Mechanism of phototransfer of hydrogen atom in the

MECHANISM OF PHOTOTRANSFER OF HYDROGEN ATOM IN THE MODEL H2CO + H20 SYSTEM I. S. Irgibaeva, B. F. Minaev, and Z. M. Muldakhmetov

UDC 539.192:541.515

A system modelling the photochemical abstraction of a hydrogen atom by ketones in alcohols is calculated by the semiempirical INDO and MINDO/3 methods with allowance for the configuration interaction in the singly and doubly excited states. The states participating in the elimination reaction and the electronic rearrangement taking place in the course of the reaction are traced on the basis of an analysis of the wave functions and the electron and spin densities. It is established that the state of the ketone which participates in hydrogen abstraction is a lowest triplet state of the nv* type, which is formed through the avoidance of intersections of several states of different orbital type ~n~*, 3no* and the charge-transfer state.

The photochemical reduction of ketones in hydrogen-donating solutions is characterized by good reaction yields and by the more ready formation of the products than in the absence of irradiation [1-9]. For example, the irradiation of benzophenone in isopropyl alcohol which has been thoroughly purified from oxygen leads to a quantitative yield of b e n z p i n a c o n e and acetone [3]. It was established by various methods that the lowest triplet state of the 3nz* type, which is formed by intercombination conversion from t h e i n i t i a l l y excited singlet state, in benzophenone is responsible for the abstraction of hydrogen. In the present work sections of the potential energy surfaces (PES) of a reaction modelling the photochemical elimination of a hydrogen atom from an alcohol by ketones were calculated by the semiempirical LCAO-MO SCF method in the version with modified intermediate neglect of differential overlap (MINDO/3) [i0] with allowance for configuration interaction (CI) in the singly and doubly excited states. As model to simplify the calculation we used the H2CO...H20 system, which contains the main functional groups of the ketone and alcohol taking part in the investigated reaction. The calculation of the sections of the PES along the reaction coordinate was conducted in the following way: The position of the H s atom was varied with a constant 01...06 distance (Fig. i). The elimination of the H s atom takes place in the plane of the carbonyl group, which remains the symmetry plane of the system throughout the reaction. Figure 2 shows sections of the PES along the reaction coordinate of the singlet ground S o state with closed shell for the H2CO...H20 complex with various R(O1...O6) distances. The most stable structure of the complex corresponds to approach to 3.5-4.0 ~. The sections of the PES on the right in Fig. 2 correspond to the initial reagents, and those on the left correspond to the products OH-...CH2OH § which can only be obtained in the singlet state with a closed shell. As seen from Fig. 2, the transition to the ionic state OH-...CH2OH § with removal of the proton is hindered as the reagents become more separated from each other. Let us discuss the results from the calculation for the distance R(O1...O6) = 4.0 ~. The change in the orbital nature of the reagents during approach can be followed by means of a Walsh diagram (Fig. 3). "Natural correlation" [ii, 12] requires that the orbitals remain as far as possible localized at the same atoms and that the phase characteristics of the orbitals correlating with each other do not change. From the energy standpoint the high-lying n orbital of the formaldehyde oxygen must strive toward correlation with the n(OH) orbital of water. However, its "natural" correlation is with the lower-lying o orbital (O is the oxygen of formaldehyde, and H is the abstracted proton of the water), whereas the n(OH) orKaraganda University. Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 25, No. 4, pp. 476-480, July-August, 1989. Original article submitted November ii, !986.

0040-5760/89/2504-0441512.50

9 1990 Plenum Publishing Corporation

441

E,eVr

!.

"R3,5 \

-80~0

\

'.o /,o

2,o

Fig. i

3.o

.

Fig. 2

Fig. i. The directions of the axes and the numbering of the atoms in the H2CO...H20 complex. Fig. 2. Sections of the PES of the ground So state of the H2CO...H20 complex in relation to the distance R(O1...Hs) for various constant values of R(Oz...06). bital of water originates from the bonding o(OH) orbital (Fig. 3, the dashed lines). Consequently, the orbitals which are correlated in the "natural" way intersect. Avoidance of the intersection leaves a "memory" of itself; the energy levels of the orbitals converge and then diverge (Fig. 3, the solid lines). In itself the "memory" is of important significance since it determines the activation energy of the photochemical n~* elimination reaction [3]. Such a situation is typical both of the py-AO and of the px-AO of oxygen. The n(OH) orbital is stabilized during removal of the proton (see Fig. 3, on the right). The energy of the HOMO, formed from the AOs of the water residue [n(OH)], increases. The energies of the bonding and virtual ~(C = O) MOs decrease, and this leads to excitation of an electron from the n(OH) orbital to the antibonding ~*(C = O) orbital. From these and from the close-lying orbitals the 40 singly and doubly excited configurations, included in the CI calculation, were formulated. With a knowledge of the expansion of the CI and the nature of the change in the orbitals with distance it is possible to analyze the rearrangement of the states in the course of the reaction. Figure 4 shows sections of the PES of the ground and some excited states of the H2CO...H20 complex. As the reagents approach in the ground So state with A' syn~netry the proton is abstracted from the H20 [at the distance R(Oz...Hs) = 2.0 ~)], and with further approach the ion pair OH-...+HOCH2 is formed. Such a conclusion emerges from analysis of the electronic structure with respect to the points of approach, and this is shown in the diagrams presented below. The initial position at R(Oz...Hs) = 3.0 ~ is characterized by the following distribution of charges at the atoms:

-~ /0

~

-~7 '

" U =

-o~

\

'

-q~ H .

In the region of 2.0 X the proton is almost separated:

-~107 H/O~H. ~o 0~3"-. - ~36~

-~o5

~ 0//H \

-oo7 N~

and at 0.948 X i t has been f u l l y removed: I,ij3

FI/O"-.. o,~ 2

oo5

H~ Ozo3/H , 037?~0 ~ C / -o;75 ~ H oo-~.

f e% ~

-~o

-moo

... n~(o~/r

I

-~7/oe)

,

/,o

,

,

Fig. 3

t

eo

,

, ~

"~176

)

40 R(4...~),;

Fig. 4

Fig. 3. The dependence of the energy of the various MOs on the distance of approach of H2CO to H20. Fig. 4. The formation of the PES, over which the abstraction of the Hs atom occurs. During the excitation of H2CO to the 3n~* state the process takes place differently. In this state the interaction between the carbonyl and the water leads to the.abstraction of the H atom. After complete transfer of the hydrogen atom the radical pair OH...HOCH 2 is formed. In Fig. 4 the curve represented by the dashed line corresponds to the formation of these radicals. It is formed from the intersections of several states with different orbital character, which avoid each other as a result of the identical A" symmetry. Initially (up to a distance of approach of 2.2 ~) it corresponds to the excited an~* state in formaldehyde. The wave function with allowance for CI has the following form: 3~ 1 =

0.99(~10----~11)

,

where ~o = O,72p~(O1)--O.24py(C2) + O,45s(H4)--O'46s(H~) ,

~ x = 0.84pz (C2) - - 0,53pz (0~). The d i s t r i b u t i o n by t h e d i a g r a m :

of electron

and sPin

(in

parentheses)

density

in this

state

is

represented

H/O~H

4z5o

oz~"- -qoz -oos,/H~ '

"'-0 =

C'C,,.

At the sections of approach to between 2.2 X and 1.2 X the form of the wave function and the nature of the orbitals change:

qr I = 0.97 (%0--~ qh 1) -k 0.20 (%o'-~ (P13), where

qho = 0,85p,, (06) - - 0.37s (Hs), % 1 = O.70s (Hs) -~- 0.43pv (06) --' 0 . 2 3 s (C2) -- O.27pu (O 1) - - 0.31s (H4),

t~13 = 0.57s (C2) - - 0.32p= (O1) - - 0.50s (Ha) - - 0.50s (H4). As seen from the presented MOs, the lowest excited state on this section is of the ~no* type; the n orbital corresponds to the unshared electron pair of the oxygen atom in the water molecule, and the o orbital is characterized by a major contribution from the s-AO of the Hs atom. The distribution of the electron and spin density for the intermediate 3no* state of hydrogen transfer has the following form:

-o,I57 H/ omo

(~g97) ~ H ' ' (o19g?'--0s85 O-

6o5~3)

.(" J os~z/H C

/a.5)"~H/QOe) 443

During the abstraction of the hydrogen atom from the water by formaldehyde (R(OI...H 5) = 0.948 ~) the wave function of the lowest SA" state is obtained in the form:

3T1 = 0,95(Tlo-'-~'~12) + 0,31(~1o~1o"~Tn~12), where

%0 = 0.90p. (06) -- 0.16s (Hs),

~ 2 = 0.88p~ (C2) -- 0.47p~ (O1).

In terms of the given scheme of calculation it can be said that in this case electron transfer takes place from the OH- anion to the HOCH~ cation. The distribution of electron and spin density in the model system indicates that the following radical pair is formed: -~i~ O/#f

'

'

"'.

-0~0

"'"0 ~

-OOZe" 'C
= 33i cm -I , and (3A" i [H~ol IA ' > = 6i cm- I , and ~3A,, [H~o[ !A, > = 0. The square of the matrix element of the spin-orbital interaction determines the rate of intercombination conversion. If the radicals decrease from the reaction on account of intercombination conversion T 1 ~ S 0 , the T x component of the triplet restores them to the initial state. (The x axis is directed along the carbonyl bond, see Fig. i.) The sublevel T z of the triplet state secures the accumulation of the radicals in the course of the reaction. The anisotropic decay of the T 1 state in the course of the reaction can explain the effect of the magnetic field on the yield of the radicals [13-15]. LITERATURE CITED i. 2. 3. 4. 5.

J. Calvert and J. Pitts, Photochemistry [Russian translation], Mir, Moscow (1968). A. Schonberg and A. Mustafa, "Photochemical synthesis of carbonyl compounds," Chem. Rev., 40, 181-185 (1947). J. N. Pitts and R. L. Letsinger, "Photochemical reactions of benzophenone in alcohols," J. Am. Chem. Sot., 81, No. 5, 1068-1077 (1959). K. R. Kopecky, G. S. Hammond, and P. A. Leermakers, "The triplet state of methylene in solution," J. Am. Chem. Sot., 84, No. 6, 1015-1019 (1962). W. M. Moor, G. S. Hammond, and R. P. Foss, "Mechanisms of photoreactions in solutions. i. Reduction of benzophenone by benzhydrol," J. Am. Chem. Sot., 83, No. 13, 2789-2799

(1961). 6.

G. O. Schenck and R. Steinmetz, "Photosensibilisierte C4-Ringsynthesen und die Haupttypen photosensibilisierter Reactionen in Losung," Bull. Soc. Chim. Belg., 71, No. 11/12, 781-

7.

W. M. Moore and M. benzophenone," J. A. Beckett and G. Photochemistry of

800 (1962).

8.

Ketchum, "The quenching effect of naphthalene on the photoreduction of Am. Chem. Sot., 84, No. 8, 1368-1371 (1962). Porter, "Primary photochemical processes in aromatic molecules. 9. benzophenone in solution," Trans. Faraday Sot., 59, No. 9, 2038-2050

(1963). 9.

i0. ii. 12.

444

S. G. Cohen, D. A. Laufer, and W. V. Sherman, "Inhibition of right-induced reactions by mercaptans and disulfides. Benzophenone-benzhydrol and acetophenone-a-methylbenzyl alcohol," J. Am. Chem. Sot., 86, No. 15, 3060-3068 (1964). M. J. S. Dewar and D. H. Lo, "Ground states of o-bonded molecules. 17. Fluorine compounds," J. Am. Chem. Sot., 94, No. 5, 5296-5310 (1972). A. Devaguet, A. Sevin, and B. Bigot, "Avoided crossings in excited states on potential energy surfaces,", J. Am. Chem. Sot., i00, No. 7, 2009-2011 (1978). C. Trindle and O. Sinanoglu, "Local orbital guide to allowed interconversions of C4H~ ions," J. Am. Chem. Sot., 91, No. 15, 4054-4062 (1969).

13. 14.

15.

A. J. Dobbs, "Experimental observations of chemically induced dynamic electron polarization (CIDEP)," Mol. Phys., 18, 290-294 (1973). P. P. Levin and V. A. Kuz'min, "The role of spin-orbital interaction in the kinetics of geminal recombination of triplet radical pairs in micelles," Dokl. Akad. Nauk SSSR, 292, No. i, 134-137 [sic]. B. F. Minaev, Yu. A. Serebrennikov, and H. D. Rempel, "Non-equilibrium spin polarization on the Si-SI center in silicon induced by spin orbit coupling," Phys. Status Solidi, 148, 689-698 (1988).

THERMODYNAMIC STABILITY AND REACTIVITY OF THE ANTIAROMATIC HETEROCYCLE IH-AZIRINE UDC 541.13

S. M. Zavoruev and R.-I. I. Rakauskas

The thermodynamic characteristics of reactions resulting in the conversion of the antiaromatic three-membered aza heterocycle iH-azirine into isomeric 2Hazirine have been calculated by the SCF-aMO-LCAO method and perturbation theory. The proton affinity has been investigated, and it has been shown that protonation occurs at a carbon atom with the subsequent transition to 2H-azirine protonated at the nitrogen atom.

Despite numerous attempts, the three-membered nitrogen-containing heterocycle iH-azirine has not yet been synthesized (see reviews [1-3]), although isomeric 2H-azirine is fairly stable. It has been postulated that iH-azirine can be an intermediate in several reactions, for example, the hydrolysis of N-phthalimido-l,2,3-triazoles or the oxidative reactions of N-aminophthalimide with alkynes [3], which probably include a common step leading to the formation of iH-azirine, which quickly isomerizes to 2H-azirine:

N

Z../\ N

(X = phthalimide) If this hypothesis is correct, the existence of iH-azirine could be detected experimentally by the methods of IR spectroscopy when the appropriate conditions are selected, since its vibrational spectrum was recently calculated by theoretical methods [4, 5]. We note that such a route was previously utilized to experimentally identify a similar antiaromatic heterocycle, viz., thiirene, whose vibrational spectrum was calculated by an ab initio method in

[4]. In the present work we determined the thermodynamic characteristics of reaction (i) in the case of the unsubstituted isomers, which may provide a basis for further kinetic calculations. The work also included an investigation of the reactivity of iH-azirine, particularly with respect to a proton, and an examination of the structural consequences of protonation. Other isomerization channels, which may be both thermodynamically and kinetically preferable, are possible [6-8], but we confined ourselves to reaction (I) in the present investigation. Calculation Method. The calculation was carried out by the SCF-MO-LCAO method with the 6-31G(d, p) basis set [9], which includes d-type polarization functions on the nitrogen and carbon atoms and p-type polarization functions on the hydrogen atoms. Partial consideration of electron correlation was carried out according to second-order Moller-Plesset perturbation theory in accordance with the algorithm in [i0] with the use of the 6-31G basis set [Ii]. The correlation corrections obtained were used together with the relative energies of the !

Vil'nyus University. Translated from Teoreticheskaya i ~ksperimental naya Khimiya, Vol. 25, No. 4, pp. 481-486, Jnly-August, 1989. Original article submitted July 13, 1987.

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9 1990 Plenum Publishing Corporation

445