Primary Dissociation Pathway Research Articles

Threshold Collision-Induced Dissociation of Hydrated Thorium(IV) Trihydroxide Cation: Experimental and Theoretical Investigation of the Binding Energies for Th(OH)3+(H2O)n Complexes (n = 1-4).

Threshold collision-induced dissociation of Th(OH)3+(H2O)n (n = 1-4) with xenon was performed using a guided ion beam tandem mass spectrometer. The primary dissociation pathway for all complexes is a loss of a single water molecule followed by the sequential loss of additional water molecules at higher collision energies. The data were analyzed using a statistical model after accounting for lifetime effects and reactant internal and kinetic energy distributions to obtain 0 K bond dissociation energies (BDEs). These were also converted using rigid rotor/harmonic oscillator approximations to yield thermodynamic values at room temperature. The 0 K BDEs of H2O ligands to Th(OH)3+ (IV) are experimentally determined for the first time as 106 ± 6, 89 ± 6, 76 ± 4, and 51 ± 4 kJ/mol for the first, second, third, and fourth water ligand added. These values agree reasonably well with values calculated at the B3LYP, B3PW91, and PBE0 levels of theory with aug-cc-pVQZ basis sets, whereas B3LYP-GD3BJ, MP2, and CCSD(T) single point energies with (without) counterpoise corrections systematically overestimate the bond energies by about 15 (20), 19 (25), and (18) kJ/mol, respectively.

Binding energies of hydrated cobalt hydroxide ion complexes: A guided ion beam and theoretical investigation.

The sequential bond energies of CoOH+(H2O)x complexes, where x = 1-4, are measured by threshold collision-induced dissociation using a guided ion beam tandem mass spectrometer. The primary dissociation pathway for all reactants consists of loss of a single water molecule. This is followed by the sequential loss of additional water molecules at higher collision energies for the x = 2-4 complexes, whereas the x = 1 reactant loses the OH ligand competitively with the H2O ligand. The kinetic energy dependent cross sections for dissociation of CoOH+(H2O)x complexes are modeled to obtain 0 and 298 K binding energies. Our experimental results agree well with theoretically determined bond dissociation energies (BDEs) at the B3LYP, B3LYP-GD3BJ, B3P86, and MP2(full) levels of theory with a 6-311+G(2d,2p) basis set using geometries and vibrational frequencies determined at the B3LYP/6-311+G(d,p) level. Thermochemical information for the loss of OH from CoOH+(H2O)x where x = 0-4 is also derived by combining the present experimental HO-Co+(H2O) and water loss BDEs from CoOH+(H2O)x with those for Co+(H2O)y from the literature. These BDEs are also compared to theory with mixed results.

Threshold collision-induced dissociation of protonated hydrazine and dimethylhydrazine clustered with water.

Threshold collision-induced dissociation using a guided ion beam tandem mass spectrometer is performed on (N2H4)H+(H2O)n, where n = 1 and 2, and on the protonated unsymmetrical 1,1-dimethylhydrazine one-water complex. The primary dissociation pathway for all clusters is a loss of a single water molecule, which for n = 2 is followed by the sequential loss of an additional water molecule at higher collision energies. The data are analyzed using a statistical model after accounting for internal and kinetic energy distributions, multiple collisions, and kinetic shifts to obtain 0 K bond dissociation energies (BDEs). These are also converted using a rigid rotor/harmonic oscillator approximation to yield thermodynamic values at room temperature. Experimental BDEs compare favorably to theoretical BDEs determined at the B3LYP, M06, mPW1PW91, PBE0, MP2(full), and CCSD(T) levels of theory with a 6-311+G(2d,2p) basis set both with and without empirical dispersion. These calculations also allow visualization of the structures of these complexes, which are simple hydrogen-bonded donor-acceptors.

Threshold Collision-Induced Dissociation of Proton-Bound Hydrazine and Dimethylhydrazine Clusters.

Threshold collision-induced dissociation (TCID) using a guided ion beam tandem mass spectrometer is performed on (N2H4)nH+ where n = 2-4 and on the proton-bound unsymmetrical 1,1-dimethylhydrazine (UDMH) dimer complex. The primary dissociation pathway for all reactants consists of loss of a single hydrazine (or UDMH) molecule followed by the sequential loss of additional hydrazine molecules at higher collision energies for n = 3 and 4. The data were analyzed using a statistical model after accounting for internal and kinetic energy distributions, multiple collisions, and kinetic shifts to obtain 0 K bond dissociation energies (BDEs). These are also converted to values at room temperature by using a rigid rotor/harmonic oscillator approximation and theoretical molecular constants. Experimental BDEs are compared to theoretical BDEs determined at the B3LYP, M06, mPW1PW91, PBE0, MP2(full), and CCSD(T) levels of theory with and without empirical dispersion with a 6-311+G(2d,2p) basis set. The structures of all clusters are explored and exhibit extensive hydrogen bonding.

Collision-induced dissociation mass spectrometry of phosphorus cluster anions [formula omitted] (3 ≤ m ≤ 20)

Phosphorus cluster anions were generated and investigated using collision-induced dissociation mass spectrometry. Results show that the primary dissociation channels of P2m+1− (3≤m≤11) are generally characterized by the loss of a P2 unit, which distinguishes them from their counterpart cations. However, P11− and P13− are two exceptions. The former is characterized by the loss of a P4 unit, and the latter is characterized by two parallel dissociation channels, which lead to the loss of P2 and P6 units. The dissociation behaviors of larger cluster anions of P8k+1− (k=3–5) are more complex. The primary dissociation pathway of P25− is the loss of a P8 unit, and those of P33− and P41− are the loss of P16 and P2 units respectively. Theoretical calculations were performed for anions of P2m+1− (3≤m≤9). Results show that their dissociation channels characterized by the loss of the P2 unit are more thermodynamically favored, which is generally consistent with the experimental results.

Hydrated copper ion chemistry: guided ion beam and computational investigation of Cu2+(H2O)n (n = 7-10) complexes.

Cross sections for the threshold collision-induced dissociation of Cu(2+)(H(2)O)(n), where n = 8 - 10, are measured using a guided ion beam tandem mass spectrometer. The primary dissociation pathway is found to be loss of a single water molecule followed by the sequential loss of additional water molecules until n = 8, at which point charge separation to form CuOH(+)(H(2)O)(4) (+) H(+)(H(2)O)(3) is observed to occur at a slightly lower energy than loss of a water molecule. Competition from charge separation prohibits the formation of appreciable amounts of the n = 7 or smaller complexes as reactants in the source. These findings indicate that Cu(2+) has a critical size of 8. Analysis of the data using statistical modeling techniques that account for energy distributions and lifetime effects yields primary and sequential bond dissociation energies (BDEs) for loss of one and two water molecules from n = 8 - 10 complexes as well as the barrier for charge separation from n = 8. More speculative analysis extends the thermochemistry obtained down to n = 5 and 6. Theoretical BDEs are determined from quantum chemical calculations using structures optimized at the B3LYP/6 311(+)G(d,p) level along with the lowest-energy isomers suggested by single point energies at the MP2(full), M06, B3LYP, and B3P86 levels of theory using a 6- 311(+)G(2d,2p) basis set. BDEs at 0K are converted to 298 K thermodynamic values using a rigid rotor/harmonic oscillator approximation. Experimental and theoretical entropies of activation suggest that a third solvent shell forms at n = 9, in accord with previous findings. The present work represents the first experimentally determined hydration enthalpies for the Cu(2+)(H(2)O)n system.

Quantum chemical characterization of the CF2(OH)CF2OONO2 and CF3CF2OONO2 peroxynitrates and related radicals

We present a detailed study of the molecular conformations, vibrational spectra and thermochemistry of the CF2(OH)CF2OONO2 and CF3CF2OONO2 peroxynitrates. In addition, we studied the CF2(OH)CF2OO, CF2(OH)CF2O, CF3CF2OO and CF3CF2O radicals, formed by the rupture of ON and OO bonds of the above peroxynitrates. The geometric structures of the most stable conformations were determined by density functional theory calculations. At the B3LYP/6-311++G(3df,3pd) level of theory, both peroxynitrates present dihedral angle COON values of about 104°. At the best levels of theory employed, G3(MP2)B3 and G4(MP2), the standard enthalpies of formation at 298K derived from isodesmic reactions, are −265.6, −248.6 and −248.5kcalmol−1 for CF2(OH)CF2OONO2, CF2(OH)CF2OO and CF2(OH)CF2O respectively; while for CF3CF2OONO2, CF3CF2OO and CF3CF2O the values of −268.2, −252.6 and −251.9kcalmol−1 were obtained. Bond dissociation enthalpies of 25.1 and 35.0kcalmol−1 have been predicted for the first time for CF2(OH)CF2OONO2 and CF2(OH)CF2OONO2 and of 23.7 and 34.2kcalmol−1 have been estimated for CF3CF2OONO2 and CF3CF2OONO2. The obtained result of the dissociation enthalpy of CF3CF2OONO2 is in excellent agreement with the available experimental determination. Therefore, the ON bond fission is the primary thermal dissociation pathway for both studied peroxynitrates.

Guided ion beam studies of the collision-induced dissociation of CuOH+(H2O)n (n = 1-4): comprehensive thermodynamic data for copper ion hydration.

Threshold collision-induced dissociation (TCID) using a guided ion beam tandem mass spectrometer is performed on CuOH(+)(H2O)n where n = 1-4. The primary dissociation pathway for the n = 2-4 reactants consists of loss of a single water molecule followed by the sequential loss of additional water molecules at higher collision energies. The n = 1 reactant departs from this trend by losing the OH ligand and the H2O ligand competitively. Loss of the OH ligand is thermodynamically favored, whereas H2O loss is the kinetically favored process, consistent with heterolytic cleavage of the dative bond. The data are analyzed using a statistical model after accounting for internal and kinetic energy distributions, multiple collisions, and kinetic shifts to obtain 0 K bond dissociation energies (BDEs). These are also converted using a rigid rotor/harmonic oscillator approximation to yield thermodynamic values at room temperature. Experimental BDEs are compared to theoretical BDEs determined at the B3LYP, cam-B3LYP, B3P86, M06, CCSD(T), and MP2(full) levels of theory with a 6-311+G(2d,2p) basis set using geometries and vibrational frequencies determined at the B3LYP/6-311+G(d,p) level. In addition, BDEs for the loss of OH from CuOH(+)(H2O)n where n = 0, 2-4 are derived using the experimental BDEs for dissociation of CuOH(+)(H2O)n and literature values for Cu(+)(H2O)n.

High-energy collision-activated and electron-transfer dissociation of gas-phase complexes of tryptophan with Na+, K+, and Ca2+

The structure and reactivity of gas-phase complexes of tryptophan (Trp) with Na+, K+, and Ca2+ were examined by high-energy collision-activated dissociation (CAD) and electron transfer dissociation (ETD) using alkali metal targets. In the CAD spectra of M+Trp (M = Na and K), neutral Trp loss was the primary dissociation pathway, and the product ion of collision-induced intracomplex electron transfer from the indole π ring of Trp to the alkali metal ion was observed, indicating a charge-solvated structure in which Trp is non-zwitterionic. The NH3 loss observed in the CAD spectrum of Ca2+Trp2 is ascribed to a CZ (mixed charge-solvated/zwitterionic)-type structure, in which one Trp is non-zwitterionic and the other Trp adopts a zwitterionic structure with an NH 3 + moiety. The H atom and NH3 losses observed in the ETD spectrum of Ca2+Trp2 indicate the formation of a hypervalent radical in the complex, R–NH3, via electron transfer from the alkali metal target to the NH 3 + group of the CZ-type structure. Ca2+ attachment to Trp cluster induces the zwitterionic structure of Trp in the gas phase, and an electron transfer to the zwitterionic Trp forms the hypervalent radical as a reaction intermediate.

CO2dissociation using the Versatile atmospheric dielectric barrier discharge experiment (VADER)

Dissociation of CO2 is investigated in an atmospheric pressure dielectric barrier discharge (DBD) with a simple, zero dimensional (0-D) chemical model and through experiment. The model predicts that the primary CO2 dissociation pathway within a DBD is electron impact dissociation and electron-vibrational excitation. The relaxation kinetics following dissociation are dominated by atomic oxygen chemistry. The experiments included investigating the energy efficiencies and dissociation rates of CO2 within a planar DBD, while the gas flow rate, voltage, gas composition, driving frequency, catalyst, and pulse modes were varied. Some of the VADER results include a maximum CO2 dissociation energy efficiency of 2.5 +/- 0.5%, a maximum CO$_2$ dissociation rate of 4 +/- 0.4*10^-6 mol CO2/s (5 +/- 0.5% percent dissociation), discovering that a resonant driving frequency of ~30 kHz, dependent on both applied voltage and breakdown voltage, is best for efficient CO2 dissociation and that TiO2, a photocatalyst, improved dissociation efficiencies by an average of 18% at driving frequencies above 5 kHz.

Sodium Cation Affinities of Commonly Used MALDI Matrices Determined by Guided Ion Beam Tandem Mass Spectrometry

The sodium cation affinities of six commonly used MALDI matrices are determined here using guided ion beam tandem mass spectrometry techniques. The collision-induced dissociation behavior of six sodium cationized MALDI matrices, Na+(MALDI), with Xe is studied as a function of kinetic energy. The MALDI matrices examined here include: nicotinic acid, quinoline, 3-aminoquinoline, 4-nitroaniline, picolinic acid, and 3-hydroxypicolinic acid. In all cases, the primary dissociation pathway corresponds to endothermic loss of the intact MALDI matrix. The cross section thresholds are interpreted to yield zero and 298 K Na+−MALDI bond dissociation energies (BDEs), or sodium cation affinities, after accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and dissociation lifetimes. Density functional theory calculations at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G* and MP2(full)/6-311+G(2d,2p)//B3LYP/6-31G* levels of theory are used to characterized the structures and energetics for these systems. The calculated BDEs exhibit very good agreement with the measured values for most systems. The experimental and theoretical Na+−MALDI BDEs determined here are compared with those previously measured by cation transfer equilibrium methods.

Adsorption and decomposition of γ-butyrolactone on Pd(1 1 1) and Pt(1 1 1)

The adsorption and thermal chemistry of γ-butyrolactone (GBL) on the (1 1 1) surface of Pd and Pt has been investigated using a combination of high resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). HREELS results indicate that GBL adsorbs at 160 K on both surfaces through its oxygenate functionality. On Pd(1 1 1), adsorbed GBL undergoes ring-opening and decarbonylation by 273 K to produce adsorbed CO and surface hydrocarbon species. On Pt(1 1 1), very little dissociation is observed using HREELS, with almost all of the GBL simply desorbing. TPD results are consistent with decarbonylation and subsequent dehydrogenation reactions on Pd(1 1 1), although small amounts of CO 2 are also detected. TPD results from Pt(1 1 1) indicate that a small proportion of adsorbed GBL (perhaps on defect sites) does undergo ring-opening to produce CO, CO 2, and H 2. These results suggest that the primary dissociation pathway for GBL on Pd(1 1 1) is through O–C scission at the carbonyl position. Through comparisons with previously published studies of cyclic oxygenates, these results also demonstrate how ring strain and functionality affect the ring-opening rate and mechanism.

Noncovalent Interactions of Ni+ with N-Donor Ligands (Pyridine, 4,4′-Dipyridyl, 2,2′-Dipyridyl, and 1,10-Phenanthroline): Collision-Induced Dissociation and Theoretical Studies

Kinetic-energy-dependent collision-induced dissociation (CID) of complexes of a variety of N-donor ligands (N-L) with Ni(+), Ni(+)(N-L)(x), is studied using guided ion beam tandem mass spectrometry. The N-donor ligands investigated include: pyridine, 4,4'-dipyridyl, 2,2'-dipyridyl, and 1,10-phenanthroline. For most of the Ni(+)(N-L)(x) complexes, CID results in endothermic loss of a single neutral N-L ligand as the primary dissociation pathway. Sequential dissociation of additional N-L ligands is observed at elevated energies for the pyridine and 4,4'-dipyridyl complexes containing more than one ligand. The cross-section thresholds for the primary dissociation pathways are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) of the Ni(+)(N-L)(x) complexes after accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and their lifetimes for dissociation. Density functional theory calculations at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G* level are performed to obtain model structures, molecular parameters, and energetics for the neutral N-L ligands and the Ni(+)(N-L)(x) complexes. In general, theory is found to overestimate the strength of binding to the first N-L ligand, and underestimate the strength of binding to additional ligands. Trends in the sequential BDEs of the Ni(+)(N-L)(x) complexes are examined and compared to complexes of Ni(+), to several other ligands previously investigated. The trends in the sequential BDEs are primarily determined by the valence electronic configuration and the effects of sd-hybridization of Ni(+) but are also influenced by repulsive ligand-ligand interactions. Natural bond orbital analyses indicate that the binding in these complexes is primarily noncovalent.

Inductive effects on cation–π interactions: Structures and bond dissociation energies of alkali metal cation–halobenzene complexes

The influence of halogenation on cation–π interactions is investigated both experimentally and theoretically. Bond dissociation energies (BDEs) of alkali metal cation–halobenzene complexes, M +(C 6H 5X), are determined using threshold collision-induced dissociation techniques in a guided ion beam tandem mass spectrometer, where M + = Na + and K +, and X = Cl, Br, and I. The primary dissociation pathway for all systems is endothermic loss of the neutral halobenzene from the cation–π complex. In addition, minor production of the ligand exchange product, M +Xe, is also observed. At elevated energies elimination of NaX is also observed for the Na +(C 6H 5Br) and Na +(C 6H 5I) systems. Ab initio calculations at the B3LYP/6-31+G* level of theory are used to determine the structures of the neutral π-ligands and cation–π complexes. Theoretical BDEs are determined from single point calculations at the MP2(full)/6-311+G(2d,2p) level using the B3LYP/6-31+G* optimized geometries. Good agreement between the theoretical and experimental BDEs is found in all cases. The trends in the BDEs of the M +(C 6H 5X) complexes are explained in terms of the varying magnitude of the electrostatic interactions in these complexes elucidated from a binding parameter model. Comparisons are also made to BDEs previously determined for the analogous M +(C 6H 6) and M +(C 6H 5F) complexes to examine the inductive effects of the halogen substituent on the strength of the cation–π interaction.

Excitations from dissociative fragments produced inH++H2Ocollisions

We report on photon emissions in the $200--800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ region resulting from collisions of $200\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ protons with ${\mathrm{H}}_{2}\mathrm{O}$. The most prominent features observed in the spectrum are the Balmer series of hydrogen and two OH molecular bands. Several less intense ${\mathrm{O}}^{+}$ as well as neutral O lines are also observed. The absolute photon emission cross sections of the major lines and bands were measured. The results indicate that a primary dissociation pathway involves the formation of ${\mathrm{H}}_{2}{\mathrm{O}}^{+}$ by removing a $1{b}_{2}$ electron. The unstable ${\mathrm{H}}_{2}{\mathrm{O}}^{+}$ ion further dissociates into ${\mathrm{H}}^{+}+\mathrm{O}\mathrm{H}$ or $\mathrm{O}{\mathrm{H}}^{+}+\mathrm{H}$. The dominant presence of neutral hydrogen lines and ${\mathrm{O}}^{+}$ lines leads to the conclusion that the subsequent dissociation of $\mathrm{O}{\mathrm{H}}^{+}$ into $\mathrm{H}+{\mathrm{O}}^{+}$ prevails over the other possible dissociation pathway leading to ${\mathrm{H}}^{+}$ and neutral oxygen fragments.

Dipole Effects on Cation−π Interactions: Absolute Bond Dissociation Energies of Complexes of Alkali Metal Cations to N-methylaniline and N,N-dimethylaniline

Threshold collision-induced dissociation of M (+)( nMA) x with Xe is studied using guided ion beam mass spectrometry, where nMA = N-methylaniline and N, N-dimethylaniline and x = 1 and 2. M (+) includes the following alkali metal cations: Li (+), Na (+), K (+), Rb (+), and Cs (+). In all cases, the primary dissociation pathway corresponds to the endothermic loss of an intact nMA ligand. The primary cross section thresholds are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) for ( nMA) x-1 M (+)-( nMA) after accounting for the effects of multiple ion-neutral collisions, the internal and kinetic energy distributions of the reactants, and the dissociation lifetimes. Density functional theory calculations at the B3LYP/6-31G* level of theory are used to determine the structures of these complexes, which are also used in single-point calculations at the MP2(full)/6-311+G(2d,2p) level to determine theoretical BDEs. The results of these studies are compared to previous studies of the analogous M (+)(aniline) x complexes to examine the effects of methylation of the amino group on the binding interactions. Comparisons are also made to a wide variety of cation-pi complexes previously studied to elucidate the contributions that ion-dipole, ion-induced-dipole, and ion-quadrupole interactions make to the overall binding.

Modeling Metal Cation−Phosphate Interactions in Nucleic Acids in the Gas Phase via Alkali Metal Cation−Triethyl Phosphate Complexes

Threshold collision-induced dissociation techniques are employed to determine the bond dissociation energies (BDEs) of complexes of alkali metal cations, Na+, K+, Rb+, and Cs+, to triethyl phosphate (TEP). The primary and lowest energy dissociation pathway in all cases is the endothermic loss of the neutral TEP ligand. Theoretical electronic structure calculations at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G* level of theory are used to determine the structures, molecular parameters, and theoretical estimates for the BDEs of these complexes. For the complexes to Rb+ and Cs+, theoretical calculations were performed using hybrid basis sets in which the effective core potentials and valence basis sets of Hay and Wadt were used to describe the alkali metal cation, while the standard basis sets were used for all other atoms. The agreement between theory and experiment is excellent for the complexes to Na+ and K+ and is somewhat less satisfactory for the complexes to the heavier alkali metal cations, Rb+ and Cs+, where effective core potentials were used to describe the cation. The trends in the binding energies are examined. The binding of alkali metal cations to triethyl phosphate is compared with that to trimethylphosphate.

Cation-π interactions with a π-excessive nitrogen heterocycle: Structures and absolute binding energies of alkali metal cation–pyrrole complexes

Threshold collision-induced dissociation techniques are employed to determine the bond dissociation energies (BDEs) of mono- and bis-complexes of alkali metal cations, Na +, K +, Rb +, Cs +, and the bis-complex of Li +, with pyrrole, C 4H 5N. The primary and lowest energy dissociation pathway in all cases is the endothermic loss of an intact pyrrole ligand. Sequential loss of a second pyrrole ligand is observed at elevated energies for the bis-complexes. Theoretical calculations at the MP2(full)/6-31G* level of theory are used to determine the structures, vibrational frequencies, and rotational constants of these complexes. Theoretical BDEs are determined from single point energy calculations at the MP2(full)/6-311+G(2d,2p) level using the MP2(full)/6-31G* optimized geometries. The agreement between theory and experiment is good for all complexes. The nature of the binding and the trends in the BDEs of these alkali metal cation–pyrrole complexes are compared to the analogous benzene and indole complexes using electrostatic potential maps and natural bond orbital analyses to examine the influence of the size of the aromatic system and the nitrogen heteroatom on the cation-π interaction. The binding of alkali metal cations to pyrrole is also compared to other metal cations and organic cations.

Influence of the d orbital occupation on the nature and strength of copper cation–π interactions: threshold collision-induced dissociation and theoretical studies

Threshold collision-induced dissociation techniques are employed to determine the bond dissociation energies of a wide variety of copper cation-pi complexes, Cu(+)(pi-ligand), where pi-ligand = benzene, flurobenzene, chlorobenzene, bromobenzene, iodobenzene, phenol, toluene, anisole, pyrrole, N-methylpyrrole, indole, naphthalene, aniline, N-methylaniline, and N,N-dimethylaniline. The primary and lowest energy dissociation pathway corresponds to the endothermic loss of the intact neutral pi-ligand for all complexes except those to N-methylpyrrole, indole, aniline, N-methylaniline, and N,N-dimethylaniline. In the latter complexes, the primary dissociation pathway corresponds to loss of the intact ligand accompanied by charge transfer, thereby producing a neutral copper atom and ionized pi-ligand. Fragmentation of the pi-ligands is also observed at elevated energies in several cases. Theoretical calculations at the B3LYP/6-311G(d,p) level of theory are used to determine the structures, vibrational frequencies, and rotational constants of these complexes. Multiple low-energy conformers are found for all of the copper cation-pi complexes. Theoretical bond dissociation energies are determined from single point energy calculations at the B3LYP/6-311+G(3df,2p) level of theory using the B3LYP/6-311G(d,p) optimized geometries. The agreement between theory and experiment is very good for most complexes. The nature and strength of the binding in these copper cation-pi complexes are studied and compared with the corresponding cation-pi complexes to Na(+). Natural bond orbital analyses are carried out to examine the influence of the d orbital occupation on copper cation-pi interactions.

Cation−π Interactions with a Model for the Side Chain of Tryptophan: Structures and Absolute Binding Energies of Alkali Metal Cation−Indole Complexes

Threshold collision-induced dissociation techniques are employed to determine bond dissociation energies (BDEs) of mono- and bis-complexes of alkali metal cations, Li+, Na+, K+, Rb+, and Cs+, with indole, C8H7N. The primary and lowest energy dissociation pathway in all cases is endothermic loss of an intact indole ligand. Sequential loss of a second indole ligand is observed at elevated energies for the bis-complexes. Density functional theory calculations at the B3LYP/6-31G level of theory are used to determine the structures, vibrational frequencies, and rotational constants of these complexes. Theoretical BDEs are determined from single point energy calculations at the MP2(full)/6-311+G(2d,2p) level using the B3LYP/6-31G* geometries. The agreement between theory and experiment is very good for all complexes except Li+ (C8H7N), where theory underestimates the strength of the binding. The trends in the BDEs of these alkali metal cation-indole complexes are compared with the analogous benzene and naphthalene complexes to examine the influence of the extended pi network and heteroatom on the strength of the cation-pi interaction. The Na+ and K+ binding affinities of benzene, phenol, and indole are also compared to those of the aromatic amino acids, phenylalanine, tyrosine, and tryptophan to elucidate the factors that contribute to the binding in complexes to the aromatic amino acids. The nature of the binding and trends in the BDEs of cation-pi complexes between alkali metal cations and benzene, phenol, and indole are examined to help understand nature's preference for engaging tryptophan over phenylalanine and tyrosine in cation-pi interactions in biological systems.