Computational studies of the reactions of CH3I with H and OH Page: 5
This article is part of the collection entitled: UNT Scholarly Works and was provided to UNT Digital Library by the UNT College of Arts and Sciences.
Extracted Text
The following text was automatically extracted from the image on this page using optical character recognition software:
and a TS could be located only at the MP2/6-31G(d) level of theory. G2 analysis revealed that the
energy of this apparent TS in fact lies between the energies of the reagents and products, which is
consistent with the idea that there is no barrier to reaction 2b beyond the endothermicity. This is
confirmed by the QCISD(T)/6-3111G(d,p) energies calculated at selected points along the
MP2/6-31 G(d) intrinsic reaction coordinate (IRC), and plotted in Fig. 3. The energy is below that
of HOI + CH3 at all points. A distinct TS therefore cannot be localized and canonical TST cannot
be applied. For this channel k2b was roughly estimated via the relation k = A exp(-Ea/RT) by
assuming a preexponential factor A = 1 x 1011 cm3 molecule' s1, equal to that measured for CF3J
+ OH [21], and an activation energy Ea = 23 kJ mol. This is equal to the endothermicity at 298 K
based on DH298(HO-I) = 216 kJ mol"[21]. Strictly this is an upper bound and any variational
effects on the fairly flat IRC of Fig. 3 would tend tolower k2b.
Visualization of the normal modes of the H-abstraction TS, 2a, showed that the lowest
frequency mode is predominantly internal rotation of the OH group. Analysis by the method of
Truhlar [22] using HF and MP2 data yielded reduced moments of inertia of 4.33 and 9.77 x 10~6
kg m2, respectively, which were employed in the TST calculations. Figure 4 shows that, unlike in
the previous system, the more reactive bond is the stronger C-H bond and that CH2I + H20
formation, the more exothermic channel, is expected to dominate at all temperatures. A check on
the reliability of the kinetic calculations is the comparison with the measurements of the total
removal rate constant k2 = k2a + k2b by Brown et al. [6]. They are seen to be in excellent
agreement, as is the room temperature k2 value obtained by Gilles et al. [20]. Again, the
geometric mean of the HF and MP2-based TST results gives the best accord with experiment and
provides a TST extrapolation of these measurements to combustion conditions (Table 3). This
extrapolation at 2000 K is about 25 times greater than the value obtained from a simple linear
Arrhenius extrapolation of the data of Brown et al. [6], which reflects the significant curvature of
the TST Arrhenius plot. This predicted curvature is similar to that measured for the analogous
reaction of OH with CH4 [23]. The TST analysis also indicates that HOI formation by OH attack
on CH3I is of only minor importance in both flames and the atmosphere.5
Upcoming Pages
Here’s what’s next.
Search Inside
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
Marshall, Paul; Misra, Ashutosh & Berry, Rajiv. Computational studies of the reactions of CH3I with H and OH, article, March 26, 1998; [Amsterdam, Netherlands]. (https://digital.library.unt.edu/ark:/67531/metadc725856/m1/5/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.