Critical analysis of the through-space transmission of NMR J FH spin-spin coupling constants

Abstract

NMR J FH coupling constants transmitted through space are known to show several unusual peculiarities. The main three are (a) in many cases their experimental values are reported to be positive and in others, negative; (b) theoretical values show that in some cases they are substantially contributed not only from the Fermi contact (FC) term but also from the paramagnetic spin orbit (PSO) term, and their respective signs could be either like or unlike; (c) in many cases it can hardly be expected that the corresponding F-H proximate interactions could be considered a "hydrogen bond," whereas in other cases it is evident that they are. For discussing points (a) and (b), characteristics of both the FC and PSO terms are discussed performing qualitative analyses based on their expressions given in terms of the polarization propagator formalism. Point (c) is discussed in terms of the well-known Bader's QTAIM method as well as recalling the known relationship between the transmissions of the Fermi hole and of the FC interaction of spin-spin coupling constants. © 2012 Wiley Periodicals, Inc. From QTAIM parameters, the presence of F-H hydrogen bonds for 1 and 2 were assessed. Considering that the sign of TSJ FH SSCC is negative for 1, whereas it is positive for 2, it is concluded that the TSJ FH SSCC sign is not indicative of the presence or absence of an F-H hydrogen bond. Instead, such different signs are indicative that their Fermi contact contributions are transmitted by exchange interactions. The actual sign is determined by the competition between the LP 1(F)/C-H and C-F/C-H overlaps, which yield different sign contributions. Copyright © 2012 Wiley Periodicals, Inc.