NN Bond Research Articles

Ab initio and DFT conformational study on N-nitrosodiethylamine, (C2H5)2N-N=O

Ab initio (MP2) and DFT (B3LYP) calculations, using the cc-pVTZ and aug-cc-pVTZ basis sets, have been performed to characterize some stationary points on the ground state potential energy surface of the title molecules. Several properties as, for instance, relative energies, the barriers for NO rotation around the NN bond, NBO charges on O and amino N atoms, as well as the dipole moments, have been calculated and analyzed in the light of the structures found. Both computational levels here employed yield three minima, in which the C(2)NNO frame is 'planar' or 'quasi-planar'. Important correlations between NBO charges and geometric parameters, as well as between some structural features and dipole moments are also discussed. A total of 17 structures have been found for the (C(2)H(5))(2)N-N=O molecule. Two ranges of values have been obtained for the dipole moment, with the largest values occurring for the structures in which the nitrogen lone pair is parallel to the NO group π system. For instance, these two ranges are from ~4.1 to 4.5 D, and from ~1.6 to 2.1 D, at the MP2/cc-pVTZ level. These ranges are consistent with a larger and a smaller contribution of a dipolar resonance structure, respectively. As the method or basis set changes the values of the dipole moments change by at most ~0.23 D.

N2 Activation in NiI–NN–NiI Units: The Influence of Alkali Metal Cations and CO Reactivity

AbstractAfter single electron reduction of the dinitrogen complex [LtBuNi(μ‐η1:η1‐N2)NiLtBu] (I) with KC8, reaction of the resulting compound K[LtBuNi(μ‐η1:η1‐N2)NiLtBu] (II) with sodium sand yields KNa[LtBuNi(μ‐η1:η1‐N2)NiLtBu] (1), which contains two different alkali metal ions. Treatment of I with two equivalents of sodium sand leads to the symmetric complex Na2[LtBuNi(μ‐η1:η1‐N2)NiLtBu] (2). Complexes 1 and 2 were investigated by single crystal X‐ray diffraction analysis as well as by Raman spectroscopy, and the results were compared with the data of K2[LtBuNi(μ‐η1:η1‐N2)NiLtBu] (III), which contains two K+ ions. Thus, it became obvious that the nature of the alkali metal ion M in compounds M2[LtBuNi(μ‐η1:η1‐N2)NiLtBu] has hardly any influence on the degree of NN bond activation. Furthermore, it was shown that treatment of the dinickel(I) complex III with CO leads to the dinickel(0) compound K2[LtBuNi(CO)]2 (4) and N2. Reaction of the unreduced dinickel(I) complex I with CO leads to a more simple replacement of the N2 ligand and formation of [LtBuNi(CO)] (3).

Photodissociation of N2O: Potential energy surfaces and absorption spectrum

The ultraviolet photodissociation of N(2)O is studied by wave packet calculations using global three-dimensional potential energy surfaces for the two lowest (1)A' states. The incorporation of all internal degrees of freedom in the dynamics calculations is essential for a realistic treatment. The room-temperature absorption cross section is well reproduced, including the weak vibrational structures. Classical periodic orbits show that the latter can be attributed to large-amplitude NN stretch motion combined with strong excitation of the bend. Weakening of the NN bond toward the N + NO channel is the necessary prerequisite. The temperature dependence of the calculated cross section is significant, as expected for a dipole-forbidden transition of a linear molecule; but it is not as strong as observed experimentally [G. S. Selwyn and H. S. Johnston, J. Chem. Phys. 74, 3791 (1981)]. This shortcoming is due to an apparent underestimation of the (0,1,0) hot band absorption. On the other hand, the calculations yield reasonable predictions of the ratios of bending-state resolved absorption cross sections, σ(0, 1, 0)∕σ(0, 0, 0) and σ(0, 2, 0)∕σ(0, 0, 0), measured at 204 nm [H. Kawamata et al. J. Chem. Phys. 125, 133312 (2006)].

Density functional theory study on the structure and properties of (HMgN3)n(n=15) clusters

We optimize the possible geometrical cluster structures and predicte relative stability of (HMgN3)n(n=15) by using the hybrid density functional theory (B3LYP) with 6-311G* basis sets. And the most stable isomers of (HMgN3)n(n=15) clusters, the bond properties, charge distributions, vibrational properties, and stability are analyzed theoretically. The calculated results show that the most stable HMgN3 has a linear structure, the (HMgN)n(n=2,5) clusters have the most stable structures in which an N atom in a sub-system and metal atom in another sub-system constitute an MgNMg structure. And the most stable structures of (HMgN3)n(n=3,4) clusters are the chain structures in which the nitrogen cardinal extremity position N atom and the Mg atom form a ring structure; the metal Mg atoms in the most stable structure show charge positivity, and H atom show charge negativity. The middle N atoms of azido show charge positivity, the N atoms on both sides of azido show charge negativity; what's more, the N atoms influenced by Mg atoms directly show a more charge negativity. MgN bond and MgH bond are the typical ionic bond; the bond between N atoms in azido is the covalent bond. The infrared spectra of the most optimized (HMgN3)n(n=15) clusters have three vibrational sections, the strongest vibrational peak lies in 22582347 cm-1, and the vibrational mode is anti-symmetric stretching vibration of NN bonds in azido. Analysis of stability shows that (HMgN3)3 clusters are more stable than other clusters.

Comparative Studies of thetrans−cisPhotoisomerizations of Azobenzene and a Bridged Azobenzene

Using density-functional-based molecular dynamics simulations, we have performed comparative studies of the trans-cis isomerizations of azobenzene and bridged azobenzene (B-Ab) 5,6-dihydrodibenzo[c,g][1,2]diazocine induced by nπ* electronic excitation. The quantum yields found in our calculations, 45% for the bridged azobenzene versus 25% for azobenzene, are consistent with the experiment. Both isomerization processes involve two steps: (1) Starting from the trans structure, each molecule moves on its S(1) excited-state potential energy surface, via rotation around the NN bond, to an avoided crossing near the S(1)/S(0) conical intersection, where de-excitation occurs. (2) Subsequently, in the electronic ground state, there is further rotation around the NN bond, accompanied by twisting of the phenyl rings around their CN bonds, until the cis geometry is achieved. Because of its lower symmetry and smaller initial CNNC dihedral angle, the bridged azobenzene has a much shorter lifetime for the S(1) excited state, about 30 fs, as compared to about 400 fs for azobenzene. However, we find that the complete isomerizations have approximately the same time scales. Although the bridging feature in trans-B-Ab does not hinder rotation around the NN bond in step 1, it makes twisting of the two phenyl rings around the CN bonds much slower in step 2.

Aromaticity and Relative Stabilities of Azines

The most refined nucleus-independent chemical shift index (NICS(0)(πzz)) and the extra cyclic resonance energies (ECREs), based on the block localized wave function (BLW) method, show that the aromaticity of all azines is like that of benzene. The same is true for aza-naphthalenes relative to naphthalene. The lower relative energies of isomers with vicinal N's are due to the weakness of NN bonds rather than to reduced aromaticity.

Sigma- and pi- electron structure of aza-azoles

The reasons behind changes of aromaticity in 10 unsubstituted aza-azoles were analysed by employing the natural bond orbital (NBO) approach at the MP2/6-311+G(d,p) level of theory. Sum of occupations of p(z) orbitals at atoms in the ring correlates well with the magnetism based aromaticity index NICS as well as with the number of nitrogen atoms in the ring. Changes of NICS depend strongly in a linear way on the number of NN bonds. Classification of azoles based on the number of pyridine-type nitrogen atoms vicinal to NH is supported by plotting the relative occupation of π orbitals (π(occ)) against the relative occupation of σ orbitals (σ(occ)) for all individual atoms in rings.

Communication: Photodissociation of N2O—Frustrated NN bond breaking causes diffuse vibrational structures

The photodissociation of N(2)O is studied by wave packet calculations using a global three-dimensional potential energy surface for the first excited (1)A(') state. It is shown that the weak vibrational structures of the absorption cross section are caused by large-amplitude NN stretch motion, combined with strong excitation of the bend as well as the O-NN stretch. Weakening of the NN bond toward the N+NO channel is the necessary prerequisite.

Ab initio kinetics for the decomposition of monomethylhydrazine (CH3NHNH2)

The decomposition kinetics of CH3NHNH2 (monomethylhydrazine) is studied with ab initio transition state theory-based master equation analyses. The simple NN and CN bond fissions to produce the radicals CH3NH+NH2 or CH3+NHNH2 are expected to dominate the decomposition kinetics. The transition states for these two bond fissions are studied with variable reaction coordinate transition state theory employing directly determined CASPT2/aug-cc-pVDZ interaction energies. Orientation independent corrections for limitations in the basis set and for the effects of conserved mode geometry relaxation are included. The bond dissociation energies are evaluated at the QCISD(T)/CBS//B3LYP/6-311++G(d,p) level. The transition state theory analysis directly provides high pressure dissociation and recombination rate coefficients. Predictions for the pressure dependence and product branching in the dissociation of CH3NHNH2 are obtained by solving the master equation.

Nonadiabatic Hybrid Quantum and Molecular Mechanic Simulations of Azobenzene Photoswitching in Bulk Liquid Environment

A nonadiabatic hybrid quantum and molecular mechanical (na-QM/MM) molecular dynamics scheme has been implemented recently combining the nonadiabatic Car-Parrinello molecular dynamics method by Doltsinis and Marx [Phys. Rev. Lett. 2002, 88, 166402] with the QM/MM coupling approach by Laio et al. [J. Chem. Phys. 2002, 116, 6941]. Here an extensive validation of the underlying, density functional theory based, electronic structure methods by comparison to CASPT2 ab initio data is presented for the case of azobenzene. The "on the fly" na-QM/MM method is then applied to study Z-->E and E-->Z photoisomerization of azobenzene in a bulk liquid environment. The isomerization mechanism is found to be a pedal motion of the central CN horizontal lineNC group in both cases. While the Z-->E reaction is barely affected by the environment, E-->Z photoisomerization is slowed down considerably in the liquid compared to the gas phase. This effect is due to the fact that reorientation of the phenyl rings is significantly hindered in the liquid by steric nearest neighbor interactions. Nonradiative decay is found to be substantially faster for Z-AB (subpicosecond regime) than for E-AB (picosecond regime). The main molecular motions responsible for nonadiabatic coupling have been identified as the oscillations in the NN and CN bond lengths, the CNN bond angles, and the CNNC dihedral angle.

Ab initio molecular dynamics simulation of photoisomerization in azobenzene in the nπ∗ state

Photoisomerization mechanism of azobenzene in the lowest excited state S(1)(n pi(*)) is investigated by ab initio molecular dynamics (AIMD) simulation with the RATTLE algorithm, based on the state-averaged complete active space self-consistent field method. AIMD simulations show that cis to trans isomerization occurs via two-step rotation mechanism, accompanying rotations of the central NN part and two phenyl rings, and this process can be classified into two types, namely, clockwise and counterclockwise rotation pathways. On the other hand, trans to cis isomerization occurs via conventional rotation pathway where two phenyl rings rotate around the NN bond. The quantum yields are calculated to be 0.45 and 0.28+/-0.14 for cis to trans and trans to cis photoisomerizations, respectively, which are in very good agreement with the corresponding experimental results.

Modeling Dinitrogen Activation by Lithium: A Mechanistic Investigation of the Cleavage of N2 by Stepwise Insertion into Small Lithium Clusters

Because of the inertness of molecular nitrogen, its practicable activation under mild conditions is a fundamental challenge. Nature can do it easily; chemists should be able to achieve comparable success. Lithium is exceptional among the main group elements in that it slowly reacts with N(2) at room temperature, leading finally to (NLi(3))(n), lithium nitride, a product of interest in its own right, because of its potential as a hydrogen storage medium. We explored this remarkably facile dinitrogen activation reaction by using model lithium clusters. Our extensive computations elucidate mechanisms for the ready reactions of N(2) with various model clusters, Li(2), Li(4), Li(6), and Li(8), leading to stepwise cleavage of the NN bond during dinitrogen reduction, N(2)(0) to 2 N(3-). Initial isomeric N(2)-Li(n) complexes, retaining NN triple bonds, undergo cluster insertion/reduction processes over generally low barriers. A minimum of eight lithium atoms are needed to cleave the triple bonded nitrogen completely in a highly exothermic process. Moreover, we provide an explanation for the exceptional reactivity of N(2) with Li, compared to the other alkali metals, e.g., Na and K. Li is a very strong reducing agent as its nitrides have the highest atomization energy, the shortest M-N bond distance, and the largest M-N charge separation as well as interaction energy. Our study delineates the general manner in which molecular nitrogen can be activated sequentially by electron transfer and bond elongation, to give a series of increasingly reduced complexes. We conclude that lithium incorporation into complexes might facilitate the development of nitrogen fixation catalysts.

Cis–trans isomerization mechanism of 4-aminoazobenzene in the S 0 and S 1 states: A CASSCF and DFT study

The isomerization mechanism of the ground-state and the first single excited-state (S 0 and S 1) for 4-aminoazobenzene (4-AAB) was studied on the basis of the DFT-B3LYP and CASSCF calculations. The results indicate that the thermal cis– trans ( E ↔ Z) isomerization of 4-AAB mainly concerns the inversion mechanism, while the photoisomerization on the S 1 state involves the rotational pathway that is characterized by the S 1/S 0 conical intersection with a twisting structure. By means of the investigation on the geometrical parameters, it was found that the relaxation modes, which determine the decay dynamics of the excited states, are different for the E- and Z-isomers upon excited in the S 1 state. Moreover, the S 1 potential energy surface of 4-AAB is found to be similar to that of parent azobenzene. During the decay process of excited E-isomer through the rotation of the NN bond, however, the activation energy barrier of 4-AAB is lower than that of azobenzene, which suggests that the corresponding decay lifetime of S 1 excited E-4-AAB is smaller than that of E-azobenzene.

The ‘belle époque’ of photochemistry

In 1908 the pioneer of photochemistry, Giacomo Ciamician, presented a comprehensive account on his ongoing work in molecular photochemistry. He reported a number of reactions caused by light and ascertained that these were caused by light alone, not heat. These included geometrical isomerization of CC, CN and NN bonds, 2 + 2 cycloaddition involving olefins as well as α,β-unsaturated ketones and esters, reactions of ketones such as α-cleavage and intermolecular hydrogen abstraction from alcohols leading to reduction or coupling, inter- and intramolecular reactions of nitro compounds, photooxygenation reactions: in fact a large fraction of the presently known reaction types. The rich perspective that photochemistry offered to synthesis was revealed by this talk. Actually, Ciamician thought that this would become a serious alternative to thermal methods and provide a less polluting way for synthesis, imitating the mild conditions of biochemical reactions. This hope did not materialize, however, and the actual development of molecular photochemistry had to wait for several decades.

Detailed mechanism of trans–cis photoisomerization of azobenzene studied by semiclassical dynamics simulation

A realistic dynamics simulation study is reported for the trans–cis photoisomerization of azobenzene. The simulation follows both nπ* and ππ* excitations and each excitation is induced by a 50 fs (FWHM) laser pulse. The simulation results show that, for both excitations, the reaction path is predominated by the rotation coordinate of the NN bond. The simulation finds that the CNN inversion angles expand as soon as the rotation starts. The expansion of the CNN bond angles permits the molecule to rotate efficiently. It is therefore suggested that the photoisomerization of trans-azobenzene follows an inversion-assisted rotation path. These simulation results are significant for understanding the mechanism of this important process.

Relation between bond order and delocalization index of QTAIM

The formal bond order is half the difference between the number of bonding and anti-bonding electrons. The relation between delocalization index (DI) from QTAIM and formal bond order is linear for CC, NN, GeGe, C–Si, C–Ge and CN bonds. The electronegativity is closely related with the correspondence between the DI and the formal bond order for the CC, NN and GeGe bonds. By using the electronegativity of C, N and Ge atoms, it was obtained a general relation between the formal bond order and the DI for CC, NN and GeGe atomic pairs.

Ultrafast dynamics and photochemistry of π–π* excited trans‐azobenzene—a comprehensive analysis of resonance Raman intensities

AbstractAzobenzene and its derivatives have widely been recognized as systems with potential applications such as optical switching elements, and the mechanism of trans‐cis photoisomerization, which is the basis for all these applications, has long been sought. The long‐standing controversy on the relaxation mechanism of π–π* (S2) excited trans‐azobenzene (TAB) is addressed in this paper. The role of S1S2 intersection and the possible involvement of a third state in the S2 photochemistry are investigated. The excited state structure and dynamics of TAB are extracted by modeling the Raman excitation profiles (REPs) of the S1 and S2 states and the absorption spectrum of TAB by Heller's time‐dependent formalism. The REPs of S2 are modeled in the single resonant state approximation by explicitly including the eight most Franck–Condon (FC) active modes. The obtained NN and CN bond lengths of S2 and the intial dynamics in these coordinates following photoexcitation to S2 are in agreement with recent theoretical calculations. The structural parameters of S1 are obtained by exploiting the nature of interference in the REPs of S1. From these parameters a three‐mode–two‐state vibronic coupling model, based on harmonic diabatic potentials and linear coupling, is formulated to study the S2S1 internal conversion dynamics. The results indicate that the S1S2 intersection is not directly accessible within electronic dephasing time of S2 to cause significant population decay from S2 to S1. The current conception of π–π* relaxation mechanisms in TAB is reviewed and revised on the basis of our observations and also from recently reported results of time‐resolved and theoretical studies. The role of a third state (S3), which appears to be an integral part of in the relaxation mechanism of S2, is pointed out and the feasibility of two competing relaxation pathways for S2 state is identified. One is the direct rotationless S2S1(C2h) internal conversion and the other is S2S3 internal conversion along rotational pathway followed by a branching of the S3 population to S0 and S1. Copyright © 2008 John Wiley & Sons, Ltd.

The role of methylene in prompt NO formation

We address the plausibility of singlet methylene (1CH2) in the prompt NO formation mechanism via examination of experimental species profiles and kinetic flame modeling of several low-pressure methane–oxygen–nitrogen flames. Existing kinetic models assuming CH as the only prompt NO precursor greatly underpredict NO formation under very fuel-lean conditions. We have constructed a kinetic pathway initiated by the recombination of singlet CH2 with molecular nitrogen to form diazomethane, CH2NN, early in the flame. Although the majority of the diazomethane is predicted to react with flame radicals to regenerate N2, a small percentage (approximately 10%) is predicted to react via cleavage of the NN bond leading to NO formation. This leads to accurate prediction of the experimental measurements of NO formation in lean, low-pressure flames. Assuming reasonable kinetic parameters for the reactions of CH2, the large underprediction of NO under lean conditions can be rectified by the inclusion of the 1CH2 prompt NO pathway in the kinetic mechanism.

Singlet Diradical Complexes of Ruthenium and Osmium: Geometrical and Electronic Structures and their Unexpected Changes on Oxidation

Reaction of HL, HLa (2-[(2-N-phenylamino)phenylazo]pyridine), HLb (2-[{2-N-(4-methylphenyl)amino}phenylazo]pyridine), or HLc (2-[{2-N-(4-chlorophenyl)amino}phenylazo]pyridine), with KRuO4 or OsO4 and PPh3 under exhaustive deoxygenation (PPh3 --> OPPh3) yields diamagnetic compounds ML2. Crystal structure determination for M(La)2 indicates the radical dianion state, L2(.-), for the ligands as evident from the typical N-N bond length of about 1.33 A for a one-electron reduced azo function. The resulting spin-coupled complexes, MIV(L(2.-))2, can be oxidized in two reversible one-electron steps, as probed by cyclic voltammetry and UV-vis-NIR spectroelectrochemistry. The paramagnetic intermediates, [M(La)2]+, are distinguished by intense NIR absorption, largely metal-centered spin as revealed by EPR, and, in the case of [Os(La)2]I3, by crystallographically determined shortening of the NN bond to about 1.30 A, corresponding to a coordinated unreduced azo function. Thus, oxidation of the complex MIV(L(2.-))2 involves partial reduction of the metal in [MIII(L-)2]+ because intramolecular double electron transfer is offsetting the external charge removal. Density-functional theory calculations were employed to confirm the structural features and to support the spectroscopic assignments.

Vibronic Structure and Ion Core Interactions in Rydberg States of Diazomethane: An Experimental and Theoretical Investigation

Vibronic transitions to the 21A2(3py <-- pi) Rydberg state of CH2N2, CD2N2, and CHDN2 were recorded by 2 + 1 REMPI spectroscopy, and kinetic energy distributions (eKE) of photoelectrons from ionization of selected vibronic levels were determined by velocity map imaging. Normal-mode frequencies were obtained for the 21A2(3py) Rydberg state and for the cation. Mixed levels of the 21A2(3py) and 21B1(3pz) of the three isotopologs were identified by photoelectron imaging and analyzed. The equilibrium geometries and harmonic vibrational frequencies of the electronic states of neutral diazomethane were calculated by CCSD(T)/cc-pVTZ, and B3LYP/6-311G(2df,p). The latter method was also used to calculate isotope shifts for the ground-state neutral and cation. Geometry and frequencies of the ground state of the cation were calculated by CCSD(T)/cc-pVTZ, using the unrestricted (UHF) reference. The equilibrium structures, frequencies, and isotope shifts of the 21A2(3py) and 21B1(3pz) Rydberg states were calculated by EOM-EE-CCSD/6-311(3+,+)G(2df). In all cases where comparisons with experimental results were available, the agreement between theory and experiment was very good allowing a full analysis of trends in structure and vibrational frequencies in going from the neutral species to the excited Rydberg states, 21A2(3py) and 21B1(3pz), and the cation. Although the 21A2(3py) and 21B1(3pz) states have planar C2v symmetry like the ion, they exhibit differences in geometry due to the specific interactions of the electron in the 3py and 3pz orbitals with the nuclei charge distributions of the ion core. Moreover, trends in normal-mode frequencies in the ground states of the neutral and ion and the 21A2(3py) and 21B1(3pz) Rydberg states are consistent with removing an electron from the bonding piCN-orbital, which also has an antibonding character with respect to NN. To explain the observed trends, the vibrational modes are divided into two groups that involve displacements mainly (i) along the CNN framework and (ii) in the CH2 moiety. Trends in the first group are due mostly to the effect of the lower CN and NN bond orders, whereas those in the second group are due to the interaction between the positively charged hydrogens and the Rydberg electron density, and the hybridization of the carbon. Within each group, marked differences in behavior between the in-plane and out-of-plane modes are observed.