Acetylene chemical shifts

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... 

Another example is the linear correlation of the 59Co chemical shifts of the catalyst with the regioselectivity of a trimerization reaction of acetylenes [15]. 

The high shift values for NO+ (474 ppm for 170 and —3 ppm for nitrogen) relative to lower values for X—N=0 compounds is good evidence for differences in chemical shift anisotropy as pertain in 13C NMR for the alkynes relative to alkanes and alkenes64. The higher symmetry of acetylene leads to a high field shift. 

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... 

However, correlation of 119Sn chemical shifts of l-substituted-2-trimethylstannyl acetylenes (set PP19, Table 6) gave the regression equation 84 ... 

Table 9.13C-Chemical Shifts of the Acetylenic Carbons in Angle Strained Cycloalkynes and Some Reference Compounds (CDC13, rel. to TMS)...

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). 

Chemical shifts have been reported for coordinated tt ligands such as olefins, acetylenes, allylic groups, dienes, cyclopentadienyl groups, and arenes. There appear to be two major factors which can cause the chemical shift of the n ligand to move from the shift found for the free ligand. 

The C Chemical Shifts of Some w-Olefinic and w-Acetylenic Complexes of Metals ... 

Chemical Shifts and Number of Signals Instrumentation Expression of Chemical Shifts Acetylene in Magnetic Field Benzene in Magnetic Field Factors Influencing Chemical Shift Solvents Used... 

Another illustrative example is that of phenylacetylene. Table 6-7 summarizes the H NMR chemical shifts of its alkyne H-atom in a variety of solvents [273], Most solvents (except aromatic solvents) decrease the shielding of the acetylenic hydrogen nuclei. The corresponding low-field shifts have been interpreted in terms of weak specific association between the alkyne as hydrogen-bond-donor and electron pair-donor groupings of the solvent [273], The high-field shifts in aromatic solvents arise from the magnetic anisotropy of the solvent molecules (see below). The order of effectiveness of the solvent... 

It is interesting to note that the rate ratios follow the spectroscopic A h (chemical shift difference of the acetylene hydrogen on the substituted vs. parent phenylacetylene) and the A8Cp (chemical shift difference of the terminal acetylene carbon in the substituted vs. parent phenylacetylene) (12). 

The spectroscopic data for 9 support the proposed structure. In particular, the wSi NMR chemical shift of 5 43.19 as a triplet (Jsi-p(C/ ) = 40.11 Hz) resembles the literature value reported for the m-NiSijPC complex. 3 Compound 9 was found to be a good reactive intermediate for the double silylation reaction. The reaction of 1 with 1-phenylprop-l-yne (1 equiv) in the presence of a catalytic amount of 9 (0.03 equiv) for 6 h afforded the double-silylated producted 10 in 94 % (GC) yield. The reaction was quite sensitive to the reaction conditions. When the same reaction was carried out at higher temperature (70-75 °C), the major component was identified as the acetylene cyclotrimerization product 11 (Scheme 2), which has been characterized by spectroscopic techniques. When hex-l-yne is employed as a terminal alkyne in the reaction with 1 under the same condition, the five-membered disilyl ring compound 12 is isolated as a colorless liquid in 71 % yield. 

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