In substituted benzenes

Table 7.53 Carbon-13 Chemical Shifts in Substituted Benzenes 7.104...

Fig. 4.3. Resonance, field, and inductive components of substituent effects in substituted benzenes.

In the benzene series, an approximately linear relationship has been obtained between the chemical shifts of the para-hydrogen in substituted benzenes and Hammett s a-values of the substituents. Attempts have been made, especially by Taft, ° to use the chemical shifts as a quantitative characteristic of the substituent. It is more difficult to correlate the chemical shifts of thiophenes with chemical reactivity data since few quantitative chemical data are available (cf. Section VI,A). Comparing the chemical shifts of the 5-hydrogen in 2-substituted thiophenes and the parahydrogens in substituted benzenes, it is evident that although —I—M-substituents cause similar shifts, large differences are obtained for -j-M-substituents indicating that such substituents may have different effects on the reactivity of the two aromatic systems in question. Differences also... 

Vertical ionization potentials of the rcs orbital in substituted benzenes were correlated with the LDRA equation (equation 70) to give the regression equation ... 

The deformation of the benzene ring in substituted benzenes is a sensitive indicator of substituent effects. Extensive experimental evidence accumulated over the past two decades, mainly from X-ray diffraction studies of solid state samples However, the first report of a ring distortion in a benzene derivative was done by Keidel and Bauer in their pioneering (1956) gas-phase electron diffraction study of the molecular structure of phenylsilane Recently a... 

Photosensitized decomposition of 9b in substituted benzenes led to similar results [83DIS(B)(44)1113]. Therefore, either spin inversion from the triplet to the singlet form of 4-diazoimidazole is faster than the decomposition of the excited diazo compound, or intersystem crossing from the triplet to singlet carbene is easier and faster than the reaction of the triplet state with substrates. 

Products (Yield %) of 4-Diazo-1,2,3-triazoles in Substituted Benzenes"... 

The vapor-phase pyrolysis of 4-hydroxy-1,2,3-triazole and its iV-methyl derivative affords methan-imine and its A-methyl analog. Analysis of the reaction path by the MNDO method shows the presence of two stable or metastable isomers, (liif)-4-hydroxy-l,2,3-triazole and its ketone protomer <89NJC551>. 4-Diazo-1,2,3-triazoles (122) thermolyze or photolyze in benzene to 4//-l,2,3-tri-azolylidenes (123) which convert benzene to 4-phenyl-1,2,3-triazoles and/or isomerize to a-diazo-nitriles (124). Intermediates (124) react with benzene via a carbene to give addition, ring expansion or substitution products (Scheme 17) <82TL5115>. The similar thermolysis of diazotriazoles in substituted benzene gives complex mixtures in which all of the components are sometimes impossible to isolate and identify <90AHC(48)65>. 

The parallelism between the behavior of nitrogen in 4-substituted pyridines and that of the corresponding carbon in substituted benzenes is well illustrated in Fig. 6.7. The results of Table 6.6 reveal another important similarity between... 

In substituted benzenes, the symmetry is lowered and the transitions into the states that correlate to the B2u and B u states of benzene become allowed by IPA, 2PA, or both. However, when the substituents induce only a weak perturbation on the benzene yr-electron system, the IPA or 2PA spectra of the substituted compounds often closely resemble the spectrum of the unsubstituted parent molecule. Various theoretical models have been developed in an attempt to predict the type of change in the band intensity and characteristics in the 2PA spectra of substituted benzenes and, more generally, of alternant hydrocarbons [34-36]. It was found that the effect of a perturbation is quite different for IP and 2P allowed transitions. In particular, 2P transitions to the state correlated to the benzene B2u state (Lb) are affected more by vibronic coupling than transitions to the state correlated to the benzene Biu state (La, in Platt notation [31,32]). In contrast, inductive perturbations enhance the La band more than the Lb band. The effects of vibronic coupling and inductive substituents are reversed for IP transitions into these states. Experimental... 

The factor ijhb is the bond order between the interacting hydrogen atoms and is unity for H2. Proton-proton bond orders in substituted benzenes as derived from coupling constants and Eq. (21) are given in Table III. The same quantitative significance should not be attached to the proton-proton bond orders of Table III that one gives to the carbon-carbon... 

In substituted benzenes (i.e., benzenes in which hydrogen is substituted by another atom or group of atoms), depending on the type and position of the substituents, the different resonance forms may exhibit somewhat different stabilities. Therefore, their contributions are different. 

What adds to our suspicion is that die steric effect of Me and COOH in substituted benzenes are so comparable M. Colomina, C. Turrion, P. Jimenez, M. V. Roux and J. F. Liebman, Struct. Chem., 5, 141 (1994). It seems very unlikely that die steric effects of COMe and COOH could be that different. 

The primary photolytic step upon UV photolysis of Cp Re(CO)3, CO loss, has been utilized to study the oxidative-addition of the putative Cp Re(CO)2 fragment in substituted benzenes. Although preparative chemistry in a gen-... 

Intracluster ion-molecule reactions have been the subject of numerous investigations (see other chapters of this book). However, in this chapter, we will focus only on aromatic nucleophilic substitutions in substituted benzene/molecule clusters where the two-photon ionization process allows us to obtain spectroscopic information on the parent cluster involved in the reaction process. 

A. Internal Conversion Electronic Relaxation in Substituted Benzenes... 

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