The nuclear shielding derivative and spin–spin coupling in nitrogen

On the performance of HRPA(D) for NMR spin-spin coupling constants: Smaller molecules, aromatic and fluoroaromatic compounds

In this study, the performance of the doubles-corrected higher random-phase approximation [HRPA(D)] has been investigated in calculations of nuclear magnetic resonance spin-spin coupling constants (SSCCs) for 58 molecules with the experimental values used as the reference values. HRPA(D) is an approximation to the second-order polarization propagator approximation (SOPPA) and is, therefore, computationally less expensive than SOPPA. HRPA(D) performs comparable and sometimes even better than SOPPA, and therefore, when calculating SSCCs, it should be considered as an alternative to SOPPA. Furthermore, it was investigated whether a coupled-cluster singles, doubles and perturbative triples [CCSD(T)] or Møller-Plesset second order (MP2) geometry optimization was optimal for a SOPPA and a HRPA(D) SSCC calculation for eight smaller molecules. CCSD(T) is the optimal geometry optimization for the SOPPA calculation, and MP2 was optimal for HRPA(D) SSCC calculations.

Deciphering the Mechanism of Base-Triggered Conversion of Ammonia to Molecular Nitrogen and Methylamine to Cyanide

We report an in-depth investigation into the ammonia oxidation mechanism by the catalyst [RuIII(tpy)(dmabpy)NH3]3+ ([Ru(NH3)]3+). Stoichiometric reactions of [Ru(NH3)]3+ were carried out with exogenous noncoordinating bases to trigger a proposed redox disproportionation reaction, which was followed using variable-temperature NMR spectroscopy. An intermediate species was identified as a dinitrogen-bridged complex using 15N NMR and Raman spectroscopy on isotopically labeled complexes. This intermediate is proposed to derive from coupling of nitridyl species formed upon sequential redox disproportion reactions. Acetonitrile displaces the dinitrogen bridge to yield free N2. DFT calculations support this lower-energy pathway versus that previously reported for ammonia oxidation by the parent [RuIII(tpy)(bpy)NH3]3+ complex. These experimental and computational results are consistent with the interpretation of redox disproportionation involving sequential hydrogen atom transfer reactions by an amide/aminyl intermediate, [Ru(NH2)-]+ ⇔ [Ru(NH2)•]+, formed upon deprotonation of the parent complex. Control experiments employing a large excess of ammonia as a base indicate this new proposed lower-energy pathway contributes to the oxidation of ammonia to dinitrogen in conditions relevant to electrocatalysis. In addition, analogous methylamine complexes, [Ru(NH2CH3)]2+/3+, were prepared to further test the proposed mechanism. Treating [Ru(NH2CH3)]3+ with a base cleanly yields two products [Ru(NH2CH3)]2+ and [Ru(CN)]+ in an ∼3:1 ratio, fully consistent with the proposed cascade of hydrogen atom transfer reactions by an intermediate.

The effect of weak intermolecular interactions on the nuclear magnetic resonance shielding constant in N2

Ab initio calculations are applied to examine the influence of the intermolecular interactions on the shielding constant in gaseous nitrogen. An accurate literature potential energy surface and the nuclear magnetic resonance shielding surface of the N₂ -N₂ complex calculated in this work provide results in satisfactory agreement with the available experimental estimates of the effect.

Catalytic Ammonia Oxidation to Dinitrogen by Hydrogen Atom Abstraction

Catalysts for the oxidation of NH₃ are critical for the utilization of NH₃ as a large-scale energy carrier. Molecular catalysts capable of oxidizing NH₃ to N₂ are rare. This report describes the use of [Cp*Ru(P^tBu ₂ N^Ph ₂ )(¹⁵ NH₃ )][BAr^F ₄ ], (P^tBu ₂ N^Ph ₂ =1,5-di(phenylaza)-3,7-di(tert-butylphospha)cyclooctane; Ar^F =3,5-(CF₃ )₂ C₆ H₃ ), to catalytically oxidize NH₃ to dinitrogen under ambient conditions. The cleavage of six N-H bonds and the formation of an N≡N bond was achieved by coupling H⁺ and e^- transfers as net hydrogen atom abstraction (HAA) steps using the 2,4,6-tri-tert-butylphenoxyl radical (^t Bu₃ ArO^. ) as the H atom acceptor. Employing an excess of ^t Bu₃ ArO^. under 1 atm of NH₃ gas at 23 °C resulted in up to ten turnovers. Nitrogen isotopic (¹⁵ N) labeling studies provide initial mechanistic information suggesting a monometallic pathway during the N⋅⋅⋅N bond-forming step in the catalytic cycle.

Cation dinitrogen complexes [N(2)...X...N(2)]+, X+=H+, Li+, Na+, Be(2+), Mg(2+)

The complexes of dinitrogen with five cations (H(+), Li(+), Na(+), Be(2+) and Mg(2+)) up to four N(2) molecules have been calculated at the MP2/6-311++G(d,p) level. Energetic and geometric aspects have been determined together with absolute shieldings (GIAO). The atoms in molecules methodology has been used to analyze energy, charge and volume of these complexes.

14N NMR spectra of molecular nitrogen in SDS/pentanol swollen lamellar phases

14N NMR spectra of air dissolved in lyotropic mesophases are reported. In order to observe the whole spectrum from a molecule in which the quadrupole coupling constant is on the order of a few megahertz, a weak alignment degree with respect to the magnetic field is mandatory. Therefore, dilute lyotropic liquid crystals, namely sodium dodecylsulphate (SDS)/pentanol swollen lamellar phases, were considered. The temperature dependence of the 14N quadrupolar splitting was followed both in the case of oil (either n-dodecane or n-heptane) and brine (a 0.2-M NaBr water solution) swelling. In the former, it paralleled the temperature dependence of the splittings of the alkane deuteria and, in both cases, it was opposite to 23Na quadrupolar splittings. Owing to the higher N2 solubility in hydrocarbons, the 14N NMR spectra provide complementary information to that obtained by means of the quadrupolar nuclei of water and hydrophilic solutes.

Spin–spin coupling constants in homonuclear polynitrogen species

Quantum chemical calculations provide new insights into the dependence of J(N,N) coupling tensors on bonding environment in a series of polynitrogen species including N(5)(+).