Reactions Of Nb Research Articles

Nitrogen Activation and Transformation on Monometallic Niobium Boron Oxide Cluster Anions at Room Temperature: A Dual-Site Mechanism.

Dinitrogen activation and transformation at room temperature is a goal that has been long sought after. Despite that, it remains underdeveloped due to being a challenging research area and the need for a better mechanistic understanding. Herein, we report that well-defined NbB3O2- gas-phase clusters can activate one N2 molecule and generate the products B3N2O- and B3N2O2-, as applying mass spectrometry and theoretical calculations. This unusual N2 activation reaction results from the different functions of the Nb and B3O2 moieties in NbB3O2-. Theoretical calculations suggest that a catalytic cycle can be completed by the recovery of NbB3O2-, which is achieved through the reactions of Nb and NbO with B3O2- and B3O-, respectively. This is the first example of N2 efficient transformation at a monometallic cluster, and this method for generating dual active sites by designing proper ligands may open the way toward the development of more effective N2 fixation and functionalization methodologies.

Reactions of Nb2 and Nb3 with CO, D2, N2, and O2: Reconciling experimental kinetics with density functional theory-calculated reaction profiles

Calculated energy profiles for the reactions of neutral Nb(2) and Nb(3) metal clusters with CO, D(2), N(2), and O(2) are presented. In each reaction path, both a physisorption energy minimum, where the reactant remains intact, and a chemisorption energy minimum, where the reactant has dissociated, are calculated and linked by saddle points. We calculate branching ratios for the forward (dissociative) and reverse reactions which we compare with the experimental kinetic data. It is found that a combination of average thermal energies and barrier heights leads to wide variation in branching ratios which compares favourably to previously determined experimental reaction rates.

Investigation of spin-flip reactions of Nb + CH3CN by relativistic density functional theory

In order to explore the details of the reaction mechanisms of Nb atoms with acetonitrile molecules, the sextet, quartet, and doublet spin state potential energy surfaces have been investigated. Density functional theory (DFT) with the relativistic zero-order regular approximation at the PW91/TZ2P level has been applied. The complicated minimum energy reaction path involves four transition states (TS), stationary states (1) to (5) and two intersystem crossings from spin sextets to quartets to doublets (indicated by ⇒): (6)Nb + NCCH3→(6)Nb η(1)-NCCH3 ((6)1) →(6)TS1/2⇒(4)Nb η(2)-(NC)CH3 ((4)2) →(4)TS2/3→(4)NbH η(3)-(NCCH2) ((4)3) →(4)TS3/4→ CNNbCH3 ((4)4) ⇒(2)TS4/5→ CN(NbH)CH2 ((2)5). The minimum energy crossing points were determined with the help of the DFT fractional-occupation-number approach. The first spin inversion leads from the sextet to an energetically low intermediate quartet ((4)2) with final insertion of Nb into the C-C bond. The second one from the quartet to the doublet state facilitates the activation of a C-H bond, lowering the rearrangement-barrier by 44 kJ mol(-1). The overall reaction is calculated to be exothermic by about 170-180 kJ mol(-1). All intermediate and product species were frequency and NBO analyzed. The species can be rationalized with the help of Lewis type formulas.

New iso-propoxides, tert-butoxides and neo-pentoxides of niobium(v): Synthesis, structure, characterization and stabilization by trifluoroheteroarylalkenolates and pyridine ligands

The synthesis and characterization of nine new heteroleptic alkoxides of niobium is described. Metathesis reactions of Nb(2)Cl(10) with (t)BuCH(2)OH and pyridine (py) or 4-dimethylaminopyridine (DMAP) affords monomeric octahedral complexes Nb(OCH(2)(t)Bu)(5)py (1) and Nb(OCH(2)(t)Bu)(5)DMAP (2), respectively, in high yields (>60%). The same reaction with (t)BuOH resulted in a chloro functionalized alkoxide Nb(O(t)Bu)(4)pyCl (3) and could not be pushed to complete removal of remaining Cl(-) ligand. The introduction of a chelating bidental ligand 3,3,3-trifluoro-1-(pyridine-2-yl)propen-2-ol (2-PyCHCOHCF(3)) (4'') in the dimeric framework of Nb(2)(O(i)Pr)(10) (4') produced a heteroleptic, monomeric niobium complex Nb(O(i)Pr)(4)(2-PyCHCOCF(3)) (4) with significantly enhanced stability and volatility. As a comparison to (4), five different heteroaryl systems (5-9) with the same side chain have been synthesized and examined in order to understand the influence upon physio-chemical properties. All the new compounds (1-9) have been characterized by microanalysis, variable temperature multinuclear NMR spectroscopy, mass spectrometry, thermal analysis and single crystal X-ray diffraction studies ((3), (4) and (9)). The molecular structure of (3) revealed mononuclear species with Nb atoms present in the distorted octahedral environment of four (t)BuO, one chloride and one pyridine ligand. Compounds (4) and (9) consisting of four (i)PrO and a trifluoroheteroarylenolate exhibited a stronger distortion in the molecular geometry due to the rigidity of chelating β-alkenolate moiety.

Study on (n,t) Reactions of Zr, Nb and Ta Nuclei

The world faces serious energy shortages in the near future. To meet the world energy demand, the nuclear fusion with safety, environmentally acceptability and economic is the best suited. Fusion is attractive as an energy source because of the virtually inexhaustible supply of fuel, the promise of minimal adverse environmental impact, and its inherent safety. Fusion will not produce CO2 or SO2 and thus will not contribute to global warming or acid rain. Furthermore, there are not radioactive nuclear waste problems in the fusion reactors. Although there have been significant research and development studies on the inertial and magnetic fusion reactor technology, there is still a long way to go to penetrate commercial fusion reactors to the energy market. Because, tritium self-sufficiency must be maintained for a commercial power plant. For self-sustaining (D-T) fusion driver tritium breeding ratio should be greater than 1.05. And also, the success of fusion power system is dependent on performance of the first wall, blanket or divertor systems. So, the performance of structural materials for fusion power systems, understanding nuclear properties systematic and working out of (n,t) reaction cross sections are very important. Zirconium (Zr), Niobium (Nb) and Tantal (Ta) containing alloys are important structural materials for fusion reactors, accelerator-driven systems, and many other fields. In this study, (n,t) reactions for some structural fusion materials such as 88,90,92,94,96Zr, 93,94,95Nb and 179,181Ta have been investigated. The calculated results are discussed andcompared with the experimental data taken from the literature.

Theoretical study on the reactions of Nb and Nb+ with CO2 in gas phase

AbstractDensity functional theory calculations have been performed to explore the potential energy surfaces of CO bond activation in CO2 molecule by gas‐phase Nb atom and Nb+ cation for better understanding the reaction mechanism of second‐row metal with CO2. The minimum‐energy reaction path is found to involve the spin inversion in the different reaction steps. This potential energy curve‐crossing dramatically affects the reaction energetic. The present results show that the mechanism is insertion‐elimination mechanism along the CO bond activation reaction. All theoretical results not only support the existing conclusions inferred from early experiment but also complement the pathway and mechanism for this reaction. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Reactions of Ti, V, Nb and Ta with N<SUB>2</SUB> Gas at 2000°C under Concentrated Solar Beam in a Solar Furnace at PROMES-CNRS

Reactions of d-group transition metals including Ti, V, Nb and Ta with N2 gas were investigated at target temperature of 2000°C under heating with concentrated solar beam at PROMES-CNRS in Odeillo (France). In the typical experimental setup using graphite crucible as the sample holder, synthesis of pure nitride with fcc (face centred cubic) structure was proved to be difficult on account of inevitable C2 radical plume with high carbon activity a(C) from the graphite components and the majority of reaction products were carbo-nitride. By insertion of Sky blue filter in the solar beam path before entering into the parabolic concentrator, C2 radical formation appeared to be efficiently suppressed and phases with high N content were synthesized. However, under conditions in which C2 radical plume yield was considerable, nitrogen content in the reaction products became suppressed and tetragonal Nb4N3 yielded in place of fcc NbN from Nb or N-deficit sub-nitride Ta2N in place of mono-nitride TaN formed.

Reactions of Laser-Ablated Nb and Ta Atoms with N2: Experimental and Theoretical Study of M(NN)x (M = Nb, Ta; x = 1−4) in Solid Neon

Reactions of laser-ablated niobium and tantalum atoms with dinitrogen in solid neon have been investigated using matrix-isolation infrared spectroscopy. The Nb(NN)(x) and Ta(NN)(x) (x = 1-4) molecules formed during sample deposition or on annealing are the major products. The products are characterized on the basis of isotopic shifts, mixed isotopic splitting patterns, stepwise annealing, and change of reagent concentration and laser energy. Density functional theory calculations have been performed to understand the structures, ground electronic states, and bonding characteristics of niobium and tantalum dinitrogen complexes. The overall agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of these species from the matrix infrared spectra. The molecular orbital analyses and plausible reaction pathways for the formation of the products are also discussed.

Reaction Mechanism Studies on Atomic Layer Deposition of Nb2O5 from Nb(OEt)5 and Water

The reaction mechanism in the atomic layer deposition of Nb(2)O(5) from Nb(OEt)(5) and deuterated water was studied in situ with a quadrupole mass spectrometer (QMS) and a quartz crystal microbalance (QCM). The responses of these in situ measurement techniques to the characteristics of the ALD processes were thoroughly clarified and the process parameters carefully optimized. Also the effect of the reaction temperature on the extent of decomposition reactions of Nb(OEt)(5) interfering with the ALD process was investigated. Decomposition did occur at 400 degrees C but not at temperatures of 350 degrees C and below. Also the reaction mechanism was studied as a function of reaction temperature and found to be about the same until decomposition of the precursor started. Deuterated ethanol was found to be the most important gaseous byproduct of the ALD process but also some diethyl ether apparently formed. About one of the five ethoxide ligands of Nb(OEt)(5) was released during the Nb(OEt)(5) pulse and the rest during the D(2)O pulse. Finally separate experiments were performed to study the adsorption of ethanol as well as 2-propanol on the surface of Nb(2)O(5). Ethanol was found to adsorb. It could also be stated that water can replace the adsorbed ethanol. On the other hand 2-propanol apparently did not adsorb on the surface of Nb(2)O(5).

Infrared Spectra of the CH3−MX, CH2MHX, and CH⋮MH2X- Complexes Formed by Reaction of Methyl Halides with Laser-ablated Group 5 Metal Atoms

Reactions of group 5 metal atoms and methyl halides give carbon-metal single, double, and triple bonded complexes that are identified from matrix IR spectra and vibrational frequencies computed by DFT. Two different pairs of complexes are prepared in reactions of methyl fluoride with laser-ablated vanadium and tantalum atoms. The two vanadium complexes (CH(3)-VF and CH(2)=VHF) are persistently photoreversible and show a kinetic isotope effect on the yield of CD(2)=VDF. Identification of CH(2)=TaHF and CH[triple bond]TaH(2)F(-), along with the similar anionic Nb complex, suggests that the anionic methylidyne complex is a general property of the heavy group 5 metals. Reactions of Nb and Ta with CH(3)Cl and CH(3)Br have also been carried out to understand the ligand effects on the calculated structures and the vibrational characteristics. The methylidene complexes become more distorted with increasing halogen size, while the calculated C=M bond lengths and stretching frequencies decrease and increase, respectively. The anionic methylidyne complexes are less favored with increasing halogen size. Infrared spectra show a dramatic increase of the Ta methylidenes upon annealing, suggesting that the formation of CH(3)-TaX and its conversion to CH(2)=TaHX require essentially no activation energy.

Nickel-Catalyzed Cross-Coupling Reaction of Niobium(III)−Alkyne Complexes with Aryl Iodides

Nickel−catalyzed cross-coupling reactions of Nb(III)−alkyne complexes with aryl iodides are reported, in which addition of lithium alkoxide is indispensable and diarylated coupling products are afforded as products.

The niobium and tantalum riddle: gas-phase monocation reactions with pyrene and pyrene-D 10

Metal monocation gas-phase reactions with pyrene (Py) have been investigated. Of those cations which bind Py by abstraction of two hydrogen atoms, there are two subgroups of which Nb + and Ta + are the most striking and puzzling representatives. The Ta +-like group binds three Py units consecutively, each by expulsion of two hydrogen atoms, whereas the Nb + group binds four Py units; however, in this latter case, the second Py attachment occurs without the expulsion of any hydrogen. When perdeuterated Py-D 10 is used, the Ta + group behavior is unaffected, but the Nb + group now exhibits a considerable degree of Py attachment in which no deuterium atoms are lost in the first reaction step. We will attempt to explain this behavior by using the results of reactions of Nb + and Ta + with Py in a Fourier transform mass spectrometer (FTMS).

Reactions of homonuclear and heteronuclear group Vb clusters with ethylene: evidence for structural isomers

Reactions of Nb n + and Nb n−1 V + ( n=2–15) with C 2H 4 were investigated in the gas phase under single collision conditions using a Fourier-transform ion cyclotron resonance mass spectrometer. Biexponential decays deviating from simple pseudo-first-order kinetics were observed for Nb 10 +, Nb 9V + and Nb 12 +. They suggest the presence of at least two structural isomers of these clusters with one of these being unreactive. The abundance ratios of the reactive to the unreactive isomers for these three clusters can be determined from the kinetic fits to be 1/4, 2/1 and 1/8, respectively.

Activation of CH 4, C 2H 6, and C 3H 8 by gas-phase Nb + and the thermochemistry of Nb–ligand complexes

The kinetic energy dependence of the reactions of Nb + ( 5 D) with methane, ethane, and propane have been studied using guided ion beam mass spectrometry. It is found that dehydrogenation is efficient and the dominant process at low energies in all three reaction systems. At high energies, products resulting from both C–H and C–C cleavage processes are appreciable. The observation of dihydride and hydrido-methyl niobium cation products provides insight into the reaction mechanisms operating in these processes. The results for Nb + are compared with those for the first-row transition metal congener V + and the differences in behavior and mechanism discussed. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies (in electron volts) of D 0(Nb–H) > 2.3 ± 0.1, D 0(Nb +–2H) = 4.64 ± 0.06, D 0(Nb +–C) = 5.28 ± 0.15, D 0(Nb +–CH) = 6.02 ± 0.20, D 0(Nb +–CH 2) = 4.44 ± 0.09, D 0(Nb +–CH 3) = 2.06 ± 0.11, D 0[Nb +–(H)(CH 3)] = 4.78 ± 0.11, D 0(Nb +–C 2H) = 4.34 ± 0.19, D 0(Nb +–C 2H 2) = 2.90 ± 0.06, D 0(Nb +–C 2H 3) = 3.43 ± 0.21, D 0(Nb +–C 2H 4) = 2.8 ± 0.3, D 0(Nb +–C 2H 5) = 2.45 ± 0.12, D 0(Nb +–C 3H 2) = 5.25 ± 0.19, and lower limits for D 0(Nb +–C 3H 3) ≥ 3.76 ± 0.23 and D 0(Nb +–C 3H 5) ≥ 1.4 ± 0.1. The observation of exothermic processes sets lower limits for the bond energies of Nb + to propyne and propene of 2.84 and 1.22 eV, respectively.

The effect of D2/H2 substitution on the Nbn+ + H2 reaction

The effect of D2/H2 substitution on the reactions of niobium cluster cations Nb n + (n = 5 − 9) with hydrogen was studied using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer combined with a laser-ablation/supersonic-jet external ion source. For most cluster sizes, it has been found that the reactions of the Nb n + clusters with H2/D2 mixture yield considerable amounts of Nb n HD+ even at small conversion of Nb n + . These results suggest a mechanism which involves the dissociative chernisorption of H2 on the niobium cluster, followed by the competing stabilization and back dissociation of the energized adduct.

Competing reaction pathways from Y+C2H2 collisions

The crossed molecular beams method with 193 and 157 nm photoionization detection was used to study the competing reaction pathways resulting from collisions of ground state Y atoms with acetylene (C2H2). Three channels, corresponding to nonreactive decay of collision complexes, H2 elimination, and H atom elimination, were studied as a function of collision energy (〈Ecoll〉=6–25 kcal/mol). Production of YC2+H2 and decay of long-lived complexes back to reactants were observed at all collision energies studied. Product translational energy distributions for the H2 elimination channel demonstrate that a substantial fraction of excess energy available to the YC2+H2 products is channeled into relative translational energy. Analogous H2 elimination channels were studied in reactions of Zr and Nb with C2H2 at 〈Ecoll〉=6.0 kcal/mol. For these reactions, the H2 elimination product translational energy distributions were found to peak near zero kinetic energy, in contrast to the behavior observed for the YC2+H2 products. This suggests that a significant potential energy barrier exists in the exit channel of the YC2+H2 elimination step, whereas no exit channel barrier exists in forming ZrC2+H2 and NbC2+H2. The reformation of Y + C2H2 reactants following decay of long-lived collision complexes was found to transfer 40%–50% of the initial relative translational energy into C2H2 internal excitation. The YC2H+H product channel was only observed to occur above a collision energy threshold of 21.5±2.0 kcal/mol. Since YC2H+H production is fully spin-allowed and involves simple Y–H bond fission in the intermediate HYC2H complex, it is unlikely that any significant potential energy barrier is present in excess of the reaction endoergicity. Additional studies of Y+C2D2 reactions confirm that the observed collision energy threshold for the H or D atom loss channel corresponds to the energetic threshold for reaction, allowing determination of D0(Y–CCH)=110.2±2.0 kcal/mol.

Reaction Dynamics of Zr and Nb with Ethylene

The reactions of transition metal (M) atoms Zr and Nb with ethylene (C2H4) were studied using the technique of crossed molecular beams. Angular and velocity distributions of MC2H2 products following H2 elimination were measured at collision energies between 5 and 23 kcal/mol using electron impact and 157 nm photoionization mass spectrometry. Photodepletion studies identify that the atomic reactants are predominantly in their ground electronic states and that the observed MC2H2 products result primarily from reactions of these ground-state atoms. Center-of-mass product angular distributions derived from the data indicate that reactions involve the formation of intermediate complexes having lifetimes longer than their rotational periods. Product translational energy distributions demonstrate that a large fraction of excess available energy is channeled into product internal excitation. Wide-angle nonreactive scattering of metal atom reactants following decay of long-lived MC2H4 association complexes was also observed for both transition metal reactants at collision energies ≥ 9 kcal/mol, with approximately 36% of the initial translational energy converted into C2H4 internal excitation. At collision energies of ≤ 6 kcal/mol, nonreactive scattering of Zr from ZrC2H4 decay was found to be negligible, whereas this channel was clearly observed for Nb. RRKM modeling of the competition between decay of MC2H4 complexes back to M + C2H4 and C−H insertion forming HMC2H3 indicates that there exists an adiabatic potential energy barrier for M + C2H4 association in the case of Zr and that the transition state for this process is tighter than for the analogous process in Nb + C2H4. The barrier for Zr + C2H4 association is attributed to the repulsive s2 ground state configuration of Zr, whereas for Nb the s1 ground state configuration results in no barrier for association. The absence of decay of ZrC2H4 back to Zr + C2H4 at low collision energies indicates that the barrier for C−H insertion forming HZrC2H3 lies below the barrier for Zr + C2H4 association. This opens up the possibility that direct C−H insertion without initial ZrC2H4 formation may play an important role.

C–H bond activation versus ring cleavage in the gas phase ion-molecule reactions of Nb + and Ta + with toluene and picoline

The reactions of Nb + and Ta + with toluene and picoline and their deuterium-labelled analogues were studied in a Fourier transform ion cyclotron resonance mass spectrometer. Methyl substitution completely changes the reactivities relative to benzene and pyridine. Both metals react to dehydrogenate toluene exclusively. In contrast to benzene, no ring cleavage is observed in the Ta +/toluene reaction. A simple explanation for this difference in reactivities is proposed based on the relative energies of the Hückel orbitals of benzene and toluene. The ( b 1) symmetric antibonding orbital is higher in energy for toluene. Population of this orbital is necessary for formation of the metallanorbornadiene intermediate and does not occur at thermal energies. Reaction with ring labelled toluene- d 5 shows exclusive H 2 or D 2 elimination in reaction with Nb + and H 2, HD, and D 2 elimination in reaction with Ta +. Reactions with the picolines show both dehydrogenation and ring cleavage. Isotope labelling studies show facile H/D scrambling occurs in the intermediate ion-molecule complexes with HCN and DCN both eliminated from the methyl- d 3-2-picoline and 4-picoline. The metals react with picoline and pyridine by different mechanisms. The isotope labelling results suggest a metal-hydrido-azepinium structure for the intermediate complex.

Reactions of niobium cluster ions Nb x± (x = 2–16) with NO and NO 2

Gas phase niobium cluster ions, Nb x ± (x = 2–16), are subjected to reactions with NO and NO 2 in a fast-flow reactor. For cationic niobium clusters, the difference mass spectra for the reactions demonstrate that dissociative chemisorption and molecular adsorption take place in different cluster size regimes. Detailed analysis of the reaction products suggests that the cluster cations are fragmented into two parts in the small cluster range: the nitrogen retained clusters and the niobium oxide species. The relative reactivity of the cluster cations shows a decreasing trend with increasing cluster size. The reactions of the anionic niobium clusters with NO and NO 2 are also studied. The results are discussed in terms of electron transfer reactions. Cluster fragmentation is also extensively observed.

Gas phase activation of carbon dioxide by niobium and niobium monoxide cations

Guided ion beam mass spectrometry is used to investigate the kinetic energy dependence of the reactions of Nb + ( 5D) and NbO + ( 3Σ −) with CO 2, and the reverse pathways, NbO + ( 3Σ −) and NbO 2 + with CO. These systems exhibit complicated behavior because the ground states of the reactants and products have different spins. To further probe the potential energy surfaces for these reaction systems, NbO 2 + and the intermediates, ONb(CO) +, ONb(CO 2) +, and O 2Nb(CO) +, are studied by collisional activation experiments with Xe. Analysis of the reaction cross sections obtained in this study yield (in eV) D 0(Nb +–CO) = 0.99 ± 0.05, D 0(ONb +–CO) = 1.10 ± 0.05, D 0 (ONb +–CO 2) = 0.88 ± 0.03, D 0(O 2Nb +–CO) = 1.11 ± 0.05, and D 0(ONb +–O) = 5.71 ± 0.17. Speculative determinations of electronic excitation energies for two states each of NbO + and of NbO 2 + are also made. Combining the results obtained from this study and those obtained from the literature, we are able to generate a fairly complete potential energy surface for the Nb + reaction system. We also compare the reactivities of Nb +, NbO +, and NbO 2 + with respect to the interconversion of CO and CO 2.