Unimolecular neutral and ion kinetics by variable-time neutralization—reionization mass spectrometry

Electron super-rich radicals. III. On the peculiar behavior of the aminodihydroxymethyl radical in the gas phase

In contrast to previously reported electron-super-rich trihydroxy-, triamino- and diaminohydroxymethyl radicals, the title aminodihydroxymethyl radical (1) generates a fraction of metastable species in the form of their deuterium isotopologues. The lifetimes of metastable radicals produced by femtosecond collisional electron transfer to aminodihydroxymethyl cations exceed 4 mus. The main fraction of 1 dissociates by fast loss of a hydroxyl hydrogen atom to form carbamic acid. Loss of an amino hydrogen atom is less facile and becomes <10% competitive at high internal energies or if the main dissociation is slowed down by deuterium isotope effects. RRKM calculations of unimolecular rate constants on a CCSD(T)/aug-cc-pVTZ potential energy surface gave a reasonably good fit for the competitive dissociations of 1 but not for the fraction of nondissociating radicals. The metastable species are attributed to excited electronic states which are predicted to have favorable Franck-Condon factors for being formed by collisional electron transfer.

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.

Simple b 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.

Modeling deoxyribose radicals by neutralization-reionization mass spectrometry. Part 2. Preparation, dissociations, and energetics of 3-hydroxyoxolan-3-yl radical and cation

The title radical (1) is generated in the gas-phase by collisional neutralization of carbonyl-protonated oxolan-3-one. A 1.5% fraction of 1 does not dissociate and is detected following reionization as survivor ions. The major dissociation of 1 (approximately 56%) occurs as loss of the hydroxyl H atom forming oxolan-3-one (2). The competing ring cleavages by O[bond]C-2 and C-4[bond]C-5 bond dissociations combined account for approximately 42% of dissociation and result in the formation of formaldehyde and 2-hydroxyallyl radical. Additional ring-cleavage dissociations of 1 resulting in the formation of C(2)H(3)O and C(2)H(4)O cannot be explained as occurring competitively on the doublet ground (X) electronic state of 1, but are energetically accessible from the A and higher electronic states accessed by vertical electron transfer. Exothermic protonation of 2 also produces 3-oxo-(1H)-oxolanium cation (3(+)) which upon collisional neutralization gives hypervalent 3-oxo-(1H)-oxolanium radical (3). The latter dissociates spontaneously by ring opening and expulsion of hydroxy radical. Experiment and calculations suggest that carbohydrate radicals incorporating the 3-hydroxyoxolan-3-yl motif will prefer ring-cleavage dissociations at low internal energies or upon photoexcitation by absorbing light at approximately 590 and approximately 400 nm.

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.

Lactone enols are stable in the gas phase but highly unstable in solution

2-Hydroxyoxol-2-ene (C(5)-1), the enol tautomer of gamma-butyrolactone, was generated in the gas phase as the first representative of the hitherto elusive class of lactone enols and shown by neutralization-reionization mass spectrometry to be remarkably stable as an isolated species. Ab initio calculations by QCISD(T)/6-311+G(3df,2p) provided the enthalpies of formation, proton affinities, and gas-phase basicities for gaseous lactone enols with four- (C(4)-1), five- (C(5)-1), and six-membered rings (C(6)-1). The acid-base properties of C(4)-C(6) lactones and enols and reference carboxylic acid enols CH(2)=C(OH)(2) (3) and CH(2)=C(OH)OCH(3) (4) were also calculated in aqueous solution. The C(4)-C(6) lactone enols show gas-phase proton affinities in the range of 933-944 kJ mol(-)(1) and acidities in the range of 1401-1458 kJ mol(-)(1). In aqueous solution, the lactone enols are 15-20 orders of magnitude more acidic than the corresponding lactones, with enol pK(a) values increasing from 5.6 (C(4)-1) to 14.5 (C(6)-1). Lactone enols are moderately weak bases in water with pK(BH) in the range of 3.9-8.1, whereas the lactones are extremely weak bases of pK(BH) in the range of -10.5 to -17.4. The acid-base properties of lactone enols point to their high reactivity in protic solvents and explain why no lactone enols have been detected thus far in solution studies.

Generation and characterization of ionic and neutral P(OH)2+/* in the gas phase by tandem mass spectrometry and computational chemistry

The bicoordinated dihydroxyphosphenium ion P(OH)2+ (1+) was generated specifically by charge-exchange dissociative ionization of triethylphosphite and its connectivity was confirmed by collision induced dissociation and neutralization-reionization mass spectra. The major dissociation of 1+ forming PO+ ions at m/z 47 involved another isomer, O=P-OH2+ (2+), for which the optimized geometry showed a long P-OH2 bond. Dissociative 70-eV electron ionization of diethyl phosphite produced mostly 1+ together with a less stable isomer, HP(O)OH+ (3+). Ion 2+ is possibly co-formed with 1+ upon dissociative 70-eV electron ionization of methylphosphonic acid. Neutralization-reionization of 1+ confirmed that P(OH)2* (1) was a stable species. Dissociations of neutral 1, as identified by variable-time measurements, involved rate-determining isomerization to 2 followed by fast loss of water. A competitive loss of H occurs from long-lived excited states of 1 produced by vertical electron transfer. The A and B states undergo rate-determining internal conversion to vibrationally highly excited ground state that loses an H atom via two competing mechanisms. The first of these is the direct cleavage of one of the O-H bonds in 1. The other is an isomerization to 3 followed by cleavage of the P-H bond to form O=P-OH as a stable product. The relative, dissociation, and transition state energies for the ions and neutrals were studied by ab initio and density functional theory calculations up to the QCISD(T)/6-311+G(3df,2p) and CCSD(T)/aug-cc-pVTZ levels of theory. RRKM calculations were performed to investigate unimolecular dissociation kinetics of 1. Excited state geometries and energies were investigated by a combination of configuration interaction singles and time-dependent density functional theory calculations.

Protonation sites in methyl nitrate and the formation of transient CH4NO3 radicals. A neutralization-reionization mass spectrometric and computational study

Protonation sites in methyl nitrate (1) were evaluated computationally at the Gaussian 2(MP2) level of ab initio theory. The methoxy oxygen was the most basic site that had a calculated proton affinity of PA = 728-738 kJ mol-1 depending on the optimization method used to calculate the equilibrium geometry of the CH3O(H)-NO2+ ion (2+). Protonation at the terminal oxygen atoms in methyl nitrate was less exothermic; the calculated proton affinities were 725, 722, and 712 kJ mol-1 for the formation of the syn-syn, anti-syn, and syn-anti ion rotamers 3a+, 3b+, and 3c+, respectively. Ion 2+ was prepared by an ion-molecule reaction of NO2+ with methanol and used to generate the transient CH3O(H)-NO2. radical (2) by femtosecond collisional electron transfer. Exothermic protonation of 1 produced a mixture of 3a(+)-3c+ with 2+ that was used to generate transient radicals 3a-3c. Radical 2 was found to be unbound and dissociated without barrier to methanol and NO2. Radicals 3a-3c were calculated to be weakly bound. When formed by vertical neutralization, 3a-3c dissociated completely on the 4.2 microseconds time scale of the experiment. The main dissociations of 3a-3c were formations of CH3O. + HONO and CH3ONO + OH.. The gas-phase chemistry of radicals 3a-3c and their dissociation products, as studied by neutralization-reionization mass spectrometry, was dominated by Franck-Condon effects on collisional neutralization and reionization. The adiabatic ionization energies of 3a-3c were calculated as 7.54, 7.57, and 7.66 eV, respectively.