L3C-NMR studies

The extent of short-chain branching in PE may be quantitatively determined by a variety of techniques including IR. 1 pyrolysis Ct., 7" and y-radiolysis."84 The most definitive information comes from l3C NMR studies.2S5"2 0 The typical... 

The H- and l3C-NMR study [86AG(E)842] shows that a rapid valence isomerization (101) <= (101a) takes place. For azete, the activation barrier of the automerization via a C2v transition state is 12.1 (MINDO/3) (80MI2),... 

The formation of adduct 40 appears to be favored with respect to attack at the halogen-bearing carbon atom. By a further detailed l3C-NMR study, the adducts of this type, as derived from 4-chIoro-2-dimethylaminopyrimidine and its 5-phenyl derivative, are reported to be the primary intermediates of a process leading to the final triazines via a number of identified ring-opening intermediates.101... 

Adduct 110 has been obtained by decomposition of 1-ethoxycarbonyl-1,4-dihydropyridine with potassium terf-butoxide. It is the ti-adduct that would be expected to form by attack of the hydride ion on the 4-position of pyridine. H- and l3C-NMR studies suggest that 110 is planar and devoid of any homoaromatic character.155 The 13C chemical shifts [6 (in DMSO) 127.9... 

Reports have recently been published of protonation at C-7 (a prerequisite for Mechanism 1) under strongly acidic conditions [16,17]. While investigating the protonation site of Rauwolfia alkaloids, Balon and co-workers [16] showed through l3C NMR studies that protonation takes place at C-7 in 18M H2S04. Experiments by Royer et al. [17] indicated that reserpine (1) and isoreserpine (2) are transformed into the corresponding 2,7-dihydro compounds (14) (55%) and (15) (87%) by NaBH3CN in TFA at room temperature, Scheme (6). This is only possible through protonation at C-7. Further evidence was provided by deuterium incorporation at C-7 when TFA-d was used. 

Several UC NMR studies on the hormone thyroliberin (TRH, Pyr-His-Pro-NH2) have appeared in the literature [822, 832-838]. The total signal assignment of TRH was achieved by comparison with the spectra of the constituent amino acids and model peptides such as Pyr-His-OCH3 or Pyr-His(NCH3)2. According to the l3C NMR studies in aqueous solution the hormons exists in a cisjtrans ratio of approximately 14 86 however, this ratio is solvent-dependent and, e.g.7 in pyridine-d5 no cis-isomer is detectable. 

Comparison of Cl and C6 l3C chemical shifts showed that the ff-participation from the 2-norbomyl ring is significantly reduced in the 2-methyl analogue, whereas in the cyclopropyl and phenyl analogues it has essentially vanished. The STO-3G calculated structures show that the spirocyclopropyl participation is mainly from the exo-C—C bond. The l3C NMR studies of these cations adequately accounted for the vanishingly low values of solvolytic keJkenio rate constants, and show that 3-spirocyclopropyl groups effectively compete with the Cl—C6 ff-bond participation in the 2-norbomyl cation framework. 

The structures of compounds (36), (37), and (38) and other products from nucleophilic additions to pyrido[3,4-e]-l,2,4-triazine (39) were deduced by H and l3C NMR studies, in some cases by comparison to related heterocyclic systems <92CJC2828>. 

The complexes trans-[RuCl(S2CNR2)2NO] (R2 = Me, Et, MePh, MeEt) are prepared by reaction of [RuCl3NO] with two equivalents of Na[S2CNR2]. A series of c s [RuX(S2CNR2)2NO] (X = Cl, Br, I, N02, SCN, N3, NCO, F) can be synthesized by treatment of the trans chloro compound with either HX or AgX. Heating some of these cis isomers to 160-210°C results in quantitative conversion to the trans isomer. Several cationic solvates frows-[Ru(S2CNR2)2NO(S)]+ (S = H20, MeOH) were also isolated and H and l3C NMR studies on all these compounds indicate stereochemical rigidity at ambient temperature.1308... 

In solution the endo-3 and exo-3 substituted compounds 233 and 234 adopt CB and CC conformations, respectively, according to 13C-NMR data (177). A variable temperature l3C-NMR study of 9-methyl-9-azabicyclo-[3.3.1]nonane has shown that the nitrogen inversion barrier is AG1 = 8.1 kcal mol"1 at — 90°C(178). For some other 9-heterobicyclo[3.3.1]nonanes studied using 13C-N M R data, the conformational behavior was shown to be governed by steric interactions (177-180). 

R.V. Law, D.C. Sherrington and C.E. Snape, Quantitative solid state l3C NMR studies of highly cross-linked poly(divinylbenzene) resins, Macromolecules, 1997, 30, 2868-2875. 

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