Benzene cation structure

Geometries, hyperfme structure, and relative stabilities of the different positional isomers of monodeuterated benzene cations have been studied theoretically by density functional theory, using the B3-LYP functional, and experimentally by ESR and ENDOR spectroscopy. A comparison between theoretical and experimental results at 30 K gives acceptable agreement, but further experiments on multiply deuterated species should improve the analysis by making the effects of deuteration larger. 

Additional experimental studies on multiply deuterated benzene cations would give more information, by enlarging the effects on the ZPVE and also by introducing new structural features in the experimental spectra which can facilitate their interpretation. This would enable a more detailed and more accurate analysis, both theoretically and experimentally. Such experiments will hopefully be carried out in the near future. 

The pyrylium cation is isoelectronic with pyridine it has the same number of electrons and, therefore, we also have aromaticity. Oxygen is normally divalent and carries two lone pairs. If we insert oxygen into the benzene ring structure, then it follows that, by having one electron in a p orbital contributing to the aromatic sextet, there is a lone pair in an sp orbital,... 

Fig. 4.23 Hiickel s rule says that cyclic n systems with An + 2 n electrons ( = 0, 1, 2,. .. An + 2 = 2, 6, 10,. ..) should be especially stable, since they have all bonding levels full and all antibonding levels empty. The special stability is usually equated with aromaticity. Shown here are the cyclopropenyl cation, the cyclobutadiene dication, the cyclopentadienyl anion, and benzene formal structures are given for these species - the actual molecules do not have single and double bonds, but rather electron delocalization makes all C/C bonds the same...

In the way it became clear that at least one or two of the criteria (1) - (6) had to be fulfilled to expect any success in the quest for free silylium cations in solution, one systematically started to use solvents with weak nucleophilic character as well as weakly coordinating counterions such as 33 or 35 shown in Scheme 5. The lead in this research was taken by Lambert, but Reed made also several important contributions and, therefore, one can say that both Lambert and Reed pushed forward the issue of silylium cations in solution despite scepticism and criticism on the usefulness of such work for general chemistry. Benzene and toluene were chosen as the ideal solvents and in 1993 Lambert and Zhang reported on trialkyl substituted silyl cations in aromatic solvents at the presence of TPFPB as a counterion. [7] From measured 29Si NMR chemical shifts (80 - 110 ppm), Lambert and Zhang concluded that they had obtained R3Si+ (R = Me, Et, Pr, Me3Si) ions with reduced electrophilic interactions with solvent or anion and, therefore, with nearly free cationic structure. 

Jahn-Teller distorted state. ENDOR results of dimeric benzene cation formed after warming in CFCI3 and present in CF3CCI3 at all temperatures indicate that the dimer had a sandwich structure in both matrices. An EPR study of the benzene radical cation in argon matrix generated by fast electron irradiation at 16 K showed favorable stabilization of big state rather than b2g state, in contrast to previous results found in Freon matrices [180]. 

Fig. 5 Results of two Ehrenfest simulations on benzene cation with fixed nuclei left side) and with nuclei moving right side). The top figures plot the evolution of the Mulliken spin densities as the function of time. The electron and nuclear motions are represented on the bottom moat diagrams (the nuclear geometry in blue and the electronic character in pink). With nuclei fixed, the electronic character of the system between a set of quinoid/antiquinoid structures. With moving nuclei, the oscillations in the electronic character seem damped until the nuclear geometry slowly catches the electronic character...

The very high resolution that can be achieved using the ZEKE-PFI technique with polyatomic molecules is illustrated in Figure 18.4, which shows the rotation-ally resolved spectrum of the benzene cation, from which its detailed structure has been determined. 

An important feature of the electronic structure of the Jt-complex XLIXa is that at the stage of its formation there occurs the complete electron transfer from the aromatic molecule to the electrophile. The total charge of the nitroso group in XLIXa equals—0.03e (MINDO/3), i.e., this structure corresponds to the pair of benzene cation radical-nitrogen oxide. 

Evans, D. R. et al. Jt-Arene/cation structure and bonding. Solvation versus ligand binding in iron(III) tetraphenylporphyrin complexes of benzene, toluene, p-xylene, and [60]fullerene. J. Am. Chem. Soc. 121, 8466-8474, 1999. 

ZEKE spectroscopy has been applied to a wide variety of molecular ions, clusters, van der Waals molecules, free radicals, reactive intermediates, and even to elusive transition states of chemical reactions. Examples of such typical applications of high-resolution ZEKE spectroscopy to molecules and clusters are given here. Compared to conventional photoelectron spectroscopy, ZEKE spectroscopy offers greatly increased spectral resolution, allowing the rotational structure of large molecular cations such as the benzene cation and the intermolecular vibrations of molecular clusters like phenol-water to be obtained. 

Figure 8. In this spectrum, only the K+ = 0 projections of even N+ are seen in the lower-energy vibronic component, whereas only those from odd N+ are seen in the higher-energy vibronic component. This effect can be attributed to nuclear spin statistics, and indicates unambiguously that the lower-energy vibronic component has B symmetry, and that the higher-energy vibronic component has B2g symmetry. Thus the rotational structure in the ZEKE spectrum has established that the B2g level in the quadratically split 6 (/ = 3/2) levels of the benzene cation, lies above the B g level.

The vibronic coupling model has been applied to a number of molecular systems, and used to evaluate the behavior of wavepackets over coupled surfaces [191]. Recent examples are the radical cation of allene [192,193], and benzene [194] (for further examples see references cited therein). It has also been used to explain the lack of structure in the S2 band of the pyrazine absoiption spectrum [109,173,174,195], and recently to study the photoisomerization of retina] [196],... 

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

Wnte a structural formula for the most stable cyclohexadienyl cation intermediate formed in each of the following reactions Is this intermediate more or less stable than the one formed by electrophilic attack on benzene" ... 

Cation (Section 1 2) Positively charged ion Cellobiose (Section 25 14) A disacchande in which two glu cose units are joined by a 3(1 4) linkage Cellobiose is oh tamed by the hydrolysis of cellulose Cellulose (Section 25 15) A polysaccharide in which thou sands of glucose units are joined by 3(1 4) linkages Center of symmetry (Section 7 3) A point in the center of a structure located so that a line drawn from it to any element of the structure when extended an equal distance in the op posite direction encounters an identical element Benzene for example has a center of symmetry Cham reaction (Section 4 17) Reaction mechanism m which a sequence of individual steps repeats itself many times usu ally because a reactive intermediate consumed m one step is regenerated m a subsequent step The halogenation of alkanes is a chain reaction proceeding via free radical intermediates... 

In the early work, benzene formed the basis of a variety of multi-armed structures. Analogs bearing from 2—6 arms were prepared and compared for cation binding ability. The only indication of mode of synthesis for the hexa-substituted benzene derivative is that it was obtained on reaction of benzene-hexakis(methanethiol) and l-bromo-3,6,9-trioxatridecane . The reaction is illustrated in Eq. (7.6), below, devoid of reaction conditions and yields which were not specified. 

The initial step is the coordination of the alkyl halide 2 to the Lewis acid to give a complex 4. The polar complex 4 can react as electrophilic agent. In cases where the group R can form a stable carbenium ion, e.g. a tert-buiyX cation, this may then act as the electrophile instead. The extent of polarization or even cleavage of the R-X bond depends on the structure of R as well as the Lewis acid used. The addition of carbenium ion species to the aromatic reactant, e.g. benzene 1, leads to formation of a cr-complex, e.g. the cyclohexadienyl cation 6, from which the aromatic system is reconstituted by loss of a proton ... 

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