NMR spectroscopic parameters of heavy-atom cotaining molecules

GUSTAVO A. AUCAR

San Andrés

Congreso; QUITEL; 2009

Comité organizador

This presentation is based on polarization propagators. They are powerful theoretical tools that were first developed within the nonrelativistic regime and applied to calculate atomic and molecular properties more than 30 years ago1. Their recent relativistic generalization2 have shown that they can play an special role for describing the subtle and still deeper quantum origin of some molecular properties like the NMR spectroscopic parameters.3 Polarization propagators: i) have a definition that is formally the same for both the NR and relativistic regime; ii) can be obtained from QED; iii) have formal solutions of its equation of motion that can be expressed in a perturbational scheme so that they may be improved in a systematic and well-defined way; iv) were implemented at different levels of approach; v) possesses an NR limit which is naturally obtained from their relativistic expressions making c goes to infinity. Within the NR regime, calculations at second-order level of approach are between the most reliables.4 Within the relativistic regime there appears several new understandings on the electronic origin of magnetic properties. As an example, diamagnetism ?only? arise as a NR approximation. Dia and paramagnetic terms are then NR concepts that do not appear as such within the relativistic regime. When working properly within the relativistic regime one should not continues thinking in the same way as one was used to within its NR counterpart. In this presentation, starting from a sketchy QED-based formulation of NMR spectroscopic parameters and the need to introduce relativistic effects on heavy-atom contaning molecules, we will show: a) the electronic origin of their paramagnetic-like and diamagnetic-like contributions; b) the smooth behaviour of such terms that appears when c goes to infinity, matching in the limit with the usual NR contributions; c) the appearance of some new and important electronic mechanisms different from the usual SO; d) some benchmark calculations of J and σ for two or more than two heavy-atom-containing molecules. 1.J. Oddershede, Polarization propagator calculations. Adv Quantum Chem 11, 275, 1978. 2.G. A. Aucar and J. Oddershede, Relativistic theory for indirect nuclear spin-spin couplings within the polarization propagator approach. Int J QuantumChern 47, 425, 1993; G. A. Aucar, T. Saue, L. Visscher and H. J. Aa. Jensen, On the origin and contribution of the diamagnetic term in four-component relativistic calculations of magnetic properties. J. Chem. Phys. 110, 6808, 1999. 3.G. A. Aucar, Understanding NMR-J couplings by the theory of polarization propagators, Concepts in Magn. Reson. Part A 32A, 88, 2008. 4.J. E. del Bene, I. Alkorta and J. Elguero, A systematic comparison of SOPPA and EOM-CC J-couplings..., J. Chem. Theory and Comput. 4, 967, 2008; T. Helgaker, M. Jaszunski and M. Pecul, The quantum-chemical calculations of NMR J-couplings. Prog. in NMR Spectrosc. 53, 249, 2008.