Conclusions

S TRUCTURES : A C ASE S TUDY *

5.4 Conclusions

In summary, significant improvements in instrumentation provided by our third generation apparatus now permit UED to study structural dynamics in chemical reactions with unprecedented temporal and spatial resolution. These experimental advances have been accompanied by improvements in data processing and use of the diffraction-difference analysis, which were described here in detail. These advances were borne out in the first application of the new apparatus—the study of the non-concerted elimination of iodine from C2F4I2. The structural changes occurring over the course of the reaction were followed with temporal resolution of ~5 ps, with spatial resolution approaching 0.01 Å, and with sensitivity to chemical change of ~1%. The high sensitivity and spatiotemporal resolution permitted the molecular structure of the transient intermediate C2F4I to be determined and refined: the radical is classical, not bridged, in nature—in quantitative agreement with quantum chemical predictions to within 0.03 Å. In the future, additional UED studies of other C2R4X intermediates and related species should provide considerable insight into the respective roles of structure and dynamics in stereochemical control.

Scheme 5-1.Non-concerted elimination reaction of C F I with the hitherto unknown reaction intermediate.

2 4 2

F F

F F I

I FF

FF I

F F FF

I

F

F F

- I ? - I

F

×

×

- I ? - I

Scheme 5-2. Schematic of dihalide elimination reactions involving C R X radical intermediates. Once the parent molecule C R X loses the first X atom, the intermediate species C R X is formed. In the case of a bridged intermediate structure (top brackets), the retention of stereochemical selectivity is derived from the inhibition of rotation about the C-C bond (top product). However, in the case of the classical structure (bottom brackets), the situation is more complex. Rotation about the C-C bond is allowed; if the time scale for the elimination of the second -X atom is much faster than the rotation, one can expect stereochemical selectivity (bottom product), whereas stereochemical control would be lost if the situation were reversed (middle product). With regard to the geometry of the radical site in the classical structure, simple rotation about the C-C bond (prior to the loss of the second X atom) would suffice for the loss of stereochemical control for structures with planar radical centers (middle intermediate), whereas species with non-planar radical centers (bottom intermediate) may require a combination of rotation and inversion.

2 4 2 4 2

2 4

Figure 5-1.Ground-state molecular diffraction image of C F I ._{2 4 2}

Figure 5-2. Refined ground-state structure of C F I . The comparison between the experimental and refined theoretical ( ) and ( ) curves is shown, along with the determined bond distances and angles for the and conformers. Distances are in ångströms, and angles are in degrees. The bond distances for the (black) and

(green) isomers are indicated by vertical lines at the bottom of the ( ) panel.

2 4 2

sM s f r

anti gauche

anti

gauche f r

0 1 2 3 4 5 6

r, Å

2 4 6 8 10 12

s, Å

^-1

sM(s) f(r)

Experiment Theory Experiment

Theory

anti

gauche _a

aa ff

(C-C-F) 109.4 ± 1.0 109.0 107.6 (C-C-I) 111.6 ± 1.0 111.9 114.8 (F-C-F) 107.8 ± 1.0 108.7 107.9

(IC-CI) 180 (fixed) 180.0

(IC-CI) 70 ± 3 67.8

anti gauche

experiment ab initio anti gauche rr

r

(C-C) 1.534 ± 0.013 1.532 1.540 (C-F) 1.328 ± 0.003 1.320 1.323 (C-I) 2.136 ± 0.007 2.159 2.147

Distances

Angles

C-I

C-FC-C I··Igauche I··Ianti

C··F C··I

I

F C

I

I I

F F

F F

F F F

C C C

Molecular Structure of C F I

_{2 4 2}

Figure 5-3. Time-resolved 2D diffraction-difference images of C F I ( = –100 ps).

Each frame is identified by the relative time delay (in picoseconds) between the laser pump and electron probe pulses. The emergence of rings in the difference images with increasing time delay reflects the ensuing molecular structural dynamics. The first image is the ground-state image.

2 4 2 tref

C F I2 4 2

+5

-5 0

-50

+15

+125

+20

+25

+225 +35

+400 +10

+50 +75

Figure 5-4.(a,b) The effect of Fourier filtering on 1-D raw diffraction-difference curves. The raw data is shown in red, and the Fourier filtered data (obtained with a 8.7-Å low-pass filter) is shown in blue. The difference between the raw and filtered data shows the noise removed by the filter. (a) ( ; ; ). (b) ( ; ; ). (c) Background fitting through the zero- crossing points of the experimental ( ; ; ) data. The experimental ( ; ; ) is shown in blue, and the background ( ; ; ), obtained by fitting a low-order polynomial through the zero-crossing points (yellow circles), is shown in red.

R 405 ps -95 ps s R ps 5 ps s

R 5 ps s R 5 ps s

R 5 ps s

E E

E E

E

¥

¥ ¥

¥

B

0 2 4 6 8 10

s (Å^-1)

Difference

(a)

0 2 4 6 8 10

s (Å^-1)

Difference

(b)

0 2 4 6 8 10

s (Å^-1)

data

background zero points (c)

Figure5-5.Time-resolvedstructuralchangesfromtheeliminationofiodinefromCFI.(a)Rawdiffraction-difference signals,(;;)(bluecurves),shownwiththebaselinecurves(red).(b)Experimental(blue)andtheoretical(red) (;;)curvesobtainedatvaryingtimedelays.(c)Correspondingexperimental(blue)andtheoretical(red)(; ;)curves.Majorpeaksarehighlightedwitharrows.

242 Rt-95pss, sMt-95pssft-95 psr

DR(t;-95ps;s) 024681012 s(Å-1 )

10ps 15ps 20ps 25ps 30ps 40ps 55ps 80ps 130ps 230ps 405ps

5ps

0ps

-45ps

(a)

DsM(t;-95ps;s) 024681012 s(Å-1 )

10ps 15ps 20ps 25ps 30ps 40ps 55ps 80ps 130ps 230ps 405ps

5ps

0ps

-45ps

(b)

Df(t;-95ps;r) 01234567 r(Å)

10ps 15ps 20ps 25ps 30ps 40ps 55ps 80ps 130ps 230ps 405ps

5ps

0ps-45ps

(c)

Figure5-6.Time-resolvedstructuralchangesinvolvingonlytheCFICF+Icontributiontothediffraction-differencesignal.(a) Rawdiffraction-differencesignals,(;;),(bluecurves),shownwiththe(nearlylinear)baselinecurves(red).(b)Experimental (blue)andtheoretical(red)(;;)curvesobtainedatvaryingtimedelays.(c)Correspondingexperimental(blue)andtheoretical (red)(;;)curves.Notetheabsenceofapeakat~5.1ÅcorrespondingtothedepletionofI···Iinternucleardistances.

2424® Rt5pss sMt5pss ft5psr

DR(t;5ps;s) 024681012 s(Å-1 )

10ps 15ps 20ps 25ps 30ps 40ps 55ps 80ps 130ps 230ps 405ps

(a)

DsM(t;5ps;s) 024681012 s(Å-1 )

(b)

10ps 15ps 20ps 25ps 30ps 40ps 55ps 80ps 130ps 230ps 405ps

Df(t;5ps;r) 01234567 r(Å)

(c)

10ps 15ps 20ps 25ps 30ps 40ps 55ps 80ps 130ps 230ps 405ps

Figure 5-7.Time dependence of the formation of C F molecules from the decay of C F I transient structures in the ( ; ; ) data. The curve is an exponential fit of the C F fraction (with the temporal pulse widths of the electron and laser pulses taken into account); the apparent time constant for the formation of C F was of 25 ± 7 ps. Each error bar represents one standard deviation.

2 4 2 4

2 4

2 4

sM t 5 ps s 0

2 4 6 8 10 12

0 50 100 150 200 250 300 350 400

time (ps)

C2F4I®C2F4(%) t2 = 25 ± 7 ps

Figure 5-8.Structural determination of the transient C F I intermediate.

Comparison of experimental ( ; ; ) and ( ; ; ) curves (blue) with corresponding theoretical curves (red) obtained via calculations of the bridged structure for C F I. Comparison of experimental ( ; ; ) and ( ; ; ) curves with theoretical curves obtained using the

classical ( and ) C F I structures.

2 4

2 4

2 4

( , ) ( , )

A B C D

sM 5 ps s f 5 ps r ab initio

sM 5 ps s

f 5 ps r ab initio

anti gauche

¥ ¥

¥ ¥

r, Å r, Å

s, Å

^-1

s, Å

^-1

D sM(s) D f(r)

Bridged Classical

A

B

C

D

Structure of C F I Transient Intermediate

_{2 4}

2 4 6 8 10

0 0 2 4 6 8 10

0 1 2 3 4 5 6 0 1 2 3 4 5 6

Figure 5-9. Refinement of the C F I radical structure. (a,b) Comparison of experimental (; ; ) (a) and (; ; ) (b) curves (blue) with corresponding theoretical curves (red) obtained from the least-squares refinement of the C F I structure (see text).

2 4

2 4

sM 5 ps s f 5 ps r

sM(¥ ps; 5 ps; s)

0 2 4 6 8 10

s (Å^-1)

(a)

f(¥ ps; 5 ps; r)

0 1 2 3 4 5 6

r (Å)

(b)

Figure5-10.CompletestructuraldeterminationoftheCFIeliminationreaction.Thebonddistancesandanglesforallthree speciesareshown,alongwiththeDFTvaluesforcomparison.Distancesareinångströms,andanglesareindegrees.Notethatthe C–Cbonddistancebecomesprogressivelyshorterfromreactantthroughintermediatetoproduct,thusreflectingthechange bondorderfromasinglebondtoadoublebond.Also,notethechangeinbondanglesofCFIwhichreflectsrelaxationofstructure.

242 24

<5ps26±7ps

a a a f f (C-C-F)109.4±1.0109.0107.6 (C-C-I)111.6±1.0111.9114.8 (F-C-F)107.8±1.0108.7107.9 (IC-CI)180(fixed)180.0 (IC-CI)70±367.8

anti gauche

experimentabinitio antigauche

r r r (C-C)1.534±0.0131.5321.540 (C-F)1.328±0.0031.3201.323 (C-I)2.136±0.0072.1592.147

Dist ances Angles

’ ’’

(C-C-F)108.6±6.0108.6109.8,108.1 (C-C-I)115.0±3.1112.7111.8 (F-C-F)108.0±11.2108.8108.0 (C-C-F)117.9±3.1114.0112.3,113.8 (F-C-F)119.8±7.8111.8111.2 a a a a Experimentabinitio antigauche

r r r r (C-C)1.478±0.0491.5031.508 (C-F)1.340±0.0371.3221.327,1.323 (C-I)2.153±0.0132.1642.149 (C-F)1.277±0.0271.3041.309,1.307’

Dist ances Angles

(C-C-F)123.8±0.6123.8

ExperimentDFT

r r(C-C)1.311±0.0211.306 (C-F)1.319±0.0061.312

Dist ances Angles

F’

F’ F F

CC

I 250fs

Table 5-1. Comparison of the experimental values of the independent structural parameters of the classical C F I radical intermediate with those obtained via quantum chemical calculations. The bond distances are in ångströms and the bond angles are in degrees.

2 4

(C=C) 1.478 ± 0.049 1.503 1.508

(C-F) 1.340 ± 0.037 1.322 1.327, 1.323

(C-I) 2.153 ± 0.013 2.164 2.149

(C-F') 1.277 ± 0.027 1.304 1.309, 1.307

CCI 115.0 ± 3.1 112.7 111.8

CCF 108.6 ± 6.0 108.6 109.8, 108.1

FCF/2 54.0 ± 5.6 54.4 54.0

CCF 117.9 ± 3.1 114.0 112.3, 113.8

F CF /2 59.9 ± 3.9 55.9 55.6

Experiment PredictedValues

anti anti gauche

r r r r Ð Ð Ð Ð Ð

' ' '

F’

F’

F F

C C

I

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6

D ^ARK S ^TRUCTURES I ^N