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Vibrational averaging of the chemical shift in crystalline α‐glycine
Author(s) -
Dračínský Martin,
Bouř Petr
Publication year - 2012
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.22940
Subject(s) - anharmonicity , molecular vibration , vibrational energy relaxation , chemistry , vibrational partition function , molecular dynamics , isotropy , density functional theory , normal mode , periodic boundary conditions , molecular physics , quantum , computational chemistry , hot band , boundary value problem , physics , molecule , vibration , quantum mechanics , organic chemistry
Averaging of the chemical shift over the molecular motion improves the simulated data and provides additional information about the temperature dependence and system dynamics. However, crystal modeling is difficult due to the limited precision of the plane‐wave density functional theory (DFT) methods and approximate vibrational schemes. On the glycine example, we investigate how the averaging can be achieved within the periodic boundary conditions at the DFT level. The nuclear motion is modeled with the vibrational configuration interaction, with other simplified quantum anharmonic schemes, and the classical Born–Oppenheimer molecular dynamics (BOMD). The results confirm a large vibrational contribution to the isotropic shielding values. Both the first and second derivatives of the shielding were found important for the quantum averaging. The first derivatives influence the shielding mostly due to the anharmonic character of the CH and NH stretching modes, whereas second derivatives produce most vibrational corrections associated with the lower‐frequency vibrational modes. Temperature excitations of the lowest‐frequency vibrational states and the expansion of the crystal cell both determine the temperature dependence of nuclear magnetic resonance parameters. The vibrational quantum approach as well as classical BOMD schemes provided temperature dependencies of the chemical shifts that are consistent with the previous experimental data. © 2012 Wiley Periodicals, Inc.