Chemical Physics Letters
Volume 491, Issues 1-3, 7 May 2010, Pages 11-16
F–H···N hydrogen bonds: Influence of substituent and hybridization of nitrogen on H-bond properties and two-bond 19F–15N spin–spin coupling constants (2hJF–N)
Ali Ebrahimi, Mostafa Habibi-Khorassani, Masoome Doosti
The effects of substituent and hybridization of nitrogen atom on hydrogen bonding in the F–H···NCX, F–H···N(H)CX, and F–H···N(H)2–CX complexes have theoretically been studied by MP2 and DFT methods with aug-cc-pVDZ and 6-311++G** basis sets. With respect to the hybridization of nitrogen, sp3-hybridized nitrogen forms the strongest bond, followed by sp2 and then sp. In equilibrium structures, the trend in the two-bond 19F–15N spin–spin coupling constants (2hJF–N) is sp <>3 <>2. The results of atoms in molecules (AIM) and natural bond orbital (NBO) analyses are in meaningful relationships with other characteristics of hydrogen bonds, especially with the 2hJF–N values.
Intactness and spatial proximity of acid–base groups in bifunctional SBA-15 as revealed by solid-state NMR
Wanling Shen, Wujun Xu, Qiang Gao, Jun Xu, Hailu Zhang, Anmin Zheng, Yao Xu, Feng Deng
The intactness and spatial proximity of acid and base groups in bifunctional mesoporous SBA-15 has been studied by various NMR techniques. The NMR results show that Brønsted acid and Lewis base groups coexist peacefully on the surface of the same support and maintain their acidity and basicity, respectively. Meanwhile, the acid groups and the base groups are in close proximity with suitable distance.
Contributions from orbital–orbital interactions to nucleus-independent chemical shifts and their relation with aromaticity or antiaromaticity of conjugated molecules
Ignacio Pérez-Juste, Marcos Mandado, Luis Carballeira
The out-of-plane components of the nucleus-independent chemical shifts (NICS) for a group of aromatic and antiaromatic [n]-annulenes have been separated into several contributions on the basis of the gauge-including atomic orbital (GIAO) method. The analysis of the orbital interactions responsible of the NICS(π)zz values shows that the large and positive values found for antiaromatic compounds are due to the predominance of π → π * rotational transitions that overcome the diamagnetic and gauge contributions. However, NICS(π)zz for aromatic compounds are dominated by contributions arising from the gauge transformation and no significant contributions are found from occupied–unoccupied orbital mixing. This analysis has been compared with previous work about ring currents in the same compounds.
Homonuclear decoupled proton NMR spectra in modest to severe inhomogeneous fields via distant dipolar interactions
Yuqing Huang, Wen Zhang, Shuhui Cai, Jianhui Zhong, Zhong Chen
On the basis of distant dipolar interactions, two new pulse sequences were proposed to obtain homonuclear broadband-decoupled proton NMR spectra in modest to severe inhomogeneous fields with time efficient acquisitions. Theoretical expressions for signals were derived according to the distant dipolar field (DDF) treatment combined with the product operator formalism. The measurements under either moderate (0.4 ppm) or severe (7 ppm) field inhomogeneity in a 500 MHz spectrometer show that the new sequences are complementary to each other and provide an attractive way to eliminate the influences of field inhomogeneities on homonuclear decoupled proton spectra.
Importance of the hybrid orbital operator derivative term for the energy gradient in the fragment molecular orbital method
Takeshi Nagata, Dmitri G. Fedorov, Kazuo Kitaura
The hybrid orbital operator is crucial in the fragment molecular orbital (FMO) method for the fragmentation across covalent bonds, however, its gradients have not been properly derived. We show that these very substantial contributions are the cause of the major part of the gradient error in FMO, impeding geometry optimizations and molecular dynamics. Capped alanine decamer (ALA)10 and chignolin (PDB: 1UAO) solvated by 157 water molecules are used to assess the accuracy of the energy gradients, and the errors are reduced by approximately one order of magnitude.
The role of cation–π interactions in ethylenic complexes: A theoretical NMR study
Ali Ebrahimi, Mostafa Habibi Khorassani, Hamid Reza Masoodi
AbstractThe effect of cation–π interactions on some NMR data of ethylenic complexes has been investigated by B97-1, PBE1KCIS and MPWKCIS1K methods using 6-311++G(3df,3pd) basis set. Unlike 3JH–H(cis), the chemical shift of ethylenic hydrogen (δH) and increases by cation–π interaction. The changes of NMR data have been considered with regard to geometry and direct electronic effects. Also, the distance dependence of NMR parameters has been studied in these complexes.
Chemical Physics Letters
Volume 494, Issues 1-3, 9 July 2010, Pages 104-110
Improved resolution in dipolar NMR spectra using constant time evolution PISEMA experiment
T. Gopinath, Gianluigi Veglia
The atomic structure of small molecules and polypeptides can be attained from anisotropic NMR parameters such as dipolar couplings (DC) and chemical shifts (CS). Separated local field experiments resolve DC and CS correlations into two dimensions. However, crowded NMR spectra represent a significant obstacle for the complete resolution of these anisotropic parameters. Using the polarization inversion spin exchange at the magic angle (PISEMA) experiment as a foundation, we designed new pulse schemes that use a constant time evolution in the dipolar (indirect) dimension to measure DC and CS correlations at high resolution. We demonstrated this approach on a 4-pentyl-4′-cyanobiphenyl (5CB) liquid crystal sample, achieving a resolution enhancement ranging from 30% to 60% for the resonances in the dipolar dimension. These new experiments open the possibility of obtaining significant resolution enhancement for multidimensional NMR experiments carried out on oriented liquid crystalline samples as well as oriented membrane proteins.
Broadband heteronuclear dipolar recoupling without 1H decoupling in solid-state NMR using simple cross-polarization methods
Morten Bjerring, Anders Bodholt Nielsen, Zdenek Tosner, Niels Chr. Nielsen
Heteronuclear dipolar recoupling experiments without 1H decoupling based on simple cross polarization are introduced for applications in biological solid-state NMR. It is shown that standard or adiabatic variants of the cross-polarization experiment with irradiation on the low-γ (e.g., 13C,15N) spins even at modest spinning frequencies enable efficient band-selective or broadband dipolar recoupling without the need for intense 1H decoupling. This facilitates experiments on expensive isotope-labelled protein samples for which sample heating by intense 1H decoupling may lead to sample detoriation. The principle is demonstrated numerically and experimentally on uniformly 13C,15N-labelled samples of GB1 and fibrils of hIAPP , , , , , , , ,  and  from the human islet amyloid labelled on the FGAIL part.
Optimal control NMR differentiation between fast and slow sodium
Jae-Seung Lee, Ravinder R. Regatte, Alexej Jerschow
Sodium ions in tissues and organs may experience motion on a variety of timescales, leading to NMR relaxation effects with quadrupolar coupling as the primary mechanism. The various effects that this fluctuating interaction has on spin dynamics can be exploited for distinguishing slow sodium ions from fast ones. Techniques such as triple-quantum filtering have been used for this purpose in the past. In this work we present optimal pulses which significantly improve the selectivity towards slow-tumbling sodium. These pulses can also be modified for robustness against magnetic field inhomogeneities, and could hence also become useful as MRI contrast methods.
NMR relaxometry: Spin lattice relaxation times in the laboratory frame versus spin lattice relaxation times in the rotating frame
Emilie Steiner, Mehdi Yemloul, Laouès Guendouz, Sébastien Leclerc, Anthony Robert, Daniel Canet
Relaxometry dispersion curves display the spin lattice relaxation rate as a function of the measurement frequency. However, as far as proton NMR is considered, dispersion curves usually start around 5 kHz and thus miss the very low frequency region. This gap can be filled by the measurement of the spin–lattice relaxation rate in the rotating frame. The issue of connecting both relaxation rates is considered for two relaxation mechanisms: (i) randomly varying magnetic fields, (ii) dipolar interaction within a system of two equivalent spins. Appropriate data processing is presented and the random field mechanism turns out to be adequate.
Metal-alkyl species are formed on interaction of small alkanes with gallium oxide: Evidence from solid-state NMR
Anton A. Gabrienko, Sergei S. Arzumanov, Alexander V. Toktarev, Alexander G. Stepanov
Abstract13C CP MAS NMR analysis of the products of the interaction of methane, ethane and propane with α-Ga2O3 or Ga-modified zeolite BEA at 523–623 K shows that dissociative adsorption of C1–C3 alkanes on the surface of gallium oxide or Ga-modified zeolite BEA results to the formation of Ga-methyl, Ga-ethyl and Ga-propyl species. This observation allows one to conclude that Ga-alkyls, rather than earlier suggested alkoxy species, could be the intermediates in small alkane dehydrogenation and aromatization on these catalysts.
Chemical Physics Letters
Volume 496, Issues 1-3, 20 August 2010, Pages 162-166
A magic-angle turning NMR experiment for separating spinning sidebands of half-integer quadrupolar nuclei
Ivan Hung, Zhehong Gan
A two-dimensional magic-angle turning NMR experiment is described for separating spinning sidebands of half-integer quadrupolar nuclei. The experiment is an alternative to quadrupolar phase-adjusted sideband separation (QPASS) [Chem. Phys. Lett. 272 (1997) 295] that yields infinite speed spectra in one dimension and spinning sideband manifolds in the other dimension. A shear transformation is introduced for processing of quadrupolar magic-angle turning (QMAT) data with time evolution of only one rotor period. The QMAT experiment has the advantages of averaging the first-order modulation of the pulse sequence efficiencies giving smaller residual spinning sidebands, linearly incrementing pulse timings for convenient practical implementation, and easily adjustable pulse spacings suitable for higher spinning frequencies.
Enantiodiscrimination and extraction of short and long range homo- and hetero-nuclear residual dipolar couplings by a spin selective correlation experiment
Nilamoni Nath, N. Suryaprakash
AbstractA two dimensional correlation experiment for the measurement of short and long range homo- and hetero- nuclear residual dipolar couplings (RDCs) from the broad and featureless proton NMR spectra including 13C satellites is proposed. The method employs a single natural abundant 13C spin as a spy nucleus to probe all the coupled protons and permits the determination of RDCs of negligible strengths. The technique has been demonstrated for the study of organic chiral molecules aligned in chiral liquid crystal, where additional challenge is to unravel the overlapped spectrum of enantiomers. The significant advantage of the method is demonstrated in better chiral discrimination using homonuclear RDCs as additional parameters.
Chemical Physics Letters
Volume 496, Issues 1-3, 20 August 2010, Pages 201-207
Indirect high-resolution detection for quadrupolar spin-3/2 nuclei in dipolar HMQC solid-state NMR experiments
Julien Trébosc, Olivier Lafon, Bingwen Hu, Jean-Paul Amoureux
We present a new kind of NMR pulse sequences to observe heteronuclear correlation (HETCOR) specifically between spin-1/2 and spin-3/2 nuclei with isotropic resolution on the quadrupolar channel. These methods, called HMQC-ST, feature a STMAS filter during the evolution period of the HMQC scheme. Compared to existing HETCOR techniques involving quadrupolar nuclei, the HMQC-ST combines high-resolution and high efficiency and allows indirect detection of spin-3/2 nuclei via sensitive nuclei. We study analytically and using simulations how through-bond and through-space HMQC-ST perform compared to regular HMQC sequence. HMQC-ST potential is demonstrated experimentally by recording through-space HETCOR 2D spectra of Na2HPO4 and NaH2PO4.
Detection of magnetic environments in porous media by low-field 2D NMR relaxometry
Cinzia Casieri, Francesco De Luca, Luca Nodari, Umberto Russo, Camilla Terenzi
The 2D 1H NMR correlation maps of longitudinal (T1) and transverse (T2) relaxation times prove sensitive in monitoring the distribution of magnetic pore environments in porous systems. The comparison with Mössbauer data establishes a direct correspondence between the susceptibility-induced effects observed in the T1–T2 maps for pore–filling water and the Fe(III)-bearing magnetic compounds.
The quadrupole moment of the As nucleus from molecular microwave data and calculated relativistic electric field gradients
Lukáš Demovič, Vladimir Kellö, Andrzej J. Sadlej
AbstractThe ‘molecular’ value of the nuclear quadrupole moment of the 75As nucleus is determined combining the experimental data for quadrupole coupling constant and the calculated electric field gradient in the AsP molecule. The accurate calculations have been carried out at the CCSD(T) level. The relativistic effects were accounted for using the infinite-order two-component method in the scalar approximation. The new recommended value of the nuclear quadrupole moment of 75As obtained in this work is 311(2) mb and is more precise than the previous ‘muonic’ value.
Chemical Physics Letters
Volume 498, Issues 1-3, 30 September 2010, Pages 42-44
Determination of chemical shift of gas-phase hydrogen molecules by 1H nuclear magnetic resonance
Hirotada Fujiwara, Junichiro Yamabe, Shin Nishimura
The precise and detailed chemical shift of gas-phase hydrogen molecules was successfully determined by 1H nuclear magnetic resonance (NMR), avoiding the intervention of neighboring molecules such as in hydrogen occluding materials (free hydrogen). The measurement was conducted with double walled NMR sample tube taking into consideration the change of hydrogen pressure. The inner tube was filled with standard substance (DHO in D2O at 4.8 ppm). The chemical shift of free hydrogen molecules was determined to be 7.40 ± 0.01 ppm at 0.18 MPa, 25 °C, which is different from previously reported chemical shifts of hydrogen gas with intervention of neighboring molecules.
Homonuclear dipolar decoupling with very large scaling factors for high-resolution ultrafast magic angle spinning 1H solid-state NMR spectroscopy
Elodie Salager, Jean-Nicolas Dumez, Robin S. Stein, Stefan Steuernagel, Anne Lesage, Bénédicte Elena-Herrmann, Lyndon Emsley
We present a new phase modulated radio-frequency pulse sequence for homonuclear dipolar decoupling in proton solid-state NMR spectroscopy, eDUMBO-PLUS-1, with a chemical shift scaling factor of 0.73. This sequence was determined by screening random sequences, and experimentally optimizing the best candidates directly on 1H NMR spectra with 60 kHz magic angle spinning. It yields efficient decoupling with linewidths as little as 150 Hz for 1.3 mm MAS probes on different spectrometers. Experiments and calculations support the hypothesis of a radio-frequency and MAS joint averaging regime, in which the large scaling factor contributes significantly to the overall performance of the decoupling sequence.
Distinguishing hydrogen bonding networks in α-d-galactose using NMR experiments and first principles calculations
Mikhail Kibalchenko, Daniel Lee, Limin Shao, Mike C. Payne, Jeremy J. Titman, Jonathan R. Yates
First principles calculations and solid-state NMR experiments are used to distinguish between possible hydrogen bonding networks in α-d-galactose. In contrast to 13C, the 1H chemical shift parameters show differences which are sufficient to allow the correct network to be identified by comparison with experiments which make use of modern homonuclear decoupling schemes. In addition, clear linear correlations are established between both 1H chemical shift and chemical shift anisotropy, and hydrogen bond length.
Petr Štěpánek, Petr Bouř, Michal Straka
The 3He chemical shifts were calculated for Hen@C84 (n = 1, 2) fullerenes to obtain characteristic NMR patterns for distinguishing their isomers in a mixture. The density functional methods were calibrated on experimental data. Accuracy within 1 ppm could be reached without further fitting of individual shifts. Such precision allows for a semi-quantitative assignment of 3He NMR spectra. Additional criteria in the identification are discussed, such as the relative energies of the isomers, positions of the satellite di-helium peaks, and the differential 3He shifts in the fullerenes reduced to anions. Endohedral 3He shifts are predicted for so far experimentally unknown He@C84 and isomers.