Loss of H2 from CH3NH3+, CH3OH2+ and CH3FH+. Reaction mechanisms and dynamics from observation of metastable ion fragmentations and ab initio calculations (original) (raw)
Related papers
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
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
1992
The classical equations of motion have been solved for the title reaction on the route leading from transition state to separated products using ab initio potential energy functions (HF and CASSCF). The calculations reproduce the experimentally observed translational energy release for both wave functions. Isotope effects on the translational energy release are also in good agreement with experiment. The calculations reveal that the translational energy release is a complicated function of the motion along the whole reaction trajectory. The situation at the transition state is not sufficient for predicting the final energy distribution.
International Journal of Mass Spectrometry and Ion Physics, 1973
The proton transfer reaction of H3+ ions with CH,NH2, CH,OH and CH,SH produces the excited intermediatecomplexes [CH3NH3i]*, [CH30H2']), and [CH,SH2+ ]* which subsequentIy decompose in two ways: by vicinai hydrogen elimination, and by C-X bond scission to give the methyl cation. The condensation reaction of methyl cations with NH,, H20, and Hz!3 is the reverse of the latter decomposition pathway and proceeds to give the same excited intermediates followed also either by vicinal hydrogen elimination or by back reaction. The Hi3 i ions formed by reaction of Hz+ with Hz are highly excited, and the presence of excess internal energy in the H, i-ion is shown to be a critical factor in the ratio of decomposition products obtained. A detailed analysis of the deactivation of * Supported by the National Science Foundation Grant No_ GP-15628 and by Contract NO. NAS 7-100 sponsored by the National Aeronautics and Space Administratioti. * D. K. Bohme (personal communication) used the flowing afterglow technique to measure the proton affinity of Hz as 4.71 e\! reiative to methane. The proton afkity of' CH, has recently been determined by photoionization~" as 5.50 eV. Cotton et al.* obtained 4.37 eV for the proton afiknity of Hz..
Biochemistry, 2003
We have studied the hydride transfer reaction catalyzed by the enzyme dihydrofolate reductase (DHFR) and the coenzyme nicotinamide adenine dinucleotide phosphate (NADPH); the substrate is 5-protonated 7,8-dihydrofolate, and the product is tetrahydrofolate. The potential energy surface is modeled by a combined quantum mechanical-molecular mechanical (QM/MM) method employing Austin model 1 (AM1) and a simple valence bond potential for 69 QM atoms and employing the CHARMM22 and TIP3P molecular mechanics force fields for the other 21 399 atoms; the QM and MM regions are joined by two boundary atoms treated by the generalized hybrid orbital (GHO) method. All simulations are carried out using periodic boundary conditions at neutral pH and 298 K. In stage 1, a reaction coordinate is defined as the difference between the breaking and forming bond distances to the hydride ion, and a quasithermodynamic free energy profile is calculated along this reaction coordinate. This calculation includes quantization effects on bound vibrations but not on the reaction coordinate, and it is used to locate the variational transition state that defines a transition state ensemble. Then, the key interactions at the reactant, variational transition state, and product are analyzed in terms of both bond distances and electrostatic energies. The results of both analyses support the conclusion derived from previous mutational studies that the M20 loop of DHFR makes an important contribution to the electrostatic stabilization of the hydride transfer transition state. Third, transmission coefficients (including recrossing factors and multidimensional tunneling) are calculated and averaged over the transition state ensemble. These averaged transmission coefficients, combined with the quasithermodynamic free energy profile determined in stage 1, allow us to calculate rate constants, phenomenological free energies of activation, and primary and secondary kinetic isotope effects. A primary kinetic isotope effect (KIE) of 2.8 has been obtained, in good agreement with the experimentally determined value of 3.0 and with the value 3.2 calculated previously. The primary KIE is mainly a consequence of the quantization of bound vibrations. In contrast, the secondary KIE, with a value of 1.13, is almost entirely due to dynamical effects on the reaction coordinate, especially tunneling.
Journal of the American Chemical Society, 1989
The bond dissociation energies (BDE) for the homolytic cleavage of a number of isoelectronic A-B dimers have been calculated to third order in Mdler-Plesset perturbation theory with the 6-31G** basis set. In particular, BDE's have been calculated for A-B -A' + 'B, with A = CH3, NH2, OH, and F and B = CH3, NH2, OH, F, N+H3, O+H2, and F' H.
The Journal of Physical Chemistry A, 2006
Direct dynamics simulations at the MP2/6-311++G** level of theory were performed to study C 2 H 5 F f HF + C 2 H 4 product energy partitioning. The simulation results are compared with experiment and a previous MP2/6-31G* simulation. The current simulation with the larger basis set releases more energy to HF vibration and less to HF + C 2 H 4 relative translation as compared to the previous simulation with the 6-31G* basis set. The HF rotation and vibration energy distributions determined from the current simulation are in overall very good agreement with previous experimental studies of C 2 H 5 F dissociation by chemical activation and IRMPA. A comparison of the simulations with experiments suggests there may be important mass effects for energy partitioning in HX elimination from haloalkanes. The transition state (TS) structures and energies calculated with MP2 and the 6-31G* and 6-311++G** basis sets are compared with those calculated using CCD, CCSD, CCSD(T), and the 6-311++G** basis set.
A DFT and ab initio direct dynamics study on the hydrogen abstract reaction of H3BNH3→H2+H2BNH2
Chemical Physics Letters, 2005
A direct ab initio dynamics study is presented on the hydrogen abstraction reaction of H 3 BNH 3 ! H 2 + H2BNH 2 . The geometries of all the stationary points are optimized at the B3LYP and MP2 levels of theory with a series of basis sets up to aug-cc-pVTZ. The energies are refined using the G3, G3MP2, G3MP2B3, CBS-Q, CBS-Q//B3, and a combined high-level (HL) method based on the geometries optimized using the B3LYP/aug-cc-pVTZ level of theory. The rate constants are evaluated using the conventional transition-state theory and canonical variational transition-state theory (CVT). The fitted Arrhenius expression calculated from the CVT/SCT method is kðT Þ ¼ 6:86  10 6  T 1:69  e ðÀ1:37Â10 4 =T Þ s À1 . The estimated apparent activation energy is in accordance with experimental results.
The Journal of Physical Chemistry
We present a recalibration of the two most recent analytical potential energy surfaces for the CH 4 + H f CH 3 + H 2 reaction. Assuming a correct functional form, the recalibration process allows one to update the original parameters in light of new experimental and theoretical data. As calibration criteria we use the reactant and product experimental properties, and the most recent ab initio saddle-point properties, which differ from the original values, especially the imaginary frequency. For both modified surfaces we use rectilinear and curvilinear coordinates to calculate the vibrational frequencies at nonstationary points. The rate coefficients are calculated with variational transition-state theory and, in general, agree with the experimental data in the temperature range 300-1500 K. They present only a slight dependence on the modified surfaces used but a major dependence on the choice of coordinate system, especially at low temperatures where the rectilinear coordinates yield rate coefficients practically double those of the curvilinear ones. We also provide a detailed analysis of the kinetic isotope effects (KIEs) for the reverse reactions. All KIEs are "normal", i.e., the ratios of the unsubstituted/substituted rate coefficients are greater than unity and in good agreement with the experimental values. Both modified surfaces reproduce the experimental dependence of the KIEs on temperature, the agreement being best when curvilinear coordinates are used.