ABC spectrum

The ABC spectrum, in which all three nuclei are closely coupled, contains up to 5 lines (four A-type, four B-type, four C-type, and three combination lines), but analysis by inspection is very difficult. Accordingly, recourse must be made to a general computer method, such as LAOCN3 or DAVINS, or to Castellano and Waugh s EXAN II program, which was designed specifically for ABC spectra and is successful in more than 90% of cases. 

At 60 MHz, the spectrum of the vinylic system of styrene becomes a borderline ABX system, but with some imagination the structure can be resolved by recognizing that the ABX spectrum can be regarded as though it were a first-order AMX spectrum—which it is at 300 MHz. Again, it must be emphasized that the order of a spectrum depends partly on the sophistication of available instrumentation. At the extreme, an ABC spectrum—a very low Ai> J ratio for all of the sets— may not be resolvable with available instrumentation. 

Vinyl Derivatives Table V). In most vinyl metallic compoimds the difference in chemical shifts of the three protons is similar in magnitude to the spin-spin coupling between these nuclei, and thus a second-order (ABC) spectrum is observed (Figs. 4 and 5). Analysis of these spectra is usually straightforward, if lengthy, and the 8 and J values indicated in Table V are readily calculated. However, in some cases two somewhat different solutions may appear to fit the data equally well. Such a discrepancy appears in this Table in the values given for (CH2 CH)4Si and for (CH2 CH)4Sn as... 

F ure 6.5 (a) Schematic representation of a 3D spectrum of a linear spin syv tern ABC with identical mixing processes Mi and M2. In a linear spin system, the transfer of magnetization between A and C is forbidden for both Mi and M2, (b) Schematic representation of a 3D spectrum of a linear spin system ABC, where transfer via Mi is possible only between A and B and transfer via M2 occurs only between B and C. (Reprinted from J. Mag. Reson. 84, C. Griesinger, et al., 14, copyright (1989), with permission from Academic Press, Inc.)... 

There is still another situation that leads to second order spectra and this one usually cannot be anticipated. For example, take a look at the proton spectrum of 3,3,3-trifluoropropene in Fig. 2.9. This spectrum is not the simple one that one would expect for a monosubstituted ethylene. However, the second order nature of this spectrum can be understood after examining the fluorine-decoupled spectrum, which is given in Fig. 2.10. The decoupled spectrum displays the expected multiplets from the ABC system, each proton appearing as a doublet of doublets. The second order spectrum seen in Fig. 2.9 derives from the fact that the protons at 5.98 and 5.93 are seen from the 19F frequency as... 

A predominant toxin (51) from water beetles of the genus llybius (Table V) shows a UV absorption corresponding to hydroxyquinoline or hydroxyiso-quinoline. The H-NMR spectrum exhibits, beside signals of methyl ester and phenol, signals of five aromatic protons as both ABC and AB systems, the latter indicating two protons at C-3 and C-4 in quinoline. Since electron pyrolysis of 51 gives radioactive 8-hydroxyquinoline, its structure is identified as methyl 8-hydroxyquinoline-2-carboxylate (51) and confirmed by synthesis from xanthurenic acid (52) (Scheme 48) (101). The precursor of this alkaloid was shown to be tryptophan (444). 

Nomenclature for coupled systems in 1H NMR. The interpretation of a spectrum of a molecule with many hydrogen atoms is simplified when signals that fall into classical cases can be observed. These particular cases are usually classified with a nomenclature that uses the letters of the alphabet, chosen in relation to the chemical shift (Fig. 9.18). Protons with identical or similar chemical shifts are designated by identical letters or neighbouring letters in the alphabet (AB, ABC, A2B2, etc.), while protons with very different chemical shifts are designated by letters such as A, M and X. 

Experimentally, one is interested first of all in the order of the reaction, its absolute rate, and the temperature dependence of that rate. In addition to these primary data of chemical kinetics, one can observe the emission spectrum of ABC under various experimental conditions. From these observations one tries to infer information about the vibrational and electronic states involved, and their interactions, with or without collisions. Theoretically, interest centers on potential surfaces. Unfortunately, these are all too often thought of as potential curves, so that two of the three internal coordinates of the molecule are ignored. The experimental and theoretical difficulties are such that, even in terms of such an oversimplified model, it is seldom possible to arrive at a unique, widely agreed upon picture of the reaction process. 

First, we describe briefly the calculation of the absorption spectrum for bound-bound transitions. In order to keep the presentation as clear as possible we consider the simplest polyatomic molecule, a linear triatom ABC as illustrated in Figure 2.1. The motion of the three atoms is confined to a straight line overall rotation and bending vibration are not taken into account. This simple model serves to define the Jacobi coordinates, which we will later use to describe dissociation processes, and to elucidate the differences between bound-bound and bound-free transitions. We consider an electronic transition from the electronic ground state (k = 0) to an excited electronic state (k = 1) whose potential is also binding (see the lower part of Figure 2.2 the case of a repulsive upper state follows in Section 2.5). The superscripts nu and el will be omitted in what follows. Furthermore, the labels k used to distinguish the electronic states are retained only if necessary. 

If two (or more) degrees of freedom are involved, it is important which mode is excited whether the spectrum shows reflection structures or not. Let us consider the linear triatomic molecule, ABC — A+BC, with Jacobi coordinates R and r as illustrated in Figure 2.1. Figure 13.2 depicts an elastic PES of the form (6.35) with coupling parameter e = 0. 

Separation of the reaction mixture by distillation is difficult as these compounds have only slightly different boiling points. Though, the starting materials could be distilled off, concentration not was possible for the respective vinyl-silanes, so mixtures were used for further reactions. Identification of the vinyl-silanes was effected via the nmr spectrum of the reaction mixture from the characteristic ABC multiplets due to the vinyl groups39. ... 

Other programs include EXAN II, a program238 that obtains nonunique sets of chemical shifts and coupling constants by explicit analysis of the experimental line-positions of ABC spectra CHEM 3, a program239 that computes a theoretical spectrum for a three-spin system and NMDRS, a program240 for computation of double-resonance spectra. 

In the 1 H-NMR spectrum (270 MHz) the methine and the two methylene protons of methylsuccinic add gave rise to a well-resolved ABC system. In the two enantiomers 38 and 39 the H atoms HR<,R/HSlS and H5(R/HWe.v are pair-wise reflection equivalent, i.e., identical in the NMR spectrum, whereas the diastereotopic geminal protons can be distinguished by their different chemical shifts. For the assignment of the signals to the diastereotopic protons, reference compounds of known configuration were synthesized by treating mesaconic and citraconic acids (40 and 42) with deuterated diimide. The known syn-addition of deuterium [35,36] afforded the racemic but stereospecifically dideuterated methylsuccinic acids (41 and 43) (Fig. 25). 

A second example is illustrated by the V-Tricine system (25). Its EXSY spectrum, revealing intramolecular exchange, is shown in Figure 10. The V-Tricine complex differs from the V-TEA complex in that the two-pendent hydroxymethyl arms are inequivalent (an ABC system) (46). The rate constants determined for the coordinated arm exchanging with each pendent arm [fc(C4b - C5f) and fc(C4b - C6f)] are very similar (25). Assuming the chemical event is the formation of a single species from which the two pendent arms and the chelated arm is derived, each rate represents only one-third of the exchange rate as shown in equation 6 ... 

Figure 9.10. Predicted and observed 60-MHz H ABC spectra of compound 9-3. (a) Predicted first order spectrum (b)-(d) multiplet slanting due to second-order effects (e) observed spectrum [From Prediction of the Appearance of Non-First-Order Proton NMR Spectra, by R. S. Macomber, Journal of Chemical Education, 60, 525 (1983). Reprinted by permission.

The spectrum of DSs exhibits two multiplets centered on 14.67 and 22.10 ppm respectively. Such a pattern differs from the ABC-type ones in MS and TS by the fact that two of the three expected quadruplets overlap here. This assumption is supported by the record of the DSs spectrum at 101.27 MHz three quadruplets are then observed which are centered on 13, 15 and 22 ppm respectively (spectrum not reported). 

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