Cyclohexane chemical shift

One property of NMR spectroscopy is that it is too slow a technique to see the mdi vidual conformations of cyclohexane What NMR sees is the average environment of the protons Because chair-chair mterconversion m cyclohexane converts each axial pro ton to an equatorial one and vice versa the average environments of all the protons are the same A single peak is observed that has a chemical shift midway between the true chemical shifts of the axial and the equatorial protons... 

A nitrogen atom at X results in a variable downfield shift of the a carbons, depending in its extent on what else is attached to the nitrogen. In piperidine (45 X = NH) the a carbon signal is shifted by about 20 p.p.m., to ca. S 47.7, while in A-methylpiperidine (45 X = Me) it appears at S 56.7. Quaternization at nitrogen produces further effects similar to replacement of NH by A-alkyl, but simple protonation has only a small effect. A-Acylpiperidines show two distinct a carbon atoms, because of restricted rotation about the amide bond. The chemical shift separation is about 6 p.p.m., and the mean shift is close to that of the unsubstituted amine (45 X=NH). The nitroso compound (45 X = N—NO) is similar, but the shift separation of the two a carbons is somewhat greater (ca. 12 p.p.m.). The (3 and y carbon atoms of piperidines. A- acylpiperidines and piperidinium salts are all upfield of the cyclohexane resonance, by 0-7 p.p.m. 

The chemical shift of a nucleus depends in part on its spatial position in relation to a bond or a bonding system. The knowledge of such anisotropic effects is useful in structure elucidation. An example of the anisotropic effect would be the fact that axial nuclei in cyclohexane almost always show smaller H shifts than equatorial nuclei on the same C atom (illustrated in the solutions to problems 37, 47, 48, 50 and 51). The y-effect also contributes to the corresponding behaviour of C nuclei (see Section 2.3.4). 

Table 14.1. Prediction of C chemical shift of c/s-l,2-dimethylcyclohexane in the frozen state, using the cyclohexane shift of 8c = 27.6 and substituent effects (e.g. Ref. 6, p. 316)...

The CH fragment which is linked to the OH group (Sh = 5.45 ) can easily be located in the H and NMR spectra. The chemical shift values Sc =74.2 for C and Sh = 3.16 for //are read from the CH COSY plot. The H signal at S,i = 3.16 splits into a triplet (11.0 Hz) of doublets (4.0 Hz). The fact that an antiperiplanar coupling of 11 Hz appears twice indicates the diequatorial configuration (trans) of the two substituents on the cyclohexane ring 5. If the substituents were positioned equatorial-axial as in 4 or 5, then a synclinal coupling of ca 4 Hz would be observed two or three times. 

Energy differences between conformations of substituted cyclohexanes can be measured by several physical methods, as can the kinetics of the ring inversion processes. NMR spectroscopy has been especially valuable for both thermodynamic and kinetic studies. In NMR terminology, the transformation of an equatorial substituent to axial and vice versa is called a site exchange process. Depending on the rate of the process, the difference between the chemical shifts of the nucleus at the two sites, and the field strength... 

For another dramatic illu.slration of chemical shifts, have. students calculate the magnetic shielding of nitrogen in pyridine and compare it to its saturated cyclohexane analogue. 

The chemical shifts of monosubstituted thiophenes relative to the a- and )8-hydrogens of thiophene at infinite dilution in cyclohexane are given in Table I and are discussed in the following. 

When CH2F is a substituent on most alicyclic rings, such as a cyclohexane ring, the 19F chemical shift of this group is not significantly altered from that of an acyclic system (Scheme 3.2). On the other hand, when it is attached to a cyclopropane ring, a unique deshielding influence is observed. 

A trifluoromethyl group attached to a cyclohexane ring is unremarkable with respect to its chemical shift, absorbing at -75 ppm, with a 3/fh = 8Hz (Scheme 5.2). There are no data available for trifluorometh-ylcyclopentane or cyclobutane. The chemical shift for trifluoromethyl-cyclopropane reflects additional shielding, such CF3 groups appearing the farthest upheld of any CF3-substituted hydrocarbon. 

The influences of SF5 groups upon H and 13C NMR spectra are also quite characteristic, and information about proton and carbon chemical shifts and the respective coupling constants will be included along with the 19F data where they are available. The proton and carbon NMR spectra for (pentafluorosulfanylmethylene)cyclohexane are provided in Figs. 7.3 and 7.4 as typical examples. 

There is a continual tendency for the values of cr/ and aR (and other substituent chemical shifts for a series of mono-substituted benzenes in very dilute solution in cyclohexane, carbon tetrachloride or deuteriochloroform were the basis for a redefinition of the aR scale and some amendment of a R values. However, the value for the nitro group was confirmed as 0.15. 

This approach has also been applied to saturated C(i) atoms. Due to the much lower polarizabilities of C-H and C-C single bonds, however, the effects are clearly smaller (82), though still detectable. Thus, some effects of substituents X on S-positioned carbon atoms in cyclohexanes and cholestanes were attributed to LEF effects of the C-X dipoles by Schneider and colleagues (76,77,85). The finding that only minor effects are to be expected in saturated molecules was confirmed by INDO-SCF calculations of electric-field effects on l3C chemical shifts of some model compounds performed by Seidman and Maciel (86). These authors conclude, further, that conformational studies on such systems are not promising (86). 

Since Eliel and Pietrusiewicz (142) have published a comprehensive review on l3C chemical shifts in saturated heterocycles, the discussion in the present report may be confined to a few basic features for the sake of completeness. It stands to reason that there is an electronegativity dependence of a-carbon chemical shifts. This was demonstrated by Lambert and co-workers (143) for a great variety of pentamethylene heterocycles, and holds also for positively charged species, such as X = (CH3)2N+, (CH3)2P+, CH3S +, Br+, and I+. The same is suggested by the good correlation of a-carbon chemical shifts in some heterocyclohexanes (143) and the a-SCS values in cyclohexanes with equatorial substituents (Figure 2)(114). 

Methyl Chemical Shift Differences (8 - 8 ) in Cyclohexane and Thia Analogs (166)... 

The preceding prediction rules are largely restricted to acyclic compounds. But there is also a considerable need for parameter sets enabling the spectroscopist to calculate I3C chemical shifts of conformationally defined cyclic molecules, especially of the cyclohexane type. Methyl-group effects in methylcyclohexanes (100,101) that are to be added to the basic value for cyclohexane itself (8 = 27.3) are listed in Table 29. Analogous methyl-group parameters in tetralins and tetra-hydroanthracenes have been reported (403). 

NMR chemical shift in cyclohexane for [Bu3Sn ][CBiiMei2] is only little lower... 

Measured in toluene-d or benzene-d. Measured in cyclohexane. All phosphorus atoms have similar /hp coupling constants, q = quartet, qui = quintet. This chemical shift belongs to the non-coordinated phosphorus atom of (LnL ). Not determined. 

In this equation, Au i 2 is the chemical shift separation between the exchanging sites, such as the protons in axial and equatorial sites in cyclohexane, and is the exchange contribution to the observed T2, T in eq. (22), which is the T2 determined from the offset-saturation experiment. The ex-... 

A plot of the 2 magnetization (M /Moo) as a function of the irradiation resonance offset is given for an offset saturation experiment on a sample of degassed cyclohexane (CeHn) in CDCI3 at a temperature of 303 K. At this temperature, the cyclohexane resonance was noticeably narrow at 280 K, the linewidth of the cyclohexane resonance was approximately 1.2 Hz, compared to a width of 0.5 Hz for the internal TMS standard. From the resulting values of T and T2, and the measured low-temperature axial-equatorial chemical shift difference of 45.4 Hz at 100 MHz used in eq. (22), the rate at 293 K was determined to be 1.94 x 10 s Thus, rates on the order of 10 or 10 s are accessible in relatively simple systems using the offset-saturation method. 

The validity of Eq. (6.1) has been carefully established for linear and branched paraffins, cyclohexane and methylated cyclohexanes, including molecules consisting of several cyclohexane rings in the chair conformation (e.g., cis- and frani-decalin, bicyclo[3.3.1]nonane, adamantane, and methylated adamantanes) as well as in boat conformation (e.g., iceane and bicyclo[2.2.2]octane) [44]. No special effect seems to contribute to the chemical shift because of the presence of cyclic stmctures. 

Density functional theory (DFT) calculations employing sum-over-states DF perturbation theory were applied to calculate both the H and chemical shifts of 1,3-dioxane, 1,3-oxathiane, 1,3-dithiane, and the parent cyclohexane <1997JMT(418)231>. Both normal and anomalous trends in the H chemical shifts could be reproduced well and. 

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