Carbon-13 Magnetic Resonance. II. Chemical Shift Data for the Alkanes

Carbon-13 n.m.r. chemical shifts for 8 anisoles and 15 diphenyl ethers are reported. paraCarbon shieldings are found to be sensitive to the degree of steric interference to conjugative electron release by the ethereal oxygen atom. An empirical correlation between the para-carbcn shifts and the average twist angle of the phenyl rings from a reference plane has been developed. Satisfactory agreement has been found between the angles of twist derived from carbon chemical shifts and those based on other physical measurements.On rapporte les dCplacements chimiques en r.m.n, du carbone-13 de 8 anisoles et de 15 Cthers diphenylCs. On a trouvC que le blindage des carbones en para est sensible au degre d'interference sterique a la scission des Clectrons de I'atonie d'oxygtne de la fonction Cthtree par conjugaison. On a dtveloppC une corrClation empirique entre les dtplacements chimiques des carbones en para et les angles rnoyens de torsion du plan des noyaux phenyles par rapport a un plan de rtfirence. On a trouve une correlation satisfaisante entre les angles de torsion derives des deplacements chiniiques du carbone et ceux bases sur d'autres methodes physiques.[Traduit par le journal]Can. J. Chern., 52,767(1974) Introduction In recent years considerable effort, using a variety of techniques, has been directed toward the elucidation of the geometries of substituted diphenyl ethers (DPO) in solution. As it would be expected, no single technique yields all the desirable information, rather the dizerent methods appear complementary.Several studies (1-6) indicate that the conformational preference of DPO can be reasonably established by 'H n.m.r. only if it is of the H-inside type (1) (the two phenyls lying perpendicular to each other). In fact, the intramolecular ring current shielding effect is sizeable for the H-inside form but sharply decreases for the other skew conformations, which become practically undetectable by this method (7,8). Skew conformations have been claimed a t times (9-12) to be consistent with dipole moment data, but it has been recently (13, 7, 6) pointed out that dipole moments often do not provide unequivocal information about the conformational preferences of ortho-substituted DPO.

Section: Resultssupporting

confidence: 87%

Section: Diphenyl Etherssupporting

confidence: 68%

See 1 more Smart Citation

Carbon-13 Nuclear Magnetic Resonance Studies of ortho-Substituted Anisoles and Diphenyl Ethers

Buchanan¹,

Montaudo²,

Finocchiaro³

1974

“…Table 6 lists the 6-and +interactions for the nonequivalent carbons C(1) and C(7) in all molecular conformations. If all rotational states in the four-bond segments C(1)-C(5), C(1)-C(8), C(7)-C(5), and C(7)-C(8) are defined, the nonequivalence of C(1) and C (7) is traced to the fact that &-interactions with C(6) occur with probabilities p,, x p,, x p,, and PZ3 x p,, x P2, for C(1), but only with probabilities p21 p22 p23 and p22 p2.1 p21 for C (7). Since the occurrence of unrealistic &-interactions, i.e.…”

Section: Summation Over Conformer Contributionsmentioning

confidence: 99%

Conformational and configurational analysis of hydrocarbon chains based on time-averaged carbon-13 chemical shifts

Beierbeck¹,

Saunders²

1977

HELMUT BEIERBECK and JOHN K. SAUNDERS. Can. J. Chem. 55, 771 (1977). The conformations of saturated hydrocarbon chains are analysed by a least-squares fit of calculated to observed time-averaged carbon-13 chen~ical shifts. The calculated resonances are the sums of products of conformer resonances and conformer probabilities. The conformer shieldings are derived with predictive parameters developed earlier for conformationally well defined systems. The conformer probabilities are taken to be products of rotamer probabilities. The rotamer probabilities are variables in the analysis. The carbon-13 resonances of saturated hydro-carbons is based on four separate reference carbon chains at ambient temperatures are the values, since a-carbon substituent effects are sums of the chemical shifts in all molecular nonadditive. The resonances of the primary, conformations, weighted by the probability of secondary, tertiary, and quaternary carbons in occurrence of each conformation. If the con-ethane, propane, isobutane, and neopentane, former resonances may be measured, or cal-respectively, served as starting estimates for culated to a good approximation, the conformer these references. Two substituent parameters, populations may be derived by fitting calculated HC and CC, account for the deshielding effect to observed time-averaged carbon-13 resonances. of P-carbons. A shift increment HC is associated Since conformational analysis is a prerequisite with every 1,3-diaxial hydrogen-hydrogen interfor the determination of the configurations of action introduced by a P-carbon. An increment mobile systems, the derivation of the conforma-CC is associated with everv carbon-carbon tional probabilities from time-averaged carbon-gauche interaction. Previously derived parameter 13 resonances also allows the determination of values (2, 3) served as starting estimates for HC configuration in asymmetric hydrocarbon chains. and CC. No y-carbon parameter is required.

“…On dCmontre ?i l'aide des rCsultats suivants qu'une relation semblable existe entre les protons des groupes mCthyles de 1 dans des solvants organiques: (i) par une synthkse de 1-rl, (8), (ii) par comparaison du spectre rmn du ' H de 1 avec ceux de ses analogues dimCthylCs en 2,2 et en 5,5 et (iii) par l'observation d'une constante de couplage i longue distance dans le spectre de 1. On a fait l'attribution des dkplacements chimiques du 13C dans 1 en se basant sur un spectre, oh les protons sont couplCs, montrant que le signal des groupes mCthyles en C-5 apparait i un champ d'environ 4 ppm plus faible que celui des carbones du groupe mCthyle en C-2.…”

unclassified

The assignment of ¹H and ¹³C chemical shifts for dihydro-3(2H)-furanone derivatives by means of specific ²H labelling experiments and ¹³C{¹H} decoupling experiments

Burke¹,

Reynolds²,

Tam³

et al. 1976

Chem. 54, 1449 (1976).The ' H nmr spectrum of dihydro-2,2,5,5-tetramethyl-3(2H)-furanoe (1) in 80% sulfuric acid shows a time-dependent decrease in the intensity of the lower-field gem-dimethyl signal relative to the upper-field gem-dimethyl signal. This is interpreted as involving reversible opening of 1 to 2-hydroxy-2,5-dimethyl-4-hexen-3-one, resulting in deuterium exchange at the C-5 methyl groups of 1. The lower-field gem-dimethyl signal of 1 in this medium is therefore assigned to these methyl groups. A similar relationship between the methyl proton signals of 1 in organic solvents has been demonstrated by (i) synthesis of 1-rl, (8), (ii) comparison of the IH nmr spectra of 1 and its 2,2-and 5,5-dimethyl analogs, and (iii) observation of long range coupling in the spectrum of 1. Assignment of l3C chemical shifts for 1 has been made based on protoncoupled spectra, showing that the signal of the C-5 methyl carbons appears at crr. 4 ppm towards lower field than that of the C-2 methyl carbons. Both the IH and 13C chemical shift assignments for 1 were confirmed from 13C (1H) spectra using low power single frequency J H decoupling, demonstrating the utility of this technique for the assignment of chemical shifts.