Minh-Tho Nguyen

The gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene--energetics, structure and interconversion of dihydrotropylium ions.

The hitherto unknown gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene (CHT) have been determined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Several independent techniques were used in order to exclude ambiguities due to proton-induced isomerisation of the conjugate cyclic C7H9+ ions, [CHT + H]+. The gas-phase basicity obtained by the thermokinetic method, GB(CHT) = 799 ± 4 kJ mol−1, was found to be identical, within the limits of experimental error, with the values measured by the equilibrium method starting with protonated reference bases, and with the values resulting from the measurements of the individual forward and reverse rate constants, when corrections were made for the isomerised fraction of the C7H9+ population. The experimentally determined gas-phase basicity leads to the proton affinity of cycloheptatriene, PA(CHT) = 833 ± 4 kJ mol−1, and the heat of formation of the cyclo-C7H9+ ion, ΔHf0([CHT + H]+) = 884 ± 4 kJ mol−1. Ab initio calculations are in agreement with these experimental values if the 1,2-dihydrotropylium tautomer, [CHT + H(1)]+, generated by protonation of CHT at C-1, is assumed to be the conjugate acid, resulting in PA(CHT) = 825 ± 2 kJ mol−1 and ΔHf0300([CHT + H(1)]+) = 892 ± 2 kJ mol−1. However, the calculations indicate that protonation of cycloheptatriene at C-2 gives rise to transannular C–C bond formation, generating protonated norcaradiene [NCD + H]+, a valence tautomer being 19 kJ mol−1 more stable than [CHT + H(1)]+. The 1,4-dihydrotropylium ion, [CHT + H(3)]+, generated by protonation of CHT at C-3, is 17 kJ mol−1 less stable than [CHT + H(1)]+. The bicyclic isomer [NCD + H]+ is separated by relatively high barriers, 70 and 66 kJ mol−1 from the monocyclic isomers, [CHT + H(1)]+ and [CHT + H(3)]+, respectively. Therefore, the initially formed 1,2-dihydrotropylium ion [CHT + H(1)]+ does not rearrange to the bicyclic isomer [NCD + H]+ under mild protonation conditions.

Decarboxylation of metastable methyl benzoate molecular ions

By using a combination of mass spectrometric methodologies and density functional theory calculations [DFT/B3LYP/6-311 ++ G(d, p)], it is proposed that the decarboxylation of metastable methyl benzoate molecular ions occurs via distonic and ion-neutral complex (INC) intermediates. The same INC involving a complex between the benzyl radical and protonated carbon dioxide is also generated upon decarboxylation of metastable phenylacetic acid molecular ions. Internal proton transfer within the INC produces in fine a mixture of toluene and isotoluene radical cations.

On the Loss of SH• from the Molecular Ions of S-Alkyl Thioformates: Experimental Evidence for the Generation of Hydroxycarbenium Ions

A series of S-alkyl thioformates, HC(O)SR, has been synthesized by formylation of alkane thiols and their behaviour upon electron impact has been investigated by using tandem mass spectrometry methodologies. When R > CH3, a prominent loss of a SH • radical is observed in the ion source as well as in the field-free regions. This reaction, which requires an important preliminary rearrangement of the molecular ion, gives hydroxycarbenium ions as evidenced by collisional-activation spectra and consecutive collisional-activation experiments. A mechanism involving ionized thietane intermediates is proposed.

Ab initio calculation of the molecular structure and electronic properties of carbodi-imide, HNCNH

Results of ab initio SCF calculations on carbodi-imide (HNCNH) are reported. The geometry is optimized by the analytical-gradient method using the split-valence 4-31 G basis set. The most stable geometry exhibits the non-linear –NCN– frame with an NCN angle of 172.8°. The calculated rotational and inversion barriers of the N–H group are shown to be very low and are almost the same (3.84 and 3.54 kcal mol–1 for rotation and inversion, respectively), confirming the configurational instability of this species. Some molecular properties have been calculated by the 6-31 G** basis set.

Localized MO analysis of the 1,2-hydrogen shift mechanism

Abstract For the 1.2-hydrogen shift, a localized SCF MO analysis shows that two different electronic rearrangement mechanisms are possible. In the first case, the migrating hydrogen behaves as a hydride along the entire reaction path; in the other case. the migrating hydrogen first behaves as a hydride, but in the second half of the reaction behaves as a naked proton. The difference between the two mechanisms appears to be related to the spatial orientation of the lone-pair orbitals.

Protonation of nitrous acid and formation of the nitrosating agent NO+ : an Ab initio study

The most favourable conformation of nitrous acid HONO have been calculated using STO-3G, 3-21 G, and 4-31 G basis sets. Protonation is predicted to occur on the hydroxylic oxygen to give (4) as the most stable structure. This shows an unusually long N–O(2) bond distance and is best represented as a complex between NO+ and H2O since the geometries are only slightly changed from the isolated molecules. Rotation about the N–O(2) bond requires only a few hundred cal mol–1 as shown by calculations on the non-planar structures (7) and (8). The site of protonation can be correctly predicted from both the molecular electrostatic potential of HONO and from a calculation of the spatial extent of the lone pairs on the oxygens. The dissociation energy for [H2ONO]+ to give NO+ is calculated as 18.2 kcal mol–1(6.31**//3-21 G), consistent with rate-determining nitrosonium formation under some reaction conditions.

Protonation of nitric acid and formation of NO2+. An Ab initio study

The molecular structures and relative stabilities of nitric acid and its protonated species have been determined by SCF calculations. Protonation takes place preferentially at the hydroxylic oxygen atom, giving a complex of NO2+ and H2O. The dissociation energy [22 kcal mol–1(92 KJ mol–1) at 6-31 G**//44-31 G level] confirms that the production of NO2+ could constitute the rate-determining step in aromatic nitration reactions.

Letter: OCCO ·+ , NNCO ·+ and NNNN ·+ radical cations

Chemical ionization of a mixture of nitrogen and carbon monoxide produces three stable isobaric species at m/z 56: OCCO, OCNN and NNNN radical cations. Separated at increased resolution, these ions are readily identified by collisional activation. Neutralization–reionization experiments performed on two different mass spectrometers have not allowed the detection of any recovery signals for the corresponding neutrals.