Danielle Leblanc

A relationship between the kinetics and thermochemistry of proton transfer reactions in the gas phase

Abstract For the proton transfer reaction [MH]+ + B → M + [BH]+ (I) a correlation is observed between the experimental reaction rate kexp and the standard free energy variation ΔG°. This correlation may be described by a relationship of the type k exp /k coll = 1 [1 + exp (ΔG° + G a °)/RT] where kcoll is the collision rate constant and ΔGa° an apparent energy barrier for reaction (I). It is found that ΔGa° is of the same order as RT and thus the preceding relationship allows the determination of unknown gas-phase basicities. Implications for the use of the “bracketing” and the “kinetic” methods are discussed.

Proton Affinity and Heat of Formation of Vinyloxy [CH2CHO]. and Acetonyl [CH2COCH3]. Radicals

Frequently found in hydrocarbon oxidation and in the photochemistry of carbonyl compounds, the β-carbonyl radicals are of interest. The experimental proton affinities of the two title radicals have been determined from proton transfer reactions (as shown) monitored in an FT-ICR mass spectrometer. This led to an estimation of their heats of formation (1: 13±3; 2: -34±3 kJ mol(-1)). Ab initio molecular orbital calculations, up to the G2 level, confirmed these results.

Determination of ionization energies by the thermokinetic method

Abstract Charge transfer reactions of the type M ·+ +B → B ·+ +M have been studied in order to test the applicability of the “thermokinetic” method to the determination of ionization energies (IEs). Three examples are presented with M=benzene, cyclopentanone, and vinyl alcohol. The correlation observed between the bimolecular rate constants and IE(B) allows the determination of IE(M) with a mean deviation of ∼0.02 eV with respect to spectroscopic values. One other advantage of the method is to provide ionization energies of unstable neutrals. The role of various corrective terms, and of possible geometry changes during charge transfer reactions are discussed.

Reactions of silyl cations with ketones in the gas phase

In the gas phase, (CH(3))(3)SiOSi(+)(CH(3))(2) and (CH(3))CH(2)SiOSi(+)(CH(3))(2) ions 1 and 2 were formed in the external source of a Fourier transform ion cyclofrom resonance (FT-ICR) spectrometer by electron impact ionization of (CH(3))(3)SiOSi(CH(3))(3). In the FT-ICR cell, the electrophilic center of these ions reacts with acetone to give product ions whose structures are probed by comparison with those of the products formed by reaction with water. The mechanisms of formation of these products, studied by labeling, involve facile 1,3-methyl transfer from silicon to silicon and cyclic intermediates. Copyright 1999 John Wiley & Sons, Ltd.

Taming halonium metathesis

The halonium metathesis reaction of ketones and aldehydes, C=O + XF + → C=F + + X=O is difficult to harness for three reasons: (1) the interchange of F + for O is so exothermic that the C=F + product ion often decomposes further; (2) the adduct of XF + to the carbonyl oxygen frequently undergoes side reactions to the virtual exclusion of metathesis; and (3) the metathesis ion is liable to rearrange to a mixture of isomers. Several approaches have been explored to make this reaction a useful source of gaseous fluoroalkyl cations. DFT calculations reinforce the intuition that cyclopentenone should give a good yield of metathesis ions in its ion–molecule reaction with CF3 + , an expectation that is borne out by experiment. Other approaches have met with less dramatic success. Variation of XF + gives a number of interesting results, but little improvement over CF3 + . Deuterium substitution at the carbon adjacent to the carbonyl (-position) does not restrain further decomposition, but deuteration of the next carbon (-position) does appear to do so. The isotopic labeling experiments indicate that the prevalent mode of decomposition requires hydrogen shift followed by a 1,3-elimination of HF. Observed side reactions of adduct ions include alkene expulsion to yield CF3-containing products (such as m/z 125 from the reaction of cycloalkanones with CF3 + ) and direct elimination of HF (as conjectured for the reaction of CFO + with diethyl ketone, with concurrent loss of CO2). Structures of ion–molecule reaction products can be reliably inferred only when the expected mechanism yields the minimum energy structure for that molecular formula. (Int J Mass Spectrom 222 (2003) 451–463)

Protonation thermochemistry of ethyl halides.

Gas-phase basicities of ethyl halides have been accurately determined from experimental proton-transfer reaction rates. Proton affinities (PA) were deduced after consideration of the entropy change associated with the protonation process and from G2 ab initio calculations. The present PA(C2H5X) assessment (653, 679, 685, and 709 kJ mol(-1) for X=F, Cl, Br and I, respectively) indicates that the currently tabulated values should be revised downward by 10 to 30 kJ mol(-1).

Is ionized cyclopropylamine cyclic

Abstract A combination of experiments based on proton transfer reactions monitored in a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer and molecular orbital calculations up to the G2 and the CBS-Q levels demonstrates that the structure of the ions produced by electron ionization of cyclopropylamine, 1 , and relaxed at thermal energy, possess the ionized 1-propene amine structure [CH 3 CHCHNH 2 ] ·+ , 3 ·+ . The experimental deprotonation enthalpy of ions 3 ·+ is equal to 915.3 ± 3.2 kJ mol −1 . CBS-Q calculations are in good agreement with experiment. A value of 919–923 kJ mol −1 is calculated for the deprotonation enthalpy of ions 3 ·+ ; 298 K heat of formation values of 852 kJ mol −1 and 783 kJ mol −1 are predicted from G2 atomization energies for ions 2 ·+ and 3 ·+ , respectively. The heat of formation of [CH 2 CHCHNH 2 ] + ions has been evaluated to 757.6 ± 5.7 kJ mol −1 from experiment and 755.8 kJ mol −1 from G2 atomization energy.

Deprotonation of α-distonic ions. Proton affinities of the α-radicals

Abstract The proton affinity at the heteroatom PAX of four α-radicals (CH2OH, CH3CHOH, CH2OCH3 and CH2NH2) was measured by studying the deprotonation of the corresponding α-distonic ions in the cell of a FTICR spectrometer. This method can only be used for α-distonic ions which are more stable than their molecular ion counterpart. It was found that the PAX of the CH2OH, CH3CHOH, CH2OCH3 and CH2NH2 α-radicals lies respectively 15.7, 14.5, 10.1 and 17.2 kcal mol−1 under that of CH3OH, CH3CH2OH, CH3OCH3 and CH3NH2. These results are in good agreement with the PA obtained by high level ab initio calculations.

Silicon vs. carbon containing ions: 1,3-proton transfers within the (CH3)(X)Si(OR)(+OHR′) units

Abstract In the cell of an FT-ICR spectrometer, (CH3)(X)C(OCR)( + OHR ′ ) and (CH3)(X)Si(OCR) + OHR ′ (R and R′=H, CH3 or C2H5; X=H or CH3) covalent ions were generated by reaction of the (CH3)(X)+SiOCR′ cations with water or alcohols. In the so-formed covalent ions, experiment shows that 1,3-H+ transfers from oxygen to oxygen are often easy in silicon containing ions while they are not observed in the corresponding ions containing only carbon. Calculations indicate that the energy required for a 1,3-H+ transfer from oxygen to oxygen is almost identical whether the transition state contains a silicon atom or not. The greater strength of the SiO bond in cations, compared to that of the CO bond or, in other words, the great electrophilic character of cations possessing a Si+, is the main factor explaining the difference in the behavior of the studied silicon containing ions and ions containing exclusively carbon.

Multiple protonation sites in aryl ethers

Abstract Molecules containing a benzene ring and an oxygen atom typically have two types of protonation sites: on the ring (where facile intramolecular hydrogen transposition from carbon to carbon probably takes place) or on an oxygen lone pair. Four aryl ethers are compared: the isomers phthalan (1, a cyclic benzylic ether) and coumaran (2, a cyclic phenyl ether), as well as isochroman (3) and isopropyl phenyl ether (iPrOPh). The proton affinities of 1–3 have been measured using FT-ICR techniques as 830, 855 and 838 kJ/mol, respectively. Comparison with model compounds and Hartree-Fock-based SCF calculations indicate that protonated phthalan (1H) and protonated isochroman (3H) have O-protonated structures. By contrast, the conjugate acids of coumaran and iPrOPh prefer ring-protonated structures. Acidification/neutralization experiments in the ICR, as well as MIKE spectra, demonstrate that chemical ionization of iPrOPh produces noninterconverting O- and ring-protonated forms. Metastable ion decompositions of protonated phthalan and protonated isochroman give evidence of separate decomposition pathways for both types of tautomers. Protonated coumaran exhibits complete randomization of hydrogen between oxygen and the ring, which is attributed to high barriers for expulsion of neutral fragments.