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Infrared Multiple Photon Dissociation (IRMPD) Spectroscopy has been used to generate gas phase vibrational spectra for six protonated tetracycline derivatives: chlortetracycline, meclocycline, minocycline, doxycycline, tetracycline, and metacycline. The IRMPD spectrum for protonated chlortetracycline is compared here to theoretical gas phase vibrational spectra obtained from geometry optimization and frequency electronic structure calculations using the B3LYP hybrid density functional method with the 6-311 + G(d,p) basis set. The most energetically favorable protonation sites, tautomer structures, and hydrogen bond orientations were determined. Protonation at the dimethyl amine functional group and an extensive hydrogen bonding network lead to the most energetically favourable structures. Calculated spectra directly resembled the IRMPD spectrum, supporting the conclusion that the probable gas phase structure of protonated chlortetracycline has been determined.

The formation of strong and very strong hydrogen bonds between NFnH3−n (n = 0.2) and F− have been computationally investigated via B3LYP/6-311++G(d,p) geometry optimizations and MP2/6-311++G(d,p) single point energy calculations. The enthalpy of deprotonation of NF2H was found to be 1518.5 kJ mol−1. The hydrogen bond energy in NH3⋯F−, NFH2⋯F− and NF2H⋯F− were calculated to be 67.9, 120.2 and 181.2 kJ mol−1, respectively, and clearly show the effect of fluorination on hydrogen bond strength in amine–fluoride systems. The change in enthalpy and entropy for the clustering of methanol to NF2H⋯F− to form the fluoride bound dimer of methanol and difluoramine has been measured via high pressure mass spectrometry to be 68.3 kJ mol−1 and 90.5 J K−1 mol−1. These values are in excellent agreement with calculated values of 70.9 kJ mol−1 and 88.5 J K−1 mol−1.

The equilibria involved in the formation of the proton-bound dimers of the three simplest amino acid methyl esters (glycine, alanine and valine methyl esters) were characterized by means of pulsed ionization high pressure mass spectrometry (PHPMS) experiments and density functional theory (DFT) calculations. Our results would indicate that, within the temperature range employed in these experiments, the most stable proton-bound dimer conformers are formed in the case of glycine and alanine whereas a more entropically favoured isomer would dominate in the case of valine. Various possible isomers of each of the proton-bound dimer species have been investigated computationally each exhibiting a combination of inter- and intramolecular hydrogen bonding. A system of nomenclature for these various species is proposed. The possibility of structures exhibiting 'salt-bridge’ interactions have also been explored, recognizing that such structures would necessarily result from highly energetic structural rearrangements.

The standard enthalpy and entropy changes (ΔH° and ΔS°) for the formation of solvated SN2 complexes (S)X−(RY) (X, Y = Cl, Br; R = (CH3)2CH; S = CH3OH, CH3CN, (CH3)2CO, CH3CF2H) have been determined by pulsed-ionization high pressure mass spectrometry. Not surprisingly, solvent effects are evident even at this mono-salvation level. Structures of solvated SN2 complexes and transition states for the Cl−(S) + CH3Cl SN2 reaction (S = H2O, H2S, NH3, PH3, SO2) have also been determined at the MP2/6-31+G(d) level of theory. A large variety of solvent dependent structures have been obtained, showing solvent reorganization upon going from the complex to the transition state. Standard binding and activation enthalpies ( Δ H 298 ∘ and Δ H 298 ‡ ) were determined at the MP2/6-311+G(3df,2p)//MP2/6-31+G(d,p) level of theory. For the Cl−(H2O) + CH3Br and Br−(H2O) + CH3Cl reactions, structures and enthalpies were calculated at the MP2/[6-31+G(d)/LanL2DZ(spd)] and MP2/[6-311+G(3df,2p)/LanL2DZ(spdf)]//MP2/[6-31+G(d)/LanL2DZ(spd)] level of theory. For the Cl− + CH3Br and Cl−(H2O) + CH3Br reactions potential energy surface scans were performed at the MP2/[6-31+G(d)/LanL2DZ(spd)] level of theory. Formation of the two possible sets of solvated products, Br−(H2O) + CH3Cl and Br− + (CH3Cl)(H2O) proceeds through two different surfaces. Water transfer to the leaving group can be facilitated by rotation of the Br−(CH3Cl) part in the exit channel Br−(CH3Cl)(H2O) complex. Finally, for the Cl− + CH3Cl reactions in the condensed phase, complexation and activation energies (ΔE(ɛ) and ΔE‡(ɛ)) were determined for a variety of solvents at the MP2/6-31+G(d) level of theory using the isodensity polarized continuum model. A linear correlation between −ΔE(ɛ) and ΔE‡(ɛ) was obtained, and a similar correlation exists for the mono-solvated gas phase SN2 reaction, indicating that mono-solvation already exhibits some features of the condensed phase reaction.

The thermochemistry of chloride ion clustering onto acetone and four fluorinated acetones (CH3C(O)CH2F, CF3C(O)CH3, CF3C(O)CF2H, and CF3C(O)CF3) under thermal equilibrium conditions has been determined by pulsed-ionization high pressure mass spectrometry (PHPMS). The standard enthalpy (ΔH°) and entropy (ΔS°) changes obtained indicate a variety of different types of bonding in these complexes. Ab initio computational methods have been used to obtain more insight into the structures and energetics. Surprisingly, in the Cl−(CF3C(O)CF2H) complex the chloride ion is not linearly hydrogen bonded, despite the presence of a very acidic CF2H bond. Instead, coordination with the carbonyl group carbon atom seems to be more pronounced, as found for the Cl−(CF3C(O)CF3) complex. For the Cl−(CF3C(O)CF3) clustering equilibrium a ΔS° value of −37.6 cal mol−1 K−1 has been measured, indicating that one or both CF3 group rotations will be hindered upon complex formation. Excellent agreement between ΔH°298 values calculated at the MP2/[6-311++G(3df,3pd)/6-311+G(2df,p)]//MP2/[6-31+G(d)/6-31G(d)] level of theory and experimental ΔH° values has been obtained. In addition, ΔacidH°298 values for a set of small to medium sized organic and inorganic acids, and the fluorinated acetones have been determined at the G3 and G3(MP2) levels of theory. Good to excellent agreement was obtained compared to experimental data, indicating that these composite methods perform well for the determination of reliable ΔacidH°298 results for medium sized molecules containing up to eight second row atoms. No linear correlation between clustering ΔH°298 and ΔacidH°298 values was found, indicating that most likely ion–dipole and ion-induced dipole interactions are determining the observed thermochemical trends. Finally, gas phase FT-IR spectra of CH3C(O)CH2F, CF3C(O)CH3, CF3C(O)CF2H, and CF3C(O)CF3 have been obtained, and the normal mode vibrational frequencies compared to results from calculations at the HF/6-31G(d) level of theory, scaled by 0.8953, and the B3LYP/6-311++G(3d,3p) level of theory. For the Cl−(CF3C(O)CF3) complex some large shifts in frequencies and IR absorption intensities are observed relative to CF3C(O)CF3, especially for the CO stretch.

The thermochemistry of the chloride ion clustering onto dimethyl ether, diethyl ether, and three fluorinated ethers (CH 3 OCF 3 , (CF 2 H) 2 O, and CF 3 OCF 2 H) under thermal equilibrium conditions has been determined using pulsed-ionization high pressure mass spectrometry. The standard enthalpy (Δ H °) and entropy change (Δ S °) values obtained indicate a variety of different types of bonding in these complexes. The mode of binding is mainly determined by the number of fluorine atoms present and by the substitution pattern. In addition ab initio computational methods have been used to obtain more insight into the structures and energetics. Δ H ° 298 value calculated at the MP2/[6-311 ++ G(3 df ,3 pd )/6-311 + G(2 df , p )]//MP2/[6-31 + G( d )/6-31G( d )] level of theory show excellent agreement with the experimentally obtained Δ H ° values.

The use of multiple overlapping gaseous ion–molecule equilibrium measurements to construct thermochemical ladders is reviewed. The types of relative thermochemical data, which may be obtained, are summarized and the types of measurements required to convert the relative equilibrium affinity scales to absolute thermochemical quantities are discussed. The important thermochemical scales produced include proton affinities, gas phase acidities, ionization energies, electron affinities, carbocation stabilities, hydrogen bond energies, Lewis acidities, and metal cation affinities.

Fourier transform ion cyclotron resonance (FTICR) measurements of solvent exchange equilibria, where the solvents differ only by a single deuterium substitution at the labile hydroxylic site, have been used to study the deuterium isotope effect on ion–molecule hydrogen-bonding interactions. The systems studied include chloride ion, fluoride ion, and alkoxide ions solvated by up to three molecules of alcohol: (ROH)n · Cl−, (ROH)n · F−, and (ROH)n · RO−. Differences in the measured isotope effects are explained on the basis of the differing hydrogen bond strengths in the adducts and by the ability of the chloride ion to partake in multiple site interactions. Keywords: Kinetic isotope effect; Hydrogen bonding; Fourier transform ion cyclotron resonance (FTICR); Ion–molecule reactions

Fourier transform ion cyclotron resonance experiments have been carried out to probe the nature of the potential energy surface for the reaction of methyl cation (CH3+) with methane (CH4) to give the ethyl cation (C2H5+). Product distributions for reactions of CH3+ and CD3+ with CD4 and CH4, respectively, are found to give a near statistical distribution of ethyl cation products, in good agreement with previous work. When the methyl cation is initially coordinated to HF in the form of a methylfluoronium ion, however, the product distributions are decidedly nonstatistical and are indicative of a reaction which is very nearly thermoneutral. Thermochemical data for relevant species suggest that the reaction is very slightly endothermic. All of the experiments support the intermediacy of a C2H7+ complex in the reactions. Ab initio calculations, in conjunction with all of the experimental data, reveal that there are likely three different stable forms of C2H7+ involved in the potential energy surface for the reaction. One of the forms, a nonclassical C2H5+ coordinated to H2 in a proton bound dimerlike structure, has not previously been considered to play a role in this reaction. The existence of this structure is supported by infrared multiphoton dissociation experiments on C2H7+ previously carried out by Lee and co-workers (J. Am. Chem. Soc. 111 (1989) 5597) and the high pressure mass spectrometric experiments of Hiraoka and Kebarle (J. Am. Chem. Soc. 98 (1976) 6119).

The gas phase equilibrium clustering reactions X−(ROH)n + ROH ⇌ X−(ROH)n+1 (X = F, Cl, Br, I; R = CH3, CH3CH2, (CH3)2CH, (CH3)3C; n = 0, 1, 2) have been investigated by using pulsed-ionization high pressure mass spectrometry (PHPMS). From the corresponding van’t Hoff plots the standard enthalpies (ΔHn,n+1O) and entropies (ΔSn,n+1O) were obtained, which are discussed in terms of the radii of the halides, the geometry of the alcohol molecules, the number of alcohol molecules, and molecular properties such as polarizability and gas phase acidity. The observed enthalpy trends can be explained on the basis of ion-dipole, ion-induced dipole, and dipole-dipole interactions within the clusters. The observed entropy trends are qualitatively discussed in terms of hindered rotations and low frequency intermolecular vibrations. In general, where available, there is good agreement between the present data and literature values obtained by various experimental techniques. In addition to the experiments, both density functional theory (DFT) calculations at the B3LYP/6–311+G(d,p) level of theory and G2 level calculations have been performed on a number of selected systems to test these methods for obtaining energetic data and to gain more insight into the structures of the investigated clusters.

Kinetic and thermodynamic isotope effects on the unimolecular dissociation of proton bound dimers were studied in the gas phase using mass spectrometry techniques. In addition proton transfer reactions were investigated using equilibrium techniques in conjunction with a theoretical study. Normal isotope effects were observed for all of the amine systems studied. The effect of label position, extent of labeling, size and structure of the proton bound dimers have been discussed with respect to (i) the kinetic and thermodynamic isotope effect on the dissociation reaction, (ii) the kinetic energy release on the dissociation reaction, (iii) the thermodynamic isotope effect on the proton exchange reaction between the labeled and unlabeled amines, and (iv) the effective temperatures and the excess energies of the metastable proton bound dimers. Other compound classes (CH3OH, (CH3)2O, CH3CN and (CH3)2CO) were studied and discussed in the same way, though not as thoroughly. All the systems show normal isotope effects, except for the proton bound dimer of CH3CN and CD3CN, which showed an inverse isotope effect.

The spontaneous dissociation of mixed proton bound dimers of nitrile bases has been studied using mass-analyzed ion kinetic energy spectra in a reverse geometry double focusing instrument coupled to a high pressure ion source. A linear correlation was found between the relative ion abundance ratios of the fragments and the ion source temperature. Proton exchange equilibria between the bases were studied using the time resolved capability of the high pressure mass spectrometry system, and relative ΔG°, ΔH°, and ΔS° values were extracted from the van't Hoff plots. The proton affinities (PA) of the nitriles studied were found to be PA(MeCN) = 186.8 kcal/mol, PA(EtCN) = 189.9 kcal/mol, PA (n-PrCN) = 191.0 kcal/mol, and PA(i-PrCN) = 192.2 kcal/mol. The effective temperatures of the metastable protonated nitrile dimers at different ion source temperatures were obtained from a plot of In(I RCN /I ref. ) versus ΔGB (relative gas phase basicity). From a plot of the effective temperature of the protonated metastable nitrile dimers reacting in the second field free region of the mass spectrometer versus the ion source temperature, it was found that the effective temperature decreases with increasing ion source temperature, which can be explained from qualitative considerations of the relevant thermal Boltzmann distributions.