Functional dependence of 1J(XH) in XHn (X=C, N, P; n=4,3) with the number of excitations energies included in the calculus

M. N. C. ZARYCZ; P. F. PROVASI

Manaus

Congreso; Internacional Conference on Molecules, Polymers and Material Physics; 2014

Currently the calculation of indirect spin-spin coupling constants, J, is performed using linear response methods, which has the inconvenient of losing information of the contribution of each excited states to the value of J. An alternative is employing the approximation of sum over states, which is capable of providing it. And also would be able to provide the physical law of the dependence of J with the number of excitation energies.In this work was studied by the first time the function dependence of the coupling 1J(X-H), X = C, N, P and of the bonds and lone-pairs contributions to them, with the number of excitation energies included in the calculation and was found the minimum number of excited states needed to reach convergence in CH4, NH3 and PH3.The J-couplings have been obtained density functional theory (DFT/B3LYP) calculated as sum over states to interval of 5 excitation energies between them with Dalton Program. It has been used the aug-cc-pVTZ-J basis set and the Foster and Boys method for localizing molecular orbitals (LMO).For the studied cases it was found that the relationship is of  the form 1J(XH)=a+b tanh(cx), where x is the number of excitation energies, a, b and c  are parameters from the curve fitting. Considering a maximum tolerance of 10% for the estimated value of the J-couplings it is possible to infer the number of excitation energies necessary to reach convergence: 70% for CH4 y PH3 and 80 % for NH3. The sign of the analyzed constant is determined by the first few excitation energies. For all compounds the FC term is the most important and the responsible of the curve shape, therefore the study of the LMO contributions is limited to it. In the studied systems the bonds which link the coupled atoms produce the dominant contributions, which also determine the shape of the curve. The other contributions have the opposite sign which is larger for the lone pair of PH3 and NH3.