Mass spectra of some azoles

The principal electron-impact fragmentation patterns of 3,4,5-triphenyl-1,2,4-triazole, 2,5-diphenyl-1,3,4-thiadiazole, their perfluorinated analogues and 2,5-di(pentafluorophenyl)-1,3,4-oxadiazole have been established from metastable ion evidence and precise mass measurements. Although ions produced by expulsion of nitrogen from the molecular-ions of these compounds are of low abundance, the simultaneous expulsion of nitrogen and a C₇X₅ radical (X = H or F) gives rise to abundant ions.     

Coordination Polyhedra OsXn(X = F, Cl, Br, I) in Crystal Structures

The Voronoi–Dirichlet polyhedra (VDP) and the method of intersecting spheres were used to perform crystal-chemical analysis of compounds containing complexes [Os _a X _b ] ^z–(X = F, Cl, Br, I). Atoms of Os(V) at X = F and Cl, of Os(IV) at X = Cl, Br, and of Os(III) at X = Br were found to exhibit a coordination number of 6 with respect to the halogen atoms and to form OsX₆octahedra. The coordination polyhedra of Os(III) for X = Cl, I are square pyramids OsX₄. Each Os(III) atom forms one Os–Os bond; as a consequence, the OsBr₆octahedra share a face in forming Os₂Br^3– ₉complexes, while the OsX₄pyramids (X = Cl, I) dimerize to produce [X₄Os–OsX₄]^2–ions. The influence of the valence state of the Os atoms and of the nature of the halogen atoms on the composition and structure of the complexes formed and some characteristics of the coordination sphere of Os were considered.     

Theoretical study on the substituent effect of halogen atom at different position of 7-azaindole-water derivatives: relative stability and excited-state proton-transfer mechanism

We have theoretically investigated the substituted effect on the first excited-state proton-transfer process of nX7AI-H₂O (n?=?2~6, X?=?F, Cl, Br) complex at the TD-M06-2X/6-31?+?G(d, p) level. Here X is the substituted halogen atom, and n value denotes the substituted position of X, such as C₂, C₃, C₄, C₅, or C₆. For the substituted 7-azaindole clusters, 6X7AI-H₂O molecule is the most stable structure in water. The replacement of halogen atom X does not affect the characters of the HOMO and LUMO, but influence the S₀?→?S₁ adiabatic transition energies of nX7AI-H₂O (n?=?2~6, X?=?F, Cl, Br). Our calculated results show that the double proton transfer occurs in a concerted but asynchronous protolysis pathway no matter which H atom is replaced by halogen atom. The halogen substitution changes the structural parameters evidently and leads to amply the asynchronousity during the proton-transfer process. The ESPT barrier height increases or decreases due to the halogen atom and substituted position.     

Etude théorique par la méthode DFT des structures non-classiques des systèmes X2H2F2 (X=Si, Ge, Sn)

The structure and the relative stability of isomers of molecules X₂H₂F₂ (X=Si, Ge, Sn) have been studied using the density functional theory (DFT). We have determined the optimised structures of the substituted isomers. The XX bond have been studied and compared to that of the parent molecules: X₂H₄. It appears that, for the planar and trans ethylenic systems, the double bond character of the XX decreases when the hydrogen atoms are substituted by fluorine atoms. The most stable structure is shown to be the one where the two fluorine atoms are fixed on the same atom. The bridged structures are also studied.     

Theoretical study of fullerene derivatives: C40H4 and C40X4 cluster molecules

Herein we demonstrate that the C₄₀ cluster molecule is easily formed to T_d symmetry structure and its ground state is ⁵A₂ open shell with four unpaired electrons. These four unpaired electrons are located at the tip points of the T_d symmetry structure. This work also indicates that these four unpaired electrons can easily react with a single valence atom, such as hydrogen or halogen atoms, to form a stable carbon hydrogen cluster molecule, C₄₀H₄, and carbon halogen cluster molecules, C₄₀X₄ (X=F, Cl, Br, I), respectively. The PM3 semiempirical molecular orbital method from Gaussian 94W computer program package was applied very well to these cluster molecules. According to the results in this study, the structures of geometrical optimization, ionization potential, energy gap, heat of formation, atomization energy, vibration frequency, and the remaining data of C₄₀H₄ and C₄₀X₄ cluster molecules. The above-calculated data prove that these unknown cluster molecules are stable and have a stable capacity similar to 1,3,5,7-tetrahaloadamantane molecules. They can be possibly synthesized experimentally in the near future. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 273–284, 1998     

Tetra­ethyl­ammonium bis­(N,N‐diethyl­dithio­carbamato‐κ2S,S′)iodo­cadmate(II)

The title compound, (C₈H₂₀N)[Cd(C₅H₁₀NS₂)₂I], containing a heteroleptic five‐coordinate mononuclear anionic cadmium complex, crystallizes in ortho­rhom­bic form in the space group Pnma. Both anion and cation lie about mirror planes. Unlike other known [Cd(dtc)₂X]‐type complexes (where dtc is dithio­carbamate and X is a halogen or pseudohalogen), the central CdS₄I core shows a square‐pyramidal configuration, with a basal plane defined by four S atoms from two chelating dithio­carbamate ligands related by a symmetry plane. The central Cd atom is displaced from the basal S₄ plane towards the apical I atom of the square pyramid.     

Molecular structure and thermal stability of α-halogen-<Emphasis Type="Italic">gem</Emphasis>-dithiols

The electronic structure of α-halogen-gem-dithiols RC(SH)₂CH₂X (R = Me, Ph; X = F, Cl, Br, I) was studied by quantum chemistry methods. Four most stable rotamers were located, differing in the mutual orientation of the thiol groups and the halogen atom. The thermodynamic and kinetic characteristics of the thermolysis of α-halogen-gem-dithiols were obtained. Thermolysis of chlorine- and bromine-substituted gem-dithiols depends on the properties of the medium, namely, in aprotic media aromatic dithiols form trithianorbornane derivatives while aliphatic dithiols form thiirane derivatives. In an aqueous medium (R = Me, Ph), water promoted elimination of hydrogen sulfide with the formation of corresponding thiones is more preferable. Thermolysis of aliphatic iodine-substituted gem-dithiols proceeds as bimolecular deiodination resulting in the formation of a new C-C bond.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 559–568, March, 2005.     

Magnitude and origin of the attraction and directionality of the halogen bonds of the complexes of C6F5X and C6H5X (X = I, Br, Cl and F) with pyridine

The geometries and interaction energies of complexes of pyridine with C₆F₅X, C₆H₅X (X=I, Br, Cl, F and H) and R_FI (R_F=CF₃, C₂F₅ and C₃F₇) have been studied by ab initio molecular orbital calculations. The CCSD(T) interaction energies (E_int) for the C₆F₅X–pyridine (X=I, Br, Cl, F and H) complexes at the basis set limit were estimated to be ?5.59, ?4.06, ?2.78, ?0.19 and ?4.37 kcal mol^?1, respectively, whereas the E_int values for the C₆H₅X–pyridine (X=I, Br, Cl and H) complexes were estimated to be ?3.27, ?2.17, ?1.23 and ?1.78 kcal mol^?1, respectively. Electrostatic interactions are the cause of the halogen dependence of the interaction energies and the enhancement of the attraction by the fluorine atoms in C₆F₅X. The values of E_int estimated for the R_FI–pyridine (R_F=CF₃, C₂F₅ and C₃F₇) complexes (?5.14, ?5.38 and ?5.44 kcal mol^?1, respectively) are close to that for the C₆F₅I–pyridine complex. Electrostatic interactions are the major source of the attraction in the strong halogen bond although induction and dispersion interactions also contribute to the attraction. Short‐range (charge‐transfer) interactions do not contribute significantly to the attraction. The magnitude of the directionality of the halogen bond correlates with the magnitude of the attraction. Electrostatic interactions are mainly responsible for the directionality of the halogen bond. The directionality of halogen bonds involving iodine and bromine is high, whereas that of chlorine is low and that of fluorine is negligible. The directionality of the halogen bonds in the C₆F₅I– and C₂F₅I–pyridine complexes is higher than that in the hydrogen bonds in the water dimer and water–formaldehyde complex. The calculations suggest that the C? I and C? Br halogen bonds play an important role in controlling the structures of molecular assemblies, that the C? Cl bonds play a less important role and that C? F bonds have a negligible impact.     

σ-Hole bond tunability in YO<Subscript>2</Subscript>X<Subscript>2</Subscript>:NH<Subscript>3</Subscript> and YO<Subscript>2</Subscript>X<Subscript>2</Subscript>:H<Subscript>2</Subscript>O complexes (X = F,Cl, Br; Y = S,Se): trends and theoretical aspects

A σ-hole is defined as an electron-deficient region on the extension of a covalently bonded group IV–VII atoms. If the electronic density in the σ-hole is sufficiently low, then this region will have a positive electrostatic potential, which allows attractive noncovalent interactions with negative sites. SO₂X₂ and SeO₂X₂ (X = F, Cl and Br) have three Lewis acid sites of σ-hole located in the outermost of chalcogen atom and X end, participating in the chalcogen and halogen bonds with NH₃ and H₂O, respectively. MP2/aug-cc-pVTZ and M06-2X/aug-cc-pVTZ calculations reveal that for a given halogen atom, SeO₂X₂ forms stronger chalcogen bond interactions than SO₂X₂ counterpart. Almost a perfect linear relationship is evident between the interaction energies and the magnitudes of the product of most positive and negative electrostatic potentials. The interaction energies calculated by M06-2X and MP2 methods are almost consistent with each other.     

Ab-inito Calculations in Reductive Bond-breaking Reaction of C-X Bond in CH3X and CH2X2 with X = F and CI

MP4/6-31+G* level calculations are performed to study the reductive bond-breaking reaction of the C-X bond in halomethanes, CH₃X and CH₂X₂ where X is a fluorine atom or chlorine atom. This type of reaction involves a radical anion, after attaching an extra electron to the halomethane molecule, in which a C-X bond-breaking takes place. Products are a radical and a halogen anion. The equilibrium geometry and bond dissociation energy of the C-X bond thus found are in good agreement with previous theoretical and experimental results. The anomeric effect, electrostatic effect, and radical re-stabilization effect, are investigated to find their influences on bond length and bond dissociation energy in CH₃X and CH₂X₂. Potential energy curves are calculated for the reductive bond-cleavage process, and trends in activation energy for various cases are discussed.     

Gas-phase C-F bond cleavage in perfluorohexane using W-, Si-, P-, Br-, and I-containing ions: Comparisons with reactions at fluorocarbon surfaces

Gas-phase reactions of W-, Si-, P-, Br-, and I-containing ions with the target molecule perfluorohexane at low collision energies (<15 eV) parallel known ion/surface reactions of the same projectile ions at fluorinated self-assembled monolayer surfaces. Charge exchange, dissociative charge exchange, and fluorine atom abstraction are observed and the majority of the projectile ions also undergo reactive charge exchange to produce specific fluorocarbon fragment ions of the target molecule in distinctive relative abundances. Abstraction of up to five fluorine atoms is observed upon collision of W⁺ with gaseous perfluorohexane, while similar experiments with CI⁺, SiCl⁺, and PCl^+· show abstraction of one or two fluorine atoms. Other projectiles, including Si^+·, PCl ₂ ⁺ , Br⁺, CBr⁺, and I⁺, abstract only a single fluorine atom. These patterns of fluorine atom abstraction are similar to those observed in ion/surface collisions. Also paralleling the ion/surface reactions, halogen exchange (Cl-for-F) reactions occur between the Cl-containing projectile ions and perfluorohexane to produce C₆F₁₂Cl⁺, a product of chemical modification of the target. Collisions of PCl^+· and PCl ₂ ⁺ also result in production of C₆F ₁₂ ^+· , indicating that the corresponding surface modification reaction involving molecular defluorination should be sought. Implications for previously proposed mechanisms, new ion/surface reactions, and for the use of gas-phase studies to guide investigations of the ion/surface reactions are discussed.     

Mass spectra of halogenated esters 6—Methyl esters of some trihalogenated propanoic and butanoic acids

The mass spectral fragmentation of trihalogenated methyl esters, formed in the reactions of monochlorinated methyl propenoates and 2-butenoates with Cl₂, BrCl and Br₂, have been investigated. In most cases α-cleavage gives the base peak, [COOCH₃]⁺, the peaks originating from the subsequent losses of one or two halogen atoms also being abundant. The primary loss of a halogen atom is more prominent in the C₄ derivatives, Br˙ and Cl˙ being preferentially lost from the 2- and 3-positions, respectively. The McLafferty rearrangement yields in one case the base peak; the 2-halo compounds could in general be distinguished by that fragmentation. Typical for all 2-bromo-substituted methyl butanoates studied is the base peak, [C₃H₃]⁺, at m/z 39, and for some 3-halo compounds the peaks at m/z 95, [C₂H₄ClO₂]⁺ and 139, [C₂H₄BrO₂]⁺.     

Electron impact fragmentation of some mixed cyclotetraphosphazenes

The electron impact fragmentations of some cyclotetraphosphazenes are reported and discussed. The major fragmentation path involves loss of two amine radicals and one chlorine radical in the series P₄N₄Cl_8-n(NMe₂)_n when n=2, and subsequent stages involve a ring contraction process with elimination of a P = N fragment, when n = 5 loss of amine radicals predominates on statistical grounds with little evidence of ring contraction. In the series P₄N₄F_8-n(NMe₂)_n fragmentation is dominated by loss of amino radicals when n = 4 and loss of fluorine radicals predominates on statistical grounds when n = 2. In the series P₄N₄F_8-nX_n (n = 2 or 4, X = Cl or Br), when n = 2 and X = Br the major fragmentation path is the loss of two bromine radicals, whereas when X = Cl the more favoured path is the loss of two chlorine radicals. In both, subsequent stages involve ring contraction reactions with elimination of a PN fragment. When n = 4 and X = Br or Cl on bond energy grounds the more favoured fragmentation pattern is the loss of bromine or chlorine radicals, respectively.     

Substitution reactions in the secondary ion mass spectrometry of N-methylpyridinium halides

Secondary ion mass spectra of N-methylpyridinium halides (C⁺X^?, where C⁺ is a pyridinium cation and X^? is a halogen anion) exhibit the C⁺ ions, a series of cluster ions ((C⁺)_n(X^?)_n–1) and, furthermore, remarkable [CX – R]⁺ ions (R = H or Me). The mechanism of the formation of [CX – R]⁺ ions was investigated by the use of deuterated compounds and B/E and B²/E constant linked-scan measurements. A possible explanation is proposed in which the ions are produced through substitution reactions between species constituting the C₂X⁺ cluster ions in the gas phase.     

Investigation of interaction in C60 embedded complexes (X@C60) (X = alkali or halogen) at a series of radial positions by Buckingham potential function

Two typical series of C₆₀ embedded complexes (X@C₆₀) (X = Li, Na, K, Rb, Cs; F, Cl, Br, I) have been chosen to study as prototypes, in which the Buckingham potential (exp-6-1) function was applied to calculating the interactions of the atom pairs. The potential parameters are obtained from related crystals by the simulations using molecular mechanics methods. To utilize the symmetry of the potential field in C₆₀, the calculation is carried out along five typical radial directions. The computational results show that the interaction between the embedded atom and the C₆₀ cage is not purely electrostatic. The repulsive energy, E_rep, accounts for from 0.2% to 6.6% (for the alkali series), and from 1.5% to 58% (for the halogen series); the dispersive energy E_dis accounts for from 1.2% to 6.5% (for the alkali series), and from 2.2% to 42% (for the halogen series); and the electrostatic energy, E_es, accounts for 99% to 87% (for the alkali series) and from 96% to 0% (for the halogen series) when the embedded atom is put at the center of the cage. E_rep reaches up to 8% ∼ 35% (alkali), and 16% ∼ 704% (halogen); E_dis up to 4% ∼ 16% (alkali) and 7% ∼ 26% (halogen); and E_es falls down to about 88% ∼ 49% (alkali), and 96% ∼ 0% (halogen), when the embedded atom deviates 1.8 A from the cage center. The total interactions, E_inter, are all attractive for X (X = Li, Na, K, Rb, Cs; F. Cl, Br), but repulsive for the I atom. It is shown that the potential field in the C₆₀ cage has nearly spherical symmetry in an area with a radius of 1.8 Å around the cage center. The same kinds of interactions for the atoms in the two individual series are compared, and some variation rules are obtained. For (Li@C₆₀), the minimum energy equilibrium point deviates from the center by about 0.5 Å. © 1996 by John Wiley & Sons, Inc.     

Mass spectrometric studies on o-terphenyl-2-13C: Identification of compounds related to its synthesis and fragmentation processes

After identification of the impurities, separated from synthesized orthoterphenyl (I) labelled in position 2 with ¹³C, i.e. o-xenylcyclohexanol (II), o-xenylcyclohexene (III), hexahydrotriphenylene (IV) and o-xenylcyclohexane (V), the fragmentation processes under electron-impact of molecules I, II and IV have been studied, advantage being taken of their ¹³C labelling. Mass spectrometric studies were carried out on an AEI MS-9 for determination of the atomic composition of the main ions and on a MAT CH-4 instrument for quantitative determination of ¹²C and ¹³C species, at low energy, assuming an equal sensitivity. The ¹³C labelling of position 2 in I, has not yet given clear evidence of a formerly assumed rearrangement into phenanthrenic structure after ring opening. The fragmentation of molecule II occurs in two ways, i.e. into a series of hydrocarbon fragments, produced from the molecular ion by loss of H₂O or into fragments still containing the oxygen atom. The difference between the fragmentation processes of the two series depends on the fact that the loss of H₂O from the molecule stabilizes the two carbon atoms adjacent to the C carrying the hydroxyl group. This is shown by the relatively higher ¹³C content of the hydrocarbon fragment in C¹⁵ compared with that of the equivalent oxygenated fragment. The labelling in IV shows how the rupture of the saturated ring occurs under electron-impact, the first C removed being the one nearest to the central ring.     

Secondary interactions in the iso­morphous compounds 2,6‐bis­(chloro­methyl)­pyridinium chloride and 2,6‐bis­(bromo­methyl)­pyridinium bromide

The title compounds, C₇H₈Cl₂N⁺·Cl⁻ and C₇H₈Br₂N⁺·Br⁻, are isomorphous. In the crystal packing, layers parallel to the ac plane are formed by a classical N⁺—H⋯X⁻ hydrogen bond (X = halogen) and two X⋯X contacts. A third X⋯X contact links the layers, and a fourth, which is however very long, completes a ladder‐like motif of halogen atoms. Hydro­gen bonds of the form C—H⋯X play at best a subordinate role in the packing.     

Divergent synthesis of 2-C-branched pyranosides and oxepines from 1,2-gem-dibromocyclopropyl carbohydrates

The ring opening of 1,2-(gem-dibromo)cyclopropyl carbohydrates by two different modes leads to either 2-C-(bromomethylene)pyranosides (using base) or 2-bromooxepines (using silver salts), as shown previously by us for a d-glucal-derived cyclopropane. The base-promoted ring opening is extended to encompass additional alcohol, thiol and amine nucleophiles, and diastereoisomeric cyclopropane precursors. Cross-coupling of the 2-C-(bromomethylene)pyranosides leads to extended 2-C-branched pyranosides. Silver-promoted ring expansion of the cyclopropyl carbohydrates in the presence of various alcohols is described. Cross-coupling of the resulting benzyl 2-bromooxepines affords 2-C-substituted oxepines.     

Behaviour of isomeric methyl ethyl and ethyl methyl halosuccinates under electron impact. Elimination of a halogen atom by a multi-step mechanism

The electron impact mass spectra of isomeric methyl ethyl and ethyl methyl halosuccinates (X = Cl and Br) are surprisingly different. Only the isomers with the ethyl group remote from the halogen give rise to [M - X]⁺ ions. A low-energy collision-induced dissociation study of deuterium-labelled analogues of the former isomers indicates that the [M - X]⁺ ions are mixtures of protonated methyl ethyl maleate (major component, > 85%) and fumarate, and the loss of the halogen atom is a multi-step process including at least two specific hydrogen transfers. Migration of a β-hydrogen atom to the carbonyl oxygen within the ethoxycarbouyl group produces a primary radical site in a distonic intermediate which, by subsequent abstraction of a hydrogen atom from C(3), triggers the ejection of X from C(2) with concomitant double bond formation. Whereas in the other isomer an [M - X]⁺ ion is absent or negligible, a characteristic double loss of C₂H₄ and CO₂ is observed.     

A study of the structures of decomposing and non-decomposing [C4H5N]+˙ ions formed from different neutral species

[C₄H₅N]^+˙ ions have been generated from eleven neutral species. From a study of their metastable transitions and the translational energy released in the fragmentation in which C₂H₂ is lost, it is concluded that [C₄H₅N]^+˙ ions with sufficient energy to decompose do so from a common structure or mixture of structures when they are generated from crotonitrile, allyl cyanide, cyclopropyl cyanide, methacrylontrile, pyrrole, 2-, 3- and 4-hydroxypyridines and 2-aminopyridine. The [C₄H₅N]^+˙ ions formed from allyl isocyanide decompose from a different structure and those given by cyclopropyl isocyanide appear to decompose from a mixture of the two structures. Non-decomposing [C₄H₅N]^+˙ ions were investigated by means of their collision induced decomposition spectra using a B/E linked scan. Six different structures or mixtures of structures are suggested to explain these observations.