Further investigation of the HCl elimination in the photodissociation of vinyl chloride at 193 nm: a direct MP2/6-31G(d,p) trajectory study
Introduction
Vinyl chloride (VCl, C2H3Cl) is a prototypical system for studying molecular eliminations in saturated olefins. The photodissociation dynamics of VCl has been extensively investigated during the last decades, and a brief history of this subject is reported in a previous paper (hereinafter referred to as paper I) [1]. At an excitation wavelength of 193 nm, elimination of hydrogen chloride (HCI) takes place on the ground electronic state following internal conversion from the optically prepared state [2], and may primarily occur by either a three-center (3C) or a four-center (4C) mechanism. On the basis of ab initio calculations, contributions from other possible HCl elimination channels are expected to be negligible [3]. The 3C channel produces HCl and vinylidene (H2CC, which then isomerizes to acetylene) and has a null or very small recombination barrier (see Fig. 1). On the other hand, the 4C channel leads directly to acetylene (HC≡CH) + HCl, and presents a large reverse barrier. The 3C elimination is found to be more important than the 4C channel. Specifically, experimental estimates yield a 3C/4C branching ratio of 2.3/1 [4] or 3/1 [5], and RRKM calculations predict a ratio of 6.7/1 [6].
An interesting feature of the photodissociation of VCl at 193 nm is the fact that the product translational energy distribution, P(Et), for the elimination of hydrogen chloride is clearly nonstatistical (it peaks at 12–13 kcal/mol) [2], [7], in spite of the small recombination barrier of the 3C channel (i.e., one would expect a near statistical P(Et) with a maximum probability less than the reverse 3C barrier height). Notice that the most probable translational energy in the P(Et) is governed by the dominating, 3C channel [1]. An explanation to this puzzling observation was first given by Gordon and co-workers [8], who proposed that the large translational energy release arises from isomerization of vinylidene to acetylene on a time scale comparable to the separation of the two photofragments; in other words, the 3C elimination of HCl and the vinylidene isomerization would take place concertedly (but nonsynchronously).
In paper I, we studied the HCl elimination in the photodissociation of VCl at 193 nm using classical trajectory calculations. The trajectories were propagated with the energy and gradients taken directly from semiempirical AM1 calculations, using specific reaction parameters (SRPs). The calculated P(Et) was found to be in good agreement with the experimental distribution, and the lifetime predicted for the isomerization of vinylidene to acetylene (58–77 fs) was within the range of the experimental estimate obtained from line width analysis of negative ion photodetachment (40–200 fs) [9]. The AM1-SRP calculations, therefore, supported that the 3C HCl elimination occurs, mainly, in concert with the isomerization of vinylidene.
Another important feature in the photodissociation of vinyl chloride at 193 nm concerns the HCl rotational distributions. Recently, Lin et al. [6] reported rotationally resolved emission spectra from HCl (up to v=7) after photolysis of VCl at 193 nm. All the explored vibrational levels appear to show a bimodal rotational distribution with one component corresponding to ≈500 K and the other to ≈9500 K. On the basis of statistical phase space theory, separate statistical ensemble, and impulse model approaches, they concluded that the components with low and high rotational temperatures correspond to HCl (v,J) produced from 4C and 3C channels, respectively. The rotational distributions calculated in paper I appear to be in reasonable accord with those reported by Lin et al. [6]. However, the calculations do not predict bimodal distributions for v=0–6, at least apparently. Furthermore, contrary to their interpretation, the calculations show that elimination through the 4C channel leads to rotationally hot HCl molecules.
Although direct AM1-SRP trajectory calculations have been successfully applied to the study of the photodissociation dynamics in halo-derivatives of ethylene [10], [11], [12], one may argue that in the case of VCl these qualitative discrepancies (concerning the HCl rotational distributions) come from a lack of accuracy of the AM1-SRP potential energy surface (PES). Actually, because in our parameterization procedure1 we only use properties of the transition state and products, and the reverse barrier height, other regions of the potential surface may not be well enough reproduced by the AM1-SRP model PES. Shortcomings of semiempirical-SRP Hamiltonians were discussed in detail by Peslherbe and Hase [13]. It would be desirable, therefore, to perform further trajectory calculations using a more rigorous PES in order to provide a more accurate description of the photodissociation dynamics of this prototypical system. In this Letter we report direct MP2/6-31G(d,p) trajectory calculations for the HCl elimination from the photodissociation of vinyl chloride at 193 nm, paying attention to the two appealing features mentioned above. As in paper I, for each channel we started the trajectories at the relevant transition states (TS3 and TS4, see Fig. 1), using microcanonical initial conditions. This is a reasonable assumption even though the process of internal conversion may lead to some selectivity of the vibrational modes excited, and, since the lifetime is short at 193 nm of excitation, intramolecular vibrational energy redistribution may not be complete before VCl decomposes.
Section snippets
Methods
The MP2/6-31G(d,p) level of theory was used for the calculation of energies and forces because it gives a good balance between accuracy and computational cost. As shown in the work by Riehl and Morokuma [3], the energetics of the 3C and 4C HCl elimination channels (in particular for the exit channel regions) predicted at the MP2/6-31G(d,p) level are in reasonably good agreement with the more accurate, single point QCISD(T)/6-311+G(d,p) data using the MP2/6-31G(d,p) optimized geometries (see
Results and discussion
Fig. 2a and b depicts the product translational energy distributions P(Et) for the 3C and 4C channels, respectively, obtained in this work and in paper I (denoted in the figure as AM1-SRP). As can be seen, for the 3C channel the MP2/6-31G(d,p) direct dynamics distribution is in very good agreement with that obtained using the AM1-SRP PES. By contrast, the P(Et) predicted by using the MP2 surface for the 4C channel is narrower and peaks at a lower energy. This may appear to be striking because
Acknowledgements
E.M.-N. and J.F.C. thank the Spanish Ministry of Science and Technology for their Ramón y Cajal research contracts. Financial support from the Spanish Ministry of Science and Technology (Grants BQU2000-0462 and BQU2002-04627-C02-02) and from the European Commission within the RT Network ‘Reaction Dynamics’ (Contract No. HPRN-CT-1999-00007) is gratefully acknowledged.
References (25)
- et al.
Chem. Phys. Lett.
(2001) - et al.
Chem. Phys. Lett.
(1992) Ann. Phys. (NY)
(1958)- et al.
Chem. Phys. Lett.
(1986) - et al.
J. Phys. Chem. A.
(2003) - et al.
J. Chem. Phys.
(1998) - et al.
J. Chem. Phys.
(1994) - et al.
J. Am. Chem. Soc.
(1979) - et al.
J. Chem. Phys.
(1993) - et al.
J. Chem. Phys.
(2001)