Units, NMR

In principle, therefore, the microstructure of such a polymer could be described by a sequence of m and r diads. However, the limits of NMR resolution is such that the microstructure can only be determined over a limited section of the polymer, usually no longer than 5-7 monomer units. NMR spectra on comparatively insoluble polymers such as polyethylene and polypropylene are usually recorded in 1,2,4-trichlorobenzene or tetrachloroethane-d2 at elevated temperatures (110-150 °C). 

Cluster Ground state metal core structure 1 Number of structurally inequivalent CuL (L = two-electron donor ligand) units NMR evidence for dynamic behavior at ambient temperature Low temperature NMR spectrum consistent with ground state structure observed Ref. 

Elemental analysis shows the decrease of N% corresponding to a partial cleavage of the biguanide units. NMR experiments performed with soluble biguanide in dr methanol proved the methanolysis of the molecule at the C=N bond.24... 

When interest was rekindled in the 1980s, the method commonly used to synthesise PAni was the oxidative coupling of aniline with ammonium persulphate in aqueous HC1. This produces partially protonated EM salt which can be deprotonated with ammonia to form EM base. The molecular mass can then be determined in dilute LiCl solution in N-methylpyrrolidone (NMP) typical values correspond to 160 of the repeat units shown in Fig. 9.6. The reaction shows the characteristics of a living polymerisation, since the molecular mass increases if further monomer and oxidant are added to the reaction mixture, when the chain length increases to a maximum of about 240 repeat units. NMR studies of the LM form show about 5% defects in the polymer backbones. When the reaction is performed at 248 K the polymer is produced in nearly 100% yield, with higher molecular mass, typically over 500 repeat units, and greatly reduced defect content (Adams et ai, 1996). Other oxidising... 

Identification of the abnormal groups in a polymer may not be sufficient it is often necessary also to determine the numbers of such groups for, say, 1000 normal units. NMR techniques can give the required information but precautions are needed if reliable results are to be obtained. The problems can be illustrated by considering the use of NMR to compare the number... 

Solvation numbers of actinide cations in non-aqueous media are almost an unexamined field. FT-IR investigations of the homologous solvates [Ln(N03)3(DMS0) ] in anhydrous acetonitrile (Biinzli etal. 1990) indicated a change in coordination number in the middle of the series [ca. Eu(III)] from nine to eight with increasing atomic number. In the presence of large excess of DMSO([DMSO]j/[Ln]( = 6), the coordination numbers can be increased by one unit. NMR spectroscopy and crystal stoichiometries have indicated a solvation number of two for the uranyl cation in pure TBP (TBP is tributylphosphate). The overall coordination number in this case should include two for TBPs coordination and four for the bidentate nitrate coordination (Siddall and Stewart 1967). 

An impressive result was obtained by Warmuth et al., reporting on the thermodynamically driven condensation of 18 molecules into nanocontainer 13 with an inner cavity of around 1700 (Figure 7). Also here, tetraformyl-cavitand 11 was used as the concave molecule, but different from the Stoddart-Cram system, ethylenediamine was used as the bridging unit. NMR spectroscopy and gas-phase chromatography (GPC) revealed the slow... 

Addition of equimolar [Ag(NCMe)4]to an acetone solution of the cyclometalated complex PtMe K (A,C)-bhq dppy-K-(P) (bhq = benzo h quinoline, dppy = diphenylphosphino-2-pyridine) affords the butterfly cluster 66, where the silver atoms occupy hinge positions bridged by a coordinated acetone unit. NMR studies indicate an equilibrium between butterfly and planar isomers of 66 accessible throught the facile dissociation of the acetone ligand." ... 

A 2-methylthio substituent decreases the basicity of thiazole pK = 2.52) by 0.6 pK unit (269). The usual bathochromic shift associated with this substituent in other heterocycles is also found for the thiazole ring (41 nm) (56). The ring protons of thiazole are shielded by this substituent the NMR spectrum of 2-methylthiothiazole is (internal TMS, solvent acetone) 3.32 (S-Me) 7.3 (C -H) 6.95 (Cj-H) (56, 270). Typical NMR spectra of 2-thioalkylthiazoles are given in Ref. 266. 

Analyzing an NMR spectrum m terms of a unique molecular structure begins with the mfor matron contained m Table 13 1 By knowing the chemical shifts characteristic of various proton environments the presence of a particular structural unit m an unknown compound may be inferred An NMR spectrum also provides other useful information including... 

The NMR spectrum of isopropyl chloride (Figure 13 17) illustrates the appearance of an isopropyl group The signal for the six equivalent methyl protons at 8 1 5 is split into a doublet by the proton of the H—C—Cl unit In turn the H—C—Cl proton signal at 8 4 2 IS split into a septet by the six methyl protons A doublet-septet pattern is char acteristic of an isopropyl group... 

NMR The most helpful signals m the H NMR spectrum of alcohols result from the O—H proton and the proton m the H—C—O unit of primary and secondary alcohols... 

H NMR The chemical shift of the proton m the H—C—O—C unit of an ether is very similar to that of the proton m the H—C—OH unit of an alcohol A range of 8 3 2-4 0 IS typical The proton m the H—C—S—C unit of a sulfide appears at higher field than the corresponding proton of an ether because sulfur is less electronegative than oxygen... 

Section 16 18 An H—C—O—C structural unit m an ether resembles an H—C—O—H unit of an alcohol with respect to the C—O stretching frequency m its infrared spectrum and the H—C chemical shift m its H NMR spectrum Because sulfur is less electronegative than oxygen the H and chemical shifts of H—C—S—C units appear at higher field than those of H—C—O—C... 

Most of the experimental information concerning copolymer microstructure has been obtained by physical methods based on modern instrumental methods. Techniques such as ultraviolet (UV), visible, and infrared (IR) spectroscopy, NMR spectroscopy, and mass spectroscopy have all been used to good advantage in this type of research. Advances in instrumentation and computer interfacing combine to make these physical methods particularly suitable to answer the question we pose With what frequency do particular sequences of repeat units occur in a copolymer. 

The use of NMR spectroscopy to characterize copolymer microstructure takes advantage of this last ability to discern environmental effects which extend over the length of several repeat units. This capability is extremely valuable in analyzing the stereoregularity of a polymer, and we shall have more to say about it in that context in Sec. 7.11. 

Poly(vinyl fluoride) [24981-14-4] (PVF) is a semicrystaltiae polymer with a planar, zig-zag configuration (50). The degree of crystallinity can vary significantly from 20—60% (51) and is thought to be primarily a function of defect stmctures. Wide-line nmr and x-ray diffraction studies show the unit cell to contain two monomer units and have the dimensions of a = 0.857 nm, b = 0.495 nm, and c = 0.252 nm (52). Similarity to the phase I crystal form of poly (vinytidene fluoride) suggests an orthorhombic crystal (53). 

Glass-forming systems other than siUca have been examined. The fraction of three- and four-coordinated boron in borate glasses can be determined by nmr (29). Both nmr and x-ray diffraction (30) results led to the suggestion that the boroxyl ring is the stmctural unit of vitreous The... 

Iron Porphyrins. Porphyrias (15—17) are aromatic cycHc compouads that coasist of four pyrrole units linked at the a-positions by methine carbons. The extended TT-systems of these compounds give rise to intense absorption bands in the uv/vis region of the spectmm. The most intense absorption, which is called the Soret band, falls neat 400 nm and has 10. The TT-system is also responsible for the notable ring current effect observed in H-nmr spectra, the preference for planar conformations, the prevalence of electrophilic substitution reactions, and the redox chemistry of these compounds. Porphyrins obtained from natural sources have a variety of peripheral substituents and substitution patterns. Two important types of synthetic porphyrins are the meso-tetraaryl porphyrins, such as 5,10,15,20-tetraphenylporphine [917-23-7] (H2(TPP)) (7) and P-octaalkylporphyrins, such as 2,3,7,8,12,13,17,18-octaethylporphine [2683-82-1] (H2(OEP)) (8). Both types can be prepared by condensation of pyrroles and aldehydes (qv). 

---