We present a global potential energy surface for the 1A″ state of NH2 based on application of the reproducing kernel Hilbert space interpolation method to high quality ab initio (multireference configuration interaction) results. Extensive quasiclassical trajectory calculations are performed on this surface to study the N(2D)+H2/D2 reaction dynamics. Comparison is made with calculations on the lower level [first order configuration interaction (FOCI)] surface of Kobayashi, Takayanagi, Yokoyama, Sato, and Tsunashima (KTYST). We find a saddle point energy of 2.3 (1.9) kcal/mol for the perpendicular approach for the second order configuration interaction (SOCI) (SOCI with Davidson correction) surfaces, and a collinear stationary point energy of 5.5 (4.6) kcal/mol. The ordering of these stationary points is reversed compared to the corresponding FOCI results, and the only true reaction path on our surface is perpendicular. The primary reaction mechanism is determined to be C2v insertion to produce short lived (100–300 fs) NH2 intermediates. Angular distributions are found to be primarily forward–backward symmetric, with a slight bias towards backward scattering at low energies. Decay of the NH2’s occurs before energy is fully randomized, so the product vibrational distributions are a little hotter than statistical—with vibrational population ratios NH(v″=1)/NH(v″=0)=0.8 and ND(v″=1)/ND(v″=0)=0.9 (near threshold). These ratios, and other aspects of the vibrational product distributions are in excellent agreement with recent laser induced fluorescence studies.
Theoretical model calculations were performed to investigate the degree of validity of the mobile proton model of protonated peptides. The structures and energies of the most important minima corresponding to different structural isomers of protonated diglycine and their conformers, as well as the barriers separating them, were determined by DFT calculations. The rate coefficients of the proton transfer reactions between the isomers were calculated using the RRKM method in order to obtain a quantitative measure of the time scale of these processes. The proton transfer reactions were found to be very fast already at and above the threshold to the lowest energy decomposition pathway. Two possible mechanisms of b2+-ion formation via water loss from the dipeptide are also discussed. The rate-determining step of the proton migration along a peptide chain is also investigated using the model compound N-formylglycylglycinamide. The investigations revealed that this process very possibly occurs via the protonation of the carbonyl oxygens of the amide bonds, and its rate-determining step is an internal rotation-type transition of the protonated C=O-H group between two adjacent C=O-HellipsisO=C bridges.
The mobile proton model was critically evaluated by using purely theoretical models which include quantum mechanical calculations to determine stationary points on the potential energy surface (PES) of a model compound, and Rice-Ramsperger-Kassel-Marcus (RRKM) calculations to determine the rate constants of various processes (conformational changes, proton transfer reactions) which occur during mass analysis of protonated peptides. Extensive mapping of the PES of protonated N-formylglycinamide resulted in various minima which were stabilized by one or more of the following types of interaction: internal hydrogen bond, charge transfer interaction, charge delocalization, and ring formation. The relative energies of most of the investigated minima are less then 20 kcal mol(-1) compared with the most stable species. More importantly, the relative energies of the transition structures connecting these minima are fairly low, allowing facile transitions among the energetically low-lying species. It is demonstrated that a path can be found leading from the energetically most stable species, protonated on an amide oxygen, to the structure from which the energetically most favorable fragmentation occurs. It is also shown that the added proton can sample all protonation sites prior to fragmentation. The RRKM calculations applied the results of ab initio computations (structures, energetics, vibrational frequencies) to the reactions (internal rotations, proton transfers) occurring in protonated N-formylglycinamide, and clearly lend additional evidence to the mobile proton model. Based on the results of the PES search on protonated N-formylglycinamide, we also comment on the mechanism proposed by Arnot et al. (Arnot D, Kottmeier D, Yates N, Shabanowitz J, Hunt D F. 42(nd) ASMS Conference on Mass Spectrometry, 1994; 470) and Reid et al. (Reid G E, Simpson R J, O'Hair R A J. J. Am. Soc. Mass Spectrom. 1998; 9:945) for the formation of b(2)(+) ions. According to the high level ab initio results, the mechanism relying on amide oxygen protonated species seems to be less feasible than the one which involves N-protonated species.
A "mixed" representation approach in conjunction with a trajectory surface-hopping method is used to study intersystem crossing effects in the S + H 2 reaction. These calculations are based on high-quality potential surfaces that we have determined for the two lowest triplet states of SH 2 and globally determined spin-orbit coupling matrix elements that are obtained from CASSCF calculations. A previously determined surface for the lowest singlet state (
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