Understanding the Dielectric Properties of Liquid Amides from a Polarizable Force Field

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• 作者：Edward Harder ; Victor M. Anisimov ; Troy Whitfield ; Alexander D. MacKerell ; Jr. ; Beno&icirc ; t Roux
• 刊名：Journal of Physical Chemistry B
• 出版年：2008
• 出版时间：March 20, 2008
• 年：2008
• 卷：112
• 期：11
• 页码：3509 - 3521
• 全文大小：286K
• 年卷期：v.112,no.11(March 20, 2008)
• ISSN：1520-5207

文摘

The role played by electronic polarization in the dielectric properties of liquid N-methyl acetamide (NMA)is examined using molecular dynamics simulations with a polarizable force field based on classical Drudeoscillators. The model presented is the first force field shown to reproduce the anomalously large dielectricconstant of liquid NMA. Details of the molecular polarizability are found to be important. For instance, allelements of the polarizability tensor, rather then just the trace, impact on the condensed phase properties.Two factors related to electronic polarizability are found to contribute to this large dielectric constant. Firstis the significant enhancement of the mean amide molecular dipole magnitude, which is 50% larger in theliquid than in the gas phase. Second is the consequent strong hydrogen bonding between molecular neighborsthat enhances the orientational alignment of the molecular dipoles. Polarizable models of amide compoundsthat have two (acetamide) and zero (N,N-dimethyl acetamide) polar hydrogen-bond donor atoms are alsoinvestigated. Experimentally, the neat liquid dielectric constants at 373 K are 100 for NMA, 66 for acetamideand 26 for N,N-dimethyl acetamide. The polarizable models replicate this trend, predicting a dielectric constantof 92 ± 5 for NMA, 66 ± 3 for acetamide and 23 ± 1 for N,N-dimethyl acetamide.