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Ion mobility spectrometry (IMS) mass spectrometry (MS) centers on the ability to separate gaseous structures by size, charge, shape, and followed by mass-to-charge (m/z). For oligomeric structures,...

Threshold collision-induced dissociation of alkali metal cation-diaza-18-crown-6 complexes, M+(da18C6), with xenon is studied using guided ion beam tandem mass spectrometry techniques. The alkali metal cations examined here include: Na+, K+, Rb+, and Cs+. In all cases, M+ is the only product observed, corresponding to endothermic loss of the intact da18C6 ligand. The cross section thresholds are analyzed to extract zero and 298 K M+ da18C6 bond dissociation energies (BDEs) after properly accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and the lifetimes for dissociation. Density functional theory calculations at the B3LYP/def2-TZVPPD and B3LYP/6-31+G* levels of theory are used to determine the structures of da18C6 and the M+(da18C6) complexes and provide molecular constants necessary for the thermodynamic analysis of the experimental data. Theoretical BDEs are determined from single point energy calculations at the B3LYP and MP2(full) levels of theory using the def2-TZVPPD and 6-311+G(2d,2p) basis sets using the B3LYP/def2-TZVPPD and B3LYP/6-31+G* optimized geometries. The agreement between B3LYP/def2-TZVPPD theory and experiment is excellent for all four M+(da18C6) complexes. The M+ da18C6 BDEs decrease as the size of the alkali metal cation increases, consistent with the electrostatic nature of the binding in these complexes. The M+(da18C6) structures and BDEs are compared to those previously reported for the analogous complexes of 18-crown-6 and hexaaza-18-crown-6, to examine the effects of the donor atoms (N versus O) on the structure and strength of binding.

The gas-phase structures of sodium cationized complexes of 5- and 6-halo-substituted uracils are examined via infrared multiple photon dissociation (IRMPD) action spectroscopy and theoretical electronic structure calculations. The halouracils examined in this investigation include: 5-flourouracil, 5-chlorouracil, 5-bromouracil, 5-iodouracil, and 6-chlorouracil. Experimental IRMPD action spectra of the sodium cationized halouracil complexes are measured using a 4.7 T Fourier transform ion cyclotron resonance mass spectrometer coupled to the FELIX free electron laser (FEL). Irradiation of the mass selected sodium cationized halouracil complexes by the FEL was carried out over the range of frequencies extending from 950 to 1900 cm −1 . Theoretical linear IR spectra predicted for the stable low-energy conformations of the sodium cationized halouracils, calculated at B3LYP/6-31G(d) level of theory, are compared with the measured IRMPD action spectra to identify the structures accessed in the experiments. Relative stabilities of the low-energy conformations are determined from single-point energy calculations performed at the B3LYP/6-311+G(2d,2p) level of theory. The evolution of IRMPD spectral features as a function of the size (F, Cl, Br, and I) and position (5 versus 6) of the halogen substituent are examined to elucidate the effects of the halogen substituent and noncovalent interactions with sodium cations on the structure of the nucleobase. Present results are compared with results from energy-resolved collision-induced dissociation and IRMPD action spectroscopy studies previously reported for the protonated and sodium cationized forms of uracil, and halo-, methyl-, and thioketo-substituted uracils. The present results suggest that only a single conformer is accessed for all of the 5-halouracil complexes, whereas multiple conformers are accessed for the Na + (6ClU) complex. In all cases, the experimental IRMPD action spectra confirm that the sodium cation binds to the O4 carbonyl oxygen atom of the canonical diketo tautomer in the ground-state conformers, and gains additional stabilization via chelation interactions with the halogen substituent in the complexes to the 5-halouracils as predicted by theory.

Threshold collision-induced dissociation (CID) of alkali metal cation-hexacyclen (ha18C6) complexes, M(+)(ha18C6), with xenon is studied using guided ion beam tandem mass spectrometry techniques. The alkali metal cations examined here include: Na(+), K(+), Rb(+), and Cs(+). In all cases, M(+) is the only product observed, corresponding to endothermic loss of the intact ha18C6 ligand. The cross-section thresholds are analyzed to extract zero and 298 K M(+)-ha18C6 bond dissociation energies (BDEs) after properly accounting for the effects of multiple M(+)(ha18C6)-Xe collisions, the kinetic and internal energy distributions of the M(+)(ha18C6) and Xe reactants, and the lifetimes for dissociation of the activated M(+)(ha18C6) complexes. Ab initio and density functional theory calculations are used to determine the structures of ha18C6 and the M(+)(ha18C6) complexes, provide molecular constants necessary for the thermodynamic analysis of the energy-resolved CID data, and theoretical estimates for the M(+)-ha18C6 BDEs. Calculations using a polarizable continuum model are also performed to examine solvent effects on the binding. In the absence of solvent, the M(+)-ha18C6 BDEs decrease as the size of the alkali metal cation increases, consistent with the noncovalent nature of the binding in these complexes. However, in the presence of solvent, the ha18C6 ligand exhibits selectivity for K(+) over the other alkali metal cations. The M(+)(ha18C6) structures and BDEs are compared to those previously reported for the analogous M(+)(18-crown-6) and M(+)(cyclen) complexes to examine the effects of the nature of the donor atom (N versus O) and the number donor atoms (six vs four) on the nature and strength of binding.

The gas-phase structures of alkali metal cationized complexes of cyclen (1,4,7,10-tetraazacyclododecane) are examined via infrared multiple photon dissociation (IRMPD) action spectroscopy and electronic structure theory calculations. The measured IRMPD action spectra of four M+(cyclen) complexes are compared to IR spectra predicted for the stable low-lying conformers of these complexes calculated at the B3LYP/def2-TZVPPD level of theory to identify the structures accessed in the experiments. The IRMPD yields of the M+(cyclen) complexes investigated increase as the size of the alkali metal cation increases, in accordance with the decrease in the strength of alkali metal cation binding. The IRMPD spectrum of the Na+(cyclen) complex is relatively simple, and the features observed are retained for all of the other alkali metal cation–cyclen complexes. New spectral features begin to appear for K+(cyclen) and become very obvious for the Rb+(cyclen) and Cs+(cyclen) complexes. The IRMPD action spectra for the complexes of cyclen to K+, Rb+, and Cs+ are well reproduced by the calculated spectra predicted for the most stable conformations computed. Overall comparisons suggest that only the ground-state conformations of the M+(cyclen) complexes were accessed in the experiments for the complexes to Na+ and K+, whereas evidence for a very small population of the first excited conformers is observed for the complexes to Rb+ and Cs+.

Threshold collision-induced dissociation of alkali metal cation–cyclen complexes, M+(cyclen), with xenon is studied using guided ion beam tandem mass spectrometry techniques. The alkali metal cations examined here include: Na+, K+, Rb+ and Cs+. In all cases, M+ is the only ionic product observed, corresponding to endothermic loss of the intact cyclen ligand. The cross section thresholds are analyzed to extract zero and 298 K M+–cyclen bond dissociation energies (BDEs) after properly accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and the lifetimes for dissociation. Density functional theory calculations at the B3LYP/def2-TZVPPD and B3LYP/6-31+G* levels of theory are used to determine the structures of cyclen and the M+(cyclen) complexes. Theoretical BDEs are determined from single point energy calculations at the B3LYP/def2-TZVPPD and MP2(full)/def2-TZVPPD levels of theory using the B3LYP/def2-TZVPPD optimized geometries, and also at the B3LYP/6-311+G(2d,2p) and MP2(full)/6-311+G(2d,2p) levels of theory using the B3LYP/6-31+G*optimized geometries. The agreement between theory and experiment is reasonably good for all levels of theory examined, but is best for the calculations performed at the B3LYP/def2-TZVPPD level of theory. The M+–cyclen BDEs decrease as the size of the alkali metal cation increases, consistent with the electrostatic nature of the binding in these complexes. The M+(cyclen) structures and BDEs are compared to those previously reported for the analogous M+(12-crown-4) complexes to examine the effects of the donor atom (N versus O) on the structures and strength of binding.

Quantitative interactions of alkali metal cations with the cyclic 12-crown-4 polyether ligand (12C4) are studied. Experimentally, Rb + (12C4) and Cs + (12C4) complexes are formed using electrospray ionization and their bond dissociation energies (BDEs) determined using threshold collision-induced dissociation of these complexes with xenon in a guided ion beam tandem mass spectrometer. The energy-dependent cross sections thus obtained are interpreted using an analysis that includes consideration of unimolecular decay rates, internal energy of the reactant ions, and multiple ion-neutral collisions. 0 K BDEs of 151.5 ± 9.7 and 137.0 ± 8.7 kJ/mol, respectively, are determined and exceed those previously measured by 60 and 54 kJ/mol, respectively, consistent with the hypothesis proposed there that excited conformers had been studied. In order to provide comparable thermochemical results for the Na + (12C4) and K + (12C4) systems, the published data for these systems are reinterpreted using the same analysis techniques, which have advanced since the original data were acquired. Revised BDEs for these systems are obtained as 243.9 ± 12.6 and 182.0 ± 17.3 kJ/mol, respectively, which are within experimental uncertainty of the previously reported values. In addition, quantum chemical calculations are conducted at the B3LYP and MP2(full) levels of theory with geometries and zero point energies calculated at the B3LYP level using both HW*/6-311+G(2d,2p) and def2-TZVPPD basis sets. The theoretical results are in reasonable agreement with experiment, with B3LYP/def2-TZVPPD values being in particularly good agreement. Computations also allow the potential energy surfaces for dissociation of the M + (12C4) complexes to be elucidated. These are used to help explain why the previous studies formed excited conformers of Rb + (12C4) and Cs + (12C4) but apparently not of Na + (12C4) and K + (12C4).

Lithium acetyl acetonate undergoes a perfectly reversible phase change from orthorhombic to non-merohedrally twinned monoclinic between 195 and 200 K. The phase transfer is associated with a sudden slight deviation of the orthorhombic γ-angle from 90°, with the initial coordinate changes following the phase transfer being very subtle. The connectivity within the lithium acetyl acetonate chains and the basic structural motif remain unchanged and the main difference between the orthorhombic and monoclinic structures is associated with a departure of the Li atoms – along with the Li(acac) chains as a whole – from a perfectly linear fashion in the room temperature structure to a slightly curved chain in the low temperature phase. Detailed descriptions of experimental procedures for the handling of non-merohedrally twinned datasets are given and discussed.

Quantitative interactions of the alkali metal cations with the cyclic 15-crown-5 polyether ligand (15C5) are studied. In this work, Rb(+)(15C5) and Cs(+)(15C5) complexes are formed using electrospray ionization and studied using threshold collision-induced dissociation with xenon in a guided ion beam tandem mass spectrometer. The energy-dependent cross sections thus obtained are interpreted to yield bond dissociation energies (BDEs) using an analysis that includes consideration of unimolecular decay rates, internal energy of the reactant ions, and multiple ion-neutral collisions. 0 K BDEs of 175.0 ± 9.7 and 159.4 ± 9.6 kJ/mol, respectively, are determined and exceed those previously measured [J. Am. Chem. Soc. 1999, 121, 417-423] by 68 and 57 kJ/mol, respectively, consistent with the hypothesis proposed there that excited conformers had been studied. Because the analysis techniques have advanced since this early study, we also reanalyze the published data for the Na(+)(15C5) and K(+)(15C5) systems to ensure a self-consistent interpretation of all four systems. Revised BDEs for these systems are 296.1 ± 15.5 and 215.6 ± 10.6 kJ/mol, respectively, which are within experimental uncertainty of the previously reported values. In addition, quantum chemical calculations are conducted at the B3LYP/def2-TZVPPD level of theory with theoretical BDEs in reasonable agreement with experiment. Computations are also used to explore features of the potential energy surfaces for isomerization of the M(+)(15C5) complexes.