NMR spectroscop

Bausch J W, Prakash G K S, Olah G A, Tse D S, Lorents D C, Bae Y K and Malhotra R 1991 Considered novel aromatic systems. 11. Diamagnetic polyanions of the Cgg and C g fullerenes. preparation, 13-C and 7-Li NMR spectroscopic observation, and alkylation with methyliodide to polymethylated fullerenes J. Am. 

It seems now established by NMR spectroscopic investigations that a change can take place in electronic structures and atomic configuration of the dyes depending on the polarity of the solvent. Parameters describing the transition from one single bond to more double bond character vary according to the nature of the solvent (107). 

The computational results (93JA2465) are consistent with the experimental findings of NMR spectroscopic studies (82JOC5132 97MRC35), which showed the presence of only the H tautomer of tetrazole in DMSO-dfi e = 49) solution. The content of H tautomer 27a in dioxane e = 22) at 30°C was estimated as 78% (82JST283) and 85% (75BSF1675) from its dipole moment 4.88 D and those of 1,5- and 2,5-dimethyltetrazoles as models for the H and 2H tautomers respectively. 

An amino group may take any of three possible positions in the five-membered isoxazole ring, giving rise to three tautomeric forms for 70 and 71 and four forms for 72 [76AHC(S1), pp. 416, 444, 445 84CHEC-I(5)1]. However, only amino structures 70a-72a have been detected using IR- or NMR-spectroscopic techniques (Scheme 33). 

Based on IR and NMR-spectroscopic studies [76AHC(S1), pp. 406, 414 94MI1027 96CHEC-II(4)267], 2-mercapto-l,3,4-oxadiazole prefers the thione form 195. 

Evidence in favor of the amino-thionic tautomeric form of 2-amino-l,3,4-thiazoline-5-thione 239 (A = X = S R = H) was obtained from X-ray structural determinations [72AX(B)1584]. A NMR-spectroscopic study (77JOC3725) of compounds 239 (A = Se X = S R = Me) demonstrated their amino-thionic structure. A similar tautomeric form 240 is also dominant for the hydrazine derivative (R = NHNHCOPh) [73JCS(P2)4]. 

Table IV. NMR Spectroscopic Comparison Between Coralyne-13-Olate 109, 13-Methoxy Derivative 112, and the Alkaloid 117...

Fig. 6-4. Minimum energy conformations of L-PA and L-phenylalanine-A -methyl-anilide (L-PMA) based on molecular mechanics calculations and UV- and NMR-spectroscopic characterizations. (From Lepisto and Sellergren [25].)...

NMR SPECTROSCOPIC INVESTIGATIONS OF SURFACE AND INTERLAYER SPECIES ON MINERALS, CLAYS AND OTHER OXIDES... 

The structural environments of cations absorbed onto clays Cs variable-temperature MAS NMR spectroscopic... 

Likewise, temperature dependent H NMR spectroscopic studies on yV-phenyl-3//-azepin-2-amine indicate that one of the C3 protons is highly shielded, the other strongly deshielded.82... 

Similarly, photolysis of l-(2-azidophenyl)-l/f-pyrazole in acetonitrile in the presence of dipropylamine affords AfN-dipropyl-7-(lF/-pyrazol-l-yl)-3//-azepin-2-amine in low yield (4%).192 Surprisingly, however, photolysis of the corresponding 1-(2-azidophenyl)-3,5-dimethyl-l//-pyrazole (84) in cyclohexane in the presence of the base yields 85 which, on the basis of H NMR spectroscopic evidence, has been formulated as a rare example of a stable 2 H-azepine. 

The anti diastereoselectivity is improved to a 6 1 ratio by the addition of triethylborane to the reaction mixture83. NMR-spectroscopic investigations indicate that a boronate complex is the decisive intermediate84, since it can also be prepared by the addition of alkyllithium to the dialkyl(2-butenyl)borane (path ). 

The selectivity for (/ ,/ )( ,S)-10 has been rationalized by invoking a synperiplanar enolate species whose conformation is enforced by a donor(enolate oxygen)- acceptor) peril uo-rophenyl) interaction depicted in structure N47. Infrared and variable temperature NMR spectroscopic studies of the neutral precursor complex 8 support the existence of such a donor-acceptor interaction. 

PMe3)2H] + (12) with NaH (Eq. 5) [37]. The conformation for these complexes as deduced from 31P NMR spectroscopic data are depicted above. Interestingly the cationic derivative [Co(Bu2Cp)(PMe3)2Me]+ (13) possesses a molecular plane of symmetry. 

Variable-temperature H and 13C NMR spectroscopic study of this seemingly crowded molecule 38 revealed that the ring rotation about the iron ring axis occurs freely down to — 95 °C, although at lower temperatures the rotation of the... 

A variable-temperature NMR spectroscopic study of the titanium(IV) complex 43 also indicated free rotation of the five-membered rings, but, as in the ferrocene derivative 38 allowed the determination of the activation barrier for the phenyl ring rotation (AG (-90 °C) = 9.8 0.5 kcal mol1). 

After the completion of this manuscript a paper concerning conformational analyses of 1,1, 3,3 -tetra-r< H-alkylmctallocene of iron and ruthenium including 6 based on thorough NMR spectroscopic measurements (line-shape analysis) has appeared in which the nature of the transition states has conclusively been discussed in detail [164]. 

More recently, Grubbs et al. obtained a refined mechanistic picture of the initiating step by conducting a 31P NMR spectroscopic study of the phosphine exchange in precatalysts 12-A. These investigations revealed that substitution of the phosphine proceeds via a dissociative-associative mechanism, i.e., a 14-electron species 12-B is involved that coordinates the alkene to give a 16-electron species 12-C (Scheme 12) [26a]. Increased initiation rates are observed if the substituents R and the phosphine ligands PR3 in precatalysts... 

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