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Artículo

NMR chemical shielding and spin-spin coupling constants of liquid NHȝ: a systematic investigation using the sequential QM/MM method

Gester, Rodrigo M.; Georg, Herbert C.; Canuto, Sylvio; Caputo, Maria CristinaIcon ; Provasi, Patricio FedericoIcon
Fecha de publicación: 09/2009
Editorial: American Chemical Society
Revista: Journal of Physical Chemistry A
ISSN: 1089-5639
e-ISSN: 1520-5215
Idioma: Inglés
Tipo de recurso: Artículo publicado
Clasificación temática:
Otras Ciencias Físicas

Resumen

The NMR spin coupling parameters, ¹J(N,H) and ²J(H,H), and the chemical shielding, σ(15N), of liquid ammonia are studied from a combined and sequential QM/MM methodology. Monte Carlo simulations are performed to generate statistically uncorrelated configurations that are submitted to density functional theory calculations. Two different Lennard−Jones potentials are used in the liquid simulations. Electronic polarization is included in these two potentials via an iterative procedure with and without geometry relaxation, and the influence on the calculated properties are analyzed. B3LYP/aug-cc-pVTZ-J calculations were used to compute the ¹J(N,H) constants in the interval of −67.8 to −63.9 Hz, depending on the theoretical model used. These can be compared with the experimental results of −61.6 Hz. For the ²J(H,H) coupling the theoretical results vary between −10.6 to −13.01 Hz. The indirect experimental result derived from partially deuterated liquid is −11.1 Hz. Inclusion of explicit hydrogen bonded molecules gives a small but important contribution. The vapor-to-liquid shifts are also considered. This shift is calculated to be negligible for ¹J(N,H) in agreement with experiment. This is rationalized as a cancellation of the geometry relaxation and pure solvent effects. For the chemical shielding, σ(15N) calculations at the B3LYP/aug-pcS-3 show that the vapor-to-liquid chemical shift requires the explicit use of solvent molecules. Considering only one ammonia molecule in an electrostatic embedding gives a wrong sign for the chemical shift that is corrected only with the use of explicit additional molecules. The best result calculated for the vapor to liquid chemical shift Δσ(15N) is −25.2 ppm, in good agreement with the experimental value of −22.6 ppm.
Palabras clave: Solvent Effect , Chemical Shielding , Spin Spin Coupling Constant
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info:eu-repo/semantics/openAccess Excepto donde se diga explícitamente, este item se publica bajo la siguiente descripción: Creative Commons Attribution-NonCommercial-ShareAlike 2.5 Unported (CC BY-NC-SA 2.5)
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URI: http://hdl.handle.net/11336/24732
URL: http://pubs.acs.org/doi/abs/10.1021/jp9050484
DOI: http://dx.doi.org/10.1021/jp9050484
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Articulos(IMIT)
Articulos de INST.DE MODELADO E INNOVACION TECNOLOGICA
Citación
Gester, Rodrigo M.; Georg, Herbert C.; Canuto, Sylvio; Caputo, Maria Cristina; Provasi, Patricio Federico; NMR chemical shielding and spin-spin coupling constants of liquid NHȝ: a systematic investigation using the sequential QM/MM method; American Chemical Society; Journal of Physical Chemistry A; 113; 52; 9-2009; 14936-14942
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