Sulfinyl cations [R-S+-O (R = CH,, Ph, Cl, CH,O and C,H,O)] have been demonstrated by MO calculations in conjunction with pentaquadrupole multidimensional (2D and 3D) MS2 and MS3 mass spectrometric experiments to be stable and easily accessible gas phase species, and their dissociation and ionlmolecule chemistry have been studied. Potential energy surface diagrams indicate that the sulfoxides (CH3)2S=O, P h , W , Cl, S==O, (CH,O),S=O and (C,H,0)2S==0 do not undergo rearrangement upon dissociative ionization, yielding the corresponding sulfinyl cations as primary fragments. Ph(CH,)S-O + ', on the other hand, is predicted to isomerize to CH,-SO -Ph+' via a four-membered ring transition state, yielding upon further CH,' loss the isomeric ion W +-P h. The sulfinyl cations were found by ab initio calculations to be much more stable than their S=O+-R isomers, hence isomerization via [ 1,2-R] shifts is not expected. Direct cleavage of the R-SO+ bonds and/or processes that are preceded by isomerization dominate the low-energy collision dissociation chemistry of the sulfinyl cations, thus providing limited structural information. On the other hand, a general and structurally diagnostic ion/molecule reaction with 2-methyl-l,3-dioxolane occurs for all the sulfinyl cations yielding abundant net oxirane (C,H,O) addition products. The reaction probably occurs via a transketalization-like mechanism that leads to cyclic 2-thia-l,3-dioxolanylium ions. This reactivity parallels that of several acylium (R<+=O) and thioacylium ions (R-Cf=S), and is not shared by the isomeric ions SO+-Ph and CH,=S+-OH. While the corresponding acylium ions react extensively with isoprene by [4 + 2'1 cycloaddition, only the phenylsulfinyl cation Ph-S+=O yields an abundant cycloadduct.

Eleven isomers with the PyC2H 5 (+·) composition, which include three conventional (1-3) and eight distonic radical cations (4-11), have been generated and in most cases successfully characterized in the gas phase via tandem-in-space multiple-stage pentaquadrupole MS(2) and MS(3) experiments. The three conventional radical cations, that is, the ionized ethylpyridines C2H5-C5H4N(+·) (1-3), were generated via direct 70-eV electron ionization of the neutrals, whereas sequences of chemical ionization and collision-induced dissociation (CID) or mass-selected ion-molecule reactions were used to generate the distonic ions H2C(·)-C5H4N(+)-CH3 (4-6), CH3-C5H4N(+)-CH 2 (·) (7-9), C5H5N(+)-CH2CH 2 (·) (10), and C5H5N(+)-CH(·)-CH3 (11). Unique features of the low-energy (15-eV) CID and ion-molecule reaction chemistry with the diradical oxygen molecule of the isomers were used for their structural characterization. All the ion-molecule reaction products of a mass-selected ion, each associated with its corresponding CID fragments, were collected in a single three-dimensional mass spectrum. Ab initio calculations at the ROMP2/6-31G(d, p)//6-31G(d, p)+ZPE level of theory were performed to estimate the energetics involved in interconversions within the PyC2H5 (+·) system, which provided theoretical support for facile 4⇌7 interconversion evidenced in both CID and ion-molecule reaction experiments. The ab initio spin densities for the a-distonic ions 4-9 and 11 were found to be largely on the methylene or methyne formal radical sites, which thus ruled out substantial odd-spin derealization throughout the neighboring pyridine ring. However, only 8 and 9 (and 10) react extensively with oxygen by radical coupling, hence high spin densities on the radical site of the distonic ions do not necessarily lead to radical coupling reaction with oxygen. The very typical "spatially separated" ab initio charge and spin densities of 4-11 were used to classify them as distonic ions, whereas 1-3 show, as expected, "localized" electronic structures characteristic of conventional radical ions.

The gas-phase reactivity of a set of halocarbocations, +CH2X (X = Cl, Br, or I), +CHXX (X1, X2 = F, Cl, or Br), and +CX3 (X = F or Cl), with four prototype aromatic compounds (benzene, furan, pyrrole, and pyridine) was investigated via double- and triple-stage mass spectrometry and compared to that of the simplest +CH3 carbocation. A rich chemistry is observed, and the reaction channels are greatly influenced by the number and type of halogen substituents (X), the strength of the C−X bonds, the nature of the aromatic compound, and the relative stabilities of the carbocation products. [Ar−CH2]+, [Ar−CHX]+, or [Ar−CX1X2]+ functionalization of the relatively inert aromatic Ar−H bonds is the main reaction channel observed. A structure-specific “methylene by hydride exchange” reaction with toluene and B3LYP/6-311G(d,p) calculations indicate that the benzylium ion and the 2-furanylmethyl cation are formed in the [Ar−CH2]+ functionalization of benzene and furan, respectively. Kinetic isotope effects for the [Ar−CHX]+ functionalization using naturally occurring halogen isotopes (35Cl/37Cl and 79Br/81Br) were measured. Using halogen-mixed halocarbocations +CHX1X2, we evaluated the intrinsic competition for either the [Ar−CHX1]+ or [Ar−CHX2]+ functionalization. In reactions with pyridine, no Ar−H functionalization occurs and either proton transfer, N-addition, or net [CH2]+• transfer due to the loss of X• from the nascent adducts is observed. Structural characterization of product ions was performed by on-line collision-induced dissociation or ion/molecule reactions, or both, and when possible by comparison with authentic ions.

For the first time [3 + 2] 1,3-cycloaddition of an ionized carbonyl ylide has been observed in gas phase ion−molecule reactions of +CH2OCH2 • (1) with several carbonyl compounds. The reaction, which competes with electrophilic addition that leads to net CH2 •+ transfer, occurs across the CO double bond of acetaldehyde and several acyclic ketones yielding ionized 4,4-dialkyl-1,3-dioxolanes as unstable cycloadducts. Rapid dissociation of the nascent cycloadducts by loss of a 4-alkyl substituent as a radical leads to the observed products, that is cyclic 4-alkyl-1,3-dioxolanylium ions. Cycloaddition of 1 with cyclic ketones yields bicyclic spiro adducts, which also undergo rapid dissociation. Cyclobutanone yields ionized 1,3-dioxaspiro[4,3]octane, which dissociates exclusively by neutral ethene loss to ionized 4-methylene-1,3-dioxolane. Ionized 1,3-dioxaspiro[4,4]nonane is formed in reactions with cyclopentanone, and its rapid dissociation by loss of C3H6 and C2H5 • yields the ionized 4-methylene-1,3-dioxolanylium and the 4-ethenyl-1,3-dioxolanylium product ions, respectively. A systematic study of this novel reaction and characterization of the product ions carried out via pentaquadrupole (QqQqQ) multiple stage (MS2 and MS3) mass spectrometric experiments provide experimental evidence for the cycloaddition mechanism. The dissociation chemistry observed for the cycloaddition products correlate well with their proposed structures and was compared to that of both isomeric and reference ions. Ab initio MP2/6-31G(d,p)//HF/6-31G(d,p) + ZPE potential energy surface diagrams for the reactions of 1 with acetone, fluoroacetone, and 1,1,1-trifluoroacetone support the operation of the two competitive reaction pathways, that is CH2 •+ transfer and [3 + 2] 1,3-cycloaddition/dissociation, and show that the cycloaddition process is favored by electron-withdrawing substituents.