently, superconcentrated electrolytes have been successfully employed in Li-ion batteries because of their enhanced reductive stability and formation of an excellent solid electrolyte interphase (SEI). Herein, we performed density functional theory-based molecular dynamics (DFT-MD) simulations to delineate the initial processes occurring at the interface of Li metal and superconcentrated electrolyte Li bis(trifluoromethanesulfonyl)imide ([Li][TFSI]) in acetonitrile (AN) and to thoroughly determine the structural evolution of the SEI that formed near the Li metal. For the first time, we show that in such superconcentrated salts the initial SEI formation primarily originates from the reductive decomposition of the TFSI anions and the AN molecules are stable against the redox processes occurring near the Li metal. We observe that the SEI layer is highly structured and inhomogeneous. While the atomized C, O, F, and S atoms prefer to stay in between the inner edge and middle portion of the SEI layer, the atoms of N–SO2–CF3fragments and AN molecules are mostly found in the middle or outer edge of the layer.

The structure and vibrational spectra of protonated Ar clusters ArnH+(n= 2–3) are studied using potential energy surfaces at the CCSD(T)/aug-cc-pVTZ level and basis set. Ar binding energies, as well as position isomerism in Ar3H+, were investigated. In our previous work, the spectra of Ar2H+reveal a strong progression of combination bands, which involves the asymmetric Ar–H+stretch with multiple quanta of the symmetric Ar–H+stretch. In this work, insights on the origin of such progression were examined using an adiabatic model. In addition, contributions from mechanical and electrical anharmonicity on the progressions’ intensities were also examined. Comparison of the calculated spectrum for the bare and Ar-tagged ions reveals that the reduction of the symmetry group, from D∞hto either C∞vor C2v, results in a richer vibrational structure in the 500–1700 cm–1region. When compared with previously reported action spectra (D. C. McDonald III, D. T. Mauney, D. Leicht, J. H. Marks, J. A. Tan, J.-L. Kuo, and M. A. Duncan, J. Chem. Phys., 2016, 145, 231,101), it appears that the position isomers, because of the binding of the weakly bound Ar messenger, are needed to account for the additional bands in the infrared photodissociation spectrum for Ar3H+. These findings demonstrate the active role of the messenger atom in relaxing some of the selection rules for the bare ion’s vibrational transitions – resulting in an augmentation of the bands in the action spectrum.

Motivated by the fundamental difference in the reactivity of hexoses and N-acetylhexosamines under collision-induced dissociation (CID) mass spectrometry conditions, we have investigated the CID of two hexosamines, glucosamine (GlcN) and galactosamine (GalN), experimentally and computationally. Both hexosamines undergo ring-opening and then dissociate via the 0,2A and the 0,3A (0,3X) cross-ring cleavage channels. The preference for the ring-opening is similar to the behavior of N-acetylhexosamines and explains why the two anomers of the same sugar give the same mass spectrum. While the spectrum for GlcN is dominated by the 0,2A signal, the signal intensities for both 0,2A and the 0,3A (0,3X) dissociation channels are comparable for GalN, which allows GlcN and GalN to be distinguished easily. Calculations at MP2 level of theory indicate that this is related to the differences in the relative barrier heights for the 0,2A and the 0,3A (0,3X) cross-ring cleavage channels. This, in return, reflects the circumstance that the 0,2A cross-ring cleavage barriers are different for the two sugars, while the barriers of all other dissociation channels are comparable. While the mechanisms of the cross-ring dissociation channels of hexoses are well described using the retro–aldol mechanism in the literature, this study proposes a new mechanism for the 0,3A (0,3X) cross-ring cleavage of hexosamines that involves the formation of an epoxy intermediate or a zwitterionic intermediate.

The mechanism for the collision-induced dissociation (CID) of two sodiated N-acetylhexosamines (HexNAc), N-acetylglucosamine (GlcNAc), and N-acetylgalactosamine (GalNAc), was studied using quantum-chemistry calculations and resonance excitation in a low-pressure linear ion trap. Experimental results show that the major dissociation channel of the isotope labeled [1-18O, D5]-HexNAc is the dehydration by eliminating HDO, where OD comes from the OD group at C3. Dissociation channels of minor importance include the 0,2A cross-ring dissociation. No difference has been observed between the CID spectra of the α- and β-anomers of the same HexNAc. At variance, the CID spectra of GlcNAc and GalNAc showed some differences, which can be used to distinguish the two structures. It was observed in CID experiments involving disaccharides with a HexNAc at the nonreducing end that a β-HexNAc shows a larger dissociation branching ratio for the glycosidic bond cleavage than the α-anomer. This finding can be exploited for the rapid identification of the anomeric configuration at the glycosidic bond of HexNAc-R′ (R′ = sugar) structures. The experimental observations indicating that the dissociation mechanisms of HexNAcs are significantly different from those of hexoses were explained by quantum-chemistry calculations. Calculations show that ring opening is the major channel for HexNAcs in a ring form. After ring opening, dehydration shows the lowest barrier. In contrast, the glycosidic bond cleavage becomes the major channel for HexNAcs at the nonreducing end of a disaccharide. This reaction has a lower barrier for β-HexNAcs as compared with the barrier of the corresponding α-anomers, consistent with the higher branching ratio for β-HexNAcs observed in experiment.

Resolving the structure–property relationships of two-dimensional (2D) organic–inorganic hybrid perovskites is essential for the development of photovoltaic and photoelectronic devices. Here, pressure (0–10 GPa) was applied to 2D hybrid perovskite flakes mechanically exfoliated from butylammonium lead halide single crystals, (C4H9NH3)2PbI4, from which we observed a series of changes of the strong excitonic emissions in the photoluminescence spectra. By correlating with in situ high-pressure X-ray diffraction results, we examine successfully the relationship between structural modifications in the inorganic PbI42–layer and their excitonic properties. During the transition between Pbca(1b) phase and Pbca(1a) phase at around 0.1 GPa, the decrease in ⟨Pb–I–Pb⟩ bond angle and increase in Pb–I bond length lead to an abrupt blue shift of the excitonic bandgap. The presence of the P21/aphase above 1.4 GPa increases the ⟨Pb–I–Pb⟩ bond angle and decreases the Pb–I bond length, leading to a deep red shift of the excitonic bandgap. The total band gap narrowing of ∼350 meV to 2.03 eV at 5.3 GPa before amorphization, facilitates (C4H9NH3)2PbI4as a much better solar absorber. Moreover, phase transitions inevitably modify the carrier lifetime of (C4H9NH3)2PbI4, where an initial 150 ps at ambient phase is prolongated to 190 ps in the Pbca(1a) phase along with enhanced photoluminescence (PL), originating from pressure-induced strong radiative recombination of trapped excitons.The onset of P21/aphase shortens significantly the carrier lifetime to 53 ps along with a weak PL emission due to pressure-induced severe lattice distortion and amorphization. High-pressure study on (C4H9NH3)2PbI4nm-thin flakes may provide insights into the mechanisms for synthetically designing novel 2D hybrid perovskite based photoelectronic devices and solar cells.

We test existing efficient schemes for the “direct-dynamics” approach in building a potential energy surface (PES) in the code MULTIMODE. These are (1) the n-mode representation (nMR) approach to the PES, (2) the exploitation of the normal mode’s symmetry to reduce the computational effort in constructing the PES, (3) the use of sparse grids for fitting the n-mode potentials, and (4) different levels of ab initio theory for these potentials. These schemes are applied to a four-dimensional calculation for the proton-bound methanol dimer (CH3OH)2H+. In addition to the major reduction in complexity obtained by considering only four modes, the combination of these schemes leads to a significant reduction in the computational effort without any major loss of accuracy. VSCF/VCI test calculations are presented for (CH3OH)2H+.

We follow the idea of using metal-decorated polyhydroxy adamantane structures as H2adsorption sites and develop it further by integrating such structures into porous framework structures. We use density functional theory calculations to investigate the hydrogen adsorption in five Sc-decorated, porous, covalent organic frameworks. Each of them consists of polyhydroxy adamantane units connected by molecular linkers. A polyhydroxy adamantane unit acting as an anchor site for four Sc atoms, in return, can bind up to four H2molecules per Sc site. At full coverage, the average H2adsorption energy is between −0.17 and −0.19 eV/H2. We use a simplified thermodynamic model to estimate the gravimetric and volumetric hydrogen uptake as a function of temperature and H2pressure. The most promising framework found here is a structure with benzene units as linkers, which is predicted to achieve 3.04 wt % or 27.28 g/L H2uptake at 233 K and 100 bar H2pressure. We also show that the relatively weak framework–H2interaction leads to the circumstance that at typical operating conditions, the hydrogen uptake deviates significantly from the full coverage, illustrating the necessity to account for the temperature and pressure dependency of the H2uptake.

We study the hydrogen adsorption in porous frameworks composed of silsesquioxane cages linked via boron substituted aromatic structures by first-principles modeling. Such polyhedral oligomeric silsesquioxane (POSS) frameworks can be further modified by decorating them with metal atoms binding to the ring structures of the linkers. We have considered Sc- and Ti-doped frameworks which bind H2via so-called Kubas interaction between hydrogen molecules and transition metal atoms. It will be demonstrated that the maximum H2gravimetric capacity can be improved to more than 7.5 wt % by using longer linkers with more ring structures. However, the maximum H2volumetric capacity can be tuned to more than 70 g/L by varying the size of silsesquioxane cages. We are optimistic that by varying the building blocks, POSS frameworks can be modified to meet the targets for the gravimetric and volumetric capacities set by the U.S. Department of Energy.

We systematically investigated the potential of single-layer VS2polytypes as Na-battery anode materials via density functional theory calculations. We found that sodiation tends to inhibit the 1H-to-1T structural phase transition, in contrast to lithiation-induced transition on monolayer MoS2. Thus, VS2can have better structural stability in the cycles of charging and discharging. Diffussion of Na atom was found to be very fast on both polytypes, with very small diffusion barriers of 0.085 eV (1H) and 0.088 eV (1T). Ab initio random structure searching was performed in order to explore stable configurations of Na on VS2. Our search found that both the V top and the hexagonal center sites are preferred adsorption sites for Na, with the 1H phase showing a relatively stronger binding. Notably, our random structures search revealed that Na clusters can form as a stacked second layer at full Na concentration, which is not reported in earlier works wherein uniform, single-layer Na adsorption phases were assumed. With reasonably high specific energy capacity (232.91 and 116.45 mAh/g for 1H and 1T phases, respectively) and open-circuit voltage (1.30 and 1.42 V for 1H and 1T phases, respectively), VS2is a promising alternative material for Na-ion battery anodes with great structural sturdiness. Finally, we have shown the capability of the ab initio random structure searching in the assessment of potential materials for energy storage applications.

Black phosphorus (P) has been considered as a promising candidate for anodes due to its ability to absorb a large amount of Li atoms. Unfortunately, lithiation of bulk black P induces huge structural deformation, which limits its application. Here, on the basis of the density functional theory calculation, we predict that the newly found two-dimensional (2D) black and blue P are good electrodes for high-capacity lithium-ion batteries. Our theoretical calculations indicate that, in contrast to bulk black P, the monolayer and double-layer black and blue P can maintain their layered structures during lithiation and delithiation cycles. Moreover, it is found that Li diffusion on the surfaces of black and blue P has relatively low energy barriers (<0.4 eV), and the single-layer blue P and double-layer black and blue P possess high charge capacities.

The search for new materials that can withstand the tough demands of practical hydrogen storage for use in automotive transportation is currently receiving a great deal of attention from the scientific community because of the urgency for replacements of traditional energy resources such as fossil fuels. In this work, transition metal (TM)-decorated boron-doped tetrahedral silsesquioxane frameworks (B-TSFs) for application in hydrogen storage are investigated using first-principles density functional theory calculations. We design this plausible hydrogen storage system based on the knowledge of previous works by other groups including metal atom decoration for quasi-molecular H2adsorption, boron substitution into benzene rings to prevent metal clustering, and assembling modified benzene rings and tetrahedral silsesquioxane cages into the framework for this study. Boron substitution substantially enhances the TM binding energy to the linker of B-TSF to suppress metal clustering as well as maintain stable hydrogen adsorption energy to TMs. The average hydrogen adsorption energy energies in Sc-, Ti-, and V-decorated B-TSF are 0.29, 0.40, and 0.69 eV, respectively, with acceptable gravimetric density of 6.9, 5.6, and 4.15 wt %. Gibbs free energy calculations are also carried out to estimate the working temperature and pressure ranges for using B-TSF as a hydrogen storage system.

In one of our previous works (Du, Y. A.; Chen, Y.-W.; Kuo, J.-K. Phys. Chem. Chem. Phys.2013, 15, 19807–19818), it had been found that nonpolar and polar facets of gallium nitride (GaN) nanoparticles may play different roles in water splitting processes because of their different redox characters for O2and H2evolution. The mechanism of removing other water adsorbates on different facets was left undetermined. In this work, we investigate the hopping processes of water adsorbates on GaN (101̅0) nonpolar and (0001) polar surfaces and also the effect of a Pt4cluster on nonpolar surface for water splitting. The hopping of O adatoms from polar to nonpolar surface is found to be feasible at room temperature and hence can facilitate the O2evolution. However, the mobility of H adatoms from nonpolar to polar surface is relatively limited for higher barriers. On the other hand, the Pt4cluster offers mediate states to facilitate water splitting and H2evolution on GaN nonpolar surface. The features of Pt4projected density of states show suitable properties of being a cocatalyst for H2evolution but also a shortcoming as an electron–hole recombination center.

We analyzed the adsorption of Li on graphene in the context of anodes for lithium-ion batteries (LIBs) using first-principles methods including van der Waals interactions. We found that although Li can reside on the surface of defect-free graphene under favorable conditions, the binding is much weaker than to graphite and the concentration on a graphene surface is not higher than in graphite. At low concentration, Li ions spread out on graphene because of Coulomb repulsion. With increased Li content, we found that small Li clusters can be formed on graphene. Although this result suggests that graphene nanosheets can conceivably have a higher ultimate Li capacity than graphite, it should be noted that such nanoclusters can potentially nucleate Li dendrites, leading to failure. The implications for nanostructured carbon anodes in batteries are discussed.

The effects of adsorbing simple aromatic molecules on the electronic structure of graphene were systematically examined by first-principles calculations. Adsorptions of different aromatic molecules borazine (B3N3H6), triazine (C3N3H3), and benzene (C6H6) on graphene have been investigated, and we found that molecular adsorptions often lead to band gap opening. While the magnitude of band gap depends on the adsorption site, in the case of C3N3H3, the value of the band gap is found to be up to 62.9 meV under local density approximationwhich is known to underestimate the gap. A couple of general trends were noted: (1) heterocyclic molecules are more effective than moncyclic ones and (2) the most stable configuration of a given molecule always leads to the largest band gap. We further analyzed the charge redistribution patterns at different adsorption sites and found that they play an important role in controling the on/off switching of the gapthat is, the energy gap is opened if the charge redistributes to between the C–C bond when the molecule is adsorbing on graphene. These trends suggest that the different ionic ability of two atoms in heterocyclic molecules can be used to control the charge redistribution on graphene and thus to tune the gap using different adsorption conditions.

With first-principles DFT calculations, the interaction between Li and carbon in graphene-based nanostructures is investigated as Li is adsorbed on graphene. It is found that the Li/C ratio of less than 1/6 for the single-layer graphene is favorable energetically, which can explain what has been observed in Raman spectrum reported recently. In addition, it is also found that the pristine graphene cannot enhance the diffusion energetics of Li ion. However, the presence of vacancy defects can increase the ratio of Li/C largely. With double-vacancy and higher-order defects, Li ion can diffuse freely in the direction perpendicular to the graphene sheets and hence boost the diffusion energetics to some extent.

The local structures between nano-TiO2and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI+TFS–) and 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMI+TFS–) were investigated using high-pressure infrared spectroscopy. No significant changes in C–H spectral features of EMI+TFS–were observed in the presence of nano-TiO2under ambient pressure. As the EMI+TFS–/nano-TiO2mixture was compressed to 0.3 GPa, the imidazolium C–H absorptions became two sharp bands at 3108 and 3168 cm–1, respectively, and the alkyl C–H stretching absorption exhibits a new band at 3010 cm–1associated with a weaker band at 3028 cm–1. It appears that pressure stabilizes the isolated conformations due to pressure-enhanced imidazolium C–H---nano-TiO2interactions. Our results also reveal that alkyl C–H groups play non-negligible roles at the conditions of high pressures. The results of BMI+TFS–/nano-TiO2are remarkably different from what is revealed for EMI+TFS–/nano-TiO2. The spectral features and band frequencies of BMI+TFS–/nano-TiO2are almost identical to those of pure BMI+TFS–under various pressures. This study demonstrates that changes to the alkyl chain length of the cation could be made to control the order and strength of ionic liquid/nano-TiO2interactions.

The structural and dynamic properties of water on the GaN(0001) polar surface are investigated via classical molecular dynamics simulations. The interfacial molecules are observed to have enhanced structural ordering and slowed-down dynamics compared to the liquid bulk; these unique properties are evidenced in the slower reorientational relaxation, smaller diffusion constant, and longer residence lifetime for water located at the surface region up to ∼7 Å from the substrate. Further analysis of the vibrational spectra at low frequencies shows that both the hydrogen bond network bending and the hydrogen bond stretching bands at the interface shift to the blue compared to those in the bulk, due to the strong coupling between the O atom of water and the Ga sites. The distinct spectral features along with the anisotropy of the hydrogen bond distributions of the interfacial water are complex results determined by both the substrate–water and water–water interactions.

The opening of an electrical band gap in graphene is crucial for its application for logic circuits. Recent studies have shown that an energy gap in Bernal-stacked bilayer graphene can be generated by applying an electric displacement field. Molecular doping has also been proposed to open the electrical gap of bilayer graphene by breaking either in-plane symmetry or inversion symmetry; however, no direct observation of an electrical gap has been reported. Here we discover that the organic molecule triazine is able to form a uniform thin coating on the top surface of a bilayer graphene, which efficiently blocks the accessible doping sites and prevents ambient p-doping on the top layer. The charge distribution asymmetry between the top and bottom layers can then be enhanced simply by increasing the p-doping from oxygen/moisture to the bottom layer. The on/off current ratio for a bottom-gated bilayer transistor operated in ambient condition is improved by at least 1 order of magnitude. The estimated electrical band gap is up to ∼111 meV at room temperature. The observed electrical band gap dependence on the hole-carrier density increase agrees well with the recent density-functional theory calculations. This research provides a simple method to obtain a graphene bilayer transistor with a moderate on/off current ratio, which can be stably operated in air without the need to use an additional top gate.

The adsorption mechanism of water on the GaN (0001) polar surface is investigated via both the Density Functional Theory (DFT) method and its derived classical force field. The physisorption binding energy and the adsorption geometry of the water molecule on the clean Ga-terminated surface are analyzed via the first-principle static calculations. The adsorption energy hypersurfaces are then extracted to be used in the fitting of the interaction potentials between water and GaN. Classical molecular dynamics (MD) simulations based on the developed force field are performed for the interfacial system of liquid water and the GaN surface slab. From our computations, the interfacial water exhibits significant oscillatory profiles for the atomic densities and the molecular orientations. Further data analysis suggests a highly confined first layer with the O being locked right upon the surface Ga atoms and the H pointing toward the neighboring O to form the weakened hydrogen bonds. A bilayer configuration with opposite dipole orientations is consequently characterized as the wetting structure on the GaN polar surface and is explained by the anisotropic perturbations from the surface polar sites. Our simulations would be helpful to provide an atomistic picture for the water adsorption configuration on this semiconductor surface and would be useful in the relevant nanofluidic and nanoengineering applications.

The structural and electronic properties of Br2/Br adsorption and intercalation of single-layer graphene (SLG) and multi-layer graphene (MLG) are studied by density-functional theory. As a result of charge transfer, the Br atom is found to be stable as adsorbed on the vertex or near bridge sites of graphene whereas the Br2molecule will be more stable when adsorbed perpendicularly on graphene. Because of the interactions between Br2molecules, the stable configurations of Br2on graphene or intercalated in MLG are parallel to graphene. With the analysis of charge difference, the experimental observation that the lowest stage of Br2intercalated graphite is the stage 2 compound is ascribed to the effect of localized dipoles on graphene induced by Br2. Although only slightly disturbing the orbitals of graphene atoms, the existence of Br2molecules or Br atoms will still affect the electronic structures of both materials. As adsorbed on the single surface of graphene, Br2will open its bandgap at the K (K′) point. While present on both surfaces, Br2molecules will induce a much larger bandgap of graphene with the Fermi level shifted down into the valence bands. If Br atoms are absorbed on graphene, the significant amount of charge will transfer from graphene to Br atoms because of the strong electronegativity of Br. More importantly, the electronic properties of SLG/MLG with the absorbed Br2molecules can be controlled by the ultraviolet light that decomposes the Br2on SLG/MLG.

We carried out a principle study on the reaction mechanism of rhodium‐catalyzed intramolecular aziridination and aziridine ring opening at a sugar template. A sulfamate ester group was introduced at different positions of glycal to act as a nitrene source and, moreover, to allow the study of the relative reactivity of the nitrene transfer from different sites of the glycal molecule. The structural optimization of each intermediate along the reaction pathway was extensively done by using BPW91 functional. The crucial step in the reaction is the Rh‐catalyzed nitrene transfer to the double bond of the glycal. We found that the reaction could proceed in a stepwise manner, whereby the N atom initially induced a single‐bond formation with C1 on the triplet surface or in a single step through intersystem crossing (ISC) of the triplet excited state of the rhodium–nitrene transition state to the singlet ground state of the aziridine complexes. The relative reactivity for the conversion of the nitrene species to the aziridine obtained from the computed potential energy surface (PES) agrees well with the reaction time gained from experimental observation. The aziridine ring opening is a spontaneous process because the energy barrier for the formation of the transition state is very small and disappears in the solution calculations. The regio‐ and stereoselectivity of the reaction product is controlled by the electronic property of the anomeric carbon as well as the facial preference for the nitrene insertion, and the nucleophilic addition.

The structural, electronic, and vibrational properties of the free finite carbon chains and those encapsulated inside carbon nanotubes (CNTs) are studied by density functional theory calculations. The end effect and chain symmetry are found to play key roles in deciding the structural characteristics of the free finite carbon chains based on the parity of the carbon numbers. Due to the potential interaction between the carbon chains and CNTs, the electrons of the chain−CNT systems will redistribute, and some charges may transfer to the inner carbon chains from CNTs. We suggest that the attractive potential of chain atoms inside CNTs could be the driving force for formation of the linear carbon inside CNTs. Unexpectedly, we find that inside CNTs the carbon chains with even-numbered carbons present almost constant bond length alternation, which is independent of the chain length. This trend of the even-numbered carbon chains in CNTs helps to explain the universal experimental observation that the Raman peaks from chains in CNTs are within 1820−1860 cm−1.

The energy landscape of protonated water clusters H+(H2O)nis thoroughly explored at the first-principle level using a hierarchical search methodology. In particular, the distinct configurational isomers of OSS2 empirical potential for n= 5−9 are uncovered and archived systematically using an asynchronous genetic algorithm and are subsequently refined with first-principle calculations. Using the OSS2 model, quantitative agreements in the thermal properties between Monte Carlo and harmonic superposition approximation (HSA) highlighted the reliability of the latter approach for the study of small- to medium-sized protonated water clusters. From the large sets of collected isomers, finite temperature behavior of the clusters can be efficiently examined at first-principle accuracy with the use of HSA. From the results obtained, evidence of structural changes from single-ring to treelike (n= 5−7) and multi-ring to single-ring structures (n= 7−9) is observed, as expected for the empirical model. Finally, the relevance of these findings to recent experimental data is discussed.

The energy transfer from fluorene-based polymers such as poly(9,9-dioctylfluoreny-2,7-diyl) (PFO) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(bithiophene)] (F8T2) to single-walled carbon nanotubes (SWNTs) is proved by photoluminescence excitation (PLE) and fluorescence lifetime studies. The excitation wavelength to cause optimal emissions from SWNTs is tunable in a wide wavelength range from 388 to 480 nm (from 488 to 520 nm) depending on the concentration of excess PFO (F8T2) polymers. The energy transfer and hence the excitation wavelength to cause optimal SWNT emissions is govern by the chain conformation and aggregation state of the polymers proximate to SWNT surfaces. The PLE mapping technique, an ideal method to monitor the energy transfer process, is adopted to study the SWNT binding competition between PFO and F8T2. We conclude that SWNTs are preferentially bound with F8T2 polymers. Moreover, the molecular dynamic simulation also agrees well with the experiment results. This study explores the photon conversion process between aromatic polymers and SWNTs and suggests a convenient method of adjusting the desired wavelength for the optimal energy conversion, useful for polymer-SWNT composites in optoelectronic applications.

The stability and the BN-substitution pattern of C 12B 6N 6fullerene have been investigated by bond-counting rules (BCR) with density functional theory (DFT) calculations (B3LYP/6−31G*). It is found that while the simple BCR by including only the nearest-neighbor terms can serve as an effective guide to look for the low-energy alloy configurations, contributions beyond simple BCR by including more than two-body terms are essential for a quantitative description. The substitutions of BN units in the cage seem to follow a “continuity” pattern in the low-energy configurations. Though all the alloy configurations have positive mixing energy, the low-energy isomers are quite stable (less than 150 meV/atom). We expect that these fullerene structures can be synthesized under the high temperature and nonequilibrium experimental conditions offered by the modern growth techniques such as laser ablation, direct-current arc, and chemical vapor deposition.

Stereocontrolled intramolecular aziridination of the glycal‐derived sulfamates offers a highly efficient strategy to divergently prepare aminoglycosides. Rhodium‐catalyzed nitrogen‐atom transfer to CC bonds formed semistable aziridines, which were subjected to various nucleophiles (C, O, S, and N) to give cyclic sulfamate‐containing aminosugar derivatives selectively. The second nucleophilic displacement of sulfonyloxy moieties of [1,2,3]‐oxathiazepane‐2,2‐ dioxides allows straightforward access to aminoglycosides with selective α‐ or β‐linkages. This approach is operationally simple, complements existing methods, and is a versatile protocol for the synthesis of polyfunctionalized amino sugars. In addition, the mechanism of the rhodium‐catalyzed intramolecular aziridination of glycals and its ring‐opening reaction was extensively studied by using DFT calculations.

Understanding the structures of medium-sized protonated water clusters [H+(H2O)n] has made a significant advancement recently thanks to the development of new experimental techniques and high-level computational methods. A combination of vibrational predissociation spectroscopy and ab initio calculations was shown to be effective in elucidating the structures of the clusters as a function of their temperature and size. The combined study revealed several intriguing features (such as symmetric hydrogen bonds) that could not be found for neutral water clusters. However, similar to its neutral counterpart, the number of stable isomers increases exponentially with cluster size (n), making direct structural identification of medium-sized clusters difficult. Despite the difficulties, both experimental and computational results indicated a smooth change in hydrogen-bond topology from tree-like, single-ring, multiple-ring to polyhedron-like structures (and their mixtures) as n increases from 5 to 28. The excess proton can be symmetrically hydrated at n = 6-8. Five-membered ring isomers can form at n = 7 and 8 as the low-lying minima. Only a single feature (∼3695 cm-1) in the free OH stretching region was observed for H+(H2O)21 and H+(H2O)28, suggesting that all surface water molecules are linked in a similar 3-coordinated (double-acceptor-single-donor, (AAD)) configuration in both "magic number" clusters. The clathrate-like structures open up at higher temperatures, as evidenced by the increased intensity of the free-OH stretching absorption band (∼3715 cm-1) of 2-coordinated (single-acceptor-single-donor, (AD)) water molecules. Further understanding of the structures and thermal properties of these clusters is gained through the studies with Monte Carlo (MC) and molecular dynamics simulations, ion reactivity and thermal dissociation measurements, as well as Ar tagging experiments.Contents PAGE1. Introduction 5542. Experiments 5552.1. Hydrogen-bond topologies 5562.2. "Magic number" clusters 5592.3. Symmetric proton hydration 5622.4. Cluster temperatures 5643. Theories 5673.1. Global minimum structures 5673.2. Thermal and dynamical effects 5724. Conclusion 574Acknowledgements 451References 451

The low-temperature proton-ordered counterparts for ice-Ih, ice-III, ice-VI and ice-VII are investigated by first principle methods in conjunction with a graph enumeration technique. Two experimentally well calibrated disorderorder transitions, ice-Ihice-XI and ice-VIIice-VIII, are used to validate the methodology we used herein and in both cases our approach is able to reproduce major experimental features. For ice-III and ice-VI, direct structural determination on the fully proton-ordered counterparts by neutron diffraction is not available and in the literature different ordering schemes have been proposed. Our calculation results serve as an independent reference, and we shall discuss our findings and their relevance to previous experimental works.

The energetics and structure of the protonated water hexamer H2O6Hhave been examined employing both model potentials and high-level ab initiomethods. To select candidate structures for this cluster, Parallel-Tempering and the OSS2 potential were used as devices to complement the set of stationary points previously optimized by Hodges and Wales, Chem. Phys. Lett., 2000, 324, 279. Structures of these local minima were successively re-optimized using OSS3, B3LYPaug-cc-pVDZ, and MP2aug-cc-pVDZ, the latter providing a reference to benchmark the performance of the empirical models and B3LYP method. We found that both OSS2 and OSS3 require a re-parameterization to adequately describe the energetics of some isomers. Zero point energy was found to be important in defining the relative stability of the optimized isomers. The effect of the anharmonicity on the vibrational ground state of H2O6Hwas also examined by means of diffusion Monte Carlo DMC and the OSS3 potential, and we found that it accounts for a decrease in total energy of roughly 0–4.4 mEh. This is a significant effect on the energetics considering that many isomers are nearly degenerate. Including the anharmonic corrections computed with DMC, the branched species were found to be the most stable isomers. The height of the barriers separating a cage or cyclic isomer from a branched one was found to vary from 1.5 to 5.8 mEh.

The infrared (IR) spectra of the O–H stretching vibrations of pyridine–water clusters (Pyd)m(H2O)n, with m, n= 1–4, have been investigated with infrared–vacuum ultraviolet (VUV) spectroscopy under a jet-cooled condition. The time-of-flight mass spectrum of (Pyd)m(H2O)n+by VUV ionization at ∼9 eV showed an unusual intensity pattern with very weak ion signals for m= 1 and 2 and stronger signals for m≥ 3. This unusual mass pattern was explained by a drastic structural change of (Pyd)m(H2O)nupon the VUV ionization, which was followed by the elimination of water molecules. Among the recorded IR spectra, only one spectrum monitored, (Pyd)2+cation, showed a well-resolved structure. The spectrum was analyzed by comparing with the simulated ones of possible stable isomers of (Pyd)2(H2O)n, which were obtained with quantum-chemical calculations. Most of the calculated (Pyd)2(H2O)nclusters had the characteristic structure in which H2O or (H2O)2forms a hydrogen-bonded bridge between two pyridines to form the π-stacked (Pyd)2, and an additional H2O molecule(s) extends the H-bonded network. The π-stacked (Pyd)2(H2O)nmoiety is very stable and is thought to exist as a local structure in a pyridine/water mixed solution. The Fermi resonance between the O–H stretch fundamentals and the overtones of the O–H bending vibrations in (Pyd)m(H2O)nwas found to be less pronounced in the case of (Pyd)m(NH3)nstudied previously.