Chemical shifts alkyne protons

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. 

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 extent of alkyne ir donation in Mo(RC=CR)(SBu )2(CNBu% (SO) is not easily quantified as discussed in the structural and molecular orbital sections. Proton shifts for HC=CH and PhC=CH ligands in these complexes are near 10.4 ppm, above the N = 4 median value and approaching an N = 3 chemical shift. The 13C chemical shifts range from 170 to 185 ppm, also above classic four-electron donor alkyne values, presumably reflecting competition with the two adjacent equatorial thiolates for donation to the two vacant metal dir acceptor orbitals. Single bond VCH values of 215 and 211 Hz are typical of terminal alkynes bound to molybdenum(II) (133). 

The development of Fourier transform NMR spectroscopy made carbon NMR (13C NMR or CMR) possible, and high-field superconducting spectrometers allowed it to become nearly as convenient as proton NMR ( H NMR). Carbon NMR determines the magnetic environments of the carbon atoms themselves. Carbonyl carbon atoms, alkyne carbon atoms, and aromatic carbon atoms all have characteristic chemical shifts in the 13C NMR spectrum. 

The distinction can be vital in structural problems. The symmetrical alkyne diol below cyclizes in acid with Hg(II) catalysis to a compound having, by proton NMR, the structural fragments shown. The product is unsymmetrical in that the two C.Me2 groups are still present, but they are now different. In addition, the chemical shift of the CH2 group shows that it is next to C-O but not next to oxygen. This leaves us with two possible structures. One is an ester and one a ketone. The C=0 shift is 218.8 p.p.m. and so there is no doubt that the second structure is correct. 

Of special relevance to the investigation of alkyne-substituted clusters is the observation of a low-field resonance, generally in the range —1 to + 1.5t, for a proton attached to a carbon atom which is either a or n bonded to the metals. Values of the chemical shift for this signal for 23 tri- and tetranuclear osmium clusters were presented by... 

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. 

Figure 3.20 clearly shows that there are some types of protons whose chemical shifts are not easily explained by simple considerations of the electronegativity of the attached groups. For instance, consider the protons of benzene and other aromatic systems. Aryl protons generally have a chemical shift as large as that of the proton of chloroform Alkenes, alkynes, and aldehydes also have protons whose resonance values are not in line with the expected magnitudes of any electron-withdrawing or hybridization effects. In each of these cases, the anomalous shift is due to the presence of an unsaturated system (one with r electrons) in the vicinity of the proton in question. 

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. 

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