Large Long-Range F−F Indirect Spin−Spin Coupling Constants. Prediction of Measurable F−F Couplings over a Few Nanometers

Abstract

Large long-range indirect nuclear spin coupling constants are of great interest for quantum computers. But they are rarely observed and are usually considered very small, unless the coupled nuclear spins are proximate in space. Looking for counterexamples, we have calculated F-F couplings in four different series of acyclic hydrocarbons (alkanes, conjugated polyenes, conjugated polyynes, and cumulenes) where the coupled fluorine nuclei are separated by up to 11 bonds or 1.4 nm. The calculations were carried out at the level of the second-order polarization propagator approximation using locally dense basis sets. This approach has, in recent years, been shown to be particularly successful in reproducing indirect nuclear spin-spin couplings in organic molecules. We find that the F-F couplings in saturated alkanes diminish very quickly with the number of bonds between the coupled fluorine atoms, whereas in the conjugated polyenes and in particular polyynes the F-F couplings can be transmitted over much longer distances. We predict that the F-F coupling over 9 bonds or 1.1 nm is 12 Hz in (1E,3E,5E,7E)-1,8-difluoroocta-1,3,5,7-tetraene and the coupling over 11 bonds or 1.4 nm is 7 Hz in difluorodecapentayne. Analyzing the four Ramsey contributions, we find that the F-F couplings in the polyenes are dominated by the spin-dipolar term, which is known to be favored by π-electronic systems, whereas in the case of the polyynes the orbital paramagnetic terms make the largest contributions, although the spin-dipolar and the Fermi contact contributions are also significant.

Understanding NMR J‐couplings by the theory of polarization propagators

“…This means that these molecular systems can be good candidates for quantum computers containing more than seven qubits as suggested in Ref. (112).…”

“…Given also that unsaturated F-substituted carbon chains were used as quantum computers with five qubits (111), it is highly recommended to have a look on this kind of general systems. We investigated the magnitude and characteristics of long-range FÀ ÀF couplings, nþ1 J FÀ ÀF in 1,n-difluoro-(alkanes, polyenes, cumulenes, and polyynes) (112). For J calculations in these kind of systems one should be very careful with the appearance of instabilities (or QIs).…”

Optimized basis sets for the calculation of indirect nuclear spin-spin coupling constants involving the atoms B, Al, Si, P, and Cl

“…PSO and SD, terms, which dominate the total coupling. This behavior is quite typical for couplings between atoms with more than one lone-pair 135,160,172, 215,216,244,245 . The PSO term is thus the largest contribution in ClF and is more than six times as large as the FC term.…”

Resolving an apparent discrepancy between theory and experiment: spin–spin coupling constants for FCCF

“…[4] The computed results prompted the authors of Ref. [2] to question the assignment of the experimental spectrum of FCCF, which yielded a value of 2.1 Hz for 3 J(F-F), [5] although the sign of this coupling constant was not determined. In an effort to resolve this discrepancy, we have reinvestigated spin-spin coupling in FCCF, with emphasis on 3 …”

“…These range from −85. 4 Hz from semiempirical calculations, [1] −43.7 [2] and −21.5 Hz [3] from second-order polarization propagator approximation (SOPPA) calculations, and −25.4 Hz from an earlier equation of motion coupled cluster singles and doubles (EOM-CCSD) calculation. [4] The computed results prompted the authors of Ref.…”