Matrix isolation substituent effects

With a few exceptions simple 1,2,3-oxadiazoles (1) are not isolable because they are less stable than their open tautomers (2) (Equation (1)). The sterically protected 1,2,3-oxadiazole (3) is the only known oxadiazole bearing alkyl substituents. It exists in the cyclic form in the crystalline state but as the diazoketone in chloroform solution <80TH 403-01 >. 1,2,3-Benzoxadiazolehas been detected in a matrix at 15 K <84AG(E)509> this and some substituted 1,2,3-benzoxadiazoles have been shown to exist in equilibrium with their open-chain tautomers <91JST(247)135>. The position of the equilibrium is strongly dependent upon solvent and substituent effects the most stable of the... 

The problem of norbomyl cation stabilities vs. solvolysis rate discrepancies in the norbomyl system has been addressed in an important paper.159 The classical and non-classical norbomyl cations do not resemble the 2-endo- and 2-exo -norbomyl solvolysis transition states very closely. The authors conclude that Brown was wrong, but that Winstein was not entirely right either.159 A substituent in the benzene ring has little effect upon the kinetics of the acid-catalysed hydrolysis of 2-exo-norbomyl phenyl ether.160 The FTIR spectra of matrix-isolated 2-methylbenzonorbomen-2-yl cations have been examined at —196 °C the structure can best be represented as (108), rather like a phenonium cation, but at higher temperatures a transition takes place to a structure that is more nearly represented as (109), with some 71-bridging.161 The stereoselectivities of some 7-methyl-7-norbom(en)yl cations have been investigated (110) has a classical structure and reacts in a stereo-random manner, whereas (111) is... 

Silicon substituted silylenes attract the chemists interest for a special reason. According to ab initio calculations 85 137 176], substituents acting as cr-donors should induce a relatively large red-shift of the UV maximum or, in other words, the n(Si) —> 3p(Si) transition energy should be relatively small. Therefore these species are potential candidates for the long-sought triplet ground state silylenes, especially if this electronic effect is supported by a steric one. Nevertheless, reports on the matrix isolation of silicon substituted silylenes are comparatively scarce. 

Jeng, M.-L. H., and Ault, B. S., Infrared matrix isolation study of hydrogen bonds involving C—H bonds substituent effects, J. Phys. Chem. 93, 5426-5431 (1989). 

Conjugative Effects Ligands acting as n-donors generally cause blue-shifts . However, the n-donor does not have to be a real substituent at the subvalent silicon atom. Lewis bases which are matrix-isolated along with the silylene also induce blue-shifts by the formation of adducts. This was shown for phosphines, sulfides, amines, alcohols, ethers and carbon monoxide . As mentioned above, in the case of methylsUylene even the use of N2 as the matrix material instead of argon led to a blue-shift of 150 nm . ... 

The electronic structure of silylenes has been discussed in depth since the first matrix isolation of dimethylsilylene by West and coworkers , who observed that dimethylsilylene produced by the irradiation of dodecamethylcyclohexasilane in an argon matrix at 10 K or in a 3-methylpentane glass matrix at 77 K showed a characteristic absorption maximum at 450 nm. All the silylenes investigated so far are known to be ground state singlets and the absorption due to the excitation of a lone-pair electron to the vacant pjr orbital of the silylene (equation 13) is observed in the UV-VIS region °. The absorption maxima depend strongly on the electronic and steric effects of the substituents. Absorption maxima... 

By use of matrix isolation infrared spectroscopy, it has been shown that the mechanism of ozonolysis of (Z)-3-methyl-2-pentene (mp) is similar to that for ozonolysis of simple alkenes. Indirect evidence for formation of one or both possible Criegee intermediates is presented. Eight fundamental vibrations of the c/s -isomer of the primary ozonide of mp are observed. UV irradiation led to the product arising from O atom addition to mp. Second-order rate coefficients for the ozonolysis of -butyl methacrylate, ethyl cro-tonate and vinyl propionate under atmospheric pressure have been determined and the effects of substituent groups on the overall rate coefficients have been analysed. Free energy relationships are presented and atmospheric lifetimes are discussed. ... 

The effect of substituents placed para to the nitrene center was investigated in a series of matrix isolation, laser flash photolysis, and product studies. Table 44.1 Ksts a number of rate constants (k,sc for intersystem crossing to the triplet ground-state and for rearrangement to the didehydroazepine). For comparison, the kinetic parameters of several other singlet aryl nitrenes are also given. 

However, the complete elimination of the radicals is problematic, because their stability and hence their lifespan is enhanced by various kinetic or thermodynamic effects. For instance, alkyl groups have a stabilizing action and there is also a steric intervention by substituents with a blocking action or reduced mobility owing to strong crosslinking. Thus, some radicals hardly have any opportunity for recombination and remain isolated in the matrix. 

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