Solvent effects on the absorption spectrum and recombination kinetics of diphenylcarbonyl oxide

Article abstract of DOI:10.1023/A:1011386421212

The absorption spectra and rate constants of diphenylcarbonyl oxide recombination in a series of solvents and their binary mixtures were determined by flash photolysis. An increase in the solvent polarity causes hypsochromic shift of the maximum in the absorption spectrum of Ph₂COO. The analysis of the solvent effect on the recombination rate constant in terms of the four-parameter Koppel - Palm equation shows that the reactivity of carbonyl oxide depends on both specific and non-specific solvations. Quantum chemical B3LYP/6-31G(d) calculations of H₂COO and PhHCOO carbonyl oxides as well as the complexes of H₂COO with acetonitrile and ethylene in different media were performed using a polarized continuum model.

Full text of DOI:10.1023/A:1011386421212

Russian Chemical Bulletin, International Edition, Vol. 50, No. 5, pp. 793—797, May, 2001
793
Solvent effects on the absorption spectrum and recombination kinetics
of diphenylcarbonyl oxide
S. L. Khursan,a A. M. Nazarov,b« E. M. Chainikova,b and V. D. Komissarov a
aBashkir State University,
32 ul. Frunze, 450074 Ufa, Russian Federation.
Tel: (347 2) 23 6727
bInstitute of Organic Chemistry, Ufa Research Center of the Russian Academy of Sciences,
71 prosp. Oktyabrya, 450054 Ufa, Russian Federation.
Fax: +7 (347 2) 35 6066
The absorption spectra and rate constants of diphenylcarbonyl oxide recombination in a
series of solvents and their binary mixtures were determined by flash photolysis. An increase in
the solvent polarity causes hypsochromic shift of the maximum in the absorption spectrum of
Ph2COO. The analysis of the solvent effect on the recombination rate constant in terms of the
four-parameter Koppel—Palm equation shows that the reactivity of carbonyl oxide depends
on both specific and non-specific solvations. Quantum chemical B3LYP/6-31G(d) calcula-
tions of H2COO and PhHCOO carbonyl oxides as well as the complexes of H2COO with
acetonitrile and ethylene in different media were performed using a polarized continuum model.
Key words: carbonyl oxides, reaction kinetics, absorption spectra, solvent effect, quantum
chemical calculations.
processing a pulse signal. A UFS-2 light filter (transmission
range is 240—380 nm) was used for the flash photolysis of the
Ph2CN2—solvent—O2 (air) system. To keep Ph2CN2 from
photochemical decay, the light intensity was weakened with an
A study of the solvent effects on the reactivity and
other physicochemical parameters of reactants1 is im-
portant for elucidating the mechanisms of chemical
reactions, especially in the case of ionic or highly polar
reactants. Carbonyl oxides R´R″COO are highly reac-
tive intermediates in the ozonolysis of unsaturated or-
ganic compounds and in photosensitized or thermal
decomposition of diazo compounds. The delocalized
three-center π−system of carbonyl oxides provides the
high dipole moments and high polarizability by the
molecular environment. Because of these properties the
chemical behavior of carbonyl oxides depends on the
solvent nature. Earlier, we reported the medium effect
on the kinetics of the reaction of diphenylcarbonyl
oxide Ph2COO with sulfoxides2 and alkenes.3
SS-15 filter (transmission range is 300—520 nm). The initial
–4
concentration [Ph2CN2]0 was (1.5—2)•10
mol dm–3. The
reaction kinetics was monitored by the decrease in the optical
density À in the maximum of the absorption band for
diphenylcarbonyl oxide.
Results and Discussion
The flash photolysis of diphenyldiazomethane in all
the solvents studied produces the short-lived optical
absorption in the range of wave lengths λ = 370—470 nm
(Fig. 1). The photolytic decomposition of Ph2CN2 is a
well studied process.8—11 The photolysis under ultravio-
let radiation and visible light initially produces triplet
carbene Ph2C whose reaction with O2 is characterized
by a great (likely diffusional) rate constant (Scheme 1).
In this work, the kinetics of Ph2COO recombination
in different solvents (previous communication, see Ref. 4)
was studied and the theoretical and experimental inves-
tigations of the medium effect on the electronic struc-
ture of carbonyl oxides was carried out.
Experimental
Scheme 1
Solvents (acetonitrile, n-pentane, n-decane, AcOEt, ben-
zene, chlorobenzene, and 1,4-dioxane (Russia)) were purified
according to the known procedure.5 Diphenyldiazomethane
Ph2CN2 was synthesized and purified by a known protocol.6
Kinetic runs were carried out on an FP setup described in
work7. An IFP 5000-2 lamp was the light source; the maximal
pulse energy was 400 J, and ∼90% of light energy was irradiated
during 50 µs. A quartz cell with l = 10 cm and ∼1 cm diameter
was used as a reactor. The setup whose parameters were re-
ported in work7, was supplemented with a device for computer
hν, λ = 280—370 nm
Ph2CN2
1(Ph2C:)
1(Ph2C:) + N2,
3(Ph2C:),
(Keq = 321, see Ref. 12)
3(Ph2C:) + O2
Ph2COO
s
–1, see Ref. 8)
(k = 5•109 L mol
–1
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 759—763, May, 2001.
1066-5285/01/5005-793 $25.00 © 2001 Plenum Publishing Corporation

794
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 5, May, 2001
Khursan et al.
–1
A/Amax
νmax•10–3/cm
6
24.6
1.0
7
24.2
0.8
0.6
0.4
3
2
3
1
2
5
23.8
4
1
–1
32
36
40
44
ET(30)/kcal mol
–1
22
23
24
25
26
ν•10–3/cm
Fig. 2. Effect of solvating ability of a solvent ET(30) on the
position of maximum of the absorption band for diphenyl-
carbonyl oxide: 1 , n-pentane; 2, benzene; 3, 1,4-dioxane;
4, chlorobenzene; 5, AcOEt; 6, acetonitrile; 7, acetonitrile with
photosensitizer (methylene blue).
Fig. 1. Gaussian shape of the optical spectra of diphenylcarbonyl
oxide at 295 K in different solvents: 1, acetonitrile; 2, chlo-
robenzene, and 3, n-pentane.
The product of this reaction is diphenylcarbonyl oxide
Ph2COO, which absorbs light with λ = 370—470 nm
and is further consumed through the reaction with an
appropriate substrate or via a bimolecular reaction. Un-
der the conditions of sensitized photolysis (Scheme 2),
the interaction of singlet dioxygen (1∆g) with a
diazo compound is a source of carbonyl oxides. For
Ph2CN2, the efficiency of Ph2COO formation is 60%
(CH3CN : CH2Cl2 = 2 : 3; 294 K).13
The signal recorded can be assigned to the Ph2COO
absorption because of coincidence of the absorption
–4
spectra and characteristic life time τ = 10 —10–2 s.14,15
In the absence of O2, no optical absorption in the above
spectral range was found.
The absorption spectrum for Ph2COO (see Fig. 1)
can be described by the Gaussian function and does not
depend on the signal intensity. This indicates that car-
bonyl oxide is the single intermediate species that ab-
sorbs light in the above range. The absorption band for
Ph2COO corresponds to the allowed π—π* transition.16
On going from acetonitrile to n-pentane, the maximum
of the absorption band λmax shifts to the long-wave
region by 20 nm (Table 1). This solvatochromic effect
correlates with the solvent parameter ET(30) (Fig. 2).1
In the absence of active additives, the main channel
of Ph2COO consumption in the solvents under study is
bimolecular decay with the formation of two molecules
of benzophenone and O2 molecule:
Scheme 2
hν, λ > 560 nm
MB
MB*,
MB* + O2
MB + 1O2(1∆g),
Ph2CO + N2O
Ph2COO + N2
s
–1, see Ref. 11).
Ph2CN2
+
1O2
Ph2COO + Ph2COO
Ph2CO + Ph2CO + O2.
(1)
(k = 1•109 L mol
–1
On going from acetonitrile to n-pentane, the rate
constant for this reaction (2k) changes from 1.8•107 to
Here MB is methylene blue, acetonitrile is the solvent.
Table 1. Maxima of absorption bands (λmax), rate constants (2k) of diphenylcarbonyl
oxide recombination in different solvents, and parameters of the Koppel—Palm equation
(Y, P, E, B)17
a
Solvent
λmax
2k
Y
P
E
B
/nm
/L mol–1 s–1
Acetonitrile
406
408
418
418
418
1.8•107
0.4803
0.2119
5.2
160
Acetonitrile (MB)b
Benzene
1.0•108
6.3•107
0.2306
0.3775
0.2231
0.3850
0.1989
0.1800
0.2947
0.3064
0.2543
0.2275
0.2488
0.2193
2.1
0
4.2
0
0
0
48
38
237
181
0
Chlorobenzene
1,4-Dioxane
AcOEt419
n-Decane
n-Pentane
3.0•107
8
1.4•10
420
424
1.2•109
2.0•109
0
a
Calculated from experimental spectra for the Gaussian shape of bands.
b
–7
Methylene blue was used as photosensitizer ([MB] = 4•10 mol L–1).

Recombination kinetics of diphenylcarbonyl oxide Russ.Chem.Bull., Int.Ed., Vol. 50, No. 5, May, 2001
795
log(2k)
licity of a solvent, respectively; y, p, e, and b are
coefficients characterizing the sensitivity of the reac-
tion to the effects of non-specific (polarity, polarizabil-
ity) and specific (electrophilicity, nucleophilicity) solva-
tions.
One can conclude that different properties of a
solvent (yY, pP, eE, bB) affect the kinetics of Ph2COO
decay. All coefficients of the Koppel—Palm equation
are negative, i.e., the enhancement of any interaction
results in a decrease in the 2k value. This can be
connected with an increase in the fraction of the zwitter-
ionic state of carbonyl oxide due to its stabilization by a
solvent. In nonpolar solvents (n-alkanes), in the ab-
sence of specific solvation, the biradical state of Ph2COO
is more probable, increasing the rate constant.
To elucidate in more detail the solvent effects on the
properties and reactivity of carbonyl oxides, we per-
formed ab initio quantum chemical calculations of the
equilibrium structure, electronic structure, and elec-
tronic absorption spectra for the simplest carbonyl oxide
H2COO and phenylcarbonyl oxide PhHCOO in n-hep-
tane, benzene, and acetonitrile, as well as in the gas
phase. Since the electron correction should be taken
into account to describe correctly the structure and
distribution of electron density of 1,3-dipoles, calcula-
tions were performed in the framework of density func-
tional theory using the valent-splitted basis set, includ-
ing the polarization d-functions for non-hydrogen at-
oms, B3LYP/6-31G(d). It has been shown18 that this
approach provides high accuracy of the calculation of
the geometric and electronic parameters of carbonyl
oxides, which is comparable with the accuracy of other
methods taking into account the correlation energy, for
example, CCSD(T)/TZ+2P.19,20
9.5
1
2
9.0
8.5
8.0
7.5
0.2
0.4
0.6
0.8
N
Fig. 3. Rate constant of the bimolecular decay (2k) of diphenyl-
carbonyl oxide as a function of the composition of a binary
mixture (N) at 295 K in different solvents: 1, n-pentane—benzene
mixture; 2, dioxane—n-decane mixture.
2.0•109 L mol–1 s–1 (see Table 1). A study of Ph2COO
recombination in the benzene—n-pentane and 1,4-di-
oxane—n-decane systems showed that the molar frac-
tion of a solvent affects the 2k value (Fig. 3)
log2kAB = NAlog2kAB + NBlog2kB,
where NA, NB are the molar fractions of the compo-
nents of a binary mixture; 2kA, 2kB, 2kAB are the rate
constants for reaction (1) in the pure and mixed sol-
vents.
The dependence of the reaction rate constant on the
solvent nature is described by the Koppel—Palm equa-
tion (correlation coefficient, r = 0.999, confidence level
is 0.95):
log(2k) = log(2k)0 + yY + pP + eE + bB,
The geometry of the carbonyl oxide moiety changes
regularly with increasing medium polarity (Table 2) in the
sequence gas phase (ε = 1)—n-heptane (ε = 1.924)—ben-
zene (ε = 2.284)—acetonitrile (ε = 37.5). The length of
the O—O bond in this sequence increases, whereas the
length of the C—O bond slightly decreases, i.e., the
log(2k)0 = 11.89±0.78, y = –2.74±0.92, p = –9.3±2.8,
–3
e = –0.161±0.057, b = –(3.1±1.4)•10
.
Here Y = (ε – 1)/(2ε + 1); P = (n2 – 1)/(n2 + 2); E and
B are the parameters of electrophilicity and nucleophi-
Table 2. Parameters of equilibrium structure (bond lengths (d), angle C—O—O), distribution of electron density (q), dipole moment
(µ), and energy characteristics (total energies (Etot) and solvation energies (∆Gsolv)) calculated by the B3LYP/6-31G(d) method for
carbonyl oxides R2C—O(1)—O(2) in different media
Medium
d/Å
C—O—O
/deg
q(C)
q(O(1))
q(O(2))
µ
/D
Etot
∆Gsolv
/kcal mol
–1
/a.u.
O(1)—O(2) C—O(1)
el.u.
H2COO
Gas phase
Heptane
Benzene
1.3433
1.3461
1.3466
1.3534
1.2653
1.2635
1.2631
1.2612
119.47
119.41
119.40
119.24
–0.024
–0.019
–0.018
–0.011
–0.025
–0.027
–0.027
–0.032
–0.338
–0.358
–0.362
–0.391
3.921
4.216
4.274
4.703
–189.576821
–189.579010
–189.577675
–189.578957
0.00
–1.37
–0.53
–1.34
Acetonitrile
PhHCOO
Gas phase
Heptane
Benzene
1.3582
1.3638
1.3650
1.3746
1.2763
1.2749
1.2747
1.2740
118.21
118.01
117.98
117.76
0.149
0.151
0.151
0.151
–0.122
–0.125
–0.126
–0.130
–0.353
–0.354
–0.354
–0.355
5.662
5.708
5.718
5.796
–420.651465
–420.651445
–420.651434
–420.651311
—
—
—
—
Acetonitrile

796
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 5, May, 2001
Khursan et al.
Table 3. Medium effects on the maxima in the absorption spectra (λmax) calculated by the
B3LYP/6-31G(d) method and the oscillator strength of electron transitions (f ) in carbonyl oxides
Medium
H2COO
PhHCOO
n—π*
max/nm
π—π*
max/nm
n—π*
max/nm
π—π*
max/nm
λ
f
λ
f
λ
f
λ
f
Gas phase
Heptane
Benzene
Acetonitrile
Ethylenea
Acetonitrilea
602.3
599.1
598.6
593.7
540.6
499.6
0.0007
0.0007
0.0007
0.0007
0.0002
0.0006
373.1
373.9
374.0
376.6
342.5
338.1
0.1778
0.1743
0.1737
0.1665
0.1736
0.1556
593.5
589.5
588.7
582.9
—
0.0005
0.0005
0.0005
0.0005
—
416.9
418.6
419.0
422.0
—
0.3913
0.3782
0.3756
0.3566
—
—
—
—
—
à Complex of carbonyl oxide with a solvent molecule modeling the specific solvation of H2COO.
contribution of the resonance structure A into the wave
function of the carbonyl oxide molecule increases with
increasing the dielectric permeability of a medium.
The non-specific solvation of carbonyl oxides does
not affect in fact the calculated energies of the electron
transitions and consequently the λmax values (Table 3).
This allows one to suggest that specific solvation mainly
affects the spectral properties of carbonyl oxides. To
simulate specific solvation, the geometric parameters of
the H2COO complexes with ethylene and acetonitrile
were calculated. The equilibrium structures and geomet-
ric parameters of the complexes are presented in Fig. 4
and Table 4. The enthalpy of complex formation was
calculated as a difference between the total energies of
the complex and initial compounds, taking into account
the correction by the zero potential energy (ZPE), as well
–
•
O
O
O
•
–
C
O+
C
O
C
O+
A
B
C
A decrease in the O—O bond order is accompanied
by the increase in the electron density on the terminal O
atom. This effect is most pronounced in the case of the
simplest carbonyl oxide: ∆q = 0.053 e.u. Conjugation
with the phenyl group stabilizes the carbonyl oxide
moiety resulting in a much less polarizability of the
PhHCOO molecule. Correspondingly, the dipole mo-
ments of carbonyl oxides increase on going from the gas
phase to acetonitrile and to a greater extent for H2COO
(∆µ = 0.782 D) than for PhHCOO (∆µ = 0.134 D).
Table 4. Geometric parameters of complexes of carbonyl oxide
with ethylene and acetonitrile
Parameter
H2COO...CH2=CH2 H2COO...MeCN
Bond
d/Å
O(1)—O(2)
O(2)—O(3)
C(3)—C(6)
C(6)—C(7)
O(1)—C(7)
C(3)—N(6)
N(6)—C(7)
1.351
1.265
3.111
1.336
2.895
1—
1.361
1.260
1—
O(1)
a
H(8)
H(9)
C(7)
1—
2.880
2.832
1.162
O(2)
1—
C(6)
H(11)
Valence angle
ω/deg
C(3)
O(1)—O(2)—C(3)
O(2)—C(3)—C(6)
C(3)—C(6)—C(7)
O(2)—O(1)—C(7)
O(1)—C(7)—C(6)
O(2)—C(3)—N(6)
C(3)—N(6)—C(7)
O(1)—C(7)—N(6)
118.4
118.3
—
—
89.3
—
92.0
101.3
100.6
H(5)
94.1
85.0
89.9
112.9
—
H(10)
N(6)
H(4)
C(8)
b
—
—
C(7)
H(4)
C(3)
H(5)
H(9)
Torsion angle
O(1)—O(2)—C(3)—C(6)
C(3)—O(2)—O(1)—C(7)
O(1)—O(2)—C(3)—H(4) –179.2
O(1)—O(2)—C(3)—H(5)
H(8)—C(7)—C(6)—H(10)
α/deg
70.8
–65.6
—
–66.6
–178.4
–1.8
—
H(10)
O(2)
O(1)
H(11)
–3.2
–1.5
Fig. 4. Equilibrium geometry of complexes of carbonyl oxide
with ethylene (a) and acetonitrile (b) calculated by the
B3LYP/6-31G(d) method.
H(8)—C(7)—C(6)—H(11) –179.1
O(1)—O(2)—C(3)—N(6)
—
67.5
—

Recombination kinetics of diphenylcarbonyl oxide Russ.Chem.Bull., Int.Ed., Vol. 50, No. 5, May, 2001
797
Table 5. Thermodynamic parameters of complex formation of H2COO with acetonitrile and
ethylene
ZPEa
H°298 – H°0
∆H°
∆G°b
KC
a
b
Compound,
complex
Etot
–1
/a.u.
/L mol
–1
kcal mol
H2COO
CH2=CH2
H2COO...CH2=CH2 –268.168703
MeCN
–189.576821
–78.587458
18.90
30.91
50.84
27.53
47.50
2.55
2.40
5.17
2.74
5.56
—
—
–1.53
—
—
—
3.54
—
—
—
0.06
—
–132.754928
–322.342812
H2COO...MeCN
–5.60 –0.53
60
a The values were corrected for anharmonicity of vibrations equal to 0.9614.21
b Changes in the Gibbs energy and concentration equilibrium constant were estimated using the
–1
value ∆S° = –17 cal mol–1 Ê
.
4. A. M. Nazarov, E. M. Chainikova, S. L. Khursan, I. A.
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as the change in temperature 0 → 298 K (H°298 – H°0)
(see Table 5). The complex of H2COO with acetonitrile
(∆H° = –5.60 kcal mol–1) is stronger than the
5. Technics of Organic Chemistry. Organic Solvents. Physical
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H2COO...CH2=CH2 complex (∆H° = –1.53 kcal mol ).
The calculation of the entropy variation upon the com-
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–1
low-frequency vibrations (λ < 400 cm ) in the com-
plexes, which are the origin of significant errors in
∆S° determination. The ∆S° values for both complexes
–1
–1
are within the range of –7—–27 cal mol
K . The
totality of the data indicates that nearly the whole
carbonyl oxide in acetonitrile occurs in the form of the
complex with the solvent (see Table 5), whereas the
presence of free carbonyl oxide is more probable in
benzene.
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The calculation of the spectral properties of the
H2COO complexes with a solvent (see Table 3) revealed
a strong hypsochromic shift (for acetonitrile ∆λmax = 38.5
(π—π*) and 94.1 (n—π*) nm). Since the phenyl ring in
carbonyl oxide decreases polarizability of the molecule,
one can expect a hypsochromic shift for the phenyl-
substituted carbonyl oxides.
Thus, both specific and non-specific solvations change
the properties of carbonyl oxide. Non-specific solvation
manifests itself in the polarization of carbonyl oxide
moiety and affects mainly the reactivity of Ph2COO,
decreasing it. Specific solvation determines the spectral
properties of carbonyl oxide. The stronger the complex
with a solvent, the greater the shift of λmax to the short-
wave region. The formation of strong complexes between
carbonyl oxide and a solvent decreases its reactivity.
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Chem., 2000, 74 (Engl. Transl.)].
Received March 18, 2000;
in revised form November 30, 2000