Acetylene chemical shifts, proton

Acetylenic hydrogens are unusual in that they are more shielded than we would expect for protons bonded to sp hybridized carbon This is because the rr electrons circulate around the triple bond not along it (Figure 13 9a) Therefore the induced magnetic field is parallel to the long axis of the triple bond and shields the acetylenic proton (Figure 13 9b) Acetylenic protons typically have chemical shifts near 8 2 5... 

In a similar approach, the aggregation of phenyl-acetylene macrocycles 65 (Chart 1) in solution have been studied with H NMR spectroscopy.169 170 They have determined the association constant for dimerization, iCssoc, by curve fitting the concentration dependence of the proton chemical shift to a model for monomer—dimer equilibrium. The results obtained from NMR studies, e.g., aggregation constants and aggregate size, have independently been verified by vapor pressure osmometry experiments. Further, it has been well documented in the literature that... 

The global utility of this H-NMR alkyne probe is decreased by the scarcity of terminal alkyne adducts relative to the abundance of internal alkyne adducts. Diphenylacetylene and dimethylacetylenedicarboxylate (DMAC) are two particularly popular alkyne ligands which have no acetylenic proton to monitor. An empirical correlation between alkyne irx donation and, 3C chemical shift for the bound alkyne carbons has been recognized (155) which partially fills this spectroscopic need. A plot of alkyne 13C chemical shifts, which span over 100 ppm (Tables II and III), versus N, the number of electrons donated per alkyne to fulfill the effective atomic number guideline, reveals both the advantages and the limitations... 

Bisalkyne d4 monomers, with N = 3 by symmetry, exhibit proton and carbon chemical shifts at higher fields than those of monoalkynes with N = 4. The proton chemical shift of 10.45 ppm for Mo(PhC=CH)2-(S2CNEt2)2 (52) falls nicely between the four-electron donor Mo(CO)-(PhC=CH)(S2CNEt2)2 case (12.6 ppm) and the two-electron donor (7r-C5H5)2Mo(HC=CH) case [7.68 ppm (Table II)]. Additional data for bisalkyne complexes, including pyrrole-N-carbodithioate derivatives, support a correlation of H chemical shifts with alkyne ttj donation, with three-electron donors typically near 10.0 0.5 ppm. Similar H values are found for cyclopentadienyl bisalkyne complexes with terminal alkyne ligands. Chemical shifts between 8.5 and 10.5 ppm characterize all the neutral and cationic bisalkynes listed in Table V except for [CpMo-(RC=CH)2(MeCN)]+ where one isomer has S near 11 ppm for the acetylenic proton (72). 

The chemical shift of acetylenic protons in conjugated polyynes shows marked solvent effects. For example, the signal for 1,3-pentadiyne appears at 8 T75, 2-80 and 3-50 in CClj, acetone and DMF, respectively . 

Considering the rather high conformational stability of the series of acetylene-cumulene bisdehydro[4/j-)-2]annulenes, it seems reasonable to assume that the bisdehydroannulenes have approximately the same planarity and essentially the same geometry. Therefore, this series of bisdehydroannulenes makes it possible to study the effect of ring size on the delocalization of a [4 -t-2] tz electron system. The differences in chemical shifts between the signals of the inner protons (tO and the lowest field signal of the outer protons (tq), which are always located at the position nearest to the centre of the molecule, is summarized in Table 9. The chemical... 

The reverse reaction is determined by the nature of R. The generating [Pt(PR3)2] moiety is a rather stable intermediate. A comparison of the results obtained with the NMR data (22) of substituted phenylacetylenes led to the conclusion that there is a relation between equilibrium constant, first-order rate constants, and chemical shifts of the acetylenic proton they all depend on substituent effects on the electron density in the triple bond. 

Variable temperature MAS NMR was used to characterize the structure and dynamics of hydrogen bonded adsorption complexes between various adsorbates and the Brpnsted acid site in H ZSM-5 the Brpnsted proton chemical shift of the active site was found to be extremely sensitive to the amount of type of adsorbate (acetylene, ethylene, CO and benzene) introduced (105). Zscherpel and coworkers performed maS NMR spectroscopic measurements in order to investigate the interaction between Lewis acid sites in H ZSM-5 and adsorbed CO. A new measure for the "overall" Lewis acidity of zeolites was derived from the C MAS NMR spectroscopic data. In addition, the chemical shift of CO adsorbed... 

Alkynes. The anisotropy of the triple bond results in a relatively low frequency (upfield) position for protons on sp-hybridized carbons. For acetylene (ethyne) itself, the chemical shift is 8 2.88, and the range is about 8 1.8-2.9. 

The large difference in the chemical shifts of H(7) (t 1.10) and H(8) (t —3.05) in their internal environments has been attributed to long range shielding by the lone triple bond. However, this must be incorrect as the analogous difference for the a- and cis- (3-protons in vinyl acetylene is predicted to be only 0.15 ppm 49>. Furthermore, planar models indicate that H(7) should be more deshielded by the di-yne system than H(8). It may well be that the observed shift of H(7) represents greater non-... 

Acetylenic hydrogens (C—H,. sp-lj) appear anomalously at 2 to 3 ppm owing to anisotropy (to be discussed in Section 3.12). On the basis of hybridization alone, as already discussed, one would expect the acetylenic proton to have a chemical shift greater than that of the vinyl proton. An sp carbon should behave as if it were more electronegative than an sp carbon. This is the opposite of what is actually observed. 

The chemical shift of any given proton depends upon the combination of effects, which are (roughly) additive the effects reinforce or cancel one another. Thus, acetylenic protons are deshielded by the inductive effect (acetylene is acidic) but shielded by the anisotropy of the triple bond a value of S = 1.80 ppm results. 

An alkyne with a triple bond at the end of a chain is called a terminal alkyne and the hydrogen atom at the end of the triple bond is referred to as an acetylenic hydrogen. This terminal proton is shielded by the anisotropy of the triple bond tt electrons, as was shown in Fig. 3.8, and so absorbs at about 1.8 ppm. The protons on the carbon next to the triple bond are affected in the same way as aUylic protons in alkenes and absorb in the same chemical shift range. 

Relatively few data are available concerning NMR spectra of protons connected to the alkyne carbon atoms in acetylene metal complexes. In contrast to olefins, coordinated acetylenes have their proton signals shifted to lower t values. The shift to lower fields generally equals 2.5-4 ppm for coordinated acetylenes. Therefore, acetylene protons in alkynes bonded to the central atom have chemical shifts which are typical for olefin hydrogen atoms. This is in agreement with theoretical predictions. X-ray data, IR spectra, and the metal-alkyne bond model. The chemical shift of protons for some alkyne complexes are given in Table 6.21. 

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