scholarly journals Theoretical studies of carbon isotopic fractionation in reactions of C with C2: dynamics, kinetics, and isotopologue equilibria

2021 ◽  
Vol 647 ◽  
pp. A142
Author(s):  
C. M. R. Rocha ◽  
H. Linnartz
Keyword(s):  
Energy Content ◽  
Equilibrium Constants ◽  
Isotopic Fractionation ◽  
Zero Point ◽  
Classical Trajectory ◽  
Rate Coefficients ◽  
Kinetic Temperature ◽  
Positive Temperature ◽  
Exchange Reactions ◽  
Point Energy

Context. Our current understanding of interstellar carbon fractionation hinges on the interpretation of astrochemical kinetic models. Yet, the various reactions included carry large uncertainties in their (estimated) rate coefficients, notably those involving C with C2. Aims. We aim to supply theoretical thermal rate coefficients as a function of the temperature for the gas-phase isotope-exchange reactions 13C+12C2(X1Σg+,a3Πu)⇌13C12C(X1Σg+,a3Πu)+12C and 13C+13C12C(X1Σg+,a3Πu)⇌13C2(X1Σg+,a3Πu)+12C. Methods. By relying on the large masses of the atoms involved, we employ a variation of the quasi-classical trajectory method, with the previously obtained (mass-independent) potential energy surfaces of C3 dictating the forces between the colliding partners. Results. The calculated rate coefficients within the range of 25 ≤ T∕K ≤ 500 show a positive temperature dependence and are markedly different from previous theoretical estimates. While the forward reactions are fast and inherently exothermic owing to the lower zero-point energy content of the products, the reverse processes have temperature thresholds. For each reaction considered, analytic three-parameter Arrhenius-Kooij formulas are provided that readily interpolate and extrapolate the associated forward and backward rates. These forms can further be introduced in astrochemical networks. Apart from the proper kinetic attributes, we also provide equilibrium constants for these processes, confirming their prominence in the overall C fractionation chemistry. In this respect, the 13C+12C2(X1Σg+) and 13C+12C2(a3Πu) reactions are found to be particularly conspicuous, notably at the typical temperatures of dense molecular clouds. For these reactions and considering both equilibrium and time-dependent chemistry, theoretical 12C/13C ratios as a function of the gas kinetic temperature are also derived and shown to be consistent with available model chemistry and observational data on C2.

scholarly journals Astrophysically motivated laboratory measurements of deuterium reacting with isotopologues of H

2019 ◽  
Vol 15 (S350) ◽  
pp. 114-115
Author(s):  
K. P. Bowen ◽  
P.-M. Hillenbrand ◽  
J. Liévin ◽  
X. Urbain ◽  
D. W. Savin
Keyword(s):  
Cross Sections ◽  
Zero Point ◽  
Rate Coefficients ◽  
Laboratory Measurements ◽  
Exchange Reactions ◽  
Point Energy ◽  
Chemical Tracers ◽  
Prestellar Cores ◽  
Order Of Magnitude ◽  
High Level

AbstractH2D+ and D2H+ are important chemical tracers of prestellar cores due to their pure rotational spectra that can be excited at the ~20 K temperature of these environments. The use of these molecules as probes of prestellar cores requires understanding the chemistry that forms and destroys these molecules. Of the eight key reactions that have been identified (Albertssonet al. 2013), five are thought to be well understood. The remaining three are the isotope exchange reactions of atomic D with H $${ + \over 3}$$ , H2D+, and D2H+. Semi-classical results differ from the classical Langevin calculations by an order of magnitude (Moyano et al. 2004). To resolve this discrepancy, we have carried out laboratory measurements for these three reactions. Absolute cross sections were measured using a dual-source, merged fast-beams apparatus for relative collision energies between ~10 meV to ~10 eV (Hillenbrand et al. 2019). A semi-empirical model was developed incorporating high level quantum mechanical ab initio calculations for the zero-point-energy-corrected potential energy barrier in order to generate thermal rate coefficients for astrochemical models. Based on our studies, we find that these three reactions proceed too slowly at prestellar core temperatures to play a significant role in the deuteration of H $${ + \over 3}$$ isotopologues.

Stereodynamics of chemical reactions: quasi-classical, quantum and mixed quantum-classical theories

Open Physics ◽  
2012 ◽  
Vol 10 (2) ◽  
Author(s):  
Wenwu Xu ◽  
Guangjiu Zhao
Keyword(s):  
Chemical Reactions ◽  
Classical Method ◽  
Zero Point ◽  
Quantum Effects ◽  
Classical Trajectory ◽  
Tunneling Effect ◽  
Point Energy ◽  
Classical Quantum ◽  
Theoretical Results ◽  
Good Agreement

AbstractIn this review, some benchmark works by Han and coworkers on the stereodynamics of typical chemical reactions, triatomic reactions H + D2, Cl + H2 and O + H2 and polyatomic reaction Cl+CH4/CD4, are presented by using the quasi-classical, quantum and mixed quantum-classical methods. The product alignment and orientation in these A+BC model reactions are discussed in detail. We have also compared our theoretical results with experimental measurements and demonstrated that our theoretical results are in good agreement with the experimental results. Quasi-classical trajectory (QCT) method ignores some quantum effects like the tunneling effect and zero-point energy. The quantum method will be very time-consuming. Moreover, the mixed quantum-classical method can take into account some quantum effects and hence is expected to be applicable to large systems and widely used in chemical stereodynamics studies.

scholarly journals Zero-point energy conservation in classical trajectory simulations: Application to H2CO

2018 ◽  
Vol 148 (19) ◽  
pp. 194113 ◽  
Author(s):  
Kin Long Kelvin Lee ◽  
Mitchell S. Quinn ◽  
Stephen J. Kolmann ◽  
Scott H. Kable ◽  
Meredith J. T. Jordan
Keyword(s):  
Energy Conservation ◽  
Zero Point ◽  
Classical Trajectory ◽  
Zero Point Energy ◽  
Point Energy ◽  
Trajectory Simulations

scholarly journals A Hessian free method to prevent zero-point energy leakage in classical trajectory simulation

2022 ◽  
Author(s):  
Saikat Mukherjee ◽  
Mario Barbatti
Keyword(s):  
Degrees Of Freedom ◽  
Zero Point ◽  
Classical Trajectory ◽  
The Other ◽  
Water Dimer ◽  
Dynamics Simulation ◽  
Zero Point Energy ◽  
Point Energy ◽  
Quantum Uncertainty ◽  
Local Vibrational Mode

The problem associated with the zero-point energy (ZPE) leak in classical trajectory calculations is well known. Since ZPE is a manifestation of the quantum uncertainty principle, there are no restrictions on energy during the classical propagation of nuclei. This phenomenon can lead to unphysical results, such as forming products without the ZPE in the internal vibrational degrees of freedom (DOFs). The ZPE leakage also permits reactions below the quantum threshold for the reaction. We have developed a new Hessian-free method, inspired by the Lowe-Andersen thermostat model, to prevent energy dipping below a threshold in the local-pair (LP) vibrational DOFs. The idea is to pump the leaked energy to the corresponding local vibrational mode, taken from the other vibrational DOFs. We have applied the new correction protocol on the ab initio ground-state molecular dynamics simulation of the water dimer (H20)2, which dissociates due to unphysical ZPE spilling from the high-frequency OH modes. The LP-ZPE method has been able to prevent the ZPE spilling of the OH stretching modes by pumping back the leaked energy into the corresponding modes while this energy is taken from the other modes of the dimer itself, keeping the system as a microcanonical ensemble.

Analysis of the zero‐point energy problem in classical trajectory simulations

1996 ◽  
Vol 104 (2) ◽  
pp. 576-582 ◽  
Author(s):  
Yin Guo ◽  
Donald L. Thompson ◽  
Thomas D. Sewell
Keyword(s):  
Zero Point ◽  
Classical Trajectory ◽  
Energy Problem ◽  
Zero Point Energy ◽  
Point Energy ◽  
Trajectory Simulations

scholarly journals The effect of imprisonment of resonance radiation in the decomposition of ammonia and of deutero-ammonia

1935 ◽  
Vol 152 (876) ◽  
pp. 325-341 ◽  
Author(s):  
Harry Work Melville ◽  
Eric Keightley Rideal
Keyword(s):  
Energy Content ◽  
Zero Point ◽  
High Energy ◽  
Mercury Atom ◽  
Point Energy ◽  
Stationary Concentration ◽  
Number Of Factors ◽  
Secondary Reactions ◽  
Kinetics Of ◽  
Kinetic Treatment

In three recent papers, the kinetics of the mercury photo-sensitized decomposition of ammonia and of deutero-ammonia have been compared in order ( a ) to obtain the velocity coefficients for the processes occurring in these reactions, ( b ) to establish correlations between these coefficients and the measured quenching radii and zero-point energy content of the molecules participating in the collisions. Calculations made upon simple premises yielded inconsistencies in the numerical values of the coefficients, thus throwing doubt upon the fundamental assumptions in the kinetic treatment of the problem. Evans and Taylor, in a later paper, have sought to surmount these difficulties by postulating the formation of mercury-ammonia complexes which have finite and different lifetimes for the two ammonias and which are subject to various deactivating collisions in the gas phase. There are, however, a number of factors to be taken into account, not hitherto considered, before recourse need be made to assumptions of this nature. It is the purpose of this paper to attempt to estimate quantitatively the magnitude of the corrections to be applied, to see then what discrepancies still exist and what probable explanations need be put forward to effect the necessary modifications to the scheme of reactions. We must distinguish between two separate phenomena occurring in these reactions. In the first place the ammonia molecule is dissociated by a collision with a mercury atom of high energy content and therefore, in order to calculate the velocity of this process, the stationary concentration of the mercury atoms as a function of ammonia and hydrogen pressure and other possible variables must be known. Secondly, the fragments of the dissociated molecule do not necessarily yield molecular hydrogen and nitrogen, for it is known in the direct photo chemical decomposition the quantum yield is less than unity and therefore reverse actions leading to the regeneration of ammonia must come into play. In comparing the behaviour of isotopic molecules, each of these factors may be different and hence they must be separately investigated. In the following pages the phenomena having to do with mercury atom collisions are described first and then the question of secondary reactions is considered in the second part of the paper.

Exploring zero-point energies and hydrogen bond geometries along proton transfer pathways by low-temperature NMR

1999 ◽  
Vol 77 (5-6) ◽  
pp. 943-949 ◽  
Author(s):  
Sergei N Smirnov ◽  
Hans Benedict ◽  
Nikolai S Golubev ◽  
Gleb S Denisov ◽  
Maurice M Kreevoy ◽  
...  
Keyword(s):  
Hydrogen Bond ◽  
Chemical Shift ◽  
Proton Transfer ◽  
Isotopic Fractionation ◽  
Zero Point ◽  
Strong Hydrogen Bond ◽  
Zero Point Energy ◽  
Acid Base ◽  
Point Energy ◽  
Fractionation Factors

We have followed by NMR the zero-point energy changes of the hydrogen bond proton in 1:1 acid-base complexes AHB triple bond {A—H···B <-–> Aδ-···H···Bδ+ <-–> A-···H—B+} as a function of the proton position between A and B. For this purpose, the isotopic fractionation factors K between the acid-base complexes AHB + Ph3COD···B –><- ADB + Ph3COH···B, where AH represents a variety of acids and B represents pyridine-15N, were measured around 110 K, using a 2:1 mixture of liquefied CDClF2-CDF3 as solvent. As under these conditions the slow hydrogen bond exchange regime is reached, the values of K could be obtained directly by integration of appropriate proton NMR signals. Using the valence-bond order concept established previously by crystallography, the fractionation factors and corresponding zero-point energy changes (ΔZPE) are related in a quantitative way to the hydrogen bond geometries, the 1H chemical shift of the hydrogen bond proton, and the pyridine-15N chemical shift. The K values are related in a quasi-linear way to the chemical shifts of the hydrogen bond proton, where the slope depends on whether the proton is closer to oxygen or nitrogen. In the region of the strongly hydrogen-bonded quasi-symmetric complexes, which are characterized by a strong hydrogen bond contraction, the variation of K is very small in spite of substantial proton displacements.Key words: NMR, isotopic fractionation, hydrogen bonding, acid-base complexes, proton transfer, geometric isotope effects.

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