Approximating correlation effects in multiconfigurational self-consistent field calculations of spin-spin coupling constants

Toward a Computational NMR Procedure for Modeling Dipeptide Side-Chain Conformation

Theoretical relationships between the vicinal spin–spin coupling constants (SSCCs) and the χ₁ torsion angles have been studied to predict the conformations of protein side chains. An efficient computational procedure is developed to obtain the conformation of dipeptides through theoretical and experimental SSCCs, Karplus equations, and quantum chemistry methods, and it is applied to three aliphatic hydrophobic residues (Val, Leu, and Ile). Three models are proposed: unimodal-static, trimodal-static-stepped, and trimodal-static-trigonal, where the most important factors are incorporated (coupled nuclei, nature and orientation of the substituents, and local geometric properties). Our results are validated by comparison with NMR and X-ray empirical data described in the literature, obtaining successful results on the 29 residues considered. Using out trimodal residue treatment, it is possible to detect and resolve residues with a simple conformation and those with two or three staggered conformers. In four residues, a deeper analysis explains that they do not have a unique conformation and that the population of each conformation plays an important role.

Natural bond orbital/natural J-coupling study of vicinal couplings

NBO-NJC decomposition of vicinal (3)J HH spin-spin coupling constants into Lewis, delocalization, and repolarization contributions are presented. A deep study allows to assign the main contributions to specific orbitals or electron delocalizations between two orbitals. (3)J HH torsional dependence and the substituent effect are analyzed according to the main orbital contributions for ethane and fluoroethane molecules using different basis sets. The torsional dependence for the energies corresponding to electron delocalization is also studied.

1JCH couplings in Group 14/IVA tetramethyls from the gas-phase NMR and DFT structural study: a search for the best computational protocol

The gas-phase ¹J_0,CHs in ‘isolated’ molecules of EMe₄ were determined and discussed in terms of their geometric/electronic properties obtained from DFT calculations.

H2O, H2, HF, F2 and F2O nuclear magnetic shielding constants and indirect nuclear spin-spin coupling constants (SSCCs) in the BHandH/pcJ-n and BHandH/XZP Kohn-Sham limits

Good performance of segmented contracted basis sets XZP, where X = D, T, Q and 5, for obtaining H(2)O, H(2), HF, F(2) and F(2)O nuclear isotropic shielding constants in the BHandH Kohn-Sham basis set limit was shown. The results of two- and three-parameter complete basis set limit extrapolation schemes were compared with experimental results, earlier literature data and benchmark ab initio results. Similar convergence patterns of shieldings obtained from calculations using general purpose XZP basis sets and from polarization-consistent basis sets pcS-n and pcJ-n, where n = 0, 1, 2, 3 and 4, designed to accurately predict magnetic properties were observed. On the contrary, the SSCCs were more sensitive to the XZP basis set size and generally less accurate than those estimated using pcJ-n basis set family. The BHandH density functional markedly outperforms B3LYP method in predicting heavy atom shieldings and SSCCs values in the studied systems.

Complete basis set prediction of methanol isotropic nuclear magnetic shieldings and indirect nuclear spin-spin coupling constants (SSCC) using polarization-consistent and XZP basis sets and B3LYP and BHandH density functionals

Efficient B3LYP and BHandH density functionals were used to estimate methanol's nuclear magnetic isotropic shieldings and spin-spin coupling constants in the basis set limit. Polarization-consistent pcS-n and pcJ-n (n = 0, 1, 2, 3 and 4), and segmented contracted XZP, where X = D, T, Q and 5, basis sets were used and the results fitted with simple mathematical formulas. The performance of the methods was assessed from comparison with experiment and higher level calculations. 1J(CH) and 3J(HH) values were determined from very diluted solutions in deuterochloroform and compared with theoretical predictions. The agreement between complete basis set (CBS) density functional theory (DFT) predicted isotropic shieldings and spin-spin values and experiment was good. The BHandH/pcS-n methanol shieldings obtained using structures optimized at the same level of theory are approaching the accuracy of the advanced coupled-cluster-singles-doubles-approximate triples (CCSD(T)) calculations.

Prediction of water's isotropic nuclear shieldings and indirect nuclear spin-spin coupling constants (SSCCs) using correlation-consistent and polarization-consistent basis sets in the Kohn-Sham basis set limit

Density functional theory (DFT) was used to estimate water's isotropic nuclear shieldings and indirect nuclear spin-spin coupling constants (SSCCs) in the Kohn-Sham (KS) complete basis set (CBS) limit. Correlation-consistent cc-pVxZ and cc-pCVxZ (x = D, T, Q, 5, and 6), and their modified versions (ccJ-pVxZ, unc-ccJ-pVxZ, and aug-cc-pVTZ-J) and polarization-consistent pc-n and pcJ-n (n = 0, 1, 2, 3, and 4) basis sets were used, and the results fitted with a simple mathematical formula. The performance of over 20 studied density functionals was assessed from comparison with the experiment. The agreement between the CBS DFT-predicted isotropic shieldings, spin-spin values, and the experimental values was good and similar for the modified correlation-consistent and polarization-consistent basis sets. The BHandH method predicted the most accurate (1)H, (17)O isotropic shieldings and (1)J(OH) coupling constant (deviations from experiment of about -0.2 and -1 ppm and 0.6 Hz, respectively). The performance of BHandH for predicting water isotropic shieldings and (1)J(OH) is similar to the more advanced methods, second-order polarization propagator approximation (SOPPA) and SOPPA(CCSD), in the basis set limit.