The barrierless TSs are possible when the potential energy surface is flat. This is usually observed in radical recombination reactions, where a small energy barrier is calculated through a loose transition state. As far as I understood, the H-atom abstraction reactions by OH radicals proceed by atleast a considerable energy barrier, unless you are studying H-atom abstraction between two radicals. How you located your transition state? By STQN procedure or opt=ts method?

Yes, locate TS by Opt=TS. I am satisfied by your answer but you  told me lower negative frequency is related to barrier less TS

Dear Gour,

Yest. There are reactions with barrierless TS. Please see the following publications:

1-Finding the Transition State of Quasi-Barrierless Reactions by a Growing String Method for Newton Trajectories:  Application to the Dissociation of Methylenecyclopropene and Cyclopropane†

Wolfgang Quapp ,‡ Elfi Kraka ,*§ and Dieter Cremer §‖

Mathematical Institute, University of Leipzig, Augustus-Platz, D-04109 Leipzig, Germany, and Departments of Chemistry and Physics, The University of the Pacific, Stockton, California 95366

J. Phys. Chem. A, 2007, 111 (44), pp 11287–11293

DOI: 10.1021/jp070736j

Publication Date (Web): August 17, 2007

Copyright © 2007 American Chemical Society

 http://pubs.acs.org/doi/abs/10.1021/jp070736j?journalCode=jpcafh

Abstract

A method for finding a transition state (TS) between a reactant minimum and a quasi-flat, high dissociation plateau on the potential energy surface is described. The method is based on the search of a growing string (GS) along reaction pathways defined by different Newton trajectories (NT). Searches with the GS-NT method always make it possible to identify the TS region because monotonically increasing NTs cross at the TS or, if not monotonically increasing, possess turning points that are located in the TS region. The GS-NT method is applied to quasi-barrierless and truly barrierless chemical reactions. Examples are the dissociation of methylenecyclopropene to acetylene and vinylidene, for which a small barrier far out in the exit channel is found, and the cycloaddition of singlet methylene and ethene, which is barrierless for a broad reaction channel with Cs-symmetry reminiscent of a mountain cirque formed by a glacier.

2-PETER POULAY SPECIAL ISSUE, editor in charge H. F. SCAEFER

The Mechanism of a Barrierless Reaction:

Hidden Transition State and Hidden Intermediates in the Reaction of Methylene with Ethene

Hyun Joo,a Elfi Kraka,a, Wolfgang Quapp,b and Dieter Cremerc

http://miserv1.mathematik.uni-leipzig.de/~quapp/JKQCPaper2.pdf

Abstract: The chelotropic addition reaction of singlet methylene to ethene yielding cyclopropane (reaction 1) was investigated with the help of the Unified Reaction Valley approach (URVA) using different levels of theory (B3LYP, MP2, MP4, CCSD(T), G3) and two basis sets (6-31G(d,p), 6-311++G(3df3pd)).

At all levels of theory, reaction (1) proceeds without barrier and transition state (TS). Nevertheless, reaction (1) possesses a distinct mechanism comprising four different reaction phases: 1) a van der Waals phase, in which the stereochemistry of the reaction is decided; 2) an electrophilic attack phase, in which charge is

transferred from ethene to methylene to establish a weak bonding interaction between the reaction partners typical of those encountered in TSs of CC bond forming reactions; c) a nucleophilic attack phase, in which charge transfer between methylene and ethene is reverted and a trimethylene biradical structure is formed;

d) a ring closure phase, in which the trimethylene structure closes to the three-membered ring. The URVA analysis identifies a hidden TS and two hidden intermediates at the transitions from one phase to the next. If methylene is replaced by difluorocarbene (reaction 2) or germylene (reaction 3), the 4-phase mechanism

is retained, however the hidden TS and one of the hidden intermediates are converted into real TS and real intermediate thus establishing 2-step mechanisms with strongly different energy profiles along the reaction path.

3-Molecular Physics: An International Journal at the Interface Between Chemistry and Physics

Volume 105, Issue 19-22, 2007

Special Issue:   A Special Issue in Honour of Professor Pĕter Pulay

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Translator disclaimer

Original ArticlesThe mechanism of a barrierless reaction: hidden transition state and hidden intermediates in the reaction of methylene with ethene

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DOI:

10.1080/00268970701620677

Hyun Jooa, Elfi Krakaa*, Wolfgang Quappb & Dieter Cremercpages 2697-2717

Publishing models and article dates explained

http://www.tandfonline.com/doi/abs/10.1080/00268970701620677

Abstract

The chelotropic addition reaction of singlet methylene to ethene yielding cyclopropane (reaction 1) was investigated with the help of the Unified Reaction Valley approach (URVA) using different levels of theory (B3LYP, MP2, MP4, CCSD(T), G3) and two basis sets (6-31G(d,p), 6-311++G(3df,3pd)). At all levels of theory, reaction (1) proceeds without barrier and transition state (TS). Nevertheless, reaction (1) possesses a distinct mechanism comprising four different reaction phases: (i) a van der Waals phase, in which the stereochemistry of the reaction is decided; (ii) an electrophilic attack phase, in which charge is transferred from ethene to methylene to establish a weak bonding interaction between the reaction partners typical of those encountered in TSs of CC bond forming reactions; (iii) a nucleophilic attack phase, in which charge transfer between methylene and ethene is reverted and a trimethylene biradical structure is formed; (iv) a ring closure phase, in which the trimethylene structure closes to the three-membered ring. The URVA analysis identifies a hidden TS and two hidden intermediates at the transitions from one phase to the next. If methylene is replaced by difluorocarbene (reaction 2) or germylene (reaction 3), the 4-phase mechanism is retained, however the hidden TS and one of the hidden intermediates are converted into a real TS and a real intermediate thus establishing 2-step mechanisms with strongly different energy profiles along the reaction path.

Hoping this will be helpful,

Rafik

Hi Gour,

I did not tell that a low negative frequency corresponds to a barrierless TS. I told that when the PES is flat, the TS may be barrierless. This you can easily understand from your IRC path. Then again optimize the end points of the IRC path to minima and then calculate the energy barrier. Because, as far as I experienced, the H-atom abstraction reactions from a molecule by hydroxyl radical did not proceed through barrierless transition state pathways.

regards,

Sandhiya. L

I would like to thank to Prof. Karaman for his explanation and the references.

We experienced several barierless TS in calculations of organic radicals binding weakly Br. radicals. In some cases this was apparently not due to thermochemical corrections (as reported in some references known to us) but due to an entropy penalty for some of the intermediates. We would like to understand the phenomenon (if not resulting from a method artifact) better, hence I am grateful for any hint.