Effect of temperature and concentration of solutions on the position of 13C NMR signals for fractions of saturated hydrocarbons forming urea adducts

A 13C NMR study of crude oils: Identification of compounds with nonisoprenane k,(k + 3)-dimethylalkyl moieties in oils and bitumenoids from the southern part of Anadyr basin

“…(2) is very high. Since the parameters that determine these effects are structure dependent [3][4][5], such accuracy is a good piece of evidence that the structure of these components of the fractions has been correctly established. Another piece of evidence was obtained by com paring of the measured and calculated coefficients L C,i at a fixed concentration of the solution C for the tem perature dependence of the chemical shifts of C atoms (δ i ) defined by Eq.…”

“…and n(k ± i) C atoms [3]: = a kMe,(k ± i) -a n(k ± i) , = b kMe,(k ± i) -b n(k ± i) , etc. Then in the additivity approximation, the chemical shift of СН 2 groups in k,(k + 3)Me 2 Alk, "internal" with respect to both chiral centers (С int ) δ{С int } is calcu lated by Eq.…”

“…Chemical shifts, in accordance with [3], were counted from the signal of the terminal methyl group of n alkyl moieties, assum ing it to be δ = 14.070 ppm for all recording tempera tures (Т rec ) and concentrations (С) of solutions. To iso late the subspectra of СН 2 and CH + СН 3 groups, a pair of spectra was used: one was with the standard APT pulse width sequence and the other was the "pseudo BB" spectrum with the same wait times and the scanning pulsewidth as those of the first spectrum.…”

“…Finally, the structure is established by adding model compounds to the fraction and comparing the spectra before and after their admixture. However, it was shown [3][4][5] that the coefficients for the following equation are also characteristic quantities for each sig nal i:…”

“…Finally, the structure is established by adding model compounds to the fraction and comparing the spectra before and after their admixture. However, it was shown [3][4][5] that the coefficients for the following equation are also characteristic quantities for each sig nal i: (1) they define the dependence of chemical shifts on both the recording temperature and the concentration of the fraction in solution or, more exactly, on their effec tive values (T eff , C eff ). It was shown that the use of this set of numerical characteristics of each signal can sig nificantly extend the opportunities for finding new groups of saturated cyclic compounds in crude oils by 13 C NMR spectroscopy, making it unnecessary to syn thesize model compounds in certain cases [5].…”