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Cisplatin (cis-diamminedichloroplatinum(II), cis-[PtCl2(NH3)2]), widely used drug in cancer treatment, has been allowed to react with simple molecular targets (L) mimicking biological functional groups. The selected molecules (L = acetamide, dimethylacetamide, urea and thiourea) react by ligand exchange leading to cis-[PtCl(NH3)2(L)]+ complexes that have been assayed by ESI-MS, IRMPD spectroscopy and computations in order to characterize their structure and platination site. Formal five-coordinate complexes are also delivered by ESI, [(PtCl(NH3)2(H2O)(L)]+, notably absent only when L is thiourea. IRMPD spectroscopy combined with computational analysis has revealed non-covalent adducts of the reactant aqua complex with an external ligand L corresponding to {cis-[PtCl(NH3)2(H2O)]+ · L}, reminiscent of the Eigen-Wilkins encounter complex invoked in the ligand displacement path in solution. The complex, successfully isolated in the gas phase, undergoes ligand exchange yielding cis-[PtCl(NH3)2(L)]+ + H2O when activated by multiple IR photons, testifying at the same time both structure and reactivity.

The structure and the dynamics of the isomerization of an ions, which are observed upon low-energy collision induced dissociation of protonated peptides in tandem mass spectrometry (MS/MS), are investigated using a combination of gas phase infrared spectroscopy and theory. IR spectra in the fingerprint region are discussed, but a particular emphasis is given to the NH stretching region which turns out to be highly structurally diagnostic. Overall, theory and infrared spectroscopy provide compelling evidence that an ions undergo cyclisation and/or rearrangement reactions. In the cases of the a2 and a3 ions of oligoglycine, the analysis of the NH stretching region is fully consistent with our previous conclusions based on the IR fingerprint spectra. In the case of the a4 ions of oligoglycine, a band observed near 3550 cm−1 provides a clear-cut signature of the permuted imine–amide structure, thus allowing for a better structural assignment. The dynamics of the rearrangement chemistry of the imine–amide structure is discussed in details, and a critical discussion on the influence of the experimental CID conditions is proposed in the case of the YG a2 ion.

S-nitrosation of cysteine plays an important role in storage and transport of NO, a key signaling molecule in vivo . An approach to detect this modification in the bare, charged amino acid is presented, based on IR multiple photon dissociation (IRMPD) spectroscopy. Protonated and deprotonated S-nitrosocysteine ions, [SNOCys+H] + and [SNOCys−H] − , have been obtained by electrospray ionization and assayed for IR activity in either the 1000–1900 cm −1 or the 3000–3600 cm −1 wavenumber range. The so-obtained IRMPD spectra display characteristic features ascribed to the presence of the S-nitrosation motif, which are missing in the corresponding IRMPD spectra of the native cysteine ions, [Cys+H] + and [Cys−H] − . In particular, the NO stretching mode is unambiguously identified by the red shift observed for the 15 N-labelled species. The interpretation of the IRMPD spectra is supported by density functional theory calculations of the optimized geometries, relative energies and IR spectra of [SNOCys+H] + and [SNOCys−H] − . Both sampled ions comprise a thermally averaged population of conformers contributing to the experimental IRMPD spectra. This notion is supported by the agreement between the calculated IR absorption spectra of the several conformers, and the recorded IRMPD spectrum. The gathered evidence points to a characteristic NO stretching mode that emerges as a pronounced feature at 1460–1490 cm −1 in the IRMPD spectrum of [SNOCys−H] − , namely in a region where [Cys−H] − displays no IRMPD activity. Conversely, the NO stretching vibration of [SNOCys+H] + is enclosed in a wide absorption including the C O stretching mode at 1780 cm −1 . The [SNOCys−H] − negative ions are thus a promising benchmark in a search for S-nitrosation features using IRMPD spectroscopy.

The first IR spectrum of a perfluorinated carbanion has been obtained in the gas phase by IRMPD spectroscopy. Quantum chemical calculations at the MP2/cc-pVTZ level were performed yielding the optimized geometries and IR spectra for a covalently bound C 2 F 5 − species and for conceivable loosely bound F − (C 2 F 4 ) complexes. Both the computational results and the IR characterization point to a covalent structure for the assayed species in agreement with the reactivity pattern displayed with selected neutrals.

IRMPD spectroscopy in the ‘fingerprint’ and X H (X = C, N, O) stretching regions was used to probe the structures of the YG a 2 ions generated from protonated YGGFL and doubly protonated YGGFLR. Our experiments indicate a mixture of cyclic and rearranged ‘imine–amide’ structures. The cyclic isomer is generated from the initially formed protonated imine terminated linear structure by head-to-tail cyclization. Proton transfer between the secondary amine of the ring and the amide nitrogen followed by ring opening leads to the rearranged ‘imine–amide’ isomer. Quantum chemical calculations demonstrate that this proton transfer is catalyzed by the tyrosine side chain ring for the YG a 2 ion. Isomer specific IRMPD bands observed in the two spectral regions clearly show the presence of the cyclic and rearranged ‘imine–amide’ isomers, the latter being characterized by an IR signature at ∼3545 cm −1 associated with the C-terminal amide NH 2 asymmetric stretch.

Protonated furan (C4H5O+, furanH+) and protonated pyrrole (C4H6N+, pyrroleH+) are generated by chemical ionization of the respective parent molecules in the cell of an FT-ICR mass spectrometer using CH5+/C2H5+ as protonating agents. The protonation site is investigated by resonant infrared multiphoton dissociation (IRMPD) spectroscopy in the 900–1700 cm−1 fingerprint range employing the free electron laser (FEL) at the Centre Laser Infrarouge Orsay (CLIO). Comparison with quantum chemical calculations at the B3LYP/6-311G(2df, 2pd) level of theory demonstrates unambiguously that only the Cα protonated isomers are observed, which correspond to the global minima on the potential energy surfaces of both protonated heterocyclic molecules. Spectroscopic features corresponding to protonation at the Cβ atom or at the heteroatom are not detected. The IRMPD spectra correspond to the first spectroscopic identification of both protonated heterocyclic molecules in the gas phase. During the course of the experiments, the IRMPD spectrum of the furan radical cation (C4H4O+, furan+) has been detected as well. Comparison of the IR spectra of the neutral molecules with the IRMPD spectra of the radical cation and the protonated species reveals the effects of both ionization and protonation on the structural properties of these fundamental heterocyclic molecules.

The protonated leucine methyl ester structure was probed using mid-infrared multiphoton dissociation (IRMPD) spectroscopy performed at CLIO, the Orsay Free Electron Laser facility. A first experimental spectrum was obtained with a Fourier-transform ion-cyclotron-resonance mass spectrometer with ions generated through a MALDI process. A second spectrum was recorded with ions generated using ElectroSpray Ionisation and trapped in a Paul ion-trap. These two experimental spectra are analyzed and compared with infrared absorption spectra derived from hybrid density functional theory calculations. The two IRMPD spectra are in excellent agreement, although the one recorded with the Paul ion-trap presents a better resolution with an fwhm of the IRMPD bands of 20–25 cm −1 . Comparison with the calculated IR absorption spectra clearly shows that only the lowest energy isomer is formed under both experimental conditions. The CO stretch, together with two vibrational modes of the ammonium group were the three infrared signatures identified in the photon energy range explored here.

Using Fourier transform ion cyclotron resonance mass-spectrometry (FT-ICR-MS) in combination with infrared multiphoton dissociation (IRMPD) spectroscopy, at the free electron laser (FEL) facility CLIO in Orsay (France), we obtain the IR spectra, in the 900–1900 cm −1 range, of two model protonated dipeptides: N -acetyl-alanine (AcNH-Ala) and alanyl-histidine (Ala-His). By comparison with simulated spectra, calculated at the B3LYP/6-31++G ** level for many low-lying possible conformers of these two species, we are able to assign the position of the protonation site, on the acetyl oxygen for AcNH-Ala and on the side-chain imidazole nitrogen for Ala-His, and to obtain some more information on the low-lying equilibrium structures of these two species in the gas phase.

The infrared spectroscopy of gaseous ions Fe(CH3OCH3)2+ and Fe(CH3OCH2CH2OCH3)n+ (n=1–2) has been studied in the 800–2000 cm−1 energy range using the coupling of the free electron laser CLIO and an FTICR mass spectrometer, and compared to spectra calculated by ab initio quantum chemistry. The match between experimental and theoretical infrared spectra appears to be very good, the experimental spectra corresponding to the most stable structure predicted by the calculations. Characteristic absorption bands of functional groups have been evidenced, allowing this coupling to be a very powerful tool for structure elucidation in the gas phase.

First example of coupling a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FTICR-MS) with an infrared Free Electron Laser (FEL) is presented. This experimental setup is ideally suited for the direct structural characterization of reactive polyatomic ions. Ultrasensitive measurements of the infrared vibrational spectrum of ionic reactive intermediate selectively prepared is allowed by the association of the high peak power of the FEL, its wide tunability, and the flexibility of FTICR-MS, where several mass selections and ion-molecule reactions can be combined. These possibilities are demonstrated in the case of Fe + complexes where two photofragmentation pathways compete. The resulting infrared spectrum is in excellent agreement, both with respect to the position and to the relative intensities of the infrared transitions, with predicted by ab initio electronic structure calculations. r 2003 Elsevier Science B.V. All rights reserved.

Analysis of the evolution of the binding energy to a late first-row transition metal cation M+ shows that the third ligand is significantly less strongly bound than the two first. The present study suggests that a new type of cluster-assisted reactions could be induced by the metal center when it is tri-coordinated. Cu+ complexes have been chosen as models for the quantum chemical calculations (density functional and post-Hartree–Fock ab initio). First, an emphasis is made on the analysis of the evolution of the binding energy as the coordination number n varies from 1 to 4 in Cu+(H2O)n and Cu+(CO)n. The evolution of the electronic repulsion between the σ donating orbitals of the ligands and the 3d orbitals is discussed in details. Second, we consider the successive substitution reactions of CO by H2O ligand from Cu+(CO)n (n=1–4). Our calculations suggest that rather than the simple substitution reaction of CO by ROR′ (R, R′=H, alkyl), formation of an acid or ester could occur in the coordination sphere of the metal. Whereas kinetics is likely to be unfavorable in the case of Cu+, such condensation reaction of ROR′ and CO could be observed with late first-row transition metal cations such as Fe+.

A valence-bond curve-crossing model is proposed to account for the shape of the energy profile of the hydrogen exchange in HCN and isovalent molecules HXY. The adiabatic energy profile associated to the 1,2-hydrogen shift as a function of the bending angle might display one, two or three minima. We propose that these energy profiles can be modeled as the avoided-crossing of three diabatic curves. Ab initio valence-bond calculations performed on HCN/CNH support this model suggesting that the preference for the bent HXY structures versus the linear ones essentially correlates with the 2 Σ – 2 Π splitting of the XY moiety.

The electronic ground state and the three first excited states of AgNH 3 have been investigated theoretically. At the ab initio CASPT2 level, there is a quasi-degeneracy of the C 3 v minima of the 2 A 1 and 2 E excited states ( D e =1.38 and 1.40 eV, respectively). While the 2 E state displays a clear Ag(5p π )+NH 3 character, the nature of the 2 A 1 is more subtle, with both charge transfer and Rydberg characters. Semi-empirical introduction of the spin–orbit coupling enables the interpretation of a few features of the intricated R2PI spectrum.

Ab initio CASPT2 calculations show that the Ag(NH3)2 cluster displays two isomers (surface and interior), both in its ground and first excited states. In the ground state the surface structure is slightly preferred, whereas in the three first (quasidegenerate) excited states the interior structure is strongly preferred, like in the ionic cluster. This feature explains the fall of the ion signal observed (on the ns scale) in the R2PI spectrum: the signal vanishes as the excited cluster passes the isomerisation barrier. Comparison with the Na(NH3)2 system is made.

In a recent study, we reported experimental determinations of bond dissociation energies and entropies for V + (H 2 ) n ( n = 1 to 6) and suggested that a spin change from quintet to triplet occurs upon addition of the sixth H 2 ligand. A very recent theoretical paper by Niu et al. disputes this and concludes that the experimental results are consistent with purely quintet clusters (J. Niu, B.K. Rao, S.N. Khanna and P. Jena, Chem. Phys. Letters 230 (1994) 299). In this Comment, we restate our reasons for the proposed spin charge and argue that important aspects of the results of Niu et al. are incorrect.

The structures of small mixed iron carbon cluster anions, Fe n C − m ( n =1, 2, 3 and m =2–8) have been studied using gas phase ion chromatography. In the one iron atom case, results show a competition between linear structures and two-dimensional (planar) clusters. The two iron atom clusters, on the other hand, are pure monocyclic rings with the two iron atoms nearest neighbors. The three iron species are three-dimensional, and several possible structures are evaluated. In all cases the carbon atoms maintain a continuous chain and no carbon dimers are observed that would be characteristic of MetCar precursors.

We have analyzed the CCCI wavefunction for the calculation of bond dissociation energies. In the first part, CCCI is reexpressed as a non-orthogonal valence bond CI wavefunction which describes bond formation as the mixing of one covalent and two ionic parent configurations. Every other configuration is considered as an improvement of one of these three building blocks. They are grouped into different classes, the contribution of which are successively analyzed. We found the dynamical correlation of ionic structures (only one for a polar bond such as HSiH 3 or HGeH 3 ) to be of prime importance in obtaining accurate dissociation energies. In the second part, we improve CCCI by extending both the one- and N -electron bases, which lead to an increase and decrease of the computed bond energies, respectively. We thus conclude that the high accuracy of CCCI at the DZP level results from a cancellation of limited errors. The highest level used is uniformly accurate and yields D e of 110.9, 96.3 and 92.5 kcal/mol for CH 4 , SiH 4 and GeH 4 respectively.

Three bond types of electron-pair bonding emerge from multi-structure valence bond (VB) computations of 10 different single bonds. The first bond type is observed in HH, LiLi, CH and SiH. These are all covalent bond types whose major bonding comes from the covalent Heitler-London (HL) configuration, with a minor perturbation from the resonance interaction between the covalent and zwitterionic (Z) configurations. The second bond type is observed for NaF. This is an ionic bond type in which the major bonding is provided by the electrostatic stabilization of the ionic configuration, Na + F − , with a slight perturbation from the HLZ resonance interaction. The third bond type is observed for FF, HF, CF and SiF. These are the resonating bond types in which the major bonding event is the resonance energy stabilization due to the HLZ mixing. No special status should be attached to either the covalency or ionicity of these last bond types, even if they may appear purely “covalent”, such as FF, or “highly ionic”, as CF, by charge distribution criterion. The phenomenon of resonating bonding is shown to emerge from weakly bound or unbound covalent HL configurations which originate when the “preparation” for bonding of the fragments becomes energy demanding, as for fluorine. The mechanism of HL bond weakening is through costly promotion energy and overlap repulsion of a lone pair with a bond pair of the same symmetry. The essential requirements for a fragment A to qualify as a resonating binder are therefore: (a) to possess two AOs which maintain a very large energy gap between them, and which by virtue of overlap capability can both enter into bonding; and (b) to have three electrons in these two AOs which thereby mutually antagonize each other's bonding. The propensity for resonating bonding is discussed, in the light of these qualifications, for the main elements across the Periodic Table. It is concluded that the elements with the highest propensity for resonating bonding are F, O and N. Any combination AB where either A or B or both are resonating binders is likely to lead to a resonating bond (e.g. OO, NF, CF, CO, and so on). The resonating bonds are shown to coincide with the group of “weakened” bonds in the classification of Sanderson, and with those bonds which exhibit negative or marginally positive deformation densities in electron density determinations. Negative or marginally positive deformation densities may serve as the experimental signature of the theoretical concept of resonating bonding. The LiH bond appears to possess a special status. While the computations tend to classify this bond among the covalent types, the results also show that the HL and ionic Li + H − configurations are nearly degenerate and maintain a very weak coupling. Therefore the LiH bond will have a metastable character, as far as ionicity-covalency, in the presence of medium perturbations which are at least of the magnitude of the coupling between the ionic and covalent structures.

Ab initio calculations have been performed at various levels on the SiH5 hypervalent radical in D3h and C4v symmetry. Optimization at UHF and UMP2 are presented for these structures and show that both of them are neither stable nor metastable species. The calculation of a potential energy surface at the CASSCF level confirms the transition state nature of the radical in its D3h geometry.