Quasiclassical trajectory calculations of acetaldehyde dissociation on a global potential energy surface indicate significant non-transition state dynamics
ABSTRACT A recent experimental study [Houston, P. L.; Kable, S. H. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 16079] of the photodissociation of acetaldehyde (CH(3)CHO) has suggested two distinct mechanisms for the production of the molecular products CH(4) + CO. One corresponds to the traditional transition state mechanism and the other to a transition state-skirting path similar to the roaming channel previously reported in formaldehyde. To investigate this theoretically, a full-dimensional potential energy surface (PES) has been constructed. The PES was fit with permutationally invariant polynomials to 135,000 points calculated using coupled cluster theory with single and double excitations and a perturbative treatment of triple excitations [CCSD(T)] and correlation consistent basis sets of double- and triple-zeta quality. To test the accuracy of the PES additional CCSD(T) and multireference configuration interaction calculations were carried out. Quasiclassical trajectory calculations were run on the PES starting at the acetaldehyde equilibrium geometry and also at the conventional transition state (TS) for the molecular products CH(4) + CO. The former calculations agree well with the experimental results of Houston and Kable; however, those from the TS do not. The implications for a non-transition state, roaming mechanism in this molecule are discussed.
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ABSTRACT: The correlated speed and rotational energy distributions of the CO fragment from photodissocia-tion of CH3CHO have been measured at a range of wavelengths from 308 to 328 nm. The distri-butions are bimodal, showing low J, slow speed, and high J, fast speed components. The cold component disappears for λ>325 nm. This threshold corresponds to C-H bond cleavage and we assign these CO products as arising from roaming of a H-atom about a CH3CO core. We attribute the hot component to CO formed through CH3-roaming. No evidence was observed for the pres-ence of a transition state mechanism. This is the first time two distinct roaming channels have been observed from the same electronic state. The results support the growing understanding that roaming can be significant in chemical reactions and outweigh traditional pathways.Chemical Science 08/2014; 5(12). DOI:10.1039/C4SC02266A · 8.60 Impact Factor
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ABSTRACT: Reaction mechanisms of the MgH+H→Mg+H2 reaction have been investigated using quantum reactive scattering methods on a global ab initio potential energy surface. There exist two microscopic mechanisms in the dynamics of this reaction. One is a direct hydrogen abstraction reaction and the other proceeds via initial formation of a HMgH complex in the deep potential well. The result of the present quantum dynamics calculations suggests that the HMgH complex formed in the reaction mainly decays into the Mg+H2 channel via a ‘roaming mechanism’ without going through the saddle point region.Chemical Physics Letters 03/2011; 504(4):130-135. DOI:10.1016/j.cplett.2011.02.002 · 1.99 Impact Factor
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ABSTRACT: We review recent progress in developing potential energy and dipole moment surfaces for polyatomic systems with up to 10 atoms. The emphasis is on global linear least squares fitting of tens of thousands of scattered ab initio energies using a special, compact fitting basis of permutationally invariant polynomials in Morse-type variables of all the internuclear distances. The computational mathematics underlying this approach is reviewed first, followed by a review of the practical approaches used to obtain the data for the fits. A straightforward symmetrization approach is also given, mainly for pedagogical purposes. The methods are illustrated for potential energy surfaces for , (H2O)2 and CH3CHO. The relationship of this approach to other approaches is also briefly reviewed.International Reviews in Physical Chemistry 10/2009; 28(4):577-606. DOI:10.1080/01442350903234923 · 4.92 Impact Factor