Distonic radical cations : Guidelines for the assessment of their stability

Ab initio molecular orbital calculations on the distonic radical cations CH₂(CH₂)_nN⁺H₃ and their conventional isomers CH₃(CH₂)_nNH₂+ (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 CH₃·H₂+CH₃^+NH₃ (representing the distonic isomer) and CH₃CH₃+CH₃NH₂+ (representing the conventional isomer). The generality of this result is assessed by using results for the component systems CH₃·Y+CH₃X⁺H and CH₃YH+CH₃X^+. (or CH₃YH^+. + CH₃X) to predict the relative energies of the distonic ions ·Y(CH₂)_nX⁺H and their conventional isomers HY(CH₂)nX^+. (X = NH₂, OH, F, PH₂, SH, Cl; Y = CH₂, 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.     

Exergonic and Spontaneous Production of Radicals through Beryllium Bonds

High‐level ab initio calculations show that the formation of radicals, by the homolytic bond fission of Y?R (Y=F, OH, NH₂; R=CH₃, NH₂, OH, F, SiH₃, PH₂, SH, Cl, NO) bonds is dramatically favored by the association of the molecule with BeX₂ (X=H and Cl) derivatives. This finding is a consequence of two concomitant effects, the significant activation of the Y?R bond after the formation of the beryllium bond, and the huge stabilization of the F^. (OH^., NH₂^.) radical upon BeX₂ attachment. In those cases where R is an electronegative group, the formation of the radicals is not only exergonic, but spontaneous.     

A computational study on [(PH<Subscript>2</Subscript>X)<Subscript>2</Subscript>]<Superscript>·+</Superscript> homodimers involving intermolecular two-center three-electron bonds

A computational study at CCSD(T) theoretical level has been carried out on radical cation [(PH₂X)₂]^·+ homodimers. Four stable minima configurations have been found for seven substituted phosphine derivatives, X = H, CH₃, CCH, NC, OH, F and Cl. The most stable minimum presents an intermolecular two-center three-electron P···P bond except for X = CCH. The other three minima correspond to an alternative P···P pnicogen bonded complex, to a P···X contact and the last one to the complex resulting from a proton transfer, PH₃X⁺:PHX^·. The complexes obtained have been compared with those of the corresponding neutral ones, (PH₂X)₂, and the analogous protonated ones, PH₃X⁺:PH₂X, recently described in the literature. The spin and charge densities of the complexes have been examined. The electronic characteristics of the complexes have been analyzed with the NBO and AIM methods. The results obtained for the spin density, charge and NBO are coherent for all the complexes.     

A study of aromatic three membered rings

The resonance energies (REs) of neutral three membered ring analogs of the cyclopropenyl cation, computed using block localized wave function (BLW) methods, reveal considerable variations. The RE's of cyclopropenes substituted with exocyclic double bonded groups C?X, (X = O, NH, CH₂, S, PH, SiH₂) increase with the electronegativity of X in the same row (SiH₂ < PH < S and CH₂ < NH < O). The extra cyclic resonance energies (ECREs) (an energetic measure of aromaticity based on comparisons with the RE's of acyclic models) of these derivatives range from +10.5 kcal/mol for cyclopropenone (X = O) (somewhat aromatic; the benzene ECRE is 29.3 kcal/mol) to ?2.4 kcal/mol (slightly antiaromatic) for X = SiH₂. Additional disubstitution of the C?C double bond by X′ groups (X′ = CH₃, NH₂, OH, SiH₃, PH₂, SH) increases the REs considerably, but has only small effects on the ECREs. Even the ECRE of deltic acid (X = O, X′ = OH) is only increased to +13.3 kcal/mol. The conclusion based on ECRE's, that all 12 of the three membered rings are only marginally aromatic/antiaromatic, is supported by the satisfactorily plot (R² = 0.92) of ECRE against values of NICS(0)_πzz (a superior nucleus chemical independent shift magnetic index of aromaticity), which range only from ?6.1 ppm (diatropic) for deltic acid (cf., ?35.5 ppm for benzene and ?14.2 ppm for the parent cyclopropenium ion) to +8.9 ppm (paratropic) for the silicon derivative, X = SiH₂, X′ = SiH₃. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

A 4 K matrix ESR study of the rearrangement and fragmentation of molecular radical cations of alkyl formates and acetates: Direct evidence for a McLafferty rearrangement

A 4 K matrix ESR study shows that the molecular radical cations of isopropyl formate and acetate, produced radiolytically in halocarbon matrices at 4.2 K, undergo spontaneous rearrangement due to a selective intramolecular hydrogen shift from the tertiary CH bond in the isopropyl group to the carbonyl oxygen atom giving RC⁺(OH)OC(CH₃)₂, where R = H or CH₃. The radical cation of tert-butyl acetate undergoes further fragmentation at the ester CO bond following a similar rearrangement to give an isobutene radical cation in CFCl₃.     

Methyl propionate radical cation

Examination of the reactions of the long-lived (>0.5-s) radical cations of CD₃CH₂COOCH₃ and CH₃CH₂COOCD₃ indicates that the long-lived, nondecomposing methyl propionate radical cation CH₃CH₂C(O)OCH ₃ ^+· isomerizes to its enol form CH₃CH=C(OH)OCH ₃ ^+· (ΔH _{isomerization} ? ?32 kcal/mol) via two different pathways in the gas phase in a Fourier-transform ion cyclotron resonance mass spectrometer. A 1,4-shift of a β-hydrogen of the acid moiety to the carbonyl oxygen yields the distonic ion ^·CH₂CH₂C⁺ (OH)OCH₃ that then rearranges to CH₃CH=C(OH)OCH ₃ ^+· probably by consecutive 1,5- and 1,4-hydrogen shifts. This process is in competition with a 1,4-hydrogen transfer from the alcohol moiety to form another distonic ion, CH₃CH₂C⁺(OH)OCH ₂ ^· , that can undergo a 1,4-hydrogen shift to form CH₃CH=C(OH)OCH ₃ ^+· . Ab initio molecular orbital calculations carried out at the UMP2/6-31G** + ZPVE level of theory show that the two distonic ions lie more than 16 kcal/mol lower in energy than CH₃CH₂C(O)OCH ₃ ^+· . Hence, the first step of both rearrangement processes has a great driving force. The 1,4-hydrogen shift that involves the acid moiety is 3 kcal/mol more exothermic (ΔH _{isomerization}=?16 kcal/mol) and is associated with a 4-kcal/mol lower barrier (10 kcal/mol) than the shift that involves the alcohol moiety. Indeed, experimental findings suggest that the hydrogen shift from the acid moiety is likely to be the favored channel.     

Ab Initio Calculations Show How 31PNMR Chemical Shifts in PXYZ Phosphines Correlate with Substituent Electronegativity

Abstract

Letcher and Van Wazers suggestion[1] that the ³¹P NMR chemical shifts of phosphines might be related to the substituent electronegativity, EN(X)[2], has not been verified subsequently by the available experimental data[3,4]. We now have explored this relationship systematically by means of reliable[5] ab initio magnetic property calculations[6] on a comprehensive set of molecules: PXY2 (Y= H, F, CH₃, Cl) and PXYZ (Y= H, Cl and Z= F) with X= H, CH₃, NH₂, OH, F, SiH₃, PH₂, SH, Cl.     

Theoretical study on the interactions of halogen‐bonds and pnicogen‐bonds in phosphine derivatives with Br2, BrCl,and BrF

The MP2 ab initio quantum chemistry methods were utilized to study the halogen‐bond and pnicogen‐bond system formed between PH₂X (X = Br, CH₃, OH, CN, NO₂, CF₃) and BrY (Y = Br, Cl, F). Calculated results show that all substituent can form halogen‐bond complexes while part substituent can form pnicogen‐bond complexes. Traditional, chlorine‐shared and ion‐pair halogen‐bonds complexes have been found with the different substituent X and Y. The halogen‐bonds are stronger than the related pnicogen‐bonds. For halogen‐bonds, strongly electronegative substituents which are connected to the Lewis acid can strengthen the bonds and significantly influenced the structures and properties of the compounds. In contrast, the substituents which connected to the Lewis bases can produce opposite effects. The interaction energies of halogen‐bonds are 2.56 to 32.06 kcal·mol^?1; The strongest halogen‐bond was found in the complex of PH₂OH???BrF. The interaction energies of pnicogen‐bonds are in the range 1.20 to 2.28 kcal·mol^?1; the strongest pnicogen‐bond was found in PH₂Br???Br₂ complex. The charge transfer of lp(P) ? σ*(Br? Y), lp(F) ? σ*(Br? P), and lp(Br) ? σ*(X? P) play important roles in the formation of the halogen‐bonds and pnicogen‐bonds, which lead to polarization of the monomers. The polarization caused by the halogen‐bond is more obvious than that by the pnicogen‐bond, resulting in that some halogen‐bonds having little covalent character. The symmetry adapted perturbation theory (SAPT) energy decomposition analysis showes that the halogen‐bond and pnicogen‐bond interactions are predominantly electrostatic and dispersion, respectively.     

Carbon?hydrogen versus carbon?heteroatom activation by a high‐valent zirconium‐imido complex

A density functional theory (DFT) study of carbon? hydrogen versus carbon? heteroatom bond activation is presented. Heteroatom groups (X) investigated include X = F, Cl, OH, SH, NH₂, PH₂. The activating model complex is a prototypical d⁰ zirconium‐imide. While C? X activation has a thermodynamic advantage over C? H activation, the former has been found to have a kinetic advantage. Implications for catalytic hydrocarbon functionalization and phosphine–ligand degradation are discussed. The present results for a high‐valent metal complex are compared/contrasted with low‐valent bond activating complexes. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Geminal interactions in ClZ(CH3)2X molecules (Z = C, Si, Ge) according to ab initio calculations

The structure and electronic parameters of ClZ(CH₃)₂X molecules (Z = C, Si, Ge, X = CH₃, OCH₃) were calculated by the RHF/6–31G(d) and RHF/6–311G(d,p) methods with full geometry optimization; calculations of ClZ(CH₃)₂OCH₃ molecules were also performed by the RHF/6–31G(d) method with partial geometry optimization. The ³⁵Cl NQR frequencies calculated from the populations of less diffuse 3p constituents of valence p orbitals of chlorine [RHF/6–31G(d)] were in agreement with the experimental values. The ³⁵Cl NQR frequencies for molecules with X = OCH₃ are lower than those for molecules with X = CH₃ (the Z atom being the same), due mainly to direct through-field polarization of the Z-Cl bond, induced by the effect of unshared electron pair of the oxygen atom in the trans position with respect to that bond. The difference in the ³⁵Cl NQR frequencies decreases in going from Z = C to Z = Si, Ge, in parallel with variation of the Z-Cl bond polarization as the size of Z increases.     

Mono- and bis-alkoxides of tris(3,5-dimethylpyrazolyl)borato molybdenum and tungsten nitrosyls

The complexes [ML^*(NO)Cl(OR)] {L^* = HB(3,5-Me₂C₃HN₂)₃; M= Mo, R = CH₂CH₂X, X = Cl, OMe or OEt; (CH₂)_nOH, n = 2, 5, 6; M = W, R = CH₂CH₂X, X = Cl, OMe or OEt; (CH₂)_nOH, n = 2–6; CH₂(CF₂)₃CH₂OH; CHMeCH₂CMe₂OH} and [ML^*(NO)(OR)₂] {M = Mo, R = CH₂CH₂X, X = Cl, OMe or OEt; (CH₂)_nOH, n = 2–6; M = W,R = CH₂CH₂X, X= Cl, OMe or OEt; (CH₂)_nOH, n = 2,4–6; CH₂(CF₂)₃CH₂OH} have been prepared from [ML^*(NO)Cl₂] and the appropriate alcohol in the presence of NEt₃ or NaCO₃, and have been characterized by IR, ¹H NMR and mass spectroscopy.     

Rational design of iron catalysts for C–X bond activation

We have quantum chemically studied the iron-mediated C X bond activation (X = H, Cl, CH₃) by d⁸-FeL₄ complexes using relativistic density functional theory at ZORA-OPBE/TZ2P. We find that by either modulating the electronic effects of a generic iron-catalyst by a set of ligands, that is, CO, BF, PH₃, BN(CH₃)₂, or by manipulating structural effects through the introduction of bidentate ligands, that is, PH₂(CH₂)_nPH₂ with n = 6–1, one can significantly decrease the reaction barrier for the C X bond activation. The combination of both tuning handles causes a decrease of the C H activation barrier from 10.4 to 4.6 kcal mol⁻¹. Our activation strain and Kohn-Sham molecular orbital analyses reveal that the electronic tuning works via optimizing the catalyst–substrate interaction by introducing a strong second backdonation interaction (i.e., “ligand-assisted” interaction), while the mechanism for structural tuning is mainly caused by the reduction of the required activation strain because of the pre-distortion of the catalyst. In all, we present design principles for iron-based catalysts that mimic the favorable behavior of their well-known palladium analogs in the bond-activation step of cross-coupling reactions.     

CH2 + transfer to pyridine nucleophiles: a means of producing α-distonic ions

The distonic radical cation C₅H₅N⁺?^·CH₂ can be generated by the reactions of neutral pyridine with the radical cations of cyclopropane, ethylene oxide, and ketene, as well as with the [C₃H₆]⁺ ion from fragmentation of tetrahydrofuran. The distonic product ion can be distinguished from isomeric methylpyridine radical cations because the former gives characteristic [M?CH₂]⁺, [M ? CH₂NCH]⁺, and a doubly charged ion, all of which are produced on collisional activation. Furthermore, the distonic species completely transfers CH₂ ⁺ to more nucleophilic, substituted pyridines. These properties are all consistent with the assigned distonic structure. Another distonic isomer, the (3-methylene) pyridinium ion, can be distinguished from the (1-methylene)pyridinium ion on the basis of their different fragmentation behaviors. The latter ion exhibits higher stability (lower reactivity) than the prototypal [·CH₂NH₃ ⁺], making available a distonic species whose bimolecular reactivity can be readily investigated.     

Phototransformations of azetidine radical cations stabilized in freonic matrices

It has been established that transformations of azetidine radical cations observed in freonic matrices under the action of light with λ = 436 nm (T = 77 K) are associated with C-N bond cleavage which corresponds to the cyclic form yielding a mixture of open distonic C-centered radical cations of the following structure: ·CH₂CH₂CH=NH ₂ ⁺      

Unusual crystal and molecular structure of tris(2-hydroxyethyl)ammonium fluoride

According to the X-ray diffraction data, the crystal and molecular structure of tris(2-hydroxyethyl) ammonium fluoride (F^?N⁺H(CH₂CH₂OH)₃, fluoroprotatrane, substantially differs from other halo protatranes X^?N⁺H(CH₂CH₂OH)₃ (X = Cl, Br, and I). At X = F, to the endo-molecular LP of the nitrogen atom the HF molecule having the minimum ionic radius in a series of X^? anions is bonded. The geometry of fluoroprotatrane and the cation packing in the crystal are analyzed.     

Relative stability of multiple bonds between germanium and arsenic. A theoretical study

Substituent effects on the potential energy surface of XGeAs (X=H, Li, Na, BeH, MgH, BH₂, AlH₂, CH₃, SiH₃, NH₂, PH₂, OH, SH, F, and Cl) were investigated by using B3LYP and CCSD(T) methods. The isomers include structures with formal double (GeAsX) and triple (XGeAs) bonds to germanium–arsenic, so a direct comparison of these types of species is possible. Our model calculations indicate that electropositively substituted GeAsX species are thermodynamically and kinetically more stable than their isomeric XGeAs molecules. Moreover, the theoretical findings suggest that F, OH, NH₂, and CH₃ substitution prefer to shift the double bond (GeAsX) by forming a triple bond (XGeAs).     

Properties and reactivity in groups of the periodic system: Ion-molecule reactions CH3X + CH3X+· (X = F through At)

The reactions indicated in the title have been studied in terms of direct processes and complex formation. Quantum-chemical methods have been applied to the passage of an acid (H⁺, CH, X⁺) from CH₃X^+· to CH₃X, and the abstraction of a radical (H^· CH, X^·) from CH₃X by CH₃X^+·. It has been shown that a complex represented by a dimer of a methyl-halide radical cation, (CH₃X), with a two-center three-electron bond X? X, has fairly high stability. These investigations were based on non-empirical quantum-chemical calculations, the results being systematically compared with experimental determinations. Some calculations included all electrons (X=F, Cl, Br), others were based on relativistic pseudopotentials (X=F through At). The two sets of calculations agree qualitatively with each other and with experimental observations.     

Cleavage of the carbon-silicon bonds in trimethylsilylacetylenes by trans[(PR3)2PtX(R′OH)]PF6 cations and formation of cationic alkoxycarbene complexes of platinum(II)

Cationic alkoxycarbene complexes of platinum(II) have been isolated in the reactions of trans-[(PR₃)₂PtX(R′OH)]PF₆ (X  H or Me; R′  Me or Et) with Me₃SiCCR′′ (R′′  H, Me or SiMe₃). In these reactions cleavage of the carbon-silicon bond by the nucleophilic attack of alcohol has been observed. These carbene complexes have been characterized by elemental analyses and by IR, ¹H and ¹³C NMR spectral data. ¹³C NMR chemical shift data for carbene carbon atoms suggest that the carbene carbon may be very positively charged.     

Cationic polymerization with p-substituted benzyl p-hydroxyphenyl methyl sulfonium salts: Effect of substituents and mechanistic aspects of initiation reaction

Various p-substituted benzyl p-hydroxyphenyl methyl sulfonium salts ( 2 ) were synthesized and their initiator activities were evaluated in bulk polymerization of glycidyl phenyl ether (PGE). The order of the activity was found to be 2b (X = CH₃) > 2a (X = H) ≈ 2c (X = Cl) > 2d (X = NO₂), indicating that the introduction of an electron-donating group enhanced the activity. In Hammett's plots, the logarithm of the ratio of the polymerization rates (log k_x/k_H) was correlated with σ⁺_ρ better than with σ_p and a negative ρ⁺ value (-1.18) was obtained. Reaction of 2a with benzyl mercaptan mainly gave dibenzyl sulfide and p-hydroxyphenyl methyl sulfide. The obtained results seemed to demonstrate that the OH group of the aryl group yielded no proton as initiator for the polymerization, whereas the benzyl group caused the polymerization, which was initiated by the corresponding benzyl cation formed by C? S bond cleavage. © 1993 John Wiley & Sons, Inc.     

Gas-phase chemistry of the methyl carbamate radical cation,H2NCOOCH. II—A test case for ion structure assignment

The unimolecular chemistry of the methyl carbamate radical cation, H₂NCOOCH, 1, has been further investigated by a combination of mass spectrometry-based experiments (metastable ion (MI), collisional activation (CA), collision-induced dissociative ionization (CIDI), neutralization-reionization (NR) Spectrometry and ²H labelling) and ab initio molecular orbital calculations, executed at the MP3/6–31G*//4–31G level of theory and corrected for zero-point vibrational energies. Apart from the previously located maxima, i.e. H₂NCOOCH₃^+·, 1, the distonic ion H₂NC(OH)OCH₃^+·, 2, hydrogen-bridged ions [H₂N? C?O…? H…?O?CH₂]^+·, 5, and [H₂N? CH?O…?…?H…?O?C? H]^+·, 7, there exist at least two other equilibrium structures, viz. the iminol ion H? N?C(OH)? OCH, la, and the hydrogen-bridged species [H₂C?O…?H…?N(H)COH], 6a, which is closely related to ion 5. Although the iminol ion la lies only 30 kJ mol^?1 above 1, our calculations indicate that the barriers for its formation either directly from ionized methyl carbamate 1 via a 1,3-hydrogen shift or indirectly via 1,4-hydrogen shifts from the distonic ion 2 are too high to allow the iminol ion to be involved in the unimolecular chemistry of ionized methyl carbamate. This explains the earlier observation that there are no H-D exchange reactions prior to decomposition of ionized labelled methyl carbamate, in contrast to the related ion methyl acetate. However, attempts to generate the iminol ion by loss of CH₃CN from CH₃CH?N? NHCOOCH₃ produced the more stable distonic ion 2 instead, but it proved very difficult to assign its structure unequivocally because 2 can rapidly interconvert with 1 and so virtually identical dissociation characteristics ensue. Only by integration of results obtained from many experiments and from ab initio calculations could structure 2 be assigned. The distonic ion 2 can undergo two transformations: after stretching of the C? OCH₂ bond the incipient formaldehyde can migrate within the electrostatic field of ionized hydroxyaminocarbene to the OH end to generate 5, but it can also migrate to the NH end to generate 6a. This explains the previous puzzling observation that H₂NCOOCD forms both CD₂OD^· and CD₂OH^· in CA and NR experiments. The calculations and experiments indicate that, although the ion is exceedingly difficult to characterize, the distonic ion 2 is the key intermediate for all the observed dissociations of methyl carbamate.