(a) (e) (b) (f) (c) (g) (h) (d) Chapter 6. The Dissociation Reaction of Acetylacetone

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1 Chapter 6. The Dissociation Reaction of Acetylacetone (e) (f) (g) (h) Fig.6-. Difference patterns created by subtracting the ratio pattern of the 77 ps time point from the ratio patterns recorded at 27 ps, +3 ps, +8 ps, +23 ps, (e) +28 ps, (f) +33 ps, (g) +43 ps, and (h) +73 ps. 6

2 Chapter 6. The Dissociation Reaction of Acetylacetone (e) (f) (g) Fig.6. Difference patterns created by subtracting the ratio pattern of the 77 ps time point from the ratio patterns recorded at +98 ps, +23 ps, +73 ps, +223 ps, (e) +423 ps, (f) +823 ps, and (g) +273 ps. 62

3 Chapter 6. The Dissociation Reaction of Acetylacetone Fig.6-3. Comparison between data recorded with and without a neutral-density filter (% transmission) in the beam path. Difference patterns are created by subtracting the ratio pattern of the 77 ps time point from the ratio patterns recorded at +98 ps with laser filter, +98 ps without laser filter, +273 ps with laser filter, and +273 ps without laser filter. The filtered data were used in the analysis reported herein. 63

4 Chapter 6. The Dissociation Reaction of Acetylacetone 64 (e) (f) (g) (h) Fig.6-4. Isomers of the enol tautomer of acetylacetone. The naming convention is defined with respect to rotations about the C single bond, the C C double bond, and the C C single bond. C = cis; T = trans. CCC, TCC, CTC CTT, (e) TTT, (f) CCT, (g) TCT, (h) TTC.

5 Chapter 6. The Dissociation Reaction of Acetylacetone 65 (e) (f) (g) C 3 (h) Fig Possible reaction products in the photolysis of CCC enolic acetylacetone. keto tautomer of acetylacetone. Four isomers of the 2-penten-4-on-yl radical + the radical TC, TT (e) CT, and (f) CC. (g) The CCC isomer of the formylacetonyl radical + the methyl radical. (h) The acetyl radical loss products.

6 Chapter 6. The Dissociation Reaction of Acetylacetone s (Å - ) s (Å - ) s (Å - ) s (Å - ) Fig Fits of possible reactions: left column sm(s) and right column f(r) having components fractional contributions and polynomial background optimized. The data (squares) is the +273 ps data point (t = 77 ps) and theory (solid line) is derived from ref the DFT structures of possible products, here the enol isomers: CCC, TCC, 2 TCT, TTC. and R are given in Table 6.

7 Chapter 6. The Dissociation Reaction of Acetylacetone s (Å - ) s (Å - ) s (Å - ) s (Å - ) Fig Fits of possible reactions: left column sm(s) and right column f(r) having components fractional contributions and polynomial background optimized. The data (squares) is the +273 ps data point (t = 77 ps) and theory (solid line) is derived from ref the DFT structures of possible products, here the enol isomers: TTT, CCT, 2 CTC, and CTT. and R are given in Table 6.

8 Chapter 6. The Dissociation Reaction of Acetylacetone s (Å - ) s (Å - ) s (Å - ) Fig Fits of possible reactions: left column sm(s) and right column f(r) having components fractional contributions and polynomial background optimized. The data (squares) is the +273 ps data point (t = 77 ps) and theory (solid line) is derived from ref the DFT structures of possible products, here a : mixture of vibrationally hot enol CCC and keto acetylacetone tautomers, the methyl radical loss products, and the 2 acetyl radical loss products. and R are given in Table 6.

9 Chapter 6. The Dissociation Reaction of Acetylacetone s (Å - ) s (Å - ) s (Å - ) s (Å - ) Fig Fits of possible reactions: left column sm(s) and right column f(r) having components fractional contributions and polynomial background optimized. The data (squares) is the +273 ps data point (t = 77 ps) and theory (solid line) is derived from ref the DFT structures of possible products, here the loss products with the 2 2-penten-4-on-yl radical isomers CC, CT, TC, TT. and R are given in Table 6.

10 Chapter 6. The Dissociation Reaction of Acetylacetone s (Å - ) s (Å - ) Fig. 6-. The data (squares) and the refined theoretical model (line) corresponding to the reaction of enolic acetylacetone to form the TC 2-penten-4-on-yl and radicals. sm(s) comparison and f(r) comparison. The experimental 2 sm(s) (squares) and the polynomial background (line). = 8.4 and R =.35.

11 Chapter 6. The Dissociation Reaction of Acetylacetone 7 (e) (f) (g) (h) (i) (j) (k) s (Å - ) Fig. 6-. The experimental (squares) and refined theoretical (solid line) sm(s) (left) and f(r) (right) at multiple time points (t ref = 77 ps). t = +28 ps, +33 ps, +43 ps, +73 ps, (e) +98 ps, (f) +23 ps, (g) +73 ps, (h) +223 ps, (i) +423 ps, (j) +823 ps, (k) +273 ps. Fitting information is given in Table 6-4.

12 Chapter 6. The Dissociation Reaction of Acetylacetone product fraction (%) time delay (ps) Fig. 6. The fit of a single-step reaction process (solid line) to the fraction of product structure at each time point (squares). The time constant for the reaction is 247 ± 34 ps.

13 Chapter 6. The Dissociation Reaction of Acetylacetone 73 ( - x)% parent x% product - ( - x)% parent (x - y) % intermediate y% product = -(x-y)% intermediate (x-y)% product Fig A schematic illustration of the process used to isolate the reaction of the formation of product from an intermediate structure. Data from a time point when the reaction is complete and only parent and product are present has data from an earlier positive time point subtracted from it. This positive point contains unreacted parent, product, and reaction intermediate. The resulting difference data has the parent contribution removed and contains only the structural information of product and intermediate molecules.

14 Chapter 6. The Dissociation Reaction of Acetylacetone 74 (e) s (Å - ) (f) S 428 K S 2256 K S K S 2 K T K Fig The calculated molecular structure of some of the electronic states of enolic acetylacetone S, S n *, S 2 *, and T *. The sm(s) and f(r) curves calculated using these structures. S at 428 K (broad solid line), S at 2256 K (narrow solid line), S at K (dashed line), S at K (dotted line), and T at 2 K (dot-dash line).

15 Chapter 6. The Dissociation Reaction of Acetylacetone 75 (e) s (Å - ) (f) s (Å - ) (g) s (Å - ) (h) s (Å - ) Fig The alternate sm(s) (left) and f(r) (right) created by subtracting the +73 ps time point from the +273 ps time point. ere the experimental (squares) data is compared with theory (solid line) corresponding to the reaction of the intermediate to the refined product. The intermediate in each is represented by enol S at 2256 K, enol S at K, enol S 2at K, and enol T at K.

16 Chapter 6. The Dissociation Reaction of Acetylacetone 76 S, * 2 S, n * Energy T, * loss products S Fig The schematic representation of the dynamics of acetylacetone. Excited molecules arrive in the S 2 state where ultrafast internal conversion sends population to the S state. Intersystem crossing to T takes place over 247 ps ( ). Dissociation from T to the radical products is then ultrafast such that its population never builds up enough to be detected by UED.