• 沒有找到結果。

Observation of transition state in Raman triggered oxidation of chloroform in the ground state by real-time vibrational spectroscopy

N/A
N/A
Protected

Academic year: 2021

Share "Observation of transition state in Raman triggered oxidation of chloroform in the ground state by real-time vibrational spectroscopy"

Copied!
6
0
0

加載中.... (立即查看全文)

全文

(1)

Observation of transition state in Raman triggered oxidation of chloroform

in the ground state by real-time vibrational spectroscopy

Izumi Iwakura

a,b

, Atsushi Yabushita

c

, Takayoshi Kobayashi

b,c,d,e,* a

JSPS Research Fellow, 8 Ichibancho, Chiyoda-ku, Tokyo 102-8472, Japan

bDepartment of Applied Physics and Chemistry and Institute for Laser Science, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan cDepartment of Electrophysics, National Chiao-Tung University, Hsinchu 300, Taiwan

d

ICORP, JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

e

Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0971, Japan

a r t i c l e

i n f o

Article history:

Received 24 October 2007 In final form 5 March 2008 Available online 10 March 2008

a b s t r a c t

A transition state (TS) during Raman triggered oxidation of chloroform was directly observed by ultrafast spectroscopy using broadband visible sub-5 fs pulses. Changes in the molecular structures including the TSs along the reaction pathways were detected by the time-dependent instantaneous frequencies of vibrational modes. The Raman triggered oxidation of chloroform was found to be initiated by the excita-tion of vibraexcita-tional levels through stimulated Raman processes involving the ground-state chloroform– oxygen complex.

Ó 2008 Published by Elsevier B.V.

1. Introduction

It was a chemists’ dream for years to precisely observe the transition states (TS). However, researches in the fields of phys-ical chemistry have recently succeeded to identify reaction inter-mediates and theoretical analysis helps us to investigate TS. The detailed knowledge of real chemical reactions including TSs will provide us revolutionary way to design reaction processes to im-prove efficiencies and varieties of products, which are hard to be found in a blind way using conventional methods. To confirm the molecular structure in the TS obtained by theoretical analy-sis, the direct observation of the TS during chemical reactions has been desired by chemists for many years and realized by ultrafast spectroscopy using ultrashort laser pulses. The develop-ment of NOPA (non-collinear optical parametric amplifier)[1–6]

in 2002 has enabled stable generation of visible-near infrared sub-5 fs laser pulses in our group [3]. As a typical application, reaction processes including TSs were studied by detecting the structural changes using the sub-5 fs pulses [7–11]. Using the ultrashort pulses, we tried to solve the reaction mechanism by observation of the TS in a chemical reaction and supporting DFT calculation.

Chloroform is known to be easily decomposed by oxygen with sunlight, and to produce poisonous phosgene, hydrochloric acid, and carbon dioxide[12]. It is one of problem in organic synthesis processes. Furthermore, chloroform may be formed in drinking water in daily life. The knowledge chloroform reaction is important not only for the organic synthesis, but also for life hazard.

The present Letter is an extension to the Raman triggered oxida-tion of chloroform in the ground electric state was fully traced along the reaction pathways including TS. The analysis has been achieved by triggering the reaction through the stimulated Raman excitation of coherent distribution of vibrational levels using a broadband laser pulse of NOPA, whose spectrum is as broad as 5200 cm1. Though the reaction is ignited by an optical pulse,

electronic excited states are just virtually involved. 2. Experimental

Chloroform was bubbled with nitrogen gas for 5 h to remove oxygen solved in chloroform to prepare deoxidized chloroform used as one of the samples in this work. Another sample, O2

satu-rated chloroform, was prepared by bubbling with oxygen gas for 5 h to solve oxygen in chloroform. Subtracting the absorption spec-trum of deoxidized CHCl3from the absorption spectrum of

oxygen-dissolved samples, we found absorption peak in the vicinity of 280 nm. Therefore, it is thought that the charge-transfer complex O2CHCl3 are contained in O2 bubbled chloroform. The width,

wavelength, and repetition rate of the pulses of NOPA are sub-5 fs, sub-52sub-5–72sub-5 nm and sub-5 kHz respectively. The pump and probe intensities of the pulses are 2200 and 480 GW/cm2, respectively.

SeeSupplementary materialfor further details. 0009-2614/$ - see front matter Ó 2008 Published by Elsevier B.V.

doi:10.1016/j.cplett.2008.03.003

* Corresponding author. Address: Department of Applied Physics and Chemistry and Institute for Laser Science, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan. Fax: +81 42 443 5825.

E-mail address:kobayashi@ils.uec.ac.jp(T. Kobayashi).

Contents lists available atScienceDirect

Chemical Physics Letters

(2)

3. Results and discussion 3.1. DFT calculation

3.1.1. Possible seven mechanism of photoreaction of CHCl3–O2

complex

Under light irradiation, chloroform 1 is photo-dissociated into CCl2 2 [13,14] andCHCl2 3 [15,16] and easily reacts with O2 4

and O 5[17]to generate ClCO, CHClO, CCl2O, etc.[18]. The C–H

insertion reaction is also known as photo-oxidation of chloroform

[19,20].

When chloroform and oxygen are photo-excited, the following seven reactions are possible: photo-oxidation of chloroform through C–H insertion (1) or C–Cl insertion (2)[21], reaction of CCl2with O2(3) or CHCl3(4), dimerization ofCHCl2(5), and

reac-tion of

CHCl2with O2(6) or O (7). The results of all ground-state

reactions calculated with the B3LYP/6-311+G** method [22] are

shown inFigs. 1 and 2. The images of activation energy barriers of TSs, which start the reaction under light irradiation, are shown as broken curves inFig. 1 [23].

Fig. 1. Reaction pathway of photo-oxidization of chloroform estimated by B3LYP/6-31G*

. Under the irradiation of light, the photon energy is converted to chemical reaction energy resulting in radical dissociation. Molecular structures of TSs for these reactions could not be estimated because of the complexity in the calculations [26]. Therefore, the images of activation energy barriers of these TSs are shown as broken curves.

Fig. 2. Calculated structure at B3LYP/6-31G*

of the species in the reaction processes discussed. In each molecular model, the Cl is colored orange, O red, C black, and H gray. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(3)

CHCl31 þ O24 ! CCl3OOH 7 ! CCl2O    HO 9 þ Cl ð1Þ CHCl31 þ O24 ! CHCl2OOCl 10 ð2Þ CCl22 þ O24 ! CCl2OO 13 ! CCl2O 15 þ O ð3Þ CCl22 þ CHCl31 ! C2Cl422 þ HCl ð4Þ  CHCl23 þ CHCl23 ! C2H2Cl423 ð5Þ  CHCl23 þ O24 ! CHCl2OO 24 ð6Þ  CHCl23 þ O 5 ! CHCl2O 25 ! CHClO 27 þ Cl ð7Þ

3.1.2. The mechanism of the photo-oxidization of chloroform (schemes

(1) and (2))

As the calculated results of activation energies only scheme(1)

is possible. The detail can be explained as follows.

When O2is dissolved in chloroform (solubility constant of O2in

chloroform being 9.8 mM atm1)[24], O

2is coordinated to the H

of chloroform and generates a coordinated complex CHCl3O26

(–0.5 kcal mol1, 16% of the dissolved O

2being coordinated in 6 [25]).

In case of scheme(1), under light irradiation, the photon energy is converted to chemical reaction energy and dissociates the C–H bond of CHCl3. Then C–H insertion of O2, proceeds to generate an

intermediate CCl3OOH 7 (–11.7 kcal mol1) via the TS of C–H

inser-tion. Then, OH and Cl are dissociated from CCl3OOH 7 with

activa-tion energy of 5.6 kcal mol1via TS 8, and the product phosgene 9

is coordinated with dissociated OH. The imaginary frequencies of TS 8 indicate that a barrier is present along the OH rotational mo-tion of CCl3O–OH 7. The intrinsic reaction coordinate were further

calculated to confirm that TS 8 lies on the saddle points of the en-ergy surface between the intermediate 7 and the product 9. When the OH of CCl3O–OH 7 rotates in such a way that OH approaches Cl

within the distance of the van der Waals radii of OH and Cl, then these two species start to dissociate from of Cl–CCl2O–OH 7.

On the other hand, in case of scheme(2), C–Cl insertion of O2,

does not proceed because its intermediate CHCl2OOCl 10 is less

sta-ble than the reactant of CHCl3O2complex 6 by 21.1 kcal mol1.

3.1.3. The mechanism of the photoreaction of CCl22 (schemes(3) and (4))

The case of CCl22 generating under photo-dissociation of

chlo-roform, as the calculated results of activation energies only scheme

(3)is possible. The detail can be explained as follows.

In case of scheme(3), the generated CCl22 will react with triplet

state O2, (reactant CCl2+ O2 161.3 kcal mol1), and a complex

CCl2O211 is formed (161.0 kcal mol1, triplet state). The complex

11 changes to TS 12 with 4.6 kcal mol1 and an intermediate

CCl2OO 13 (147.3 kcal mol1, triplet state) in series. Then, O is

dis-sociated from CCl2O–O 13 via TS 14 with 1.1 kcal mol1followed

by phosgene 15 being produced. It looks less stable than the reac-tant energy of CHCl3 and O2by 100 kcal mol1or more because

photo-dissociation products

H and

Cl have high energies. In case the triplet state intermediate 13 is converted to a singlet state 16 by intersystem crossing, neither ‘O-dissociated from CCl2O–O 14 TS 17’ nor ‘TS 18 react to generate dichlorocarbon

per-oxide 19’, because of their high activation energies of 44.5 and 18.4 kcal mol1, respectively.

If case of scheme(4), CCl22 reacts with CHCl3, their complex

CCl2–CHCl320 (157.8 kcal mol1) is produced. However, the

acti-vation energy of TS 21 to generate CCl2–CCl2 22 is so high,

40.4 kcal mol1, that the reaction is unlikely to take place.

3.1.4. The mechanism of the photoreaction of

CHCl23 (schemes(5)– (7))

The case of 

CHCl2 3 generating under photo-dissociation of

chloroform, as the calculated results, all schemes(5)–(7)cannot exclude. The detail can be explained as follows.

In case of schemes(5)–(7), when

CHCl2is generated under light

irradiation on CHCl3, dimerization process 

CHCl2 3 producing

CHCl2–CHCl223, scheme(5), the reaction between 

CHCl2and O2

producing CHCl2OO 24, scheme (6), and the reaction between 

CHCl2and O generating 

CHCl2O 25, scheme(7), have heats of

reac-tion of 73.0, 22.6, 88.4 kcal mol1, respectively. Radicals

CHCl2and

O react rapidly with each other to form

CHCl2O 25. After that, one

of the 

Cl is easily dissociated with activation energy lower than 1 kcal mol1via TS 26 and changes to CHClO 27.

3.2. Pump–probe experiment

3.2.1. Real-time trace and FFT analysis

The above theoretical calculations show that schemes(1), (3),

(5)–(7)are likely to take place. We have thus used sub-5 fs pulses

[2,3]to observe the real-time dynamics of the photo-oxidization of chloroform discussed above.

The oscillatory absorbance change (DA) signal of deoxidized chloroform around finite positive value of DA (Fig. 3a) is ascribed to the electronic excited state excited by multi-photon absorption. The Fourier transform (FT) of the real-time trace has peaks at 260 (dC–Cl: deformation), 366 (dC–Cl), 667 (mC–Cl: stretching), and

1237 cm1(d

C–H) (Fig. 3c and d), which agree well with the

corre-sponding Raman frequencies[26]. The signal of O2-dissolved

chlo-roform around 0 (Fig. 3c) indicates that there is no electronic excitation. And so, the signal observed can be concluded to be due to CHCl3O2 complex. The reason why CHCl3O2 complex

was observed can be given as follows. The formation of oxygen– chloroform complex reduces the electronic transition energy from 7.3 eV (170 nm) to 4.4 eV (280 nm) by the charge-transfer interac-tion between chloroform and O2. The transition of 4.4 eV is due to

the charge-transfer band, which is expected to increase polarizabil-ity change upon excitation resulting in the increase in the Raman cross section. The transition is still non-resonant even after the complex formation, but the detuning energy is reduced from 4.8 to 1.9 eV than that of deoxidized chloroform, therefore CHCl3O2

complex was more efficiently observed.

Chloroform does not absorb the laser, but the sub-5 fs pulses ex-cited vibrational levels in the ground state by stimulated Raman process, and the oxidation of O2coordinated complex 6 is triggered.

Concerning the amount of population of the vibrational levels in the 5200 cm1 range, photo-irradiation by the laser pulse is almost

equivalent to thermal-irradiation at 7500 K of vibrational (non-equilibrium) temperature by the stimulated Raman process, be-cause the laser spectrum has a broad spectral width of 5200 cm1,

extending from 525 to 725 nm. Usually, the vibrational ladders can be climbed up one by one by large number of multi-photon res-onant IR photons from high power laser via high-order nonlinear interaction. However, the photo-irradiation of the broadband pulse used in the present study can excite to vibrational levels equivalent to about 7500 K through the stimulated Raman process. The Raman process can excites the sample to single quanta at one time. Of course the state after the stimulated Raman excitation is not equilib-rium state, in which temperature cannot be defined. However, the state has populations in the vibrational level, which can only be reached in molecular systems in equilibrium state at 7500 K.

It is impossible to excite the electronic state in chloroform by one photon absorption process even though the laser spectrum used in the present work has a broad frequency width of 5200 cm1corresponding to 7500 K. FFT amplitude of the

absor-bance change of O2-dissolved chloroform is linearly proportional

to the pump intensity (Fig. 4), which shows that the state to be ob-served is not electronic excited states excited by multi-photon absorption, but the lowest excited vibrational level of v = 1.

It may sound difficult to believe single quanta would trigger a chemical reaction. However, there might not be report example

(4)

of reaction triggered by single quanta, the reaction never has the positive proof of not being triggered by single quanta. The experi-mental result obtained in this work can be explained only by the reaction triggered by single quanta from the linearity relation be-tween the vibrational amplitude of the O2CHCl3and pump

inten-sity. Because of the broad bandwidth of the laser pulse, the excitation of the vibrational modes is equivalent to the excitation by heat of 4500 K (=3000 cm1). Therefore, it is reasonable the

excitation does not occur within seconds or less in the natural world at room temperature. There can be a possibility that the reaction in solution discussed in this work may take place more efficiently because of the following reasoning than that in a gas phase. Molecular vibration modulates the atomic bond length

and charge on atoms. When the molecule is polarized under the modulation of the charge, surrounding solvent molecules work as a ligand and increase the bond distance, which accelerates the reaction. It shows that the solvent is converting the vibration en-ergy into the enen-ergy for reaction. The FT signal of the O2-dissolved

chloroform has an additional peak to those of deoxidized chloro-form, which is observed at 1782 cm1(m

C@O) (Fig. 3c and d).

3.2.2. Spectrogram analysis

The calculated spectrogram at 666 nm using the Blackman win-dow of 240 fs FWHM shows a gradual change in the FT spectrum from the beginning of the reaction to 1300 fs after excitation. The time and frequency resolutions of the spectrogram were estimated Fig. 3. Experimental results of the real-time trace of induced absorbance change probed at 660670 nm (1.881.85 eV): (a) deoxidized CHCl3and (c) O2-dissolved CHCl3, and

(5)

as 100 fs and 30 cm1, respectively. The data in the vicinity of 0 fs

up to 250 fs could not be obtained with high precision because of the finite window width and the effect of interference (Fig. 5).

The spectrogram of deoxidized chloroform,Fig. 5a, shows only a small change in the red-shift of the C–H bending mode around 1237 cm1up to 1300 fs after the photo-excitation. This red-shift

may be explained by the following two possible mechanisms: (1) The process of SFC?SR. (2) Two modes with a small difference

corresponding to a low-frequency mode of 30 cm1 around

1237 cm1are coupled through this low-frequency mode.

In comparison with a small change in the spectrogram of deox-idized chloroform, that of the O2-dissolved sample exhibits a peak

around 1755–1785 cm1 at 600 fs after the photo-excitation

(Fig. 5b). This peak can be assigned to mC@O, and hence, it implies

reaction between chloroform and O2 600 fs after the

photo-exci-tation.Fig. 3c shows that each component in this reaction process has a specific vibrational frequency.

The transfer of such vibrational coherence or even the genera-tion of coherence by chemical reacgenera-tion has also been explained previously in other systems[27–31]. The condition required for the coherence transfer is a shorter vibrational period than the vibrational dephasing time and the reaction time. The vibrational dephasing time is estimated to be longer than 100 fs from

Fig. 5b. Some photo-dissociation processes can generate products without vibrations when the reaction is too fast to be measured by real-time spectroscopy. This reaction mechanism analysis ob-serves the dynamics change of molecular structure by detecting the time-dependent instantaneous frequencies of vibrational modes. The analysis method can only detect a process slower than a molecular vibration period. Hereafter, we discuss only the pro-cesses observed on the spectrogram (Fig. 5a).

3.3. Comparison of the experimental results and calculated results The reaction dynamics can be explained by comparison of the experimental results with the following five calculated processes. Scheme(5)and (6)can be excluded because of the lack of C@O generation. The spectrograms to be observed under schemes(1), (3), and (7)were predicted by calculations as follows (Table 1)[32]. Scheme(1): The vibration frequencies of CHCl3O2 6 do not

change significantly from those of the reactant. Light irradiation to 6 starts C–H insertion, and generates CCl3OOH 7 gives mO–O,

mC–O, and dO–H around 910, 1025, and 1400 cm1, respectively.

Then, it is converted to TS 8, which thereafter dissociates OH and Cl, and mC–Oand dO–Hare observed at 1090 and 1435 cm1(both

blue-shifted), respectively. Then mO–O vanishes, because it is an

imaginary frequency mode of the TS. Finally, product phosgene 9 gives mC@Oaround 1773 cm1(as for mC@O, our calculated

frequen-cies were rescaled for comparison with the experimental results

[33–35]with a scaling factor of 0.96 for mC@O).

The experiment spectrogram result is not contradicted by scheme(1).

Scheme(3): In the reaction of CCl22 and O2, mC–Cland dC–Clof

CCl2O211 are blue-shifted from those of CCl2. Then, 11 changes

to TS 12 and mO–O appears around 1435 cm1 because O@O in

CCl2O@O 11 is converted to the O–O in CCl2O–O 13 via TS 12.

In 13, the appearance of mC–Oaround 1050 cm1and the symmetry

of 13 cause the disappearance of mO–O. No peak appears in the

range higher than 1050 cm1. Likewise, the frequency of m C–O

(1182 cm1) is also the highest frequency in TS 14. The final

prod-uct phosgene 15 shows mC@Oaround 1809 cm1(the scaling factor

for mC@Obeing 0.96).

The absence of signal corresponding to mO@O (1635 cm1) of

CCl2O211 with longer lifetime than that of CCl2OO 13 rules out

the possibility of scheme (3). Furthermore, the existence of the mode whose frequency is higher than 1182 cm1in the

spectro-gram also verifies that scheme(3)does not take place. Fig. 4. Pump intensity dependence of vibrational amplitude of CHCl3O2with the

same probe intensity of 480 GW cm2.

Fig. 5. Spectrograms calculated from the DA trace probed at 666 nm using the Blackman window function whose FWHM is 240 fs: (a) deoxidized CHCl3probed at

666 nm (1.86 eV) and (b) O2-dissolved CHCl3probed at 666 nm (1.86 eV).

Table 1

Calculated wavenumbers of eight vibrational modes of molecules relevant in the reactionsa

Molecule Wavenumbers (cm1)

mC@O mO@O dO–H dC–H mC–O mO–O mC–Cl dC–Cl

1 1248 663 365 4 1571 6 1573 1255 663 365 7 1343 1026 913 546 8 1378 1090 529 9 1773 579 307 2 660 299 11 1571 708 329 12 1436 659 320 13 1051 14 1182 15 1809 555 302 3 1251 748 25 1261 1132 679 26 1287 1327 695 27 1773 1330 721 a

(6)

Scheme(7): In

CHCl23, mC–Cland dC–Clappear around 750 and

1250 cm1, respectively. After react O, newly generated m C–O

around 1130 cm1of the intermediate

CHCl2O 25 is then

blue-shifted up to 1330 cm1in

ClCHClO TS 26. Another final product CHClO 27 shows mC@O and dC–H around 1773 and 1330 cm1,

respectively (scaling factor for mC@Obeing 0.96).

And so, the observed signal around 1410 cm1cannot be

ex-plained. In addition, generation of

CHCl23 and O from the same

CHCl3O2 6 seems improbable, because each radical generation

is a multi-photon process. Therefore, ps timescale diffusion in the solvent has to be considered for scheme(7), which cannot explain the reaction of 600 fs shown inFig. 5b.

Therefore, the change observed around 600 fs after the photo-excitation in the experimental result is most likely due to the C–H insertion process, scheme (1). The calculated results of scheme (1) described earlier and the experimentally observed spectrograms can be construed as follows. At 250 fs after the photo-excitation, the intermediate CCl3OOH 7 was generated.

Be-cause, the peaks observed at 923, 1050, 1220, and 1330 cm1were

assigned to mO–Oin 7, mC–Oin 7, dC–Hin CHCl3, and dO–Hin 7,

respec-tively. After that, the signal of 923 cm1(m

O–O) vanished because it

is an imaginary frequency mode of the TS 8 and the signals of 1050 (mC–O) and 1330 cm1(dO–H) were blue-shifted to dissociate OH and

Cl in TS 8, as predicted by the calculation. At 600 fs after the photo-excitation, a new peak appeared at 1755 cm1(m

C@O), and

then it was gradually blue-shifted. This shift can be ascribed to the increase in mC@Oas OH dissociates away from CCl2OOH 9.

The dC–Cl(307 cm1) and mC–Cl(578 cm1) of the product phosgene

9 were not observed; this is probably because the signal strengths are weak and overlapped with the strong broad signal of chloro-form in these frequency regions.

4. Conclusion

Raman triggered oxidation of chloroform has been observed by ultrafast time-resolved spectroscopy using sub-5 fs pulses. Though chloroform has no absorption band in the spectral range of this la-ser, the broadband pulse excited the vibrational modes in the ground state by stimulated Raman processes via virtual excitation of the electronic state, and the reaction between chloroform and oxygen was triggered by the population of these vibrational levels. An analysis of the spectrogram enabled direct observation of the real-time dynamics of the oxidation process. These results have demonstrated that the observation of transition states by sub-5 fs time-resolved spectroscopy is applicable for ‘ground-state reac-tion’ as well as ‘excited state reacreac-tion’ via Raman excitation in a wide variety of chemical reactions.

Acknowledgments

This work was partially supported by JSPS to I.I., and a Grant MOE ATU Program in NCTU to A.Y. and T.K. The authors are grateful to the ITC of the UEC for their support of the DFT calculations. Appendix A. Supplementary material

Supplementary data associated with this article can be found, in the online version, atdoi:10.1016/j.cplett.2008.03.003.

References

[1] A. Shirakawa, I. Sakane, M. Takasaka, T. Kobayashi, Appl. Phys. Lett. 19 (1999) 2268.

[2] A. Baltuska, T. Kobayashi, Appl. Phys. B 75 (2002) 427. [3] A. Baltuska, T. Fuji, T. Kobayashi, Opt. Lett. 27 (2002) 306. [4] G. Cerullo, M. Nisoli, S. De Silvestri, Appl. Phys. Lett. 71 (1997) 3616. [5] T. Wilhelm, J. Piel, E. Riedle, Opt. Lett. 22 (1997) 1494.

[6] J. Piel, M. Beutter, E. Riedle, Opt. Lett. 25 (2000) 180. [7] T. Kobayashi, T. Saito, H. Ohtani, Nature 414 (2001) 531.

[8] T. Kobayashi, H. Wang, Z. Wang, T. Otsubo, Chem. Phys. Lett. 426 (2006) 105. [9] T. Kobayashi, H. Wang, Z. Wang, T. Otsubo, J. Chem. Phys. 125 (2006) 044103. [10] Z. Wang, T. Otsubo, T. Kobayashi, Chem. Phys. Lett. 430 (2006) 45. [11] T. Kobayashi, A. Yabushita, T. Saito, H. Ohtani, M. Tsuda, Photochem. Photobiol.

83 (2007) 363.

[12] N. Schoorl, L.M. Berg, Chem. Centr. II (1905) 1623. [13] M.L. Azcárate, E.J. Quel, J. Phys. Chem. 93 (1989) 697.

[14] V.M. Freytes, J. Codnia, M.L. Azcárate, Photochem. Photobiol. 81 (2005) 789. [15] W.H.S. Yu, M.H.J. Wijnen, J. Chem. Phys. 52 (1970) 2736.

[16] A.J. Eskola, W.D. Geppert, M.P. Rissanen, R.S. Timonen, L. Halonen, J. Phys. Chem. A 109 (2005) 5376.

[17] B. Walker, M. Saeed, T. Breeden, B. Yang, L.F. DiMauro, Phys. Rev. A 44 (1991) 4493.

[18] H. Hou, B. Wang, Y. Gu, J. Phys. Chem. A 103 (1999) 8075. [19] A.M. Clover, J. Am. Chem. Soc. 45 (1923) 3133.

[20] S. Gab, W.V. Turner, Angew. Chem. Int. Ed. Engl. 24 (1985) 50. [21] M.G. Rosenberg, U.H. Brinker, J. Org. Chem. 68 (2003) 4819. [22] GAUSSIAN03, Revision D.02, Gaussian, Inc., Wallingford CT, 2004. [23] K.A. Black, S. Wilsey, K.N. Houk, J. Am. Chem. Soc. 125 (2003) 6715. [24] B.M. Monroe, Photochem. Photobiol. 35 (1982) 863.

[25] H. Tsubomura, R.S. Mulliken, J. Am. Chem. Soc. 82 (1960) 5966.

[26] W.G. Rothschild, G.J. Rosasco, R.C. Livingston, J. Chem. Phys. 62 (1975) 1253. [27] J.M. Jean, G.R. Fleming, J. Chem. Phys. 103 (1995) 2092.

[28] F. Rosca, A.T.N. Kumar, X. Ye, T. Sjodin, A.A. Demidov, P.M. Champion, J. Phys. Chem. A 104 (2000) 4280.

[29] M.H. Vos, F. Rappaport, J.-C. Lambry, J. Breton, J.-L. Martin, Nature 363 (1993) 320.

[30] R.J. Stsnley, S.G. Boxer, J. Phys. Chem. 99 (1995) 859.

[31] Q. Wang, R.W. Schoenlein, L.A. Peteanu, R.A. Mathies, C.V. Shank, Science 266 (1994) 422.

[32] A.P. Scotto, L. Radon, J. Phys. Chem. 100 (1996) 16502. [33] H. Schnöckel, R.A. Eberlein, H.S. Plitt, J. Chem. Phys. 97 (1992) 4. [34] D.L. Joo, D.J. Clouthier, A.J. Merer, J. Chem. Phys. 101 (1994) 31.

[35] E.A. Wade, K.E. Reak, B.F. Parsons, T.P. Clemes, K.A. Singmaster, Chem. Phys. Lett. 365 (2002) 473.

數據

Fig. 2. Calculated structure at B3LYP/6-31G *
Fig. 5. Spectrograms calculated from the DA trace probed at 666 nm using the Blackman window function whose FWHM is 240 fs: (a) deoxidized CHCl 3 probed at

參考文獻

相關文件

6 《中論·觀因緣品》,《佛藏要籍選刊》第 9 冊,上海古籍出版社 1994 年版,第 1

The first row shows the eyespot with white inner ring, black middle ring, and yellow outer ring in Bicyclus anynana.. The second row provides the eyespot with black inner ring

• helps teachers collect learning evidence to provide timely feedback & refine teaching strategies.. AaL • engages students in reflecting on & monitoring their progress

Robinson Crusoe is an Englishman from the 1) t_______ of York in the seventeenth century, the youngest son of a merchant of German origin. This trip is financially successful,

fostering independent application of reading strategies Strategy 7: Provide opportunities for students to track, reflect on, and share their learning progress (destination). •

Strategy 3: Offer descriptive feedback during the learning process (enabling strategy). Where the

How does drama help to develop English language skills.. In Forms 2-6, students develop their self-expression by participating in a wide range of activities

(c) If the minimum energy required to ionize a hydrogen atom in the ground state is E, express the minimum momentum p of a photon for ionizing such a hydrogen atom in terms of E