DFT-D Study of 14N Nuclear Quadrupolar Interactions in Tetra-n-alkyl Ammonium Halide Crystals

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• 作者：Eddy Dib ; Bruno Alonso ; Tzonka Mineva
• 刊名：Journal of Physical Chemistry A
• 出版年：2014
• 出版时间：May 15, 2014
• 年：2014
• 卷：118
• 期：19
• 页码：3525-3533
• 全文大小：339K
• 年卷期：v.118,no.19(May 15, 2014)
• ISSN：1520-5215

文摘

The density functional theory-based method with periodic boundary conditions and addition of a pair-wised empirical correction for the London dispersion energy (DFT-D) was used to study the NMR quadrupolar interaction (coupling constant C_Q and asymmetry parameter 畏_Q) of ¹⁴N nuclei in a homologous series of tetra-n-alkylammonium halides (C_xH_2x+1)₄N⁺X^{鈥?/sup> (x = 1鈥?), (X = Br, I). These 14N quadrupolar properties are particularly challenging for the DFT-D computations because of their very high sensitivity to tiny geometrical changes, being negligible for other spectral property calculations as, for example, NMR 14N chemical shift. In addition, the polarization effect of the halide anions in the considered crystal mesophases combines with interactions of van der Waals type between cations and anions. Comparing experimental and theoretical results, the performance of PBE-D functional is preferred over that of B3LYP-D. The results demonstrated a good transferability of the empirical parameters in the London dispersion formula for crystals with two or more carbons per alkyl group in the cations, whereas the empirical corrections in the tetramethylammonium halides appeared to be inappropriate for the quadrupolar interaction calculation. This is attributed to the enhanced cation鈥揳nion attraction, which causes a strong polarization at the nitrogen site. Our results demonstrated that the 14N C_Q and 畏_Q are predominantly affected by the molecular structures of the cations, adapted to the symmetry of the anion arrangements. The long-range polarization effect of the surrounding anions at the target nitrogen site becomes more important for cells with lower spatial symmetry.}