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Abstract: Binding to proteins and peptides in condensed phases, it is normal for alkali and alkaline earth metal ions to interact preferentially with Lewis-basic oxygen, nitrogen and similar open chelation points, while late transition metals like cobalt, nickel and copper characteristically deprotonate and bind to amide nitrogens along the peptide chain. Parallel to these contrasting condensed-phase binding-mode alternatives, metal ions in the gas phase can form complexes with small peptides in several complexation modes, among them the charge-solvated (CS) and the Iminol patterns. Reported here is a computational study of the factors determining the choice between these patterns in the gas phase for model ligands, dialanine and trialanine, also including illustrative experimental spectroscopic results for Ag+(Ala)3 using the infrared multiple photon dissociation (IRMPD) technique (which has also provided previous experimental results for many of the ions studied here). Across a survey of 29 metal ions in normal oxidation states (+1, +2 and +3), unexpectedly strong correlations are found (for each charge state) between the preference for CS versus Iminol binding and the overall binding energies of the ions. Ions of +1 charge invariably prefer CS binding, while those with higher charge exhibit variable preferences. Within a given charge state, Iminol binding is more favorable, and overall binding is stronger, for light metal ions and for metal ions (“transition metals”) late in the periodic table. The tendency to go from CS to Iminol in the gas phase is generally parallel to the tendency to bind deprotonated amide nitrogens in condensed-phase, but with possible divergence between the differing environments at the point where the tendencies cross over near Mg(II). Hard/soft character of the metal ions correlates to some extent with the binding preferences, but this correlation shows numerous discrepancies. For “main-group” metal ions, electrostatic character of the binding is suggested by excellent scaling of binding energies with a scaling parameter q/R, while a contribution of enhanced binding in addition to the electrostatic binding energy is indicated for “transition” metals. [Copyright &y& Elsevier]

A gas-phase radical rearrangement through intramolecular hydrogen-atom transfer (HAT) was studied in the glutathione radical cation, [γ-ECG]+., which was generated by a homolytic cleavage of the protonated S-nitrosoglutathione. Ion-molecule reactions suggested that the radical migrates from the original sulfur position to one of the α-carbon atoms. Experiments on the radical cations of dipeptides derived from the glutathione sequence, [γ-EC]+. and [CG]+., pointed to the glutamic acid α-carbon atom as the most likely site of the radical migration. Infrared multiple-photon dissociation (IRMPD) spectroscopy was employed to generate complementary information. IRMPD of [γ-ECG]+. in the approximately 1000-1800 cm−1 region was inconclusive owing to the relatively broad, overlapping absorption bands. However, the IRMPD spectrum of [γ-EC]+. in this region was consistent with the radical migrating from the sulfur to the α-carbon atom of glutamic acid. IRMPD in the 2800-3700 cm−1 region performed on [γ-ECG]+. is consistent with a mixture of both the original sulfur-based radical and the resulting glutamic acid α-carbon-based species. Comparisons are made with previously published condensed and gas-phase studies on intramolecular HAT in glutathione. [ABSTRACT FROM AUTHOR]

Gas phase infrared dissociation spectra of the radical cation, deprotonated and protonated forms of the hormone melatonin, and its complexes with alkali (Li+, Na+, and K+) and alkaline earth metal ions (Mg2+, Ca2+, and Sr2+) are measured in the spectral range 800-1800 cm−1. Minimum energy geometries calculated at the B3LYP/LACVP++** level are used to assign structural motifs to absorption bands in the experimental spectra. The melatonin anion is deprotonated at the indole-N. The indole-C linking the amide chain is the most favored protonation site. Comparisons between the experimental and calculated spectra for alkali and alkaline earth metal ion complexes reveal that the metal ions interact similarly with the amide and methoxy oxygen atoms. The amide I band undergoes a red shift with increasing charge density of the metal ion and the amide II band shows a concomitant blue shift. Another binding motif in which the metal ions interact with the amide-O and the π-electron cloud of the aromatic group is identified but is higher in energy by at least 18 kJ/mol. Melatonin is deprotonated at the amide-N with Mg2+ and the metal ion coordinates to the amide-N and an indole-C or the methoxy-O. These results provide information about the intrinsic binding of metal ions to melatonin and combined with future studies on solvated melatonin-metal ion complexes may help elucidate the solvent effects on metal ion binding in solution and the biochemistry of melatonin. These results also serve as benchmarks for future theoretical studies on melatonin-metal ion interactions.ᅟ [ABSTRACT FROM AUTHOR]

Complexes of divalent metal ions (Cu 2+ and Ni 2+ ) with histidine tripeptide complexes (HAA, AHA and AAH) are interesting gas-phase models for some of the most widely observed patterns of metal ion binding to peptides and proteins. Gas-phase structures were characterized using infrared multiple photon dissociation (IRMPD) spectroscopy in ion-trapping mass spectrometers, along with density functional theory (DFT) computations. Ground states are square-planar with two deprotonated amide nitrogens bound to the metal ion via a double iminol rearrangement (IM binding mode), but contrary to expectations based on solution behavior, the histidine imidazole group is not bound to the metal, but instead is hydrogen bonded remote from the metal ion. The alternative “charge-solvated” (CS) binding mode (amide carbonyl oxygens binding the metal ion) lies higher in energy, but in many cases was observed to be present as conformationally unrelaxed ions with an abundance (relative to the IM conformation) that was dependent on instrument configuration and source and trap conditions. Taking advantage of the ability to form and trap both IM and CS conformations for a few of the complexes, the infrared spectroscopy of both conformations was explored in the fingerprint (1000–1800 cm −1 ) and hydrogen-stretching (3200–3800 cm −1 ) regions. For the fingerprint region, agreement is very good between the observed IRMPD spectra and the spectra predicted by DFT calculations at the B3LYP/6–311 + +g(d,p) level. Agreement in the H-stretching region is not perfect, but the characteristic IM and CS spectral patterns are evident. [ABSTRACT FROM AUTHOR]

Synthesis and spectra of isocytosino-8-trifluoromethylquinolone ( 1 ), as well as the gas phase InfraRed Multiple Photon Dissociation (IRMPD) spectra in the fingerprint region of the corresponding deprotonated anion ( 3 ), its d 3 analogue, the monodeprotonated homodimer ( 2 ), and its d 7 analogue are reported here. The anions represent nucleobase analogues having the hydrogen bonding pattern ADAAD (where A stands for acceptor and D stands for donor), in which the site of negative charge is unambiguous (as opposed to guanine, which has more than one acidic nitrogen). The match between experimental vibrational spectra and calculation is good, except for the out-of-plane HNH bends of the undeuterated and deuterated monomer anions between 400 and 600 cm −1 . The anionic homodimers form in a parallel orientation. [ABSTRACT FROM AUTHOR]

Evidence is presented for cyclization to yield 2-bora-1,3-diazacycloalkanium cations in the gas phase. While the neutral compounds in solution and solid phase are known to possess an acyclic structure (as revealed by X-ray diffraction), the gaseous cations (from which borohydride BH4- ion has been expelled) have a cyclic structure, as revealed by InfraRed Multiple Photon Dissociation (IRMPD) spectroscopy and collisionally activated decomposition (CAD). The IRMPD decomposition of the monocyclic ions proceeds principally via H2 expulsion, although CAD experiments show additional pathways. Pyrolyses of solid monomeric salts and small oligomers produce higher polymers that are consistent with H2 expulsion as the major pathway. Deuterium labeling experiments show that scrambling occurs prior to IRMPD or CAD decomposition in the gas phase. [ABSTRACT FROM AUTHOR]

In the complex formed between the calcium cation (Ca 2+ ) and a deprotonated HisHis dipeptide, the complex adopts a charge solvation (CS) structure. Ca 2+ , a weak binding main group metal cation, interacts with the oxygens of the peptide carbonyl moiety and the deprotonated C-terminus. In contrast, the much stronger binding Ni 2+ cation deprotonates the peptide nitrogen and induces an iminolate (Im) ligand structure in the [Ni(HisHis-H)] + complex ion. The combination of infrared multiple-photon dissociation (IRMPD) spectroscopy and quantum chemistry evidence these two representative binding motifs. The iminolate coordination pattern identified and characterized in the [Ni(HisHis-H)] + complex serves as a model case for nickel complexes of poly-histidyl-domains and is thereby also of interest to better understand the fundamentals of immobilized metal ion affinity chromatography as well as of Ni co-factor chemistry in enzymology. [ABSTRACT FROM AUTHOR]

Infrared multiple-photon dissociation (IRMPD) spectroscopy has been used to examine the gas-phase conformations of a series of short protonated polyproline ions (Pro 3 –Pro 6 ), their CID/IRMPD fragmentation pathways, and the associated fragment identities. Consistent with previous findings, and in combination with density functional theory (DFT) and MM/MD methods, a series of conformers for the protonated parent ions having their first peptide bond in the cis conformation has been identified. This conformation maximizes the solvation of the protonated N-terminus and stabilizes these compact globular-type conformations. This is in contrast to the PPI and PPII polyproline-type helices reported for larger polyproline peptides in solution. As well, this conformation leads to a unique fragmentation pattern upon collisional or multiple-photon activation. We report observation of the uncommon, but thermodynamically favored, diketopiperazine-type b 2 + fragment ion. Formation of b 2 + ions along the diketopiperazine pathway is in line with a cis configuration of the first amide linkage in the protonated parent ion. Additionally, the parent ion conformations, fragmentation pathways, and proton affinities of the resulting fragments have been related to the observed proline-effect in CID mass spectra. [ABSTRACT FROM AUTHOR]

Gas-phase complexes of five metalions with the dipeptide HisGlyhave been characterized by DFT computations and by infrared multiplephoton dissociation spectroscopy (IRMPD) using the free electron laserFELIX. Fine agreement is found in all five cases between the predictedIR spectral features of the lowest energy structures and the observedIRMPD spectra in the diagnostic region 1500–1800 cm–1, and the agreement is largely satisfactory at longer wavelengthsfrom 1000 to 1500 cm–1. Weak-binding metal ions(K+, Ba2+, and Ca2+) predominantlyadopt the charge-solvated (CS) mode of chelation involving both carbonyloxygens, an imidazole nitrogen of the histidine side chain, and possiblythe amino nitrogen. Complexes with Mg2+and Ni2+are found to adopt iminol (Im) binding, involving the deprotonatedamide nitrogen, with tetradentate chelation. This tetradentate coordinationof Ni(II) is the preferred binding mode in the gas phase, againstthe expectation under condensed-phase conditions that such bindingwould be sterically unfavorable and overshadowed by other outcomessuch as metal ion hydration and formation of dimeric complexes. TheHisGly results are compared with corresponding results for the PheAla,PheGly, and PhePhe ligands, and parallel behavior is seen for thedipeptides with N-terminal Phe versus His residues. An exception isthe different chelation pattern determined for PhePhe versus HisGly,reflecting the intercalation-type cation binding pocket of the PhePheligand. The complexes group into three well-defined spectroscopicpatterns: nickel and magnesium, calcium and barium, and potassium.Factors leading to differentiation of these distinct spectroscopiccategories are (1) differing propensities for choosing the iminolbinding pattern, and (2) single versus double charge on the metalcenter. Nickel and magnesium ions show similar gas-phase binding behavior,contrasting with their quite different patterns of peptide interactionin condensed phases. [ABSTRACT FROM AUTHOR]

The reactivity of the cysteine (Cys) and homocysteine(Hcy) radicalcation was studied using ion–molecule reactions. The radicalcations were generated via collision-induced dissociation (CID) oftheir S-nitrosylated precursors. Cleavage of the S–NO bondled to the formation of the radical initially positioned on the sulfuratom. The reactions of the radical cations with dimethyl disulfiderevealed that the cysteine radical cation reacts more quickly thanthe homocysteine radical cation. Infrared multiple photon dissociation(IRMPD) spectroscopy and density functional theory (DFT) calculationswere used to determine the structure of the homocysteine radical cation,which was compared to the previously published structure of the cysteineradical cation (Sinha et al.Phys. Chem. Chem. Phys.2010, 12, 9794–9800). IRMPDspectroscopy and DFT calculations revealed that this difference inradical reactivity was not a result of a radical rearrangement forthe homocysteine radical cation but rather that the reactivity wasmodulated by stronger hydrogen bonding. [ABSTRACT FROM AUTHOR]

Abstract: Infrared multiple-photon dissociation (IRMPD) spectroscopy was used to probe the conformations of gas-phase metal-ion complexes between a series of five metal ions and six small peptide ligands. This report is presented in recognition and tribute for the Armentrout group''s long and hugely productive interest in metal-ion binding to gas-phase ligands. The metal ions (K+, Ba2+, Ca2+, Mg2+, Ni2+) span a range of ligand binding strengths, and the ligands include several dipeptides and tripeptides, and one tetrapeptide. The weaker metal ions generally form charge-solvated (CS) complexes binding amide carbonyl oxygen, while the strongest metal ion, nickel, deprotonates the amide nitrogens, probably through iminol tautomerization, and binds to the amide nitrogens. The Amide II vibrational mode (1500–1550cm−1) is found to be an excellent marker for the presence or absence of protons on amide nitrogens in a complex. The magnesium ion marks a boundary between these two structural motifs, forming iminol complexes with the dipeptides and switching to CS complexes for the tripeptides FGG and FGGF. Compared with solution-phase behavior, the iminol binding mode shown by Mg2+ for the smallest peptides is surprising, since this ion is considered as generally binding in a CS mode in solution. The present results for the larger peptides reconcile this surprising difference, showing that larger peptide ligands revert to the expected CS binding pattern for gas-phase Mg2+. [Copyright &y& Elsevier]

Abstract: Collision-induced dissociation (CID) of protonated 2-hydroxynicotinic acid (2-OHNic) generates a dominant product ion through loss of 18mass units, presumably the elimination of water. Subsequent isolation and storage of this product ion in the gas-phase environment of an ion trap mass spectrometer, without imposed collisional activation, shows that the species undergoes addition reactions to furnish new products that are higher in mass by 18 and 32units. Density functional theory (DFT) calculations suggest that an acylium ion (i.e., loss of H2O from the acid group) is energetically more favored than is a species generated by elimination of H2O from the hydroxypyridine ring. Formation of the acylium product is confirmed by comparing the infrared multiple photon dissociation (IRMPD) spectrum to theoretical spectra from (DFT) harmonic calculations for several possible isomers. A thorough DFT study of the reaction dynamics suggests that the acylium ion is generated from the global minimum for the protonated precursor along a pathway that involves proton transfer from the hydroxypyridine ring and elimination of he acid group. [Copyright &y& Elsevier]

Abstract: The gas-phase IR spectrum of the anionic chromophore of the green fluorescent protein (p-hydroxy-benzylidene-2,3-dimethylimidazolidinone, HBDI) is recorded in the 800–1800cm−1 frequency range using the free electron laser FELIX in combination with an electrospray ionization (ESI) Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. The spectrum is substantially different from IR spectra of the anion recorded previously in solution, which were found to be difficult to interpret on the basis of electronic structure calculations involving the polarisable continuum model (PCM) method. In contrast, the IR spectrum of the isolated anion recorded here matches favourably with its DFT calculated counterpart if diffuse functions are included in the basis set. IR photo-fragmentation of the HBDI anion proceeds via loss of a methyl radical (CH3 ng in an odd-electron product anion. The IR spectrum of this radical anion photoproduct is also recorded, which indicates that the radical site resides on the imidazolinone nitrogen atom where the methyl group is detached. [Copyright &y& Elsevier]

In uranyl coordination complexes, UO 2 (L) n 2+ , uranium in the formally dipositive [O=U=O] 2+ moiety is coordinated by n neutral organic electron donor ligands, L. The extent of ligand electron donation, which results in partial reduction of uranyl and weakening of the U=O bonds, is revealed by the magnitude of the red-shift of the uranyl asymmetric stretch frequency, ν 3 . This phenomenon appears in gas-phase complexes in which uranyl is coordinated by electron donor ligands: the ν 3 red-shift increases as the number of ligands and their proton affinity (PA) increases. Because PA is a measure of the enthalpy change associated with a proton-ligand interaction, which is much stronger and of a different nature than metal ion-ligand bonding, it is not necessarily expected that ligand PAs should reliably predict uranyl-ligand bonding and the resulting ν 3 red-shift. Here, ν 3 was measured for uranyl coordinated by ligands with a relatively broad range of PAs, revealing a surprisingly good correlation between PA and ν 3 frequency From computed ν 3 frequencies for bare UO 2 cations and neutrals, it is inferred that the effective charge of uranyl in UO 2 (L) n 2+ complexes can be reduced to near zero upon ligation by sufficiently strong charge-donor ligands. The basis for the correlation between ν 3 and ligand PAs, as well as limitations and deviations from it, are considered. It is demonstrated that the correlation evidently extends to a ligand that exhibits polydentate metal ion coordination. [ABSTRACT FROM AUTHOR]

Gas-phase organoactinyl complexes possessing discrete An-C bonds (An = U, Np, Pu) were synthesized in a quadrupole ion trap by endothermic decarboxylation of [AnO2(O2C-R)3]- anion complexes in which a formally AnO22+ actinyl core is coordinated by three carboxylate ligands, with R = CH3 (methyl), CH3CC (1-propynyl), C6H5 (phenyl), C6F5 (pentafluorophenyl). Decarboxylation and competing ligand loss were studied computationally by density functional theory complementing experiment. Although decarboxylation was computed to be the energetically most favorable process in all cases, reduction from An(VI) to An(V) via neutral ligand loss was often prevalent, particularly for An = Np, Pu, presumably resulting from barriers associated with decarboxylation. Comparative hydrolysis rates of the An-C bonds were experimentally determined, and the chemical properties of these bonds were analyzed by the quantum theory of atoms in molecules. The measured hydrolysis rates differed by up to 3 orders of magnitude: the fastest was for [(CH3CC)UO2(O2C-CCCH3)2]- and the slowest for [(C6F5)PuO2(O2C-C6F5)2]-. There is a general correlation between hydrolysis exothermicity and hydrolysis rate. Prototypical hydrolysis reaction pathways computed for R = CH3 (An = U, Np) reveal a mechanism in which an outer-sphere water becomes inner-sphere concomitant with transfer of an H atom to yield an OH ligand and CH4, with a net energy release of 170 kJ mol-1 and a transition state barrier of 45 kJ mol-1 for An = U. Infrared multiphoton dissociation spectra of selected complexes were acquired to confirm the predicted structures by agreement between the computed and observed vibrational frequencies. The experiment and theory results provide an evaluation of the comparative propensities for formation of the organoactinyls as a function of actinide and carboxylate and an assessment of the nature and stability toward hydrolysis of the primarily ionic An-C bonds. [ABSTRACT FROM AUTHOR]

Four isomeric sugar methylglycosides (α- and β-d-gluco- and galactopyranosides) were evaluated as rubidium cation coordination adducts in the gas phase using variable-wavelength multiple-photon dissociation in the range from 2750 to 3750 cm–1. The adducts dissociated following photon absorption to yield neutral sugars and the rubidium cation, resulting in infrared “action” spectra. Well-resolved hydroxyl stretching bands clearly differentiate stereoisomers that vary solely in their asymmetry at single carbons. Density functional theory calculations of the lowest-energy gas-phase complexes indicate that rubidium coordinates with lone pairs of oxygen atoms as either bi- or tridentate complexes and that more than one positional coordination isomer could adequately account for most of the O–H stretch frequencies observed for each methylglycoside. [ABSTRACT FROM AUTHOR]

The syntheses of cationic nickel complexes using N,N'-dimethyl piperazine 2,3-dithione (Me2Dt0) and N,N'-diisopropyl piperazine 2,3-dithione (iPr2Dt0) ligands are reported. These ligands were used in synthesizing bis and tris(dithione)Ni(II) complexes as tetrafluoroborate or hexafluorophosphate salts, i.e., [Ni(iPr2Dt0)2][BF4]2 ([1a][BF4]2), [Ni(iPr2Dt0)2][PF6]2 ([1a][PF6]2), [Ni(Me2Dt0)2][BF4]2 ([1b][BF4]2), [Ni(iPr2Dt0)3][BF4]2 ([2a][BF4]2), and [Ni(iPr2Dt0)3][PF6]2 ([2a][PF6]2), respectively. Complex [2a][PF6]2 was isolated from a methanolic solution of [1a][PF6]2. Compound [1a][BF4]2 crystallizes in a trigonal crystal system (space group, P31/c) and exhibits unique packing features, whereas [2a][BF4]2 crystallizes in a monodinic (P21/n) space group. Cyclic voltammograms of [1a][BF4]2 and [1b][BF4]2 are indicative of four reduction processes assodated with stepwise single-electron reduction of the ligands. Spectroelectrochemical experiments on [1a][BF4]2 exhibit an intervalence charge transfer (IVCT) transition as a spectroscopic signature of the mixed-valence [Ni(iPr2Dt0)-(iPr2Dt1-)]- spedes. Analysis of this IVCT band suggests that this ligand based mixed valence complex, [Ni(iPr2Dt0)-(iPr2Dt1-)]-, behaves more like a traditional dass II/IU metal based mixed-valence complex. The density functional theory (DFT) and time dependent DFT calculations provide a theoretical framework for understanding the electronic structures and the nature of exdted states of the target compounds that are consistent with their spectroscopic and redox properties. Vibrational spectra of [1a]2+ and [2a]2+ were investigated as discrete spedes in the gas phase using infrared multiple photon dissodation (IRMPD) spectroscopy. [ABSTRACT FROM AUTHOR]

Thegas-phase complex UO2(TMOGA)22+(TMOGA= tetramethyl-3-oxa-glutaramide) prepared by electrosprayionization was characterized by infrared multiphoton dissociation(IRMPD) spectroscopy. The IRMPD spectrum from 700–1800 cm–1was interpreted using a computational study basedon density functional theory. The predicted vibrational frequenciesare in good agreement with the measured values, with an average deviationof only 8 cm–1(<1%) and a maximum deviationof 21 cm–1(<2%). The only IR peak assigned tothe linear uranyl moiety was the asymmetric ν3mode,which appeared at 965 cm–1and was predicted byDFT as 953 cm–1. This ν3frequencyis red-shifted relative to bare uranyl, UO22+, by ca. 150 cm–1due to electron donation fromthe TMOGA ligands. Based on the degree of red-shifting, it is inferredthat two TMOGA oxygen-donor ligands have a greater effective gas basicitythan the four monodentate acetone ligands in UO2(acetone)42+. The uranyl ν3frequency wasalso computed for uranyl coordinated by two TMGA ligands, in whichthe central Oetherof TMOGA has been replaced by CH2. The computed ν3for UO2(TMGA)22+, 950 cm–1, is essentiallythe same as that for UO2(TMOGA)22+, suggesting that electron donation to uranyl from the Oetherof TMOGA is minor. The computed ν3asymmetric stretchingfrequencies for the three actinyl complexes, UO2(TMOGA)22+, NpO2(TMOGA)22+and PuO2(TMOGA)22+, are comparable.This similarity is discussed in the context of the relationship betweenν3and intrinsic actinide-oxygen bond energies inactinyl complexes. [ABSTRACT FROM AUTHOR]

DNA nucleobases make use of hydrogen bonding, whether in associating to form the Watson–Crick double-helix or in producing alternative structures such as the G-quadruplex or the i -motif. Nucleoside proton-bound dimers provide an avenue for investigating characteristics that they possess within the i -motif and related non-Watson–Crick conformations. In addition, several nucleosides are approved antiviral and anticancer agents. The nucleosides under investigation (2′-deoxycytidine, gemcitabine, and decitabine) are capable of forming proton-bound dimers (PBDs) with their conjugate acids. Protonated monomers of 2′-deoxycytidine, gemcitabine, and decitabine and proton-bound dimers of gemcitabine and decitabine have been produced in the gas phase using electrospray ionization (ESI). This paper reports proton affinities of the neutral nucleosides as well as their infrared multiple photon dissociation (IRMPD) spectra from 2800 to 3800 cm −1 collected using an Optical Parametric Oscillator (OPO) laser. In the case of the conjugate acid of 2′-deoxycytidine, a partial deuteration experiment suppresses overtones and combination bands, leading to the inference that a single tautomer predominates in the protonated monomer. IRMPD spectra of proton-bound dimers of gemcitabine and decitabine suggest furanose sugar ring puckering to be in the South orientation, as they are in the protonated monomers. [ABSTRACT FROM AUTHOR]

Electrosprayionization (ESI) in the negative ion mode was usedto create anionic, gas-phase oxo-molybdenum complexes with dithioleneligands. By varying ESI and ion transfer conditions, both doubly andsingly charged forms of the complex, with identical formulas, couldbe observed. Collision-induced dissociation (CID) of the dianion generatedexclusively the monoanion, while fragmentation of the monoanion involveddecomposition of the dithiolene ligands. The intrinsic structure ofthe monoanion and the dianion were determined by using wavelength-selectiveinfrared multiple-photon dissociation (IRMPD) spectroscopy and densityfunctional theory calculations. The IRMPD spectrum for the dianionexhibits absorptions that can be assigned to (ligand) CC,C–S, CCN, and MoO stretches. Comparisonof the IRMPD spectrum to spectra predicted for various possible conformationsallows assignment of a pseudo square pyramidal structure with C2vsymmetry, equatorial coordinationof MoO2+by the S atoms of the dithiolene ligands, anda singlet spin state. A single absorption was observed for the oxidizedcomplex. When the same scaling factor employed for the dianion isused for the oxidized version, theoretical spectra suggest that theabsorption is the MoO stretch for a distorted square pyramidalstructure and doublet spin state. A predicted change in conformationupon oxidation of the dianion is consistent with a proposed bondingscheme for the bent-metallocene dithiolene compounds [Lauher, J. W.; Hoffmann, R. J. Am.Chem. Soc.1976, 98, 1729−1742], where a largefolding of the dithiolene moiety along the S···S vectoris dependent on the occupancy of the in-plane metal d-orbital. [ABSTRACT FROM AUTHOR]

In an effort to understand the chemical factors thatstabilize dianions, experimental and theoretical studies on the stabilityof the tartrate dianion were performed. Quantum chemical calculationsat the coupled cluster level reveal only a metastable state with apossible decomposition pathway (O2C–CH(OH)–CH(OH)–CO2)2–→ (O2C–CH(OH)–CH(OH))•– CO2 e–explainingthe observed gas-phase instability of this dianion. Further theoreticaldata were collected for the bare dianion, this molecule complexedto water, sodium, and a proton, in both the meso and lformsas well as for the uncomplexed radical anion and neutral diradical.The calculations suggest that the l-tartrate dianion is morethermodynamically stable than the dianion of the meso stereoisomerand that either dianion can be further stabilized by association witha separate species that can help to balance the charge of the molecularcomplex. Mass spectrometry was then used to measure the energy neededto initiate collisionally induced dissociation of the racemic tartratedianion and for the proton and sodium adducts of both the racemicand meso form of this molecule. Infrared action spectra of the dianionstereoisomers complexed with sodium were also acquired to determinethe influence of the metal ion on the vibrations of the dianions andvalidate the computationally predicted structures. These experimentaldata support the theoretical conclusions and highlight the instabilityof the bare tartrate dianion. From the experimental work, it couldalso be concluded that the pathway leading to dissociation is underkinetic control because the sodium adduct of the racemic stereoisomerdissociated at lower collisional energy, although it was calculatedto be more stable, and that decomposition proceeded via C–Cbond dissociation as computationally predicted. Taken together, thesedata provide insight into the gas-phase stability of the tartratedianion and highlight the role of adducts in stabilizing this species. [ABSTRACT FROM AUTHOR]