Electron photodetachment spectroscopy of solvated anions. A probe of structure and energetics

Electron photodetachment of solvated anions of the form ROHF^- has been undertaken. Observation or non-observation of photodetachment provides information about the location of the bridging proton, i.e. whether the complex has the structure ROH·F^- or RO^-·HF. Where photodetachment is observed, the experimental detachment threshold energy yields the solvation energy for the corresponding free anion.     

Negative hydrogen and helium in a variety of debye plasmas

Aspects of the influence of a strong Debye plasma environment on the negative hydrogen ion and the neutral helium atom have been studied. Contrary to earlier work, in the present calculation all interactions have been screened. This increases the stability of the systems to the extent that neither the H^- ion nor the ground state of helium will lose an electron by pressure ionization. It has been found that the charge distribution of H^- remains remarkably constant over a vast range of values of the Debye parameter D. © 1996 John Wiley & Sons, Inc.     

Molecular dynamics of CO in few-cycle laser fields

We experimentally study the ionization, fragmentation and Coulomb explosion of CO using 6 fs laser pulses. Different from previous observations in tens or hundreds of femtoseconds laser pulses, strong charge asymmetric dissociation and CO²⁺ are observed in the current intense few-cycle laser field.     

Dihydrogen Splitting by Intramolecular Borane-Phosphane Frustrated Lewis Pairs: A Comprehensive Characterization Strategy Using Solid State NMR and DFT Calculations

Four hydrogenated intramolecular phosphane-borane frustrated Lewis pair (B/P FLP) compounds bearing unsaturated cyclic or aromatic carbon backbones have been synthesized and structurally characterized using ¹¹B, ³¹P, ¹H and ²H solid-state NMR spectroscopy. A comparison of the spectra with those of the corresponding free B/P FLPs shows that both ¹¹B isotropic chemical shifts as well as nuclear electric quadrupolar coupling constants decrease significantly upon FLP hydrogenation, revealing the breakage of the partial B−P bond present in the starting materials. Likewise, the ³¹P isotropic chemical shift, the chemical shift anisotropy, and the asymmetry parameter decrease significantly upon FLP hydrogenation, reflecting the formation of a more symmetric, C_3v-like local environment. ¹¹B{³¹P} rotational echo double resonance (REDOR) experiments can be used to measure the B−P internuclear distance (about 3.2 Å) of these compounds. Observation of the hydrogen atoms bound to the Lewis centers is best accomplished via ³¹P{¹H} and ¹¹B{¹H} cross-polarization-heteronuclear correlation experiments or by direct observation of the ²H MAS NMR signals on especially prepared FLP-D₂ adducts. For accurately measuring the phosphorus-deuterium distance via ³¹P{²H} rotational echo adiabatic passage double resonance (REAPDOR), it is essential to take the secondary dipolar coupling of ³¹P with the boron-bonded ²H nuclei explicitly into consideration, by simulating a ²H_P-³¹P-²H_B three-spin system based on structural input. All of the experimental NMR interaction parameters are found in excellent agreement with values calculated by DFT methods, using the geometries obtained either by energy optimization or from single-crystal structures.     

Conformational analysis of cyclic phosphates derived from 5-C′ substituted 1,2-O-isopropylidene-α-d-xylofuranose derivatives

Twelve 2-phenoxy-2-oxo-1,3,2-dioxaphosphorinanes fused with a 1,2-O-isopropylidene-α-d-xylofuranose moiety in cis orientation and substituted at the C′5 position were prepared in two steps from commercially available diacetone-α-d-glucose. Their conformations, and configurations were determined by ¹H and ³¹P NMR and X-ray crystallographic techniques. Both, chair-twisted-chair and chair-boat equilibria were observed in solution. We observed that the strong anisotropic shielding effect of the benzene ring in the phenoxy group generates an upfield shift of the H¹ hydrogen atom, when the cyclic phosphates adopt a boat conformation. This is due to a relative cis-orientation of the P-phenoxy group and the H¹ proton of the 1,2-O-isopropylidene-α-d-xylofuranose moiety. Therefore, the configuration of the phosphorus center (S_P or R_P) can be determined by ¹H NMR spectroscopy. Interestingly, the crystal structure of one of the cyclic phosphates exhibits two independent molecules in the asymmetric unit, one with a chair and the other one with a boat conformation.     

Quantum-chemical calculation of NMR chemical shifts of organic molecules: III. Intramolecular coordination effects on the 31P NMR chemical shifts of phosphorylated N-vinylazoles

Theoretical and experimental studies on magnetic shielding of the phosphorus nucleus in trichloro-[2-(1H-pyrazol-1-yl)ethenyl]phosphonium hexachlorophosphate(V) and 1,1,1,1-tetrachloro-1H-1λ⁶-pyrazolo-[1,2-a][1,2,3]diazaphosphol-8-ium-1-ide showed that intramolecular coordination of the phosphorus atom in the chlorophosphonium group to the nitrogen atom in the pyrazole ring leads to upfield shift of the phosphorus signal (to δ_P 170 ppm) and that the contribution of the spin-orbital contribution to the ³¹P chemical shift reaches 15%. Relativistic effects and effects of the medium are determining in the theoretical calculation of ³¹P NMR chemical shifts.     

Classification of partially reflecting surfaces using photodetached electron spectrum

Photodetachment spectra of H^? near a partially reflecting surface is investigated by closed orbit theory. A reflection parameter K is introduced for the analysis of the photodetached‐electron spectrum. Factor K controls the spectra on the screen and strongly depends on the surface properties of a material. This parameter can effectively be used for the classification of surfaces using photodetachment microscopy. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Differentiation among the four diastereomers of benzyloxycarbonyl-protected γ-hydroxyornithine in negative-ion fast atom bombardment mass spectrometry

Discrimination among the four γ-hydroxyornithine diastereomers was studied by fast atom bombardment mass spectrometry (FABMS). It is impossible to distinguish among the four diastereomers of this amino acid by positive- and negative-ion FAB and collisionally activated dissociation MS, but benzyloxycarbonyl group protection of the α- and δ-amino groups in γ-hydroxyornithine allows differentiation among the diastereomers in negative-ion FABMS. The negative-ion mass spectra of benzyloxycarbonyl-protected γ-hydroxyornithine diastereomers showed differences among the abundances of the molecule ion [M – H]^-, the dehydrated ion [M — H — H₂O]^- due to the loss of the γ-hydroxyl group and the fragment ions formed from both [M — H]^- and [M — H — H₂O]^- ions. On the other hand, no difference was found between the fragmentations of the benzyloxycarbonyl-protected enantiomers of ornithine in negative-ion FABMS. These results indicate that the orientation of the γ-hydroxyl group and the existence of two benzene rings in the benzyloxycarbonyl group are important factors which are responsible for the fragmentations of the four benzyloxycarbonyl-protected γ-hydroxyornithine diastereomers in negative-ion FABMS. These studies also showed that the negative-ion FABMS for benzyloxycarbonyl-protected γ-hydroxyornithine diastereomers is a useful method for determining the configuration of each diastereomer of γ-hydroxyornithine.     

Pyridinethiones. VI. The synthesis of 2H-Thiopyrano[2,3-b]pyridin-2-ones

A general and versatile method for the preparation of 2H-thiopyrano[2,3-b]pyridin-2-ones is described. The starting materials, the β, β-disubstituted vinyl-1-t-butyl-2-(1H)pyridinethiones were prepared from the synthon 3-formyl-1-t-butyl-2-(1H)pyridinethione by condensation. ¹³H nmr spectra showed the vinyl double bond of the condensation products to have the trans configuration with the smallest group close to the sulfur atom. Some reactions of these new azaanalogues of thiocoumarins are reported.     

Relationships between NMR shifts and interaction energies in biphenyls,alkanes, aza-alkanes,and oxa-alkanes with X─H…H─Y and X─H…Z (X,Y = C or N; Z = N or O) hydrogen bonding

Hydrogen–hydrogen C─H^…H─C bonding between the bay-area hydrogens in biphenyls, and more generally in congested alkanes, very strained polycyclic alkanes, and cis-2-butene, has been investigated by calculation of proton nuclear magnetic resonance (NMR) shifts and atom–atom interaction energies. Computed NMR shifts for all protons in the biphenyl derivatives correlate very well with experimental data, with zero intercept, unit slope, and a root mean square deviation of 0.06 ppm. For some congested alkanes, there is generally good agreement between computed values for a selected conformer and the experimental data, when it is available. In both cases, the shift of a given proton or pair of protons tends to increase with the corresponding interaction energy. Computed NMR shift differences for methylene protons in polycyclic alkanes, where one is involved in a very short contact (“in”) and the other is not (“out”), show a rough correlation with the corresponding C─H^…H─C exchange energies. The “in” and “in,in” isomers of selected aza- and diaza-cycloalkanes, respectively, are X─H^…H─N hydrogen bonded, whereas the “out” and “in,out” isomers display X─H^…N hydrogen bonds (X = C or N). Oxa-alkanes and the “in” isomers of aza–oxa-alkanes are X─H^…O hydrogen bonded. There is a very good general correlation, including both N─H^…H─Y (Y = C or N) and N─H^…Z (Z = N or O) interactions, for NH proton shifts against the exchange energy. For “in” CH protons, the data for the different C─H^…H─Y and C─H^…Z interactions are much more dispersed and the overall shift/exchange energy correlation is less satisfactory.