12152
J. Phys. Chem. 1994, 98, 12152-12157
Visible Light-Induced Oxygen Transfer from Nitrogen Dioxide to Ethyne and Propyne in a Cryogenic Matrix. 2. Mechanism and Regioselectivity James A. Harrison?and Heinz Frei' Laboratory of Chemical Biodynamics, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 Received: December 6, 1993; In Final Form: August 17, 1994@ The kinetics of visible light-induced oxidation of ethyne and propyne by NO2 in solid Ar has been monitored as a function of photolysis wavelength. CW dye and Ar ion laser emission was used for irradiation, and FT-infrared spectroscopy for the measurement of product growth. Determination of the wavelength dependence of quantum efficiencies from the kinetic measurements revealed that the photolysis threshold of HCSCH NO2 lies approximately 6 kcal mol-' higher than that of CH3CzCH N02. This and other parameters derived from the product growth measurements, and the formyl methyl iminoxy radical trapped in the case of the CH3CsCH NO2 reaction are interpreted in terms of a direct 0 transfer mechanism. The proposed path involves large-amplitude 0 transfer from photoexcited NO2 to the C Z C group to yield a transient ketocarbene. Formation of iminoxy radical is attributed to trapping of the ketocarbene by NO cage coproduct, which is in competition with Wolff rearrangement to yield ketene. From the structure of the iminoxy radical, it is inferred that the photoinduced 0 transfer from NO2 to the unsymmetrical C W bond of propyne is completely regioselective, leading exclusively to the carbene transient with oxygen at the terminal carbon. This regiochemical outcome is interpreted in terms of potential energy profiles for central and terminal carbon attack of the triple bond.
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I. Introduction In a previous study of the visible light-induced chemistry of ethyne-NO;?and propyne.NO2 pairs isolated in solid Ar matrices, we have found that ketene and methylketene, respectively, are the sole final oxidation products.' In the case of CH3CECH N02, an intermediate could be trapped. Vibrational analysis based on D, l8O, and 15Nisotopic substitution revealed that this species is an iminoxy radical with constitution CH3C(=NO')C(=O)H. Monitoring of the chemistry at various visible wavelengths showed that this radical was generated in four diastereomeric forms (isomerism with respect to the central C-C and the C=N bond). In this paper we will report measurement of the product growth kinetics of the HCECH NO2 and CH3CWH NO2 photoreactions at various wavelengths. The photochemistry is induced with light from a continuously tunable CW dye laser, and products are monitored by FT-infrared spectroscopy. Reaction quantum efficiencies and their photolysis wavelength dependence are determined. These data, combined with the interpretation of the observed iminoxy radical in terms of a NOtrapped ketocarbene transient, will be used to propose a detailed reaction mechanism. The regiochemistry of 0 transfer to CH3CECH will be inferred from the structure of the observed iminoxy radical, and the complete regioselectivity will be explained by the potential energy profiles for central and terminal carbon attack of the triple bond.
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11. Results
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1. Wavelength Dependence of Propyne NO2 Kinetics. Figure 1 shows the growth kinetics of four infrared bands at three photolysis wavelengths, namely 552, 514, and 476 nm. Each absorption represents a different propyne NO2 product (or product isomer). Experiments were conducted with matrices CH~CCH/N02/Ar= 2.5/1/100. A fresh matrix was prepared
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Present address: Department of Chemistry, Oregon State University, Corvallis, OR 97311. Abstract published in Advance ACS Absrracfs, November 1, 1994. @
0022-365419412098-12152$04.5010
for photolysis at each wavelength. Details regarding experimental setup, chemicals, and procedures have been described in our previous report. l s 2 Since measured infrared bandwidths were found to be constant over the photolysis periods, peak absorbances were used in place of integrated absorbances. The products shown in Figure 1 are formyl methyl iminoxy radical , (MK), and propynol (P). The isomers 1.42 and 1 ~ 1methylketene methylketene and the 1 ~ 1bands exhibit growth behavior characteristic of a single-photon process. This holds also for the growth of 1A2 upon irradiation at 552 nm. The lack of growth of the latter at 514 and 476 nm is due to efficient secondary photolysis at these higher photon energies, a result that was already used in the preceding paper for assignment purposes.' Comparison of parts a-c Figure 1 shows that the product yields increase sharply at higher photolysis photon energies, a behavior that is common to all hydrocarbon NO2 photoreactions investigated thus far.3 By contrast to the products IA2, 1 ~ 1and , MK, the propynol absorbance growth shows an induction period (zero initial slope) at all photolysis wavelengths, indicating that the alcohol is produced by secondary photolysis. A second species exhibiting an induction period in its growth absorbs at 1821, 1119, 1115, and 1038 cm-'. While the 1821 cm-' absorption points to a carbonyl species, no definitive assignment can be made for this product based on the available data. Corresponding growth curves are displayed in Figure 2a. The only product band that exhibits kinetic behavior different from that of the species already mentioned is the methylketene site absorbing at 2145 cm-'. This absorption shows sigmoidal behavior with a nonzero initial slope (Figure 2b). 2. Wavelength Dependence of Reaction Quantum Efficiency. 2.1. Ethyne NOz. Recording of infrared product growth under negligible depletion of the reactant reservoir coupled with measurement of the visible absorption of the C2H2/ NOdAr matrix allows us to determine the photolysis wavelength dependence of the reaction quantum efficiency. Figure 3a shows the absorption spectrum of NO2 isolated in solid Ar in the region 500-640 nm (N02/Ar = 1/30). Previous work revealed that
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0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 47, 1994 12153
Reaction of Ethyne*NOn and Propyne*NOZ Pairs O.O2
____.
Q,
'"*" 3430cm-1(P) 2125cm-I(MK) ___. 1588Cm-1( I B l ) ....D... 1570 cm-1 (IAz)
0.16.
.
: 0.12. e
a)
1
C
r
n ~
B
-
1038 cm-'
I
0.01
a
;0.08-
a
t 119cm-'
0.00
0 500
0
1000 1500 2000 2500
2000
4000
6000
1
0.02 4
.___.
. -..._
*-... 3430 cm-1 (P)
2125 cm-1 (MK) ___. 1588 Cm-1(IB1)
0.16.
......0.....
: 0.12E
1570 Cm-1 (IA2) .,-*
+..--
mWh cm-2
1
h)
..*--.
0
2000 4000
6 0 0 0 m W h cm'2
Figure 2. (a) Infrared absorbance growth kinetics of bands originating
from secondary photolysis products. Photolysis was conducted at 552 nm in a matrix CH~CCH/NOdAr= 2.5/1/200. (b) Growth of the 2145 cm-' methylketene band. 476 nm Photolysis
3 0
n L
SI
n
0
5 0 0 1000 1500 2000 2500 Photon Flux ( m W h Cm-2)
500
Figure 1. Infrared absorbance growth kinetics of four species upon photolysis of a matrix CH3CCWNOdAr = 2.5/1/100 at 552,514, and 476 nm. MK, methylketene; P, propynol; IAZ, I B ~formyl , methyl
iminoxy radical isomers.
540
HC"CH
580
620nm
+ NO2
+(A)
N 5 1 4 nm)
r 1.0
the presence of an unsaturated hydrocarbon molecule in the matrix cage containing NO2 does not have a discernible effect 0.8 on the NO2 visible absorption ~ p e c t r u m .The ~ triangles in the same graph represent the absorbance growth of the 2141 cm-I 0.6 band of ketene at four different photolysis wavelengths, namely 582, 560, 545, and 514 nm. The product growth at each 0.4 wavelength was measured upon irradiation of one and the same 0.2 matrix (C2H2/N02/Ar = 2/1/100) with the same number of photons per square centimeter (8.4 x 1021). The ratio of the I absorbance growth of ketene at two different photolysis 500 540 580 620 nm wavelengths dl and d2 is equal to c ~ ( ~ + & I I ) / ( c ~ ( ~ ~ ) Figure 3. Onset of the ethyne NO2 photoreaction in a matrix 4(d2)). Therefore, the ratio of the reaction quantum efficiencies HC=CH/NOdAr = 2/1/100. (a) Absorption spectrum of NO2 susat the two wavelengths is simply the ratio of the ketene pended in solid Ar (scale on the right-hand side); absorbance growth absorbance growth multiplied by the factor e:?(d2)k:?(d& of the 2141 cm-' ketene band (triangles, scale on the left-hand side). These data can be taken directly from Figure 3a. The resulting The ketene growth at each photolysis wavelength was measured by irradiation of the matrix with 8 x loz1photons cm-2. (b) Wavelength relative quantum efficiencies to reaction, normalized to the value dependence of quantum efficiency to reaction, referenced to the value at 514 nm, are presented in Figure 3b. at 514 nm. 2.2. Propyne NOz. As in the case of ethyne NO2 the photolysis laser wavelength between 540 and 630 nm is reaction, recording of infrared product growth under negligible shown in Figure 4a. Irradiations were conducted with one and depletion of the reservoir of reactant pairs allowed us to determine the wavelength dependence of the quantum efficiency the same matrix CH3CCH/N02/Ar = 2.5/1/200 and with 8.6 x lo2' photons at each wavelength. The same figure also shows of the CH3CCH NO2 reaction. The absorbance growth of the NO2 absorption profile in this spectral region. As described the methylketene product band at 2125 cm-' as a function of -
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8
Harrison and Frei
12154 J. Phys. Chem., Vol. 98, No. 47, 1994 in the preceding section, we can readily determine the wavelength dependence of the reaction quantum efficiency from these data. The result is shown in Figure 4b, with values referenced to the quantum efficiency at 545 nm. Although methylketene is only one of two single-photon products of the CH3CCH NO2 reaction (the other is formyl methyl iminoxy radical),' the curve of Figure 4b reflects faithfully the dependence of the reaction quantum efficiency on wavelength. This is because the branching ratio of methylketene to iminoxy radical was found to be constant over this wavelength range. The ratio of the integrated v,(C=C=O) absorption of CH3CH=C=O to the sum of the integrated v(C=O) absorption intensities of all four CH3C(=NO')C(=O)H isomers was determined as b = 2.6 f 0.8 at 552, 514, and 476 nm. Three independent intensity measurements were made at each wavelength, and photolysis periods were kept short in order to avoid any distortion of the result by secondary photolysis of iminoxy radicals. This treatment assumes that the C=O stretch extinction coefficient is the same for all four iminoxy radical isomers. The branching ratio between IA and IBaround the reaction threshold wavelength (where secondary photolysis of iminoxy radical is slow) was determined to be 1.5.2 The constancy of the methylketene to iminoxy radical branching ratio allowed for a straightforward comparison between product yields of the ethyne NO2 and propyne f NO2 reactions. This was accomplished by expressing the total yield of the CH3CCH NO2 reaction in terms of CH3CH=C=O growth by multiplying the observed methylketene absorbance growth by the factor (1 lh). The branching ratio r between CH3CH=C=O and CH3C(=NO)C(=O)H is equal to b*E(I,v(C=O))k(MK,v,(C=C=O))= 0.4. The extinction coefficient E of the methylketene absorption was taken as 2000 L mol-' cm-', reflecting a typical value of a ketene v,(C=C=O) mode.5 The assumption of equal C=C=O stretch extinction coefficients for ketene and methylketene is justified by the fact that both absorptions exhibit the same l8O isotope frequency shift, indicating that these noma1 modes have identical potential energy distribution. The C=O stretch extinction coefficient of the iminoxy radical was chosen as 320 L mol-' cm-', an average value reported for v(C=O) of a,P-unsaturated carbon y k 6 These ketene and methylketene absorbance growths were normalized for irradiation with 8.6 x lo2' photons and corrected for the concentration of C2HrN02 and CH3CCEN02 pairs in the two matrices. Ratioing against the absorbance of NO2 affords a direct comparison to ethyne NO2 and propyne -t NO2 quantum efficiencies, shown in Figure 5. The figure shows that the C2H2 f NO2 product growth at 550 nm is equivalent to that of CH3CCH NO2 under 615 nm (threshold) irradiation. The fact that CHz=C=O growth can already be observed with 582 nm light is merely due to the much higher sensitivity in the case of the ethyne NO2 system, where ketene, with its very high absorption cross section, is the sole photoproduct. We conclude that the true separation of the ethyne NO2 and propyne NO2 reaction thresholds is around 6 kcal mol-' (corresponding to the energy difference between 550 and 615 nm photons). 3. Absolute Quantum Yields. The absolute quantum yield to reaction is equal to product growth per photolysis photon absorbed. Calculation is based on the measured product absorbance growth, the absorption of NO2 at the photolysis wavelength, and the laser light intensity. In the case of HC'CH NO2 initiated at 514 nm, for example, the ketene infrared absorbance growth shown in Figure 3a corresponds to 1.6 x mol cm-2 of product. The visible absorption spectrum of the same HCECWN02IAr matrix has an absorbance of 0.10 at
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h
r
(5n N
n, v
P)
C
0.04 L
si n
r:
0.02
W
I
0, r 0
560
540
580
600
620nm
CH3CrCH t NO2
T
'.O
1R
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b)
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+
O.*
1 560
540
+
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600
620nm
+
3'0
; A
I i
2.0
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580
Figure 4. Onset of the propyne NO2 photoreaction in a matrix CH3C=CH/NO*/Ar = 2.5/1/200. (a) Absorption spectrum of NO1 suspended in solid Ax (scale on the right-hand side); absorbance growth of the 2125 cm-' methylketene band (triangles, scale on the left-hand side). The methylketene growth at each photolysis wavelength was measured by irradiation of the matrix with 8 x loz1photons cm-2. (b) Wavelength dependence of the quantum efficiency to reaction, referenced to the value at 542 nm.
HCECH
N
OK?
z>
1.0 .
+
500
+ NO2
\ 540
580
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620nm
Figure 5. Comparison of HC=CH NO2 and CH3CsCH reaction quantum efficiencies (arbitrary scale).
+ NO2
5 14 nm. Assuming a statistical distribution of the reactants in the matrix,' one-third of all NO2 species would have an ethyne nearest neighbor. Hence, 7.5% of the photolysis light is absorbed by the reactants. Since irradiation with 1.24 x mol cm-2 of photons was required to produce the 1.6 x lo-* mol cm-2 of ketene, the quantum yield of the HCECH NO2 photolysis at 514 nm is 2 x lop5. This value may be off in
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Reaction of Ethyne-NO2 and Propyne-NO2 Pairs
J. Phys. Chem., Vol. 98, No. 47, 1994 12155
either direction by as much as a factor of 3, primarily because of uncertainties in the true absorption of NO2 in the visible (baseline, scattering), the extinction coefficient of the product (the measured extinction coefficient of ketene refers to the gas p h a ~ e ) and , ~ the assumption of a statistical distribution of the reactants in the matrix. A similar estimate in the case of the CH3CZCH NO2 reaction can be made on the basis of the methylketene infrared growth shown in Figure 4a. For example, at 542 nm, a CH3CH=C-O growth of 1.8 x mol cm-2 is measured upon irradiation with 1.33 x mol cm-2 of photons. This corresponds to 6.3 x mol cm-2 of total product if the iminoxy radical growth is taken into account (see section 2.2). The absorbance of the reactive matrix CH3C=CWN02/Ar = 2.5/1/200 at 542 nm was determined as 0.0435, of which 16% would originate from reactant pairs according to the statistical model.7 This means that 1.5% of the 1.33 x lo-* mol cm-2 of incident photons are absorbed by the reactants. Hence, the quantum yield of the CH~CECH NO2 reaction at 542 nm is 3 x lop4,with a large uncertainty of a factor of 3 due to reasons already given.
E (kcal mol")
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6o
t
(37) (29)
+
NO2
.20
111. Discussion 1. Reaction Mechanism. The experimental results of the ethyne NO2 and the propyne NO;? studies reported here lend strong support and add new insight to the mechanism of photochemical 0 transfer from NO2 to CC triple bonds proposed in our previous 2-butyne NO2 work.8 The mechanism involves 0 transfer from visible light-excited NO2 to the CEC group of an alkyne cage partner with which it is in van der Waals contact. Excitation energies lie tens of kilocalories per mole below the 398 nm NO2 O(3P) -t NO dissociation threshold. Hence, the 0 transfer is in essence a large-amplitude N-O-C antisymmetric stretching motion, as formation of free O(3P) atoms is not accessible energetically. The primary photoproduct so formed was assigned as a singlet excited ketocarbene in the lA'(8) state.2*8 The two observed products, ketene and iminoxy radical, emerge from a competition between Wolff rearrangement of the ketocarbenegand combination with the NO cage neighbor (the latter channel exists only in the case of the methyl-substituted ethynes). Hence, the transient ketocarbene acts as a common precursor of ketene and iminoxy radical. The potential energy diagram of the H C s C H NO2 reaction (Figure 6) shows the lowest electronic states of HCC(0)H biradicallOsl that are energetically accessible to the visible light-excited HC=CH.N02 pairs. The energy of the biradical ground electronic state relative to that of HCGCH NO2 was derived from the ab initio value of the potential energy change for O(3P) HC=CH HCC(O)H(X) reported by Harding.12 Energies of the three excited states of HCC(0)H in the immediate vicinity of the ground state were taken from the same ab intio work12 as well as calculations by Bargon and Y0~himine.l~ According to these theoretical studies, the triplet ground state (3A"(ucxc)) and the first excited singlet state (lA'(8)) of the biradical both have a ketocarbene electron configuration featuring a CO double bond, a central CC single bond, and a carbene C, as shown in Figure 6. Symbols in parentheses indicate orbitals occupied by nonbonding electrons; o means in-plane (sp2) and x means perpendicular (p) orbital with respect to the CCO plane. In contrast to these two lowest states, the two next higher states of the biradical that are energetically accessible by visible light-induced 0 transfer to HCECH have electron configurations corresponding to a 1,3biradical HC=C(O)H designated lA'(ucuo)and 3A'(ucu,). These are also shown in Figure 6.
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-.
n
+ H-C&-H
(0)
t
c
\\
H\ H/c=c=O
-60
+ NO (*n)
Reaction Coordinate
+
Figure 6. Potential energy diagram for the ethyne NO2 system. The energy of each species relative to the reactant level is indicated in parentheses (in kcal mol-'). AHo of NO2 HCGCH CH2=C=O NO was calculated as -52 kcal mol-' from standard thermodynamic data.28 The energy of the lowest triplet state of CH2=C=O (55 kcal mol-' above ground state) was taken from ab initio work.29
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One important result of the ethyne NO2 and propyne NO2 study presented here is that it adds substantial new evidence in terms of assignment of the observed iminoxy radical as a chemically trapped ketocarbene transient. We observed a monotonic increase of the branching between ketene and iminoxy radical with decreasing CH3 substitution of the CEC group. The branching ratio is 0.07 for CH3CrCCH3 NO2, 0.4 for CH3CECH NOz, and 2 5 for H C W H NOz. These ratios were determined as described in section 11.2.2. The methyl substitution effect on the competition between ketene and iminoxy radical formation constitutes strong evidence that the two products share not only a common precursor but even a common state of the precursor. Moreover, the direction of the effect is one that would be predicted for competitionbetween Wolff rearrangement of a ketocarbene biradical and combination with NO: the 1,2-H shift is more facile than l,2-CH3 shift.14-16 Furthermore, CH3-substituted carbenes are more stable than unsubstituted carbene. As a consequence, longer lifetimes and, hence, higher probability for trapping by NO cage neighbor are expected with increasing CH3 substitution, as observed. The fast rise of the reaction quantum efficiency above the photolysis threshold both in the case of ethyne NO2 and propyne NO2 (Figures 3b and 4b, respectively) signals reaction of the vibrationally unrelaxed NO2. In fact, excitation by visible light prepares a highly vibrationally excited NO2 molecule with predominant electronic ground state character" (the upper state is the 2B2 state, so excited NO2 has mixed *B2%A1 electronic character with ZA1dominating). The observed wavelength dependence of the quantum yields is consistent with this photophysical property of the excited reactant.18 Electronic
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Harrison and Frei
12156 J. Phys. Chem., Vol. 98, No. 47, 1994 SCHEME 1
E (kcal mol") ON '
[cH3-e7!-H 0
terminal addition CHI-C-C-H
+N
'
,j
+NO
/
0
w
II
C H r C- C-H 4 isomers
CH3, 40
H/-o 0
2
L
+NO
( 1
20
0
cenwal addition
state correlations based on a Salem diagram analysis2-*indicate that the HCC(0)H (CH3CC(O)H) state accessed by largeamplitude 0 transfer from excited N02(2B2-2A1) to the CEC group is the lA'(u,uo) biradical state, as indicated in Figure 6. This biradical state is presumed to be strongly coupled to the nearby 'A'($) ketocarbene state of same symmetry.* Hence, very fast relaxation to HCC(O)H('A'($)) followed by 1,2-H migration (and combination with NO in the case of CH3CC(0)H) is the most probable path leading to the observed products. The fact that no iminoxy radical at all is produced in the H C W H NO2 photochemistryimplies that combination of HC(O)CH(IA'(d)) with NO can not compete with 1,2-H shift. This is consistent with the result of ab initio calculations, which predict no barrier for 1,2-H migration of singlet HC(0)CH19320to yield CH2=C=O. Furthermore, the lack of any HC(=O)C(=NO')H formation implies that the NO coproduct of the 0 transfer reaction, even though a radical itself, does not affect intersystem crossing of HC(O)CH('A'($)) to the lower lying triplet ground state. The latter would almost certainly be trapped as an iminoxy radical because of the large barrier to 1,2-H ~ h i f t ' ~(Figure , ' ~ 6), yet none is observed. All hydrocarbon photooxidations by NO2 conducted thus far indicate that the continued chemistry of transient biradicals is always fast on the time scale of 2NO-inducedsinglet-triplet state ~oupling.~ It is interesting to note that visible light-induced 0 transfer from NO2 to allene, a constitutional isomer of propyne, yields an altogether different product, namely the nitrite radical CH2=C(ONO)CH2.* Final oxidation products cyclopropanone and allene oxide are also completely different from that of the CH3C"CH NO2 photochemistry. This implies that no interconversion between transient oxirane biradical CH2=C(O)CH2 (allene NO2 reaction) and formyl carbene CH3CC(=O)H (propyne reaction) takes place along the 0 transfer paths. 2. Regioselectivity. The structure of the formyl methyl iminoxy radical trapped in the case of the CH3CsCH NO2 reaction allows us to infer the regiochemistry of 0 transfer to this unsymmetrical CC triple bond. Scheme 1 shows that 0 transfer from NO2 to the terminal carbon of CH3CFCH would yield transient formyl carbene CH3CC(=O)H (labeled isomer 1 in the scheme). On the other hand, oxygen attack at the central carbon would lead to the CH3C(=O)CH isomer (labeled 2). Combination of formyl carbene (1) with NO gives the observed iminoxy radical CH3C(=NO')C(=O)H,'4.21 while trapping of CH3C(=O)CH (2) by NO would lead to CH3C(=O)C(=NO')H, a species that is not formed. It is worth adding that observation of the secondary photolysis product CH3C=COH lends strong
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-20
-40
'.....*
-60
O$Y@
Reaction Coordinate
n/
C=C=O (So) (-50)
+ NO
(2n)
+
Figure 7. Potential energy diagram for the propyne NO2 system. It shows the regiochemical paths corresponding to 0 transfer to the
terminal carbon (bold front curve, assumed to be identical with the energy path of the 2-butyne NO2 reaction)8and to the central carbon (rear curve, assumed to be identical with the energy path of the ethyne NO2 reactions (Figure 6)). The energy of each species is given in parentheses (in kcal mol-'). The two surfaces are connected by an 0 flip coordinate.
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support to the attribution of the observed iminoxy radical to the aldehyde structure CH3C(=NO')C(=O)H rather than the keto form CH3C(=O)C(=NO')H because only the former can yield propynol upon photoexpulsion of NO. It would appear, therefore, that only carbene isomer CH3CC(=O)H is formed along the reaction path but no CHsC(=O)CH. We conclude either that transient acetylcarbene (2) is not generated at all or that it isomerizes to CH3CC(O)H on a time scale which is short compared to trapping by N0.22,23 A more detailed view of the origin of the observed regioselectivity can be obtained by considering the CH~CECH NO2 potential energy diagram shown in Figure 7 . Comparison of the photolysis thresholds of HC'CH, CH3CECH, and CH3=CCH3 NO2 reactions points to distinct differences in the stability of the isomeric carbenes 1 and 2. CH~CECH NO2 and C H ~ C E C C H ~ NO2 reactions have the same threshold of 615 nm, and both lead to a methyl-substituted carbene CH3CC(=O)H or CH3CC(=O)CH3. By contrast, HCECH has an approximately 6 kcal mol-' higher threshold (at 550 nm) with HCC(=O)H as the proposed transient photoproduct. The finding that formation of CH3CC(=O)H and CH3CC(=O)CH3 occurs at the same threshold energy suggests that the nature of the carbonyl moiety (aldehyde vs keto group) of the ketocarbene does not influence significantly the energy of the carbene (more precisely, the energy of the transition state leading to CH3CC(O)H or CH3CC(O)CH3). By analogy, we would expect that CH3C(=O)CH and HC(=O)CH have similar thresholds to formation as well. If this holds, the threshold to
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Reaction of Ethyne*NOz and PropyneNOa Pairs
J. Phys. Chem., Vol. 98, No. 47, 1994 12157
0 transfer to the central carbon of CH~CECHwould lie around 550 nm. Thus, we are considering two separate energy profiles
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in the potential energy diagram of the CH3C"CH NO2 system. One corresponds to 0 transfer to the terminal C atom of CH3CECH (bold front curve of Figure 7), and the other to attack of the central carbon of propyne (rear profile in Figure 7). Comparison of the two profiles suggests that only CH3CC(=O)H but not CH3C(=O)CH is accessible upon excitation of CH3CWHN02 pairs at yellow and longer wavelengths. Hence, at photolysis wavelengths longer than 550 nm the observed complete regioselectivity may be attributed entirely to differences in the potential energy profiles of the two paths. On the other hand, the energy diagram raises the question of whether some central carbon attack occurs at photolysis photon energies above 550 nm. It it does occur, it may be followed by fast rearrangement of transient CH3C(O)CH to CH3CC(O)H by 0 flip before trapping by NO can take place. Oxygen exchange between the two carbons initiated by photoexcitation of CH2=C=O has been observed in the gas p h a ~ e . In ~ ~the, ~most ~ recent work by Moore,250 flip has been found to be very fast (nanoseconds) for HC(0)CH generated at excess energies that are comparable with those accessible in our HCZCH NO2 photolysis experiments. The corresponding path is indicated in Figure 7 by dashed lines.26 We conclude that if central carbon attack occurs, 0 flip would explain why no CHF(=O)C(=NO')H is trapped. However, this remains a speculation, and it is equally possible that the observed regioselectivity is entirely due to an inaccessible barrier to central C attack at all visible wavelengths.
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IV. Conclusions Chemical trapping of transient carbene by NO cage coproduct and determination of the structure of the iminoxy radical so formed has allowed us to uncover for the first time the regiochemical path of 0 transfer to an unsymmetrical CEC bond. The reaction turns out to be completely regioselective, leading to the transient biradical that corresponds to attack of the terminal CZC carbon. Comparison of the potential energy paths for transfer of 0 from NO2 to the central and terminal carbons suggests that the principal controlling factor is preferential coupling of the photoexcited CH3CECH.NO2 pairs into the more stable CH3CC(O)H NO rather than the CH3C(O)CH NO product states. It is possible that at higher photolysis photon energies (A 550 nm) coupling does occur into CH3C(0)CH NO states as well. If this is the case, the preservation of regioselectivity at these higher photon energies may be explained by fast 0 flip, leading from initially produced CH&(O)CH to CH3CC(O)H biradical on a time scale short compared to chemical trapping of CH3C(O)CH by NO. An analogous 0 transfer path is proposed for the HCeCH NO2 system, although no NO-trapped transient was detected in this case. The lack of carbene trapping is attributed to the instability of HCC(0)H with respect to Wolff rearrangement to CHz-CIO. Beyond the mechanistic insight gained by the photoinduced ethyne and propyne NO2 reactions, this work demonstrates that long-wavelength photochemistry of 0 donorhydrocarbon pairs in inert gas matrices offers a unique opportunity to explore new details of hydrocarbon oxidation pathways. As discussed in a previous report, the novel point of entry to singlet ketocarbene (formyl carbene) hypersurfaces opened up by 0 transfer from NO?;may be typical for CC triple-bond oxidation by common closed shell oxidizers.8 If this is the case, the results reported here furnish a first insight into the factors determining the regiochemical path of the practically important oxidation of alkynes by p e r o x y a c i d ~ . At ~ ~ the same time, the result of
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+
the study points to long-wavelength photochemistry of collisional pairs sustained by a solid matrix cage as a promising method for very mild oxidation of unfunctionalized abundant hydrocarbons.
Acknowledgment This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division, of the U.S. Department of Energy under contract No. DE-AC03-76-SF00098. J.A.H. thanks the Fannie and John Hertz Foundation for a predoctoral fellowship. References and Notes (1) Harrison, J. A.; Frei, H. J . Phys. Chem., preceding paper in this issue. (2) Harrison, J. A. Ph.D. Thesis, University of Califomia, Berkeley, 1991. (3) Frei, H. In Vibrational Spectra and Structure; Dung, J. R., Ed.; Elsevier: Amsterdam, The Netherlands, 1992; Vol. 20, p 1 and references therein. (4) Fitzmaurice, D. J.; Frei, H. Chem. Phys. Lett. 1992, 192, 166. (5) Nadzhimutdinov, S.; Slovokhotova, N. A.; Kargin, V. A. Russ. J . Phys. Chem. 1966,40, 479. (6) Noack, K. Spectrochim. Acta 1962, 18, 1625. (7) Rytter, E.; Gruen, D. M. Spectrochim. Acta 1979, 35A, 199. (8) Nakata, M.; Frei, H. J . Am. Chem. SOC. 1992, 114, 1363. (9) March, J. Advanced Organic Chemistry, 4th ed.;Wiley: New York, 1992; p 1083. (10) The term biradical is used here for any even-electron species that has one bond less than the number permitted by the standard rules of valence." (11) Salem, L. Electrons in Chemical Reactions: First Principles; Wiley: New York, 1982; p 60. (12) Harding, L. B.; Wagner, A. F. J . Phys. Chem. 1986, 90, 2974. (13) (a) Bargon, J.; Tanaka, K.; Yoshimine, M. In Computationnl Methods in Chemistry; Bargon, J., Ed.; Plenum: New York, 1980; p 239. (b) Tanaka, K.; Yoshimine, M. J . Am. Chem. SOC. 1980, 102, 7655. (14) Bachmann, C.; N'Guessan, T. Y.; Debu, F.; Monnier, M.; Pourcin, J.; Aycard, J. P.; Bodot, H. J . Am. Chem. SOC. 1990, 112, 7488. (15) Csizmadia, I. G.; Gunming, H. E.; Gosavi, R. K.; Strausz, 0. P. J . Am. Chem. SOC.1973, 95, 133. (16) Meier, A.; Zeller, K. P. Angew. Chem., Int. Ed. Engl. 1975, 14, 32. (17) Nakata, M.; Shibuya, K.; Frei, H. J . Phys. Chem. 1990, 94, 8168. (18) Hsu, D. K.; Monts, D. L.; Zare, R. N. Spectral Atlas ofNitrogen Dioxide; Academic Press: New York, 1978. (19) Bouma, W. J.; Nobes, R. H.; Radom, L.; Woodward, C. E. J. Org. Chem. 1982, 47, 1869. (20) Vacek, G.; Colegrove, B. T.; Schaefer, H. F. Chem. Phys. Lett. 1991, 177, 468. (21) Independent support for competition between 1,2-H shift and trapping by NO of CH$C(=O)H in the 'A'(
