Theoretical Frequency Calculations Research Articles

Complex with Linear B–B–B Skeleton Trapped in Dinitrogen Matrix: Matrix Infrared Spectra and Quantum Chemical Calculations

The neutral (NN)-B-B-B-(N2) complex has been trapped in low-temperature dinitrogen matrix and identified by isotopic substitution and theoretical frequency calculations. The linear B-B-B skeleton is stabilized by two inequivalent N2, namely, one end-on and other side-on N2. The structure of linear B-B-B skeleton illustrates much difference from previously reported triangle configuration of B3 clusters. Frontier orbital analysis demonstrates that the σ orbital of end-on NN and the π-bonding orbital of side-on N2 acts as the donor orbital. π bonding character across B-B-B skeleton donates to the antibonding π* orbital of end-on NN and out of phase the B-B-B features π back-donation to antibonding π* orbital of side-on N2. The combination of strong σ-donating capacity coupled with a greater ability for accepting π-back-donation of the N2 ligand leads to the formation of (NN)-B-B-B-(N2) complex with linear B-B-B skeleton. In addition, complexes of (NN)B(NN), (NN)BB(NN), and (NN)B4(NN) have been identified in our experiments.

Nitrogen Trifluoride Complexes of Group 10 Transition Metals M(NF3) (M = Pd, Pt).

The group 10 transition metal atoms Pd and Pt react with nitrogen trifluoride (NF3) forming N-coordination M(NF3) complexes in solid neon and argon matrices. The M(NF3) complexes isomerize to more stable fluoronitrenoid FNMF2 isomers via fluorine migration upon blue LED (λ = 470 nm) light irradiation. These products are characterized on the basis of infrared absorption spectroscopy with isotopic substitutions and theoretical frequency calculations. The analysis of the electronic structure of nitrogen trifluoride complexes indicates that the bonding between metal and nitrogen trifluoride can be described as σ donation from the HOMO of nitrogen trifluoride to the empty metal dz2 orbital and π back-donation from the metal dxz/yz orbitals to the LUMO of nitrogen trifluoride, the latter of which stabilized the metal ligand bond and destabilized the ligand N-F bond. In FNMF2, the FN ligand doubly bonded to the metal and bear imido character.

Molecular-scale study of Cr(vi) adsorption onto lepidocrocite facets by EXAFS, in situ ATR-FTIR, theoretical frequency calculations and DFT+U techniques

Lepidocrocite, as a ubiquitous iron mineral, is widely detected in different morphologies in natural environments, controlling the mobility and availability of heavy metal ions (HMIs).

Infrared Spectra and Theoretical Calculations of BSe2 and BSe2-: The Pseudo-Jahn-Teller Effect.

Matrix isolation infrared spectroscopy has been employed to study the reaction of laser-ablated boron atoms with hydrogen selenide in a 5 K solid argon matrix. On the basis of the isotopic shifts as well as the theoretical frequency calculations, triatomic molecule BSe2 and its anion BSe2- were identified. Both BSe2 and BSe2- were predicted to have D∞h symmetric structures with nearly identical structure parameters and bond strengths by quantum chemical calculations. Whereas the observed antisymmetric B-Se stretching frequency of BSe2 is much lower than that of BSe2-, the anomalously low antisymmetric B-Se stretching frequency of BSe2 is attributed to a pseudo-Jahn-Teller effect due to the mixing of the X2Πg ground state with the A2Πu excited state. In addition, H2BSe, HBSe, and HSeBSe molecules were produced in the reaction.

Counter Cations Affect Transport in Aqueous Hydroxide Solutions with Ion Specificity.

The anomalously high mobility of hydroxide and hydronium ions in aqueous solutions is related to proton transfer and structural diffusion. The role of counterions in these solutions, however, is often considered to be negligible. Herein, we explore the impact of alkali metal counter cations on hydroxide solvation and mobility. Impedance measurements demonstrate that hydroxide mobility is attenuated by lithium relative to sodium and potassium. These results are explained by ab initio molecular dynamics simulations and experimental vibrational hydration shell spectroscopy, which reveal substantially stronger ion pairing between OH- and Li+ than with other cations. Hydration shell spectra and theoretical vibrational frequency calculations together imply that lithium and sodium cations have different effects on the delocalization of water protons donating a hydrogen bond to hydroxide. Specifically, lithium leads to enhanced proton delocalization compared with sodium. However, proton delocalization and the overall diffusion process are not necessarily correlated.

Co-Crystal Formation of Antibiotic Nitrofurantoin Drug and Melamine Co-Former Based on a Vibrational Spectroscopic Study.

The co-crystallization of active pharmaceutical ingredients (APIs) has received increasing attention due to the modulation of the relative physicochemical properties of APIs such as low solubility, weak permeability and relatively inferior oral bioavailability. Crystal engineering plays a decisive role in the systematic design and synthesis of co-crystals by means of exerting control on the inter-molecular interactions. The characterization and detection of such co-crystal formations plays an essential role in the field of pharmaceutical research and development. In this work, nitrofurantoin (NF), melamine (MELA) and their hydrated co-crystal form were characterized and analyzed by using terahertz time-domain spectroscopy (THz-TDS) and Raman vibrational spectroscopy. According to the experimental THz spectra, the hydrated co-crystal form has characteristic absorption peaks at 0.67, 1.05, 1.50 and 1.73 THz, while the THz spectra for the two raw parent materials (NF and MELA) are quite different within this spectral region. Similar observations were made from the experimental Raman vibrational spectra results. Density functional theory (DFT) calculation was performed to help determine the major vibrational modes of the hydrated co-crystal between nitrofurantoin and melamine, as well as identify the structural changes due to inter- and/or intra-molecular hydrogen bonding motifs between NF and MELA. The results of the theoretical frequency calculations corroborate the THz and Raman experimental spectra. The characteristic bands of the NF–MELA-hydrated co-crystal between nitrofurantoin and melamine were also determined based on the DFT simulated calculation. The reported results in this work provide us with a wealth of structural information and a unique vibrational spectroscopic method for characterizing the composition of specific co-crystals and inter-molecular hydrogen bonding interactions upon pharmaceutical co-crystallization.

M-S Multiple Bond in HMSH, H2MS, and HMS Molecules (M = B, Al, Ga): Matrix Infrared Spectra and Theoretical Calculations.

Reaction products of B, Al, and Ga atoms with H2S have been identified in solid argon using matrix isolation infrared absorption spectroscopy. The results show that the ground state B atom reaction with H2S gives the H2BS molecule while for the Al atom the HAlSH molecule forms first, which then further isomerizes to H2AlS upon >500 nm irradiation. The reaction of the Ga atom with H2S only takes place upon photolysis to produce HGaSH in the matrix. The assignments of the major modes for these products were confirmed by appropriate 10B, 11B, D2S, and H234S isotopic shifts and theoretical frequency calculations. Topological analysis of the electron density suggests that both HBSH and H2BS molecules possess covalent B-S bond with significant double bond character, while the M-S bond in the heavier group 13 homologues (Al, Ga) was characterized as a polar covalent strong interaction.

Raman bandshape analysis and DFT study of C[dbnd]O and C[sbnd]H stretching modes of NN-Dibutyl formamide in DMSO solvent`

The study reports the inter-molecular interactions and dynamics of CO and CH stretching modes of NN- Dibutyl Formamide (DBF) in DMSO solvent using Raman spectroscopic technique. The Raman band of CO and CH stretching modes have been deconvoluted into two distinct bands for neat as well as in DMSO solvent. Peak wavenumbers of CO stretching modes show red shift while CH stretching modes shows blue shift with the increase in solvent concentrations. The optimized geometric parameters, vibrational wavenumbers, Mulliken atomic charges and natural bond orbitals of the molecule has been computed using Density Functional Theory (DFT) method with basis set 6-31 +G (d, p). In addition, the same basis set has been carried out with counterpoise keyword accounting BSSE calculation on monomer and dimer states with DMSO solvent to present an appropriate interacting environment. IEF-PCM solvation model has also been computed using the same basis set and compares the geometrical parameters and vibrational wavenumbers of the molecules and in their complexes. In order to get a complete study of the DBF + DMSO complexes, explicit solvation model has also been calculated for Monomer DBF in two solvent molecules. Theoretical calculations of frequencies have been compared with the experimental findings and the results are found in good agreement.

Isocyanate Formation from Reactions of Early Lanthanide Metal Atoms with NO and CO in Solid Argon.

The reactions of early lanthanide metal atoms (Ce, Pr, and Nd) with carbon monoxide and nitric oxide mixtures are studied by infrared absorption spectroscopy in solid argon. The reaction intermediates and products are identified via isotopic substitution as well as theoretical frequency calculations. The results show that the reactions proceed with the initial formation of inserted NLnO molecules, which subsequently react with CO to form the NLnO(CO) complexes on annealing. The NLnO(CO) complexes further isomerize to the more stable isocyanate OLnNCO species under UV light excitation.

Infrared spectra of HMSH and HMMSH (M = Zn, Cd, Hg) in solid argon

Reactions of zinc, cadmium and mercury atoms with hydrogen sulfide were studied by matrix-isolation infrared spectroscopy. Insertion products HMSH (M=Zn, Cd, Hg) and HMMSH (M=Zn, Cd) were identified by H234S and D2S isotopic substitutions and by theoretical vibrational frequency calculations (B3LYP, BPW91, CCSD(T)). The absorptions of HZnSH and HCdSH molecules increased on annealing in solid argon, suggesting the reactions occur without activation energy; however, the HHgSH molecule was observed on full-arc irradiation. In addition our HMMSH molecules provide simple model to understand the bonding of {M2}2+ core complexes.

Synthesis, spectroscopic and structural characterisation of two p-nitrobenzamide compounds: experimental and DFT study

The title molecules, N-(1,5-dimethyl-3-oxo-2-phenyl-1H-3-(2H)-pyrazolyl)4-nitrobenzamide (C18H16N4O4·H2O) (I) and 2,2-dimethyl-3-(4-nitrobenzoyl)-5-(phenylamino)-2,3-dihydro-1,3,4-thiyadiazole (C17H16N4O3S) (II), were prepared and characterised by 1H NMR, 13C NMR, infrared spectroscopy (IR) and structural X-ray diffraction (XRD) techniques. The molecular geometries, vibrational frequencies of the title compounds in the ground state have been calculated by using the density functional theory (DFT) method with 6-31G(d) basis set, and compared with the experimental data. The calculated results showed that the optimised geometries from the DFT method agree with the X-ray structures well for both compounds. Theoretical calculations of harmonic vibration frequencies are in good agreement with experimental results. To determine conformational flexibility, the molecular energy profiles of the title compounds were obtained. Besides, molecular electrostatic potential (MEP), frontier molecular orbitals (FMO) analysis and thermodynamic properties of the title compounds were investigated by theoretical calculations.

Theoretical transition frequencies beyond 0.1 ppb accuracy inH2+,HD+, and antiprotonic helium

We present improved theoretical calculations of transition frequencies for the fundamental transitions $(L\!=\!0,v\!=\!1)\to(L'\!=\!0,v'\!=\!0)$ in the hydrogen molecular ions H$_2^+$ and HD$^+$ with a relative uncertainty $4\cdot10^{-11}$ and for the two-photon transitions in the antiprotonic helium atom with a relative uncertainty $10^{-10}$. To do that, the one-loop self-energy correction of order $\alpha(Z\alpha)^6$ is derived in the two Coulomb center approximation, and numerically evaluated in the case of the aforementioned transitions. The final results also include a complete set of other spin-independent corrections of order $m\alpha^7$. The leading order corrections of $\alpha^2\ln^3(Z\alpha)^{-2}(Z\alpha)^6$ are also considered that allows to estimate a magnitude of yet uncalculated contributions.

Matrix Infrared Spectroscopy and Quantum‐Chemical Calculations for the Coinage‐Metal Fluorides: Comparisons of ArAuF, NeAuF, and Molecules MF2 and MF3

The reactions of laser-ablated Au, Ag, and Cu atoms with F(2) in excess argon and neon gave new absorptions in the M-F stretching region of their IR spectra, which were assigned to metal-fluoride species. For gold, a Ng-AuF bond was identified in mixed neon/argon samples. However, this bonding was much weaker with AgF and CuF. Molecules MF(2) and MF(3) (M=Au, Ag, Cu) were identified from the isotopic distribution of the Cu and Ag atoms, comparison of the frequencies for three metal fluorides, and theoretical frequency calculations. The AuF(5) molecule was characterized by its strongest stretching mode and theoretical frequency calculations. Additional evidence was observed for the formation of the Au(2) F(6) molecule.

Investigation of Chromate Coordination on Ferrihydrite by in Situ ATR-FTIR Spectroscopy and Theoretical Frequency Calculations

Chromate mobility, reactivity, and bioavailability in soil environments are affected by adsorption reactions on iron oxide minerals, but the adsorption mechanisms remain controversial. In this study, we employed in situ attenuated total reflectance Fourier transform infrared spectroscopy and theoretical frequency calculations to characterize chromate adsorption on 2-line ferrihydrite. The effects of pH, aqueous chromate concentration, ionic strength, and deuterium exchange were investigated. Results suggest the formation of monodentate and bidentate surface complexes. It was determined that monodentate complexes are dominant at low surface coverage and pH ≥ 6.5 and that bidentate complexes form at high surface coverage and pH < 6. Deuterium exchange experiments indicated that the inner-sphere complexes are not protonated. Difference spectra revealed that monodentate complexes are particularly susceptible to ionic strength effects under acidic conditions.

Tantalum atom reactions with ammonia: Matrix infrared spectra and DFT calculations of the H2TaNH and H2Ta(NH2)2 molecules

Laser-ablated tantalum atoms react with ammonia to form the metal-ammonia complexes Ta(NH3) and Ta(NH3)2 spontaneously on annealing. These complexes underwent photochemical rearrangement to form the imido molecule H2TaNH and amido complex H2Ta(NH2)2. The reaction products have been identified by isotopic substitutions as well as density functional theoretical frequency calculations. In addition, reaction mechanism of the possible reaction paths for these reactions is discussed.

Acoustic cross sections and resonance frequencies of large fish schools

A prerequisite for stable inversion of bioacoustic parameters of fish schools from large-scale broadband acoustic observations is a theoretical model that permits fast and accurate calculations of acoustic cross sections and school resonance frequencies based on realistic geometrical models of fish schools. The schools of commercially important species, such as sardines, anchovies, and herring, may be characterized by dense nuclei which contain tens of thousands of individuals (N) with separations (S) on the order of one fish length, and diffuse “fuzz” regions with fish at significantly larger separations. Numerical computations of cross sections and school resonances based on the fundamental equations of multiple scattering for point scatterers for these fish school geometries will be presented. Initial results indicate that bubble cloud frequencies of large schools depend primarily on the average spacing between fish in the nuclei and are essentially independent of school size and shape. It will be shown that theoretical calculations of bubble cloud frequencies (based on previously reported values of N and S of sardine schools) are consistent with Diachok's (1999) observations of the average resonance frequency of sardine schools which were derived from broadband transmission loss measurements. [Work is supported by the Office of Naval Research.]

Infrared Spectra of the Chloromethyl and Bromomethyl Cations in Solid Argon

Infrared spectra of chloromethyl and bromomethyl cations isolated in solid argon are reported. Cocondensation of dichloromethane and dibromomethane with high-frequency discharged argon at 4 K produces the dichloromethane and dibromomethane cations, which dissociate upon visible light irradiation to form the chloromethyl and bromomethyl cations. On the basis of isotopic substitutions ((13)C and deuterium) as well as theoretical frequency calculations, photosensitive absorptions are assigned to different vibrational modes of the chloromethyl and bromomethyl cations. Theoretical calculations predict that the halomethyl cations have a planar C(2v) symmetry with a shorter C-X bond and longer C-H bonds relative to those of the halomethyl free radicals.

Two-dimensional fluorescence spectroscopy for the identification of discharge intermediates

A protocol is developed for the identification of discharge intermediates from complex electronic spectra. The protocol involves resonant ionisation mass spectrometry to determine the mass of the unknown carrier followed by a two-dimensional fluorescence spectrum which is used as a "map" to guide higher resolution techniques. High resolution dispersed fluorescence provides accurate ground electronic state vibrational frequencies which are then compared with theoretical calculations of frequencies and structures of a range of isomers consistent with the observed mass. The protocol is demonstrated in the first identification of the cis and trans-1-vinylpropargyl radicals as a ubiquitous radical in hydrocarbon discharges.

Infrared Spectra of Transition-Metal Dioxide Anions: MO2−(M = Rh, Ir, Pt, Au) in Solid Argon

Transition-metal dioxide anions RhO(2)(-), IrO(2)(-), PtO(2)(-), and AuO(2)(-) were produced from cocondensation of laser-ablated metal atoms and electrons with dioxygen in excess argon at 6 K. Photosensitive absorptions were assigned to the antisymmetric stretching vibration (nu(3)) of the metal dioxide anions from isotopic shifts and splittings as well as theoretical frequency calculations. On the basis of the observed nu(3) vibrational frequencies for M(16)O(2) and M(18)O(2), the anions are estimated to be linear or near linear. Density functional calculations at the DFT/B3LYP level predicted that the third-row transition-metal dioxide anions IrO(2)(-), PtO(2)(-), and AuO(2)(-) have singlet, doublet, and triplet ground states with linear structures, while the RhO(2)(-) anion was predicted to have a slightly bent geometry.

Infrared Spectrum of Carbon Trisulfide in Solid Argon

Cocondensation of carbon disulfide with high-frequency discharged argon at 4 K produced carbon monosulfide and atomic sulfur, which reacted spontaneously upon annealing to form the carbon trisulfide molecule as identified from the multiplets observed in mixed (12)C, (13)C and (32)S, (34)S isotopic spectra. On the basis of isotopic substitution and theoretical frequency calculations, infrared absorptions at 1263.3 and 570.1 cm(-1) were assigned to predominantly CS stretching and bending vibrations of CS(3) in solid argon. The CS(3) molecule, which was calculated to have a singlet ground state with C(2v) symmetry, dissociated to form the weakly bound SCS-S complex upon visible light irradiation.