Atom-atom potential in rotational line broadening for molecular gases: Application to CO lines broadened by CO,N2, O2 and NO

Linewidths in CO-CO, CO-N₂, CO-C₂ and CO-NO collisions have been calculated using an improved potential for atom-atom interactions in addition to the first-order terms of the electrostatic interaction. The energy parameters characterizing the Lennard-Jones potential have been estimated previously from second virial coefficients. Overall agreement between calculated and experimental linewidths (particularly for CO-O₂ mixtures for which the electrostatic contributions are weak) is thus obtained without any adjustable parameters. It is concluded that the atom-atom potential describes the angular dependence of the short-range forces in molecular collisions fairly adequately.     

Structures and fragmentations of cobalt(II)-cysteine complexes in the gas phase

The electronebulization of a cobalt(II)/cysteine(Cys) mixture in water/methanol (50/50) produced mainly cobalt-cationized species. Three main groups of the Co-cationized species can be distinguished in the ESI-MS spectrum: (1) the cobalt complexes including the cysteine amino acid only (they can be singly charged, for example, [Co(Cys)n- H]+ with n = 1-3 or doubly charged such as [Co + (Cys)2]2+); (2) the cobalt complexes with methanol: [Co(CH3OH)n- H]+ with n = 1-3, [Co(CH3OH)4]2+; and (3) the complexes with the two different types of ligands: [Co(Cys)(CH3OH) - H]+. Only the singly charged complexes were observed. Collision-induced dissociation (CID) products of the [Co(Cys)2]2+, [Co(Cys)2 - H]+ and [Co(Cys) - H]+ complexes were studied as a function of the collision energy, and mechanisms for the dissociation reactions are proposed. These were supported by the results of deuterium labelling experiments and by density functional theory calculations. Since [Co(Cys) - H]+ was one of the main product ions obtained upon the CID of [Co(Cys)2]2+ and of [Co(Cys)2 - H]+ under low-energy conditions, the fragmentation pathways of [Co(Cys) - H]+ and the resulting product ion structures were studied in detail. The resulting product ion structures confirmed the high affinity of cobalt(II) for the sulfur atom of cysteine.     

Noncovalent complexes between DNA and basic polypeptides or polyamines by MALDI-TOF

MALDI-MS was evaluated as a method for the study of noncovalent complexes involving DNA oligonucleotides and various polybasic compounds (basic polypeptides and polyamines). Complexes involving single-stranded DNA were successfully detected using DHAP matrix in the presence of an ammonium salt. Control experiments confirmed that the interactions involved basic sites of the polybasic compounds and that the complexes were not formed in the gas phase but were pre-existing in the matrix crystals. Moreover, the pre-existence in solution was probed by isothermal titration calorimetry at concentration and ionic strength similar to those used for mass spectrometry. Spectra showed no important difference between negative and positive ion modes. The influence of nature and size of DNA and polybasic compound on the relative intensities and stoichiometries of the complexes was investigated. Despite the fact that relative intensities can be affected by ionization yields and the gas-phase stabilities of the different species, numerous trends observed in the MALDI study were consistent with the expected in-solution behaviors. Experimental conditions related to sample preparation were investigated also. Complex abundance generally decreased when increasing the ammonium acetate concentration. It was dramatically decreased when using ATT instead of DHAP. Penta-L-arginine is an exception to these observations. Lastly, in the case of complexes involving DNA duplex, the ATT matrix was shown to favor the observation of specific DNA duplex but not that of its complex with polybasic compounds. Inversely, DHAP was appropriate for the conservation of DNA-polybasic compound interaction but not for the transfer of intact duplex.     

Perturbation of the intramolecular hydrogen bonds of glucose by Cu+ association

A density functional theory study of glucose and glucose–Cu⁺ complexes has been performed to investigate the changes undergone by the set of intramolecular hydrogen bonds of the neutral system upon Cu⁺ association. The geometries of the different species investigated were optimized at the B3LYP/6‐31G(d,p) level. The same level of theory was used to obtain the harmonic vibrational frequencies and to analyze the electron charge density by means of the atoms in molecules theory. We have shown that the interaction with Cu⁺ strongly perturbs the set of intramolecular hydrogen bonds of the neutral. Some of these changes are a direct consequence of the conformational changes induced by the metal, which result in the breaking of some of the existing bonds or in the formation of new ones. The most important point, however, is that the intramolecular hydrogen bonds that remain are perturbed to a different extent. In general, all hydrogen bonds in which the OH donor interacts directly with the metal cation are significantly stabilized while the remaining ones become weaker. These changes influence the relative stability of the complexes as well as its capacity to interact with other systems. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Synthese totale et stereoselective du dimethyl-6,10 methylidene-9 undecene-5 (e) one-2: un des constituants de l'huile de Costus

The first total and stereoselective synthesis of (E)-6, 10 dimethyl-9 methylidene undec-5-en-2-one: a constituant of Costus Root oil is reported. It was conducted with an overall yield of 22% from 1-methyl-2-oxo-methylcyclopentane carboxylate (α-methylated Dieckmann-ester) $\text{2}$, used as a starting material.     

Vibrational dephasing effects and motional narrowing in the isotropic Raman Q branch of HD

Important motional narrowing has been previously experimentally observed by Dion and May in the isotropic Raman Q branch of pure gaseous HD in the fundamental vibrational band. A recent theoretical calculation of the present authors, in good agreement with these experiments, has shown that for pure HD only rotational mechanisms are efficient. In this paper, an infinite order treatment of the vibrational dephasing effects for HD perturbed by rare gases indicates an important role of these last effects in the broadening of the individual lines (particularly in the 0–2 band) leading to a significant decrease of the motional narrowing in the resulting band profile at high density.     

Carbon-13 NMR studies of nitrogen compounds. I—substituent effects of amino,acetamido, diacetamido,ammonium and trimethylammonium groups

¹³C NMR chemical shifts are reported for several alkyl or aryl amines, acetamides, diacetamides and ammonium and trimethylammonium salts. Substituent effects, determined with respect to chemical shifts of amino derivatives, are rationalized in terms of γ effects, steric interactions and for benzene derivatives, in terms of competition between inductive (σ₁) and resonance (σ_R) effects. These perturbations can be used to assign chemical shifts of complex structures and to study certain chemical processes.     

Gas-phase deprotonation of uracil-Cu2+ and thiouracil-Cu2+ complexes

The deprotonation of Cu2+ complexes with uracil, 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil has been investigated by means of B3LYP/ 6-311+G(2df,2p)//6-31G(d) calculations. The most stable [(uracil-H)Cu]+ and [(thiouracil-H)Cu]+ complexes correspond to bidentate structures in which Cu interacts with the deprotonated ring-nitrogen atom and with the oxygen or the sulfur atom of the adjacent carbonyl or thiocarbonyl group. For 2- and 4-thiouracil derivatives, the structures in which the metal cation interacts with the thiocarbonyl group are clearly favored with respect to those in which Cu interacts with the carbonyl group. This is at variance with what was found to be the most stable structure of the corresponding Cu2+ complexes, where association to the carbonyl oxygen was always preferred over the association to the thiocarbonyl group. The [(uracil-H)Cu]+ and [(thiouracil-H)Cu]+ complexes can be viewed as the result of Cu+ attachment to the uracil-H and thiouracil-H radicals formed by the deprotonation of the corresponding uracil+* and thiouracil+* radical cations. As a matter of fact their relative stability is dictated by the intrinsic stability of the corresponding uracil-H and thiouracil-H radical and by the fact that, in general, the N3-deprotonated site is a better electron donor than the N1. In all complexes, the bonding of Cu both to nitrogen and sulfur and to nitrogen and oxygen has a significantly large covalent character.