Ryan P. Dain

IRMPD Spectroscopy of Anionic Group II Metal Nitrate Cluster Ions

Anionic group II metal nitrate clusters of the formula [M2(NO3)5]−, where M2 = Mg2, MgCa, Ca2, and Sr2, are investigated by infrared multiple photon dissociation (IRMPD) spectroscopy to obtain vibrational spectra in the mid-IR region. The IR spectra are dominated by the symmetric and the antisymmetric nitrate stretches, with the latter split into high and low-frequency components due to the distortion of nitrate anion symmetry by interactions with the cation. Density functional theory (DFT) is used to predict geometries and vibrational spectra for comparison to the experimental spectra. Calculations yield two stable isomers: the first one contains two terminal nitrate anions on each cation and a single bridging nitrate (“mono-bridging”), while the second structure features a single terminal nitrate on each cation with three bridging nitrate ligands (“tri-bridging”). The tri-bridging isomer is calculated to be lower in energy than the mono-bridging one for all species. Theoretical spectra of the tri-bridging structure provide a better qualitative match to the experimental infrared spectra of [Mg2(NO3)5]− and [MgCa(NO3)5]−. However, the profile of the low-frequency ν3 band for the Mg2 complex suggests a third possible isomer not predicted by theory. The IRMPD spectra of the Ca2 and Sr2 complexes are better reconciled by a weighted summation of the spectra of both isomers suggesting that a mixture of structures is present.

Infrared multiple-photon dissociation spectroscopy of group II metal complexes with salicylate

Ion trap tandem mass spectrometry with collision-induced dissociation, and the combination of infrared multiple-photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations, were used to characterize singly charged, 1:1 complexes of Ca(2+), Sr(2+) and Ba(2+) with salicylate. For each metal-salicylate complex, the CID pathways are: (a) elimination of CO(2) and (b) formation of [MOH](+) where M = Ca(2+), Sr(2+) or Ba(2+). DFT calculations predict three minima for the cation-salicylate complexes which differ in the mode of metal binding. In the first, the metal ion is coordinated by O atoms of the (neutral) phenol and carboxylate groups of salicylate. In the second, the cation is coordinated by phenoxide and (neutral) carboxylic acid groups. The third mode involves coordination by the carboxylate group alone. The infrared spectrum for the metal-salicylate complexes contains a number of absorptions between 1000 and 1650 cm(-1), and the best correlation between theoretical and experimental spectra is found for the structure that features coordination of the metal ion by phenoxide and the carbonyl O of the carboxylic acid group, consistent with the calculated energies for the respective species.

Infrared multiple photon dissociation spectroscopy of group I and group II metal complexes with Boc‐hydroxylamine

RATIONALE Hydroxamates are essential growth factors for some microbes, acting primarily as siderophores that solubilize iron for transport into a cell. Here we determined the intrinsic structure of 1:1 complexes between Boc-protected hydroxylamine and group I ([M(L)](+)) and group II ([M(L-H)](+)) cations, where M and L are the cation and ligand, respectively, which are convenient models for the functional unit of hydroxamate siderphores. METHODS The relevant complex ions were generated by electrospray ionization (ESI) and isolated and stored in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Infrared spectra of the isolated complexes were collected by monitoring (infrared) photodissociation yield as a function of photon energy. Experimental spectra were then compared to those predicted by density functional theory (DFT) calculations. RESULTS The infrared multiple photon dissociation (IRMPD) spectra collected are in good agreement with those predicted to be lowest-energy by DFT. The spectra for the group I complexes contain six resolved absorptions that can be attributed to amide I and II type and hydroxylamine N-OH vibrations. Similar absorptions are observed for the group II cation complexes, with shifts of the amide I and amide II vibrations due to the change in structure with deprotonation of the hydroxylamine group. CONCLUSIONS IRMPD spectroscopy unequivocally shows that the intrinsic binding mode for the group I cations involves the O atoms of the amide carbonyl and hydroxylamine groups of Boc-hydroxylamine. A similar binding mode is preferred for the group II cations, except that in this case the metal ion is coordinated by the O atom of the deprotonated hydroxylamine group.

Infrared multiple photon dissociation spectroscopy

RATIONALE Hydroxamates are essential growth factors for some microbes, acting primarily as siderophores that solubilize iron for transport into a cell. Here we determined the intrinsic structure of 1:1 complexes between Boc-protected hydroxylamine and group I ([M(L)](+)) and group II ([M(L-H)](+)) cations, where M and L are the cation and ligand, respectively, which are convenient models for the functional unit of hydroxamate siderphores. METHODS The relevant complex ions were generated by electrospray ionization (ESI) and isolated and stored in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Infrared spectra of the isolated complexes were collected by monitoring (infrared) photodissociation yield as a function of photon energy. Experimental spectra were then compared to those predicted by density functional theory (DFT) calculations. RESULTS The infrared multiple photon dissociation (IRMPD) spectra collected are in good agreement with those predicted to be lowest-energy by DFT. The spectra for the group I complexes contain six resolved absorptions that can be attributed to amide I and II type and hydroxylamine N-OH vibrations. Similar absorptions are observed for the group II cation complexes, with shifts of the amide I and amide II vibrations due to the change in structure with deprotonation of the hydroxylamine group. CONCLUSIONS IRMPD spectroscopy unequivocally shows that the intrinsic binding mode for the group I cations involves the O atoms of the amide carbonyl and hydroxylamine groups of Boc-hydroxylamine. A similar binding mode is preferred for the group II cations, except that in this case the metal ion is coordinated by the O atom of the deprotonated hydroxylamine group.

Infrared spectrum of potassium-cationized triethylphosphate generated using tandem mass spectrometry and infrared multiple photon dissociation

Tandem mass spectrometry and wavelength-selective infrared photodissociation were used to generate an infrared spectrum of gas-phase triethylphosphate cationized by attachment of K(+). Prominent absorptions were observed in the region of 900 to 1300 cm(-1) that are characteristic of phosphate P=O and P-O-R stretches. The relative positions and intensities of the IR absorptions were reproduced well by density functional theory (DFT) calculations performed using the B3LYP functional and the 6-31+G(d), 6-311+G(d,p) and 6-311++G(3df,2pd) basis sets. Because of good correspondence between experiment and theory for the cation, DFT was then used to generate a theoretical spectrum for neutral triethylphosphate, which in turn accurately reproduces the IR spectrum of the neat liquid when solvent effects are included in the calculations.

A study of fragmentation of protonated amides of some acylated amino acids by tandem mass spectrometry: observation of an unusual nitrilium ion

A tandem mass spectrometric study of a series of secondary amides of acetylglycine and hippuric acid utilizing electrospray ionization (ESI) was conducted. Among the fragment ions observed was an unusual one, which we have determined to be a nitrilium ion having the structure CH3-C≡N⊕-Ph or Ph-C≡N⊕-Ph by loss of the full mass of glycine as a neutral fragment. A mechanism that we propose involves an initial protonation of the oxygen atom at the N-terminus, followed by cyclization to a five-membered imidazolium ring, and its subsequent collapse to the nitrilium ion. This mechanism is supported by extensive isotopic labels and considerable variation of substituents. A similar study of the amides of acyl β-alanine and acyl γ-aminobutyric acid revealed that the former furnishes the same nitrilium ion, but not the latter. Thus, a six-membered intermediate is also possible and capable of losing the full mass of β-alanine as a neutral fragment. When the size of the ring is forced to be seven-membered, this pathway is blocked. When this study was expanded to include a variety of N-acylproline amides, the nitrilium ion was observed in 100% abundance only when the acyl group was acetyl. Thus a proline effect (involvement of a strained bicyclic [3.3.0] structure) is being observed.

Infrared multiple photon dissociation spectroscopy of sodium and potassium chlorate anions

The structures of gas-phase, metal chlorate anions with the formula [M(ClO(3))(2)](-), M = Na and K, were determined using tandem mass spectrometry and infrared multiple photon dissociation (IRMPD) spectroscopy. Structural assignments for both anions are based on comparisons of the experimental vibrational spectra for the two species with those predicted by density functional theory (DFT) and involve conformations that feature either bidentate or tridentate coordination of the cation by chlorate. Our results strongly suggest that a structure in which both chlorate anions are bidentate ligands is preferred for [Na(ClO(3))(2)](-). However, for [K(ClO(3))(2)](-) the best agreement between experimental and theoretical spectra is obtained from a composite of predicted spectra for which the chlorate anions are either both bidentate or both tridentate ligands. In general, we find that the overall accuracy of DFT calculations for prediction of IR spectra is dependent on both functional and basis set, with best agreement achieved using frequencies generated at the B3LYP/6-311+g(3df) level of theory.