Ultraviolet photodissociation of the N-methylpyridinium ion: action spectroscopy and product characterization.

The ultraviolet photodissociation of gas-phase N-methylpyridinium ions is studied at room temperature using laser photodissociation mass spectrometry and structurally diagnostic ion-molecule reaction kinetics. The C5H5N-CH3(+) (m/z 94), C5H5N-CD3(+) (m/z 97), and C5D5N-CH3(+)(m/z 99) isotopologues are investigated, and it is shown that the N-methylpyridinium ion photodissociates by the loss of methane in the 36,000 - 43,000 cm(-1) (280 - 230 nm) region. The dissociation likely occurs on the ground state surface following internal conversion from the S1 state.

Guidelines for radical reactions: some thirty years on.

In this viewpoint article we reflect on the state of play of organic free radical chemistry before the contributions of Beckwith et al. to our understanding of the factors that control intramolecular homolytic addition chemistry, and the rapid rise in the use of this chemistry once the impact of the "guidelines for radical reactions" became fully appreciated.

Structure and Reactivity of the Glutathione Radical Cation: Radical Rearrangement from the Cysteine Sulfur to the Glutamic Acid α-Carbon Atom.

A gas-phase radical rearrangement through intramolecular hydrogen-atom transfer (HAT) was studied in the glutathione radical cation, [γ-ECG] , which was generated by a homolytic cleavage of the protonated S-nitrosoglutathione. Ion-molecule reactions suggested that the radical migrates from the original sulfur position to one of the α-carbon atoms. Experiments on the radical cations of dipeptides derived from the glutathione sequence, [γ-EC] and [CG] , pointed to the glutamic acid α-carbon atom as the most likely site of the radical migration.

New insights into 1,2,4-trioxolane stability and the crucial role of ozone in promoting polymer degradation.

Quantum chemical calculations reveal that ozone is capable of instigating a series of degradative processes in poly(methyl methacrylate). The crux lies in the self-decomposition of 1,2,4-trioxolane, which undergoes a spin intersystem crossing process and O-O bond cleavage to unravel a triplet biradical intermediate that facilitates auto-degradation of the polymer chain.

Chloroform as a hydrogen atom donor in Barton reductive decarboxylation reactions.

The utility of chloroform as both a solvent and a hydrogen atom donor in Barton reductive decarboxylation of a range of carboxylic acids was recently demonstrated (Ko, E. J. et al.

Direct observation of photodissociation products from phenylperoxyl radicals isolated in the gas phase.

Gas phase peroxyl radicals are central to our chemical understanding of combustion and atmospheric processes and are typically characterized by strong absorption in the UV (λ(max) ≈ 240 nm). The analogous maximum absorption feature for arylperoxyl radicals is predicted to shift to the visible but has not previously been characterized nor have any photoproducts arising from this transition been identified. Here we describe the controlled synthesis and isolation in vacuo of an array of charge-substituted phenylperoxyl radicals at room temperature, including the 4-(N,N,N-trimethylammonium)methyl phenylperoxyl radical cation (4-Me3N([+])CH2-C6H4OO(•)), using linear ion-trap mass spectrometry.

Surface analysis of lipids by mass spectrometry: more than just imaging.

Mass spectrometry is now an indispensable tool for lipid analysis and is arguably the driving force in the renaissance of lipid research. In its various forms, mass spectrometry is uniquely capable of resolving the extensive compositional and structural diversity of lipids in biological systems. Furthermore, it provides the ability to accurately quantify molecular-level changes in lipid populations associated with changes in metabolism and environment; bringing lipid science to the "omics" age.

Accurate Computation of Cohesive Energies for Small to Medium-Sized Gold Clusters.

High-level CCSD(T)-F12-type procedures have been used to assess the performance of a variety of computationally less demanding methods for the calculation of cohesive energies for small to medium-sized gold clusters. For geometry optimization for small gold clusters, the PBE-PBE/cc-pVDZ-PP procedure gives structures that are in close agreement with the benchmark geometries. We have devised a CCSD(T)-F12b-based composite protocol for the accurate calculation of cohesive energies for medium-sized gold clusters.

Rapid differentiation of isomeric lipids by photodissociation mass spectrometry of fatty acid derivatives.

Rationale: Both traditional electron ionization and electrospray ionization tandem mass spectrometry have demonstrated limitations in the unambiguous identification of fatty acids. In the former case, high electron energies lead to extensive dissociation of the radical cations from which little specific structural information can be obtained. In the latter, conventional collision-induced dissociation (CID) of even-electron ions provides little intra-chain fragmentation and thus few structural diagnostics.

Does addition of NO2 to carbon-centered radicals yield RONO or RNO2? An investigation using distonic radical ions.

Nitrogen dioxide is used as a "radical scavenger" to probe the position of carbon-centered radicals within complex radical ions in the gas phase. As with analogous neutral radical reactions, this addition results in formation of an [M + NO2](+) adduct, but the structural identity of this species remains ambiguous. Specifically, the question remains: do such adducts have a nitro- (RNO2) or nitrosoxy- (RONO) moiety, or are both isomers present in the adduct population? In order to elucidate the products of such reactions, we have prepared and isolated three distonic phenyl radical cations and observed their reactions with nitrogen dioxide in the gas phase by ion-trap mass spectrometry.

Evaluation of the heats of formation of corannulene and C60 by means of high-level theoretical procedures.

In this study, we address the issues associated with predicting usefully accurate heats of formation for moderately-sized molecules such as corannulene and C(60). We obtain a high-level theoretical heat of formation for corannulene through the use of reaction schemes that conserve increasingly larger molecular fragments between the reactants and products. The reaction enthalpies are obtained by means of the high-level, ab initio W1h thermochemical protocol, while accurate experimental enthalpies of formation for the other molecules involved in the reactions are obtained from the Active Thermochemical Tables (ATcT) network.

Effect of substituents on the preferred modes of one-electron reductive cleavage of N-Cl and N-Br bonds.

In this study, we investigate the effect of substituents in determining the modes of one-electron reductive cleavage of X-NRR' (X = Cl and Br) molecules. We achieve this through comparison of the calculated gas-phase electron affinities (EAs) and aqueous-phase one-electron reduction potentials (E°'s) for a range of nitrogen-centered radicals ((•)NRR') with the corresponding EA and E° values for (•)Cl and (•)Br. The gas-phase EAs have been obtained using the benchmark-quality W1w thermochemical protocol, whereas E° values have been obtained by additionally applying free energy of solvation corrections, obtained using the conductor-like polarizable continuum (CPCM) model.

Reaction of aromatic peroxyl radicals with alkynes: a mass spectrometric and computational study using the distonic radical ion approach.

The reaction of the aromatic distonic peroxyl radical cations N-methyl pyridinium-4-peroxyl (PyrOO(.+)) and 4-(N,N,N-trimethyl ammonium)-phenyl peroxyl (AnOO(.+)), with symmetrical dialkyl alkynes 10a-c was studied in the gas phase by mass spectrometry.

Heteroatomic deprotonation of substituted methanes and methyl radicals: theoretical insights into structure, stability, and thermochemistry.

High-level W1w ab initio calculations have been used to investigate the structural and thermochemical changes that result from heteroatomic deprotonation of CH(3)YH molecules and (•)CH(2)YH radicals (YH = BH(2), CH(3), NH(2), OH, AlH(2), SiH(3), PH(2), and SH). The thermochemical quantities considered include gas-phase acidities, various bond dissociation enthalpies, and heats of formation. The high-level ab initio results are compared with available experimental data and generally show excellent agreement.

Performance of Gradient-Corrected and Hybrid Density Functional Theory: Role of the Underlying Local Density Approximation and the Gradient Correction.

We have analyzed the performance of several widely used density functional theory procedures, namely B-P86, B-PW91, B-LYP, B3-P86, B3-PW91, and B3-LYP, for the E3 set of thermochemical properties. Each of these procedures employs a local density approximation (LDA) functional and a gradient correction for the correlation energy. We find that the VWN3 LDA functional in B-P86, B-PW91, B3-P86, and B3-PW91 leads to extremely large deviations from benchmark values for heats of formation (as large as -455.

W1X-1 and W1X-2: W1-Quality Accuracy with an Order of Magnitude Reduction in Computational Cost.

We have examined a number of approaches for reducing the computational requirements of the W1w and W1-F12 procedures, while maintaining the accuracy. A key finding is that MP2/cc-pCVTZ provides a reliable means for the evaluation of core-correlation effects at a cost that is negligible in the context of W1-type procedures. This greatly reduces the overall computational cost, since calculations for core-correlation represent the most time-consuming steps for both W1w and W1-F12.

Prohormone-substrate peptide sequence recognition by peptidylglycine α-amidating monooxygenase and its reflection in increased glycolate inhibitor potency.

The interactions of nineteen peptide substrates and fifteen analogous peptidomimetic glycolate inhibitors with human peptidylglycine α-amidating monooxygenase (PAM) have been investigated. The substrates and inhibitors are the prohormones of calcitonin and oxytocin and their analogues. PAM both secreted into the medium by and extracted from DMS53 small lung carcinoma cells has been studied.

Heats of Formation for CrO, CrO2, and CrO3: An Extreme Challenge for Black-Box Composite Procedures.

In the present study, we use composite methods, denoted CM(5)Λ and CM5, with post-CCSD(T) terms up to CCSDTQ(5)Λ and CCSDTQ5, respectively, to evaluate the atomization energies for CrO, CrO2, and CrO3. The heats of formation (ΔHf,298) based on our best estimated atomization energies are 198.3 ± 5 kJ mol(-1) (CrO), -81.

Determination of Barrier Heights for Proton Exchange in Small Water, Ammonia, and Hydrogen Fluoride Clusters with G4(MP2)-Type, MPn, and SCS-MPn Procedures-A Caveat.

Calculation of accurate water-water interaction energies is of fundamental importance in computational modeling of many biological and chemical phenomena. We have obtained benchmark barrier heights for proton-exchange reactions and complexation energies in water clusters (H2O)n (n = 1-6) by means of the high-level W1-F12 procedure. We find that lower-cost composite procedures (e.

Ultraviolet action spectroscopy of iodine labeled peptides and proteins in the gas phase.

Structural investigations of large biomolecules in the gas phase are challenging. Herein, it is reported that action spectroscopy taking advantage of facile carbon-iodine bond dissociation can be used to examine the structures of large molecules, including whole proteins. Iodotyrosine serves as the active chromophore, which yields distinctive spectra depending on the solvation of the side chain by the remainder of the molecule.

Differentiation of complex lipid isomers by radical-directed dissociation mass spectrometry.

Contemporary lipidomics protocols are dependent on conventional tandem mass spectrometry for lipid identification. This approach is extremely powerful for determining lipid class and identifying the number of carbons and the degree of unsaturation of any acyl-chain substituents. Such analyses are however, blind to isomeric variants arising from different carbon-carbon bonding motifs within these chains including double bond position, chain branching, and cyclic structures.

7-Selenabicyclo[2.2.1]heptane.

Thermolysis of a benzene solution of N-[4-(p-(methoxybenzyl)seleno)cyclohexanoyl]-N,S-dimethyldithiocarbonate affords the hitherto unknown 7-selenabicyclo[2.2.1]heptane in 48% conversion and in 20% yield after chromatography.

Understanding (the lack of) homolytic substitution chemistry of sulfones.

High level calculations suggest that homolytic substitution (S(H)2) by alkyl radicals at sulfur proceeds through a mechanism that is assisted and dominated by LP → SOMO interactions; in the absence of these interactions, S(H)2 chemistry at sulfur is predicted to be virtually impossible. G3(MP2)-RAD calculations suggest that cyclization of the tert-butylsulfonylbutyl radical 2 (n = 2) proceeds with a rate constant of 1.7 × 10(-24) s(-1) at 80°, some 28 orders of magnitude slower than its sulfide cousin (n = 0).

BDE261: a comprehensive set of high-level theoretical bond dissociation enthalpies.

We have used the high-level W1w protocol to compile a comprehensive collection of 261 bond dissociation enthalpies (BDEs) for bonds connecting hydrogen, first-row and second-row p-block elements. Together they cover 45 bond types, and we term this the BDE261 set. We have used these benchmark values to assess the performance of computationally less demanding theoretical procedures, including density functional theory (DFT), double-hybrid DFT (DHDFT), and high-level composite procedures.

Assessment of theoretical procedures for calculating barrier heights for a diverse set of water-catalyzed proton-transfer reactions.

Accurate electronic barrier heights are obtained for a set of nine proton-transfer tautomerization reactions, which are either (i) uncatalyzed, (ii) catalyzed by one water molecule, or (iii) catalyzed by two water molecules. The barrier heights for reactions (i) and (ii) are obtained by means of the high-level ab initio W2.2 thermochemical protocol, while those for reaction (iii) are obtained using the W1 protocol.