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Nuclear magnetic resonance spectrometers presently available are unable to recognize the two mirror-image forms of a chiral molecule, because in the absence of a chiral solvent, the NMR spectral parameters (chemical shifts and spin-spin coupling constants) are identical for the two enantiomers. This paper discusses how chirality may nevertheless, at least in theory, be recognized in liquid-state NMR spectroscopy by applying strong d.c. electric fields and measuring a pseudoscalar contribution to nuclear spin-spin coupling polarizability. Calculations are reported for medium-size chiral molecules, (2R)- N-methyloxaziridine, ( R)-1,3-dimethylallene, and (2R)-2-methyloxirane. The very small contributions provided by the pseudoscalar of nuclear spin-spin coupling polarizability seem rather difficult to detect via NMR experiments in disordered phase. [ABSTRACT FROM AUTHOR]

The formation of excimers and exciplexes is relevant to a number of photochemical and photophysical processes such as fluorescence quenching and exciton trapping. Theoretical evaluation of the properties and dynamics of such systems often relies on approximate methods to reduce computational time. In this work, we present an assessment of calculations of the excited states of pyrene–pyrene and pyrene–naphthalene pairs with RI-SOS-ADC(2) and the TD-DFT functionals CAM-B3LYP-D3 and ωB97-XD. We find that ADC(2) yields dissociation energies with relatively small errors for the pyrene–pyrene excimer in S1 and a consistent picture of the splitting of monomer La and Lb states into the La−, La+, Lb−, and Lb+ states as the rings approach. TD-DFT, on the other hand, predicts too large dissociation energies for the excimer and is unfit for the description of the pyrene–pyrene dimer. The pyrene–naphthalene exciplex is predicted by RI-SOS-ADC(2) to have a very small dissociation energy, comparable to that of the benzene–benzene excimer. [ABSTRACT FROM AUTHOR]

The basis set convergence of nuclear spin-spin coupling constants (SSCC) calculated at the coupled cluster singles and doubles level has been investigated for ten difficult molecules. Eight of the molecules contain fluorine atoms, and nine contain double or triple bonds. Results obtained using the ccJ-pVXZ, X = D,T,Q,5, hierarchy of basis sets of Benedikt et al. (J Chem Phys 129:064111, 2008), converge rather slowly towards the basis set limit, but fast convergence can be obtained by adding diffuse functions, in particular for couplings across several bonds. The pcJ-n basis sets of Jensen (J Chem Theory Comput 2:1360-1369, 2006) exhibit large contraction errors for one-bond couplings, but in their uncontracted form they perform better than the ccJ-pVXZ basis sets. For multi-bond couplings, however, diffuse functions should be used, and when including these the two hierarchies show similar performance. The ccJ-pVXZ basis sets with diffuse functions included (aug-ccJ-pVXZ) show consistent performance across all types of SSCCs, with the triple-zeta (X = T) basis set yielding sufficiently good results for most purposes. [ABSTRACT FROM AUTHOR]

The nuclear spin-spin coupling constants in a series of acetylene derivatives HC≡CXH3, where X is C, Si, Ge, Sn, Pb, have been calculated employing both coupled-cluster theory (CC) and density functional theory (DFT), the latter in nonrelativistic and relativistic four-component approach with different exchange-correlation functionals and basis sets. In addition, property derivatives with respect to molecular geometry parameters have been computed with nonrelativistic and relativistic Hamiltonians in order to evaluate the usefulness of the calculated nonrelativistic vibrational corrections. Generally, the CC method reproduces the experimental values somewhat better than DFT. In the case of the latter, the performance of B3LYP functional was the most satisfactory. The relativistic effects become noticeable for the couplings including heavy atom in tin- and lead-containing molecules. The calculations of the derivatives of the coupling constants indicate that these derivatives are even more sensitive to the relativistic effects than the corresponding coupling constants. [ABSTRACT FROM AUTHOR]