Anharmonic IR spectra of solvated ammonium and aminium ions: resemblance between water and bisulfate solvations.

In this work, we analyze the vibrational spectra of ammonium, methylammonium, and dimethylammonium ions solvated by either water molecules or bisulfate anions using anharmonic vibrational algorithms. Rich and complicated spectral features in the 2700-3200 cm region of the experimental spectra of these clusters are attributed to originate from strong Fermi resonance between hydrogen-bonded NH stretching fundamentals and NH bending overtones. Additional weaker bands around 2500-2600 cm in solvated aminium ions are assigned to the combination tones involving the CH-NH (methyl-amino) rocking modes.

An anharmonic approach to IR, Raman and SFG spectra of the solvated methylammonium ion.

The methylammonium ion (CHNH, or noted as MA-H) is one of the smallest organic ammonium ions that play important roles in organic-inorganic halide perovskites. Despite the simple structure, the vibrational spectra of MA-H exhibit complicated features in the 3 μm region which are sensitive to the solvation environment. In the present work, we have applied the anharmonic algorithm at the CCSD/aug-cc-pVDZ level to simulate the IR and Raman spectra of the solvated methylammonium ion, MA-H⋯X, where X denotes the solvent molecules, to understand the Fermi resonance mechanism in which the overtones of NH bending modes are coupled with the fundamentals of NH stretching modes.

Anharmonic Coupling Revealed by the Vibrational Spectra of Solvated Protonated Methanol: Fermi Resonance, Combination Bands, and Isotope Effect.

Intriguing vibrational features of solvated protonated methanol between 2400-3800 cm are 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.

Vibrational spectroscopic signatures of hydrogen bond induced NH stretch-bend Fermi-resonance in amines: The methylamine clusters and other N-H⋯N hydrogen-bonded complexes.

The appearance of multiple bands in the N-H stretching region of the infrared spectra of the neutral methylamine dimer and trimer is a sign of NH bend-stretch anharmonic coupling. Ab initio anharmonic calculations were carried out in a step-wise manner to reveal the origin of various bands observed in the spectrum of the methylamine dimer. A seven-dimensional potential energy surface involving symmetric and asymmetric stretching and bending vibrations of both the hydrogen bond donor and the acceptor along intermolecular-translational modes was constructed using the discrete variable representation approach.

Anharmonic coupling behind vibrational spectra of solvated ammonium: lighting up overtone states by Fermi resonance through tuning solvation environments.

Studies on the vibrational spectra of various ammonium-centered clusters under different solvation environments have raised interest over the last thirty years. The gas-phase infrared photodissociation spectroscopy (IRPD) experiments showed that these NH4+Xn clusters exhibit rich spectral features from 2600 to 3400 cm-1. In this work, we have simulated the vibrational spectra and analyzed couplings among vibrational quantum states in the aforementioned frequency range using ab initio anharmonic algorithms.

Strong Fermi Resonance Associated with Proton Motions Revealed by Vibrational Spectra of Asymmetric Proton-Bound Dimers.

Infrared spectra for a series of asymmetric proton-bound dimers with protonated trimethylamine (TMA-H ) as the proton donor were recorded and analyzed. The frequency of the N-H stretching mode is expected to red shift as the proton affinity of proton acceptors increases. The observed band, however, shows a peculiar splitting of approximately 300 cm with the intensity shifting pattern resembling a two-level system.

Vibrational spectroscopy of protonated amine-water clusters: tuning Fermi resonance and lighting up dark states.

Strong coupling between stretching fundamentals and bending overtones of vibrational modes, known as Fermi resonance (FR), has been observed for proton motions in the protonated trimethylamine-water cluster. To investigate the role of FR, we examined the vibrational spectra of other three protonated ammonia/amine-water clusters, including the NH4+ ion and its mono- and di-methylated analogues, respectively, with and without argon tagging. In these systems, a simple frequency-scaled harmonic oscillator model will predict only one strong band between 2600 and 3200 cm-1 uniquely due to the hydrogen-bonded NH stretching fundamental for a given conformer.

Improved agreement between experimental and computational results for collision-induced dissociation mass spectrometry of cation-tagged hexoses.

To model the collision-induced dissociation mass spectrometry (CID-MS) of Na+-tagged hexoses, it is not only required to perform an extensive sampling of the conformational space as addressed in our previous work [Huynh et al., Phys. Chem.

Pyridine vs N-Hydrogenated Pyridine Moieties: Theoretical Study of Stability and Spectroscopy of Nitrogen-Contained Heterocyclic Aromatic Compounds and Graphene Nanoflakes.

Nitrogen is one of the most common heteroatom appearing in heterocyclic aromatic compounds (HACs) as well as the frequently applied dopant in graphene nanoflakes/nanoribbons. The pyridine moiety is an intuitive and stable common feature of these compounds; but interestingly, using density functional theory calculations, we found that the N-hydrogenated pyridine moiety could be even more stable in large HACs and in N-doped graphene nanoflakes considering their formation reaction energies. The hydrogenation reaction of the pyridine moiety was calculated to be exothermic for models of four and more fused aromatic rings with specific substitutional positions of nitrogen.

First-principles study on sum-frequency generation spectroscopy of methanol adsorbed on TiO(110) surface: Effects of substrate and molecular coverages.

In this work, starting from the general theory of sum-frequency generation (SFG), we proposed a computational strategy utilizing density functional theory with periodic boundary conditions to simulate the vibrational SFG of molecules/solid surface adsorption system. The method has been applied to the CHOH/TiO(110) system successfully. Compared with the isolated molecule model, our theoretical calculations showed that the TiO substrate can significantly alter the second-order susceptibilities of a methanol molecule which is directly related to the SFG intensity.

The dp type π-bond and chiral charge density waves in 1T-TiSe.

Based on the atomic electronic configuration and Ti-Se coordination, a valence bond model for the layered transition metal dichalcogenide (TMDC) 1T-TiSe2 is proposed. 1T-TiSe2 is viewed as being composed of edge-sharing TiSe4-plaquettes as TiSe2-ribbon chains in each layer via a directional valence shell electron distribution as chemical bonds, in contrast to the conventional layer view of face-sharing TiSe6-octahedra. The four valence electrons per Ti in the hybridized dsp2-orbitals of square coordination form σ-bonds with the four nearest neighbor Se atoms in the chain.

Theoretical study of nitrogen-doped graphene nanoflakes: Stability and spectroscopy depending on dopant types and flake sizes.

As nitrogen-doped graphene has been widely applied in optoelectronic devices and catalytic reactions, in this work we have investigated where the nitrogen atoms tend to reside in the material and how they affect the electron density and spectroscopic properties from a theoretical point of view. DFT calculations on N-doped hexagonal and rectangular graphene nanoflakes (GNFs) showed that nitrogen atoms locating on zigzag edges are obviously more stable than those on armchair edges or inside flakes, and interestingly, the N-hydrogenated pyridine moiety could be preferable to pure pyridine moiety in large models. The UV-vis absorption spectra of these nitrogen-doped GNFs display strong dependence on flake sizes, where the larger flakes have their major peaks in lower energy ranges.

Generalizability Theory With One-Facet Nonadditive Models.

In generalizability theory (G theory), one-facet models are specified to be additive, which is equivalent to the assumption that subject-by-facet interaction effects are absent. In this article, the authors first derive estimators of variance components (VCs) for nonadditive models and show that, in some cases, they are different from their counterparts in additive models. The authors then demonstrate and later confirm with a simulation study that when the subject-by-facet interaction exists, but the additive-model formulas are used, the VC of subjects is underestimated.

The visible spectrum of zirconium dioxide, ZrO2.

The electronic spectrum of a cold molecular beam of zirconium dioxide, ZrO(2), has been investigated using laser induced fluorescence (LIF) in the region from 17,000 cm(-1) to 18,800 cm(-1) and by mass-resolved resonance enhanced multi-photon ionization (REMPI) spectroscopy from 17,000 cm(-1)-21,000 cm(-1). The LIF and REMPI spectra are assigned to progressions in the Ã(1)B(2)(ν(1), ν(2), ν(3)) ← X̃(1)A(1)(0, 0, 0) transitions. Dispersed fluorescence from 13 bands was recorded and analyzed to produce harmonic vibrational parameters for the X̃(1)A(1) state of ω(1) = 898(1) cm(-1), ω(2) = 287(2) cm(-1), and ω(3) = 808(3) cm(-1).

The role of the nπ* 1A(u) state in the photoabsorption and relaxation of pyrazine.

The geometric, energetic, and spectroscopic properties of the ground state and the lowest four singlet excited states of pyrazine have been studied by using DFT/TD-DFT, CASSCF, CASPT2, and related quantum chemical calculations. The second singlet nπ* state, (1)A(u), which is conventionally regarded dark due to the dipole-forbidden (1)A(u)←(1)A(g) transition, has been investigated in detail. Our new simulation has shown that the state could be visible in the absorption spectrum by intensity borrowing from neighboring nπ* (1)B(3u) and ππ* (1)B(2u) states through vibronic coupling.

Symmetry forbidden vibronic spectra and internal conversion in benzene.

The spectra of symmetry-forbidden transitions and internal conversion were investigated in the present work. Temperature dependence was taken into account for the spectra simulation. The vibronic coupling, essential in the two processes, was calculated based on the Herzberg-Teller theory within the Born-Oppenheimer approximation.

A theoretical study on the spectroscopy and the radiative and non-radiative relaxation rate constants of the S(0)(1)A(1)-S(1)(1)A(2) vibronic transitions of formaldehyde.

We have carried out a close examination on the mathematical treatments and the first-principle computations concerning the vibronic transitions between the S(0)(1)A(1) and the S(1)(1)A(2) states of formaldehyde. The simulation of absorption spectrum was presented with peak intensities calculated according to vibronic-coupled transition dipole moments and Franck-Condon factors. The radiative and non-radiative transition rate constants from the excited to the ground states were calculated with formulas based on Fermi's golden rule.

One- and two-photon absorption properties of diamond nitrogen-vacancy defect centers: A theoretical study.

The negatively charged nitrogen-vacancy defect center, (NV)(-), in diamond has been investigated theoretically for its one- and two-photon absorption properties involving the first excited state with the (3)A(2)-->(3)E transition. Time-dependent density functional theory (TD-DFT), configuration interaction with single excitation (CIS), and complete active space self-consistent field (CASSCF) were employed in this investigation along with the 6-31G(d) basis set. Diamond lattice models containing 24-104 carbon atoms were constructed to imitate the local environment of the defect center.

Symmetric double-well potential model and its application to vibronic spectra: studies of inversion modes of ammonia and nitrogen-vacancy defect centers in diamond.

In this paper, we have studied the vibronic transitions between two symmetric double-well potentials by proposing a model Hamiltonian consisting of a harmonic oscillator and a parturition described by a Gaussian function that leads to a double minima potential with a barrier between the two energy minima. Making use of the contour integral form of Hermite polynomials, we present a new formula that can calculate Franck-Condon factors of the system rigorously. The simulated vibronic spectra of ammonia and the negatively charged nitrogen-vacancy center in diamond are presented as an application of the formula.

Calculation of the vibrationally non-relaxed photo-induced electron transfer rate constant in dye-sensitized solar cells.

In this paper we shall show how to calculate the single vibronic-level electron-transfer rate constant, which will be compared with the thermal averaged one. To apply the theoretical results to the dye-sensitized solar cells, we use a simple model to describe how we model the final state of the electron-transfer process. Numerical calculations will be performed to demonstrate the theoretical results.

Vibrational predissociation spectra and hydrogen-bond topologies of H+(H2O)9-11.

Vibrational predissociation spectra of protonated water clusters H+(H2O)n, n = 9-11, are presented. Examination of the spectra in the free-OH stretching region revealed predominance of a single absorption band at approximately 3690 cm(-1) for three-coordinate H2O acting as a double-proton-acceptor/single-proton-donor in the n = 11 cluster. In contrast, the intensity of the absorption band of two-coordinate H2O acting as a single-proton-acceptor/single-proton-donor at approximately 3715 cm(-1) decreases with cluster size, and that of one-coordinate H2O acting as a single-proton-acceptor at approximately 3740 and approximately 3650 cm(-1) diminishes nearly entirely at n > 10 in the spectrum.

Protonated clathrate cages enclosing neutral water molecules: (H+)(H2O)21 and (H+)(H2O)28.

This paper describes a systematic study on the clathrate structure of (H+)(H2O)21 using tandem mass spectrometry, vibrational predissociation spectroscopy, Monte Carlo simulations, and density functional theory calculations. We produced (H+)(H2O)n from a continuous corona-discharged supersonic expansion and observed three anomalies simultaneously at the cluster temperature near 150 K, including (1) the peak at n=21 is more intense than its neighboring ions in the mass spectrum, (2) the size-dependent dissociation fractions show a distinct drop for the 21-mer, and (3) the infrared spectrum of (H+)(H2O)21 exhibits only a single feature at 3699 cm(-1), corresponding to the free-OH stretching of three-coordinated water molecules. Interestingly, the anomalies appear or disappear together with cluster temperature, indicating close correlation of these three observations.

Photoisomerization and photodissociation of toluene in molecular beam.

The photodissociation of isotope-labeled toluene C(6)H(5)CD(3) and C(6)H(5)(13)CH(3) molecules at 6.4 eV under collision-free conditions was studied in separate experiments by multimass ion imaging techniques. In addition to the major dissociation channels, C(6)H(5)CD(3) --> C(6)H(5)CD(2) + D and C(6)H(5)CD(3) --> C(6)H(5) + CD(3), the respective photofragments CD(2)H, CDH(2), and CH(3) and their heavy fragment partners C(6)H(4)D, C(6)H(3)D(2), and C(6)H(2)D(3) were observed from C(6)H(5)CD(3) dissociation.