Alkanes increment system

Similarly to alkanes, an increment system has been proposed in order to predict carbon-13 shifts of olefinic carbons from the reference value of ethene (122.1 ppm) [237] and the increments A, for a, f , y, and 6 alkylation, as well as multiple substitution corrections S (Table 4.11). Accuracy is about 1 ppm. 

Carbon-13 shifts of alkynes (Table 4.13) [246-250] are found between 60 and 95 ppm. To conclude, alkyne carbons are shielded relative to olefinic but deshielded relative to alkane carbons, also paralleling the behavior of protons in proton NMR. Shielding relative to alkenes is attributed to the higher electronic excitation energy of alkynes which decreases the paramagnetic term according to eq. (3.4), and to the anisotropic effect of the triple bond. An increment system can be used to predict carbon shieldings in alkynes... 

Two empirical increment systems (Table 4.22) derived from experimental data as collected in Table 4.23 permit prediction of alkanol carbon-13 shifts. One is related to the shift value B of the hydrocarbon R —H and involves, as usual, addition of the increments Z, = <5,(r oh) — r)(- R > according to eq. (4.8 a) [268]. The other employs a linear equation (4.8 b), correlating the shifts of an alkanol R — OH and the corresponding methyl-alkane R —CH3 by a constant bk and a slope ak, which is 0.7-0.8 for a and about 1 for ji and y positions [269]. Specific parameter sets characterize primary, secondary, and tertiary alcohols (Table 4.22). The magnitudes of Z) increments in eq. (4.8 a) decrease successively from primary to tertiary alcohols (Table 4.22), obviously as a result of reduced populations of conformers with yliauche interactions in the conformational equilibrium when the degree of alkylation increases. 

Carbon-13 shift values of aliphatic amines [337-339] collected in Table 4.42 indicate that the y effect is about — 4.5 ppm for primary, secondary, and tertiary amines. The a effect, however, increases while the ft effect decreases with an increasing degree of alkylation at the amino nitrogen. Analogously to alkanols, two increment systems can be used to predict alkyl carbon shifts of amines according to eq. (4.12 a, b), based on the reference shifts of corresponding alkanes (R —H) or the methyl homologs (R —CH3) [337, 338] (Table 4.43). 

Fig. 4.17 illustrates the potential of carbon-13 NMR to detect the presence of isotactic (a), syndiotactic (b), and atatactic (c) vinyl polymers with polypropylene as sample [521], The spectrum of atactic polypropylene (Fig. 4.17(c)) displays the signals of all possible stereosequences including iso- and syndiotactic ones. Using the empirical increment systems for alkane carbon shift prediction [85, 201, 202] and including y effects of Zy = — 5 ppm specifically obtained by analysis of stereoisomeric polypropylene partial sequences between 3,5-dimethylheptane and 3,5,7,9,11,13,15-heptamethylheptadecane as a heptad model, the methyl carbon-13 shifts of all 36 possible heptads can be calculated... 

The relative contributions of the paramagnetic shielding and the diamagnetic shielding constants within this increment system have been discussed Empirical systems of this kind allow an estimation of chemical shifts with a mean deviation of less than one ppm. Modern data bank systems, e.g. SPECINFO or CSEARCH, are able to reproduce the chemical shifts of any given alkane even more accurately within a few seconds. In a slightly different approach, a substituent constant for 34 different alkyl groups was defined, and it was shown that the chemical shift of a nucleus X in a compound XR4 obeys equation 1, where m and b are constants. 

TABLE 2. Increment system for NMR chemical shifts of alkanes ... 

Several reviews have been devoted to a detailed presentation of the fragmental method of Rekker." This method, which was the first of its kind, was revised recently, leading to more precise hydrophobic fragmental increments for the octanol/water system as well as for the alkane/water system." ... 

Over the past decade it has been established that for various substituents the i C chemical shift increment is a constitutive property. This applies to many systems e.g. benzenes, alkanes and alkenes. The availability of over 200 allenes, randomly substituted with groups of different nature, enabled us to prove that in the case of allenes the chemical shift increment is a constitutive property too, thus establishing a convenient method for estimating i ( C) values for allenes. 

Despite these structural features, the adamantane ring system is not strain-free 142). Comparison of an estimated heat of formation for adamantane based on group increments derived from acyclic alkanes in completely skew-free conformations 142> with the experimentally determined value 143 indicates that adamantane is strained to the extent of 6.48 kcal/mole. 

The specific contribution of B in the DBB-value can be taken into account by two supplementary dimensionless parameters. While psB stands for a structural difference of B in comparison to a hypothetical n-alkane with the same relative molecular mass MrB, the number pBB = pBP + pJ)p/T represents an interaction increment between B and P, due to the different polarities of the solute and plastic. This interaction is generally a function of temperature. The two parameters, pvB and pBB can be considered as relative mass increments (positive or negative) which vanish in the reference system of n-alkanes in polymethylene. 

Contrary to the alkane series, in this case, the higher the cluster size, the lower the energy per cluster unit. This is expected for a system bound by hydrogen bonds, due to the cooperative nature of the hydrogen bonding network. As the cluster size increases, the increments in energy per cluster unit tend to increase linearly. For instance, at n = 2, we obtain a value of 1.9 kcal/mol, and at n = 8, a value of 4.9 kcal/mol, at the PNOF5/cc-pVDZ level of theory. Remind that these structures are frozen. 

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