13C-’H coupling constants

The H and 13CNMR spectra of various cyclopentazepines have been recorded (Tables 1 and 2), as has the HNMR spectrum of 7V,7V-dimethylcyclopent[e]azepin-l-amine.68 A detailed analysis of geminal and long-range 13C-H coupling constants for cyclopent[c]azepine is also available.87 The HNMR spectra of 9//-pyrrolo[l,2-a]azepin-9-one (8b) and its fully delocalized cation have been recorded in various solvents.7... 

The 13C—H coupling constants of methyl (213 Hz23 ) and phenyl (216 Hz55 ) cyclopropenone are in the order of those obtained for cyclopropene vinylic protons (200/201 218 Hz/221 Hz174 ) and reflect an s-contribution of more than 40% in the carbon hybrid orbital of the vinyl C—H bond. 

Geminal, and vicinal, 13C-H coupling-constants are 8 Hz or less, and difficulties have been reported in observing splittings in the 13C-... 

Table 1. 13C-H coupling constant of chloroform in various solvents...

Douglas, A. W., Dietz, D. 13C-H Coupling Constants. III. 13C-H Coupling in the Vapor Phase and its Dependence on Medium Effects. J. Chem. Phys. 46, 1214 (1967). 21) Watts, V. S., Goldstein, J. H. Solvent Effects on 13C-H Coupling Parameters and Chemical Shifts of Some Halomethanes. J. Phys. Chem. 70, 3887 (1966). 

Watts, V. S., Loemker, J., Goldstein, J. H. Solvent and Concentration Effects on 13C—H Coupling Constants and Chemical Shifts in some Dihaloethylenes. J. Mol. Spectry 17, 348(1965). 

Aside from 13C-H coupling constants (often obtained from 13C satellites in 1H NMR spectra) not very much information is available on the 13C NMR spectra of small heterocycles. A recent review (B-79MI50101) devotes only seven pages to the 13C NMR spectra of all three- and four-membered heterocyclics. 

Another quantity of interest is the 13C H coupling constant. These vary considerably with bond type, and some typical values are given in Figure 12.111... 

Single-bond 13C- H coupling constants for six-membered heteroaromatic compounds lie in the approximate range 150-220 Hz, the magnitude varying with substituent electronegativity. Data for... 

Table 19b 13C- h Coupling Constants for the Furan Nucleus in Furocoumarins and Furochromones (Benzofuran... 

Configurational assignments for 5,6-dihydro-2H-pyrans have also been made from the magnitude of the geminal 13C-H coupling constants (73TL1037) and from the sensitivity of the chemical shift to steric hindrance (B-72MI22202). 

The magnitude of 13C— H coupling constants depends on the hybridization of the carbon atom (sp3, sp2, sp) coupled to the proton. This is indicative of a mechanism of spin-spin coupling involving the bonding electrons, as is illustrated in Fig. 1.11 for a group AX with nuclei A and X, each with a total spin quantum number of / =... 

Correlation of spectral studies for these classes of compounds involve K-band shifts in the UV spectra of 6-substituted 2,4-diamino-pyrimidines,165 NMR chemical shifts of the two amide protons and the 6-proton in 5-substituted uracils170 and of protons in pyrazines,171 MeO 13C—H coupling constants in methoxy-s-triazines,172 and 3iCl NQR in chloropyrimidines.173... 

C- H coupling constants, 4, 569 crystal data, 3, 623 mass spectra, 3, 610 structure, 4, 541 Furo[ 1,2]diazepines synthesis, 7, 598 Furo[ 1,3]diaze pines synthesis, 7, 607 Furodysin... 

The P—O bond in (CII3)sPO has been reported to be well described as a double bond, since the 13C—H coupling constant of this compound is closer to that of (CH3)3P than to that of (CHa) P+ (31). 

In most cases, the stereochemistry of the quaternary carbon atom in branched-chain carbohydrates cannot be elucidated from H-n.m.r. spectra, but 13C-chemical shifts, or long-range, 13C- H coupling-constants, may often yield valuable information.114-118 Likewise, the stereochemistry of acetal carbon atoms of benzylidene derivatives,103, 104 and of acetals derived from pyruvic acid,105,106 may be determined from l3C-chemical shifts. 

The chemical shift of H-2 of benzoselenazole has been compared with those of its thia and oxa analogs. The observed deshielding effect of the heteroatom is opposite to the order of electronegativity. 13C- H coupling constants have also been measured (Table 3) (67JHC139). 

The delay, r = (2J) 1 with J = 150 Hz, is selected from the average J values (13C— H coupling constants) found for lignin structures. The three pure subspectra CH, CH2, and CH3, are obtained from appropriate linear combinations of the three basic spectra ... 

Spin-spin coupling constants involving, 3C nuclei are usually dominated by the Fermi contact term (72) and therefore depend in part upon the hybridization of the interacting nuclei. The 13C- H coupling constants, /(CH), are a useful indication of the hybridization of a given carbon atom. Typical ranges for hydrocarbons are approximately 125 Hz for sp3 carbon atoms in alkanes, 155-160 Hz for sp2 carbon atoms in alkenes and arenes, and approximately 250 Hz for alkynes (8). Applications to structural determinations are mentioned throughout this review. 

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