Binuclear metal carbonyl dab complexes : VII. A 13C NMR investigation of MM′(CO)6(DAB) complexes (M = M ′ = Fe,Ru; M = Mn,Re and M′ = Co; DAB = 1,4-diazabutadiene). Dynamic behaviour of σ2-N, σ2-N′, η2-C=N coordinated dab in MnCo(CO)6(DAB) and local scrambling of the carbonyl groups

The ¹³C NMR spectra of MM′(CO)₆(DAB) complexes (M = M′ = Fe, Ru; M = Mn, Re and M′ = Co; DAB = 1,4-diazabutadiene) show very characteristic features which are directly related with the bonding mode of the DAB ligand to the binuclear metal carbonyl fragment. In these complexes the DAB ligand is σ²-N, μ²-N′, η²-C=N or σ²-N, σ²-N′, η²-C=N coordinated. Chemical shifts of about 175 ppm are observed for the σ-coordinated imine fragments and about 60 or 80 ppm for the η²-C=N coordinated imine fragments.In MnCo(CO)₆[diacetylbis(cyclopropylimine)] the DAB ligand is fluxional, and the changes in the spectra when recorded at various temperatures can be interpreted in terms of an exchange between the σ- and π-coordinated part of the DAB ligand.The homodinuclear M₂(CO)₆(DAB) complexes (M = Fe or Ru) contain M(CO)₃ fragments on which the carbonyl groups are involved in a local scrambling process with very different activation parameters (T_c = ?50°C and +85°C).MCo(CO)₆(DAB) complexes (M = Mn, Re), which contain a semi-bridging carbonyl group according to the crystal structure, show rapid interchange of this carbonyl group with the terminal carbonyl groups on cobalt. The electronic balance is kept in equilibrium by an internal compensation within the DAB ligand.     

C Anisotropic chemical shift in a single crystal of benzophenone

The ¹³C resonance of the carbonyl carbon in a single crystal of benzophenone was studied by Fourier transform NMR at room temperature. Rotation patterns about the three cyrstallographic axes yielded the orientations of the major axis systems of the chemical shift tensor σ relative to the crystallographic axes for the four molecules in the unit cell. The principal elements of σ were found to be: σ_xx = −79 ± 4, σ_yy = −36 ± 4, and σ_zz = +94 ± 4 ppm , relative to CS₂. The Z axis of the nearly axially symmetric σ tensor was found to be perpendicular to the plane spanned by the carbonyl carbon and the carbon and oxygen atoms directly bound to it.     

Appearance-ionization potentials differences as a guide to the relative stabilities of the cis- and trans-A/B-steroids

The ionization potentials of cis- and trans-A/B-steroids of the androstane and pregnane series, as well as the appearance potentials of the M⁺-Me ions have been determined.The approximate activation energies (obtained as the AP-IP differences) for the loss of an angular Me from the molecular ion were found to be higher in the case of the more stable trans-A/B (5α)-steroids as compared to their cis (5β)-isomers.     

Ionization and appearance potentials in organic chemistry. II—Appearance potentials and enthalpy differences for the stereoisomers of methylbicyclo[4.4.0]decanes

The ionization potentials for the stereoisomers of trans-fused 2- and 3-methylbicyclo[4.4.0]decanes and the appearance potentials for the ions at m/e 137 [M–CH₃]⁺, 109, 96, 95 and 82 were measured by the electron impact method. The ionization potentials and appearance potentials of the [M–CH₃]⁺ ions appeared to be equal for each of the epimers. Appearance potentials of the other ions were always lower for the less stable epimer. No quantitative correlation was observed between the difference in the appearance potentials for any ions and the differences in enthalpies of the ground states.     

Studies in negative ion mass spectrometry: XIV—Electron attachment reactions of a series of η6-arenetricarbonylchromium(O) complexes

Electron attachment reactions of a series of (η⁶-arene)tricarbonylchromium(O) complexes have been examined in the gas phase. The electron capture process has been shown to be influenced by the structure of the η⁶-arene ligand and its substituents. Whereas (η⁶-benzene)- and (η⁶-mesitylene)tricarbonylchromium(O) undergo dissocative electron capture, or reductive decarbonylation, yielding [M? CO]^?˙ ions of highest abundance in their negative ion mass spectra, [η⁶-(2,2-dimethylindan-1,3-dione)]tricarbonylchromium(O) forms a molecular negative ion which undergoes sequential CO eliminations and finally a demetallation to give the arene radical ion. A localization of charge on the coordinated arene ligand is proposed for the formation of [M]^?˙ in this case. (η⁶-Methylbenzoate)tricarbonylchromium(O) also forms a molecular negative ion by secondary electron attachment which decomposes by simultaneous and consecutive eliminations of up to four CO molecules. The elucidation of a mechanism and sequence for these CO eliminations has been achieved by synthesizing and examining the negative ion mass spectrum of [η⁶-(C₆H₅·¹³CO₂Me)]Cr(CO)₃. The first CO loss in the principal fragmentation pathway occurs solely from the –Cr(CO)₃ group of [M]^?˙. The effect of para substituent groups on the stabilities of molecular negative ions and their fragmentations has been ascertained using a series of para-substituted (η⁶-methylbenzoate)tricarbonylchromium(O) compounds containing the groups NH₂, OH, OCH₃, CL and COOMe. The stabilities of the [M]^?˙ ions have been rationalized in terms of the Hammett and Taft parameters σ_P, σ_I, σ_R^P, σ_P^O and σ_R^O. The overall electronic substituent effect transmitted to the carbonyl groups of the –Cr(CO)₃ unit involves both resonance and inductive components. In this series of compounds the stability of [M]^?˙ decreases as the electron withdrawing capacities of the para substituents increase.     

Substituent effects in mass spectrometry—III: Substituent effects in the dissociation of the molecular ions of para and meta substituted benzoic acids

The effect of substituents on the electron-impact-induced fragmentation of the molecular ions of para and meta substituted benzoic acids has been examined. The substituent is observed to exert an effect on the ionisation potential of the molecular ion, on the appearance potentials of the primary daughter ions and on the amount of H/D scrambling in the molecular ion of the carboxyl-d₁ analogues prior to the loss of hydroxyl therefrom. The energy of activation for the loss of hydroxyl from the molecular ion is in general dependent upon the nature but not the position of the substituent, while the amount of H/D scrambling in the molecular ion of the carboxyl-d₁ derivative is dependent upon both the nature and the position of the substituent. No correlation of the relative ion abundances with σ⁺ constants was observed. The results are consistent with the molecular ions of each compound having a dissimilar energy distribution, which could arise either by different energy transfers from the electron beam to the molecule or by the participation of different isolated electronic excited states (or similar states but to varying extents) in the dissociation of the molecular ions.     

Ionization and appearance potentials in organic chemistry. I—energetic aspects of the mass spectra of stereoisomeric 4-substituted N-alkyl-2-methyldecahydroquinol-4-ols

The ionization potentials for the stereoisomers of trans-fused 1,2-dimethyl- and 1-ethyl-2-methyl-4-R-decahydroquinol-4-ols (R?C?CH, CH?CH₂ or C₂H₅) and the appearance potentials for the [M–CH₃]⁺ and [M–C₂H₅]⁺ ions (loss of 2-CH₃ and 4-C₂H₅ groups potential, respectively) were measured by using the electron impact method. The ionization and appearance potential for [M–CH₃]⁺ are always lower for the isomers with the axial 2-CH₃ group. For the C-2 epimers, the difference between the appearance potentials for the [M–CH₃]⁺ ion values is likely to be equal to the enthalpy differences between the ground states of the epimers and the dissociation energy differences between the axial and equatorial C₂–CH₃ bonds. The appearance potentials for [M–C₂H₅]⁺ for the C-4 epimers possessing the 4-C₂H₅ group were very similar. At the same time, the appearance potentials for the [M–CH₃]⁺ ions were lower for less stable epimers which had an axial 4-C₂H₅ group.     

Mass spectrometry of π-complexes of transition metals : III. Mass spectra of some π-cyclopentadienyl complexes of seven—coordinate rhenium

The mass spectra of π-RC₅H₄Re(CO)₂XY (R = H, CH₃ COOCH₃, X, Y = H, CH₃, COCH₃, Br, I, HgCl) have been studied. A successive expulsion of carbonyl groups, a common fragmentation path for cyclopentadienyl metal carbonyls, is observed in addition to the parallel elimination of substituents X and Y from the molecular ions of seven-coordinate rhenium complexes. Loss of a hydrogen molecule from the [M  2CO]⁺ ion is a characteristic of the fragmentation of σ-methyl complexes.     

An experimental and computational investigation on the fragmentation behavior of enaminones in electrospray ionization mass spectrometry

The dissociation pathways of protonated enaminones with different substituents were investigated by electrospray ionization tandem mass spectrometry (ESI‐MS/MS) in positive ion mode. In mass spectrometry of the enaminones, Ar? CO? CH?CH? N(CH₃)₂, the proton transfers from the thermodynamically favored site at the carbonyl oxygen to the dissociative protonation site at ipso‐position of the phenyl ring or the double bond carbon atom adjacent to the carbonyl leading to the loss of a benzene or elimination of C₄H₉N, respectively. And the hydrogen? deuterium (H/D) exchange between the added proton and the proton of the phenyl ring via a 1,4‐H shift followed by hydrogen ring‐walk was witnessed by the D‐labeling experiments. The elemental compositions of all the ions were confirmed by ultrahigh resolution Fourier transform ion cyclotron resonance tandem mass spectrometry (FTICR‐MS/MS). The enaminones studied here were para‐monosubstituted on the phenyl ring and the electron‐donating groups were in favor of losing the benzene, whereas the electron‐attracting groups strongly favored the competing proton transfer reaction leading to the loss of C₄H₉N to form a benzoyl cation, Ar‐CO⁺. The abundance ratios of the two competitive product ions were relatively well‐correlated with the σ_p⁺ substituent constants. The mechanisms of these reactions were further investigated by density functional theory (DFT) calculations. Copyright © 2010 John Wiley & Sons, Ltd.     

Mass spectrometry of π-complexes of transition metals XIX. Cymantrene-cyclopentadienylmetal carbonyl derivatives

Mass spectra of the complexes π-C₅H₅M(CO)_nXC₅H₄Mn(CO)₃ have been studied. The presence of intense ions C₅H₅M(CO)_nMn⁺ suggests that synthesis of new compounds with charged bivalent manganese of π-C₅H₅W(CO)₃Mn⁺X^? type is possible. In addition, the presence of these ions indicates preferential loss of the first three carbonyl groups from the manganese atom.     

On the photoionization and fragmentation of ammonia clusters using TPEPICO

Measurements of the appearance potentials of ammonia cluster ions (NH₃) _n ⁺ and (NH₃) _n ?2NH ₄ ⁺ using the threshold photoelectron photoion coincidence (TPEPICO) time-of-flight method are performed. The results agree well with other available data, however, solvation energies derived from these and from thermochemical data show significant discrepancies. The energetics for ammonia clusters are elucidated from various points of view.     

Mass spectra of aryl substituted N-t- butylbenzamides: Substituent effects on unimolecular ion decomposition

The major reactions of aryl substituted N-t-butylbenzamides upon electron-impact involve direct cleavage of a methyl radical, the loss of a butene molecule with the transfer of one hydrogen, or the loss of a butenyl radical with the transfer of two hydrogens. The last of these processes parallels the mass spectral behavior of aliphatic amides. Substituent effects indicate that electron-withdrawing groups on the aromatic ring enhance the two hydrogen transfer process, while electron-donating groups enhance the single hydrogen transfer process. Ion abundances, ionization potentials and appearance potentials are discussed with respect to correlation with σ⁺ values.     

Empirical relations between σI, σoR and taft σ* values of alkyl,acyl, benzoyl and substituted phenyl groups

The X(X) values¹ of the halogens (which resemble the Pauling electronegativities) and of some oxa substituents can be interpreted in terms of the inductive and resonance parameters σ_I and σ^o_R according to the regression equation

and η*_R=η(X)?η(R) it is found that for some substituted methyl, phenyl and benzoyl groups [σ*]^X_R=αη^X_R where α equals ?10.6 and ?10.9 for R = Me and R = Ph, CHO and PhCO respectively. Thus [σ*]^X_Rand η^x_r represent Taft σ* and [σ_I(X)?σ_oR(X)] values relative to that of the parent R group. The hydroxyl frequencies of phenol, and benzoic, acrylic, acetic and formic acids measured in dilute carbon tetrachloride solutions correlate with σ_I(X) and σ^o_R(X) according to the equations v(OH) = ?423.0 σ_I(X) + 3654.7 v(OH) = ?270.0 σ⁰_R(X) + 3586.7 where X = Ph, PhCO, CH₂=CHCO, MeCO and HCO. From these results, it is inferred that the σ* values of substituents having an α sp² hybridized carbon atom are proportional to σ⁰_R according to the equation σ*(X) = 3.97 σ⁰_R(X) + 1 New σ_I σ^o_R and σ* values of some acyi, benzoyl and substituted phenyl groups are presented.     

Proton magnetic shielding in malonic acid by multiple pulse techniques

The proton NMR in single crystals of malonic acid has been studied by multiple pulse line narrowing techniques. The nuclear magnetic shielding tensors σ⁽ⁱ⁾ of all protons in malonic acid could be determined from the spectra. There are two magnetically distinct carboxyl protons. The principal components of their shielding tensors are found to be σ⁽¹⁾_ZZ= ?0.8 ppm, σ⁽¹⁾_YY = ?19.2 ppm, σ⁽¹⁾_XX = ?21.8 ppm, and σ⁽²⁾_ZZ = ?1.0 ppm, σ⁽²⁾_ZZ = ?21.3 ppm relative to adamantane. The error limits are estimated to be ± 1 ppm. The most shielded directions lie along the hydrogen bond directions to within 8 degrees. The least shielded directions are essentially perpendicular to the plane of the carboxyl groups. Within experimental accuracy the shielding of the aliphatic protons is axially symmetric about the CH bond axes. The anisotropy Δσ = σ_? ? σ_⊥ is (4 ± 1) ppm. The gross features of the anisotropy of the carboxyl protons are shown to be governed by the diamagnetic effect.     

Photoelectrochemistry of HfSe2 involving two condition bands

During the investigation of n-type HfSe₂ (ΔE_g = 1.13 eV, with n = 1.2 × 10¹⁹ cm^?3) in contact with aqueous electrolytes, both anodic and cathodic photocurrents were detected. This inversion of the polarity of the photocurrents is only observed during illumination with photon energies larger than hν = 2.5 eV. For photons with hν<2.5 eV only anodic photocurrents were found. These results are interpreted in terms of two narrow d-conduction bands placed at different distances from the Fermi level, the existence of which is compatible with data on optical transmission. Additional results obtained concern the photocorrosion of HfSe₂ to Hf(IV) and elemental selenium, as well as a very characteristic transient photocurrent response arising at potentials considerably negative of the flat-band potential on a newly cleaved surface. The latter effect is interpreted as a photo-deintercalation phenomenon arising from hafnium atoms pushed on to intercalation or adsorption sites during cleavage, and released as ions during illumination.     

Dissociation of Multisubunit Protein–Ligand Complexes in the Gas Phase. Evidence for Ligand Migration

The results of collision-induced dissociation (CID) experiments performed on gaseous protonated and deprotonated ions of complexes of cholera toxin B subunit homopentamer (CTB₅) with the pentasaccharide (β-D-Galp-(1→3)-β-D-GalpNAc-(1→4)[α-D-Neu5Ac-(2→3)]-β-D-Galp-(1→4)-β-D-Glcp (GM1)) and corresponding glycosphingolipid (β-D-Galp-(1→3)-β-D-GalpNAc-(1→4)[α-D-Neu5Ac-(2→3)]-β-D-Galp-(1→4)-β-D-Glcp-Cer (GM1-Cer)) ligands, and the homotetramer streptavidin (S₄) with biotin (B) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(biotinyl) (Btl), are reported. The protonated (CTB₅ + 5GM1)ⁿ⁺ ions dissociated predominantly by the loss of a single subunit, with the concomitant migration of ligand to another subunit. The simultaneous loss of ligand and subunit was observed as a minor pathway. In contrast, the deprotonated (CTB₅ + 5GM1)^n- ions dissociated preferentially by the loss of deprotonated ligand; the loss of ligand-bound and ligand-free subunit were minor pathways. The presence of ceramide (Cer) promoted ligand migration and the loss of subunit. The main dissociation pathway for the protonated and deprotonated (S₄ + 4B)^n+/– ions, as well as for deprotonated (S₄ + 4Btl)^n– ions, was loss of the ligand. However, subunit loss from the (S₄ + 4B)ⁿ⁺ ions was observed as a minor pathway. The (S₄ + 4Btl)ⁿ⁺ ions dissociated predominantly by the loss of free and ligand-bound subunit. The charge state of the complex and the collision energy were found to have little effect on the relative contribution of the different dissociation channels. Thermally-driven ligand migration between subunits was captured in the results of molecular dynamics simulations performed on protonated (CTB₅ + 5GM1)¹⁵⁺ ions (with a range of charge configurations) at 800 K. Notably, the migration pathway was found to be highly dependent on the charge configuration of the ion. The main conclusion of this study is that the dissociation pathways of multisubunit protein–ligand complexes in the gas phase depend, not only on the native topology of the complex, but also on structural changes that occur upon collisional activation.

Infrared intensities as a quantitative measure of intramolecular interactions XXXVIII [1]. Selenophenes

The infrared intensities of the three ring stretching modes; v₅, v₆ and v₁₅ of a series of 2- and 3-substituted selenophenes have been measured. Using a previously developed valence bond approach these intensities are rationalised in terms of the σ⁰_R constants of the substituent groups.     

Correlations of substituent parameters with the carbonyl stretching force constant in arenetricarbonylchromium complexes

Correlations are established between selected substituent parameters (σ_I,σ_P, σ_R^P,σ_P^O,σ_R^O) and the carbonyl stretching force constant, k(CO), for 28 mono and poly-substituted tricarbonylchromium-complexed arene compounds. On the basis of the statistical results it is concluded that the overall electronic substituent effect transmitted to the carbonyl groups involves both mesomeric and inductive mechanisms. Within the restricted domain, including substituent group and benzene ring, transmission proceeds largely by resonance, with a minor inductive (through-bond and field) effect operative in the same domain. Further transmission from the substituted arene ring to the chromium atom predominantly involves an inductive mechanism. This result, in support of existing literature data, suggests appreciable participation of the ring carbon σ framework in the metal—ring bond formation.     

Translational energy release and stereochemistry of steroids. VII—the loss of angular methyl groups after the dehydration of molecular ions of cholesterol and related C(5)-unsaturated 3β-hydroxy steroids

The translational energy, T, released during the loss of the angular 18- and 19-methyl groups both from metastable molecular ions and metastable [M ? H₂O]⁺ and [M ? 2H₂O]⁺ ions, in C(5)-unsaturated mono-and di-hydroxy steroids, as well as in their 19-nor and deuterated analogues bearing the label in the 19-methyl group, has been measured. It was found that, while the T values for the 19-CH₃ loss, following the dehydration of the molecular ions, are increased substantially when compared to those for the same loss from the molecular ions, the T values for the 18-CH₃ loss are increased much more moderately. Nevertheless, the amounts of translational energy released in the [M ? H₂O]⁺˙ ? 18-CH₃˙ and [M ? 2 H₂O]⁺˙ ? 18-CH₃˙ transitions are still higher than those found for the respective 19-methyl loss, in accordance with the general rule established recently.     

New possibilities of dimethylformamide dimethylacetal as a derivatization agent for gas chromatography/mass spectrometry analysis

It is shown that the use of dimethylformamide dimethylacetal for the derivatization of analytes in gas chromatography/mass spectrometry cannot be restricted by the known conversion of carboxylic acids, phenols, and thiols into their methyl esters (ethers), as well as by the conversion of non-volatile amino acids (and C-amino compounds of other classes) into their dimethylaminomethylene derivatives. The application of this reagent to the derivatization of hydrazine derivatives and volatile carbonyl-containing analytes is considered. In the last case, the reaction proceeds selectively via CH₂ and/or CH₃ groups in the α-position to the carbonyl fragment. The principal predestination of the derivatization of such analytes is their characterization by differences of gas-chromatographic retention indices (ΔRI) of reaction products and initial substrates. The ranges of variation of such increments, ΔRI, appeared to be different for different subgroups of carbonyl compounds; this allowed us to determine their structures more precisely. The mass spectra of C-dimethylaminomethylene derivatives of some carbonyl compounds, preferably 2-substituted 1-methyl- and 1-aryl-3-(dimethylamino) prop-2-en-1-ones, revealed intense [M–17] peaks. The appearance of these signals can be explained by the migration of a hydrogen atom and the formation of [М–ОН]⁺ ions.