A comparison of quantum chemical models for calculating NMR shielding parameters in peptides: Mixed basis set and ONIOM methods combined with a complete basis set extrapolation

Abstract

This article compares several quantum mechanical approaches to the computation of chemical shielding tensors in peptide fragments. First, we describe the effects of basis set quality up to the complete basis set (CBS) limit and level of theory (HF, MP2, and DFT) for four different atoms in trans N-methylacetamide. For both isotropic shielding and shielding anisotropy, the MP2 results in the CBS limit show the best agreement with experiment. The HF values show quite a different tendency to MP2, and even in the CBS limit they are far from experiment for not only the isotropic shielding of carbonyl carbon but also most shielding anisotropies. In most cases, the DFT values differ systematically from MP2, and small basis-set (double- or triple-zeta) results are often fortuitously in better agreement with the experiment than the CBS ones. Second, we compare the mixed basis set and ONIOM methods, combined with CBS extrapolation, for chemical shielding calculations at a DFT level using various model peptides. From the results, it is shown that the mixed basis set method provides better results than ONIOM, compared to CBS calculations using the nonpartitioned full systems. The information studied here will be useful in guiding the selection of proper quantum chemical models, which are in a tradeoff between accuracy and cost, for shielding studies of peptides and proteins.

DFT RX3LYP and RPBEPBE studies on the structural, electronic, and vibrational properties of some amino-alcohol ligands

“…These two functionals were employed in conjunction with three basis sets, 6-31G(d,p), 6-31+G(d,p) and 6-311++G(d,p), to investigate the equilibrium bond lengths and valence bond angles, the topology of the electron density and the vibrational properties of the three amino-alcohol ligands, BHEEN, Cyp 2 -EN, and Cy 2 -EN (Scheme 1). It is known that both standard and hybrid exchange-correlation functionals can be used for modeling the structures and properties of chemical systems with acceptable accuracy [30][31][32][33][34] and the B3LYP hybrid functional is one of the many such functionals which has been extensively applied. It has been suggested, however, that the B3LYP functional is inadequate for the description of weak interactions in chemical systems (and this is what we expected to be present in our ligands) because of the lack of electron-electron correlation [35][36][37].…”

Complete basis set B3LYP NMR calculations of CDCl3 solvent's water fine spectral details

“…[8,11 -14] The best absolute NMR shielding parameters are obtained from fitting the results of systematic ab initio calculations with correlation-consistent [15,16] or polarization-consistent [17,18] basis sets toward the basis set limit. [13,14,19,20] Such results do not depend on accidental error cancellation and the use of reference molecule. However, the use of correlation-consistent basis sets for complete basis set (CBS) approximation to the Hartree-Fock or Kohn-Sham limit at self consistent field (SCF) and/or density functional theory (DFT) level is not widely recognized.…”

Calculations of gas phase 1H NMR chemical shifts of alcohols: An approach to optimize basis functions using factorial design

“…To reduce the rms and avoid using different methods, we have used the hybrid method of ONIOM for calculating the chemical shifts of these hydrogens (32,33).…”