Ketalization of phosphonium ions by 1,4-dioxane: selective detection of the chemical warfare agent simulant DMMP in mixtures using ion/molecule reactions

Phosphonium ions CH(3)P(O)OCH(3)(+) (93 Th) and CH(3)OP(O)OCH(3)(+) (109 Th) react with 1,4-dioxane to form unique cyclic ketalization products, 1,3,2-dioxaphospholanium ions. By contrast, a variety of other types of ions having multiple bonds, including the acylium ions CH(3)CO(+) (43 Th), CH(3)OCO(+) (59 Th), (CH(3))(2)NCO(+) (72 Th), and PhCO(+) (105 Th), the iminium ion H(2)C[double bond]NHC(2)H(5)(+) (58 Th) and the carbosulfonium ion H(2)C[double bond]SC(2)H(5)(+) (75 Th) do not react with 1,4-dioxane under the same conditions. The characteristic ketalization reaction can also be observed when CH(3)P(OH)(OCH(3))(2)(+), viz. protonated dimethyl methylphosphonate (DMMP), collides with 1,4-dioxane, as a result of fragmentation to yield the reactive phosphonium ion CH(3)P(O)OCH(3)(+) (93 Th). This novel ion/molecule reaction is highly selective to phosphonium ions and can be applied to identify DMMP selectively in the presence of ketone, ester, and amide compounds using a neutral gain MS/MS scan. This method of DMMP analysis can be applied to aqueous solutions using electrospray ionization; it shows a detection limit in the low ppb range and a linear response over the range 10 to 500 ppb.     

Endgroup-assisted siloxane bond cleavage in the gas phase

Unimolecular dissociation of H(2)N(CH(2))(3)SiOSi(CH(2))(3)NH(3)(+) generates SiC(5)H(16)NO(+) and SiC(5)H(14)N(+). The formation of SiC(5)H(16)NO(+) involves dissociation of a Si[bond]O bond and formation of an O[bond]H bond through rearrangement. The fragmentation mechanism was investigated utilizing ab initio calculations and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry in combination with hydrogen/deuterium (H/D) exchange reactions. Sustained off-resonance irradiation collision-induced dissociation (SORI-CID) studies of the fully deuterated ion D(2)N(CH(2))(3)SiOSi(CH(2))(3)ND(3)(+) provided convincing evidence for a backbiting mechanism which involves hydrogen transfer from the terminal amine group to the oxygen to form a silanol-containing species. Theoretical calculations indicated decomposition of H(2)N(CH(2))(3)SiOSi(CH(2))(3)NH(3)(+) through a backbiting mechanism is the lowest energy decomposition channel, compared with other alternative routes. Two mechanisms were proposed for the fragmentation process which leads to the siloxane bond cleavage and the SORI-CID results of partially deuterated precursor ions suggest both mechanisms should be operative. Rearrangement to yield a silanol-containing product ion requires end groups possessing a labile hydrogen atom. Decomposition of disiloxane ions with end groups lacking labile hydrogen atoms yielded product ions from direct bond cleavages.     

Ion-molecule reactions of ammonia clusters with C60 aggregates embedded in helium droplets

Helium nanodroplets are co-doped with C(60) and ammonia. Mass spectra obtained by electron ionization reveal cations containing ammonia clusters complexed with up to four C(60) units. The high mass resolution of Δm/m≈ 1/6000 makes it possible to separate the contributions of protonated, unprotonated and dehydrogenated ammonia. C(60) aggregates suppress the proton-transfer reaction which usually favors the appearance of protonated ammonia cluster ions. Unprotonated C(x)(NH(3))(n)(+) ions (x = 60, 120, 180) exceed the abundance of the corresponding protonated ions if n < 5; for larger values of n the abundances of C(60)(NH(3))(n)(+) and C(60)(NH)(n-1)NH(4)(+) become about equal. Dehydrogenated C(60)NH(2)(+) ions are relatively abundant; their formation is attributed to a transient doubly charged C(60)-ammonia complex which forms either by an Auger process or by Penning ionization following charge transfer between the primary He(+) ion and C(60). The abundance of C(x)NH(3)(+) and C(x)NH(4)(+) ions (x = 120 or 180) is one to two orders of magnitude weaker than the abundance of ions containing one or two additional ammonia molecules. However, a model involving evaporation of NH(3) or NH(4) from the presumably weakly bound C(x)NH(3)(+) and C(x)NH(4)(+) ions is at odds with the lack of enhancement in the abundance of C(120)(+) and C(180)(+). Mass spectra of C(60) dimers complexed with water complement a previous study of C(60)(H(2)O)(n)(+) recorded at much lower mass resolution.     

Cluster ions derived from sodium and potassium tetrafluoroborate and their collision induced dissociation in an ion trap mass spectrometer

Electrospray ionization was used to produce distributions of gas-phase cluster ions from solutions of sodium and potassium tetrafluoroborate. The majority of the cluster species followed the trend (MBF(4))(n)M(+), where M=Na and K. The values of n, for both salts, ranged from 1-15. Collision induced dissociation (MS/MS and MS(n)) in an ion trap mass spectrometer was used to determine the dissociation pathways for the cluster ions. The (NaBF(4))(n)Na(+) cluster ions fragmented via two pathways: (a) the loss of one or multiple neutral BF(3) molecules and (b) the loss of one or more NaBF(4) units. Of the two, the product ions corresponding to the loss of BF(3) units were more prominent. Unlike the Na salt, the (KBF(4))(n)K(+) cluster ions decomposed primarily by the loss of one or multiple KBF(4) units. Similar differences in dissociation behavior were observed when the heated transfer capillary, normally used to desolvate ions, was used to investigate cluster ion stability to thermal degradation and dissociation. The dissociation profiles (decrease in ion abundance with increasing activation amplitude) for several (NaF)(n)Na(+) and (KF)(n)K(+) cluster ions were measured and compared to probe the influence of the relative stability of the alkali fluorides (NaF and KF) on the dissociation behavior exhibited by the tetrafluoroborate cluster distributions. We found that the (NaF)(n)Na(+) cluster ions required higher activation amplitudes to induce fragmentation than the corresponding (KF)(n)K(+) species, indicative of stronger ionic bonding and higher gas-phase stability for the former. This in turn indicates that the reaction pathway involving only the loss of one or multiple units of BF(3), favored for the (NaBF(4))(n)Na(+) cluster series, but not for the analogous (KBF(4))(n)K(+) series, may be due to the high gas-phase stability of NaF, and relatively lower stability of KF, towards dissociation.     

Structures and dissociation channels of protonated mixed clusters around a small magic number: infrared spectroscopy of ((CH3)3N)n-H(+)-H2O (n = 1-3)

The magic number behavior of ((CH(3))(3)N)(n)-H(+)-H(2)O clusters at n = 3 is investigated by applying infrared spectroscopy to the clusters of n = 1-3. Structures of these clusters are determined in conjunction with density functional theory calculations. Dissociation channels upon infrared excitation are also measured, and their correlation with the cluster structures is examined. It is demonstrated that the magic number cluster has a closed-shell structure, in which the water moiety is surrounded by three (CH(3))(3)N molecules. The ion core (protonated site) of the clusters is found to be (CH(3))(3)NH(+) for n = 1-3, but coexistence of an isomer of the H(3)O(+) ion core cannot be ruled out for n = 3. Large rearrangement of the cluster structures of n = 2 and 3 before dissociation, which has been suggested in the mass spectrometric studies, is confirmed on the basis of the structure determination by infrared spectroscopy.     

C4H5N-(NH3)n氢键团簇的多光子电力与从头计算

在３５５和５３２ｎｍ激光波长下用ＴＯＦ质谱仪研究了Ｃ４Ｈ５Ｎ－（ＮＨ３）ｎ系列氢键团簇体系的多光子电离,实验发现,两波长下除了得到一系列团簇离子Ｃ４Ｈ５Ｎ－（ＮＨ３）ｎ＾＋外,还观测到一系列质子化产物Ｃ４Ｈ５Ｎ－（ＮＨ３）ｎ－Ｈ＾＋,这些质子化产物来自于光电离过程中团簇内部的质子转移反应;Ｃ４Ｈ５Ｎ－（ＮＨ３）ｎ＾＋系列离子出现反常强度变化,即Ｃ４Ｈ５Ｎ－（ＮＨ３）２＾＋离子强度较Ｃ４Ｈ５Ｎ－（Ｎ     

Structure and unimolecular chemistry of protonated sulfur betaines, (CH3)2S(+)(CH2)(n)CO2H (n = 1 and 2)

The fixed charge zwitterionic sulfur betaines dimethylsulfonioacetate (DMSA) (CH(3))(2)S(+)CH(2)CO(2)(-) and dimethylsulfoniopropionate (DMSP) (CH(3))(2)S(+)(CH(2))(2)CO(2)(-) have been synthesized and the structures of their protonated salts (CH(3))(2)S(+)CH(2)CO(2)H···Cl(-) [DMSA.HCl] and (CH(3))(2)S(+)(CH(2))(2)CO(2)H···Pcr(-) [DMSP.HPcr] (where Pcr = picrate) have been characterized using X-ray crystallography. The unimolecular chemistry of the [M+H](+) of these betaines was studied using two techniques; collision-induced dissociation (CID) and electron-induced dissociation (EID) in a hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer. Results from the CID study show a richer series of fragmentation reactions for the shorter chain betaine and contrasting main fragmentation pathways. Thus while (CH(3))(2)S(+)(CH(2))(2)CO(2)H fragments via a neighbouring group reaction to generate (CH(3))(2)S(+)H and the neutral lactone as the most abundant fragmentation channel, (CH(3))(2)S(+)CH(2)CO(2)H fragments via a 1,2 elimination reaction to generate CH(3)S(+)=CH(2) as the most abundant fragment ion. To gain insights into these fragmentation reactions, DFT calculations were carried out at the B3LYP/6-311++G(2d,p) level of theory. For (CH(3))(2)S(+)CH(2)CO(2)H, the lowest energy pathway yields CH(3)S(+)=CH(2)via a six-membered transition state. The two fragment ions observed in CID of (CH(3))(2)S(+)(CH(2))(2)CO(2)H are shown to share the same transition state and ion-molecule complex forming either (CH(3))(2)S(+)H or (CH(2))(2)CO(2)H(+). Finally, EID shows a rich and relatively similar fragmentation channels for both protonated betaines, with radical cleavages being observed, including loss of ˙CH(3).     

Infrared photodissociation spectroscopy of protonated ammonia cluster ions, NH4+(NH3)n (n=5-8), by using infrared free electron laser

Infrared photodissociation action spectra of protonated ammonia cluster ions, NH(4) (+)(NH(3))(n) (n=5-8), were measured in the range of 1020-1210 cm(-1) by using a tunable infrared free electron laser. Analyses by the density functional theory (DFT) show that the spectral features observed can be assigned to the nu(2) vibrational mode of the NH(3) molecules in NH(4) (+)(NH(3))(n). Size dependence of the spectra supports structural models obtained by the DFT calculations, in which the NH(4) (+) ion is solvated by the four nearest-neighbor NH(3) molecules. For NH(4) (+)(NH(3))(5), the spectrum between 1000 and 1700 cm(-1) was measured. The nu(4) bands of the NH(3) molecules and the NH(4) (+) ion were found in the range of 1420-1700 cm(-1).     

Modeling of the gas-phase ion chemistry of protonated arginine

Arginine is often involved at the C-terminus of peptides obtained from tryptic digests of proteins. The very basic guanidine group of the side-chain of arginine has a large effect on the backbone fragmentation of protonated peptides. Furthermore, arginine exhibits specific fragmentation reactions involving its side-chain. Various tautomerization states, conformers and side-chain dissociation channels of protonated arginine were studied using theoretical methods. The guanidine loss of protonated arginine is proved to be an S(N)2 substitution on the delta-carbon of the side-chain, starting from species containing the N(epsilon)H-C(+)(N(eta)H(2))(N(eta')H(2)) or -N(epsilon) (+)H(2)-C(N(eta)H)(N(eta')H(2)) moieties and leads to formation to either protonated guanidine or protonated proline. In the corresponding transition structures the proline moiety is protonated. Under low-energy collision conditions the extra proton transfers to the guanidine moiety, leading to the formation of C(+)(NH(2))(3). On the other hand, the lifetime of the fragmenting species under high-energy collision conditions is shorter, resulting in enhanced formation of protonated proline and its dissociation products. The first step of ammonia loss is the leaving of a preformed NH(3) from tautomers containing the -N(epsilon)H-C(N(eta)H(3) (+))(N(eta')H) or -N(epsilon)-C(N(eta)H(3) (+))(N(eta')H(2)) moieties. The resulting protonated carbodiimide group can be stabilized by intramolecular nucleophilic attack, leading to ring formation. Overall, reactions involved in the ammonia loss from protonated arginine can be considered as an S(N)1 substitution on the central zeta-carbon of the guanidine group.     

Evaluation of a novel approach for peptide sequencing: laser-induced acoustic desorption combined with P(OCH(3))(2)(+) chemical ionization and collision-activated dissociation in a Fourier transform ion cyclotron resonance mass spectrometer

A novel mass spectrometric method has been developed for obtaining sequence information on small peptides. The peptides are desorbed as intact neutral molecules into a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR) by means of laser-induced acoustic desorption (LIAD). Reactions of the neutral peptides with the dimethoxyphosphenium ion, P(OCH(3))(2)(+), occur predominantly by addition of the peptide to P(OCH(3))(2)(+) followed by the loss of two methanol molecules, thus yielding product ions with the composition (peptide + P - 2H)(+). Upon sustained off-resonance irradiation for collision-activated dissociation (SORI-CAD), the (peptide + P - 2H)(+) ions undergo successive losses of CO and NHCHR or H(2)O, CO, and NHCHR to yield sequence-related fragment ions in addition to the regular a(n)- and b(n)-type ions. Under the same conditions, SORI-CAD of the analogous protonated peptides predominantly yields the regular a(n)- and b(n)-type ions. The mechanisms of the reactions of peptides with P(OCH(3))(2)(+) and the dissociation of the (peptide + P - 2H)(+) ions were examined by using model peptides and molecular orbital calculations.     

Secondary ion emission induced by fission fragment impact in CO-NH(3) and CO-NH(3)-H(2)O ices: modification in the CO-NH(3) ice structure

CO-NH(3) and CO-NH(3)-H(2)O ices at 25-130 K were bombarded by (252)Cf fission fragments ( approximately 65 MeV at the target surface) and the emitted secondary ions were analyzed by time-of-flight mass spectrometry (TOF-SIMS). It is observed that the mass spectra obtained from both ices have similar patterns. The production of hybrid ions (formed from CO and NH(3) molecules) emitted from CO-NH(3) ice has already been reported by R. Martinez et al., Int. J. Mass. Spectrom. 262 (2006) 195; here, the secondary ion emission and the modifications of the CO--NH(3) ice structure during the temperature increase of the ice are addressed. These studies are expected to throw light on the sputtering from planetary and interstellar ices and the possible formation of new organic molecules in CO-NH(3)-H(2)O ice by megaelectronvolt ion bombardment. The presence of water in the CO-NH(3) ice mixture generates molecular ion series such as (NH(3))(p-q)(H(2)O)(q)CO(+) and replaces the cluster series (NH(3))(n)NH(4) (+) emission by the hybrid series (NH(3))(I-i)(H(2)O)(i=1, 2...I)H(+). The distribution of NH(3) and H(2)O molecules within the cluster groups indicates that ammonia and water mix homogeneously in the icy condensate at T = 25 K. The desorption yield distribution of the cluster series (NH(3))(n)NH(4) (+) is described by the sum of two exponential functions: one, slow-decreasing, attributed to the fragmentation of the solid target into clusters; and another, fast-decreasing, due to a local sublimation followed by recombination of ammonia molecules. The analysis of the time-temperature dependence of these two yield components gives information on the formation process of molecular ions, the transient composition of the ice target and structural changes of the ice. Data suggest that the amorphous and porous structure of the NH(3) ice, formed by the condensation of the CO--NH(3) gas at T = 25 K, survives CO sublimation until the occurrence of a phase transition around 80 K, which produces a more fragile ice structure.     

Fragmentation of protonated dipeptides containing arginine. Effect of activation method

The fragmentation reactions of the protonated dipeptides Gly-Arg and Arg-Gly have been studied using collision-induced dissociation (CID) in a quadrupole ion trap, by in-source CID in a single-quadrupole mass spectrometer and by CID in the quadrupole cell of a QqTOF mass spectrometer. In agreement with earlier quadrupole ion trap studies (Farrugia, J. M.; O'Hair, R. A. J., Int. J. Mass Spectrom., 2003, 222, 229), the CID mass spectra obtained with the ion trap for the MH(+) ions and major fragment ions are very similar for the two isomers indicating rearrangement to a common structure before fragmentation. In contrast, in-source CID of the MH(+) ions and QqTOF CID of the MH(+), [MH - NH(3)](+) and [MH <23 HN = C(NH(2))(2)](+) ions provide distinctly different spectra for the isomeric dipeptides, indicating that rearrangement to a common structure has not occurred to a significant extent under these conditions even near the threshold for fragmentation in the QqTOF instrument. Clearly, under normal operating conditions significantly different fragmentation behavior is observed in the ion trap and beam-type experiments. This different behavior probably can be attributed to the shorter observation times and concomitant higher excitation energies in the in-source and QqTOF experiments compared to the long observation times and lower excitation energies relevant to the ion trap experiments. Based largely on elemental compositions derived from accurate mass measurements in QqTOF studies fragmentation schemes are proposed for the MH(+), [MH - NH(3)](+), and [MH - (HN = C(NH(2))(2))](+) ions.     

吡啶团簇的飞秒光电离和从头计算研究

用飞秒激光电离飞行时间质谱研究了吡啶分子团簇在400 nm波长下的多光子光电离,实验观测到一系列的质子化和非质子化团簇离子.结果表明,质子转移也能发生在弱氢键结合的分子间.通过分析离子峰宽和离子信号强度随气源压力的变化,得到质子化团簇离子来源于大团簇离子的碎裂,而非质子化团簇离子是中性团簇直接电离的结果.从头计算结果表明,吡啶团簇是通过弱氢键C-H…N 结合在一起的,并且团簇离子离解倾向于生成质子化产物.     

Fragmentation reactions of protonated peptides containing glutamine or glutamic acid

A variety of protonated dipeptides and tripeptides containing glutamic acid or glutamine were prepared by electrospray ionization or by fast atom bombardment ionization and their fragmentation pathways elucidated using metastable ion studies, energy-resolved mass spectrometry and triple-stage mass spectrometry (MS(3)) experiments. Additional mechanistic information was obtained by exchanging the labile hydrogens for deuterium. Protonated H-Gln-Gly-OH fragments by loss of NH(3) and loss of H(2)O in metastable ion fragmentation; under collision-induced dissociation (CID) conditions loss of H-Gly-OH + CO from the [MH - NH(3)](+) ion forms the base peak C(4)H(6)NO(+) (m/z 84). Protonated dipeptides with an alpha-linkage, H-Glu-Xxx-OH, are characterized by elimination of H(2)O and by elimination of H-Xxx-OH plus CO to form the glutamic acid immonium ion of m/z 102. By contrast, protonated dipeptides with a gamma-linkage, H-Glu(Xxx-OH)-OH, do not show elimination of H(2)O or formation of m/z 102 but rather show elimination of NH(3), particularly in metastable ion fragmentation, and elimination of H-Xxx-OH to form m/z 130. Both the alpha- and gamma-dipeptides show formation of [H-Xxx-OH]H(+), with this reaction channel increasing in importance as the proton affinity (PA) of H-Xxx-OH increases. The characteristic loss of H(2)O and formation of m/z 102 are observed for the protonated alpha-tripeptide H-Glu-Gly-Phe-OH whereas the protonated gamma-tripeptide H-Glu(Gly-Gly-OH)-OH shows loss of NH(3) and formation of m/z 130 as observed for dipeptides with the gamma-linkage. Both tripeptides show abundant formation of the y(2)' ion under CID conditions, presumably because a stable anhydride neutral structure can be formed. Under metastable ion conditions protonated dipeptides of structure H-Xxx-Glu-OH show abundant elimination of H(2)O whereas those of structure H-Xxx-Gln-OH show abundant elimination of NH(3). The importance of these reaction channels is much reduced under CID conditions, the major fragmentation mode being cleavage of the amide bond to form either the a(1) ion or the y(1)' ion. Particularly when Xxx = Gly, under CID conditions the initial loss of NH(3) from the glutamine containing dipeptide is followed by elimination of a second NH(3) while the initial loss of H(2)O from the glutamic acid dipeptide is followed by elimination of NH(3). Isotopic labelling shows that predominantly labile hydrogens are lost in both steps. Although both [H-Gly-Glu-Gly-OH]H(+) and [H-Gly-Gln-Gly-OH]H(+) fragment mainly to form b(2) and a(2) ions, the latter also shows elimination of NH(3) plus a glycine residue and formation of protonated glycinamide. Isotopic labelling shows extensive mixing of labile and carbon-bonded hydrogens in the formation of protonated glycinamide.     

Bond energies and structures of ammonia-sulfuric acid positive cluster ions

New particle formation in the atmosphere is initiated by nucleation of gas-phase species. The small molecular clusters that act as seeds for new particles are stabilized by the incorporation of an ion. Ion-induced nucleation of molecular cluster ions containing sulfuric acid generates new particles in the background troposphere. The addition of a proton-accepting species to sulfuric acid cluster ions can further stabilize them and may promote nucleation under a wider range of conditions. To understand and accurately predict atmospheric nucleation, the stabilities of each molecular cluster within a chemical family must be known. We present the first comprehensive measurements of the ammonia-sulfuric acid positive ion cluster system NH(4)(+)(NH(3))(n)(H(2)SO(4))(s). Enthalpies and entropies of individual growth steps within this system were measured using either an ion flow reactor-mass spectrometer system under equilibrium conditions or by thermal decomposition of clusters in an ion trap mass spectrometer. Low level ab initio structural calculations provided inputs to a master equation model to determine bond energies from thermal decomposition measurements. Optimized ab initio structures for clusters up through n = 3, s = 3 are reported. Upon addition of ammonia and sulfuric acid pairs, internal proton transfer generates multiple NH(4)(+) and HSO(4)(-) ions within the clusters. These multiple-ion structures are up to 50 kcal mol(-1) more stable than corresponding isomers that retain neutral NH(3) and H(2)SO(4) species. The lowest energy n = s clusters are composed entirely of ions. The addition of acid-base pairs to the core NH(4)(+) ion generates nanocrystals that begin to resemble the ammonium bisulfate bulk crystal starting with the smallest n = s cluster, NH(4)(+)(NH(3))(1)(H(2)SO(4))(1). In the absence of water, this cluster ion system nucleates spontaneously for conditions that encompass most of the free troposphere.     

Metastable decompositions of gem-dialkoxyalkanes upon electron impact. III. Diethoxymethane (CH(2)(OCH(2)CH(3))(2))

Unimolecular metastable decomposition of diethoxymethane (CH(2)(OCH(2)CH(3))(2), 1) upon electron impact has been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and theD-labeling technique in conjunction with thermochemistry. The m/z 103 ion ([M - H](+) : CH(OCH(2)CH(3)) = O(+)CH(2)CH(3)) decomposes into the m/z 47 ion (protonated formic acid, CH(OH) = O(+)H) by consecutive losses of two C(2)H(4) molecules via an m/z 75 ion. The resulting product ion at m/z 47 further decomposes into the m/z 29 and 19 ions by losses of H(2)O and CO, respectively, via an 1,3-hydroxyl hydrogen transfer, accompanied by small kinetic energy release (KER) values of 1.3 and 18.8 meV, respectively. When these two elimination reactions are suppressed by a large isotope effect, however, another 1,1-H(2)O elimination with a large KER value (518 meV) is revealed. The m/z 89 ion ([M - CH(3)](+) : CH(2)(OCH(2)CH(3))O(+) = CH(2)) decomposes into the m/z 59 ion (CH(3)CH(2)O(+) = CH(2)) by losing CH(2)O in the metastable time window. The source-generated m/z 59 ion ([M - OCH(2)CH(3)](+) : CH(2) = O(+)CH(2)CH(3)) decomposes into the m/z 41 (CH(2) = CH(+)CH(2)) and m/z 31 (CH(2) = O(+)H) ions by losses of H(2)O and C(2)H(4), respectively, with considerable hydrogen scrambling prior to decomposition. Copyright 2000 John Wiley & Sons, Ltd.     

Gas-phase reactions of protonated tryptophan

The gas phase reactions of protonated tryptophan have been examined in a quadrupole ion trap using a combination of collision induced dissociation, hydrogen-deuterium exchange, regiospecific deuterium labeling and molecular orbital calculations (at the B3LYP/6-31G* level of theory). The loss of ammonia from protonated tryptophan is observed as the primary fragmentation pathway, with concomitant formation of a [M + H - NH(3)](+) ion by nucleophilic attack from the C3 position of the indole side chain. Hydrogen-deuterium exchange and regiospecific deuterium labeling reveals that scrambling of protons in the C2 and C4 positions of the indole ring, via intramolecular proton transfer from the thermodynamically preferred site of protonation at the amino nitrogen, precedes ammonia loss. Molecular orbital calculations have been employed to demonstrate that the activation barriers to intramolecular proton transfer are lower than that for NH(3) loss.     

Metastable ion study of organosilicon compounds. Part XIII: dimethoxydimethylsilane, (CH(3))(2)Si(OCH(3))(2), and dimethoxymethylsilane, CH(3)SiH(OCH(3))(2)

Unimolecular metastable fragmentations of dimethoxydimethylsilane, (CH(3))(2)Si(OCH(3))(2) (MW 120, 1), and dimethoxymethylsilane, CH(3)SiH(OCH(3))(2) (MW 106, 2), upon electron impact ionization have been studied by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and the D-labeling technique in conjunction with thermochemistry. The results have been compared with those of the corresponding carbon analogues, 2,2-dimethoxypropane, (CH(3))(2)C(OCH(3))(2) (MW 104, 3) and 1,1-dimethoxyethane, CH(3)CH(OCH(3))(2) (MW 90, 4). In analogy with the cases of 3 and 4, both molecular ions from 1 and 2 are formed at very low abundance at 70 eV, and begin to decompose by the expulsion of the substituents (H, CH(3) or OCH(3)) on the central silicon atom. These decompositions are followed by the loss of a formaldehyde molecule (CH(2)O), as commonly observed in the mass spectra of methoxysilanes. Further, an ethylene (C(2)H(4)) or a dimethyl ether (CH(3)OCH(3)) molecule loss is observed in the fragmentation of some intermediate ions generated from 1(+)* and 2(+)*, but the mechanisms are different than those in the cases of 3 and 4. Some of these fragmentations are also different than those reported previously. The relative abundance of the ions in many MIKE spectra is explained by the extension of the Stevenson-Audier rule. The reaction, which is in contrast to the rule, however, is rationalized by the energy of the transition state for the reaction, estimated by semi-empirical molecular orbital calculation. The peak at m/z 59 from 2(+)* consists only of CH(3)OSi(+) ion, whereas the peak from 1(+)* consists of two different ions, CH(3)OSi(+) and (CH(3))(2)Si(+)H. The ions CH(3)OSi(+) from 1(+)* and 2(+)* are generated by at least two and three separate routes respectively.     

Binding energies and isomerization in metallocene ions from threshold photoelectron photoion coincidence spectroscopy

Metallocene ions (Cp(2)M(+), M = Cr, Co, Ni) were studied by threshold photoelectron photoion coincidence spectroscopy (TPEPICO) to investigate the mechanism, energetics, and kinetics of the ionic dissociation processes. The examined energy-selected Cp(2)M(+) ions fragment by losing the neutral cyclopentadienyl ligand. In addition, CH and C(2)H(2) losses appear as minor channels, while the cobaltocene ion also loses an H atom. A possible isomerization pathway has also been observed for Cp(2)Ni(+), yielding a complex with pentafulvalene (C(10)H(8)) with a loss of H(2). In order to determine the 0 K appearance energies for the CpM(+) fragment ions, the asymmetric time-of-flight peak shapes and the breakdown diagrams of the energy-selected metallocene ions were modeled by both the rigid activated complex (RAC) Rice-Ramsperger-Kassel-Marcus (RRKM) theory and the simplified statistical adiabatic channel model (SSACM). The following appearance energies were obtained with SSACM, which is more reliable for loose transition states: 10.57 ± 0.14, 11.01 ± 0.13, and 10.18 ± 0.13 eV for M = Cr, Co, and Ni, respectively. These values combined with the corresponding adiabatic ionization energies yield M-Cp bond dissociation energies in Cp(2)M(+) ions of 5.04 ± 0.16, 5.77 ± 0.15, and 3.96 ± 0.15 eV. Density functional calculations at the B3LYP/6-311G(d,p) level of theory were used to determine the structures of these complexes and to provide parameters necessary for the analysis of the experimental data. The trends in the M-Cp bond energies can be related to the electronic structures of the metallocene ions based on a simple molecular orbital picture.     

Long-range electronic coupling of MM quadruple bonds (M = Mo or W) via a 2,6-azulenedicarboxylate bridge

The preparation of 2,6-azulenedicarboxylic acid (I) from its diester, 2-CO(2)(t)Bu-6-CO(2)-C(10)H(6) (II), is reported together with the crystal and molecular structure of the ester, II. From the reactions between the dicarboxylic acid I and the MM quadruply bonded complexes M(2)(O(2)C(t)Bu)(4), where M = Mo or W, the azulenedicarboxylate bridged complexes [M(2)(O(2)C(t)Bu)(3)](2)(mu-2,6-(CO(2))(2)-C(10)H(6)) have been isolated, III (M = Mo) and IV (M = W). The latter compounds provide examples of electronically coupled M(2) centers via a polar bridge. The compounds show intense electronic absorptions due to metal-to-bridge charge transfer. This occurs in the visible region of the spectrum for III (M = Mo) but in the near-IR for IV (M = W). One electron oxidation with Ag(+)PF(6)(-) in THF generates the radical cations III(+) and IV(+). By both UV-vis-NIR and EPR spectroscopy the molybdenum ion III(+) is shown to be valence trapped or Class II on the Robin and Day classification scheme. Electrochemical, UV-vis-NIR, and EPR spectroscopic data indicate that, in the tungsten complex ion IV(+), the single electron is delocalized over the two W(2) centers that are separated by a distance of ca. 13.6 A. Furthermore, from the hyperfine coupling to (183)W (I = (1)/(2)), the singly occupied highest molecular orbital is seen to be polarized toward one W(2) center in relationship to the other. Electronic structure calculations employing density functional theory indicate that the HOMO in compounds III and IV is an admixture of the two M(2) delta orbitals that is largely centered on the M(2) unit having proximity to the C(5) ring of the azulenedicarboxylate bridge. The energy of the highest occupied orbital of the bridge lies very close in energy to the M(2) delta orbitals. However, this orbital does not participate in electronic coupling by a hole transfer superexchange mechanism, and the electronic coupling in the radical cations of III and IV occurs by electron transfer through the bridge pi system.