epr study of nitroxides formed from the reaction of nitric oxide with photolyzed amides

MAGNETIC RESONANCE IN CHEMISTRYMagn. Reson. Chem. 2003; 41: 647–659Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/mrc.1229

EPR study of nitroxides formed from the reactionof nitric oxide with photolyzed amides

Fan Wang, Jing Jin and Longmin Wu∗

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

Received 17 March 2003; Revised 12 May 2003; Accepted 13 May 2003

Free radicals generated from UV irradiation of simple aliphatic amides in anaerobic and nitric oxide(NO)-saturated liquid mixtures or solutions gave EPR spectra of nitroxides. The application of isotopiceffects to EPR spectra and the generation of radicals by transient radical attack on substrate moleculesor by photolysing amine or acetoin were used to help identify photochemically produced radicals fromthe amides. The aliphatic amides used were formamide, acetamide and their N-methyl- or deuterium-substituted derivatives. Transient radicals used to attack the amides via hydrogen-atom abstraction weregenerated from the initiator AIBN or AAPH. The observation of various nitroxides indicates the reactivityof NO for trapping acyl, carbamoyl and other carbon-centered radicals. Possibly mechanistic pathwaysdiagnosed with this trap are proposed. Copyright 2003 John Wiley & Sons, Ltd.

KEYWORDS: EPR; nitric oxide; nitroxide; amides; acyl; carbamoyl

INTRODUCTION

It has been well established that nitric oxide (NO) is a highlystable free radical under chemical conditions. NO shows notendency to dimerize or disproportionate. It does not abstracta hydrogen atom nor add itself to an inactivated double bond.However, increasing experimental facts have indicated thatNO can be used as a long-lived paramagnetic free radicalscavenger in certain cases.1 – 5 The range of reactions of NOwith alkyl radicals is complicated. The (RNOR)2 dimer orR2NOR was found to be its major product.6,7 This suggestedthat an oxyaminyl radical or a nitroxide appeared, althoughthe reaction yielding oxyaminyl-type radicals was ruledout in some cases.8 There were also indications of aminylformation.5,9 In previous studies,4,5 we found that NO tendedto couple with carbon (C)-centered and less stereo-hinderedalkyl radicals to give nitroso compounds. The compoundsformed could trap other C-centered radicals, sulfinyl radicalsor thiyl radicals to yield nitroxides (i.e. aminoxyls) oroxyaminyls, but they did not seem to trap alkyloxy or phenylradicals.4,5 The long-lived nitroxides thus formed wereobserved by the EPR spectroscopic technique. Furthermore,we were encouraged to investigate the availability of NOfor trapping other kinds of transient radicals, particularlythe ‘non-�’ aliphatic acyl radicals,9 although Forrester et al.suggested that nitrosoacyls,9a which stemmed from acyloxyamidyl radicals generated by hydrogen-atom abstractionfrom N-acyloxyamides, might be useful traps, especially fornucleophilic radicals.

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.

‘Non-�’ acyl radicals can be generated by different pro-cesses and trapped by nitroso compounds, yielding acylalkylnitroxides.9 – 11 The EPR spectroscopic features of acylalkylnitroxides are characterized by a significantly smaller nitro-gen hyperfine splitting constant (HFSC), generally in therange 0.7–0.8 mT, and by a larger g-value than those of nor-mal dialkyl nitroxides.10 Therefore, they will be importantcriteria determining whether an acyl group bears directlythe nitroxide function or not. Generally, the EPR databank, especially for acylalkyl nitroxides, seems limited. Itis well known that the UV photolysis of aliphatic amides cangenerate aliphatic acyl radicals.12 – 14 Other radicals derivedfrom amides, carbamoyl radicals in particular, could beformed by various methods, such as sonolysis,15 radiolysis,16

radicals or excited triplet organic molecule attack17 andphoto-oxidation,18 etc. The radicals so formed were directlyobserved in the matrix at a low temperature,18,19 by the spintrapping technique9,15,17e or by the flow technique.17a,d

Amides have long been of practical importance and offundamental interest. The amide group is a ubiquitous moi-ety in biologically important macromolecules. Amides canserve as a linkage in proteins and act as a building blockfor many polymers. For instance, the simplest formamideis the smallest model molecule of the peptide prototypeNH—C O linkage. It has been reported that some dipep-tide amides showed selective inhibition effects on nitric oxidesynthases (NOS) and exhibited therapeutic potential in thetreatment of some diseases resulting from NO overproduc-tion, such as septic shock and inflammation.20 In particular,organic radicals derived from DMF played a significantrole in cell killing, with possible implications for cancertreatment.21 This is another reason why we are interested inspin trapping reactions of NO with acyl and other radicalsgenerated or derived from aliphatic amides.

Copyright 2003 John Wiley & Sons, Ltd.

648 F. Wang, J. Jin and L. Wu

Table 1. EPR parameters of nitroxides R1R2N(Ož)

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

1CO

H C HO Benzene 1N: 0.962 2.0062

2CO

D C DO Benzene

Aqueous1N: 0.9831N: 1.028

2.0060

3 (CH3)2N N(CH3)2 Aqueous 1N: 1.6782N: 1.447

2.0053

4CO

HC(CH3)2CN Benzene 1N: 1.080 2.0061

5CO

DC(CH3)2CN Benzene 1N: 1.116 2.0060

6CO

H3C C CH3

O Acetoin 1N: 0.614 2.0064

7COH

H3CH

C CH3

O Acetoin 1N: 0.6141H: 1.064

2.0068

8CO

NH

H

C(CH3)2CN Benzene 1N: 1.0101H: 0.0981H: 0.047

2.0062

9CO

ND

D

C(CH3)2C(ND2� NH D2O 1N: 0.962 2.0060

10CO

NH

H

C(CH3)2C(NH2� NH Aqueous 1N: 0.9701H: 0.105

2.0061

11CO

HNCH3

C(CH3)2C(NH2� NH Aqueous 1N: 0.956 2.0060

12CO

DNCH3

C(CH3)2C(ND2� NH D2O 1N: 1.035 2.0060

13CO

HNCH3

C(CH3)2CN Benzene 1N: 1.001 2.0060

14CH

NH

CH3C

HO C(CH3)2CN Benzene 1N: 1.438

1N: 0.2191H: 1.7131H: unresolved

2.0061

15CH

NH

CH3C

HO C(CH3)2C(NH2� NH Aqueous 1N: 1.476

1N: 0.2181H: 1.8211H: unresolved

2.0059

16CD

ND

CD3C

DO C(CH3)2CN Benzene 1N: 1.550

1N: 0.2501D: 0.132

2.0061

17CD

ND

CD3C

DO C(CH3)2C(NX2� NH

(X D H or D)Aqueous or D2O 1N: 1.479

1N: 0.2201D: 0.1371D: 0.127

2.0060

18CO

HH

H

HCN

C(CH3)2C(NH2� NH Aqueous 1N: 1.4001N: 0.2291H: 1.3241H: unresolved

2.0059

Copyright 2003 John Wiley & Sons, Ltd. Magn. Reson. Chem. 2003; 41: 647–659

EPR study of nitroxides from NO and photolyzed amides 649

Table 1. (Continued)

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

19CO

CH2

CH3

H3C NC(CH3)2CN Benzene 1N: 1.475

1N: 0.2712H: 0.685

2.0059

20CH

HNC

OH3C

H3C

C(CH3)2CN Benzene 1N: 1.5021N: 0.1321H: 2.1961H: unresolved

2.0059

21CO

N CH3CCH3

CH2

C(CH3)2C(NH2� NH Aqueous 1N: 1.4621N: 0.1842H: 1.027

2.0060

22CH

HNC

OH3C

H3C

C(CH3)2C(NH2� NH Aqueous 1N: 1.6511N: 0.1591H: 2.0441H: unresolved

2.0060

23CO

CH2

(X=H or D)

X2NC(CH3)2C(NX2� NH(X D H or D)

Aqueous or D2O 1N: 1.4102H: 0.796

2.0056

24CO

N CH2X

(X=H or D)

H3CC(CH3)2C(NX2� NH(X D H or D)

Aqueous or D2O 1N: 1.5021N: 0.2192H: 0.915

2.0059

25CNH

H

XCH3CO

(X=H or D)

C(CH3)2C(NX2� NH(X D H or D)

Aqueous or D2O 1N: 1.4611N: 0.2291H: 1.4161H: unresolved

2.0059

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

EXPERIMENTAL

N,N-Dimethylformamide (DMF) (Xian Chemicals, AP) wasdried over magnesium sulfate for 24 h and then treatedwith potassium hydroxide to remove water and formicacid. It was distilled and the middle fraction was col-lected for use.22 N,N-Dimethylacetamide (DMA) (TianjinChemicals, AP) was distilled at reduced pressure from bar-ium oxide.22 Formamide (Xian Chemicals, AP) was treatedaccording to the literature.22 Acetamide (Xian Chemicals,AP) was recrystallized from acetone.22 N-Methylformamide(NMF) (J&K Chemicals, 99%), N-methylacetamide (NMA)(J&K Chemicals, 99%), 2,20-azobis(2-methylpropionamidine)dihydrochloride (AAPH) (Aldrich), dimethylamine (Shang-hai Chemicals, 33%) and 3-hydroxy-2-butanone (acetoin)(Aldrich) were used as received. 2,20-Azobisisobutyronitrile(AIBN) (4th Shanghai Chemicals, AP) was recrystallizedfrom ethanol. NO was prepared and purified fully accordingto the procedure described previously.4

Benzene for sample preparation was treated by standardprocedures.22 Water for aqueous samples was distilledand deionized. Deuterium oxide (D2O, 95.5%) (DeuChem,Leipzig, Germany) was used as received.

The sample preparation, UV irradiation performance,EPR measurements and calculations on EPR spectra were

described previously.4,5 All the EPR determinations werecarried out at ambient temperature. EPR parameters such asHFSCs and g-values are given in Table 1.

RESULTS AND DISCUSSION

Nitroxides generated from reaction of NO withphoto-produced acyl radicalsThe EPR spectrum shown in Fig. 1(a) was recorded duringthe UV photolysis of NO-saturated DMF mixed with benzene(DMF : benzene D 4 : 1, v/v). This spectrum consists of a0.962 mT 1 : 1 : 1 triplet with a broad linewidth and a g-valueof 2.0062. It might be assigned to a nitroxide (1). The smallernitrogen HFSC (0.962 mT) implies that one or more electron-withdrawing substituents are attached to the nitrogen atomof the nitroxide 1. An EPR spectrum [Fig. 1(b)] having almostthe same features as the nitroxide 1 was obtained when anNO-saturated DMF-d7 solution mixed with benzene (DMF-d7 : benzene D 4 : 1, v/v) was UV photolyzed. The radicalspecies might also be assigned to a nitroxide (2). The EPRspectrum of the nitroxide 1 differs from that of the nitroxide2 in linewidth, that of the latter being much narrower thanthat of the former.

It is well known that DMF primarily undergoes pho-todissociation after excitation via a ��Ł transition upon UV



(a)

(b)

(c)

1 mT

Figure 1. EPR spectra of radicals from the UV photolysis ofNO-saturated (a) DMF, (b) DMF-d7 mixed with benzene and(c) AIBN-containing (0.03 M) mixture of DMF andbenzene (1 : 1).

(a)

(b)

1 mT

Figure 2. (a) EPR spectrum generated by the UV irradiation ofan NO-saturated aqueous solution of dimethylamine atambient temperature and (b) its simulation.

absorption,12,14 giving a formyl radical and a dimethylaminoradical. In order to confirm whether dimethylamino rad-icals build the nitroxide 1 or not, the UV photolysis ofan NO-saturated aqueous solution of dimethylamine (33%)was conducted. It gave rise to the EPR spectrum shown inFig. 2(a). The EPR lines were not intense, but clearly charac-teristic of a 1.447 mT 1 : 2 : 3 : 2 : 1 quintet superimposed on a1.678 mT 1 : 1 : 1 triplet [Fig. 2(b)] and with a g-value of 2.0053,which clearly manifested a hyperfine coupling to two equiv-alent nitrogen nuclei and to another nitrogen nucleus. Thecorresponding radical is assigned to the nitroxide 3. It hasbeen well established that UV irradiation of dimethylamineyields a dimethylamino radical and a hydrogen atom.23

Therefore, the nitroxide 3 may be formed from the additionof dimethylamino radical to N-nitroso-N,N-dimethylamine,which is formed from the coupling reaction of a photochem-ically produced dimethylamino radical with NO. However,it should be pointed out that N—N ( O) bond forma-tion is energetically less favorable than C—N ( O) bondformation.24 As a result, the formyl radical reacts periodicallywith NO. Therefore, no nitroxide-containing dimethylaminomoiety was observed in Fig. 1. In the experiments involv-ing the amine, the situation is that no formyl radical is in

competition with dimethylamino radical for coupling withNO. The observation of the nitroxide 3 indicates that adialkylamino nitrogen atom, attached directly to the nitrox-ide function, would characteristically contribute to a largernitrogen HFSC of ¾1.4 mT. Otherwise, it has been observedthat the aN value of an acylalkyl nitroxide has a magnitudeof 0.7–0.8 mT.10 Nevertheless, to our best knowledge, the aN

value of a formylalkyl nitroxide is still unknown. Misik andRiesz15b assigned a broad triplet with aN D 0.95 mT to the3tBNB (2,4,6-tri-tert-butylnitrosobenzene) spin adduct of thedimethylcarbamoyl radical when DMF was UV photolyzedin the presence of H2O2 and the spin trap 3tBNB. As describedabove, the linewidths of the nitroxides 1 and 2 are sharplydifferent, so it seems to us that dimethylcarbamoyl doesnot build the nitroxides 1 or 2, because dimethylcarbamoyl-d6 does not influence the triplet linewidth of a nitroxide.Therefore, the greatest possibility in the present case is thattwo formyl radicals react with NO in sequence to producenitroxides 1 and 2. Their structures are assumed to be asillustrated.

C N C X

OO

X

1: X = H

2: X = D

N(H3C)2N N(CH3)2

3

O. O.

The production of the nitroxides 1, 2 and 3 is suggestedin Schemes 1 and 2. Two issues could be considered from theabove observation: (a) formyl directly bearing the nitroxidefunction causes an aN value of ¾1 mT, somewhat largerthan those of other acyl radicals;9b and (b) NO reacts morefavorably with formyl radical than with dimethylaminoradical under the present conditions.

Clearly, unresolved HFSCs of two formyl protonscontribute a broadening linewidth to the nitroxide 1, whereasthe deuteron HFSCs of two formyls in the nitroxide 2, whichis about 1/6.5 times smaller than that of the correspondingprotons in the nitroxide 1, make the EPR lines of 2 reasonablymuch sharper than those of 1.

In order to gain more evidence to identify the structures ofthe nitroxides 1 and 2, the UV photolysis of an NO-saturatedand AIBN-containing (0.03 M) mixture of DMF and benzenewas tested. At a lower volume fraction of DMF in benzene(DMF : benzene D 1 : 1, v/v), a 1.080 mT 1 : 1 : 1 triplet EPRspectrum was generated with a g-value of 2.0061 and witha broad linewidth [Fig. 1(c)]. The corresponding radical isassigned to the nitroxide 4. Its significantly larger aN valueshows that it should be different from the nitroxide 1. Threeprimary radicals should exist in the experiment involvingAIBN: formyl, dimethylamino and 2-cyano-2-propyl. Asdiscussed above, the dimethylamino moiety should beexcluded from the nitroxide 4. Compared with aN values ofthe nitroxides 1 and 2, another substituent is supposed to bearthe nitroxide function. Most likely, it is 2-cyano-2-propyl.Therefore, the nitroxide 4 may be 2-cyano-2-propylformyl



Chν

+ (CX3)2N .

+ N CC

O

X

OO

X

.1 or 2

O

X N(CX3)2 C

O

X .

C

O

X .

(1)

(2)

X = H or D

C N C

O

CN

O

H

.4

CH3

CH3

NN

CC

CH3

CH3

CN

CH3

NC

H3C hνC

CH3

CH3

NC .2 + N2

C

CH3

CH3

NC . + NO C

CH3

CH3

NC NO

C

CH3

CNON+C

O

H .CH3

(3)

(4)

(5)

In the presence of AIBN

+ NOC

O

X . N

O

C X

O

N

O

C X

O

(6)

Scheme 1

.N

3

(CH3)2N + N(CH3)2.

(CH3)2NHhν

(CH3)2N. + H .

NO

(H3C)2N

O

N(CH3)2

(7)

(8)

(9)

(CH3)2N. + NO (CH3)2N NO

Scheme 2

nitroxide. This implies that the photodissociation of DMF is amajor process in the present case, although a hydrogen-atomabstraction from DMF by 2-cyano-2-propyl may occur.17e

C N C CN

O

H

4

CH3

CH3

O.

The UV irradiation of an NO-saturated benzene mixtureof DMF-d7 and AIBN (0.06 M) gave the EPR spectrum shownin Fig. 3(a). It seemed to be a mixture of three signals: twosets of strong triplets and a set of less intense multilines.The latter [sticks in Fig. 3(a)] disappeared when light was

masked [Fig. 3(b)]. The two triplets [open and solid circlesin Fig. 3(b), respectively] with overlapping central lines maybe assigned to two different nitroxides with the same g-value and different aN values. The aN values (0.983 and1.116 mT) enable us to assign them to the nitroxides 2 and 5,respectively.

C N C

O

CN

O

D

.

5

CH3

CH3

Similar experiments involving DMA were carried out.A very complicated and unresolved EPR spectrum with aweak line intensity was obtained during UV irradiation ofNO-saturated DMA mixed with benzene (DMA : benzene D9 : 1, v/v). It seemed to consist of several nitroxides. Theassignment of these radicals is very difficult. When an NO-saturated mixture of DMA and benzene (DMA : benzene D

(a)

(b)

(c)

1 mT

Figure 3. (a) EPR spectrum of radicals from the UV photolysisof an NO-saturated benzene mixture of DMF-d7 and AIBN,(b) the EPR spectrum recorded when light was masked and(c) the simulation for the lines marked with sticks inspectrum (a).



9 : 1, v/v) containing AIBN (0.03 M) was UV photolyzed, acomplicated EPR spectrum was obtained (see below). EPRspectrum assignments indicated a lack of acetyl nitroxides.In order to provide more spectroscopic evidence for acetylnitroxides, a supplementary experiment was performedusing acetoin. The UV photolysis of NO-saturated acetoindisplayed a mixed EPR spectrum (Fig. 4). Lines marked withcircles in Fig. 4 are a 0.614 mT 1 : 1 : 1 triplet with a g-value of2.0064. Lines with sticks in Fig. 4 exhibit a hyperfine couplingto a nitrogen atom (aN D 0.614 mT) and a hydrogen atom(aH D 1.064 mT) with a g-value of 2.0068. These two radicalsare assigned to the nitroxides 6 and 7, respectively. Primaryphotoreaction of acetoin quantitatively undergoes Norrishtype I homolytic ˛-cleavage from its n�* excited triplet state,giving an acetyl radical and an ˛-hydroxyethyl radical.25

The acetyl radical may secondarily decompose into amethyl radical and CO.13 Probably, the radical CH3C(O)C(ž)(OH)CH3 may be secondarily generated by radical attack onacetoin. However, there are a series of experimental facts, (a)the lack of nitrosomethane adducts,4 (b) the larger g-valuesof the radicals 6 and 7 and (c) the existence of a single ˇ-proton in the nitroxide 7, that indicate that the secondaryprocesses may be ignored and that the primary process isthe major pathway for acetoin photochemistry in the presentcase. Consequently, both acetyl and ˛-hydroxyethyl radicalsreact with NO to produce nitroxides. They are easily assignedto the nitroxides 6 and 7, respectively. The reason why NO isable to trap the acetyl radical in this case, but not in the case ofDMA, may most likely be attributed to a higher concentrationof acetyl radicals in the system. The dissociation of acetoinis induced by UV light at 313 nm with a quantum yield of1.0,25b whereas that of the acetamides is achieved by UV lightat 254 nm with a yield of 0.1.12a Otherwise, the quantum fluxof the lamp at 313 nm is at least about five times strongerthan that at 254 nm. This causes the acetyl concentrationfrom acetoin to rise higher than that from the acetamides.

C N C

O

CH3

OO

H3C

.6

C N C

O

CH3

OOH

H3C

.7

H

Nitroxides generated from reaction of NO withphoto-produced carbamoyl radicalsThe UV photolysis of NO-saturated pure formamide or amixture of formamide and benzene (7 : 3) did not give anEPR spectrum. However, in the presence of AIBN (0.04 M),the EPR spectrum shown in Fig. 5(a) was obtained. The g-value is 2.0062. The simulation [Fig. 5(b)] exhibits a hyperfinecoupling to one nitrogen nucleus (aN D 1.010 mT) and twonon-equivalent protons (aH D 0.098 and 0.047 mT). Certainly,this radical species (8) could be assigned to a nitroxide. Theprimary step for free radical production in the photolysisof formamide yields H-atoms and žCONH2 radicals.19

However, it appeared that they did not produce the nitroxide8, because no nitroxide was observed in the absence of AIBN.Therefore, 2-cyano-2-propyl radicals played a key role in the

1 mT

Figure 4. EPR spectrum generated by the UV photolysis ofNO-saturated acetoin.

1 mT

(a)

(b)

(c)

(d)

(e)

Figure 5. (a) EPR spectrum of the radical from the UVphotolysis of NO-saturated formamide mixed with benzene inthe presence of AIBN, (b) its simulation, (c) the EPR spectrumof the radical from the UV photolysis of NO-saturatedAAPH-containing formamide mixed with D2O and (d) mixedwith water and (e) the simulation for spectrum (d).

production of the nitroxide 8. It abstracted a hydrogen-atomfrom H—C(O),17e,19 giving the radical žCONH2 and coupledperiodically with NO to give 2-cyano-2-nitrosopropane.4,5,26



Because of its less steric block, 2-cyano-2-nitrosopropanetrapped favorably carbamoyl radicals, giving the nitroxide 8.This assumption is supported by two facts: (a) there exist twomuch smaller hydrogen HFSCs (aH D 0.098 and 0.047 mT)in the nitroxide 8, which indicates that these two hydrogenatoms should stand in a position far away from the nitroxidefunction and at a similar but distinct position; and (b) thedouble-bond character of the C(O)—N bond in the moietyC(O)NH2 could make two amino hydrogen atoms non-equivalent in the nitroxide 8.17 Therefore, its structure maybe drawn as illustrated.

C N C

O

CN

O

N

.8

CH3

CH3H

H

It is surprising that N-HFSC, which have a magnitude of0.05 mT,9b is not observed. In order to obtain more evidenceto assign the structure, two supplementary experimentswere performed: UV photolysis of NO-saturated formamidemixed with (a) D2O (1 : 1, v/v) and (b) water (1 : 1, v/v)containing AAPH (0.03 M). Experiment (a) gave a 0.962 mT1 : 1 : 1 triplet [Fig. 5(c)], each line of which had no furthersplitting, even at a very low instrumental modulationamplitude, e.g. 0.005 mT. It has been well establishedthat only two amino hydrogen atoms of formamide areeasily exchanged by two deuterium atoms in D2O.17d Asexpected theoretically, the D-HFSC is 6.5-fold smaller thanthe corresponding hydrogen HFSC. This led to an unresolvedfine splitting of each triplet line in Fig. 5(c). This experimentalresult provided the evidence for assigning one group of thenitroxides 8 and 9 to be carbamoyl. Experiment (b) gave theEPR spectrum shown in Fig. 5(d), in which each triplet linewas split in two relatively broad lines spaced by 0.105 mT[Fig. 5(e)]. The corresponding radical species can be referredto as the nitroxide 10 owing to its g-value of 2.0061. Theapparently single H-HFSC may be explained in the following:(a) the double bond character of the C(O)—N bond causesthe two amino hydrogen atoms to be non-equivalent; theH-HFSC arising from one of them is commonly much largerthan that of the other,17b,d,19 and like the nitroxide 8, one aH

is double the other in the pure formamide or in a mixture offormamide and benzene; and (b) probably the unobservedH-HFSC of the other hydrogen is due to a stronger EPRline broadening effect caused by a significant decrease in thespin–spin relaxation time of the nitroxide 10 in the watermedium. The generation of the nitroxides 8, 9, and 10 issuggested in Scheme 3.

C N C

O

C

O

N

.

9: X = D

CH3

CH3

NH

NX2

10: X = H

X

X

.+ +

C

O O

X2N H X2N Hhν

C*

*

(10)

(11)

(12)

RC

O

X2N X2NH*

RH

R = C(CH3)2(CN), or

C(CH3)2(C(NH2)) = NH

C

O

X2N .

C

O

.

+

X = H or D

C

O

X2N N R

O.8, 9 or 10

NO + .R RNO

(13)RNO

Scheme 3

Five experiments were conducted on NMF. (a) In theUV photolysis of NO-saturated NMF mixed with water(1 : 2, v/v) containing AAPH (0.04 M), a 0.956 mT 1 : 1 : 1triplet with unresolved hyperfine splitting and apparentlybroadening linewidth was obtained [Fig. 6(a)]. The g-valuewas measured as 2.0060 (assigned to the nitroxide 11). Aftera few minutes, a 1.462 mT 1 : 1 : 1 triplet appeared [Fig. 6(b)],which was bis(2-amido-2-propyl) nitroxide.4 After 15 min,lines [circles in Fig. 6(c)] of a new radical became moreand more intense (see below). (b) A similar experiment to(a) was conducted but in D2O. A 1.035 mT 1 : 1 : 1 tripletwith unresolved fine structures was obtained. The lineseparation was estimated to be ¾0.005 mT. (c) The UVphotolysis of an NO-saturated benzene mixture of NMF(1 : 1, v/v) containing AIBN (0.04 M) was carried out, givingalso a 1 : 1 : 1 triplet, but with a little larger aN value of1.001 mT and unresolved fine structures (assigned to thenitroxide 13). (d) In the UV photolysis of NO-saturated pureNMF containing AIBN (0.07 M), a 1.001 mT 1 : 1 : 1 triplet wasobtained. Very weak lines belonging to the other radical wereseen. (e) In the UV photolysis of NO-saturated pure NMF, noradical was observed. The g-values of these radicals enableus to assign all of them to nitroxides. The photochemicalreactions of NMF have been considered to be similar to thoseof formamide.19 Based on the above experimental facts, itis most likely that the radical žC(O)NH(CH3� generatedby hydrogen-atom abstraction from NMF constructs thesenitroxides. They are assigned to the nitroxides 11,12 and13,respectively.

C N C

O

C

O

XN

.

11: x = H

CH3

CH3

NH

NX2CH3

C N C

O

CN

O

HN

.

13

CH3

CH3

CH3

12: x = D



1 mT

(a)

(b)

(c)

(d)

Figure 6. EPR spectra recorded (a) during the UV photolysisof NO-saturated NMF mixed with water containing AAPH, (b) afew minutes after irradiation and (c) ca 15 min after irradiationand (d) the simulation for spectrum (c).

Nitroxides generated from reaction of NO withC-centered amide radicalsAt a higher volume fraction of DMF in benzene (DMF : ben-zene D 9 : 1, v/v) and a higher AIBN content (0.07 M), theexperiment gave a mixed EPR spectrum [Fig. 7(a)]. It seemsto be a superposition of four radicals with the same g-valueof 2.0061. The main radical species is marked with a stick,the EPR linewidth of which is relatively sharp. Its simulation[Fig. 7(b)] was completed by overlapping of three radicalspecies in concentration proportions of 35 : 4 : 1 in the orderof (a) the nitroxide 14 [Fig. 7c] which consists of two non-equivalent nitrogen atoms (aN D 1.438 and 0.219 mT) andone hydrogen atom (aH D 1.713 mT), (b) the nitroxide 4[arrows in Fig. 7(a)] and (c) bis(2-cyano-2-propyl) nitroxide[circles in Fig. 7(a)].4 The EPR spectroscopic features ofthe nitroxide 14 are similar to those of an isomer of thenitroxide ButN(Ož) CH2N(CH3�C(O)H.17e Accordingly, theresolved large proton HFSC of 1.713 mT in the nitroxide 14should arise from one of the two ˇ-protons of the moietyCH2N(CH3�C(O)H and the unresolved HFSC arise from theother ˇ-proton with a dihedral angle (� ³ 90°) of the C—Hbond with respect to the p�-orbital of the unpaired electron onthe nitroxide nitrogen.17e,27 Both formyl and dimethylaminomoieties could be excluded from the nitroxide 14 becausethey would contribute a ca 1 mT N-HFSC and a ca 1.4 mT

N-HFSC to the nitroxide EPR spectrum, respectively. Thus,the nitroxide 14 is assumed most likely to be as illustrated.

C N C

O

CN

H

N

.

14

CH3

CH3

H

C

H3C

H

O

The representation of —C(H)(H)— in the structuraldrawing of the nitroxide 14 indicates that the two ˇ-protonsare non-equivalent. A possible mechanism is suggestedin Scheme 4. The lines indicated by question marks inFig. 7(a) are unresolved because of the smaller amount ofspectroscopic information. They may belong to a nitroxide-containing a moiety formed by hydrogen-atom abstractionfrom the N-methyl of DMF.

The photolysis of an NO saturated aqueous solutionof DMF (15%, v/v) containing AAPH (0.025 M) gave theEPR spectrum shown in Fig. 8(a). It clearly consists of tworadical species: (a) the more intense 1 : 1 : 1 triplet (arrows),which is assigned to bis(2-amido-2-propyl) nitroxide;4 and(b) the nitroxide 15. The calculation [Fig. 8(b)] for the whole

(a)

1 mT

(b)

(c)

Figure 7. (a) EPR spectrum generated by the UV irradiation ofNO-saturated DMF in benzene (9 : 1) containing AIBN (0.07 M),(b) its simulation and (c) the simulation for the EPR spectrumassigned to the nitroxide 14.



C

CH3

CH3

NC . + CHCH3

CH3

NC +C H

O

(H3C)2N (14)

(15)+ NO + C

CH3

CH3

CN.

C H

O

N

CH3

H2C.

C

O

NH

CH3

CH2

.

C N C

O

CN

H

N

. CH3

CH3

H

C

H3C

H

O

14

Scheme 4

EPR spectrum was carried out by superimposing a 1.475 mT1 : 1 : 1 triplet on a set of lines with a concentration proportionof 2 : 1. The latter, i.e. the nitroxide 15, exhibits hyperfinecoupling to two non-equivalent nitrogen nuclei (aN D 1.476and 0.218 mT) and one proton (aH D 1.821 mT). These valuesare very close to those of the nitroxide 14. Its structure couldbe similarly assumed to be as illustrated. The mechanism forits generation is closely similar to that illustrated in Scheme 4,replacing 2-cyano-2-propyl by 2-amido-2-propyl.

N

O.CC

CH3

H

H

15

CH3

C NH

NH2

NC

H3C

H

O

The lines marked with sticks in Fig. 3(a) overlap othertwo triplets. Although some details are not clear enoughowing to superposition, a principal triplet feature of thenitroxide 16 is clear. Compared with experiments concerningDMF described above, the nitroxide 16 should be logicallysimilar to the nitroxide 14, except that the deuteron HFSCin the former replaces the proton HFSC in the latter.

(a)

(b)

1 mT

Figure 8. (a) EPR spectrum of radicals from the UV photolysisof an NO-saturated aqueous solution of DMF containing AAPHand (b) its simulation.

Theoretically, aD is about 1/6.5 times smaller than thecorresponding aH assuming that no other changes occur.Its simulation [Fig. 3(c)] exhibits a hyperfine coupling tothree non-equivalent nuclei, all with I D 1 (aID1 D 1.550,0.250 and 0.132 mT). Its EPR spectroscopic features shouldbe very similar to those of the nitroxide 14. Thus, two valuesaID1 D 1.550 and 0.250 mT are assigned to two non-equivalentnitrogen atoms, whereas the other aID1 D 0.132 mT has to beassigned to the ˇ-D. Accordingly, the ˇ-D HFSC in thenitroxide 16 should be expected to be ca. 0.26 mT. In fact,aD D 0.132 mT. It is half the predicted value of aD. Obviously,the deuteron exhibits a distinct and additional effect onHFSC. The much smaller aD value in the nitroxide 16 may bereferred to as a different equilibrium conformation from thenitroxide 14. The aD assignment will be supported below.

C N C

O

CN

D

N

.

16

CH3

CH3

D

C

D3C

D

O

The photolysis of an NO-saturated aqueous solution ofDMF-d7 (25%, v/v) gave a 1.028 mT 1 : 1 : 1 triplet, whichis the nitroxide 2. Furthermore, the photolysis of an NO-saturated aqueous or D2O solution of DMF-d7 (25%, v/v)containing AAPH (0.015 M) gave an EPR spectrum [Fig. 9(a)]of a pure single radical (17). Its perfect simulation [Fig. 9(b)]manifests a hyperfine coupling to four nuclei with I D 1.They are assigned to two non-equivalent nitrogen atoms(aN D 1.479 and 0.220 mT) and two non-equivalent deuterons(aD D 0.127 and 0.137 mT). The two very similar deuteriumatoms suggest that one moiety of the nitroxide 17 should beCD2N(CD3�C(O)D generated by deuterium-atom abstractionfrom the N-methyl of DMF-d7.15 Hence the radical 17 is easilyassigned to the nitroxide 17, as depicted. It should be notedthat two non-equivalent deuterium atoms of the methylenegroup are resolved, in contrast to the case of the nitroxides14, 15 and 16.

C

O

D N C

D3C .

17

D

D

X = H or D

N C

O

CH3

CH3

C NH

NX2



(a)

(b)

1 mT

Figure 9. (a) EPR spectrum of the radical from the UVphotolysis of an NO-saturated aqueous or D2O solution ofDMF-d7 containing AAPH and (b) its simulation.

The simulation for the lines marked with circles inFig. 6(c) manifests a hyperfine coupling to two non-equivalent nitrogen atoms (aN D 1.400 and 0.229 mT) anda proton (aH D 1.324 mT). Its g-value of 2.0059 indicatesthat it is a nitroxide (18). Its EPR spectroscopic patterns arevery similar to those of the nitroxide 15. Reactions involvingthe formation of the nitroxide 18 are matchable to the reac-tion mechanism in Scheme 4, replacing DMF by NMF and2-cyano-2-propyl by 2-amido-2-propyl.

C

O

H N C

H .

H

H

N C

O

CH3

CH3

C NH

NH2

18

A complicated EPR spectrum [Fig. 10(a)] was obtainedwhen an NO-saturated mixture of DMA and benzene(DMA : benzene D 9 : 1, v/v) containing AIBN (0.03 M) wasUV photolyzed. When light was masked, only a very weak1.496 mT 1 : 1 : 1 triplet with a g-value of 2.0058 existed,which was bis(2-cyano-2-propyl) nitroxide.4 It seemedthat the spectrum shown in Fig. 10(a) consisted of twonitroxides with the same g-value (2.0059) but differentlinewidths. Its simulation [Fig. 10(b)] was completed byan overlapping of two nitroxides [Fig. 10(c) and (d), thenitroxides 19 and 20, respectively] with a concentrationproportion of 1 : 2.5. The calculation [Fig. 10(c)] for thenitroxide 19 [broader lines in Fig. 10(a), i.e. all lines exceptthose marked by sticks] reveals that it is composed of

(a)

(b)

(c)

(d)

1 mT

Figure 10. (a) EPR spectrum of radicals from the UV photolysisof an NO-saturated mixture of DMA and benzene containingAIBN, (b) a spectrum generated by an overlapping of spectra(c) and (d), (c) the simulation for one component of spectrum (a)and (d) the simulation for the other component of spectrum (a).

two non-equivalent nitrogen atoms and two equivalenthydrogen atoms (aN D 1.475 and 0.271; aH D 0.685 mT).This suggests that one moiety of the nitroxide 19 should beCH2N(CH3�C(O)CH3.15b,16a,17d,e The calculation [Fig. 10(d)]for the nitroxide 20 [sticks in Fig. 10(a)] shows a hyperfinecoupling to two non-equivalent nitrogen atoms and onehydrogen atom (aN D 1.502 and 0.132; aH D 2.196 mT).Like the nitroxide 14, the large H-HFSC (2.196 mT) of thenitroxide 20 arises from one of the two ˇ-protons. Thenitroxides 19 and 20 are conformational isomers. It is wellknown that DMA undergoes a primary photodissociation toyield alternatively (a) an acetyl radical and a dimethylaminoradical or (b) a methyl radical and a dimethylcarbamoylradical, following excitation via a ��Ł transition upon UVabsorption.12,13 Sequentially, the acetyl radical decomposesinto a methyl radical and carbon monoxide (CO) and thedimethylcarbamoyl radical into a dimethylamino radical andCO.13 As the N-methyl hydrogen is more easily abstracted,17d

the radical žCH2N(CH3�C(O)CH315b,16a,17d,e can be generated

by hydrogen-atom abstraction from an N-methyl of DMA.It is well established that NO reacts favorably with the



methyl radical to produce nitrosomethane, which provesto be a reactive spin trap in solution.4 The EPR spectrumof a methyl nitroxide formed from nitrosomethane bytrapping the other radical is characteristic of a 1 : 3 : 3 : 1quartet separated by ca 1.1 mT.1 The lack of a methylnitroxide implies that methyl radicals are not likely to beproduced in this photolysis. Therefore, we may concludethat the significantly major step for free-radical productionin the photolysis of DMA is the scission of the C—N bondunder the present conditions, giving an acetyl radical andan amino radical. Thus, except for the photochemicallyformed acetyl radical and dimethylamino radical, otherradicals possibly present in the system are 2-cyano-2-propyland žCH2N(CH3�C(O)CH3. Owing to the characteristic EPRspectroscopic features of the nitroxide 3 and acyl nitroxides,10

both acetyl and dimethylamino moieties are excludedfrom the nitroxides 19 and 20. In the structural drawingillustrated, —CH2 — denotes two equivalent ˇ-protons and—C(H)(H)— indicates two non-equivalent ˇ-protons.

N

O.CC

H

H

N

20

N

O

C

O

H3C N CH

CH3

C

CH3

CN

CH3

.19

CN

CH3

CH3

H3C

CH3C

O

R

H1

H2O

R'

R = C(CH3)2CNR' = N(CH3)C(O)CH3

RO

R'

H H

2

The photolysis of an NO-saturated aqueous solution ofDMA (15%, v/v) containing AAPH (0.03 M) gave the anEPR spectrum shown in Fig. 11(a). Its simulation [Fig. 11(b)]indicates that the whole spectrum consists of two radicalcomponents with a concentration proportion of 3.5 : 1; (1) one[Fig. 11(c), the nitroxide 21] manifests a hyperfine couplingto two non-equivalent nitrogen atoms and two equivalenthydrogen atoms (aN D 1.462 and 0.184; aH D 1.027 mT);and (2) the other one [Fig. 11(d), the nitroxide 22] exhibitsa hyperfine coupling to two non-equivalent nitrogen atomsand a hydrogen atom (aN D 1.651 and 0.159; aH D 2.044 mT).These EPR parameters are very similar to those of thenitroxides 19 and 20, respectively. The unique differencebetween the above systems lies in the initial radical: 2-cyano-2-propyl radical in the former and 2-amido-2-propyl radicalin the latter. Accordingly, the structures of the nitroxides 21and 22 are assumed to be as depicted.

Mechanisms for the generation of the nitroxides 21 and22 are similar to that shown in Scheme 4, replacing DMF byDMA and 2-cyano-2-propyl by 2-amido-2-propyl. The aboveobservation implies that NO does not react favorably withboth acetyl and dimethylamino radicals in this case.

N

O.CC

CH3

H

H

21

N

O

C

O

H3C N CH

CH3

C

CH3

C

CH3

.

22

CH3NH2

C NH

NH2

NH

NC

H3C

H3C

O

2

The UV photolysis of an NO- and acetamide-saturatedbenzene solution did not give EPR signals, whereas in thepresence of AIBN (0.04 M), the system gave an EPR spectrum.It consists of two sets of triplets with the same g-value of2.0060: one is a stronger 1.483 mT 1 : 1 : 1 triplet and theother is a weaker 1.01 mT 1 : 1 : 1 triplet. The former isbis(2-cyano-2-propyl) nitroxide.4 The latter is likely to bethe nitroxide 8, although the splitting arising from twoprotons of H2N—C(O) is not observed because of themuch lower intensity. Various primary free-radical processesinvolving acetamide have been reported: (a) acetamide wasphotolyzed to produce methyl and carbarmonyl radicals inrigid matrices;19 (b) the photolysis of an aqueous acetamidesolution, however, yielded acetyl and žNH2 radicals;28 and(c) the radical žCH2CONH2 could be formed by radical attackon acetamide.17a,d,e However, both acetyl and CH2CONH2

could be ruled out as moieties in the above nitroxide with thespectroscopic pattern of a 1.01 mT 1 : 1 : 1 triplet, because ofthe lack of an aN value of about 0.7 mT and an HFSC arisingfrom two ˇ-protons.

The UV photolysis of an NO-saturated aqueous or D2Osolution of acetamide (0.3 M) containing AAPH (0.05 M) gavean EPR spectrum which exhibited hyperfine coupling to twoequivalent protons (aH D 0.796 mT) and one nitrogen nucleus(aN D 1.410 mT). The corresponding radical is assigned tothe nitroxide 23. Its EPR behaviors are similar to those ofthe nitroxide Bu

tN(Ož)CH2C(O)NH2.17e Therefore, it is mostlikely that the nitroxide 23 may be assumed to have thestructure shown.

23

N

O

C

O

X2N CH

.C C

CH3

CH3

NH

NX2

X = H or D

2

Figure 12(a) shows the EPR spectrum obtained duringthe photolysis of an NO-saturated aqueous solution of NMA(0.5 M) containing AAPH (0.025 M). Its perfect simulation[Fig. 12(b)] fits the experiment very well. It is composedof two radical species with a concentration proportion of2 : 1; (a) a 1.465 mT 1 : 1 : 1 triplet, which is bis(2-amido-2-propyl) nitroxide;4 and (b) the nitroxide 24, which exhibitsa hyperfine coupling to two equivalent hydrogen atoms



(a)

(b)

(c)

(d)

1 mT

Figure 11. (a) EPR spectrum of radicals from the UVphotolysis of an NO-saturated aqueous solution of DMAcontaining AAPH, (b) the calculated spectrum generated byoverlapping of spectra (c) and (d), (c) the simulation for onecomponent of spectrum (a) and (d) the simulation for the othercomponent of spectrum (a).

(aH D 0.915 mT) and two non-equivalent nitrogen atoms(aN D 1.502 and 0.219 mT). When the above solutioncontained a smaller amount of AAPH (0.01 M), the EPRspectrum shown in Fig. 13(a) was first recorded. Thesimulation [Fig. 13(b)] for lines in Fig. 13(a) except thosemarked with a stick indicates that the radical species is thenitroxide 24. The EPR spectrum shown in Fig. 13(c) wasobtained in ca 10 min after UV irradiation. It implies theappearance of a new radical. Its simulation [Fig. 13(d)] was

(a)

(b)

1 mT

Figure 12. (a) EPR spectrum of radicals from the UVphotolysis of an NO-saturated aqueous solution of NMAcontaining AAPH and (b) its simulation.

completed by an overlapping of two radical componentswith a concentration proportion of 1 : 1.4; one is the nitroxide24 and the other is the nitroxide 25 [Fig. 13(e)], which exhibitsa hyperfine coupling to one hydrogen atom (aH D 1.416 mT)and two non-equivalent nitrogen atoms (aN D 1.461 and0.229 mT). When the above experiment was carried out inD2O instead of in water, the same EPR spectra as shownin Fig. 13 were obtained although the N-hydrogen atomof NMA was replaced by the deuterium atom in D2O.This means that the deuteron does not contribute HFSC toEPR spectra. This phenomenon indicates that the structuralassignment of the nitroxides 24 and 25 is reasonable, wherethe N-X (X D H or D) is far away from the nitroxide function.Reactions that occur here couple with that in Scheme 4,replacing NMF by NMA and 2-cyano-2-propyl by 2-amido-2-propyl only.

CH3C

O

N CH N

O.C C

CH3

CH3

NH

NX2X

X = H or D

24

N

O.C C

CH3

CH3

NH

NX2

CN

H

H

25

X

X = H or D

CH3C

O

2

AcknowledgementsThe authors to express their gratitude to the Natural ScienceFoundation of China for its financial support (grant No. 20072013).



(a)

(b)

(c)

(d)

(e)

1 mT

Figure 13. (a) EPR spectrum of radicals from the UVphotolysis of an NO-saturated aqueous solution of NMAcontaining a small amount of AAPH, (b) its simulation, (c) theEPR spectrum recorded ca 10 min after UV irradiation, (d) theEPR spectrum generated by overlapping of spectra (b) and (e)and (e) the simulation for the other component of spectrum (c).

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epr study of nitroxides formed from the reaction of nitric oxide with photolyzed amides

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