Bonding and Structures of Reactive Intermediates

The following representative examples of TRIR studies are not meant to be an exhaustive treatment of the various organic reactive intermediates that have been investigated by TRIR methods, but rather to demonstrate the unique insight that such studies can provide. The direct observation of organic intermediates in solution at room temperature by IR spectroscopy can reveal fundamental information related both to bonding and structure of reactive intermediates as well to mechanisms of product formation. 

The major carbon centered reaction intermediates in multistep reactions are carboca-tions (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [ ]+, [ ]-, [ ], and [ CI I21-... 

A second role for mass spectrometry in the investigation of reactive intermediates involves the nse of spectroscopy. Althongh an important nse of ion spectroscopy is the determination of thermochemical properties, including ionization energies (addition or removal of an electron), as in photoelectron or photodetachment spectroscopy, and bond dissociation energies in ions, as in photodissociation methods, additional spectroscopic data can also often be obtained, inclnding structural parameters such as frequencies and geometries. 

Metabolites formed during the decolourization of the azo dye Reactive red 22 by Pseudomonas luteola were separated and identified by HPLC-DAD and HPLC-MS. The chemical structures of Reactive red 22 (3-amino-4-methoxyphcnyl-/fhydroxyl-sulphonc sulphonic acid ester) and its decomposition products are shown in Fig. 3.92. RP-HPLC measurements were carried out in an ODS column using an isocratic elution of 50 per cent methanol, 0.4 per cent Na2HP04 and 49.6 per cent water. The flow rate was 0.5 ml/min, and intermediates were detected at 254 nm. The analytes of interest were collected and submitted to MS. RP-HPLC profiles of metabolites after various incubation periods are shown in Fig. 3.93. It was concluded from the chromatographic data that the decomposition process involves the breakdown of the azo bond resulting in two aromatic amines [154],... 

Unfortunately, most of the structural information of IR spectra is contained in the often very crowded region of 500-1600 cm which was hardly exploited for diagnostic purposes except in the case of very small molecules with few vibrations, or for pattern matching of spectra of reactive intermediates obtained independently from different precursors. The reason for this was that the prediction of IR spectra was only possible on the basis of empirical valence force fields, and the unusual bonding situations that prevail in many reactive intermediates made it difficult to model the force fields of such species on the basis of force constants obtained from stable molecules. 

Amidst all the enthusiasm about this versatile new tool that quantum chemistry has put at the hands of practioners of IR spectroscopy in matrices, one should not forget its limitations. First, a valid prediction can only come from a calculation based on a correct structure. In the case of reactive intermediates, this is not always as evident as one might wish. A famous example is given in Chapter 16 in this volume Much of the recent discussion on the correct assignment of the IR spectrum of m-benzyne was caused by the fact that different theoretical methods predict different structures, with more or less bonding between the radical centers, for this species. The DFT methods appear to overestimate this bonding, and hence are unsuitable for the prediction of the IR spectrum of m-benzyne. 

In 1975, we discovered that photolysis of aryldisilanes produces a novel type of the silicon-carbon double-bonded intermediate (83). The structure of this intermediate is quite different from that of the diphenylsilaeth-ene reported by Sommer et al. A transient formation of the reactive intermediate can be confirmed by trapping experiments. Thus, the photolysis of p-tolylpentamethyldisilane with a low-pressure mercury lamp in the presence of methanol- affords 1,4- and 1,6-adducts, 1-methoxydimeth-ylsilyl-4-methyl-5-deutero-6-trimethylsilyl-1,3-cyclohexadiene, and 1-methoxydimethylsilyl-3-deutero-4-methyl-6-trimethylsilyl-l,4-cyclohexa-diene in 27 and 28% yield (Scheme 11). In this photolysis, monodeutero methoxydimethylsilyltoluene to be expected from the reaction of the sila-ethene intermediate, p-CHsC6H4Si(Me)=CH2, with methanol- produces in only 2% yield (84). 

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