Whether transition metals can be entrapped inside fullerenes has remained unclear for a long time. Here mass spectrometric proof of entrapment of the group VIII transition-metal platinum (Pt) in fullerenes is first reported. Theoretical calculations on the example of La2PtC90 show that La2Pt@C2(99915)-C90 is the most stable isomer. Unlike other reported endohedral metal atoms, the entrapped Pt atom is negatively charged. This work provides valuable clues for the synthesis of some important missing endohedral metallofullerenes.

•A new strategy to obtain IRPD spectra of complex ions generated by MALDI.•The method is combined with H/D exchange to identify the chromophores.•Ions of [Arg + Rb]+ generated by MALDI are hotter than those generated by ESI.A new strategy to obtain IRMPD spectra of metal cationized ions by combining MALDI and IRMPD methods is presented here, in which graphene is selected to be the matrix to generate the metal cationized ions. [Arg + Rb]+ were studied here as the sample ions. Based on the experiential IRMPD spectrum and theoretical calculations, it is suggested that the ions of [Arg + Rb]+ generated here had more internal energies than those generated by ESI method reported previously by Williams et al. (J. Phys. Chem. A 2007, 111, 11759–11770). The method is also combined with the H/D exchange to identify the chromophores for the observed IR peak. The method provides a new way to obtain structural information of metal cationized ions, and also makes the identification of isomeric compounds feasible.Download high-res image (119KB)Download full-size image

Threonine-substituted serine octamer ions were generated by electrospray ionization (ESI) and investigated by mass spectrometry and infrared photodissociation (IRPD) spectroscopy. IRPD spectra of [L-Ser7 + L/D-Thr1]H+ and [L-Ser6 + L/D-Thr2]H+ were obtained in the range of 2700–3600 cm−1. Chiral differentiation was achieved by comparing their IRPD spectra. The main difference located in the range of 3300–3500 cm−1. And the results indicate the substitution of L-Ser by D-Thr could weaken the intermolecular H-bonds and loosen the original structures of serine octamers.Chiral differentiation was achieved by comparing IRPD spectra of L-Thr and D-Thr substituted serine octamers.

The gaseous chiral differentiation of alanine by permethylated β-cyclodextrin was studied using IRMPD spectroscopy and density functional theory calculations. The protonated non-covalent complexes of permethylated β-cyclodextrin and D- or L-alanine were mass-selected and investigated by IR laser pulses in the wavelength region of 2650–3800 cm−1. The remarkably different features of the IRMPD spectra for D- and L-alanine are described, and their origin is elucidated by quantum chemical calculations. We show that the differentiation of the experimentally observed spectral features is the result of different local interactions of D- and L-alanine with permethylated β-cyclodextrin. We also assign the extremely high-frequency (>3700 cm−1) bands in the observed spectra to the stretch motions of completely isolated alanine –OH groups.

•Generation of EMF ions of LunC2m+ (n = 1–9, 50 ≤ n ≤ 198) by laser ablation of graphene.•Series of Lu3C2m+ and Lu7C2m+ have the highest and lowest intensities, respectively.•For Er, Tm and Yb, only limited species of MnC2m+(n ≤ 4) were observed.Endohedral metallofullerene (EMF) ions of LunC2m+ (n = 1–9, 50 ≤ 2m ≤ 198) were generated by laser ablation of graphene and LuCl3. Some mono-EMF ions, such as LuC66+, show noticeable relative intensities, indicating the significant stabilization of IPR-violating cages by the encaged metal atom. Among those multimetallic EMF ions, Lu3C2m+ and Lu7C2m+ have the highest and lowest signal intensities, respectively. For every increase of one Lu atom inside the fullerene, the size of the cage increases 12 carbon atoms on average. Top-down mechanism was applied to explain the formation of these large-sized, multimetallic EMF ions. However, for other lanthanide metals of Er, Tm and Yb, only very limited species of MnC2m+ (n ≤ 4) were observed in the mass spectra. Thus whether the formation of the multimetallic EMF ions with n ≥ 4 is performable for other lanthanide metals still needs a further study.Download high-res image (301KB)Download full-size image

The geometric and thermodynamic stability of the M2C100 (M = La, Y, and Sc) series was systematically investigated using density functional theory calculations on the level of B3LYP/6-31G(d) ∼ Lanl2dz. In all the cases, M2@D5(285913)-C100 isomers are the lowest-energy species. However, carbide endohedral fullerenes M2C2@C1(230933)-C98 present excellent thermodynamic stabilities, except for those with La metal. The main product in electric arc experiments at temperatures lower than 3500 K for La2C100 should be La2@D5(285913)-C100, which was successfully synthesized previously; for Y and Sc, the predicted main products in these experiments should be M2C2@C1(230933)-C98. Further analysis of the geometric structures of the M2C100 series showed that the dimetallofullerenes M2@C100 have greater effects on the shapes of cages than M2C2@C98. These results provide some valuable guidance for the synthesis and characterization of large endohedral fullerenes including La, Y or Sc.

Combining with electrospray ionization (ESI) mass spectrometry, infrared photodissociation (IRPD) spectroscopy is a powerful method to study structures of cluster ions in the gas phase. In this paper, infrared photodissociation spectrum of Pro4H+ in the range of 2700–3600 cm−1 was obtained experimentally. Both theoretically predicted spectra of the two most stable isomers of Pro4-1 and Pro4-2 obtained at the level of M062X/6-31 + G(d, p) are in good consistent with the experimental results. The two isomers have similar structures and close energies. Both of them only consist of zwitterionic units, indicating the strong salt-bridged interactions inside the clusters. And the calculated collision cross section (ccs) of Pro4-1 is found to be very close to the experimental result previously reported.IRPD Spectroscopy of Pro4H+ was obtained in the gas phase and studied by theoretical calculations.

For homodimers of amino acids, their salt-bridged structures are gradually stabilized as the proton affinity of the component amino acid increases. Threonine has a proton affinity value located in the middle of the list of 20 natural amino acids. Thus, identifying whether the most stable isomer of protonated threonine dimer (Thr2H+) has a charge-solvated or salt-bridged structure is important and helpful for understanding the structures of other homodimers. By combining infrared photodissociation (IRPD) spectroscopy and theoretical calculations, the structures of Thr2H+ were investigated. Based on calculations at the M062X/6-311++G(d,p)//M062X/6-311++G(d,p) level, the most stable isomer of Thr2H+ was computed to be a charge-solvated structure, with an energy 3.87 kcal/mol lower than the most stable salt-bridged isomer. The predicted infrared spectrum is in good agreement with the experimental spectrum. To evaluate the temperature effect on the distribution of different isomers, the relative concentrations of the six isomers of Thr2H+ were calculated at different temperatures, according to their partition functions and enthalpies. The results show that the isomers are dominated by charge-solvated structures at a temperature of 300 K.

Chiral preferences exist in proline-substituted serine octamers. For ions of [L-Ser6 + Pro2]H+, the stability preference is [L-Ser6 + L-Pro2]H+ > [L-Ser6 + D-Pro2]H+ > [L-Ser6 + L-Pro1 + D-Pro1]H+. Infrared photodissociation (IRPD) experiments were performed for the observed proline-substituted octamer ions in the range from 2700 to 3750 cm−1. Chiral differentiation was achieved using the IRPD method, and the progressive changes in IRPD spectra due to the substitution were also reflected.

•Generation of M@C2n+ (M = Ca, Sr, Ba) by laser ablation of graphene.•Large monometallofullerene ions up to 2n = 272 were observed.•The productivities of M@C2n+ decrease with the size of the metal atoms.Metallofullerene ions of M@C2n+ (M = Ca, Sr, Ba, 2n = 50–230) were generated by laser ablation of graphene and corresponding metal salts using a commercial MALDI source without any modification. Large monometallofullerene ions up to Ba@C272+, and di-metallofullerene ions were observed in the mass spectra. The endohedral structures are confirmed by on resonance collisionally activated dissociation experiments on the selected ions. Although the distributions of the endohedral metallofullerene (EMF) ions for three alkaline earth metals are similar, the productivities decrease with the size of the metal atoms. Comparing with previously applied sources for EMF ions, this method is characterized by the direct usage of a commercial MALDI source, rapid and simple sample preparation, and the generation of large-sized EMF ions.

To better understand inconsistencies between the predicted infrared (IR) spectra of previously suggested isomers of Lys2H+ reported by Wu et al. (J. Am. Soc. Mass Spectrom. 22:1651–1659, 18) and the experimental IR photon dissociation (IRPD) spectrum obtained by Oh et al. (J. Am. Chem. Soc. 127:4076–4083, 4), the structure of Lys2H+ was reinvestigated using IRPD spectroscopy in the extended region 2700–3700 cm−1 and theoretical calculations. The new experimental IRPD spectrum is in good agreement with Oh’s spectrum in the corresponding wavelength range. Based on calculations at the MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p) and MP2/6-31 + G(d,p)//MP2/6-31 + G(d,p) levels, a new salt-bridged isomer, ZW1, was found to be the most stable isomer; it is more energetically favored than the previously suggested charge-solvated isomer LL-CS01 by 10 or 26 kJ mol−1. Although the calculated IR spectrum of ZW1 is in good agreement with the experimental one in the range 2700–3700 cm−1, it is in poor agreement with the previous IRPD spectrum in the range 1000–1900 cm−1. This investigation shows that the intermolecular interactions inside the dimer are more complex than previously supposed. It is possible that both salt-bridged and charge-solvated isomers of Lys2H+ are stable in the gas phase, and the isomers generated during ionization are sensitive to the experimental conditions.

The collisionally activated dissociation mass spectra of the protonated and alkali metal cationized ions of a triazole-epothilone analogue were studied in a Fourier transform ion cyclotron resonance mass spectrometer. The fragmentation pathway of the protonated ion was characterized by the loss of the unit of C3H4O3. However, another fragmentation pathway with the loss of C3H2O2 was identified for the complex ions with Na+, K+, Rb+, and Cs+. The branching ratio of the second pathway increases with the increment of the size of alkali metal ions. Theoretical calculations based on density functional theory (DFT) method show the difference in the binding position of the proton and the metal ions. With the increase of the radii of the metal ions, progressive changes in the macrocycle of the compound are induced, which cause the corresponding change in their fragmentation pathways. It has also been found that the interaction energy between the compound and the metal ion decreases with increase in the size of the latter. This is consistent with the experimental results, which show that cesiated complexes readily eject Cs+ when subject to collisions.

The formation of large even-numbered carbon cluster anions, \( {\text{C}}_{\text{n}}^{ - } \), with n up to 500 were observed in the mass spectra generated by laser ablation of graphene and graphene oxide, and the signal intensity of the latter is much weaker than that of the former. The cluster distributions generated from graphene can be readily altered by changing the laser energy and the accumulation period in the FT - ICR cell. By choosing suitable experimental conditions, weak signals of odd-numbered anions from \( {\text{C}}_{{125}}^{ - } \) to \( {\text{C}}_{{211}}^{ - } \), doubly charged anions from \( {\text{C}}_{{70}}^{{2 - }} \) to \( {\text{C}}_{{230}}^{{2 - }} \) and triply charged cluster anions from \( {\text{C}}_{{80}}^{{3 - }} \) to \( {\text{C}}_{{224}}^{{3 - }} \) can be observed. Tandem MS was applied to some selected cluster anions. Though no fragment anions larger than \( {\text{C}}_{{20}}^{ - } \) can be observed by the process of collisional activation with N2 gas for most cluster ions, several cluster anions can lose units of C2, C4, C6 or C8 in their collision process. The differences in their dissociation kinetics and structures require further calculations and experimental studies.

Chiral preferences exist in proline-substituted serine octamers. For ions of [L-Ser6 + Pro2]H+, the stability preference is [L-Ser6 + L-Pro2]H+ > [L-Ser6 + D-Pro2]H+ > [L-Ser6 + L-Pro1 + D-Pro1]H+. Infrared photodissociation (IRPD) experiments were performed for the observed proline-substituted octamer ions in the range from 2700 to 3750 cm−1. Chiral differentiation was achieved using the IRPD method, and the progressive changes in IRPD spectra due to the substitution were also reflected.

The gaseous chiral differentiation of alanine by permethylated β-cyclodextrin was studied using IRMPD spectroscopy and density functional theory calculations. The protonated non-covalent complexes of permethylated β-cyclodextrin and D- or L-alanine were mass-selected and investigated by IR laser pulses in the wavelength region of 2650–3800 cm−1. The remarkably different features of the IRMPD spectra for D- and L-alanine are described, and their origin is elucidated by quantum chemical calculations. We show that the differentiation of the experimentally observed spectral features is the result of different local interactions of D- and L-alanine with permethylated β-cyclodextrin. We also assign the extremely high-frequency (>3700 cm−1) bands in the observed spectra to the stretch motions of completely isolated alanine –OH groups.