Model Compound Comparisons

Tsutsumi Y, Islam A, Anderson CD, Sarkanen KV (1990) Acidic permanganate oxidations of lignin and model compounds comparison with ozonolysis Holzforschung 44 59-66... 

The initial demonstration of the SIMBA experiment utilized the simple alkaloid norharmane (5) as a model compound. Comparison spectra showing... 

As mentioned, we also carried out IR studies (a fast vibrational spectroscopy) early in our work on carbocations. In our studies of the norbornyl cation we obtained Raman spectra as well, although at the time it was not possible to theoretically calculate the spectra. Comparison with model compounds (the 2-norbornyl system and nortri-cyclane, respectively) indicated the symmetrical, bridged nature of the ion. In recent years, Sunko and Schleyer were able, using the since-developed Fourier transform-infrared (FT-IR) method, to obtain the spectrum of the norbornyl cation and to compare it with the theoretically calculated one. Again, it was rewarding that their data were in excellent accord with our earlier work. 

One of the most sensitive tests of the dependence of chemical reactivity on the size of the reacting molecules is the comparison of the rates of reaction for compounds which are members of a homologous series with different chain lengths. Studies by Flory and others on the rates of esterification and saponification of esters were the first investigations conducted to clarify the dependence of reactivity on molecular size. The rate constants for these reactions are observed to converge quite rapidly to a constant value which is independent of molecular size, after an initial dependence on molecular size for small molecules. The effect is reminiscent of the discussion on the uniqueness of end groups in connection with Example 1.1. In the esterification of carboxylic acids, for example, the rate constants are different for acetic, propionic, and butyric acids, but constant for carboxyUc acids with 4-18 carbon atoms. This observation on nonpolymeric compounds has been generalized to apply to polymerization reactions as well. The latter are subject to several complications which are not involved in the study of simple model compounds, but when these complications are properly considered, the independence of reactivity on molecular size has been repeatedly verified. 

Polyanilines. Initial preparations of polyaniline (PANI) led to insoluble materials that were difficult to characterize. Use of model compounds and polymers (124,125) allowed for definitive stmctural analysis. Poly( phenylene amineimine) (PPAI) was synthesized directiy to demonstrate that PANI is purely para-linked (126). The synthesis was designed so as to allow linkage through the nitrogen atoms only (eq. 9). Comparison of the properties of PPAI and PANI showed PPAI to be an excellent model both stmcturaHy and electronically. 

Figure 3 Comparison of the densities (in g/cm ) of model compounds for membrane lipids computed from constant-pressure MD simulations with the coiTespondmg experimental values. The model compounds include solid octane and tricosane, liquid butane, octane, tetradecane, and eico-sane, and the glycerylphosphorylcholme, cyclopentylphosphorylcholme monohydrate, dilauroly-glycerol, anhydrous cholesterol, cholesterol monohydrate, and cholesterol acetate crystals. (Models from Refs. 18, 42, and 43).

Buckles et al. suggested tentative structural assignments for 53a and 53b and their respective benzamido acids on the basis of ultraviolet spectral data and by comparison of physical properties with those of model compounds. They pointed out that it is not possible to establish structural relationships from configurations of the diastereomeric 2-benzamido-3-methoxy-3-phenylpropionic acids (54), each of which, on treatment with acetic anhydride, give mixtures of the azlactones. Similar observations have been made by others. ... 

Katritzky et al. have studied, using pK measurements, the tautomerism of aza[10]annulenone (2-azabicyclo[4.4.1]undeca-4,6,8,10-tetraen-3-one) 68 (93H2483). Comparison of the pX s of 68 and of model compounds 69 and 70 suggests that 68a is the dominant tautomer pKr = -0.70).Tliis implies a significantly diminished preference for the oxy form in compound 68 compared to 2-pyridone pKr = -3.0). This result indicated that aza[10]-annulenones 68a and 70 should be viewed as 677-homoaromatic species. NMR spectra support this conclusion (93H2483). 

A comparison of the absorption and emission spectra of Ooct-OPV5 with those of the fully conjugated, similarly substituted polymer Ooct-PPV shows that the absorption and luminescence maxima of the five-ring model compound are only slightly blue-shifted relative to those of the polymer (see Fig. 16-11). Hence, the... 

Treatment of the precursors with decylamine resulted in a high level of chemiluminescence activity. Taking the activity obtained with decylamine as 100%, the activities obtained with other amines were methylamine, 5% hexylamine, 23% octylamine, 39% and dode-cylamine, 55%. For comparison, the Panellus precursors, PS-A and PS-B, are best activated with methylamine, and a synthetic model compound, K-l, is best activated with hexylamine. 

The formation of compound (1) has been established under well-defined laboratory conditions in such reaction mixtures [15,26-35]. Comparison of nuclear magnetic resonance (NMR) spectra of model compounds prepared by Bakker and Cerfontain [29] with those of the reaction mixture has also clearly shown the presence of (1). p-Sultones (1) have also been identified in commercial scale equipment under less well-defined conditions [21-24]. 

The intermediate product S3 (see above) will also take part in this polymerization. Infrared spectra of the polysulfuroxide show a strong absorption at 1123 cm which was assigned to the structural unit -S-S(=0)-S- by comparison with model compounds like trisulfane-2-oxides (RS)2S=0 [31] and SsO (see above). In chloroform solution S2O reacts at —60 °C to S5O and SO2 (see above). 

This review has shown that the analogy between P=C and C=C bonds can indeed be extended to polymer chemistry. Two of the most common uses for C=C bonds in polymer science have successfully been applied to P=C bonds. In particular, the addition polymerization of phosphaalkenes affords functional poly(methylenephosphine)s the first examples of macromolecules with alternating phosphorus and carbon atoms. The chemical functionality of the phosphine center may lead to applications in areas such as polymer-supported catalysis. In addition, the first n-conjugated phosphorus analogs of poly(p-phenylenevinylene) have been prepared. Comparison of the electronic properties of the polymers with molecular model compounds is consistent with some degree of n-conjugation in the polymer backbone. 

The determination of the absolute configuration of a carotenoid is only possible by circular dichroism (CD) measurement. The spectrum interpretation can only be done by comparison with reference or model compounds with known chiralities. The sample requirement is as low as 5 to 50 pg, but CD facilities are not so commonly available. Buchecker and Noack reported experimental aspects and discussion of the relationships of carotenoid structures and CD spectra. 

For comparison purposes, some model compounds and model polymers were reacted with the gases. Liquid models were used as dilute solutions in hexane or hexadecane model polymers were used as solid films. n-Peroctanoic acid was synthesised from n-octanoic... 

Table XVIII contains the 13C-n.m.r.-spectral data for the model compounds a,/ -Araf — Hyp and a,/ -Ara/— Hyp. The assignments of Qfi and Cs of Hyp were readily obtained19 by comparison with 13C data for L-Hyp in the free amino acid form and for L-Hyp residues of peptides.109 The chemical shift of C 5 was, as expected, 7-8 p.p.m. downfield from the values reported for nonglycosylated L-Hyp. The chemical shifts of C and Cy appeared in the same region as carbohydrate resonances and were therefore identified through their chemical shift-pH dependence.19 The chemical shift of the / -anomeric carbon atom in compound 60 is rather up-field, in agreement with a syn orientation for the C-1 - O-1 and...

C-2-0-2 bonds.7 45 The assignments of the carbohydrate-carbon resonances of Arap model compounds (61, 62) were based on comparisons with the 13C chemical-shift data for methyl a- and / -arabinopyrano-side.89-110... 

Very recently, photoelectron spectra have been recorded for disilenes 1,3, and 25.54 Consistent with the electronic spectra, the lowest ionizations were assigned to the Si=Si 7r-orbital. The He(I) photoelectron spectrum for 1 is shown in Fig. 4. The low-energy ionization appears at 7. 53 eV. The more intense, higher energy bands were shown to be due to ionizations from mesityl 7r-orbitals by comparison with the photoelectron spectra of model compounds without a Si=Si bond. 

Comparison of solution pH with the pKa of a side chain informs about the protonation state. A unique pKa, termed the standard or model pKa, can be experimentally determined for each ionizable side chain in solution when it is incorporated in a model compound, often a blocked amino acid residue [73] (Table 10-1). In a protein environment, however, the pKa value of an ionizable side chain can substantially deviate from the standard value, due to desolvation effects, hydrogen bonding, charge-charge, charge-dipole, and other electrostatic interactions with the... 

The absorption bands measured by the flash spectrographic method are often assigned by (a) comparison with known singlet-singlet absorption spectra, (b) comparison of the lifetime of the species responsible for the absorption with the phosphorescence lifetime, (c) comparison with calculated energies and intensities of the various possible absorptions by semi-empirical molecular orbital methods, and (d) comparison with published triplet absorption spectra and decay kinetics of model compounds. 

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