Verbrückte und unverbrückte norbornyl-kationen in der gasphase. Berechnungen zur stabilität isomerer norbornyl-kationen mit hilfe quantenmechanischer verfahren

The geometry and the relative stability of bicyclic compounds 1–20 have been calculated by standard quantum mechanics methods.MINDO/3 yields the following stability order of isomeric norbornyl cations (relative energies in $\text{kcal}\text{mole}$): 1-norbornyl cation 9 (0.0); 1.7 σ-bridged cation 6 (0.7); 7-norbornyl cation (nonplanar) 7 (1.1); 2-norbornyl cation (classical) 2 (4.2); 7-norbornyl cation (planar) 8 (4.3); 2-norbornyl cation (bridged) 1 (6.1). The stability of the same ions calculated by ab initio methods (STO-3G, MINDO/3-geometry) leads to an order more nearly consistent with experimental results: 2-norbornyl cation (classical) 2 (0.0); 2-norbornyl cation (bridged) 1 (5.9); 7-norbornyl cation (planar) 8 (11.1); 1-norbornyl cation 9 (14.6); 7-norbornyl cation (nonplanar) 7 (21.2). For the secondary 7-norbornyl cation, MINDO/3 gives a pyramidal configuration, 3.2 $\text{kcal}\text{mole}$ more stable than the planar form. In contrast, the ab initio results (complete optimization of all geometrical parameters) indicate the planar cation to be the most stable form. The bridged structure of 2-norbornyl cation 1 is calculated (STO-3G, partly optimized) to be 4.3 $\text{kcal}\text{mole}$ less stable than the classical counterpart, 2. For the lower homologues 12 and 13 (STO-3G, complete geometry optimization), this difference is 6.4 $\text{kcal}\text{mole}$. However, more extended basis sets should favour the bridged structures. The hydrogen bridged norbornyl cations 3, 4, and 5 have been calculated (STO-3G, partly optimized) to be 14.4, 23.6 and 29.9 $\text{kcal}\text{mole}$ less stable than 2. The stability differences between the corresponding tertiary bicyclic ions 10 vs 11, and 14 vs 15 are calculated (ab initio) to be 15.3 and 19.0 kcal/mole, respectively, in favour of classical structures. The influence of methyl substitution at positions C₁ and C₆ (exo) on bridged and unbridged structure of 2-norbornyl cation is calculated. Pyramidal secondary and tertiary 2-norbornyl cations 19 (a; R=H, b; R=CH₃) and 20 (a; R=H, b; r=CH₃) have been used to model the electrical effects in the solvolysis transition states of epimeric 2-norbornyl esters. Due to more efficient hyperconjugation the pyramidal exo cation is stabilized more than the endo cation by 5.2 $\text{kcal}\text{mole}$ for the secondary series and 3.5 $\text{kcal}\text{mole}$ for the tertiary series. Bonding of endo cation 20 with a nucleophile should be stronger than bonding of exo cation 19 due to more efficient HOMO-LUMO interaction.     

A unique transition metal-stabilized silicon cation

Trivalent silicon cations are exceptionally strong electron pair acceptors that react, either desired or undesired, with almost any σ and π basic molecule. One way of intramolecular attenuation of the Lewis acidity of these superelectrophiles is by installation of a ferrocene unit at the electron-deficient silicon atom. While well-understood for isoelectronic α-ferrocenyl-substituted carbenium ions and also boranes, the stabilizing interactions between the ferrocene backbone and a positively charged silicon atom are not clear due to the challenge of crystallizing such cations. The structural characterization of our ferrocene-stabilized silicon cation now reveals an unprecedented bonding motif different from its analogues. An extreme dip angle of the silicon atom toward the iron atom is explained by two three-center-two-electron (3c2e) bonds through participation of both the upper and the lower aromatic rings of the ferrocene sandwich structure. The positive charge is still localized at the silicon atom that also retains a quasi-planar configuration.     

L-shaped three-center two-electron (C-C-C)+ bonding array

The structures and reactivities of the complexes between carbenium ions (R(+)) and acetylene or propyne have been investigated with the aid of electron-correlated quantum mechanical calculations (hybrid density functional, perturbation theory, and coupled cluster methods). Depending on the R group, the acetylene/carbenium ion interaction can produce either an "open" 3c-2e structure or the conventional vinyl cation structure. The "open" 3c-2e C-C-C bonding geometry exists as a minimum for R = methyl and primary/secondary/tertiary alkyl, and hence is the most notable. The alignment of three carbon centers is neither bridged nor linear, but L-shaped, and represents a new type of "open" 3c-2e bonding that has so far escaped proper attention.     

Electron impact induced fragmentation of dihydro-1,4-dithiines. 2,3-Substituted 5,6-dihydro-l,4-dithiines

In the electron impact induced fragmentation of 2,3-disubstituted 5,6-dihydro-1,4-dithiines, the most important cleavage of the heterocyclic ring occurs via the retro-Diels-Alder type of reaction. The further fragmentation of the resulting radical cation gives rise to substitutes thiocarbonyl cations. This fragmentation favours formations of ions in which the electron-deficient carbon-sulphur triple bond is stabilized by an electron-releasing group. The ring fragmentation was observed to be dependent on the nature of the 3- and 2-substituents of the ring when a series of 3- and 2-substituted 5,6-dihydro-l,4-dithiines was investigated. The fragmentation of dihydro-l,4-dithiines partly resembles the fragmentation of the corresponding dihydro-l,4-oxathiines. The presence of the two sulphur atoms in dihydro-l,4-dithiines give rise to more stable molecular ions than observed in the case of the dihydro-1,4-oxathiine analogues. Fragmentations of dihydro-l,4-dithiines can also involve extrusions of alkylradicals or HS-radical from the heterocyclic rings of molecular ions.     

Multicenter bonding in organic chemistry. Geometry-sensitive 3c-2e bonding in (C...H...C) fragments of organic cations

This paper reports the application of a new general tool for the study of multicenter bonding, namely the so-called generalized population analysis, to the investigation of interesting geometry dependent variation of 3c-2e bonding in the (C...H...C) fragments of ingeniously designed organic cations I and II. This phenomenon was previously characterized by the correlation between experimental (1)H NMR chemical shifts of the central hydrogen in the (C...H...C) fragment and the changes in the corresponding C-H-C bond angle. The observed values of both chemical shifts and C-H-C angles are shown herein to correlate with the calculated 3-center bond indices but the dependence displays splitting into two separate lines according to the type of corresponding cation.     

Dialane anion: three-center two-electron or two-center one-electron bonded

Dialane anions can be formed via a single three-center two-electron (3c-2e) or two-center one-electron (2c-1e) bond. The 2c-1e bonded anion Al(2)H(6)(-)(D(3)(d)) and the 3c-2e bonded anion Al(2)H(6)(-)(C(s)) have significant thermodynamic stabilities with respect to the neutral Al(2)H(6)(D(2)(h)) and correspond to 0.22 and 0.32 eV of the adiabatic electron affinities, respectively. In particular, the 2c-1e bond plays an essential role in stabilizing the Al(2)H(6)(-)(D(3)(d)) anion.     

On the addition of 4-(N,N-dimethylamino)phenyl cation to norbornene

The photochemically generated 4-(N,N-dimethylamino)phenyl cation adds to norbornene giving a phenylnortricyclene and various 2-exo substituted phenylnorbornanes (main isomers, 3-endo-, 7-anti- and 7-syn-phenyl). Acetamides are obtained in MeCN and ethers in alcohols (MeOH, iso-PrOH, tert-BuOH, CF3CH2OH). The product distribution is closely reminiscent of that obtained in the solvolysis of 2-norbornyl derivatives, supporting that the reaction offers a novel access to a 'non classical' 2-norbornyl cation. The fate of this cation is determined by the basicity/nucleophilicity of the solvent.     

Lewis acid-catalyzed rearrangement of 2,2-dichloronorbornane to 1-chloronorbornane

The mechanism for the unusual AlCl(3)-catalyzed rearrangement of 2,2-dichloronorbornane to 1-chloronorbornane in pentane has been elucidated; the reaction, which also yields four isomeric dichloronorbornanes, occurs in three steps: (1). ionization to form the 2-chloro-2-norbornyl cation, which was fully characterized by two-dimensional (1)H and (13)C NMR in SbF(5)/SO(2)ClF; (2). Wagner-Meerwein shift to yield the 1-chloro-2-norbornyl cation, which was partially characterized by (1)H NMR; and (3). hydride abstraction.     

The mass spectra of benzamide and thiobenzamide

The mass spectra of benzamide, thiobenzamide and their N-d₂ analogues have been studied In addition to fragmenting by simple bond cleavages the molecular ions dissociate partly from their imide forms. Equilibration or ‘scrambling’ of ortho ring hydrogen atoms with amide hydrogen did not occur in benzamide but such exchange must take place in the thiocompound whose fragmentation behaviour is very complex. Neither labelled compound produced the label-retaining benzoyl (thiobenzoyl) cation which has been the subject of much interest in the mass spectrum of O-d₁ benzoic acid.     

Ab initio MO and DFT study for the isomerisation of bicyclo[1.1.0]tetrasilane and the germanium analogues

The mechanism of the unimolecular isomerisation reaction of the silicon and germanium analogues of bicyclo[1.1.0]butane with various kinds of substituents (X₄R₆; X?=?Si and Ge, R=H, CH₃, t-Bu and SiH₃) to the corresponding cyclobutene analogues has been investigated by ab initio molecular orbital and DFT methods. Several reaction mechanisms were considered. They are roughly divided into two types; (1) skeletal rearrangement and (2) substituent migration. It was found that substituents (R) have the leading effect on the reaction mechanism but the partial or full replacement of the skeletal silicon atoms by germanium atoms has some important effects as well. Furthermore, the character of the bridge bond of the long-bond and short-bond isomers of these bicyclic compounds was investigated and discussed in comparison with the ?? bond in ethene and disilene by the CiLC analysis.     

Water-trapping of unstable carbocations taking place into the inverted region of the Marcus equation. First experimental and computational evidence

The reaction rates of the water-trapping of unstable 1-norbornyl cations in relation to the rates of their rearrangement (ring contraction) have been experimentally determined. The activation barrier of the rearrangement reactions was calculated with the QST2 method, whereas the Marcus theory was applied to predict the activation barrier of the trapping reaction. All computations in gas phase were performed with the DFT B3LYP/6-31G(d) method, while the PCM model was used for the computation of unspecific solvent effects. The established methodology was able to predict the major product formed in the solvolysis of the corresponding triflate precursors of the studied carbocations. The agreement between experiment and theory supports, for the first time, the interesting conclusion that the very exergonic trapping-reaction of unstable 1-norbornyl cations takes place with relatively high barriers of activation and, hence, into the Marcus inverted region. Additionally, a noteworthy relationship between the free energy of activation for the carbocation rearrangement in gas phase and the length of the rearranging bond in the ground state is reported.     

Structure,bonding, and spectra of cyclic dithia radical cations: a theoretical study

Ab initio molecular orbital and hybrid density functional theory methods are employed to characterize the structure, bonding and properties of several cyclic dithia radical cation systems, particularly in the context of intra molecular two-center three-electron (2c-3e) bonding between two sulfur atoms. The calculated results are able to interpret the time-resolved transient optical spectra obtained from pulse radiolysis technique for these positively charged dithia systems in aqueous solution. Visualization of the appropriate molecular orbital (MO) in the systems is able to depict the presence of a 2c-3e bond between two sulfur atoms and its sigma character. Geometry optimizations of these doublet systems are carried out at restricted open shell Becke's half-and-half (BHH) nonlocal exchange and Lee-Yang-Parr (LYP) nonlocal correlation functionals (BHHLYP) with 6-311+G(d,p) basis set including solvent effects adopting Onsager's reaction field model. Hessian calculations are done at the same level to check the nature of the equilibrium geometry. Energy data are further improved by performing MP2/6-311+G(d,p) calculations on these radical cation systems. Excited-state calculations are done following configuration interaction with single-electron excitation (CIS) method and the optical transition wavelength from the highest doubly occupied molecular orbital (HDOMO) to the lowest singly occupied molecular orbital (LSOMO) is seen to correspond and match to the position of the absorption maxima (lambda(max)) obtained from the experimental spectra for all these radical cation systems in aqueous solution. These calculations are able to resolve a long-standing ambiguity in the assignment of intra molecular 2c-3e bonding in the case of the 3-methyl-2,4-dithiapentane radical cation system and to provide new insights into bonding features of this odd electron system as well as of other cyclic dithia systems studied.     

C6F5Xe]+ and [C6F5XeNCCH3]+ salts of the weakly coordinating borate anions, [BY4]- (Y = CN, CF3, or C6F5)

New examples of [C6F5Xe]+ salts of the weakly coordinating [BY4]- (Y = CN, CF3, or C6F5) anions were synthesized by metathesis of [C6F5Xe][BF4] with MI[BY4] (MI = K or Cs; Y = CN, CF3, or C6F5) in CH3CN at -40 degrees C, and were crystallized from CH2Cl2 or from a CH2Cl2/CH3CN solvent mixture. The low-temperature (-173 degrees C) X-ray crystal structures of the [C6F5Xe]+ cation and of the [C6F5XeNCCH3]+ adduct-cation are reported for [C6F5Xe][B(CF3)4], [C6F5XeNCCH3][B(CF3)4], [C6F5Xe][B(CN)4], and [C6F5XeNCCH3][B(C6F5)4]. The [C6F5Xe]+ cation, in each structure, interacts with either the anion or the solvent, with the weakest cation-anion interactions occurring for the [B(CF3)4]- anion. The solid-state Raman spectra of the [C6F5Xe]+ and [C6F5XeNCCH3]+ salts have been assigned with the aid of electronic structure calculations. Gas-phase thermodynamic calculations show that the donor-acceptor bond dissociation energy of [C6F5XeNCCH3]+ is approximately half that of [FXeNCCH3]+. Coordination of CH3CN to [C6F5Xe]+ is correlated with changes in the partial charges on mainly Xe, the ipso-C, and N, that is, the partial charge on Xe increases and those on the ipso-C and N decrease upon coordination, typifying a transition from a 2c-2e to a 3c-4e bond.     

Structure and conformation properties of 1-alkyl-3-methylimidazolium halide ionic liquids: a density-functional theory study

The structures and conformational properties of 1-alkyl-3-methylimidazolium halide ionic liquids have been studied with a Becke's 3 Parameter functional method. The interaction mechanisms between the cation and the anion in 1-ethyl-3-methylimidazolium (Emim+) halide and 1-butyl-3-methylimidazolium (Bmim+) halide ionic liquids were investigated using 6-31G*, 6-31++G**, and 6-311++G** basis sets. Forty structures of different ion pairs were optimized and geometrical parameters of them have been discussed in details. Halide ions (Cl- or Br-) have been gradually placed in different regions around imidazolium cation and the interaction energies between the anion and the cation have been calculated. Theoretical results indicate that there are four activity regions in the vicinity of the imidazolium cations, in these regions the imidazolium cations and the halide anions formed stable ion pairs. Imidazolium cations can form hydrogen bond interactions with one, two or three but no more than three nearest halide anions. The halide ions are situated in hydrogen bond positions rather than at random.     

‘Lone Pair’ and ‘CI Bond’ Ionization Energies of exo- and endo-2-Norbornyl Iodides

A PE-spectroscopic study of exo- and endo-2-norbornyl iodides suggests that the relative ability of the 2-norbornyl group to stabilize an electron deficiency on a substituent X (e.g. I) in exo- or endo-position depends on the location of the positive charge. There is no difference if the positive hole is strongly localized on on the substituent X (e.g. the 5p^?1 state of the title compounds). On the other hand, our results indicate that teh positive hole semi-localized in an exo-C? X bond is better stabilized by the 2-norbornyl group than a semi-localized, positive hole in an endo-C? X bond.     

Steric Effects on Reaction Rates. Part XII. Force-Field Calculations for the Solvolysis of Cyclobutyl and Tricyclyl Derivatives

Force-field parameters have been developed for the molecular-mechanics calculation of tertiary carbenium ions with tricyclane structure, for tertiary cyclobutyl and cubyl cations. The cyclobutyl parameters are also applicable to tertiary 7-norbornyl cations. Satisfactory plots are obtained for correlation of the rates of solvolysis with the differences in steric energies between carbenium ions and the corresponding bromides.     

Bonding along a linear B...N...B triad

The synthesis of tris(2,6-dihydroxyphenyl)amine diborate, 4, is reported. This compound contains a linear B...N...B array for which a symmetrical three-center two-electron (3c-2e) bond is possible. The X-ray crystal structure of 4 shows that 3c-2e bonding is, in fact, absent. Rather, the B-N-B array of 4 is unsymmetrical, having a 2c-2e B-N dative bond with the remaining boron pyramidalized outward and bonded to the oxygen of THF, i.e., 4 x THF. In THF solution, 4 displays temperature-dependent 13C NMR spectra from which a DeltaG++ of 11.6 kcal/mol at 262 K may be calculated. The dynamic process observed in solution corresponds to a bond-switching equilibrium in which the B-N bond oscillates between the two borons ("bell clapper"). Ab initio calculations indicate that the most likely pathway for the bond switch does not involve a 3c-2e B...N...B bond, but rather occurs by nucleophilic attack of THF on the datively bonded boron to generate 4 x (THF)2, lacking any B-N interactions, followed by loss of one THF. The B-N-B system of 4 sans the perturbing effect of solvent was also investigated computationally. The form of 4 containing a 3c-2e bond is found to be a transition state in the solvent-free bond-switch reaction of 4, lying 2.66 kcal/mol above 4. The stability of three-center bonds to in-line distortion (viz., X...Y...X --> X-Y.........X) is discussed from the point of view of the second-order Jahn-Teller effect.     

A novel access to 3-aryl-2-norbornyl cation

A novel access to a 2-norbornyl cation under mild, non acidic conditions is found in the addition of photochemically generated 4-dimethylaminophenyl cation to 2-norbornene. Deprotonation to nortricyclene or nucleophile addition ensue depending on the solvent characteristics.     

Electrospray ionization mass spectrometric study of mercury complexes of N-heterocyclic carbenes derived from 1,2,4-triazolium salt precursors

By mixing 1,2,4-triazolium salts (precursors of N-heterocyclic carbenes 1–6) with mercury acetate, a number of complexes have been obtained under electrospray ionization condition. Carbenes 1 and 2 contain one carbene center; therefore, they are able to bond only one mercury cation. Carbenes 3–5 contain two carbene centers; therefore, they can bond two mercury cations. Mercury complexes of 1–5 always contain an acetate anion attached to a mercury cation. Carbene 6 also contains two carbene centers; however, its structure allows formation of a complex containing mercury bonded simultaneously to both centers, therefore, the complex that does not contain an acetate anion. The MS/MS spectra taken for complexes of carbenes 1–5 have shown formation of a cation corresponding to N1 substituent (adamantyl or benzyl), and those of complexes of carbenes 3–5 (doubly charged ions) have also shown the respective complementary partner ions. Mercury complex of 2 has yielded some other interesting fragmentation pathways, e.g. a loss of the HHgOOCCH₃ molecule. The fragmentation pathway of the mercury complexes of 6 was found to be complicated.