Towards persistent cyclobutadiene radical cations? The tetra-1-adamantyl-, and tetraneopentylcyclobutadiene radical cations.

The persistence of alkyl substituted cyclobutadiene radical cations strongly depends on the method of generation and the size of the alkyl substituents used. Hindered rotation, the consequence of bulky substituents, is observed in the title compounds.     

Hydrogen shifts in the decomposition of branched chain alkane radical cations

Heat of formation data are consistent with formation of the t-butyl ion when C₃H₇ is eliminated from the molecular ion of 2-methylhexane. The 1–2 hydrogen shift that must accompany this reaction is confirmed via a deuterium isotope effect upon the low energy metastable decomposition.     

Intense laser-induced decomposition of mass-selected 2-, 3-, and 4-methylaniline cations

Photodecomposition processes of 2-, 3-, and 4-methylaniline cations induced by a moderately intense (10¹¹ W/cm²) visible nanosecond laser field and an intense (10¹⁵ W/cm²) UV femtosecond laser field have been investigated using a tandem mass spectrometer. Highly unsaturated fragment cations such as and are dominantly produced by the nanosecond laser, while less unsaturated smaller fragment cations such as , and are produced mainly by the femtosecond laser. Ab initio calculations have also been performed to estimate the stable geometrical structures of and and those of possible intermediate ring compounds for discussing the photodecomposition pathways in intense laser fields.     

Reactions of radical cations of acetals: Evidence for unimolecular decomposition

The thermal and photochemical reactions of the methylal radical cation /I/ in freon matrices were studied using selective deuteration for elucidating the structure of the resulting species. /I/ has been shown to decay by unimolecular reaction upon heating to 140 K as well as upon photolysis in CFCl₃ matrix and the product of decay has been assumed to be the complex of formaldehyde radical cation with CFCl₃. Such decay reaction has been demonstrated for 1,3-dioxolan radical cation as well.     

Electronic spectra of 1-methyl and 2-methyl phenanthrenes and their radical cations

Electronic spectra of phenanthrene (P), 1-methyl phenanthrene (1-MeP), 2-methyl phenanthrene (2-MeP) and their monopositive ions are investigated experimentally as well as theoretically. The ions were produced by photo-oxidation of the hydrocarbons in boric acid matrix. The electronic absorption spectrum of 2-methyl phenanthrene cation (2-MeP+) is entirely new. For the interpretation of the electronic spectra of neutral and ionized MePs, semi-empirical AM1 (Austin Model 1) calculations are carried out for the first time. The bathochromic shifts in the spectral bands of the neutral and ionized MePs are attributed to 'conjugative' effect. The present experiments reveal that the 448 nm band of 1-methyl phenanthrene cation (1-MeP+) and the 486 nm band of 2-MeP+ show close matching with the respective 450 nm and 488 nm 'diffuse interstellar bands'. This suggests the possibility of the existence of such ionic species in the interstellar matter.     

Charge dispersal in 1- and 2-adamantyl cations

Substituents at the remoter 5-position of 2-adamantyl arylsulfonates control solvolysis rates more strongly than substituents at the 4-position.     

The formation and structure of radical cations

ions generated from a number of different presursors have been studied by high kinetie energy ion—molecule reations. It has been shown that at least four distinct stable species oeeur, of which acetonitrile and methyl isoeyanide retain their original structure. With imidazole or pyrazole as precursors, a mixture of open thain radical cations, not identical to the above species and probably interconvertible via the 1H-azirine radocal cation, is formed. From butrynitrile, pyrrole, crotonitrile, allyl interconvertible via the and cyanocyopropane a fourth species, probably the vinylidenimine ion, is formed.     

The structure of diphosphine radical cations

When R₂NNR₂ molecules lose an electron to give (R₂NNR₂)^+· radical cations, the whole unit becomes planar, with a(π₁)²(π₂)¹ configuration. However, because R₃P molecules are far more strongly pyramidal than R₃N molecules, this flattening on electron loss is less, and phosphorous centred radical cations do not achieve planarity. This is clearly so for (R₂PPR₂)⁺ centres, whose liquid and solid state spectra analysed herein in terms of two equivalent ³¹P hyperfine couplings, show ca. 9% 3s character. This indicates considerable bending at each phosphorous centre. Furthermore, the form of the spectra, with no x — y splitting of the ‘perpendicular’ lines, suggests that each ³¹P coupling shares a common axis. This means that a trans conformation is required, as expected because this relieves steric strain and favours “π” type orbital overlap.     

NMR spectra of seleninium and 1- and 2-benzoseleninium cations

The analysis of the NMR spectra of seleninium and 1-and 2-benzoseleninium cations, recorded in deuterotrifluoroacetic acid, is reported. The effects of condensation on the spectral parameters of the protons of the seleninium ring were examined with particular reference to the shift to higher values of δ: this was consistent with the q values calculated by the HMO method. The ³J(HH) and ⁴J(HH) variations, including an interesting variation regarding the through-selenium ⁴J coupling, are also discussed.     

The variety of reactions of radical cations derived from 2-diphenylaminothiophene oligomers

ESR spectroelectrochemical measurements of 2-diphenylamino-substituted oligothiophenes 8_m proved the existence of radical cations upon oxidation. Their stability and dimerization depend significantly on the number m of thiophene units. The radical cations and are very reactive and dimerize spontaneously to yield either 2,5-bis(diphenylamino)-2,2′-bithiophene 10₂ or 2,5-bis(diphenylamino)-5,5′-bis(2-thienyl)-3,3′-bithiopene 11₂, respectively. In contrast, the radical cations are highly stable and do not dimerize at all.     

Thermal decomposition mechanisms of tert-alkyl peroxypivalates studied by the nitroxide radical trapping technique

The thermolysis of a series of tert-alkyl peroxypivalates 1 in cumene has been investigated by using the nitroxide radical-trapping technique. tert-Alkoxyl radicals generated from the thermolysis underwent the unimolecular reactions, beta-scission, and 1,5-H shift, competing with hydrogen abstraction from cumene. The absolute rate constants for beta-scission of tert-alkoxyl radicals, which vary over 4 orders of magnitude, indicate the vastly different behavior of alkoxyl radicals. However, the radical generation efficiencies of 1 varied only slightly, from 53 (R = Me) to 63% (R = Bu(t)()), supporting a mechanism involving concerted two-bond scission within the solvent cage to generate the tert-butyl radical, CO(2), and an alkoxyl radical. The thermolysis rate constants of tert-alkyl peroxypivalates 1 were influenced by both inductive and steric effects [Taft-Ingold equation, log(rel k(d)) = (0.97 +/- 0. 14)Sigmasigma - (0.31 +/- 0.04)SigmaE(s)(c), was obtained].     

The resonance energy of amides and their radical cations

Structural Chemistry - The resonance energy of an amide can be calculated through comparison with a model amine and a model ketone (or aldehyde) with subtraction of the “residual”...     

Kinetics of the unimolecular decomposition of the 2-chloroallyl radical

The thermal decomposition of the 2-chloroallyl radical, CH(2)CClCH(2) --> CH(2)CCH(2) + Cl (1), was studied using the laser photolysis/photoionization mass spectrometry technique. Rate constants were determined in time-resolved experiments as a function of temperature (720-840 K) and bath gas density ([He] = (3-12) x 10(16), [N(2)] = 6 x 10(16) molecule cm(-3)). C(3)H(4) was observed as a primary product of reaction 1. The rate constants of reaction 1 are in the falloff, close to the low-pressure limit, under the conditions of the experiments. The potential energy surface (PES) of reaction 1 was studied using a variety of quantum chemical methods. The results of the study indicate that the minimum energy path of the CH(2)CClCH(2) dissociation proceeds through a PES plateau corresponding to a weakly bound Cl-C(3)H(4) complex; a PES saddle point exists between the equilibrium CH(2)CClCH(2) structure and the Cl-C(3)H(4) complex. The results of quantum chemical calculations, the rate constant values obtained in the experimental study, and literature data on the reverse reaction of addition of Cl to allene were used to create a model of reactions 1 and -1. The experimental dependences of the rate constants on temperature and pressure were reproduced in RRKM/master equation calculations. The reaction model provides expressions for the temperature dependences of the high-pressure-limit and the low-pressure-limit rate constants and the falloff broadening factors (at T = 300-1600 K): k(infinity)(1) = 1.45 x 10(20)T(-1.75) exp(-19609 K/T) s(-1), k(infinity)(-)(1) = 8.94 x 10(-10)T(-0.40) exp(481 K/T) cm(3) molecule(-1) s(-1), k(1)(0)(He) = 5.01 x 10(-32)T(-12.02) exp(-22788 K/T) cm(3) molecule(-1) s(-1), k(1)(0)(N(2)) = 2.50 x 10(-32)T(-11.92) exp(-22756 K/T) cm(3) molecule(-1) s(-1), F(cent)(He) = 0.46 exp(-T/1001 K) + 0.54 exp(-T/996 K) + exp(-4008 K/T), and F(cent)(N(2)) = 0.37 exp(-T/2017 K) + 0.63 exp(-T/142 K) + exp(-4812 K/T). The experimental data are not sufficient to specify all the parameters of the model; consequently, some of the model parameters were obtained from quantum chemical calculations and from analogy with other reactions of radical decomposition. Thus, the parametrization is most reliable under conditions close to those used in the experiments.     

Simultaneous occurrence of electron transfer initiated radical cage collapse and chain mechanisms in the reactions of diaryliodonium cations with 2-nitropropanate anion

Simultaneous occurrence of free radical cage and chain mechanisms initiated by single electron transfer in the reactions of diaryliodonium cations with 2-nitropropanate anion has been established.     

Reactivity of triarylphosphine peroxyl radical cations generated through the reaction of triarylphosphine radical cations with oxygen

One-electron oxidation of triarylphosphines (Ar3P, Ar = phenyl and substituted phenyl) in benzonitrile (PhCN) has been studied using pulse radiolysis technique. One-electron oxidation of Ar3P occurred to yield the radical cation (Ar3P*+) which showed an intense absorption with a peak at 360-370 nm together with a broad band at 500-600 nm. The addition of molecular oxygen (O2) to the phosphorus atom of Ar3P*+ took place at the second-order rate constant of 10(7)-10(9) dm(3) mol(-1) s(-1) to yield the peroxyl triarylphosphinyl radical cation (Ar3P+OO*). It is found that the electron-releasing substituents on the para position of the phenyl ring of Ar3P influence the rate constants of the reaction of Ar3P*+ with O2 and that o-methyl substituents on the phenyl ring influence the reactivity of Ar3P+OO*.     

Experimental and theoretical study on the structure and reactions of 1-methoxypropane radical cations

Structure and mechanism of thermal and photochemical reactions of radical cations of methyl n-propyl ether (MPE) were studied in irradiated freonic matrices CFCl₃, CF₂ClCFCl₂, and CF₃CCl₃ at 77 K. The quantum chemical calculations of the structure of radical cations and products of their transformations were carried out with methods based on the density functional theory (DFT). Experimental and calculation results show that the MPE radical cations are characterized by substantial delocalization of spin density to the propyl group. The action of light on the MPE radical cations in a CF₃CCl₃ matrix at 77 K results in intramolecular rearrangement yielding the distonic radical cation ^.CH₂CH₂CH₂(OH⁺)CH₃. It was found that the primary MPE radical cations underwent irreversible transformation to CH₃CH₂CH₂OCH ₂ ^. radical as a result of an ion-molecule reaction that occurred in a CF₂ClCFCl₂ matrix upon heating the sample to 110–120 K or in a CFCl₃ matrix upon increasing the solute concentration.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 2, 2005, pp. 105–113.Original Russian Text Copyright © 2005 by Belevskii, Feldman, Tyurin.This revised version was published online in April 2005 with a corrected cover date.     

The question of tautomerism of alkylnitrile and isonitrile radical cations

The isomeric pairs [CH₃CN]^+˙ and [CH₂CNH]^+˙ and [CH₃NC]^+˙ and [CH₂NCH]^+˙ have been established as stable, noninterconverting structures. The conclusion derives from studies of collision induced decomposition spectra. The same conclusion pertains for the ions [CH₃CH₂CN]^+˙ and [CH₃CHCNH]^+˙, and for [NCCH₂CH₂CN]^+˙, [HNCCHCH₂CN]^+˙ and [HNCCHCHCNH]^+˙. The energy barrier of a [1,3]-hydrogen shift, a possible isomerization mechanism, is determined to be at least 163 kJ mol^?1 for the [CH₃CN]^+˙ and [CH₂CNH]^+˙ pair, and the barrier may be as high as 318 kJ mol^?1. The C₃H₅N and C₄H₄N₂ radical cations decompose before they can be activated with 318 kJ mol^?1 of internal energy.     

The role of aromatic radical cations and benzylic cations in the 2,4,6-triphenylpyrylium tetrafluoroborate photosensitized oxidation of ring-methoxylated benzyl alcohols in CH2Cl2 solution

A steady-state and laser flash photolysis (LFP) study of the TPPBF(4)-photosensitized oxidation of ring-methoxylated benzyl alcohols has been carried out. Direct evidence on the involvement of intermediate benzyl alcohol radical cations and benzylic cations in these reactions has been provided through LFP experiments. The reactions lead to the formation of products (benzaldehydes, dibenzyl ethers, and diphenylmethanes) whose amounts and distributions are influenced by the number and relative position of the methoxy substituents. This behavior has been rationalized in terms of the interplay between the stabilities of benzyl alcohol radical cations and benzyl cations involved in these processes. A general mechanism for the TPPBF(4)-photosensitized reactions of ring-methoxylated benzyl alcohols has been proposed, where the alpha-OH group of the parent substrate acts as the deprotonating base promoting alpha-C-H deprotonation of the benzyl alcohol radical cation (formed after electron transfer from the benzyl alcohol to TPP) to give a benzyl radical and a protonated benzyl alcohol, precursor of the benzylic cation. This hypothesis is in contrast with previous studies, where formation of the benzyl cation was suggested to occur from the neutral benzyl alcohol through the Lewis acid action of excited TPP(+) (TPP).     

Theoretical study of the competitive decomposition and isomerization of 1-hexyl radical

The ground-state potential energy surface of the 1-hexyl system, including the main decomposition and isomerization processes, has been calculated with the MPW1K, BB1K, MPWB1K, MPW1B95, BMK, M05-2X and CBS-QB3 methods. On the basis of these data, thermal rate coefficients of different reaction channels and branching ratios were then calculated using the master equation formulation at 250–2,500 K. The results clearly point out that the 1,5 H atom transfer reaction of 1-hexyl radical with exothermicity proceeds through the lowest reaction barrier, whereas the decomposition processes are thermodynamically unfavorable with large endothermicity. The temperature effect is important on the relative importance of different reactions in the 1-hexyl system. In the low-temperature range of 250–900 K, isomerization reactions, especially 1,5 H atom transfer reaction of 1-hexyl radical, are dominating and responsible for over 82.17% of all the reactions, due to their smaller reaction barriers than those of the decomposition reactions. Furthermore, an equilibrium process involving the isomeric forms of the hexyl radicals appearing at relative low temperature was validated theoretically. However, isomerization and decomposition processes are kinetically competitive and simultaneously important under normal pyrolysis conditions.