Hung Thanh Le

Density functional study of the decomposition pathways of nitroethane and 2-nitropropaneElectronic supplementary information (ESI) available: The structure of minima on the PES of nitroethane (Fig. S1) and 2-nitropropane (Fig. S2). See http://www.rsc.org/suppdata/cp/b3/b300275f/

relatively high barriers, 42.0 kcal mol � 1 and 39.2 kcal mol � 1 for 1 and 2 respectively. These values agree well with the activation energies predicted by Benson and co-workers on the basis of experimental information. The enthalpies of formation of 1 and 2 were also determined, both applying the G3 model chemistry to the atomisation reactions on the one hand, and the G3 and B3LYP methods to isodesmic reactions involving nitromethane on the other. Supporting evidence was found that the enthalpies of formation calculated in this way for 1, � 24.5 � 1 kcal mol � 1 , and 2, � 34.0 � 1 kcal mol � 1 , are indeed accurate enough for practical purposes. The accuracy of the density functional calculations is discussed at the light of the available experimental results.

Potential energy surfaces related to thioxy-hydroxy-carbene (HSνCνOH) and its radical cation

Thioxy-hydroxy-carbene (HS–C–OH) and its radical cation have been generated in the gas phase upon dissociative ionization of ethyl thioformates and characterized by various MS-MS-MS experiments. In the present study, their energies and unimolecular rearrangements have further been determined with the aid of abinitio molecular orbital calculations. Potential energy surfaces for both neutral [CH2OS] and ionized [CH2OS]+ species constructed at the QCISD(T)/6-311++G(d,p)//(U)MP2/6-31(d,p) level confirm that in both states, the carbene form is kinetically stable. While HS–C–OH is 179 kJ mol-1 less stable than the thiol acid HC(O)SH, [HS–C–OH]+ becomes even more stable than [HC(O)SH]+ and lies at 77 kJ mol-1 higher in energy than the thion acid [HC(S)OH]+, the most stable ion isomer. While it is not involved in the unimolecular chemistry of neutral thioformic acids, carbene plays a key role in that of ionized isomers. Some thermochemical parameters of HS–C–OH are estimated as follows: heat of formation ΔHf0=63 kJ mol-1 at 0 K and 57 kJ mol-1 at 298 K; ionization energy Ei=8.6 eV, and single–triplet gap ΔES-T=-156 kJ mol-1 in favour of the singlet state. For thioformic acid, its heat of formation is evaluated to be ΔHf,2980[HC(O)SH]=-124 kJ mol-1 and proton affinity PA[HC(O)SH]=773 kJ mol-1, with errors of ±10 kJ mol-1.

Molecular and electronic structure of zwitterionic diamino-meta-quinonoid molecules

Quantum chemical calculations using molecular orbital (HF, CASSCF) and density functional theory (B3LYP) methods, in conjunction with the 6–311 + + G(d,p) basis set, have been applied to investigate the molecular and electronic structure of two diamino-meta-quinonoid molecules 1a and 2a, each containing a six-membered ring coupled with two exocyclic C = O bonds situated in a meta position, along with two amino substituents (NH2 and NH-CH3). It is confirmed that these substituted meta-quinone systems exhibit a zwitterionic structure in which the positively charged N-C-C(H)-C-N subunit, containing the two amino-groups, forms two chemical bonds with the negatively charged O-C-C(H)-C-O subunit. The negative charge amounts to about half of an electron. The charge separation has been approached in terms of geometries, vibrational frequencies, and electronic distribution. The ionization energy for molecule 1a is about 7.8±0.3eV. The quinonoid systems are essentially non-aromatic characterized by the NICS(+1) values of around 1.2 ppm.

Protonation and methylation of thiophenol, thioanisole and their halogenated derivatives: mass spectrometric and computational study

The protonation and methylation of thiophenol (C6H5SH), thioanisole (C6H5SCH3) and 4-bromo derivatives have been studied using both tandem mass spectrometric techniques and ab initio quantum chemical calculations. Protonated 4-bromo thio-compounds produced by chemical ionization (CI) are found to be collisionally dehalogenated in an rf-only quadrupole collision cell in the low (20–30 eV) kinetic energy regime giving essentially thiophenol or thioanisole radical cations. This is indicated by MS/MS/MS experiments performed in an hybrid sector-quadrupole-sector mass spectrometer. B3LYP and CCSD(T) calculations using the 6-311++G(d,p) basis set consistently confirm that protonation of either (bromo)thiophenol or thioanisole takes place on the ring; the C 4-protonated thiophenol lies about 13 kJ mol −1 below the S-protonated isomer. However, under similar conditions, protonated thioanisole is also readily demethylated generating thiophenol radical cation, but no isomer has been detected. On the other hand, experimental and theoretical results reveal a regiospecific cationization (methylation) at the sulfur atom of the title compounds. The proton and methyl cation affinities are estimated as follows: PA (thiophenol) = 812 ± 10 kJ mol −1 , MCA (thiophenol) = 397 ± 10 kJ mol −1 , PA (thioanisole) = 839 ± 10 kJ mol −1 , and MCA (thioanisole) = 454 ± 10 kJ mol −1 . The available experimental value of 873 kJ mol −1 for PA (thioanisole) appears to be overestimated. Calculated PAs at various sites of halogeno (F, Cl and Br) thiophenols (o, m, p) are also reported.

Ionized aniline and its distonic radical cation isomers

Abstract A multi-sector tandem mass spectrometer fitted with a radiofrequency (rf)-only quadrupole collision cell (Qcell) has allowed fast and unambiguous identification of dehydroanilinium ions, the distonic isomers of ionized aniline. These ions were prepared by collisional deiodonation of protonated iodoanilines previously generated by chemical ionization (CI) or even better by liquid secondary ion mass spectrometry (LSIMS) conditions. Ab initio quantum chemical calculations were also used to probe the protonation of aniline and halogeno-anilines, X–C6H4–NH2 ( X = F , Cl, Br and I) and the relative stabilities of ionized aniline and its distonic radical cation isomers. Proton affinities (PAs) of anilines at nitrogen and carbon sites were found rather dependent on the nature and the position of the substituents.

Collisional activation of protonated halogeno-pyridines: different behaviour of target gases

Abstract Protonated 2-Cl and 2-Br-pyridines undergo facile dehalogenation upon high energy (8 keV range) collisional activation provided the target is NO, or even better O 2 , instead of He or Ar. In the low energy regime (20–30 eV range), debromination occurs more readily than dechlorination, but the peculiar behaviour of O 2 in favouring an X-loss over a HX-elimination, is no longer detected (using a MS 3 instrument). B3LYP/6-31G(d) calculations on X-pyridines with X=F, Cl and Br on positions 2, 3 and 4, suggest that, if a protonated pyridine is formed in the lower-lying triplet state following strong interaction with O 2 or NO having higher spin, the loss of Cl or Br becomes almost spontaneous (F-loss is more difficult). Proton affinity PA(pyridine)=940±15 kJ/mol, is decreased by 5–15 kJ/mol upon halogenation.

Distonic isomers of ionized benzaldehyde

Dissociative ionization of phthalaldehyde 1 and collisional deiodination of protonated isomeric iodobenzaldehydes 2–4 were attempted in searching for the possible production of distonic isomers (b–d) of ionized benzaldehyde a. These distonic species were initially produced in these experiments, but most of them readily isomerized to ionized benzaldehyde. Density functional theory B3LYP/6-31+G(d,p) calculations indicate that the distonic ions are surprisingly low energy species being 50–60 kJ mol −1 above a, but kinetically metastable with respect to b → a isomerization that has an energy barrier of only 70 kJ mol −1 . It is also suggested that low energy hydrogen migrations or a benzyl/tropylium isomerization likely precede collisional deiodination of protonated iodobenzaldehydes. (Int J Mass Spectrom 217 (2002) 65–73)

Collisional activation of protonated C-halogenopyrazoles

Abstract Collisional activation of protonated 3-halogenopyrazoles (X–Pz, X=Cl, Br and I) in the high or low translational energy regime induced an intense loss of X giving C 3 H 4 N 2 + radical cations whose structure depends on the nature of the halogen. Protonated 3-I–Pz generated thus ionized pyrazole a , whereas protonated 3-Cl–Pz was a precursor of an isomeric species ascribed to a dehydropyrazolium distonic structure b . A mixture of C 3 H 4 N 2 + ions was formed in protonated 3-Br–Pz. B3LYP/6-31++G(d,p) computations confirmed a regiospecific N 2 -protonation, and a low energy content of the distonic ions b or c (50 kJ mol −1 above a and lying in deep energy wells). Two competitive C–H and C–X bond cleavages were invoked to explain the contrasting behaviour of various protonated X–Pz under dehalogenation conditions.

On the triplet-singlet energy gap of acetylene

The triplet–singlet energy gap of acetylene, Te(3B2–1Σg+), was calculated using the coupler-cluster theory including all single and double excitations plus perturbative corrections for the triples, and multiconfigurational second-order perturbation theory methods with large basis sets. The cis-bent triplet state (a 3B2) is calculated to lie 30 500±500 cm−1 above the ground singlet state (1Σg+); the latter value differs somewhat from the most recent evaluation of T0=28 900 cm−1 [Amed et al., J. Chem. Phys. 110, 4248 (1999)] but agrees well with an earlier theoretical estimate of 30 270 cm−1 [Yamaguchi et al., Theor. Chim. Acta 86, 97 (1993)]. Thus the discrepancy of 1000 cm−1 may well arise from an interpretation of experimental results rather than a shortcoming of theoretical calculations.

Dehalogenation of protonated C-halogeno-1,2,4-triazoles: synthesis of new heterocyclic carbenic and ylid radical cations and contrasting behaviour of collision gases ☆

Abstract C(3,5)-halogeno-1,2,4-triazoles, protonated under chemical ionization conditions, are found to undergo easy dehalogenation upon 8 keV collisional activation conditions, provided the collision gas is oxygen, not helium. The ions produced under these reactions are demonstrated to be five-membered cyclic carbenic ions or ylid ions, isomers of more conventional molecular ions of 1,2,4-triazoles. The same unconventional radical cations can also be produced in the low kinetic energy regime (∼20–30 eV) if the halogen is bromine, not chlorine. These conclusions were derived from tandem mass spectrometric measurements (collisional activation, neutralization–reionization, and specific ion molecule reactions) performed on a hybrid tandem mass spectrometer of sector–quadrupole–sector configuration. Quantum chemical calculations using the density functional theory (DFT) at the B3LYP/6-31G(d,p) + ZPE level were also carried out on the protonated C-halogenated-1,2,4-triazoles in both singlet and triplet states and their fragmentation products (halogen = Cl and Br). Calculated results suggest that the dehalogenation, occurring when oxygen gas was employed, is likely to arise from an excitation of protonated species into their lowest-lying triplet state prior to dissociation. Ionization energies and proton affinities of triazoles were also evaluated.