Unimolecular reactions of protonated hydrazine. Reaction mechanisms and dynamics from observation of metastable ion fragmentations and ab initio calculations (original) (raw)
Related papers
1995
The distributions of the translational energy (T) released during loss of H 2 from metastable CH 3 NH 3 + and CH 3 OH 2 + ions have been measured. For both reactions the most probable T value accounts for approximately 3/4 of the reaction's reverse critical energy. Subject to the same experimental conditions CH 3 FH + ions do not give rise to any measurable signal for H 2 loss. The relevant parts of the potential energy surfaces of all three reactions were investigated using various ab initio quantum chemical computational schemes. Ab initio direct dynamics calculations were performed to obtain representative reaction trajectories. Translational energy releases computed at the end of these trajectories (where the fragments have separated) agree with the corresponding experimental figures. The three reactions follow a common polar mechanism which involves an initial transfer of a proton from the most basic centre (N, O or F) towards one of the hydrogen atoms of the methyl group. During this stage the proton polarizes the electrons around the methyl hydrogen to give it some hydride character, and in the transition state this has resulted in an embryonic H-H bond. Further electron reorganization during the concerted bond breaking and bond making process leads to a strong repulsive force along the reaction coordinate as the two fragments depart. This accounts for the highly non-statistical partitioning of the available potential energy into relative translation between the two fragments formed.
The Journal of Physical Chemistry, 1995
Using ab initio direct dynamics, selected reaction trajectories were calculated for the title reaction. The lifetime of the intermediate ion-molecule complex formed upon encounter of the reactants depends strongly on their initial relative orientation. When the proton to be transferred is properly lined up between the oxygen and the nitrogen, rapid transfer is observed. This leads to deposition of a high and nonstatistical fraction of the reaction enthalpy into the product ammonium ion. Less favorable initial orientations appear to give a more statistical distribution of the energy. A strong basis set dependence of the dynamics is observed. It is concluded that moderately large basis functions including polarization functions should be used for future dynamical studies. Alternatively, precise analytical surfaces may be used. Dynamics Calculations of H 3 O + + NH 3 f NH 4 + + H 2 O
1996
Using ab initio direct dynamics, selected reaction trajectories were calculated for the title reaction. The lifetime of the intermediate ion-molecule complex formed upon encounter of the reactants depends strongly on their initial relative orientation. When the proton to be transferred is properly lined up between the oxygen and the nitrogen, rapid transfer is observed. This leads to deposition of a high and nonstatistical fraction of the reaction enthalpy into the product ammonium ion. Less favorable initial orientations appear to give a more statistical distribution of the energy. A strong basis set dependence of the dynamics is observed. It is concluded that moderately large basis functions including polarization functions should be used for future dynamical studies. Alternatively, precise analytical surfaces may be used. Dynamics Calculations of H 3 O + + NH 3 f NH 4 + + H 2 O
Formation and Decomposition of Chemically Activated and Stabilized Hydrazine
Journal of Physical Chemistry A, 2010
Recombination of two amidogen radicals, NH 2 (X 2 B1), is relevant to hydrazine formation, ammonia oxidation and pyrolysis, nitrogen reduction (fixation), and a variety of other N/H/X combustion, environmental, and interstellar processes. We have performed a comprehensive analysis of the N 2 H 4 potential energy surface, using a variety of theoretical methods, with thermochemical kinetic analysis and master equation simulations used to treat branching to different product sets in the chemically activated NH 2 + NH 2 process. For the first time, iminoammonium ylide (NH 3 NH), the less stable isomer of hydrazine, is involved in the kinetic modeling of N 2 H 4 . A new, low-energy pathway is identified for the formation of NH 3 plus triplet NH, via initial production of NH 3 NH followed by singlet-triplet intersystem crossing. This new reaction channel results in the formation of dissociated products at a relatively rapid rate at even moderate temperatures and above. A further novel pathway is described for the decomposition of activated N 2 H 4 , which eventually leads to the formation of the simple products N 2 + 2H 2 , via H 2 elimination to cis-N 2 H 2 . This process, termed as "dihydrogen catalysis", may have significant implications in the formation and decomposition chemistry of hydrazine and ammonia in diverse environments. In this mechanism, stereoselective attack of cis-N 2 H 2 by molecular hydrogen results in decomposition to N 2 with a fairly low barrier. The reverse termolecular reaction leading to the gas-phase formation of cis-N 2 H 2 + H 2 achieves non-heterogeneous catalytic nitrogen fixation with a relatively low activation barrier (77 kcal mol -1 ), much lower than the 125 kcal mol -1 barrier recently reported for bimolecular addition of H 2 to N 2 . This termolecular reaction is an entropically disfavored path, but it does describe a new means of activating the notoriously unreactive N 2 . We design heterogeneous analogues of this reaction using the model compound (CH 3 ) 2 FeH 2 as a source of the H 2 catalyst and apply it to the decomposition of cis-diazene. The reaction is seen to proceed via a topologically similar transition state, suggesting that our newly described mechanism is general in nature.
HAL (Le Centre pour la Communication Scientifique Directe), 2021
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Entropy Effects in the Fragmentation of 1,1-Dimethylhydrazine Ions
The Journal of Physical Chemistry A, 2007
The 1,1-dimethylhydrazine ion ((CH 3 ) 2 NNH 2 +• ) has two low-energy dissociation channels, the loss of a hydrogen atom to form the fragment ion m/z 59, (CH 3 )(CH 2 )NNH 2 + , and the loss of a methyl radical to form the fragment ion m/z 45, the methylhydrazyl cation, CH 3 NNH 2 + . The dissociation of the 1,1-dimethylhydrazine ion has been investigated using threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy, in the photon energy range 8.25-31 eV, and tandem mass spectrometry. Theoretical breakdown curves have been obtained from a variational transition state theory (VTST) modeling of the two reaction channels and compared to those obtained from experiment. Seven transition states have been found at the B3-LYP/6-31+G(d) level of theory for the methyl radical loss channel in the internal energy range of 2.32-3.56 eV. The methyl loss channel transition states are found at R N-C ) 4.265, 4.065, 3.965, 3.165, 2.765, 2.665, and 2.565 Å over this internal energy range. Three transition states have been found for the hydrogen atom loss channel: R H-C ) 2.298, 2.198, and 2.098 Å. The ∆S q (45) value, at an internal energy of 2.32 eV and a bond distance of R N-C ) 4.265 Å, is 65 J K -1 mol -1 . As the internal energy increases to 3.56 eV the variational transition state moves to lower R value so that at R N-C ) 2.565 Å, the ∆S q decreases to 29 J K -1 mol -1 . For the hydrogen atom loss channel the variation in ∆S q is less than that for the methyl loss channel. To obtain agreement with the experimental breakdown curves, ∆S q (59) ) 26-16 J K -1 mol -1 over the studied internal energy range. The 0 K enthalpies of formation (∆ f H 0 ) for the two fragment ions m/z 45 and m/z 59 have been calculated from the 0 K activation energies (E 0 ) obtained from the fitting procedure: ∆ f H 0 [CH 3 NNH 2 + ] ) 906 ( 6 kJ mol -1 and ∆ f H 0 [(CH 3 )(CH 2 )NNH 2 + ] ) 822 ( 7 kJ mol -1 . The calculated G3 values are ∆ f H 0 -[CH 3 NNH 2 + ] ) 911 kJ mol -1 and ∆ f H 0 [(CH 3 )(CH 2 )NNH 2 + ] ) 825 kJ mol -1 . In addition to the two lowenergy dissociation products, 21 other fragment ions have been observed in the dissociation of the 1,1dimethylhydrazine ion as the photon energy was increased. Their appearance energies are reported.
Journal of the American Chemical Society, 1999
An excess proton can migrate from a solute to solvent molecules upon asymmetric solvation. The migration depends sensitively on solvation number, solvation structure, and proton affinity differences between solute and solvent molecules. The present study demonstrates this intriguing solvation-induced effect using protonated dimethyl ether-water clusters as the benchmark system. An integrated examination of H + [(CH 3 ) 2 O]-(H 2 O) n by vibrational predissociation spectroscopy and ab initio calculations indicates that the excess proton is localized on (CH 3 ) 2 O at n ) 1, (2) equally shared by (CH 3 ) 2 O and (H 2 O) 2 at n ) 2, and (3) completely transferred to (H 2 O) n at n g 3. The dynamics of proton transfer is revealed by the characteristic free-and hydrogen-bonded-OH stretching vibrations of the water molecules in direct contact with the excess proton. Both hydrogen bond cooperativity and zero-point vibrations have crucial influences on the final position of the proton in the clusters. Further insight into this remarkable phenomenon of intracluster proton transfer is provided by a comparison between H + [(CH 3 ) 2 O](H 2 O) n and its structural analogues, H + (H 2 O) n+1 and H + [(C 2 H 5 ) 2 O](H 2 O) n .
Journal of Molecular Structure: THEOCHEM, 1988
level were performed on hydrazinecarboxamide and hydraxinecarbothioamide and on their protonated forms (at the hydrazinic amino group). The two molecules show very close structural characteristics and similar bebaviour for the process of internal rotation around the different bonds. For the groundstates of the free-bases the E-form turns out to be the more stable one, while the Z-form is more stable in the case of the cations. The energy difference between conformers indicates that, in physical conditions where an equilibrium is possible, the amount of one form should predominate. These trends are in agreement with a number of experimental results, especially those relative to hydrazinecarbothioamide, which is accurately known. In the bases the barriers for internal rotation around the C-N bonds are higher for the amino than for the hydrazino group and in the sulphur higher than in the oxygen compound, yet the transition states occur for almost perpendicular conformations. For the cations two transition states are found, one in the proximity of the perpendicular conformation and the other at the planar E-conformation, separated by a flat minimum. The energy barriers in the cations are lower than in the corresponding bases. The calculated properties enable a homogeneous picture of the chemical behaviour of these molecules to be drawn, even in the absence of exhaustive experimental studies.
Conjugate base of a cyclic hydrazine: formation and energetics of N-diaziridyl anion
International Journal of Mass Spectrometry, 1999
N-Diaziridyl anion (1), the conjugate base of a simple hydrazine, has been generated in a Fourier transform mass spectrometer (FTMS) by reduction of the nitrogen-nitrogen double bond in diazirine. The acidity of its conjugate acid (390 Ϯ 3 kcal mol Ϫ1) and the electron affinity of its corresponding radical (0.50 Ϯ 0.10 eV) were measured. These data are combined in a thermodynamic cycle to afford the N-H bond strength in trans-diaziridine (2, 88 Ϯ 4 kcal mol Ϫ1). This value represents one of the few such bond energies reported for a hydrazine. High-level ab initio complete basis set-quadratic configuration interaction/atomic pair natural orbital (CBS-QCI/APNO) calculations on 1 and the parent hydrazyl anion (NH 2 NH Ϫ) are presented. These results provide strong confirming evidence for Berkowitz's recent determination of the N-H bond strength in hydrazine.
Bulletin of the Chemical Society of Japan, 2008
To investigate the effect of hydration and self-association on the reaction mechanism of proton transfer in methimazole (3-methyl-1H-imidazole-2(3H)-thione) and 1H-imidazole-2(3H)-thione, quantum chemical calculations were performed at the B3LYP/6-311++G(2d,2p) level of theory. The binding energies of complexes formed in self-assisted reaction are greater than H 2 O-assisted reactions. The results show that the thione complexes are more stable than corresponding thiols. The energy barrier for direct proton-transfer tautomerization reaction is significantly greater than selfassisted and H 2 O-assisted transfer tautomerization. Direct transition is more difficult than the water-assisted and self-assisted processes both thermodynamically and dynamically. The small negative value of HðrÞ obtained by AIM analysis at the B3LYP/6-311++G(2d,2p) level reveals some contribution of sharing interaction (partially covalent) to the SÁÁÁHN bond. AIM data also reveal the partially covalent nature of SÁÁÁH5 interaction and electrostatic nature of OÁÁÁH6 interaction in the hydrated complexes. In the present complexes, results obtained by NBO analysis show that there is an increase in the à population of the N-H bond in A(C) and that of O-H bond in W upon dimerization.