Charge distributions and chemical effects. XXX. Relationships between nuclear magnetic resonance shifts and atomic charges

Abstract

In a series of closely related compounds, it may occur that the change in total (diaplus para-) magnetic shielding of atomic nuclei is nearly that of the local paramagnetic term, because of cancellation effects involving nonlocal dia-and paramagnetic contributions. This offers a justification for relationships between nuclear magnetic resonance shifts and local atomic populations, which are occasionally observed. It is important, however, to consider the type (2s, 2p, , or x) of electrons which are responsible for the variations in atomic charges. Correlations between UC NMR shifts and atomic populations of aromatic compounds, for example, should not be interpreted in terms of x electrons only, because the slope of shift vs. x charge (i.e., the ~160-ppm/electron value which is usually invoked) does not describe an intrinsic effect of x charges on magnetic shielding but accounts for the fact that a and x charges vary in opposite directions in this class of compounds. The explicit consideration of the inverse variations of a and x charges, where appropriate, offers an explanation for the observation that charge-shift correlations can have positive or negative slopes. It appears, indeed, that an increase in total electronic population is accompanied (i) by a high-field shift when the electron enrichment results from a gain in x charge prevailing over the concurrent loss in electrons (aromatic and vinyl C, carbonyl O atoms) or (ii) by a downfield shift when the increase in charge is dictated by that of the population (sp1 23 C, carbonyl C, and dialkyl ether O atoms).

Nuclear magnetic shielding calculations in the finite perturbation method involving GIAO's at the INDO level and London's approximation. II. Carbon-13 and nitrogen-15

“…Indeed, accurate correlations of this type are occasionally observed in well-defined series of compounds (15), suggesting, in agreement with the present results, that the local charge density determines the major part, by far, of the differences in shielding between nuclei in comparable situations.…”

Charge distributions and chemical effects. XXXI. Molecular energies of ethylenic compounds

“…Recent studies (4,5) indicate that, indeed, this is possible. If we write for vinyl carbons 'Moreover, it must be considered that this type of uncertainty may possibly be due to a systematic error in the estimates of the zero-point plus heat content energies which are described further below.…”

“…The important point is that the de:ivatives (aE:/aNi)O which are part of the a,'s (eq. [4]) ~ancel~exactly in A; and A; . This is no longer the case with the 2acaHCAq, term of eq.…”

Correlations between CNDO/2 charge distribution and 13C NMR chemical shift in 7-acylamino side chains of cephalosporins.

“…CNDO/2 and other molecular orbital methods are known to be sometimes useful in analyzing NMR data [26][27][28][29][30][31][32][33][34][35][36]. The success of this study in finding strong correlations between quantum mechanical indices from CNDO/2D and NMR chemical shifts further establishes this semi-empirical method as a guide to understanding the physical and chemical properties of cephalosporins2).…”