no spin trapping and epr studies on the photochemistry of aliphatic aldehydes

MAGNETIC RESONANCE IN CHEMISTRYMagn. Reson. Chem. 2005; 43: 156–165Published online 1 December 2004 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/mrc.1513

NO spin trapping and EPR studies on thephotochemistry of aliphatic aldehydes

Fan Wang, Liandi Lei and Longmin Wu∗

State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China

Received 21 June 2004; Revised 12 September 2004; Accepted 20 September 2004

Radicals, such as acyl, hydrated acyl, alkyl and ketyl radicals, from aliphatic aldehyde photochemistrywere detected by NO spin trapping and EPR techniques. Deuterium effects on EPR spectra and thegeneration of radicals by 2-amido-2-propyl radical attack on substrate molecules in aqueous solution viahydrogen-atom abstraction were applied to identify radicals produced photochemically from aldehydes.Aliphatic aldehydes used in the present investigation were formaldehyde, acetaldehyde, acetaldehyde-d4, propionaldehyde, isobutyraldehyde, isopentanal and tert-pentanal. Possible reaction mechanisms aresuggested. Copyright 2004 John Wiley & Sons, Ltd.

KEYWORDS: EPR; 1H; 14N; aldehydes; acyl; hydration; nitroxide; nitric oxide

INTRODUCTION

The photochemistry of aliphatic aldehydes is among themost familiar and thoroughly studied organic processes.1,2

This kind of compound has been known to be importantconstituents in various atmospheric chemical cycles, espe-cially in the formation of photochemical smog. Saturatedaliphatic aldehydes all possess a weak absorption band inthe region of 240–360 nm as a result of an electric dipoleforbidden but vibronically allowed n, �Ł transition local-ized on the —C O chromophore. Formaldehyde is the onlymolecule in the series in which internal conversion (IC) dom-inates over intersystem crossing (ISC) across a large rangeof excitation energies, in contrast to propionaldehyde, whichexhibits large-molecule or statistical-limit behavior and isalmost completely dominated by ISC, certainly at lower exci-tation energies.1 Acetaldehyde is in the intermediate regime,exhibiting almost 100% ISC at very low excitation energies(¾330 nm) and about 80% ISC near 315 nm excitation. Hencethe photochemistry of formaldehyde is different from thatof its larger counterparts. Studies that have been carried out,theoretically, spectroscopically, and photochemically, indi-cate that there is a high degree of similarity in the seriesof aliphatic aldehydes from acetaldehyde to longer chaincompounds. Theoretical calculations on the ground-statestructure of acetaldehyde, propionaldehyde and butyralde-hyde reveal that the structural parameters (bond lengths andangles) of the aldehyde group are quantitatively uniform.3

Based on the previous studies, we have encountered sev-eral types of photoreactions of aliphatic aldehydes. First,aliphatic aldehyde photolysis undergoes an autoxidation via

ŁCorrespondence to: Longmin Wu, State Key Laboratory ofApplied Organic Chemistry, Lanzhou University, Lanzhou 730000,China. E-mail: [email protected]/grant sponsor: Natural Science Foundation of China;Contract/grant number: 20072013.

homolysis of the bond hydrogen atom (H) to carbonyl group[Eqn (1)] or H-abstraction by other radicals, providing acylradicals [Eqn (2)].

RCHOh��!R

žCO C žH �1�

RCHO C žR1 ��! R

žCO C R1H �2�

A second type of aliphatic aldehyde photoreactioninvolves a primary �-bond cleavage, affording alkyl andformyl radicals:

RCHOh��!Rž C H

žCO �3�

The third photochemical reaction channel of aliphaticaldehydes is photoreduction, in which the 3(n, �Ł) state alde-hyde molecule undergoes quenching via an H-abstractionfrom the ground-state aldehyde molecule to give a ketyl andan acyl radical:

RCHOh��!R

žCH(OH) C R

žCO �4�

The radical-induced decarbonylation of aliphatic alde-hydes is another type of photoreaction [Eqns (2) and (5)],yielding an acyl and an alkyl radical.4,5

RžCO ��! Rž C CO �5�

It is worth pointing out that the fragmentation [Eqn (5)] isusually negligible if R is a simple primary alkyl group. Onother hand, fragmentations to give stabilized radicals arerapid.6

As a special example, photolysis of isopentanal under-goes fragmentation and a consequent disproportionation:

�CH3�2CHCH2CHOh��!�CH3�2

žCH C žCH2CHOH

��! CH3CH CH2 C CH3CHO�6�

Copyright 2004 John Wiley & Sons, Ltd.

Photochemistry of aliphatic aldehydes 157

Since a typical n, �Ł state energy of aliphatic aldehydesis >72 kcal mol�1 �1 kcal D 4.184 kJ�,7 the strong bonds[bond dissociation energies (BDEs) >80 kcal mol�1] undergoinefficient cleavage, whereas the relatively weaker bonds(BDEs between 70 and 80 kcal mol�1) undergo intermedi-ately efficient cleavage from the T(n, �Ł) state at roomtemperature. The average BDE of the H—C( O)—bondis commonly ¾87 kcal mol�1 and that of the C—CHO bond81 kcal mol�1.8 – 11 This predicts that the T(n, �Ł) state alde-hydes tend to fragment at the C—CHO bond [Eqn (3)] inpreference to the H—C( O)—bond [Eqn (1)].

Among the radicals from aliphatic aldehyde photoreac-tions, many have been confirmed by theoretical methods12

spectroscopic techniques, such as fluorescence,8 cavity ring-down13 and IR,14,15 and as product analysis by GC,16 – 18

HPLC9,19 or MS.20,21 However, these methods gave only indi-rect information on their structures. A few of them have beendirectly determined by ESR or CIDNP techniques.22,23 There-fore, we wanted to obtain more direct and detailed structuralevidence for radicals from aldehyde photoreactions. In thispaper, we present some new structural information deducedfrom nitroxides produced by NO trapping24 – 26 of tran-sient radicals appearing during the photolysis of aliphaticaldehydes.

EXPERIMENTAL

Formaldehyde (Xian Chemicals, 37–40%), acetaldehyde(Xian Chemicals, 40%), acetaldehyde (Aldrich, 99%),

acetaldehyde-d4 (Aldrich, 98 at% D), propionaldehyde(Aldrich, 99%), isobutyraldehyde (Aldrich, redistilled, 99.5 C%), isopentanal (Aldrich, 97%), tert-pentanal (Aldrich, 97%)and 2, 20-azobis(2-methylpropionamidine) dihydrochloride(AAPH, Aldrich) were used as received. NO was pre-pared and purified according to the procedure describedpreviously.24 Water for aqueous samples was distilled anddeionized.

The sample preparation, EPR measurements and calcu-lations for EPR spectra were described previously.24,25 Ingeneral, the sample was irradiated in situ with light from a1 kW compact arc Hg–Xe lamp. EPR spectra were recordedwhen the lamp was turned on or off with a duration of about2–5 min, unless indicated otherwise. All the EPR deter-minations were undertaken at ambient temperature. EPRparameters such as hyperfine splitting constants (HFSCs)and g-values are given in Table 1.

RESULTS AND DISCUSSION

AcetaldehydeThe EPR spectrum [Fig. 1(a)] was primarily obtained fromthe UV photolysis of an NO-saturated pure acetaldehyde.This spectrum appears to be a mixture of two signals withdifferent g-values, one marked with sticks and anotherwith dots. Two of their lines overlap each other. Thespectrum displays two sets of 1 : 1 : 1 triplets of a doublet.When light was masked, the lines marked as sticks inFigure 1(a) disappeared, whereas those marked as dots

Table 1. EPR parameters of nitroxides R1R2N�Ož�

No. R1 R2 Solvent HFSC (mT)a g-Valueb

1 Acetaldehyde 1N: 0.6321H: 1.107

2.0069

2 Acetaldehyde 1N: 0.7381H: 0.211

2.0064

3 Acetaldehyde-d4 1N: 0.738 2.0064

4 Water 1N: 1.3616H: 0.061

2.0064

5 Water 1N: 1.4231H: 1.871

2.0056

6 C�CH3�2C�NH2� NH Water 1N: 0.751 2.0066

7 C�CH3�2C�NH2� NH Water 1N: 1.4761H: 1.792

2.0058

8 C�CH3�2C�NH2� NH Water 1N: 1.4231H: 0.985

2.0060

9 Propionaldehyde 1N: 0.6321H: 1.080

2.0067

10 Propionaldehyde 1N: 0.7381H: 0.211

2.0064

Copyright 2004 John Wiley & Sons, Ltd. Magn. Reson. Chem. 2005; 43: 156–165

158 F. Wang, L.-D. Lei and L.-M. Wu

Table 1. (Continued)

No. R1 R2 Solvent HFSC (mT)a g-Valueb

11 CH2CH3 Propionaldehyde 1N: 0.7382H: 0.685

2.0064

12 Water 1N: 1.4231H: 1.976

2.0061

13 Isobutaldehyde 1N: 0.7591H: 0.264

2.0059

Water 1N: 0.804 2.00641H: 0.290

14 Isobutaldehyde 1N: 0.7511H: 0.580

2.0059

15 Isobutaldehyde 1N: 0.738 2.0059

16 Isopentanal 1N: 0.7381H: 0.184

2.0066

17 Water 1N: 0.830 2.0059

18 tert-Pentanal 1N: 0.7111H: 0.132

2.0067

19 C�CH3�3 tert-Pentanal 1N: 0.791 2.0064

a Absolute accuracy š0.003 mT.b Absolute accuracy š0.0001.

Figure 1. (a) EPR spectrum generated by the UV irradiation ofNO-saturated pure acetaldehyde; (b) EPR spectrum recordedwhen light was masked; (c) EPR spectrum of radicals from theUV photolysis of NO-saturated acetaldehyde-d4 when lightwas masked.

remained [Fig. 1(b)]. The former clearly exhibits hyperfinecoupling to one N atom (aN D 0.632 mT) and one H atom(aH D 1.107 mT) with a g-value of 2.0069 and the latter to oneN atom (aN D 0.738 mT) and one H atom (aH D 0.211 mT)

with a g-value of 2.0064. The g-values suggest that the radicalspecies should be assigned to nitroxides, numbered 1 and 2,respectively. The spectroscopic features of 1 are very close tothose of CH3C(O)NO

žCH(OH)CH3.26 The assignment of 1 to

CH3C(O)NOžCH(OH)CH3 is reasonable when the substrate

photoreaction occurs by the reaction channel indicated inEqn (4). The smaller N-HFSC (0.738 mT) and a single H-HFSC of 2 enable us to assign an acetyl group and a groupwith an ˛-H atom as being attached to the nitroxide functionof 2.26 Interestingly, its ˇ-proton HFSC is much smallerthan that of 1. In order to confirm the nitroxide structure,the UV photolysis of NO-saturated acetaldehyde-d4 wasconducted. It gave rise to an EPR spectrum as shown inFig. 1(c). It, termed the nitroxide 3, consists of a 0.738 mT1 : 1 : 1 triplet with a g-value of 2.0064 and a relatively broadlinewidth. The lack of a doublet caused by a single H atomis understandable because the single H contribution of 2 isreplaced by a 6.5-fold smaller deuterium (D) triplet of 3. Asa result, the D atom contributes here an unresolved tripletfine splitting (¾0.032 mT) to each triplet line in Fig. 1(c) only.Does a formyl group bear the nitroxide function of 2? Inour previous work26 we did not observe the resolved H-HFSC of the formyl group, but we found that the EPR lineswere very broad, Hpp D 0.28 mT for HC(O)N�Ož

�C(O)Hand 0.26 mT for HC(O)N�O

ž�C�CH3�2CN, respectively.

When the H atom of the formyl moiety in substrates wasreplaced by D, the corresponding EPR lines became narrower



(Hpp D 0.09 mT). We assigned the nitroxide structureswith a formyl group bearing the nitroxide function and theH-HFSC of formyl group as contributing line broadening.The mechanism of line broadening is unknown. It might beattributed to the media or other reasons. We compared thespectrum [Fig. 1(b)] with that of HC(O)N�NOž

�C�CH3�2CN26

and found that the doublet of the former was completelycovered with one triplet line of the latter. Therefore,the doublet of 2 can be assigned to the formyl proton.This reveals that the photolysis of acetaldehyde occurs asshown in Eqn (3). The BDE of the CH3 —CHO bond is79.1 kcal mol�1.11 Unfortunately, a methyl nitroxide was notobserved.24 Hence the structures of 1,2 and 3 are assumedto be as illustrated. The time behavior of 1,2 and 3 duringspectrum recording showed that 1 was a short-lived radicaldue to its ˇ-proton, whereas 2 and 3 were fairly stable.The stability of 2 and 3 may be attributed to a larger spindelocalization formed by overlap of pz-orbitals across thenitroxide function and two acyl groups.

The UV photolysis of an NO-saturated and acetaldehyde-saturated (40%) aqueous solution gave a 1 : 1 : 1 triplet withvisibly small hyperfine splittings and with a g-value of2.0064 [Fig. 2(a)]. The corresponding radical is numberedas the nitroxide 4. It is well known that in acetaldehyde-saturated (40%) aqueous solution, acetaldehyde is presentin a 4 : 5 mixture of the carbonyl form and the hydrate(K1,2 D 1.246 at 20 °C).27 The hydrate form HC(OH)2CH3

is dominant in this circumstance. Radicals such as methylradical generated from photolysis of the carbonyl form ofacetaldehyde quickly abstract an H atom from HC(OH)2CH3

to yield an ethanediol radical, žC(OH)2CH3. The rateconstant is 1.2 ð 109 dm3 mol�1 s�1.28 On the other hand,in aqueous solution the acetyl radical rapidly hydrates

Figure 2. (a) EPR spectrum of the radical generated from theUV photolysis of NO- and acetaldehyde-saturated (40%)aqueous solution; (b) simulation for the EPR spectrum (a).

to form žC(OH)2CH3, about 2 ð 106 times faster than thehydration of an acetaldehyde molecule.28 These reactionsundoubtedly lead to the presence of ethanediol radicalsin the system. The simulation [Fig. 2(b)] exhibits hyperfinecoupling to one N atom (aN D 1.361 mT) and six equivalentprotons (aH D 0.061 mT), which indicates that 4 possesses sixequivalent H atoms at a position far away from the nitroxidefunction. Hence we could deduce two ethanediols bearingthe nitroxide function of 4. The mechanism for the generationof 4 is suggested in Scheme 1.

Interestingly, the photolysis of an NO-saturated aqueoussolution of acetaldehyde (20%, v/v) provided an EPRspectrum [Fig. 3(a)] of a pure single radical 5. It clearlydisplays hyperfine coupling to one N atom (aN D 1.423 mT)and one H atom (aH D 1.871 mT) with a g-value of 2.0056.These allow us to assign the structure 5. It consists ofethanediol, ethylol and NO. When the solution containedAAPH (0.02 M), the EPR spectrum [Fig. 3(b)] was primarilyrecorded. It consists of a 0.751 mT 1 : 1 : 1 triplet with ag-value of 2.0066. The corresponding radical is numberedas the nitroxide 6. From its smaller N-HFSC (0.751 mT),6 is assigned as indicated. One of its moieties is the 2-amido-2-propyl radical generated from AAPH under UVlight. When light was masked, the strong lines of 6 slowlydecayed and finally disappeared, but other weak EPR linesappearing under UV light became much clearer than before.The lines shown in Fig. 3(c) are a superposition of tworadicals, indicated by open circles and sticks, respectively,with the same g-value of 2.0058. One, indicated by sticks, isassigned to bis(2-amido-2-propyl) nitroxide.24 The other (thenitroxide 7) displays a triplet (1 : 1 : 1) of doublets, indicatedby open circles. It manifests hyperfine coupling to one Natom (aN D 1.476 mT) and one H atom (aH D 1.792 mT).

The HFSCs of 7 are very similar to that of 5, but theN-HFSC of 7 is larger than that of 5. The HFSCs of 5 and 7lead us to assign both nitroxides as possessing a ˇ-H atom.The corresponding moiety of 5 can be easily assigned to theethylol coming from the reaction shown in Eqn (4). The othermoiety of 5 is ethanediol, having no contribution to HFSCs.The corresponding moiety of 7 should be ethylol and another



(7)

(8)

(9)

(10)

(11)

(12)

(13)

(14)

(15)

(16)

Scheme 1

moiety of 7 is assumed to be 2-amido-2-propyl radicalgenerated from AAPH under UV light. One question arises.The formyl radical can be generated from the primary �-bond cleavage [Eqn (3)] of acetaldehyde and is consequentlyhydrated to the methanediol radical in aqueous solution. Ifanother moiety of 5 or 7 were methanediol, it would alsocontribute a single H-HFSC. How do we distinguish ethylolfrom methanediol in 5 or 7?

In order to solve the problem, the UV photolysis of anNO-saturated aqueous solution of formaldehyde (37–40%)containing AAPH (0.02 M) was undertaken. It gave rise toa mixed EPR spectrum (Fig. 4). It is a superposition of tworadicals with the same g-value of 2.0060. The stronger signalis a 1.449 mT 1 : 1 : 1 triplet which is clearly bis(2-amido-2-propyl) nitroxide.24

The weaker one (the nitroxide 8) marked with sticksconsists of one N atom (aN D 1.423 mT) and one H atom

(aH D 0.985 mT). As is well known, formaldehyde producesa formyl radical and a hydrogen radical by photolysis in thegas phase.29 – 32 However, in aqueous solution formaldehydeis present in a 1 : 999 mixture of the carbonyl form and thehydrate, CH2�OH�2, (K ³ 2 ð 103 at 20 °C).33 – 35 H-abstractionleading to the methanediol radical žCH(OH)2 can be realizedby transient radicals such as 2-amido-2-propyl generatedfrom the UV photolysis of AAPH. Hence two moieties of8 are most likely assumed to be the 2-amidio-2-propyl andmethanediol group, respectively. Clearly, the H-HFSC of8 is significantly smaller than that of both 5 and 7. Thisimplies that an ethylol group is attached to the nitroxidefunction of both 5 and 7 rather than a methanediol group.Accordingly, the structures of 5, 6, 7 and 8 are assumedto be as depicted. Mechanisms for their generation areshown in Schemes 1 and 2. No signal was detected withoutAAPH.



Figure 3. (a) EPR spectrum of the radical from the UVphotolysis of NO-saturated aqueous solution of acetaldehyde(20%, v/v); (b) EPR spectrum of the radical from the UVphotolysis of NO-saturated aqueous solution of acetaldehyde(20%, v/v) containing AAPH (0.02 M); (c) EPR spectrumrecorded when light was masked in the case of (b).

Figure 4. EPR spectrum of radicals from the UV photolysis ofan NO-saturated AAPH-containing (0.02 M) aqueous solution offormaldehyde (37%).

(17)

(18)

(19)

Scheme 2

Figure 5. (a) EPR spectrum generated by UV irradiation ofNO-saturated pure propionaldehyde; (b) EPR spectrumrecorded when light was masked; (c) simulation for the EPRspectrum (b).

PropionaldehydeThe UV photolysis of an NO-saturated pure propionalde-hyde primarily produced an EPR spectrum as shown inFig. 5(a). It seems to be a mixture of three nitroxides withdifferent g-values: one is marked with sticks, one with dotsand the third with two open circles. It should be pointedout that the third radical species is very weak and stronglyoverlapped by other radical lines. Only two lines can be dis-tinguished. When light was masked, the first (the nitroxide 9)disappeared, the second (the nitroxide 10) remained and thethird (open circles) became intense and much more distinct[Fig. 5(b)].

Compound 9 manifests hyperfine coupling to one N atom(aN D 0.632 mT) and one H atom (aH D 1.080 mT) with a g-value of 2.0067. These spectroscopic patterns are similar tothat of CH3C(O)NO

žCH(OH)CH3.26 As indicated in Eqn (4),

propionaldehyde can undergo a self-quenching process likeacetaldehyde to give an acyl radical and a ketyl radical underUV light. Both radicals construct 9 with NO. Compound 10is characteristic of a triplet (1 : 1 : 1) of doublets, being closelysimilar to 2, which displays hyperfine coupling to one Natom (aN D 0.738 mT) and one H atom (aH D 0.211 mT)with a g-value of 2.0064. The single H-HFSC comes fromthe formyl moiety generated by the photofragmentationreaction of propionaldehyde, expressed in Eqn (3). The BDEof the CH3CH2 —CHO bond is 82.2 kcal mol�1. Anotherfragment is the ethyl radical. The simulation [Fig. 5(c)] for



the spectrum [Fig. 5(b)] was completed by overlapping tworadicals in concentration proportions of 2 : 1 in the orderof (a) 10 and (b) the third nitroxide, marked with opencircles in Fig. 5(b). The third (the nitroxide 11) manifestsone N atom (aN D 0.738 mT) and two equivalent H atoms(aH D 0.685 mT) with a g-value of 2.0064.

Its smaller N-HFSC allows us to assign it as contain-ing an acyl group bearing the nitroxide function. Whichgroup contributes the two equivalent H-HFSCs, the CH2CH3,CH2CHO or CH2CH2CHO group? The higher BDE of theCH3 —CH2CHO bond (85.6 kcal mol�1)36 makes it difficultfor it to undergo efficient fission. Therefore, the formation ofžCH2CHO may be negligible. Experiments37 have indicatedthat more than 95% H abstraction from aliphatic aldehydesproceeds through abstraction of the aldehydic hydrogen andmethyl substitutions on the vicinal carbon to aldehydic car-bon increase this rate constant slightly. Thus, H abstractionfrom propionaldehyde mainly leads to žC(O)CH2CH3 ratherthan žCH2CH2CHO or CH3

žCHCHO. As shown in Eqn (5),

žC(O)CH2CH3 may undergo decarbonylation to yield anethyl radical,4,5 although it may not occur readily.6 The ther-mal lifetime of žC(O)CH3 at 298 K is reported to be of theorder of several seconds,38 whereas those of žC(O)C2H5 andother carbonyl radicals with four or five carbon atoms areof the order of 10�5 –10�6 s.39 – 42 With the decrease in activa-tion energy needed from 17.1 to 12.1 kcal mol�1 for decar-bonylation from žC(O)CH3, žC(O)C2H5, žC(O)CH�CH3�2 tožC(O)C�CH3�3, studies43 on the kinetics of decarbonyla-tion show that there is a large and noticeable increase indecomposition rate constants from žC(O)CH3 �7.4 s�1� tožC(O)C2H5 �2.5 ð 102 s�1� and other carbonyl radicals withmore than three carbon atoms of the order of 103 –105 s�1.Therefore, we believe that alkyl radicals in this case aremainly from decarbonylation of acyl radicals. As a result,one moiety of 11 is propionyl and the other is ethyl ratherthan CH2CHO or CH2CH2CHO.

The photochemistry of an NO-saturated aqueous solutionof propionaldehyde (20%, v/v) is very similar to that ofacetaldehyde. HFSCs and the g-value of the generatednitroxide 12 are listed in Table 1 and its structure can bedrawn as illustrated. The mechanism for the generation of12 is closely similar to that illustrated in Scheme 1, replacinga methyl group by an ethyl group.

IsobutyraldehydeThe same experiment was performed on pure isobutyralde-hyde. The EPR spectrum (Fig. 6) seems to be a mixture of

Figure 6. EPR spectrum generated by the UV irradiation ofNO-saturated pure isobutyraldehyde; (b) simulation for theEPR spectrum (a).

three signals with the same g-value. One (the nitroxide 13)marked with dots is characteristic of a 1 : 1 : 1 triplet of dou-blets, being closely similar to the nitroxide 2. It consists of oneN atom (aN D 0.759 mT) and one H atom (aH D 0.264 mT)with a g-value of 2.0059.

One moiety of 13 may be the formyl group comingfrom a photofragmentation reaction of isobutyraldehyde[Eqn (3)]. The BDE of the �CH3�2CH—CHO bond is ca82 kcal mol�1. The second radical marked with sticks (termedthe nitroxide 14) clearly manifests hyperfine coupling to oneN atom (aN D 0.751 mT) and one H atom (aH D 0.580 mT)with a g-value of 2.0064. Its smaller N-HFSC allows us toassign it to a nitroxide containing an acyl group bearingthe nitroxide function. The single H-HFSC (0.580 mT) of14 could be interpreted in terms of the methynyl proton ofthe iso-C3H7 group generated mainly from decarbonylationof an isobutyryl radical like a propionyl radical, indicatedin Eqn (5). The third radical marked with open circles(termed the nitroxide 15) manifests a 0.738 mT 1 : 1 : 1triplet. Its smaller N-HFSC and no H-HFSC allow us toassign it to a nitroxide containing two acyl groups bearingthe nitroxide function. The simulation [Fig. 6(b)] for thespectrum [Fig. 6(a)] was completed by overlapping thesethree radicals in concentration proportions of 3.4 : 1 : 3 in theorder (a) 13, (b) 14 and (c) 15. The structures of 13, 14 and 15are assumed to be as depicted.



The UV photolysis of an NO- and isobutaldehyde-saturated aqueous solution gave an EPR spectrum (Fig. 7) ofa pure single radical. It manifests hyperfine coupling to oneN atom (aN D 0.804 mT) and one H atom (aH D 0.290 mT)with a g-value of 2.0064. They are close to those of 13. Thedifferences in HFSCs may be attributed to solvent effects.Unfortunately, a nitroxide containing a hydrated isobutyrylgroup was not observed here. There may be two reasons forthis: (a) the solubility of isobutaldehyde in water is muchlower than those of acetaldehyde and propionaldehyde and(b) the hydration of isobutyryl radical occurs much moreslowly than that of acetyl radical.

IsopentanalThe EPR spectrum [Fig. 8(a)] was obtained from the UVphotolysis of NO-saturated pure isopentanal. It is a mixtureof two nitroxides with different g-values. One is markedwith sticks, whose HFSCs and g-value are the same as thoseof 1. The other, marked with dots (termed the nitroxide16), is closely similar to 2 and characteristic of a triplet ofdoublets: aN D 0.738 mT, aH D 0.184 mT, g D 2.0066. The H-HFSC of 16 corresponds to the formyl proton coming fromthe photofragmentation reaction of isopentanal, expressedin Eqn (3). The BDE of the �CH3�2CHCH2 —CHO bond is83.6 kcal mol�1. The question arises: of why we observedthe nitroxide 1 in this case. As mentioned above, except forthe primary �-bond cleavage, isopentanal also undergoes aNorrish II process to form acetaldehyde [Eqn (6)] under UVlight owing to the lower enthalpy of reaction, fH298° D21.6 kcal mol�1.9 The relative quantum yield of NorrishII process is 0.7 for isopentanal.9 Hence acetaldehyde isproduced. The observation of 1 is reasonable. Another moietyof 16 is assumed to be an isovaleryl group. UV photolysisof NO-saturated isopentanal containing 10% acetaldehydewas conducted [Fig. 8(b)]. The same EPR signal as assignedto the radical 1 was observed. There was an indication ofonly slight changes in signal intensity in the presence ofacetaldehyde. The EPR lines indicated by question marks[Fig. 8(a)] are very weak and strongly overlap other lines.The lack of information makes assignment difficult.

The UV photolysis of an NO- and isopentanal-saturatedaqueous solution gave a 0.830 mT 1 : 1 : 1 triplet EPR spectrumwith a g-value of 2.0059. It might be assigned to the nitroxide

Figure 7. EPR spectrum of the radical generated from the UVphotolysis of NO-saturated and isobutyraldehyde-saturatedaqueous solution.

Figure 8. EPR spectrum generated by the UV irradiation of (a)NO-saturated pure isopentanal and (b) NO-saturatedisopentanal containing 10% acetaldehyde.

17. Its smaller N-HFSC (0.830 mT) implies that an acylgroup is attached to the N atom of 17. As mentioned above,acetaldehyde can be favorably produced by UV photolysisof isopentanal. The acetaldehyde so formed undergoesphotodecomposition to give an acetyl radical. The acetylradical is quickly hydrated to form its hydrate radicalžC(OH)2CH3. It reacts with NO and isovaleryl radical toyield 17.

tert-PentanalThe UV photolysis of NO-saturated pure tert-pentanal gavean EPR spectrum (Fig. 9). It seems to be a mixture of threenitroxides. (a) One marked with dots (termed nitroxide 18)is composed of a triplet (1 : 1 : 1) of doublets with a g-valueof 2.0067, being similar to nitroxide 2. It consists of one Natom (aN D 0.711 mT) and one H atom (aH D 0.132 mT).The single H-HFSC is attributed to the formyl group



Figure 9. EPR spectrum generated by the UV irradiation ofNO-saturated pure tert-pentanal.

coming from photofragmentation reaction of tert-pentanal,expressed in Eqn (3). The BDE of the �CH3�3C—CHO bondis 81.6 kcal mole�1. The other fragment is tert-butyl. (b) Thesecond one marked with open circles is a 0.791 mT 1 : 1 : 1triplet with a g-value of 2.0064. This leads us to assign anacyl group and another group without ˛-H atoms as beingattached to the nitroxide function. They are most likelytert-butanoyl and tert-butyl in this case. The correspondingradical species is numbered as the nitroxide 19. (c) The thirdradical marked with arrows is a 1.555 mT 1 : 1 : 1 tripletsignal with a g-value of 2.0058, which is clearly di-tert-butylnitroxide. Therefore, we inferred the structures of nitroxide18 and 19 as shown.

The UV photolysis of an NO- and tert-pentanal-saturatedaqueous solution did not gave any EPR signals owing tothe low solubility of tert-pentanal in water. It did not givedetectable EPR signals.

CONCLUSION

The UV photochemical behavior of aliphatic aldehydesderived from EPR observations of nitroxides generatedby NO trapping is mainly related to both a bimolecularphotoreduction to form a ketyl and an acyl radical and aunimolecular fission at the C—CHO bond to give a formyland an alkyl radical.

In the present work, ˇ-proton HFSCs estimated forradicals 1, 5, 7, 8, 9, 11 and 12 vary over the range0.685–1.976 mT (Table 1). The relationship between the ˇ-H HFSC and the dihedral angle of the C—H bond for dialkylnitroxides has been established and used approximatelyto predict the conformation.44 Thus, ˇ-H HFSCs observedin nitroxides 5, 7, 8 and 12 may provide approximateinformation about their conformations. Radicals in which anacyl group is directly linked to the nitroxide function, as in 1,9 and 11, exhibited smaller 14N-HFSCs (0.7–0.8 mT) with spindelocalization induced by electron-withdrawing acyl groups.Such stronger spin delocalization may be formed by overlapof p�-orbitals across the nitroxide function and the carbonylgroup. The most stable conformation may be controlled by

the C O bond lying on a horizontal plane related to thep�-orbital. It leads to higher stability of nitroxides containingacyl group, as observed in 2, 3, 6, 10, 13, 15, 16, 17, 18 and 19.

As a result, the lower spin density on nitrogen atom leadsto smaller ˇ-H HFSCs. However, an attempt to rationalizethe conformation of such radicals on the basis of EPR spectrastill requires a large amount of spectroscopic data and moreexperiments and calculations.

AcknowledgementThe authors thank the Natural Science Foundation of China for thefinancial support (grant No. 20072013).

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no spin trapping and epr studies on the photochemistry of aliphatic aldehydes

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