AIM/NBO Analysis of the Geminal Coupling Constants in the Stabilization of A-Type Dimeric Proanthocyanidin: Angular Dependence.

The angular dependence of the indirect short-range spin-spin coupling constants (SSCC), the geminal J ( C 3 , C 1 ' ) $$ J\left({C}_3,{C}_{1^{\prime }}\right) $$ , J ( O 1 , O ) $$ J\left({O}_1,O\right) $$ , and J ( O , C 1 ' ) $$ J\left(O,{C}_{1^{\prime }}\right) $$ in A-type dimeric proanthocyanidin, was investigated using density functional theory. We studied the rotation of ring B around the C 2 - C 1 ' $$ {C}_2\hbox{--} {C}_{1^{\prime }} $$ bond. Therefore, we calculated hyperconjugative charge transfers and bond polarizations within the natural bond orbital (NBO) approach, performing a topological study based on Bader's theory, AIM (atoms in molecules), and analyzing the angular dependence of AIM/NBO parameters. The results describe a relationship between the geminal coupling that changes with angular variation and NBO charge transfers to the bonds involved in the coupling pathways that can explain the behavior of the former property. Based on AIM/NBO data, inductive and mesomeric effects were described and quantified, showing a clear correlation with the stabilization of the structure, demonstrating a resonance-assisted inductive effect. We also set out strong hyperconjugative interactions (anomeric effect) involving nonbonding electron pairs of oxygen atoms. This analysis of coupling constants supports previous models by other authors and shows the application in this particular case. Moreover, the SSCCs studied herein are used for identifying stable structures and conformational search analysis of flavonoids. Finally, our results show the relationship between SSCCs and the structure stabilization and charge delocalization effects.

Molecular Oxygen Trimer: Multiplet Structures and Stability.

We present a detailed theoretical study of the molecular oxygen trimer where the potential energy surfaces of the seven multiplet states have been calculated by means of a pair approximation with very accurate dimer ab initio potentials. In order to obtain all the states a matrix representation of the potential using the uncoupled spin representation has been applied. The S = 0 ${S = 0}$ and S = 1 ${S = 1}$ states are nearly degenerate and low-lying isomers appear for most multiplicities. A crucial point in deciding the relative stabilities is the zero-point energy which represents a sizable fraction of the electronic well-depth. Therefore, we have performed accurate diffusion Monte Carlo studies of the lowest state in each multiplicity. Analysis of the wavefunction allows a deeper interpretation of the cluster structures, finding that they are significantly floppy in most cases.

The influence of relativistic effects on nuclear magnetic resonance spin-spin coupling constant polarizabilities of H2 O2 , H2 S2 , H2 Se2 , and H2 Te2.

Relativistic and nonrelativistic calculations have been performed on hydrogen peroxide, dihydrogen disulfide, dihydrogen diselenide, and dihydrogen ditelluride, H2 X2 (X = O, S, Se, Te), to investigate the consequences of relativistic effects on their structures as well as their nuclear magnetic resonance (NMR) spin-spin coupling constants and spin-spin coupling constant polarizabilites. The study has been performed using both one-component nonrelativistic and four-component relativistic calculations at the density functional theory (DFT) level with the B3LYP exchange-correlation functional. The calculation of nuclear spin-spin coupling constant polarizabilities has been performed by evaluating the components of the third order tensor, nuclear spin-spin coupling polarizability, using quadratic response theory. From this, the pseudoscalar associated with this tensor has also been calculated. The results show that relativistic corrections become very important for H2 Se2 and H2 Te2 and hint that a new chiral discrimination technique via NMR spectroscopy might be possible for molecules containing elements like Se or Te. © 2018 Wiley Periodicals, Inc.

Limiting Values of the one-bond C-H Spin-Spin Coupling Constants of the Imidazole Ring of Histidine at High-pH.

Assessment of the relative amounts of the forms of the imidazole ring of Histidine (His), namely the protonated (H+) and the tautomeric Nε2-H and Nδ1-H forms, respectively, is a challenging task in NMR spectroscopy. Indeed, their determination by direct observation of the 15N and 13C chemical shifts or the one-bond C-H, 1JCH, Spin-Spin Coupling Constants (SSCC) requires knowledge of the "canonical" limiting values of these forms in which each one is present to the extent of 100%. In particular, at high-pH, an accurate determination of these "canonical" limiting values, at which the tautomeric forms of His coexist, is an elusive problem in NMR spectroscopy. Among different NMR-based approaches to treat this problem, we focus here on the computation, at the DFT level of theory, of the high-pH limiting value for the 1JCH SSCC of the imidazole ring of His. Solvent effects were considered by using the polarizable continuum model approach. The results of this computation suggest, first, that the value of 1JCε1H = 205 ± 1.0 Hz should be adopted as the canonical high-pH limiting value for this SSCC; second, the variation of 1JCε1H SSCC during tautomeric changes is minor, i.e., within ±1Hz; and, finally, the value of 1JCδ2H SSCC upon tautomeric changes is large (15 Hz) indicating that, at high-pH or for non-protonated His at any pH, the tautomeric fractions of the imidazole ring of His can be predicted accurately as a function of the observed value of 1JCδ2H SSCC.

Investigation of the role of stereoelectronic effects in the conformation of piperidones by NMR spectroscopy and X-ray diffraction.

This paper reports the synthesis of a series of piperidones 1-8 by the Mannich reaction and analysis of their structures and conformations in solution by NMR and mass spectrometry. The six-membered rings in 2,4,6,8-tetraphenyl-3,7-diazabicyclo[3.3.1]nonan-9-ones, compounds 1 and 2, adopt a chair-boat conformation, while those in 2,4-diphenyl-3-azabicyclo[3.3.1]nonan-9-ones, compounds 3-8, adopt a chair-chair conformation because of stereoelectronic effects. These stereoelectronic effects were analyzed by the (1) J C-H coupling constants, which were measured in the (13)C satellites of the (1)H NMR spectra obtained with the hetero-dqf pulse sequence. In the solid state, these stereoelectronic effects were investigated by measurement of X-ray diffraction data, the molecular geometry (torsional bond angles and bond distances), and inter- and intramolecular interactions, and by natural bond orbital analysis, which was performed using density functional theory at the ωB97XD/6311++G(d,p) level. We found that one of the main factors influencing the conformational stability of 3-8 is the interaction between the lone-pair electrons of nitrogen and the antibonding sigma orbital of C(7)-Heq (nN→σ*C-H(7)eq), a type of hyperconjugative interaction.