ISSN 0012-5008, Doklady Chemistry, 2008, Vol. 420, Part 2, pp. 133–136. © Pleiades Publishing, Ltd., 2008.Original Russian Text © K.K. Kalninsh, E.F. Panarin, 2008, published in Doklady Akademii Nauk, 2008, Vol. 420, No. 4, pp. 488–491.

133

According to time-resolved laser spectroscopy [1, 2],the phototransfer of a hydrogen atom takes place inexcited complexes, exciplexes, and involves successiveelectron and proton transfer steps. The thermal transfer ofhydrogen can also be regarded as an excited state reaction[3, 4] because the activation energy of the reaction

E

a

incrystalline quinone–hydroquinone (quinhydrone) com-plexes is close to the optical charge transfer energy; how-ever, the reaction mechanism has not been adequatelystudied. In particular, the reasons for the low efficiency ofhydrogen phototransfer in quinhydrones compared to athermal redox reaction are unknown [5]. A certain stage ofthe elementary event might require close proximity of theinteracting donor D–H and acceptor A, which is absent incrystalline complexes at room temperature.

In this paper, we report a kinetic study of thermalhydrogen transfer and ab initio calculations of excitedcomplexes at successive stages of hydrogen atom trans-fer using some quinhydrone complexes as examples.

In solution (Fig. 1), the equilibrium concentrationsof all four components in hydrogen transfer reaction (1)

(1)

OH

OH

Cl

O

O

OH

OH

Cl

O

OHQ CBQ CHQ BQ

… kT …

are rapidly established (HQ is hydroquinone; CBQ ischloro-

p-

benzoquinone; CHQ is chlorohydroquinone;BQ is

p-

benzoquinone), which is indicative of a lowactivation energy

E

a

.

However, in the solid phase, reaction (1) proceedsonly toward increasing difference between the ioniza-tion potential and the electron affinity of components,

I

D

–

E

A

, and has a higher

E

a

value (1.5–1.8 eV) [4, 6]found for the slow step of the obviously two-step kinet-ics (Fig. 2). The quinhydrone complexes used in theseexperiments were prepared by solid-phase mixing ofcomponents; the crystal structure of these complexescontains a large number of defects, resulting in atwo-step reaction kinetics. Therefore, we attemptedto grow crystalline complexes from a CH

2

Cl

2

solu-tion in which the structure contains a minimum num-ber of defects. The procedure of kinetic measure-ments included preliminary heating of the crystals ofcomplexes followed by grinding for recording the IRspectra.

In the case of two crystalline phenylhydroquinone(PHQ) complexes with BQ and CBQ, a strictly one-step kinetics was observed (Fig. 2), indicating the exist-ence of only one chemical process.

(2)

OH

OH

R

O

O

OH

OH

RO

OPHQ (C)BQ (C)HQ PBQ

… kT …

R = H, Cl

Mechanism of Thermal Hydrogen Transfer in Quinone–Hydroquinone Complexes

K. K. Kalninsh and

Corresponding Member of the RAS

E. F. Panarin

Received December 26, 2007

DOI:

10.1134/S0012500808060025

Institute of Macromolecular Compounds, Russian Academy of Sciences, Bol’shoi pr. 31, St. Petersburg, 199004 Russia

CHEMISTRY

134

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2008

KALNINSH, PANARIN

The reaction gives two conjugated complexes,resulting from transfer of two hydrogen atoms. Theactivation energy

E

a

for hydrogen transfer is42.3 kcal/mol (14 780 cm

–1

) in the former case, whilefor the PHQ–CBQ complex this value is 37.3 kcal/mol(13 033 cm

–1

). The

E

a

values found are close to the fre-quencies of broad featureless charge transfer bands,16 000 and 15 400 cm

–1

, respectively. Similar correla-tions between the activation energy of the thermalhydrogen transfer and the optical charge transferenergy have been obtained previously for a series ofquinhydrone complexes [3, 4, 7], which providesgrounds for claiming the participation of electronicallyexcited states in the thermal process. Reaction (2) takesplace at high temperatures (>130

°

C), exceeding themelting point of one component of the complex by 20–30

°

C (mp

BQ

= 115–116

°

C). The redox transformationPHQ + BQ

→

HQ + PBQ is accompanied by anincrease in the charge transfer energy from 15 400 to16 000 cm

–1

due to weakening of the electron donor–acceptor properties of the components and by a low-frequency shift of the

ν

OH

band by 50 cm

–1

, indicatingan increase in the strength of the hydrogen bond in theconjugated HQ–PBQ complex.

A study of time-resolved transient spectra of thePHQ–BQ complex recorded on a pulsed laser spec-trometer (pulse energy 0.05–03 mJ) [8] did not showany noticeable yield of the photoreaction productHQ

−

PBQ, although the action of each of more than1000 pulses in the region of the charge transfer bandbrings about 20% of the complex into the excited state.Only irradiation of quinhydrone with a defocused beamof a 1-W argon laser induces a weak photoreactioneffect detected in the IR spectrum. In order to elucidatethe reasons for the low photochemical activity of crys-

talline quinhydrone, we carried out ab initio calcula-tions of the structure of the reaction complex (Fig. 3) inwhich the quinone and hydroquinone molecules arehydrogen bonded. The ROHF/DH optimization of thegeometry of the complex in the triplet excited stategives an exceptionally low activation energy for thehydrogen atom transfer,

E

a

= –0.48 kcal/mol. In theexcited state, an electron migrates between the PHQ

and BQ molecules to give the PH radical cation and

the B radical anion. During quantum-chemicaloptimization, the electron transfer is followed by pro-ton transfer (Fig. 3b), which gives in total hydrogenatom transfer (electron + proton), and the PHQ and BQmolecules are converted into two semiquinone radicals.The highest free valence indices (0.87 and 0.89) arelocalized on two oxygen atoms, one being involved in amoderately strong intermolecular hydrogen bond(

R

O…O

= 2.88

Å).

In the equilibrium configurations of the initialPHQ–BQ and final HQ–PBQ complexes, the hydroxylgroup is hydrogen bonded to the carbonyl oxygen atom(

OH…O=C

). The

R

O

· · ·

O

distance is the same in bothcases, being equal to

2.82

Å; the experimental value forthe quinhydrone BQ–HQ is

2.72

Å [9]. During quan-tum-chemical optimization, the PHQ and BQ mole-cules approach each other, the minimal

R

O

· · ·

O

distanceat the proton transfer point being

2.40

Å (Fig. 3). Thesecalculation results provide grounds for concluding that,for the hydrogen transfer event in the quinone–hydro-quinone complex to occur, the molecules shouldapproach each other to the distance

R

O

· · ·

O

≈

2.4

Å,which is about

0.4

Å shorter than the equilibrium value

Q+.

Q–.

0.25

2

D

t

/10

–3

, s

0.50

4 6 8

0.75

1.00

CBQ + HQ

BQ + CHQ

Fig. 1.

Equilibrium reaction CBQ + HQ

→

BQ + CHQ inacetonitrile at 6

°

C; the initial CBQ concentration is0.08 mol/l,

t

is time, and

D

is optical density.

0.4

4000

ln(1 +

D

2

/

D

1

)

t

, s

0.8

2000

1.2

1 2

Fig. 2.

Thermokinetics of the reaction BQ + PHQ in quin-hydrone: (

1

) crystals grown from a solution, 120

°

C;(

2

) solid-phase mixing of BQ + PHQ, 70

°

C;

t

is time, and

D

1

and

D

2

are the optical densities for the bands at 872 cm

–1

(BQ) and 900 cm

–1

(PBQ), respectively.

DOKLADY CHEMISTRY

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MECHANISM OF THERMAL HYDROGEN TRANSFER 135

in the crystalline complex at room temperature. Pre-sumably, it is this fact, i.e., the lack of free migration ofmolecules in the crystal, that is responsible for the lowefficiency of hydrogen phototransfer. As the tempera-ture increases, the O–H vibration amplitude and,hence, the probability of proton

H

+

transfer grows,which may account for the experimentally observedthermal transfer of hydrogen in crystalline complexes.No direct data on the O–H vibration amplitudes athigh temperatures have been reported, although theorder of magnitude can be judged from the root-mean-square deviations of the heavier atom B found for the

LiB

3

O

5

crystal at

227°C

, which are approximately

0.2

−

0.3

Å [10].

The calculated energy diagram that models thehydrogen transfer in solution shows very low activationenergies

E

a

. This is in contrast with the high values

E

a

=1.5–1.8 eV in the crystal, which are close to the ener-gies of optical charge transfer transitions. Positive

E

a

values were obtained for

p-

benzoquinone complexeswith amines (Fig. 4).

Thus, in hydrogen-bonded systems like quinhy-drones, transfer of a hydrogen atom (electron + proton)occurs along the hydrogen bond at a planar orientationof molecules, the electron (charge) transfer energy

2.82

Å

2.40

Å

2.82

Å

H

(a)

(b)

(c)

Fig. 3.

Calculated (RHF, ROHF/DH) molecular scheme of the reaction BQ + PHQ

→

PBQ + HQ; (a, c) the initial and final states (

S

0

);(b) the electronically excited state

T

1

at the hydrogen transfer stage.

70

60

50

40

30

20

10

0

–10

E, kcal/mol

BQ + Anl BQ + pPDCBQ + HQ

Groundstate

BQ…Anl

CBQ…HQ

BQ…pPDCHQ + BQ

Reactionproducts

Excitedstate

Electron–protoneffect

Fig. 4. Energy diagram for hydrogen transfer in the systemsquinone–hydroquinone, p-phenylenediamine (pPD), andaniline (Anl) in vacuum according to ab initio calculationdata (RHF, ROHF/DH).

136

DOKLADY CHEMISTRY Vol. 420 Part 2 2008

KALNINSH, PANARIN

being crucially dependent on the state of the H bond.The enormous lowering of the excited levels (Fig. 4) tothe thermal region (<1 eV) is determined by a pro-nounced electron–proton interaction (electron–protoneffect) [11].

REFERENCES

1. Mataga, N., Bull. Chem. Soc. Jpn., 1958, vol. 31, no. 4,pp. 487–491.

2. Miyasaka, H., Tabata, A., Kamada, K., and Mataga, N.,J. Am. Chem. Soc., 1993, vol. 115, no. 16, pp. 7335–7342.

3. Kalninsh, K.K., J. Chem. Soc., Faraday Trans. 2, 1984,vol. 80, no. 12, pp. 1569–1538.

4. Kalnin’sh, K.K. and Shchukareva, V.V., Izv. Akad. NaukSSSR, Ser. Khim., 1985, no. 1, pp. 95–102.

5. Kalninsh, K., Gadonas, R., Krasauskas, V., and Pug-zlys, A., Chem. Phys. Lett., 1993, vol. 209, no. 1, pp. 63–66.

6. Kalnin’sh, K.K. and Panarin, E.F., Vozbuzhdennye sos-toyaniya v khimii polimerov (Excited States in PolymerChemistry), St. Petersburg: Izd.-poligraf. tsentr SPGUTD,2007.

7. Kalninsh, K.K., J. Chem. Soc., Faraday Trans. 2, 1982,vol. 78, pp. 327–338.

8. Kalninsh, K., Gadonas, R., Krasauskas, V., Pelakaus-kas, A., and Pugzlys, A., J. Photochem. Photobiol.,1994, vol. 77, pp. 9–16.

9. Matsuda, H., Osaki, K., and Nitta, J., Bull. Chem. Soc.Jpn., 1958, vol. 31, pp. 611–626.

10. Shepelev, Yu.F., Bubnova, R.S., Filatov, S.K.,Sen-nova, N.A., and Pilneva, N.A., J. Solid State Chem.,2005, vol. 178, pp. 2987–2997.

11. Kalnin’sh, K.K., Opt. Spektrosk., 2007, vol. 103, no. 4,pp. 569–587.