Noncovalent complexes between dimethyl ether and formic acid--an ab initio and matrix isolation study.

The complexes formed by noncovalent interactions between formic acid and dimethyl ether are investigated by ab initio methods and characterized by matrix isolation spectroscopy. Six complexes with binding energies between -2.26 and -7.97 kcal mol(-1) (MP2/cc-pVTZ+zero point vibrational energy+basis set superposition erros) are identified. The two strongest bound complexes are, within a range of 0.3 kcal mol(-1), isoenergetic. The binding in these six dimers can be described in terms of OH...O, C=O...H, C-O...H and CH...O interactions. Matrix isolation spectroscopy allowed to characterize the two strongest bound complexes by their infrared spectra.     

Gas‐Phase Preparation of Carbonic Acid and Its Monomethyl Ester

Carbonic acid (H₂CO₃), an essential molecule of life (e.g., as bicarbonate buffer), has been well characterized in solution and in the solid state, but for a long time, it has eluded its spectral characterization in the gas phase owing to a lack of convenient preparation methods; microwave spectra were recorded only recently. Here we present a novel and general method for the preparation of H₂CO₃ and its monomethyl ester (CH₃OCO₂H) through the gas‐phase pyrolysis of di‐tert‐butyl and tert‐butyl methyl carbonate, respectively. H₂CO₃ and CH₃OCO₂H were trapped in noble‐gas matrices at 8 K, and their infrared spectra match those computed at high levels of theory [focal point analysis beyond CCSD(T)/cc‐pVQZ] very well. Whereas the spectra also perfectly agree with those of the vapor phase above the β‐polymorph of H₂CO₃, this is not true for the previously reported α‐polymorph. Instead, the vapor phase above α‐H₂CO₃ corresponds to CH₃OCO₂H, which sheds new light on the research that has been conducted on molecular H₂CO₃ over the last decades.     

Infrared Spectra of Cyanoacetaldehyde (NCCH2CHO): A Potential Prebiotic Compound of Astrochemical Interest

Cyanoacetaldehyde (NC? CH₂CH?O) and its isomer, cyanovinylalcohol (NC? CH?CH? OH), as possible components of the interstellar medium, comets, or planetary atmospheres, exist in equilibrium in the gas phase, although the latter compound is very much in the minority (2 %). The recording and analysis of the gas‐phase infrared spectrum of the former compound within the 4000–500 cm^?1 spectroscopic range and the potential presence of the latter isomer, which could be vital for their detection in these media, are reported. CCSD(T) and G4 high‐level ab initio methods, as well as density functional theory calculations, predict the existence of two stable rotamers of cyanoacetaldehyde. The global minimum has a structure with an unusual O‐C‐C‐C dihedral angle (150°) that falls between the antiperiplanar (180°) and anticlinal forms (120°). The second rotamer, which is about 4.0 kJ mol^?1 less stable in terms of free energy, has a planar structure that corresponds to the synperiplanar form (O‐C‐C‐C dihedral angle: 0°). The absorption vibrational bands of the two aldehyde rotamers that are present in the mixture lead to a spectrum with a very complex structure in the region of deformation movements, in which several low‐intensity bands overlap. A complete and unambiguous assignment of the experimental spectrum has been achieved by using the calculated harmonic and anharmonic vibrational frequencies.     

On the Extreme Oxidation States of Iridium

It has recently been suggested that the oxidation states of Ir run from the putative ?III in the synthesized solid Na₃[Ir(CO)₃] to the well‐documented +IX in the species IrO₄⁺. Furthermore, [Ir(CO)₃]^3? was identified as an 18‐electron species. A closer DFT study now finds support for this picture: The orbitals spanned by the 6s,6p,5d orbitals of the iridium are all occupied. Although some have considerable ligand character, the deviations from 18 e leave the orbital symmetries unchanged. The isoelectronic systems from Os^?IV to Au^?I behave similarly, suggesting further possible species. To paraphrase Richard P. Feynmann “there is plenty of room at the bottom”.     

IRMPD Spectroscopy of a Protonated,Phosphorylated Dipeptide

The protonated, phosphorylated dipeptide [GpY+H]⁺ is characterized by mid‐infrared multiple‐photon dissociation (IRMPD) spectroscopy and quantum‐chemical calculations. The ions are generated in an external electrospray source and analyzed in a Fourier transform ion cyclotron resonance mass spectrometer, and their fragmentation is induced by resonant absorption of multiple photons emitted by a tunable free‐electron laser. The IRMPD spectra are recorded in the 900–1730 cm^?1 range and compared to the absorption spectra computed for the lowest energy structures. A detailed calibration of computational levels, including B3LYP‐D and coupled cluster, is carried out to obtain reliable relative energies of the low‐energy conformers. It turns out that a single structure can be invoked to assign the IRMPD spectrum. Protonation at the N terminus leads to the formation of a strong ionic hydrogen bond with the phosphate P?O group in all low‐energy structures. This leads to a P?O stretching frequency for [GpY+H]⁺ that is closer to that of [pS+H]⁺ than to that of [pY+H]⁺ and thus demonstrates the sensitivity of this mode to the phosphate environment. The COP phosphate ester stretching mode is confirmed to be an intrinsic diagnostic for identification of which type of amino acid is phosphorylated.     

Generation and Identification of the Linear OCBNO and OBNCO Molecules with 24 Valence Electrons

Two structural isomers containing five second-row element atoms with 24 valence electrons were generated and identified by matrix-isolation IR spectroscopy and quantum chemical calculations. The OCBNO complex, which is produced by the reaction of boron atoms with mixtures of carbon monoxide and nitric oxide in solid neon, rearranges to the more stable OBNCO isomer on UV excitation. Bonding analysis indicates that the OCBNO complex is best described by the bonding interactions between a triplet-state boron cation with an electron configuration of (2s)⁰(2p_σ)⁰(2p_π)² and the CO/NO⁻ ligands in the triplet state forming two degenerate electron-sharing π bonds and two ligand-to-boron dative σ bonds.     

Photo‐Induced Hydrogen Exchange Reaction between Methanol and Glyoxal: Formation of Hydroxyketene

We study the structure and photochemistry of the glyoxal–methanol system (G–MeOH) by means of FTIR matrix isolation spectroscopy and ab initio calculations. The FTIR spectra show that the non‐hydrogen‐bonded complex, G–MeOH‐1, is present in an inert environment of solid argon. MP2/aug‐cc‐pVDZ calculations indicate that G–MeOH‐1 is the most stable complex among the five optimized structures. The interaction energy partitioned according to the symmetry‐adapted perturbation theory (SAPT) scheme demonstrates that the dispersion energy gives a larger contribution to the stabilization of a non‐hydrogen‐bonded G–MeOH‐1 complex than compared to the hydrogen‐bonded ones. The irradiation of G–MeOH‐1 with the filtered output of a mercury lamp (λ>370 nm) leads to its photo‐conversion into the hydroxyketene–methanol complex HK–MeOH‐1. The identity of HK–MeOH‐1 is confirmed by both FTIR spectroscopy and MP2/aug‐cc‐pVDZ calculations. An experiment with deuterated methanol (CH₃OD) evidences that hydroxyketene is formed in a photo‐induced hydrogen exchange reaction between glyoxal and methanol. The pathway for the photo‐conversion of G–MeOH‐1 to HK–MeOH‐1 is studied by a coupled‐cluster method [CR–CC(2,3)]. The calculations confirm our experimental findings that the reaction proceeds via hydrogen atom exchange between the OH group of methanol and CH group of glyoxal.     

Infrared-spectroscopic and density-functional-theory investigations of the LaCO, La2[eta2(mu2-C,O)], and c-La2(mu-C)(mu-O) molecules in solid argon

Reactions of laser-ablated lanthanum atoms with CO molecules in solid argon have been studied. The neutral lanthanum monocarbonyl (LaCO), produced upon sample deposition at 7 K, exhibits a C-O stretching frequency of 1772.7 cm(-1); to the best of our knowledge this is the lowest yet observed for a terminal CO in a neutral metal-carbonyl molecule (MCO, M = metal atom), implying anomalously enhanced metal-to-CO back-bonding. The infrared (IR) absorption band at 1145.9 cm(-1) is assigned to the C-O stretching mode of the side-on-bonding CO in the La2[eta2(mu2-C,O)] molecule. This CO-activated molecule undergoes an UV/Vis-photoinduced rearrangement to the CO-dissociated molecule, c-La2(mu-C)(mu-O). Density functional theory (DFT) calculations have been performed on these molecules, the results of which lend strong support to the experimental assignments of the IR spectra. LaCO is predicted to have a quartet ground state, corresponding to a linear geometry. Its formation involves La 6s-->4f promotion, which increases the strength of La-CO bonding by decreasing the sigma repulsion and, remarkably, by increasing the La 5d and 4f-->CO 2pi back-bonding. The observations schematically depict the whole process, starting with the interaction of CO with metal and ending with CO dissociation by the lanthanum dimer.     

A Computational Study on Closed-Shell Molecular Hexafluorides MF6 (M=S,Se, Te,Po, Xe,Rn, Cr,Mo, W,U) – Molecular Structure,Anharmonic Frequency Calculations,and Prediction of the NdF6 Molecule

Quantum chemical methods were used to study the molecular structure and anharmonic IR spectra of the experimentally known closed-shell molecular hexafluorides MF₆ (M=S, Se, Te, Xe, Mo, W, U). First, the molecular structures and harmonic frequencies were investigated using Density Functional Theory (DFT) with all-electron basis sets and explicitly considering the influence of spin-orbit coupling. Second, anharmonic frequencies and IR intensities were calculated with the CCSD(T) coupled cluster method and compared, where available, with IR spectra recorded by us. These comparisons showed satisfactory results. The anharmonic IR spectra provide means for identifying experimentally too little studied or unknown MF₆ molecules with M=Cr, Po, Rn. To the best of our knowledge, we predict the NdF₆ molecule for the first time and show it to be a true local minimum on the potential energy surface. We used intrinsic bond orbital (IBO) analyses to characterize the bonding situation in comparison with the UF₆ molecule.     

Spectroscopy of Dibenzorubicene: Experimental Data for a Search in Interstellar Spectra

We report on the characterization of dibenzo[cde,opq]rubicene (C₃₀H₁₄). The molecule was studied in solution at room temperature with absorption spectroscopy in the visible (vis) and ultraviolet (UV) wavelength ranges, and with emission spectroscopy. The infrared (IR), visible, ultraviolet, and vacuum ultraviolet (VUV) absorption spectra of a thin film were measured also at room temperature. In addition, the UV/vis absorption spectrum was measured at cryogenic temperatures using the matrix isolation spectroscopy technique. The interpretation of spectra was supported by theoretical calculations based on semiempirical and ab initio models, as well as on density functional theory. Finally, the results of the laboratory study were compared with interstellar spectra.     

Reactions of Th and U atoms with C2H2: infrared spectra and relativistic calculations of the metallacyclopropene, actinide insertion, and ethynyl products

Reactions of laser-ablated Th and U atoms with C(2)H(2) during condensation with excess argon at 7 K give several new product species. The metallacyclopropene, inserted hydride, and actinide ethynyl are identified from isotopic frequencies and relativistic DFT calculations. The higher-energy vinylidine isomer was not observed. These actinide metallacyclopropenes exhibit substantially stronger bonding interactions than found recently for the Pd and Pt metals. In the case of Th(C(2)H(2)) the argon matrix interaction is strong enough to reverse the computed order of states (MR-CISD) in favor of a triplet ground state for the (Ar)(n)(Th(C(2)H(2))) complex. The nature of the electronic interactions between various metal atoms and acetylene is compared and the origin of the particularly strong interaction for U and Th is traced to the higher energy of their 6d orbitals. The ThCCH and UCCH actinide ethynyl products are also observed and characterized by C[triple bond]C stretching modes 38+/-2 cm(-1) lower than acetylene itself.     

Gas‐Phase Generation and Decomposition of a Sulfinylnitrene into the Iminyl Radical OSN

The dipolar oxathiazyne‐like sulfinylnitrene RS(O)N, a highly reactive α‐oxo nitrene, has been rarely investigated. Upon flash vacuum pyrolysis of sulfinyl azide CF₃S(O)N₃ at 350 °C, an elusive sulfinylnitrene CF₃S(O)N was generated in the gas phase in its singlet ground state and was characterized by matrix‐isolation IR spectroscopy. Further fragmentation of CF₃S(O)N at 600 °C produced CF₃ and a novel iminyl radical OSN, an SO₂ analogue, which were unambiguously identified by IR spectroscopy. Consistent with the experimental observations, DFT calculations clearly support a stepwise decomposition mechanism of CF₃S(O)N₃.     

Infrared spectrum and structure of thorimine (HN=ThH2)

Laser-ablated thorium atoms react with ammonia to form thorimine (NH=ThH(2)), the first actinide imine to be reported. This work underscores the high reactivity of thorium atoms, particularly for N-H bond activation, reveals a new type of multiple bond to actinide atoms, and shows that this bond is strong for thorium as a result of an important contribution from the f orbitals.     

Reactions of uranium atoms with ammonia: infrared spectra and quasi-relativistic calculations of the U:NH3, H2N--UH, and HN==UH2 complexes

Ammonia molecules interact with U atoms, and the resulting U:NH3 complex rearranges upon visible irradiation to form the H2N--UH and HN==UH2 molecules in excess argon. These products are identified by functional group frequencies, 15NH3 and ND3 isotopic shifts, and comparison to frequencies calculated by using density functional theory. The N==U pi bond in HN==UH2 is enhanced by partial triple-bond character through N(2p) to U(5f) conjugation, which is comparable to that found in the analogous HN==ThH2 molecule. These products also form complexes with additional ammonia molecules in the matrix. The interesting higher-energy N[triple chemical bond]UH3 complex is not formed.