J.J. van Houte

Gas phase substitution reactions by radical cations: I. Reaction of the CC ring-opened oxirane radical cation with pyridine

Abstract Under conditions of chemical ionization the CC ring-opened oxirane radical cation transfers a methylene group to pyridine. This reaction leads to the formation of the 1-methylene-pyridinium radical cation with a high reaction selectivity. Experimental proof for this ion arises from: collisionally induced dissociation (CID) spectroscopy of the reaction product ions of the reaction of either pyridine, pyridine- d 5 , pyridine- 15 N with oxirane; CID spectroscopy of this reaction product compared with the 2-, 3-, 4-methyl pyridine radical cations. Further evidence for this ion and the reaction selectivity of its formation is obtained from MNDO calculations of the minimal energy requirement paths for the formation of the 3-methyl pyridine radical cation versus the 1-methylene-pyridinium radical cation.

The determination of collision cross-sections

Abstract Collision cross-sections of ions, σ0t, can be accurately obtained by extrapolating σt to zero target gas pressure, thus eliminating the error in the collision path length, l(n), caused by target gas leaking out of the collision cell. σ0t values for various ions have been determined and contain structural information. For normal alkanes (n = 1–10) σOt steadily increases from 3.4 to 14.2A2 by 1.2A2 per methylene unit. The observation that ions with cyclic structures have smaller total collision cross-sections than corresponding non-cyclic species could be general, as observed for cyclohexane, cyclohexanone and the C2H3O+ ions from oxirane with respect to acetyl cations. The benzene and pyridine dications cannot be considered as pure six-membered ring species since their σ0t values are significantly larger than those of the singly charged ions.

Gas phase substitution reactions by radical cations. Part 2. Reaction of the CC ring-opened oxirane radical cation with benzonitrile and benzoic acid

Abstract Under conditions of chemical ionization in the high pressure source of a mass spectrometer, the CC ring-opened oxirane radical cation transfers a methylene group to benzonitrile and benzoic acid. This transfer reaction leads to the formation of [M + 14] + product ions. The structures of these ions have been established by collisionally induced dissociation of these ions compared with isomeric reference ions. Methylene transfer to benzonitrile yields the N -methylenebenzonitrile radical cation with a high selectivity. In the case of benzoic acid, a β-distonic ion is formed in which the methylene group is attached to the carbonyl O atom.

Gas phase substitution reactions by radical cations. Part 3. Methylene transfer of the CC ring-opened oxirane radical cation to pyrazole and imidazole

Abstract In the chemical ionization source of a mass spectrometer the CC ring-opened oxirane radical cation transfers a methylene group to imidazole and pyrazole. The structures of the [M + M] ṡ+ reaction product ions and those of the ionized methylpyrazole and methylimidazole isomers have been established by collisionally induced dissociation (CID) spectrometry. The methylene transfer reaction of oxirane to pyrazole and imidazole yields 1-methylene-2 H -pyrazolium and 1-methylene-3 H -imidazolium α-distonic radical cations, respectively. Their relative stabilities, and those of three other possible isomeric α-distonic product ions, have been calculated by ab initio methods at the #UHF/6-31G//3-21G level of theory.

Gas phase substitution reactions by radical cations Part 5: Reaction of the CC ring-opened oxirane radical cation with 1,2-, 1,3- and 1,4-dichlorobenzene

Abstract Under conditions of chemical ionization in the high pressure source of a mass spectrometer, the α-distonic CC ring-opened oxirane radical cation transfers a methylene group to 1,2-, 1,3- and 1,4-dichlorobenzene. The structures of the M + 14] ·+ product ions have been established by collisionally induced dissociation of these ions compared with reference ions and application of principal component analysis. 1,2-Dichlorobenzene yields 80% 2-chlorobenzyl chloride, 5% 2,3-dichlorotoluene and 15% 3,4-dichlorotoluene. The [M + 14] ·+ ions from 1,3-dichlorobenzene are 64–67% 3-chlorobenzyl chloride, 27–28% 2,6-dichlorotoluene and 7% 2,4- or 3,5-dichlorotoluene. From 1,4-dichlorobenzene mainly 4-chlorobenzyl chloride is formed, together with some 2,5-dichlorotoluene. In this case there is also an unidentified contribution, probably by 1,4-dichlorocycloheptatriene ions. Possible formation of distonic product ions does not occur in the cases of 1,2- and 1,3-dichlorobenzene, and from 1,4-dichlorobenzene it is considered to be unlikely.

Collision cross-sections of [C,H,O] cations and radical cations from aliphatic [C,H,O] compounds

Over 260 collision cross-section σot, expressed in »ngstroms squared, have been determined for the studied ions at 20 and 70 eV by extrapolation of σt to zero target gas pressure, and these yield two types of structural information. The first type concerns occurrence and detection of cyclic ions, the second isomerization of parent molecular ions and different product ion distributions at 20 and 70 eV. In addition, examples of two distinct fragmentation mechanisms operative in the formation of identical daughter ions from a given precursor could be traced. Formation of cyclic daughter ions is, for instance, observed for C2H3O+ from oxirane, C3H5O+ from oxetane, C4H7O+2 from 4-methyl-1,3-dioxolane. Cyclic molecular ions are formed in varying proportions from oxirane, tetrahydrofuran, 2- and 4-methyl-1,3-dioxolane but not from porpylene oxide, oxetane and 1,3-dioxolane. Isomerization of the parent molecular ion is proposed for the following fragmentations: CH2H+ from allyl alcohol, CHO2+ from formic acid, C2H2O·+ from oxirane, and C3H6O·+ from 3-methyl butanal and 2-methyl pentanal. Different product ion distributions at 20 and 70 eV were found for C3H5O+ from ethyl propionate and 2-pentanone, C2H4O·+ and C4H8O·+ from butane-1,3-diol, and C3H6O·+ from 2- and 4-methyl-1,3-dioxolane. Two distinct fragmentation mechanisms were traced for the following processes: CH2H+OH, C2H2O·+ and C2H3O+ from methyl vinyl ether, CH2H+ and C2H5O+ from butane-1,3-diol and C2H2O·+ from butanone. Self protonation of acetaldehyde also appears to take place by two mechanisms. Energy partitioning is evident in the formation of formyl cations HCO+ but wears off for processes in which larger daughter ions are formed. For formyl cations from straight chain aldehydes, the 70 eV collision cross-section is linearly related to the logarithm of the reciprocal of the number of degrees of freedom in the parent molcule, log (1/DFp). One example of a proton-bound dimer is given, that of acetaldehyde. Its cross-section value is exceptionally high, more than three times than that of its monomer. Such behaviour is probably typical of this type of cation.

Gas phase substitution reactions by radical cations part 4. Methylene transfer by the CC ring-opened oxirane radical cation to chlorobenzene; product ion analysis

Abstract Under the conditions of chemical ionization, the α-distonic CC ring-opened oxirane radical cation transfers a methylene group to chlorobenzene. The [M + 14] ·+ product ions formed are benzyl chloride (82%) and meta (16%), and para (2%) chlorotoluene radical cations. The structures of these products have been established from their collisionally induced dissociation in comparison with reference ions. Application of principal component analysis (PCA) shows that no other product ions are being formed. The possible formation of the distonic methylene (phenyl)chloronium radical ion C 6 H 5 ClCH ·+ 2 can therefore be excluded; moreover, its calculated (#UHF/3-21G * //MNDO) heat of formation is 561 kJ mol −1 higher than that of benzyl chloride.