A MNDO study of the structural and electronic properties of polysilane model compounds

The results of MNDO geometry optimizations on selected H? (SiH₂)_n? H polysilane model compounds are presented. Near energetic degeneracy is indicated for all-trans(T), alternating gauche–trans (GT), and all-gauche (G⁺G⁺) models (n = 10). The most stable (T) and least stable (G⁺G⁺) conformations are separated by only ca. 0.11 eV. The existence of low-energy barriers to moderate structural distortion is also suggested. Orbital localizations and charge density distributions along the “polymer” backbone are found to be sensitive functions of such distortion. The ground-state electronic distribution of the saturated all-trans silane chains are calculated to be considerably more polarizable than the fully conjugated H? (CH)_n? H π-electron framework of comparable length. The one-electron HOMO → LUMO excitation can be viewed essentially as an in-plane Si 3p → Si3s + H1s intramolecular charge transfer transition. The qualitatively different atomic orbital character of the HOMO and LUMO levels yields transition moment components for the separate repeat units which are relatively small. In the case of the rigidly trans conformation, the phase relationships of the transition moment terms are such as to constructively sum to a large net value reflecting strong optical absorption, as is observed experimentally.     

Nitrogen incorporation in saturated aliphatic C6–C8 hydrocarbons and ethanol in low‐pressure nitrogen plasma generated by a hollow cathode discharge ion source

Ion/molecule reactions of saturated hydrocarbons (n‐hexane, cyclohexane, n‐heptane, n‐octane and isooctane) in 28‐Torr N₂ plasma generated by a hollow cathode discharge ion source were investigated using an Orbitrap mass spectrometer. It was found that the ions with [M+14]⁺ were observed as the major ions (M: sample molecule). The exact mass analysis revealed that the ions are nitrogenated molecules, [M+N]⁺ formed by the reactions of N₃⁺ with M. The reaction, N₃⁺ + M → [M+N]⁺ + N₂, were examined by the density functional theory calculations. It was found that N₃⁺ abstracts the H atom from hydrocarbon molecules leading to the formation of protonated imines in the forms of R′R″C?NH₂⁺ (i.e. C–H bond nitrogenation). This result is in accord with the fact that elimination of NH₃ is the major channel for MS/MS of [M+N]⁺. That is, nitrogen is incorporated in the C–H bonds of saturated hydrocarbons. No nitrogenation was observed for benzene and acetone, which was ascribed to the formation of stable charge‐transfer complexes benzene????N₃⁺ and acetone????N₃⁺ revealed by density functional theory calculations. Copyright © 2016 John Wiley & Sons, Ltd.     

Multiple ionization,charge separation and charge stripping reactions involving polycyclic aromatic compounds

The 70 eV electron ionization mass spectra of polycyclic aromatic compounds are characterized by the presence of relatively stable multiply charged molecular ions [M]ⁿ⁺ (n=2–4). When generated from the compounds benzene, napthalene, anthracene, phenanthrene, 2,3-benzanthracene, 1,2-benzanthracene, chrysene, 9,10-benzophenanthrene and pyrene, the relative abundances of the multiply charged ions increase dramatically with the number of rings. These compounds form multiply charged molecular ions (n=2, 3) which undergo unimolecular decompositions indicative of considerable ionic rearrangement. The main charge separation processes observed here [M]²⁺→m₁⁺+m₂⁺, [M]³⁺˙→m₃⁺+m→+m₄²⁺) involve, in almost every case, one or more of the products [CH₃]⁺, [C₂H₃]⁺˙ and [C₃H₃]⁺. This suggests the existence of preferred structures amongst the metastable parent ions. Information on the relative importance of the various fragmentation pathways is presented here along with translational energy release data. Some tentative structural information about the metastable ions has been inferred from the translational energy release on the assumption that the released energy is due primarily to coulombic repulsion within the transition state structure. For the triply charged ions these interpretations have necessitated the use of a coulombic repulsion model which takes account of an extra charge. Vertical ionization energies for the process [M]ⁿ⁺+G→[M](ⁿ⁺¹)+G+e^? (charge stripping) have also been determined where possible for n=1 and 2 and the results from these experiments allow the derivation of simple empirical equations which relate successive ionization energies for the formation of [M]²⁺ and [M]³⁺˙ to the appearance energy of [M]⁺˙.     

Structure and stability of oligomeric anions [M n X3 n + 1]? (M = Al, Ga, In; X = F, Cl, Br, I; n = 2–4): A quantum-chemical study

The structural and thermodynamic properties of oligomeric anions [M _n X_{3n+ 1}]⁻ (M = Al, Ga, In; X = F, Cl, Br, I; n = 2, 3, 4) have been obtained by the density functional theory B3LYP method with the LAN2DZ(d) and LAN2DZ(d)+ basis sets. A wide diversity of structural isomers was found for trimeric fluoride anions M₃F₁₀⁻. Among the trimers, except In₃F₁₀⁻, the most stable is a linear isomer composed of two MX₃ molecules coordinated to the MX₄⁻ anion. The formation of tetrameric anions M₄X₁₃⁻ was demonstrated to be thermodynamically allowed at low temperatures at MX₃: X⁻ > 4: 1. The existence of higher oligomers is less probable. The affinity of oligomer halides (MX₃) _n for halide ions increases with an increase in n. The propensity to form oligomeric anions decreases in the series F > Cl ≥ Br > I. The fluoride systems show a tendency to form structures with CN = 5 and 6, these structures for In being the most stable. Original Russian Text ? A.Yu. Timoshkin, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 1, pp. 87–100.     

Amidine mass spectral fragmentation patterns

Electron impact mass spectrometry of a range of amidines (R′NC(R)NHR′) including formamidines, acetamidines, benzamidines and tert-butylamidine, has been undertaken, and comparisons made of the fragmentation pathways followed by the different families of compounds. Fragmentation of all the molecular ions is characterized by skeletal carbon-nitrogen bond cleavage to form [R′NCR]⁺ and [R′NH]⁺ fragments, both of which are observed. For formamidines (R?H), the positive charge remains with the [R′NH]⁺ fragment which leads to the base peak at m/z93 corresponding to [R′NH₂]⁺˙. In contrast, for acetamidines and benzamidines the charge prefers to remain with the [R′NCR]⁺ fragment which gives the base peak for these compounds. The spectra of unsubstituted amidines (HNC(R)NH₂) are characterized by cleavage of the carbon substituent from the NCN skeleton, [CN₂H₃]⁺ (m/z 43) being produced in all cases.     

Collisional activation study of cyclic polysulfides

Cyclic polysulfides isolated from higher plants, model compounds and their electron impact induced fragment ions have been investigated by various mass spectrometric methods. These species represent three sets of sulfur compounds: C₃H₆S_x (x=1?6), C₂H₄S_x (x=1?5) and CH₂S_x (x=1?4). Three general fragmentation mechanisms are discussed using metastable transitions: (1) the unimolecular loss of structural parts (CH₂S, CH₂ and S_x); (2) fragmentations which involve ring opening reactions, hydrogen migrations and recyclizations of the product ions ([M? CH₃]⁺, [M? CH₃S]⁺, [M? SH]⁺ and [M? CS₂]^+˙); and (3) complete rearrangements preceding the fragmentations ([M? S₂H]⁺ and [M? C₂H₄]^+˙). The cyclic structures of [M]^+˙ and of specific fragment ions have been investigated by comparing the collisional activation spectra of model ions. On the basis of these results the cyclic ions decompose via linear intermediates and then recyclizations of the product ions occur. The stabilities of the fragment ions have been determined by electron efficiency vs electron energy curves.     

Electron impact and chemical ionization mass spectra of alkyldiphenylphosphine oxides

In the 70 e V electron impact mass spectra of a series of alkyldiphenylphosphine oxides (R?₂PO, R = Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, t-Bu, neopentyl, n-decyl), molecular ions of low abundance are observed and [M + H]⁺ ions are formed to a small extent at high sample pressures. The major ions include [?₂PO]⁺, [?₂POH]⁺; [?₂CH₂PO]⁺ and [?₂CH₂POH]⁺ which are formed by rearrangement and cleavage processes. The chemical ionization mass spectra obtained with methane and isobutane reagents consist of [M + H]⁺ ions. The proton affinity of R?₂PO was found to be 219 ± 2.5 kcal mol^?1.     

Mass spectral behavior of n-alkynes

The 70 e V-electron impact mass spectra of the C₇–C₁₀ n-alkynes have been determined as well as the metastable ion spectra of the molecular ions and the [CS₂]⁺ and [N₂O]⁺ charge exchange mass spectra of the C₇-C₉ n-alkynes. The metastable ion mass spectra provide only a limited opportunity to distinguish between isomers; however, the 70-eV EI mass spectra of isomeric compounds permit a ready distinction between isomers. The [CS₂]⁺ charge exchange mass spectra of isomeric compounds also show substantial differences. The [N₂O]⁺ charge exchange mass spectra do not show the enhancement of β-fission fragments observed in field ionization experiments, despite representing ions of similar internal energy, and it is concluded that field dissociation is responsible for the β-fission fragments in the field ionization experiments.     

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.     

Reversible cyclomerization and long range hydrogen abstraction by electron impact of 7-coumarinyl diesters ROOC(CH2)nCOOR

The mass spectra of 4-methyl-7-coumarinyl and 7-coumarinyl diestes ROOC(CH₂)_nCOOR (n = 2-12) have ben studied by appearance potential measurements, deuterium labelling and by comparison with suitable reference compounds such as the mised diestes ROOC(CH₂)_nCOOR′ (R=4-methyl-7-coumarinyl and R′ = methyl and phenyl) and 3.4-dihydro-4-methyl-coumarinyl diestes. Observations on the fragment ions of m/e 324, produced from the 7-coumrinyl diestes and their photocyclomers, by elimination of the central bridge as O?C?CH? (CH₂)_n–2? CH?C?O, demonstrate the existence and reversible formation of cyclomeric molecular ions. A stable bound system between the coumrin end groups is formed only at high internal energies by expulsion of a hydrogen atom, followed by elimination of the central bridge from the [M? H]⁺ ion. It is also shown that the lifetime of the open form molecular ions decreases remarkably for chain lengths with n larger than 6.     

Polymerization of acrylonitrile by catalytic systems consisting of triphenylphosphine and a lewis acid

Polymerization of acrylonitrile in the presence of systems that consisted of triphenylphosphine (PPh₃) and a Lewis acid R_mMX_n (ZnCl₂, Me₃Al, Et₃Al, Et₂AlCl, EtAlCl₂, AlCl₃) was studied. The systems that contained Me₃Al and Et₃Al (i.e., Lewis acid of moderate acidity) were the most efficient catalysts. Conductometric measurements carried out in the polymerization systems showed the presence of ions. The presence of phosphonium cation in the polyacrylonitrile chain formed by the PPh₃–R_mMX_n catalytic systems was determined by IR, ¹H-NMR, and ³¹P-NMR spectroscopy. The average molecular weight measurements and kinetic chain lengths of polyacrylonitrile formed within the reaction time in the presence of PPh₃–Et₃Al showed that transfer reactions occur. According to the results obtained, the polymerization reaction of acrylonitrile by PPh₃–R_mMX_n involved a zwitterion formed by the attack of PPh₃ on acrylonitrile complexed by Lewis acid [Ph₃P^⊕? CH₂? C^?H? C?N → MR_mX_n] and the anion [CH₂?C^?? C?N] formed by the proton abstraction from the monomer.     

Redistribution reactions in the ion source of a mass spectrometer of complexes of titanium (iv), tin (iv) and germanium (iv) with 8-quinolinolato and halo or ethoxy ligands

The relative intensities of peaks in the mass spectra of the compounds MX_4?nox_n (oxH = 8-quinolinol; n = 2; M = Ti; X = F, Cl, Br or OEt; M = Sn; X = F, Cl, Br or I; M = Ge; X = Cl or Br; n = 1; M = Ti; X = OEt) depend on the insertion temperature and the residence time of the sample in the mass spectrometer. In most cases ions which cannot arise by fragmentation of the respective molecular ions are observed. These ions arise from the ionisation and fragmentation of species which are due to redistribution reactions in the mass spectrometer. The fragmentation pattern of the compounds MX₂ox₂ (X = halogen), mainly involving loss of ligand radicals, is related to the common oxidation states of the metals and reflects the metal-halogen bond strength. The molecular ions of the compounds Ti(OEt)_4?nox_n (n = 0, 1 or 2) fragment by loss of intact ligand radicals.     

A comparison of positive and negative ion collision‐induced dissociation for model heptapeptides with one basic residue

The effects of the identity and position of basic residues on peptide dissociation were explored in the positive and negative modes. Low‐energy collision‐induced dissociation (CID) was performed on singly protonated and deprotonated heptapeptides of the type: XAAAAAA, AAAXAAA, AAAAAXA and AAAAAAX, where X is arginine (R), lysine (K) or histidine (H) residues and A is alanine. For [M + H]⁺, the CID spectra are dominated by cleavages adjacent to the basic residues and the majority of the product ions contain the basic residues. The order of a basic residue's influence on fragmentation of [M + H]⁺ is arginine > histidine ≈ lysine, which is also the order of decreasing gas‐phase basicity for these amino acids. These results are consistent with the side chains of basic residues being positive ion charge sites and with the more basic arginine residues having a higher retention (i.e. sequestering) of the positive charge. In contrast, for [M ? H]^? the identity and position of basic residues has almost no effect on backbone fragmentation. This is consistent with basic residues not being negative mode charge sites. For these peptides, more complete series of backbone fragments, which are important in the sequencing of unknowns, can be found in the negative mode. Spectra at both polarities contain C‐terminal y‐ions, but y_n″⁺ has two more hydrogens than the corresponding y_n^?. Another major difference is the production of the N‐terminal backbone series b_n⁺ in the positive mode and c_n^? in the negative mode. Thus, comparison of positive and negative ion spectra with an emphasis on searching for pairs of ions that differ by 2 Da (y_n″⁺ vs y_n^?) and by 15 Da (b_n⁺ vs c_n^?) may be a useful method for determining whether a product ion is generated from the C‐terminal or the N‐terminal end of a peptide. In addition, a characteristic elimination of NH?C?NH from arginine residues is observed for deprotonated peptides. Copyright © 2010 John Wiley & Sons, Ltd.     

Mass spectra of complexes of 8-quinolinol with trivalent metals (AL,Ga, In,Sc, Cr and Fe)

The main fragmentation of the compounds MX_3-nox_n (oxH=8-quinolinol. n = 3; M=AL, Ga, In, Sc, Cr or Fe. n = 2; M=In or Fe; X=Cl or Br. InIox₂. n = 1; M=AL, In or Fe; X= Cl or Br) involves loss of X and intact ox_. radicals. The comparative abundances of the fragments are primarily related to the common oxidation states of the metals. For example, all the Mox₃ compounds show the ions [Mox₃]⁺ and [Mox₂]⁺. The ions [Mox]⁺ and [M]⁺ are present when M=Ga, In, Cr or Fe but for the elements with only one oxidation state (Al or Sc) [M]⁺ is absent and [Mox]⁺ has only very low abundance. When M= Cr or Fe metal-containing ions arising from loss of species such as CO, H₂O, HX, C₂H₂, H and OH by fragmentation of the ox ligand are also present; this behaviour is rationalised in terms of the ability of these metals to undergo a unit change in oxidation state. When n=1 the ions [MXox₂]⁺ and [Mox₂]⁺ and when n= 2 the ions [MX₂ox]⁺ and [Mox₃]⁺ are present; these ions arise by ionization and fragmentation of species formed by redistribution reactions in the mass spectrometer.     

Self‐Assembled Multimetallic/Peptide Complexes: Structures and Unimolecular Reactions of [Mn(GlyGly−H)2n−1]+ and Mn+1(GlyGly−H2n]2+ Clusters in the Gas Phase

The unimolecular chemistry and structures of self‐assembled complexes containing multiple alkaline‐earth‐metal dications and deprotonated GlyGly ligands are investigated. Singly and doubly charged ions [M_n(GlyGly?H)_n‐1]⁺ (n=2–4), [M_n+1(GlyGly?H)_2n]²⁺ (n=2,4,6), and [M(GlyGly?H)GlyGly]⁺ were observed. The losses of 132 Da (GlyGly) and 57 Da (determined to be aminoketene) were the major dissociation pathways for singly charged ions. Doubly charged Mg²⁺ clusters mainly lost GlyGly, whereas those containing Ca²⁺ or Sr²⁺ also underwent charge separation. Except for charge separation, no loss of metal cations was observed. Infrared multiple photon dissociation spectra were the most consistent with the computed IR spectra for the lowest energy structures, in which deprotonation occurs at the carboxyl acid groups and all amide and carboxylate oxygen atoms are complexed to the metal cations. The N?H stretch band, observed at 3350 cm^?1, is indicative of hydrogen bonding between the amine nitrogen atoms and the amide hydrogen atom. This study represents the first into large self‐assembled multimetallic complexes bound by peptide ligands.     

Mass spectrometry of homoadamantane derivatives

Homoadamantane derivatives can be divided into two groups according to their mass spectra. To the first group belong compounds with electron attracting substituents (COOH, CI, COOCH₃, Br); compounds with electron releasing substituents (OCH₃, OH, NH₃, NHCOCH₃) constitute the second group. The most characteristic feature of the first group compounds is the splitting off of the substituent. The hydrocarbon fragment [C₁₁H₁₇]⁺ thus formed then loses olefin molecules with the formation of corresponding ionic species C_11?nH_17?2n. The 3-substituted compounds of this group undergo thermal Wagner-Meerwein type rearrangements into adamantane derivatives, resulting in the [C₁₀H₁₅]⁺ (m/e 135) ion formation; this is the main difference between 1- and 3-substituted homoadamantanes. The series of [C_nH_2n?6X]⁺ ions (where X = OCH₃, OH, NH₂, NHCOCH₃, n = 6 to 10) are characteristic of the mass spectra of the second group compounds, the ion [C₆H₆X]⁺, [M ? C₅H₁₁]⁺ being the most abundant. The intensity ratio of [M ? C₅H₁₁]⁺ to [M ? C₄H₉]⁺ ions is 10:1 for 1-substituted and 3:1 for 3-substituted compounds of this group, allowing the location of the substituent. Some individual features of the spectra are also reported.     

Living cationic polymerization of vinyl ethers and styrene derivatives: Design of initiating systems based on added salts with halogen anions

This paper overviews three living cationic polymerization systems (for styrene, p-methoxystyrene, and isobutyl vinyl ether) that are, in common, featured by: (i) specifically in nonpolar solvents, the use of the hydrogen halide/metal halide initiating systems (HX/MX_n; X: I, Br, Cl; MX_n: ZnX₂, SnCl₄), which generate a living growing carbocation stabilized by a nucleophilic counteranion (X⁻…MX_n); (ii) specifically in polar solvents, the use of externally added ammonium salts (nBu₄N⁺Y⁻; Y⁻: I⁻, Br⁻, Cl⁻), which permit the generation of living species from HX/MX_n by providing nucleophilic halogen anions Y⁻, either the same as or different from the halogen X⁻ in HX.     

Structural relationships,crystal chemistry and anion substitution processes for M(III)X3 systems of the lanthanides and actinides

An examination of data for lanthanide and actinide phases with UCl₃-type and PuBr₃-type M(III)X₃ structures has shown that these systems are conveniently described by alternating layers of [MX₂]ⁿ⁺_n and [X]^n?_n. The relationships between the UCl₃- and PuBr₃-type structures are described and expanded to include a variety of anion substitution systems, M(III)X_3?xY_x. The two different types of [MX₂]ⁿ⁺_n layers observed in these systems are consistent with the existence of a novel structural unit, [M₂X₄]²⁺. The effects of radius ratio constraints and layering mechanisms on the phase equilibria and anionic substitution processes, polymorphism and crystal growth in the MX_3?xY_x systems are discussed.     

Mass spectra of sulfinamide derivatives and identification of their thermal decomposition products

The mass spectra of 30 sulfinamide derivatives (RSONHR', R' alkyl or p-XC₆H₄) are reported. Most of the spectra had peaks attributable to thermal decomposition products. For some compounds these were identified by pyrolysis under similar conditions to be: RSO₂NHR', RSO₂SR, RSSR and NH₂R' (in all kinds of sulfinyl amides); RSNHR' (in the case of arylsulfinyl arylamides); RSO₂C₆H₄NH₂, RSOC₆H₄NH₂ and RSC₆H₄NH₂ (in the case of arylsulfinyl arylamides of the type of X = H) The mass spectra of the three thermally stable compounds showed that there are several kinds of common fragment ions. The mass spectra of the thermally labile compounds had two groups of ions; (i) characteristic fragment ions of the intact molecules and (ii) the molecular ions of the thermal decomposition products. It was concluded that the sulfinamides give the following ions after electron impact: [M]⁺, [M ? R]⁺, [M ? R + H]⁺, [M ? SO]⁺, [RS]⁺, [NHR']⁺, [NHR' + H]⁺, [RSO]⁺, [RSO + H]⁺, [R]⁺, [R + H]⁺, [R']⁺ and [M ? OH]⁺, and that the thermal decomposition products give the following ions: [RSO₂SR]⁺, [RSSR]⁺, [M ? O]⁺, [M + O]⁺ and [RSOC₆H₄NH₂]⁺.     

Concerning the silicon-germanium bond: mass spectral fragmentations of isomeric SiGe compounds R3EE′R3 (E  Si,E′  Ge,R  Ph,Me,(η5-C5H4)Fe (C5H5), (η5-C5H5)Fe(CO)2 and (η5-C9H7) Fe(CO)2)

Isomeric pairs of silicon-germanium compounds containing a SiGe bond (Me₃SiGePh₃ (I) and Ph₃SiGeMe₃ (II); FpSiMe₂GeMe₃ (III) and FpGeMe₂SiMe₃ (IV) (Fp = (η⁵-C₅H₅)Fe(CO)₂); IFpSiMe₂GeMe₃ (V) and IFpGeMe₂SiMe₃ (VI) (IFp = (η⁵-C₉H₇)Fe(CO)₂); IFpSiMe₂GePh₃ (VII) and IFpGeMe₂SiPh₃ (VIII) and the complex FcSiMe₂GeMe₂Fc (IX) (Fc = ferrocenyl) have been synthesized and examined by mass spectrometry.The R₃SiGeR′₃ compounds I and II exhibit considerable exchange of R groups to produce [R_3-nR′_nSi]⁺ and [R′_3-nR_nGe]⁺ ion in progressively lesser amounts as n = 1 → 2 → 3. For the metal-substituted complexes containing the grouping FeSiGe fragmentation occurs predominantly via SiGe bond cleavage with formation of ions containing the silylene ligand [FeSiR₂]⁺. Complexes with the FeGeSi backbone undergo preferential scission of the FeGe bond, illustrating the general bond strength trend FeSi > SiGe. Upon direct cleavage of the SiGe bond in R₃SiGeR₃ compounds, the percentage of the charge carried by [R₃Si]⁺ ions significantly exceeds that carried by [R₃Ge]⁺ ions, reflecting the greater electronegativity of Ge polarizing the SiGe bond.