Modeling deoxyribose radicals by neutralization-reionization mass spectrometry. Part 1. Preparation,dissociations, and energetics of 2-hydroxyoxolan-2-yl radical,neutral isomers,and cations

Collisional neutralization of several isomeric C(4)H(7)O(2) cations is used to generate radicals that share some structural features with transient species that are thought to be produced by radiolysis of 2-deoxyribose. The title 2-hydroxyoxolan-2-yl radical (1) undergoes nearly complete dissociation when produced by femtosecond electron transfer from thermal organic electron donors dimethyl disulfide and N,N-dimethylaniline in the gas phase. Product analysis, isotope labeling ((2)H and (18)O), and potential energy surface mapping by ab initio calculations at the G2(MP2) and B3-PMP2 levels of theory and in combination with Rice-Ramsperger-Kassel-Marcus (RRKM) kinetic calculations are used to assign the major and some minor pathways for 1 dissociations. The major (approximately 90%) pathway is initiated by cleavage of the ring C-5[bond]O bond in 1 and proceeds to form ethylene and *CH(2)COOH as main products, whereas loss of a hydrogen atom forms 4-hexenoic acid as a minor product. Loss of the OH hydrogen atom forming butyrolactone (2, approximately 9%) and cleavage of the C-3[bond]C-4 bonds (<1%) in 1 are other minor pathways. The major source of excitation in 1 is by Franck-Condon effects that cause substantial differences between the adiabatic and vertical ionization of 1 (5.40 and 6.89 eV, respectively) and vertical recombination in the precursor ion 1(+) (4.46 eV). (+)NR(+) mass spectra distinguish radical 1 from isomeric radicals 2-oxo-(1H)oxolanium (3), 1,3-dioxan-2-yl (9), and 1,3-dioxan-4-yl (10) that were generated separately from their corresponding ion precursors.     

Bond dissociations in hypervalent ammonium radicals prepared by collisional neutralization of protonated six-membered nitrogen heterocycles

Hypervalent organic ammonium radicals were generated by collisional neutralization with dimethyl disulfide of protonated 1,4-diazabicyclo[2.2.2]octane (1H⁺), N,N′-dimethylpiperazine (2H⁺) and N-methylpiperazine (3H⁺). The radicals dissociated completely on the 5.1 μs time-scale. Radical 1H^• underwent competitive N−H and N−C bond dissociations producing 1,4-diazabicyclo[2.2.2]octane and small ring fragments. Dissociations of radical 2H^• proceeded by N−H bond dissociation and ring cleavage, whereas N−CH₃ bond cleavage was less frequent. Radical 3H^• underwent N−H, N−CH₃ and N−C_ring bond cleavages followed by post-reionization dissociations of the formed cations. The pattern of bond dissociations in the hypervalent ammonium radicals derived from six-membered nitrogen heterocycles is similar to those of aliphatic ammonium radicals. © 1997 John Wiley & Sons, Ltd.     

Electron-rich radicals by neutralizationreionization mass spectrometry. Generation, dissociations and energetics of the hydrogen atom adduct to acetamide

Protonated acetamide exists as two planar conformers, the more stable anti-form (anti-1(+)) and the syn-form (syn-1(+)), DeltaG(degree) (298) (anti-->syn) = 10.8 kJ mol(-1). Collisional neutralization of 1(+) produces 1-hydroxy-1-amino-1-ethyl radicals (anti-1 and syn-1) which in part survive for 3.7 micros. The major dissociation of 1 is loss of the hydroxyl hydrogen atom (approximately 95%) which is accompanied by loss of one of the methyl hydrogen atoms (approximately 3%) and loss of the methyl group (approximately 2%). The most favorable dissociation of the OH bond is calculated to be only 34 kJ mol(1) endothermic but requires 88 kJ mol(-1) in the transition state. Other dissociations of 1, e.g., loss of one of the amide hydrogens, methyl hydrogens, and loss of ammonia are calculated to proceed through higher- energy transition states and are not kinetically competitive if proceeding from the ground doublet electronic state of 1. The unimolecular dissociation of 1 following collisional electron transfer is promoted by large Franck-Condon effects that result in 8090 kJ mol(-1) vibrational excitation in the radicals. Radicals 1 are calculated to exoergically abstract hydrogen atoms from acetamide in water, but not in the gas phase. The different reactivity is due to solvent effects that favor the products, (.)CH(2)CONH(2) and CH(3)CH(OH)NH(2), over the reactants.     

Simple <Emphasis Type="Italic">b</Emphasis> ions have cyclic oxazolone structures. A neutralization-reionization mass spectrometric and computational study of oxazolone radicals

The 2-methyloxazol-5-on-2-yl radical (3) and its deuterium labeled analogs were generated in the gas-phase by femtosecond electron-transfer and studied by neutralization-reionization mass spectrometry and quantum chemical calculations. Radical 3 undergoes fast dissociation by ring opening and elimination of CO and CH(3)CO. Loss of hydrogen is less abundant and involves hydrogen atoms from both the ring and side-chain positions. The experimental results are corroborated by the analysis of the potential energy surface of the ground electronic state in 3 using density functional, perturbational, and coupled-cluster theories up to CCSD(T) and extrapolated to the 6-311 ++ G(3df,2p) basis set. RRKM calculations of radical dissociations gave branching ratios for loss of CO and H that were k(CO)/k(H) > 10 over an 80-300 kJ mol(-1) range of internal energies. The driving force for the dissociations of 3 is provided by large Franck-Condon effects on vertical neutralization and possibly from involvement of excited electronic states. Calculations also provided the adiabatic ionization energy of 3, IE(adiab) = 5.48 eV and vertical recombination energy of cation 3(+), RE(vert) = 4.70 eV. The present results strongly indicate that oxazolone structures can explain fragmentations of b-type peptide ions upon electron capture, contrary to previous speculations.     

Dissecting the proline effect: dissociations of proline radicals formed by electron transfer to protonated Pro-Gly and Gly-Pro dipeptides in the gas phase

We report a combined experimental and computational study of the proline effect in model dipeptides Pro-Gly and Gly-Pro. Gas-phase protonated peptide ions were discharged by glancing collisions with potassium or cesium atoms at 3 keV collision energies, and the peptide radical intermediates and their dissociation products were analyzed following collisional ionization to anions. The charge reversal (+CR-) mass spectra of (Pro-Gly + H)+(1a+) and (Gly-Pro + H)+ (2a+) showed dramatic differences and thus provided a sensitive probe of ion structure. Whereas 1a+ completely dissociated upon charge inversion, 2a+ gave a nondissociated anion as the most abundant product. Ab initio and density functional theory calculations provided structures and vertical recombination energies (REvert) for 1a+ and 2a+. The recombination energies, REvert = 3.07 and 3.36 eV for 1a+ and 2a+, respectively, were lower than the alkali metal ionization energies and indicated that the collisional electron transfer to the peptide ions was endoergic. Radical 1a* was found to exist in a very shallow local energy minimum, with transition state energies for loss and migration of H indicating very facile dissociation. In contrast, radical 2a* was calculated to spontaneously isomerize upon electron capture to a stable dihydroxycarbinyl isomer (2e*) that can undergo consecutive and competitive isomerizations by proline ring opening and intramolecular hydrogen atom transfers to yield stable radical isomers. Radical 2e* and its stable isomers were calculated to have substantial electron affinities and thus can form the stable anions that were observed in the +CR- mass spectra. The calculated TS energies and RRKM kinetic analysis indicated that peptide N-C alpha bond dissociations compete with pyrrolidine ring openings triggered by radical sites at both the N-terminal and C-terminal sides of the proline residue. Open-ring intermediates were found in which loss of an H atom was energetically preferred over backbone dissociations. This provided an explanation for the proline effect causing low incidence of electron capture dissociations of N-C alpha bonds adjacent to proline residues in tryptic peptides and also for some peculiar behavior of proline-containing protein cation-radicals.     

Gas-phase tautomers of protonated 1-methylcytosine. Preparation, energetics, and dissociation mechanisms

Tautomers of 1-methylcytosine that are protonated at N-3 (1+) and C-5 (2+) have been specifically synthesized in the gas phase and characterized by tandem mass spectrometry and quantum chemical calculations. Ion 1+ is the most stable tautomer in aqueous and methanol solution and is likely to be formed by electrospray ionization of 1-methylcytosine and transferred in the gas phase. Gas-phase protonation of 1-methylcytosine produces a mixture of 1+ and the O-2-protonated tautomer (3+), which are nearly isoenergetic. Dissociative ionization of 6-ethyl-5,6-dihydro-1-methylcytosine selectively forms isomer 2+. Upon collisional activation, ions 1+ and 3+ dissociate by loss of ammonia and [C,H,N,O], whose mechanisms have been established by deuterium labeling and ab initio calculations. The main dissociations of 2+ following collisional activation are losses of CH2=C=NH and HN=C=O. The mechanisms of these dissociations have been elucidated by deuterium labeling and theoretical calculations.     

Isomerisierung von Methoxy- und Carbomethoxysubstituierten Cycloalkyl- und Alken-radikalkationen

The very complex isomerization patterns of methoxy and carbomethoxy substituted cycloalkanes (3- to 7-membered rings) have been investigated using collisional activation, metastable ion characteristics and field ionization kinetics. The extent of isomerization depends on both the ring size and the substituent. Irrespective of the electronic properties of the substituent, ring opening involves exclusively the C-1? C-2 bond whereby linear alkene radical cations are formed. In the case of OCH₃- and COOCH₃ substituents the position of the resulting double bond (terminal or α,β-unsaturated) is determined more by the ring size of the precursor molecules and less by the electronic properties of the substituents. Contrary to these findings alklyl substituted cycloalkanes (3- to 5-membered rings) rearrange exclusively to terminal alkene radical cations. The barrier for double bond isomerization seems to be substantially influenced by substituents.     

Collision-induced dissociation of uracil and its derivatives

The collision-induced dissociation of protonated uracil has been studied by tandem mass spectrometry using models extensively labeled with stable isotopes, and derivatives of the kinds found in nucleic acids. Following collisional activation at 30 eV translational energy, protonated uracil dissociates through two principal pathways which do not occur in electron ionization mass spectra: (1) elimination of NH3 almost entirely from N-3, followed by loss of CO from C-4, 0⁴; (2) loss of H2O, equally from 0² and 0⁴. Elimination of HNCO, also the principal dissociation process from odd-electron molecular ions, proceeds primarily by loss of N-3, C-Z, O² and 10% from N-l, C-Z, 0². Several secondary dissociation products are formed with quantitative site specificity of skeletal atoms: C,HO+ (4-C0, C-5, C-6); H2CN+ (N-l, C-6); C2H2+ (N-l, C-5, C-6). First-step dissociation reactions are interpreted in terms of pyrimidine ring opening at likely sites of protonation after collisional activation of MH+. Collision-induced dissociation mass spectra of uracils with structural themes common to nucleic acids (methylation, replacement of 0 by S, C-5 substitution) follow analogous reaction paths which permit assignment of sites of substitution, and exhibit ion abundance changes attributed to differences in substituent basicity and electron density.     

Electron transfer to protonated beta-alanine N-methylamide in the gas phase: an experimental and computational study of dissociation energetics and mechanisms

Ammonium radicals derived from protonated beta-alanine N-methyl amide (BANMA) were generated by femtosecond collisional electron transfer to gas-phase cations prepared by chemical ionization and electrospray. Regardless of the mode of precursor ion preparation, the radicals underwent complete dissociation on the time scale of 5.15 micros. Deuterium isotope labeling and product analysis pointed out several competitive and convergent dissociation pathways that were not completely resolved by experiment. Ab initio calculations, which were extrapolated up to the CCSD(T)/6-311++G(3df,2p) level of theory, provided the proton affinity and gas-phase basicity of BANMA as PA = 971 kJ mol-1 and GB = 932 kJ mol-1 to form the most stable ion structure 1c+ in which the protonated ammonium group was internally solvated by hydrogen bonding to the amide carbonyl. Ion 1c+ was calculated to have an adiabatic recombination energy of 3.33 eV to form ammonium radical 1c*. The potential energy surface for competitive and consecutive isomerizations and dissociations of 1c* was investigated at correlated levels of theory and used for Rice-Ramsperger-Kassel-Marcus (RRKM) calculations. RRKM unimolecular rate constants suggested that dissociations starting from the ground electronic state of radical 1c* were dominated by loss of an ammonium hydrogen atom. In contrast, dissociations starting from the B excited state were predicted to proceed by reversible isomerization to an aminoketyl radical (1f*). The latter can in part dissociate by N-Calpha bond cleavage leading to the loss of the amide methyl group. This indicates that apparently competitive dissociations observed for larger amide and peptide radicals, such as backbone cleavages and losses of side-chain groups, may originate from different electronic states and proceed on different potential energy surfaces.     

The arginine anomaly: arginine radicals are poor hydrogen atom donors in electron transfer induced dissociations

Arginine amide radicals are generated by femtosecond electron transfer to protonated arginine amide cations in the gas phase. A fraction of the arginine radicals formed (2-amino-5-dihydroguanid-1'-yl-pentanamide, 1H) is stable on the 6.7 micros time scale and is detected after collisional reionization. The main dissociation of 1H is loss of a guanidine molecule from the side chain followed by consecutive dissociations of the 2-aminopentanamid-5-yl radical intermediate. Intramolecular hydrogen atom transfer from the guanidinium group onto the amide group is not observed. These results are explained by ab initio and density functional theory calculations of dissociation and transition state energies. Loss of guanidine from 1H is calculated to require a transition state energy of 68 kJ mol(-)(1), which is substantially lower than that for hydrogen atom migration from the guanidine group. The loss of guanidine competes with the reverse migration of the arginine alpha-hydrogen atom onto the guanidyl radical. RRKM calculations of dissociation kinetics predict the loss of guanidine to account for >95% of 1H dissociations. The anomalous behavior of protonated arginine amide upon electron transfer provides an insight into electron capture and transfer dissociations of peptide cations containing arginine residues as charge carriers. The absence of efficient hydrogen atom transfer from charge-reduced arginine onto sterically proximate amide group blocks one of the current mechanisms for electron capture dissociation. Conversely, charge-reduced guanidine groups in arginine residues may function as radical traps and induce side-chain dissociations. In light of the current findings, backbone dissociations in arginine-containing peptides are predicted to involve excited electronic states and proceed by the amide superbase mechanism that involves electron capture in an amide pi* orbital, which is stabilized by through-space coulomb interaction with the remote charge carriers.     

Electron predators are hydrogen atom traps. Effects of aryl groups on N—Cα bond dissociations of peptide radicals

Effects of substituted aryl groups on dissociations of peptide aminoketyl radicals were studied computationally for model tetrapeptide intermediates GXD^?G where X was a cysteine residue that was derivatized by S‐(3‐nitrobenzyl), S‐(3‐cyanobenzyl), S‐(3,5‐dicyanobenzyl), S‐(2,3,4,5,6‐pentafluorobenzyl), and S‐benzyl groups. The aminoketyl radical was placed within the Asp amide group. Aminoketyl radicals having the S‐(3‐nitrobenzyl) group were found to undergo spontaneous and highly exothermic migration of the hydroxyl hydrogen atom onto the nitro group in conformers allowing interaction between these groups. Competing reaction channels were investigated for aminoketyl radicals having the S‐(3‐cyanobenzyl) and S‐(3,5‐dicyanobenzyl) groups, e.g. H‐atom migration to the C and N atoms of the C≡N group, migration to the C‐4 position of the phenyl ring, and dissociation of the radical‐activated N? C_α bond between the Asp and Gly residues. RRKM kinetic analysis on the combined B3LYP and ROMP2/6‐311++G(2d,p) potential energy surface indicated > 99% H‐atom transfer to the C≡N group forming a stable iminyl intermediate. The N? C_α bond dissociation was negligible. In contrast, peptides with the S‐(2,3,4,5,6‐pentafluorobenzyl) and S‐benzyl groups showed preferential N? C_α bond dissociation that outcompeted H‐atom migration to the C‐4 position and fluorine substituents in the phenyl ring. These computational results are used to suggest an alternative mechanism for the quenching effect on electron‐based peptide backbone dissociations of benzyl groups with electron‐withdrawing substitutents, as reported recently. Copyright © 2010 John Wiley & Sons, Ltd.     

Neutralization-Reionization of alkenylammonium cations: An experimental and ab initio study of intramolecular N-H … C=C interactions in cations and hypervalent ammonium radicals

A series of isomeric hexenylammonium and hexenyldimethylammonium cations were neutralized by collisional electron transfer in the gas phase in an attempt to generate hypervalent ammonium radicals. The radicals dissociated completely on the 4.8–5.4 µs time scale. Radicals in which the hexene double bond was in the 3-, 4-, and 5-positions dissociated by competitive N-H and N=C bond cleavages. Allylic 2-hexen-1-ylammonium and 2-hexen-1-yldimethylammonium radicals underwent predominant cleavages of allylic N-C bonds. Deuterium labeling experiments revealed no intramolecular hydrogen transfer from the hypervalent ammonium group to the hexene double bond. Ab initio and density functional theory calculations showed that alkenylammonium and alkenylmethyloxonium ions preferred hydrogen bonded structures in the gas phase. The stabilization through intramolecular H bonding in 3-buten-1-ylammonium and 3-buten-1-yl methyloxonium ions was calculated by B3LYP/6-311G(2d,p) at 26 and 18 kJ mol^?1, respectively. No intramolecular hydrogen bonding was found for the allylammonium ion. The hypervalent 3-buten-1-yl-methyloxonium radical was calculated to be unbound and predicted to dissociate exothermically by O-H bond cleavage. This dissociation may provide kinetic energy for the hydrogen atom to overcome a small energy barrier for exothermic addition to the double bond. The 3-butten-1-ylammonium and allylammonium radicals were found to be bound and preferred gauche conformations without intramolecular hydrogen bonding. Vertical neutralization of alkenylammonium ions was accompanied by small Franck-Condon effects. The failure to detect stable or metastable hypervalent alkenylammonium radicals was ascribed to the low activation barriers to exothermic dissociations by N-H and N-C bond cleavages.     

Reduction of 1,4-dien-3-one steroids with LiAl2H4 or NaB2H4: stereospecific deuterium-labeling at the c-1alpha position of a 4-en-3-one steroid

Reduction of a double bond at C-1 of 1,4-dien-3-one steroids 7 and 8 with LiAl2H4 in THF or NaB2H4 in MeOH and H2O gave stereospecifically [1alpha-2H]-labeled 4-en-3-one steroids 9 and 10, respectively. When the deuterated solvents, MeO2H and 2H2O, were used for the reaction of steroid 8 with NaB2H4, [1alpha,2xi-2H2]-labeled compound 10 was produced. This indicates that the reaction proceeds through the initial hydride attack at the C-1alpha position, followed by ketonization of the 2-en-3-ol intermediate.     

Modeling deoxyribonucleic acid and ribonucleic acid damage in the gas phase

This short review outlines the tandem mass spectrometric methods for the generation and analysis of transient nucleobase radicals relevant to deoxyribonucleic acid and ribonucleic acid damage. Radical hydrogen atom adducts to uracil, adenine, cytosine and N-methylcytosine were generated by femtosecond electron transfer to the corresponding gas-phase cations in fast beams at 8 keV kinetic energy. Radical unimolecular dissociations were monitored by product analysis following collisional ionization to cations or anions using neutralization-reionization mass spectrometry. The radical energetics and dissociation kinetics were further analyzed by mapping the potential energy surfaces by high-level ab initio calculations in combination with Rice-Remsberger-Kassel-Marcus calculations of unimolecular rate constants. This first- principles-based approach allows one to model radical dissociations occurring from doublet ground electronic states of radical intermediates, assign reaction mechanisms and derive quantitative branching ratios for dissociation channels that are in agreement with experiments. Theoretical analysis also provides distinction between radical dissociations occurring on the ground and excited electronic state potential energy surfaces. Specific characterization of excited state dissociations of nucleobase and other polyatomic radicals remains a challenging topic for both experimentalists and computational chemists.     

Where does the electron go? Electron distribution and reactivity of peptide cation radicals formed by electron transfer in the gas phase

We report the first detailed analysis at correlated levels of ab initio theory of experimentally studied peptide cations undergoing charge reduction by collisional electron transfer and competitive dissociations by loss of H atoms, ammonia, and N-C alpha bond cleavage in the gas phase. Doubly protonated Gly-Lys, (GK + 2H) (2+), and Lys-Lys, (KK + 2H) (2+), are each calculated to exist as two major conformers in the gas phase. Electron transfer to conformers with an extended lysine chain triggers highly exothermic dissociation by loss of ammonia from the Gly residue, which occurs from the ground ( X ) electronic state of the cation radical. Loss of Lys ammonium H atoms is predicted to occur from the first excited ( A ) state of the charge-reduced ions. The X and A states are nearly degenerate and show extensive delocalization of unpaired electron density over spatially remote groups. This delocalization indicates that the captured electron cannot be assigned to reduce a particular charged group in the peptide cation and that superposition of remote local Rydberg-like orbitals plays a critical role in affecting the cation-radical reactivity. Electron attachment to ion conformers with carboxyl-solvated Lys ammonium groups results in spontaneous isomerization by proton-coupled electron transfer to the carboxyl group forming dihydroxymethyl radical intermediates. This directs the peptide dissociation toward NC alpha bond cleavage that can proceed by multiple mechanisms involving reversible proton migrations in the reactants or ion-molecule complexes. The experimentally observed formations of Lys z (+*) fragments from (GK + 2H) (2+) and Lys c (+) fragments from (KK + 2H) (2+) correlate with the product thermochemistry but are independent of charge distribution in the transition states for NC alpha bond cleavage. This emphasizes the role of ion-molecule complexes in affecting the charge distribution between backbone fragments produced upon electron transfer or capture.     

First generation and characterization of the enol of glycine, H(2)N--CH=C(OH)(2), in the gas phase

The enol of glycine, H(2)N-CH&dbond;C(OH)(2), is generated in the gas phase by neutralization of the corresponding radical cation, which is available by dissociative electron ionization of isoleucine. Reionization approximately 0.6 micros later shows that the isolated enol (2) exists and does not isomerize to the significantly more stable glycine molecule, H(2)N--CH(2)--COOH (1); hence the intramolecular tautomerization 2-->1 must be associated with high barriers. The neutralization-reionization reactivity of 1(+*) further confirms that neutral glycine has a canonical structure (1) and is not a zwitterion. The unimolecular chemistry of 1(+*) is dominated by C--C bond cleavage to the immonium ion (+)H(2)NCH(2); in sharp contrast, 2(+*) primarily loses H(2)O. The ylide ion (+)H(3)N--CH(*)--COOH, an intermediate in the water loss from 2(+*), is found to readily equilibrate to 2(+*) prior to dissociation. Tautomers 1(+*) and 2(+*) differ in their charge-stripping behavior, with only 2(+*) forming a stable dication. The radical anions 1(-*) and 2(-*), formed by charge reversal of 1(+*) and 2(+*), respectively, dissociate extensively to (mainly) different closed-shell fragment anions. An important channel is H(*) loss; 1(-*) yields the carboxylate ion H(2)N--CH(2)--COO(-) whereas 2(-*) yields the enolate ion H(2)N--CH=C(OH)O(-).     

Adenine radicals in the gas phase: an experimental and computational study of hydrogen atom adducts to adenine

The elusive hydrogen atom adduct to the N-1 position in adenine, which is thought to be the initial intermediate of chemical damage, was specifically generated in the gas phase and characterized by neutralization-reionization mass spectrometry. The N-1 adduct, 1,2-dihydroaden-2-yl radical (1), was generated by femtosecond electron transfer to N-1-protonated adenine that was selectively produced by electrospray ionization of adenine in aqueous-methanol solution. Radical 1 is an intrinsically stable species in the gas phase that undergoes specific loss of the N-1-hydrogen atom to form adenine, but does not isomerize to the more stable C-2 adduct, 1,2-dihydroaden-1-yl radical (5). Radicals 1 that are formed in the fifth and higher electronically excited states of DeltaE > or = 2.5 eV can also undergo ring-cleavage dissociations resulting in expulsion of HCN. The relative stabilities, dissociation, and transition state energies for several hydrogen atom adducts to adenine have been established computationally at highly correlated levels of theory. Transition state theory calculations of 298 K rate constants in the gas phase, including quantum tunnel corrections, indicate the branching ratios for H-atom additions to C-8, C-2, N-3, N-1, and N-7 positions in adenine as 0.68, 0.20, 0.08, 0.03, and 0.01, respectively. The relative free energies of adenine radicals in aqueous solution point to the C-8 adduct as the most stable tautomer, which is predicted to be the predominating (>99.9%) product at thermal equilibrium in solution at 298 K.     

Mono and double polar [4 + 2(+)] Diels-Alder cycloaddition of acylium ions with O-heterodienes

Gas-phase reactions of acylium ions with alpha,beta-unsaturated carbonyl compounds were investigated using pentaquadrupole multiple-stage mass spectrometry. With acrolein and metacrolein, CH(3)-C(+)(double bond)O, CH(2)(double bond)CH-C(+)(double bond)O, C(6)H(5)-C(+)(double bond)O, and (CH(3))(2)N-C(+)(double bond)O react to variable extents by mono and double polar [4 + 2(+)] Diels-Alder cycloaddition. With ethyl vinyl ketone, CH(3)-C(+)(double bond)O reacts exclusively by proton transfer and C(6)H(5)-C(+)(double bond)O forms only the mono cycloadduct whereas CH(2)(double bond)CH-C(+)(double bond)O and (CH(3))(2)N-C(+)(double bond)O reacts to great extents by mono and double cycloaddition. The positively charged acylium ions are activated O-heterodienophiles, and mono cycloaddition occurs readily across their C(+)(double bond)O bonds to form resonance-stabilized 1,3-dioxinylium ions which, upon collisional activation, dissociate predominantly by retro-addition. The mono cycloadducts are also dienophiles activated by resonance-stabilized and chemically inert 1,3-dioxonium ion groups, hence they undergo a second cycloaddition across their polarized C(double bond)C ring double bonds. (18)O labeling and characteristic dissociations displayed by the double cycloadducts indicate the site and regioselectivity of double cycloaddition, which are corroborated by Becke3LYP/6-311++G(d,p) calculations. Most double cycloadducts dissociate by the loss of a RCO(2)COR(1) molecule and by a pathway that reforms the acylium ion directly. The double cycloadduct of the thioacylium ion (CH(3))(2)N-C(+)(double bond)S with acrolein dissociates to (CH(3))(2)N-C(+)(double bond)O in a sulfur-by-oxygen replacement process intermediated by the cyclic monoadduct. The double cycloaddition can be viewed as a charge-remote type of polar [4 + 2(+)] Diels-Alder cycloaddition reaction.     

Conformational Behaviour of Substituted 2-Methoxy-2-Oxo-1,2-Oxaphospholan-3-Ols

Abstract

Conformational behaviour of about 30 2-methoxy-2-oxo-1,2- oxaphospho l an-3-0 1 s containing various substituents was examined by ¹H and ¹³C NMR. Vicinal coupling constants J(HCCH), J(HCCP), J(HCOP), J(CCOP) and J(CCCP) were employed in this study. Conformation of the 1,2-oxaphospholane ring is governed almost exclusively by substituents at C-3, C-4 and C-5, as we l l as by their orientation. The configuration of the P atom has little or no influence on conformation of the ring in diastsreomeric pairs. Strong preference of phenyl, methyl and substituted methyl groups to occupy the equatorial or pseudoequatoria l positions was observed for all but one compounds studied. In the cis-fused bicyclic syst ems conformat ionally rigid 6-membered rings forced the 1,2-oxaphospholane rings to adopt an enve l ope-l ike (E₄) conformation. No influence of the p=o……HO-C-3 hydrogen bond on conformation of the 1,2-oxaphospholane ring was found. Preferred conformations for (2R, 3R, 4R)-3-(hydroxymethyI)-2-methoxy-2-oxo-1,2-oxaphospho lane-3,4-diol and its triacetate are shown below.     

Protonation Sites in Gaseous Pyrrole and Imidazole: A Neutralization–Reionization and ab initio Study

Mild gas-phase acids C₄H₉⁺ and NH₄⁺ protonate pyrrole at C-2 and C-3 but not at the nitrogen atom, as determined by deuterium labeling and neutralization–reionization mass spectrometry. Proton affinities in pyrrole are calculated by MP2/6–311G(2d, p) as 866, 845 and 786 kJ mol^-1 for protonation at C-2, C-3 and N, respectively. Vertical neutralization of protonated pyrrole generates bound radicals that in part dissociate by loss of hydrogen atoms. Unimolecular loss of hydrogen atom from C-2-and C-3-protonated pyrrole cations is preceded by proton migration in the ring. Protonation of gaseous imidazole is predicted to occur exclusively at the N-3 imine nitrogen to yield a stable aromatic cation. Proton affinities in imidazole are calculated as 941, 804, 791, 791 and 724 for the N-3, C-4, C-2, C-5 and N-1 positions, respectively. Radicals derived from protonated imidazole are only weakly bound. Vertical neutralization of N-3-protonated imidazole is accompanied by large Franck–Condon effects which deposit on average 183 kJ mol^-1 vibrational energy in the radicals formed. The radicals dissociate unimolecularly by loss of hydrogen atom, which involves both direct N-H bond cleavage and isomerization to the more stable C-2 H-isomer. Potential energy barriers to isomerizations and dissociations in protonated pyrrole and imidazole isomers and their radicals were investigated by ab initio calculations.