A systematic comparison of second-order polarization propagator approximation (SOPPA) and equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin-spin coupling constants for selected singly bonded molecules, and the hydrides NH3, H2O, and HF and their protonated and deprotonated ions and hydrogen-bonded complexes
Second-order polarization propagator approximation (SOPPA) and equation-of-motion coupled cluster singles and doubles (EOM−CCSD) methods have been employed for the calculation of one-bond spin−spin coupling constants in series of small molecules and ions, and of one- and two-bond coupling constants across X−H···Y hydrogen bonds. For isolated molecules, one-bond SOPPA coupling constants 1J(X-Y) involving 13C, 15N, 17O, and 19F have larger absolute values than corresponding EOM−CCSD coupling constants, with the EOM−CCSD values being in significantly better agreement with available experimental data. The difference between SOPPA and EOM−CCSD tends to increase as the number of nonbonding electrons on the coupled atoms increases, and the SOPPA values for O−F coupling are significantly in error. Similarly, the absolute values of SOPPA one-bond coupling constants1J(X−H) for the hydrides NH3, H2O, and FH and their protonated and deprotonated ions are greater than EOM−CCSD values, with the largest differences occurring for F−H coupling. One- and two-bond coupling constants 1J(X-H), 1hJ(H−Y), and 2hJ(X−Y) across X−H···Y hydrogen bonds in neutral, protonated, and deprotonated complexes formed from the hydrides are similar at SOPPA and EOM−CCSD, with the largest differences again found for 1J(F−H) in complexes with F−H as the proton donor, and 2hJ(F−F) for (FHF)−. The signs of 1J(X−H), 1hJ(H−Y), and 2hJ(X−Y) are the same at both levels of theory, as is their variation across the proton-transfer coordinate in F−H···NH3. SOPPA would appear to provide a reliable and more cost-effective alternative approach for computing coupling constants across hydrogen bonds, although couplings involving F may be problematic.