Conventional Isomers Research Articles

Möbius and Hückel Cyclacenes with Dewar and Ladenburg Defects.

Cyclacene nanobelts have not been synthesized in over 60 years and remain one of the last unsynthesized building blocks of carbon nanotubes. Recent work has predicted that Hückel-cyclacenes containing Dewar benzenoid ring isomers are the most stable isomeric forms for several of the smaller sizes of cyclacene belts. Here, we give a more complete picture of the isomers that are possible within these nanobelt systems by simulating embedded Ladenburg (prismane) benzenoid rings in Hückel-[n]cyclacenes (n = 5-14) and embedded Dewar benzenoid rings in twisted Möbius-[n]cyclacenes (n = 9-14). The Möbius-[9]cyclacene isomer containing one Dewar benzenoid defect and the Hückel-[5]cyclacene isomer containing two maximally spaced Ladenburg benzenoid defects are found to be more stable than their conventional Kekulé benzenoid ring counterparts. The isomers that contain Dewar and Ladenburg benzenoid rings have larger electronic singlet-triplet energy gaps and lower polyradical character when compared with the conventional isomers.

Site‐Selectivity in the Protonation and Related Reactions of Chalcogenophosphinidene‐Bridged Dimolybdenum Cyclopentadienyl Complexes

AbstractReactions of the chalcogenophosphinidene‐bridged complexes syn‐[Mo2Cp2(μ‐κ2P,Z:κ1P,η4‐ZPMes*)(CO)2L] (L = CO, CNtBu; Z = O, S, Se), anti‐[Mo2Cp2(μ‐κ2P,S:κ1P,η4‐SPMes*)(CO)2L] (L = CO, CNtBu) and [Mo2Cp2{μ‐κ2P,Z:κ1P‐ZPH}(η6‐HMes*)(CO)2] (Z = S, Se, Te) towards sources of H+, Me+, and AuP(pTol)3+ cations were investigated (Mes* = 2,4,6‐C6H2tBu3; Cp = η5‐C5H5). The latter two electrophiles invariably added to the chalcogen atom to give corresponding derivatives [Mo2Cp2{μ‐κ2P,Z:κ1P,η4‐Mes*PZ(CH3)}(CO)3]BX4 [X = F, Z = O, S, Se; P–S 2.144(1) Å when Z = S and X = Ar′ = 3,5‐C6H3(CF3)2], [Mo2Cp2{μ‐κ2P,Z:κ1P,η4‐Mes*PZ(CH3)}(CNtBu)(CO)2]BF4 (Z = S, Se), and [AuMo2Cp2(μ3‐κ1S:κ2P,S:κ1P,η4‐SPMes*){P(pTol)3}(CO)3]PF6 [P–S 2.113(2), S–Au 2.320(2) Å]. Even when syn and anti isomers of the neutral precursor were used, the corresponding products were invariably characterized by their syn conformation (Z atom close to L ligand). Besides this, methylated derivatives of the chalcogenophosphinidene complexes bearing the formula [Mo2Cp2{μ‐κ2P,Z:κ1P‐HPZ(Me)}(η6‐HMes*)(CO)2](CF3SO3) (Z = S, Se, Te), were found in solution to exist as an equilibrium of corresponding cis and trans isomers differing, in each case, in the relative positioning of the Cp rings with respect to the MoPZ plane. In contrast to the above results, the protonation of all these compounds was quite sensitive to the particular chalcogenophosphinidene ligand and conformation of the complex. Protonation of the HPZ‐bridged complexes led to complex mixtures of products that could not be isolated or properly characterized. In contrast, the aryl‐bearing substrates reacted selectively to give corresponding complexes [Mo2Cp2{μ‐κ2P,Z:κ1P,η5‐ZP(C6H3tBu3)}(CO)3]BX4 (Z = S, X = F; P–S 2.032(2) Å when X = Ar′; Z = Se, X = F) and [Mo2Cp2{μ‐κ2P,Z:κ1P,η5‐ZP(C6H3tBu3)}(CNtBu)(CO)2]BAr′4 (Z = S, Se). The reaction of anti‐[Mo2Cp2(μ‐κ2P,S:κ1P,η4‐SPMes*)(CO)3] with HBF4·OEt2 at 213 K initially gave unstable intermediate anti‐[Mo2Cp2{μ‐κ2P,S:κ1P,η4‐Mes*PS(H)}(CO)3]BF4, which then rapidly converted at room temperature to the conventional isomer with the S atom close to the metallocene‐bound carbonyl ligand (syn isomer). This transformation is in agreement with Density Functional Theory calculations for neutral thiophosphinidene complexes; electrophilic attack at the sulfur atom, which is both an orbital‐ and charge‐favoured event, affords products that are more stable than those resulting from protonation at the metal sites. All these protonation reactions eventually result in an endo addition of H+ to the C6 atom of the Mes* ring with concomitant η4→η5 haptotropic shift of the resulting HMes* group. This conversion involves an easy H migration from S to the C6 atom, computed to take place with a low activation barrier of about 70 kJ/mol.

The same but different

Electromerism is an unfamiliar concept to many chemists and refers to molecules that are not conventional isomers but instead differ in how the electrons are distributed across their structure. A novel example of such electromers has now been demonstrated.

Exploring the potential energy surface of ion–molecule pairs by experiment and by theory: acetaldehyde and methanol

The energy barrier for the keto–enol isomerization of the isolated acetaldehyde ion to vinyl alcohol lies above its lowest dissociation limit and so the spontaneous isomerization is never observed. However, it has been shown that the distonic methanol ion can ionize the acetaldehyde molecule and greatly lower the energy barrier of the keto–enol isomerization. The present study involved two systems in which the acetaldehyde was associated with the distonic methanol cation, CH 2OH 2 +, and with its conventional isomer, CH 3OH + , in stable and metastable adducts. The isomerization barrier for acetaldehyde was lowered by association with CH 2OH 2 +; in contrast, the CH 3OH + ion did not facilitate the rearrangement of acetaldehyde to its enol ion. Tandem mass spectrometry combined with ab initio calculations was applied to investigate the two systems. Potential energy surface diagrams were obtained by MP2/6-31+g(d) and B3-LYP/6-31+g(d) calculations to further elucidate the reaction mechanisms.

Distonic ions of the “Ate” class

The gas phase synthesis, structure, and reactivity of distonic negative ions of the “ate” class are described. “Ate”-class negative ions are readily prepared in the gas phase by addition of neutral Lewis acids, such as BF 3, BH 3, and AlMe 3, to molecular anions, carbene negative ions, and radical anions of biradicals. The ions contain either localized σ- or delocalized π-type radical moieties remote from relatively inert borate and aluminate charge sites. The free radical reactivity displayed by these ions appears to be independent of the charge site. As an example, the distonic alkynyl radical (·CCBF 3 −) is highly reactive and undergoes radical coupling reactions with NO 2, NO, H 2CCH–CN, and H 2CCH–CH 3. Radical-mediated group and atom transfers are observed with O 2, CS 2, and CH 3SSCH 3. Furthermore, H-atom abstraction reactions are observed, in accordance with the predicted high C–H bond strength of this species [DH 298(H–C 2BF 3 −) = 130.8 kcal mol −1]. High level ab initio molecular orbital calculations on the prototype “ate”-class distonic ion · CH 2BH 3 − and its conventional isomer CH 3BH 2 ·− reveal that CH 3BH 2 ·− is 3.2 kcal/mol more stable than the α-distonic form. However, the calculations also show that CH 3BH 2 ·− is unstable with respect to electron detachment, and only the α-distonic form · CH 2BH 3 − should be experimentally observed in the gas phase.

Isotope effect of hydrated clusters of hydrogen chloride, HCl(H2O) n and DCl(H2O) n (n = 0–4): application of dynamic extended molecular orbital method

The recently proposed dynamic extended molecular orbital (DEMO) method is applied to the HCl(H2O) n and DCl(H2O) n (n = 0–4) clusters in order to explore the isotope effect on their structures, wavefunctions, and energies, theoretically. Since the DEMO method determines both electronic and nuclear wavefunctions simultaneously by optimizing all parameters including basis sets and their centres variationally, we can get the different nuclear orbitals for proton and deuteron as well as their electronic wavefunctions. The positions of the centres of nuclear orbitals show that the deuteron has weaker hydrogen bonding than the proton. There are three isomers in the case of n = 3 clusters, and less stable isomers have hydrogen transferred and non-transferred structures. In the conventional MO calculation, both hydrogen transferred and non-transferred isomers are calculated to be energy minima. When we have applied the DEMO method, only the hydrogen transferred structure is obtained for HCl(H2O)3, while both structures are optimized for DCl(H2O)3. Such strong H/D dependence on the structures of the HCl(H2O) n and DCl(H2O) n clusters can be expressed directly by using the DEMO method. The present application demonstrates that the DEMO method is a useful tool for analysing the anharmonicity and vibronic effects of a hydrogen bonding system.

Spontaneous Hydrogen Atom Transfer on Ionization: Characterization of Enol Radical Cations in Cryogenic Matrices

Organic radical cations are important intermediates in a wide variety of chemical and biological processes. Because of the unpaired spin and charge, radical cations are very reactive and undergo transformations typical for both free radicals and/or cations. Unimolecular processes in radical cations such as valence isomerization, rearrangement, and fragmentation are often significantly different than those of their neutral precursors. The deficit of one electron leading to a reduced strength of key bonds manifests itself in unusual structures and reactivities of radical cations.1 A special feature of radical cation chemistry is the possibility of hydrogen atom transfer which may occur spontaneously upon ionization if a cyclic transition state with five members can be formed. Mass spectrometric experiments showed that this occurs in many heteroatomcontaining compounds leading to so-called distonic ions whose structure often does not correspond to that of a persistent neutral species, but which are nevertheless usually more stable than their conventional isomers.2 Distonic ions have also been investigated in condensed phase where they may occur as products of radiation or radical chemistry (cf. Section VI of ref 2c). However, apart from a few exceptional cases, the only distonic ions which form by spontanous intramolecular hydrogen atom transfer in solid media are those arising from aliphatic esters3 where it is thought to occur by quantum mechanical tunneling.3c This Account is concerned with another class of compounds that show a similar behavior, namely enones (or related compounds) which are disposed for 1,5hydrogen transfers via 6-membered ring transition states to yield dienols as shown here:

Methylene-iodonium ylide: an isomer of diiodomethane

Iso-diiodomethane, H 2CII is a minimum at the B3LYP/6–31G(d) and MP2/6–31G(d) levels and is separated from conventional isomer, diiodomethane, by a barrier of 134.6 kJ mol −1 calculated using G2 theory. The G2 calculated heats of formation for diiodomethane and iso-diiodomethane are 136.1 and 317.8 kJ mol −1, respectively. The enthapy change of the H 2CII → H 2CI + I( 3P) dissociation is only 19.4 kJ mol −1. The charge distribution for H 2CII does not agree with the description of this species as a hypervalent structure with a positive charge on the carbon and a negative charge on the central iodine.

Distinguishing conventional and distonic radical cations by using dimethyl diselenide

Dimethyl diselenide is demonstrated to be among the most powerful reagents used to identify distonic radical cations. Most such ions readily abstract CH3Se from dimethyl diselenide. The reaction is faster and more exclusive than CH3S(·) abstraction from dimethyl disulfide, a reaction used successfully in the past to identify numerous distonic ions. Very acidic distonic ions, such as HC(+)(OH)OCH 2 (·) , do not undergo CH3Se(·) abstraction, but instead protonate dimethyl diselenide. In sharp contrast to the reactivity of distonic ions, most conventional radical cations were found either to react by exclusive electron transfer or to be unreactive toward dimethyl diselenide. Hence, this reagent allows distinction of distonic and conventional isomers, which was demonstrated directly by examining two such isomer pairs. To be able to predict whether electron transfer is exothermic (and hence likely to occur), the ionization energy of dimethyl diselenide was determined by bracketing experiments. The low value obtained (7.9±0.1 eV) indicates that dimethyl diselenide will react with many conventional carbon-, sulfur-, and oxygen-containing radical cations by electron transfer. Nitrogen-containing conventional radical cations were found either to react with dimethyl diselenide by electron transfer or to be unreactive.

The Prototypical Organophosphorus Ylidion •CH2PH3+

The reactivity of the prototypical phosphorus-containing ylidion (α-distonic ion) •CH2PH3+ has been investigated in the gas phase by using a dual cell Fourier-transform ion cyclotron resonance mass spectrometer. The ion •CH2PH3+ and its more stable conventional isomer CH3PH2•+ show distinctly different reactivities toward neutral reagents. This observation contrasts the facile interconversion of the analogous sulfur- and oxygen-containing distonic ions •CH2SH2+ and •CH2OH2+ with their conventional isomers CH3OH•+ and CH3SH•+, respectively, within collision complexes in the gas phase. Bracketing experiments yield a proton affinity of 190.4 ± 3 kcal mol-1 for the phosphorus atom in •CH2PH2. Together with a calculated heat of formation for •CH2PH2, this value yields a heat of formation of 217 ± 3 kcal mol-1 (at 298 K) for the distonic ion •CH2PH3+.

A reappraisal of the structures and stabilities of prototype distonic radical cations and their conventional isomers

High levels of ab initio molecular orbital theory have been used to reexamine the structures and thermochemical properties of distonic radical cations CH 2 X + H and their conventional isomers (CH 3 X .+ ) (X=F, OH, NH 2 , Cl, SH, and PH 2 ). It is found that a generally satisfactory description of structures is provided by MP2/6-311+G(2df,p) optimized geometries and of zero-point vibrational energies by appropriately scaled MP2/6-311G(d,p) harmonic vibrational frequencies. Energies are calculated using a modification of the G2 procedure

Bimolecular reactions of ˙CH2CH2OCH: A comparison with the chemical properties of CH3CH2OCH and ionized trimethylene oxide

AbstractThe chemical properties of the distonic radical cation ˙CH2CH2OCH are compared with those of its conventional isomer, ionized trimethylene oxide, and its even‐electron analog, the ethoxymethyl cation (CH3CH2OCH). The rate constants and branching rations have been determined for the gas‐phase reactions of the three ions with several neutral reagents in a dual‐cell Fourier‐transform ion cyclotron resonance device. The reactivity of each ion is found to be quite distinct. Fast electron transfer dominates most of the bimolecular reactions of ionized trimethyleene oxide. The ethoxymethyl cation reacts slowly or not at all with the reagents studied, and the only reaction observed is ethyl cation transfer. The distonic ion ˙CH2CH2OCH undergoes vedry fast reactions with all the neutral reagents, and, in sharp contrast to the other two ions, shows remarkably versatile reactivity. This ion rapidly transfers ionized ethylene to most of the neutral molecules studied. Facile electron transfer occurs with some of the neutral reagents while others abstract a pproton from the ion. Abstraction of a radical by the distonic ion‐a reaction reported earlier for dimethyl disulfide—is not observed for any other reagent.

The distonic HC + (OH) OĊH 2 radical cation: a stable isomer of ionized methyl formate

The distonic radical cation HC + (OH)OĊH 2, an isomer of ionized methyl formate HC(O)OCH + 3, has been generated from fragmentation of ionized isobutyl formate and ionized 2-methylbutyl formate and characterized by collisional activation (CA) and neutralization—reionization (NR) mass spectrometries. A mixture of both methyl formate and distonic radical cations has been detected following ionization of methyl formate. MO calculations at the UHF/6-31G ** level showed that the distonic species is more stable its conventional isomer and that the energy difference between both isomers is essentially due to the difference of the OH and CH energies.

ChemInform Abstract: Collisional Activation of Distonic Radical Cations and Their Conventional Isomers in Quadrupole Tandem Mass Spectrometry.

ChemInform Abstract: Collisional Activation of Distonic Radical Cations and Their Conventional Isomers in Quadrupole Tandem Mass Spectrometry.

Collisional activation of distonic radical cations and their conventional isomers in quadrupole tandem mass spectrometry

Collisional activation of distonic radical cations and their conventional isomers in quadrupole tandem mass spectrometry

F2PN: a remarkably stable species

Ab initio MO calculations show that F2PN is remarkably stable, lying about 70 kJ mol–1 lower in energy than its conventional valence isomer difluoro-iminophosphane (FPNF) and could thus be a candidate for an actual synthesis in the laboratory.

Distonic radical cations : Guidelines for the assessment of their stability

Ab initio molecular orbital calculations on the distonic radical cations CH 2(CH 2) nN +H 3 and their conventional isomers CH 3(CH 2) nNH 2+ ( n = 0,1, 2 and 3) indicate a preference in each case for the distonic isomer. The energy difference appears to converge with increasing n towards a limit which is close to the energy difference between the component systems CH 3·H 2+CH 3 +NH 3 (representing the distonic isomer) and CH 3CH 3+CH 3NH 2+ (representing the conventional isomer). The generality of this result is assessed by using results for the component systems CH 3·Y+CH 3X +H and CH 3YH+CH 3X +. (or CH 3YH +. + CH 3X) to predict the relative energies of the distonic ions ·Y(CH 2) nX +H and their conventional isomers HY(CH 2) nX +. (X = NH 2, OH, F, PH 2, SH, Cl; Y = CH 2, NH, O) and testing the predictions through explicit calculations for systems with n = 0,1 and 2. Although the predictions based on component systems are often close to the results of direct calculations, there are substantial discrepancies in a number of cases; the reasons for such discrepancies are discussed. Caution must be exercised in applying this and related predictive schemes. For the systems examined in the present study, the conventional radical cation is predicted in most cases to lie lower in energy than its distonic isomer. It is found that the more important factors contributing to a preference for distonic over conventional radical cations are the presence in the system of a group(X) with high proton affinity and the absence of a group (X, Y or perturbed (C—C) with low ionization energy.

Novel gas-phase ions. The radical cations [CH2XH]+• (X = F, Cl, Br, I, OH, NH2, SH) and [CH2CH2NH3]+•

The title ions have been generated by dissociative ionisation of appropriate precursor molecules and have been unequivocally identified by collisional activation and collisional ionisation mass spectrometry. Their heats of formation, [Formula: see text], have been measured and were found to be similar to those for their ionic isomer of conventional structure, [CH3X]+•. The exception is [CH2NH3]+• whose [Formula: see text], is ca. 30 kcal mol−1 higher than that of [CH3NH2]+•. The shapes and appearance energies of metastable peaks in conjunction with isotopic labelling experiments, show that these novel ions do not interconvert with their conventional isomer at internal energies below the threshold for their fragmentation of lowest energy requirement, namely, loss of H•. However, unimolecular H• loss from all [CH2XH]+• ions is preceded by a rate determining isomerisation to [CH3X]+•, followed by fast dissociation. The results are generally in good agreement with recent abinitio calculations. Also described are the properties of the homologous ion [CH2CH2NH3]+•.

Unusual low-energy isomers for simple radical cations

The radical cations of formaldimine, methylamine, formaldehyde, methanol, diazene, hydrazine, nitroxyl, hydroxylamine and hydrogen peroxide, and of isomers derived formally from these systems by means of a 1,2-hydrogen shift have been studied using ab initio molecular orbital theory, including electron correlation. For the ions of formaldimine, methylamine and methanol, evidence is presented that the 1,2-hydrogen-shifted species lie lower in energy than the conventional isomers.

Characterization of ion-dipole complexes by collisional activation and collisional ionization spectrometry

Complexes between a radical cation and a dipole molecule are taking their place beside the conventional isomeric ion structures in mass spectra. Complexes of H 2O, NH 3, H 2S, PH 3, HF, HCl, HBr, CH 3CN and CH 3NC with CH 2 + ions and a number of higher homologues are formed via dissociative ionization and/or ion molecule reactions. Structures were established by Collisional Ionization and Activation Mass spectra. A number of stabilities established by measurements and quantum mechanical calculations show that they fall in the range of those of conventional isomers and that conversion barriers are generally high.