The Anomalous Deuterium Isotope Effect in the NMR Spectrum of Methane: An Analysis in Localized Molecular Orbitals

Abstract

Isotope effects [1] on NMR indirect nuclear spin-spin coupling constants (SSCC) are less frequently studied than the corresponding effects on chemical shifts. Nevertheless, unusual effects have been observed for them and still lack a satisfying explanation. Raynes and co-workers [2] measured the temperature dependence of the SSCC of the isotopomers of methane and found it puzzling that the reduced one-bond carbon-deuterium SSCC is larger than the carbon-proton coupling constant for all mixed isotopomers. This implies that the secondary isotope effect [Eq. (1)] on the carbon-hydrogen SSCC

Communication: Localized molecular orbital analysis of the effect of electron correlation on the anomalous isotope effect in the NMR spin-spin coupling constant in methane

“…In addition to the decomposition at the equilibrium geometry also the changes in each LMO contribution on extending or contracting the bond between the central atom X and the first of the hydrogen atoms, H1, are shown. 16,[25][26][27] In the CHF calculations the change in K(C-H2) is about 40% larger than at the correlated levels, but only about 53% (on contracting) or 68% (on extending) of the change in K(C-H1). In the case of NH 3 in Table III, methods.…”

“…Furthermore, from Table II one can see that the largest change on extending or contracting the C-H1 bond happens for the contribution from the bonding orbital between these atoms, σ (C−H1) , and not for any of the other bond contributions, σ (C−H2/3/4) , implying that at the level of the individual localized orbital contributions there is no unexpected sensitivity of the individual orbital contributions, i.e., orbital UDS, as previously pointed out. [25][26][27] Comparing the LMO contributions in the three correlated calculations for both CH 4 and NH 3 , one can see that with a few exceptions DFT/B3LYP gives the largest (in absolute values) LMO contributions and changes in these contributions followed by SOPPA while SOPPA(CCSD) typically gives the smallest LMO contributions.…”

“…22 Based on the comparison of molecules with and without UDS it was proposed that the absence of lone pairs would be a prerequisite for the UDS. 23,24 Later, through an analysis of the one-bond couplings in CH 4 , NH 3 , and H 2 O in terms of contributions from localized occupied molecular orbitals (LMO), carried out at the coupled perturbed density functional theory (DFT) level with the B3LYP functional, Provasi and Sauer [25][26][27] could show that the lone pairs are at least not directly responsible for the lack of UDS in neither NH 3 nor H 2 O, as removing the lone-pair contributions to the couplings in these two molecules did not provoke an UDS. On the other hand, it turned out to be a subtle balance in the sensitivity of the bond-core, bond-other bond, and other bond-other bond contributions to changes in the bond length between the coupled atoms, R own , which is responsible for the appearance of the UDS in methane and its absence in ammonia or water.…”

“…[25][26][27] Hence the present work is devoted to an analysis of the effect of electron correlation on each individual localized occupied molecular contribution to the onebond coupling constants in CH 4 and NH 3 at the CHF and SOPPA(CCSD) levels, which will shed light on the failure of the CHF calculations.…”

Investigation of the resonance‐assisted hydrogen bond in modelβ‐diketones through localized molecular orbital analysis of the spin–spin coupling constants related to the O–H · · · O hydrogen bond

“…To obtain the contribution of each occupied LMO to the spin–spin coupling constants, equation 9 of the work of Zarycz and Aucar was applied. As described in this reference, the procedure differs from that implemented by Sauer and Provasi within Hartree–Fock and DFT . Hence, the calculations using LMOs in the present work give only approximated results.…”