Alkynes chemical shifts

Relatively few 13C-NMR data have been reported for bisalkyne complexes. The range of 13C alkyne chemical shifts is roughly 170-190 ppm for Mo(RC=CR)2(S2CNEt2)2 complexes (87). Terminal alkynes exhibit VCh values near 210 Hz in these three-electron donor roles, similar to values reported for four-electron donor alkynes. For cationic cyclopentadienyl bisalkyne derivatives an alkyne carbon 13C range of 140-180 is seen in Table VI. 

Alkynes have chemical shifts between 63 and 95 ppm. The chemical shifts for some alkynes are presented in Table 4.13. The effect of carbon substituents on alkyne chemical shifts are presented in Table 4.14. It may be noted that the resonance of the non-hydrogen-bearing carbon in alkynes is less intense due to lower NOE and is therefore usually readily recognized. 

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 the 1H noise-decoupled 75.5 MHz 13C NMR spectrum of 72, the signals of the sp-hybridized carbon atoms C15 and C. 15 are found at 98.3 and 97.3 ppm. This is in the expected region for substituted alkynes and the chemical shifts agree very well with those of other didehydrocarotenoids. As can be seen in Table 21, the 15,15 -triple bond leads to an upheld shift of ca 22 ppm for the directly connected C14 and C. 14. The chemical shifts of the other carbon atoms of the polyene chain are also affected a downheld shift is observed for the odd carbon atoms and a (shght) upheld shift for the even carbon atoms, both decreasing with increasing distance from the central part. 

The same situation is encountered with a,/ -alkynic phosphonium salts in which C(a) carbons are strongly shielded vs C p) carbons 47.8-73.0 vs 119-127 ppm36,97. The linear correlation of the C(a) and C(P) chemical shifts with a para Hammet constant was illustrative of the polarization of the 7r-electron system in the C=C triple bond (Figure 8). In the same sense, the observed C(flt) and C(/J) chemical shifts were used to argue for the existence of either resonance hybrid structures as in the case of enaminophosphonium salts98,99 or incipient ylide contribution40. 

Table 4.13. 13C Chemical Shifts of Alkynes [246, 247], Polyalkynes [248], Alkenynes [249] and Cycloalkynes [250] (<5C in ppm).

Figure 27-8 Observed (upper) and calculated (lower) nmr spectra of 4-deuterio-1-buten-3-yne (CH2=CH—C=CD) at 60 MHz. The calculated spectrum is based on chemical shifts of 300, 297, and 283 Hz and coupling constants of 18.0, 11.5, and 2.0 Hz. The deuterium substitution was made to simplify the spectrum by eliminating small long-range couplings involving the double-bond hydrogens and the alkyne hydrogen.

Chiral Metal Atoms in Optically Active Organo-Transition-Metal Compounds, 18, 151 13C NMR Chemical Shifts and Coupling Constants of Organometallic Compounds, 12, 135 Compounds Derived from Alkynes and Carbonyl Complexes of Cobalt, 12, 323 Conjugate Addition of Grignard Reagents to Aromatic Systems, I, 221 Coordination of Unsaturated Molecules to Transition Metals, 14, 33 Cyclobutadiene Metal Complexes, 4, 95 Cyclopentadienyl Metal Compounds, 2, 365... 

C Chemical Shift Increments for A, the Shielding Term for Alkynes... 

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