Quantum chemical topology description of the hydrogen transfer between the ethynyl radical and ammonia (C2Hradical dot + NH3) – The electron localization function study

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Abstract

The topological analysis of the Electron Localization Function (ELF) has been carried out for the hydrogen abstraction pathway between ethynyl radical (HCCradical dot) and ammonia (NH3). The IRC path, minima (HCC⋯HNH2, HCCH⋯NH2) and transition structure have been calculated at the UB3LYP/6-311++G(2d, 2p) computational level. The ‘closed-shell’ and ‘spin-polarized’ formula of ELF are used for an analysis. A detailed study of the topology of ELF (the basin population and spin density redistribution) shows that the reaction consists of three main steps distinguished on the IRC path. First, the N–H bond in ammonia is broken, then the hydrogen atom with a population of ca. 0.7 e is transferred and finally the C–H bond in acetylene is formed. It is interesting to note that all chemically important changes occur after the transition state.

Graphical abstract

The topological analysis of the Electron Localization Function (ELF) shows that the reaction of the hydrogen transfer between ethynyl radical (HCCradical dot) and ammonia (NH3) proceeds in four steps with ‘dressed proton’ for which the basin population equals 0.7 e. Calculations are performed using ‘closed-shell’ and ‘spin-polarized’ formula for ELF.

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Introduction

Chemical reactions between radicals and closed-shell molecules could play very important role in chemical processes, e.g. in interstellar space. Very good example of reactive radicals is ethynyl radical (C2H·) which has been found in planetary atmospheres [1], [2].

Recently Carl et al. [3] experimentally studied the reaction between the ethynyl radical and ammonia (C2H + NH3) over the temperature range 295–765 K. It was showed that the reaction proceeds without barrier, is unusually fast and preceded with an initial HCC⋯HNH2 alignment. Theoretical work by Nguyen et al. [4], mainly using the DFT method (B3LYP), revealed two main reaction routes: the H-abstraction which is favored and condensation. The barrier-less transformation has been confirmed since all stationary points on potential energy surface (PES) were found lower in energy than the entry point, i.e. C2H + NH3. The reaction starts with formation of weak complex HCC⋯H3N and after H transfer weak HCCH⋯NH2 complex is observed.

Quantum chemical studies of chemical reactions usually are restricted for determination of structures of reactants and transition state. Very little attention is paid to the analysis of chemical bonds which are important during the chemical reaction. From this point of view very important role could play quantum topological methods which have been developed using the electron density [5] and electron localization function (ELF) [6]. An adoption of the topological analysis of ELF function to study chemical processes led Krokidis et al. [7] to elaboration of the Bonding Evolution Theory (BET) which merges the topological analysis of ELF and Thom’s catastrophe theory [8]. For example, the proton transfer mechanisms were analyzed by Krokidis et al. [9] in malonaldehyde and by Alikhani and Silvi [10] in the [HF(HCl)⋯OH] complexes.

In this Letter, mechanism of H-abstraction in the reaction between ethynyl radical and ammonia is analyzed from a point of view of the Bonding Evolution Theory. We focus on: (1) a comparison of results achieved using two different formulas for ELF – ‘closed-shell’[11] and ‘spin-polarized’[12], (2) an analysis of the spin density redistribution following the hydrogen transfer, and (3) a determination of catastrophes on the IRC path in respect to position of the transition state.

Section snippets

The electron localization function (ELF) and bonding evolution theory (BET)

The topological analysis of the Electron Localization Function [11] and an extension to chemical reactions via introduction of a control space (e.g. reaction paths) [7] is well documented in the literature [13], [14], [15], [16], [17], [18]. Here, we present only two approximations for ELF used in our Letter. Savin et al. [19] highlighted the physical meaning of the ELF expressing it in terms of local kinetic energy density which increases due to the Pauli repulsion TS(r)  TvW(r):η(r)=1+TS(r)-TvW

Computational details

Optimizations of geometrical structures of all molecules and calculation of the IRC path were performed with Gaussian 03 [20] using the B3LYP functional and 6-311++G(2d, 2p) basis set as referenced in the program. All stationary points were characterized to have the correct type of vibrational eigenvalues, and saddle points were verified to connect to the correct minima using the internal reaction coordinate (IRC) method [21]. For the BET analysis a reaction path was taken from an IRC

Isolated molecules

The population of basins (N¯) localized in the gradient field of ELF for isolated HCCH, HCC, NH3, and NH2 molecules are collected in Table 1, Table 2 using the ‘closed-shell’ and ‘spin-polarized’ formula for ELF. In HCCH there are found two protonated V(H, C) basins, corresponding to the C–H bonds, with large basin populations 2.31(2.30) e and single V(C1, C2) basin corresponding to formally triple bond C1triple bondC2 with 5.26(5.22) e calculated with the ‘closed-shell’ (‘spin-polarized’) formula for ELF.

Conclusions

Presented results clearly show that the Bonding Evolution Theory is very important tool for study of hydrogen abstraction reactions. Analysis of consequent steps on the reaction path gives new detailed information on reaction mechanism. Breaking of old bonds and formation of new bonds as a result of chemical reaction are visible due to analysis of the topology of ELF on the IRC path. In case of studied reaction of hydrogen abstraction from NH3 to C2H radical it has been noted of several steps

Acknowledgement

The authors thank the Wrocław Centre for Networking and Supercomputing for generous allocation of computer time. The Marie Curie European Reintegration Grant supported the work of S.B. – Contract No. MERG-CT-2004-006330; ‘The authors are solely responsible for the information communicated and it does not represent the opinion of The European Community. The European Community is not responsible for any use that might be made of data appearing therein.’ Z. L. gratefully acknowledge support from

References (26)

  • D.F. Strobel

    Planet. Space Sci.

    (1982)
  • M. Allen et al.

    Icarus

    (1992)
  • S. Noury et al.

    Comp. Chem.

    (1999)
  • S.A. Carl et al.

    J. Phys. Chem. A

    (2004)
  • H.M.T. Nguyen et al.

    Phys. Chem. Chem. Phys.

    (2004)
  • U. Koch et al.

    J. Phys. Chem.

    (1995)
  • F. Fuster et al.

    Theor. Chem. Acc.

    (2000)
  • X. Krokidis et al.

    J. Phys. Chem. A

    (1997)
  • R. Thom

    Stabilité Structurelle et Morphogénèse

    (1972)
  • X. Krokidis et al.

    J. Phys. Chem. A

    (1998)
  • M.E. Alikhani et al.

    J. Mol. Struct.

    (2004)
  • A.D. Becke et al.

    J. Chem. Phys.

    (1990)
  • M. Kohout et al.

    Int. J. Quantum Chem.

    (1996)
  • Cited by (0)

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