H NMR Studies

H-NMR studies were performed on a Bruker MSL-400 spectrometer operating in the Fourier transform mode, using a static multinuclei probehead operating at 400.13 MEtz. A pulse length of 1 iis is used for the 90° flip angle and the repetition time used (1 second) is longer than five times Tjz ( H) of the analyzed samples. 

As shown by an H NMR study (77BSB967), A-methyl derivatives of 5-mercaptoimidazole exist in the thione form 176a in chloroform. According to results obtained in (82KGS957), the zwitterionic form 176b should not be excluded from consideration for the compounds 176 (R" = CONH2, CSNH2). 

Pyrido[2,3-h]pyrazin-3- and -2-ones exhibit temperature-dependent equilibria in DMSO-dg solution ( H NMR studies), with gradual increase in the proportion of the imino forms 173b and 174b as the temperature increases, although the enamino forms 173a and 174a remain predominant (97JHC773). 

Whereas the pyrrolo[l,2-a]azepinium cation, a 107t-aromatic system, is unknown, 5-(cyano-methyl)pyrrolo[1,2-a]azepinium perchlorate (7) is available by protonation of the cyanomethy-lene derivative 6 in perchloric acid.7 H NMR studies reveal that the cation has a completely delocalized -electron system. 

One of the most dramatic examples of a solvent effect on propagation taken from the early literature is for vinyl acetate polymerization.78,79 Kamachi el al.n reported a ca. 80-fold reduction in kp (30aC) on shifting from ethyl acetate to benzonilrile solvent (Table 8.1). Effects on polymer structure were also reported. Hatada ef a m conducted a H NMR study on the structure of the PVAc formed in various solvents. They found that PVAc (M n 20000) produced in ethyl acetate solvent has 0.7 branches/chain while that formed in aromatic solvents is essentially unbranched. 

Reliable information on the thermodynamic stability of group 13/15 adducts is usually obtained by gas phase measurements. However, due to the lability of stibine and bismuthine adducts in the gas phase toward dissociation, temperature-dependent H-NMR studies are also useful for the determination of their dissociation enthalpies in solution [41b], We focussed on analogously substituted adducts t-BusAl—E(f-Pr)3 (E = P 9, As 10, Sb 11, Bi 12) since they have been fully characterized by single crystal X-ray diffraction, allowing comparisons of their thermodynamic stability in solution with structural trends as found in their solid state structures. 

X-ray crystallography and variable temperature H NMR studies show that the conformation of the coordinated imidazolidin-2-ylidene, in both the neutral and cationic complexes 70, is anti, anti with respect to the Ph of the backbone of the NHC, exclusively in the solid state and predominantly in solution at lower temperatures (-75°C). At room temperature in solution, possible conformer interconversion by the rotation around the phenyl-N bond of the NHC substituent is apparent from the broadness of the peaks in the NMR spectra. Hydrosilylation of acetophenone by Ph SiH catalysed by 70 at room temperature or at -20°C results in maximum ee of 58%. However, at lower temperatures the reaction rates are much slower [55]. 

Claxson, A.W.D., Hawkes, G.E., Richardson, D.P., Naughton, D.P., Haywood, RM., Chander, C.L., Atherton, M., Lynch, E.J. and Grootveld, M.C. (1994). Generation of lipid peroxidation products in culinary oils and frits during episodes of thermal stressing a high field H NMR study FEBS Lett. 355, 81-90. 

Variable-temperature H NMR studies of MesP Ga(Trip)2 278 and t-Bu2-GaP(Mes )SiPh379,80 show barriers to rotation around the Ga—P bond of... 

The results of H-NMR studies on a series of solutions of TBSM/MA mixture are summarized in Table 1. 

The complex [imH][IrCl4(im)2], imH = imidazole, is prepared by the reaction of either H2IrClg or IrCl3 with imidazole and HC1.118 X-ray diffraction confirmed the trans geometry around the Ir center and showed that the im ligands are nearly perpendicular to one another (84°). The complex is stable in aqueous solution pH = 7.4, as confirmed by H NMR studies. 

H NMR studies of the zinc complex formed with the hexadentate aniline functionalized triazacyclononane ligand (70) showed C3 symmetry in solution. It is suggested a six-coordinate complex is formed with all A-donors bound.664 This compares with the benzyl amino derivative,... 

If, on the other hand, unsymmetrically substituted carbonyl compounds such as monosubstituted benzophenones (X = OCH3, CH3, Cl), tert-butyl methyl ketone, acetophenone, acetaldehyde, or benzaldehyde are used for trapping 39a, diastere-omeric mixtures are formed in each case they could all be resolved except for the products obtained with p-methoxybenzophenone and acetophenone 33>. An X-ray structure analysis has been performed for the E-isomer 57g 36) which, in conjunction with H-NMR studies, permitted structural assignment in cases 56 and 57e, g and h35>. Additional chemical evidence for the structure of the six-membered heterocycles is provided by the thermolysis of 56 a considered in another context (see Sect. 3.1). In general the reaction 39a- 56 or 57 is accompanied by formation of phosphene dimers, presumably via [4 + 4]- and via [4 + 2]-cycloaddition 35). 

Titanatranes such as (319) and (320) polymerize 300 equivalents of L-LA or rac-LA to 70-100% conversion within 24h at 130 °C.922 H NMR studies show that the polymer bears the bulky di-i.vo-propylphenolate end-group, indicating that initiation occurs in a controlled manner. However, molecular weight distributions are relatively broad (1.4-2.0), due in part to transesterification. 

The formation of cationic nickel hydride complexes by the oxidative addition of Brdnsted acids (HY) to zero-valent nickel phosphine or phosphite complexes (method C,) has already been discussed in Section II. Interesting in this connection is a recent H NMR study of the reaction of bis[tri(o-tolyl)phosphite]nickelethylene and trifluoroacetic acid which leads to the formation of a square-planar bis[tri(o-tolyl)phosphite] hydridonickel trifluoroacetate (30) (see below) having a cis arrangement of the phosphite ligands (82). 

Brookhart s group has reported a related rhodium-catalyzed olefin insertion. To gain insight into the mechanism of this process, labeling studies were carried out under conditions where no coupling product was observed ( H NMR study at 80 °C). Deuterium loss occurred in both meta- and para-sites of the aromatic group, and deuterium incorporation was observed in the olefin (Equation (95)).89... 

A rare and biogenetically interesting example of the ethyl chain functionalization of the iboga skeleton is represented by 18-hydroxycoronaridine (al-bifloranine) (128, C21H26N203, MP 192-194°C, [a]D -210°) isolated from T. albiflora (28). The base peak in its mass spectrum appeared at m/z 323 (M+--31), and this suggested that a hydroxy group was present at C-18. A detailed H-NMR study of 128 in comparison with 97 and heyneanine (122) was reported and shown in Table IV. 

Choi, S.-G. and Kerr, W.L. 2003a. H NMR studies of molecular mobility in wheat starch. Food Res. Int. 36, 341-348. 

The use of both 8(CH3) values and iftM- H) magnitudes does, however, allow determination of the coordination mode in many instances (e.g., 462). Problems due to ligand dissociation are frequently evident in H-NMR studies of sulfoxide complexes. Thus the dimethyl sulfoxide adduct of niobium oxychloride (409) undergoes solvolysis in acetoni-... 

Complexes of the ligands (CH2)4SO (129), (CH2)5SO (243), Ph2SO (241), and (PhCH2)2SO (497) have been synthesized, and spectroscopic data indicate O-bonding in all cases. H-NMR studies of Ph2SO complexes, including those of nickeldl), have appeared (498). 

The anion [H3Ru4(CO)12] exists in two isomeric forms. The initial evidence for this was from infrared and H-NMR studies (182). Recently, the X-ray structure of two modifications has been determined (184). The two isomeric structures are shown in Fig. 37a and b, and this has established, for the first time, that the polynuclear hydridocar-bonyls may exist in more than one isomeric form. 

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