\~olurnc 96. number 1 CHEMICAL PHYSICS LETTERS 25 March 1983

RADIOFREQUENCY LABELLING OF MOLECULES IN CHEMICAL REACTIONS

R.Z. SAGDEEV. T.V. LESHINA, N.E. POLYAKOV, V.I. MARYASOVA, A V. YURKOVSKAYA and A.A. OBYNOCHNY Immure of CI~emicaI Kinerics and Combustion. Siberim Branclr. USSR Academy of Sciences. SOI osibirsk 630090. USSR

Rrcrwed 13 October 1981: in final form 22 December 1982

.A method of radlofrequency labellin_r of molecules in chemical reactions has been proposed. It is based on selective RF pumpmg of molecules and on tracing rhe transformation of rhis label in the reaction products by the NYR spectra. Appli- Z.IIIOIIS of this merhod in srudying one-\rdy reaction mechanisms are illustrated by some experimental examples.

I_ Introduction

V,~rious methods of selective labelling of molecules are widely used at present in studying chemical reac- 11011 kinetics and mechanisms. The methods are based on characierrstic probes (spin labels. radioactive labels. stable isotopes) that are introduced into either defmile molecules or particular positions in molecules IIII~ cm be traced during chemical transformarions.

Unfortun&ely. chemtcal ways of introducing such probes are rather ldbour-consuming and not always reahr.#bk. Forsl;n and Hoffman [l] proposed an ele- gmr merhod IO inrroduce 13hels not by chemical IIISJIIS but by ~pplymg a rAdiofrequency field to par- ricu1.u groups in moleculss (the so-called saturation- tr.msfer techmque). This method finds extensive appli- CJIIWIS 111 studymg euhmge process kinetics_ The dim 0l‘Ihe present paper is 10 show that the applica- tion oft selectee rJdiofrcquency field can be em- ployed 111 one-way chenwxl reactions (in non-sready- WIT suntluions) as ,I tine radiofrequency probe yield- III!: II~~XJI~III~ mformation on the reaction mech3nisni.

2. Method

Let ub cons~dcr the reJcrion scheme in the general form A-B-C. (1)

where A is a reagent, B intermediates (molecules, rad- ical ions, free radicals, u complexes), C a reaction product_ The signals from A and C are, as a rule, readi- ly distinguished in highly resolved NMR spectra. This fact allows one to apply selectively a radiofrequency field to the initial compound signals. As a result, the signal intensity can either diminish (signal saturation in the case of homonuclear decoupling) or increase due fo the nuclear Overhauser effect (in the case of heteronuclear decoupling, e.g. { 1H}-13C). Hence, a selective effect of a radiofrequency field can label (e.g. as a complete saturation of one of the signals) either a nucleus or a group of nuclei in a certain posirion in the compound A.

To ensure the possibility of tracing the transforma- tion of such a radiofrequency label (RF label) to the product C by the NMR spectra, it is necessary to meet some requirements:

(1) First of all. the lifetime of B (T& must obey the condition

where Tp is the nuclear spin-lattice relaxation time. This condition permits the RF-label transfer to the product C.

(2) In the course of reaction the products are ac- cumulated, with the RF label in a certain position. On the other hand, this label also vanishes with a charac- teristic time equal to the C relaxation time TF.

108 0 00%2614/83/0000-0000/s 03.00 0 1983 North-Holland

Volume 96, number 1 CHEMICAL PHYSICS LETTERS 25 hiarch 1983

At long times t 2 Tf, the C signal intensity is mainly contributed to by the accumulated equilibrium product, the fraction of RF-labelled molecules being small.

Hence, the selective labelling of A molecules by a radiofrequency field is manifested in the product C NMR spectra most pronouncedly at short times (t d T:).

(3) The last requirement concerns the reaction rate and means that the technique in question may be ap- plied to relatively fast reactions yielding, at t = TF = 10 s, a product concentration that suffices to be detected by NMR. This condition follows directly from the previous one. Note that similar requirements to intermediate’s lifetimes, relaxation times, and reac- tion rates are imposed upon CIDNP experiments [2]_ The only difference is that in the latter case the non- equilibrium nuclear polarization transferred to the reaction product is induced by S-T conversion in the intermediate radical pairs rather than by radiofrequen- cy field effects in the initial compound_

Thus. under the above conditions the radiofrequen-

Fig. 1. Scheme of the apparatus for UV irradiation of samples inside the probe of an XL-200 NMR spectrometer_ (1) High- pressure mercury lamp DRSh-1000. (2) quartz lenses, (3) mir- ror, (4) quartz lightguide. (5) quartz &htguide of the sample tube in the spinner turbine, (6) sample (0.3 ml). (7) receiver and decoupler coils, (8) superconducting solenoid, (9) mov-

cy labelling allows one to use NMR spectra for tracing the fate of a definite atom or of a group of atoms in reactions (from reagents to products) and hence for determining the general scheme of the reaction under study. Note that selective effects upon various mole- cules in a solution gives information on such subtle details of the reaction mechanism as discrimination between in-cage disproportionation and reactions with solvents. As applied to H-atom abstraction processes. the above method may prove to be extremely useful for determining relative rate constants of H abstraction from various donors.

In conclusion, it should be noted that the radiofre- quency labelling (unlike spin labelling and even iso- topic substitution) induces no change in the reaction mechanism under study.

3. Experimental

The proton magnetic resonance spectra were de- tected by a NMR “XL-200” spectrometer (Varian) with a superconducting solenoid (magnetic field 47 kG)_ The samples were irradiated by the full light of a DRSh-1000 mercury lamp mounted at the top of a mobile support (see fig. 1). The light from lamp 1 was

able stand.

focused by two quartz lenses fixed on the support and by fitting mirror 3 to the end of lightguide 4 (I = 60 cm) placed into the warm opening of the solenoid.

Through lightguide 4 the light went to a special spinning sample tube consisting of quartz lightguide 5 and a hollow quartz tube (Z = 3 mm) soldered to the bottom of lightguide 5. When loaded with a solution to be studied, the tube wzss locked with a teflon plug in the lower end_ The air gap between lightguides 4 and 5 was 0.5 mm. The proton signals were saturated by a radiofrequency Geld under homonuclear decou- pling_ The H, amplitude and the damping band were chosen experimentally to saturate completely the sig- nal required. leaving the rest of the spectrum un- changed_ To prevent noise from the radiofrequency field in the spectrum, the homonuclear decoupling was used only during the photolysis time (1 O-20 s) and switched off 0.01-0.03 s before the radiofrequen- cy pulse (observing pulse)_

4. Results and diicussion

We now present some applications of RF labelling

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Volume 96. number 1 CHEMICAL PHYSICS LETTERS 25 March 1983

to studies of organic reaction mechanisms. We have chosen photoinduced reactions for reasons of conve- nience: they can be instantaneously stopped by switching off the light.

Generally speaking. the proposed RF-Iabelling method is applicable to any fast reaction - either thermally initiated or running under mixing of the

redgents. The RF-label transfer from reagent molecules to

reactton products was detected in fwo ways. Firstly,

the stgnal intensity of the products (generated under

trradiation with 3 simultaneous saturation of one of

the lines in the NMR spectrum for the reagents) im-

rnedtately after photolysis was compared with that

detected at a time f * 10 T, _ The criterion of the RF- label transfer to the product was an increase in the

signal intensity at t > T1. Secondly, the NMR signal

of the product observed under continuous saturation of a certain line of the reagents was compared with that detected without decoupling.

The latter method 1s more universal and can be apphed to investigations of thermally induced and mixing reactions.

To rllustrate the general Idea of the method, let us consider the migration of a RF label from azo-bis-

isobutyrotutril (AIBN) methyl protons to the products

of photolysis in CD3CN in the presence of diphenyl- methane (DPhl). The well-studied [3.4] AIBN photo- 11 sis mechanism is shown in scheme I_

Frg. 2 depicts the PhlR spectra of products (I) and (II) detected by using AIBN methyl proton saturation

under hght (a). and also the dark spectrum (after photoly SE) (b).

The reduction of the signal intensity for methyl

IbJ

20 15 pm_ ‘O H 0

ppm

Fig. 2. PhlR spectra of AIBN (0.05 hl) photolysis products in CDaCN in the presence of diphenyhnethane (0.01 M) under conditions of AIBN methyl proton saturation (arrows indicate the position of decoupling) (a) under Ii&t. (b) after photolysis.

protons of (I) and (II) under light compared to the dark spectrum is indicative of the RF-label transfer to these molecules and proves reaction scheme 1.

The next example (fig. 3) concerns the application of RF labelling to the novel [5] photochemical reac- tion of triethylallyltin (III) with trichlorobromoethane (IV). It is assumed in ref. [6] that the initial stage of

(CI~j)2C-N=N-;-(CHj)2 4 (CH,),-C- ‘C-(CH,), + N, I I I CN CN

/

CN CN 1

(CD3)2-7 - y--(CD,)2

CN CN

(I1

(CH3)2-C’ + (C&,)2CH, + (CH,)2-y I

CN CN WI

Scheme I_

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Volume 96, number 1 CHEMICAL PHYSICS LEITERS 25 hbrch 1983

Fig. 3. PMR spectra for the reaction of (C2Hs)$nCH2CH=CH2 (0.05 hi) with C&Br (0.10 M) in deuterocyclohesane. The ethyl part is omitted. Below: the reaction mkture before irradiation; “light” and “dark” PhlR spectra taken under saturation of protons 3 of triethylallylstannane (an arrow shows the position of decoupling). Above: spectrum of 1.1.1~trichlorobutene (V) reaction product shown for comparison.

this reaction is the homolytic decomposition of (IV) and the following addition of Ccl, radicals to (III):

CCl,Br 2 Ccl, + I& ,

(C,H5),SnC;r,-C&=C&, + C’cl,

(1111

+ (C,H,),S~CH,-~H-CH,CC~~ + CCl,Br

. . + (C2H5)3SnBr + CC13-C~2-C&=C62 -t dC13.

W) 00

The alternative mechanism might be that including simultaneously the “Sn-C” and “C-B? bond break- ing and (IV) and (V) formation (concerted mecha- nism).

We made an attempt to choose between these mechanisms using RF labelling. The PMR spectra for the mixture of CC!i,Br with triethylallyltin (III) are

in fig. 3. The RF pumping of the protons 3 of (III) reduces by 30-4096 the peak intensity of the protons 1’ of(v), see fig. 3. Some distortion of the signal shape for protons 2 of (III), 3’ and 3’ of(V) in the PMR spectra taken under continuous saturation of protons 3 of (111) is induced by an additional RF field. The RF pumping of protons 1 of (III) leads, in its turn, to a decrease of the signal intensity corre- sponding to position 3’ of(V)_ The chemical shifts of 2 and 2’ in (III) and (V) nearly coincide. which makes it impossible to use RF labelling.

The transformation of groups 1 and 3 of (111) to positions 3’ and 1’ of (V) (transitions 1.3 --, 3’. 1’) completely confirms the above reaction scheme and excludes the alternative mechanism. Indeed, in the latter case the transitions 1,3 -+ 1’,3’ would occur_

The application of RF labelling to studies of the in-cage disproportionation (intramolecular abstraction) proves to be demonstrative in the case of cyclohepta- none (VII) photolysis.

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Volume 96, number 1 CHEMICAL PHYSICS LETTERS 25 March 1983

0 CH2= 0 CHzS

! dH2 c* : .I

\ / (CH& 1 \,H2r

i I (VIIa)

0 1 II

HC-(CH2)4-CH=CH,

(VIII)

Scheme 2. I

Cyclic ketone photolysis has been fairly well stud- ied [-iI, in particular. by the CIDNP technique 181.

The reaction runs according to scheme 2 [7.8] _ Aldeliyde (VIII) is believed to result from H abstrac- uon frown the p position of the intermediate biradical (VIIa). Under light. a solution of (VII) in CDCI, gives polarized lines (a, 13. and -) protons of (VII)), as well ds (Vlll) (aldehyde and vinyl protons). The RF label- ling can easily confirm reaction scheme 2 (fig. 4) Indeed. saturation of polarized 13 protons of cyclohep- tanonc (VII) reduces the intensity of the (VIII) alde- hydr proton. At Ibe sanx time. the protonsaturation in the Q position of (VII) does not appreciably affect the .rldehyde proton signal.

The easmple discussed needs a special comment. As a rule, RF labelling is inapplicable to reactions characterized by CIDNP effects. The fact is that the polartred Ime intensity of the product in no way depends on the population difference of the nuclear sublevels of the reagent molecules and hence is insensi- tive to RF-sxuration effecrs. The only exceptions are reversible processes when polarized molecules of the inittal reagents cm once again enter the reaction during the polariration lifetime (TI). In this case, the polari- z.ttion arising in the first stage contributes appreciably to rbe tots1 polxization of the final product. Hence, the saturation of the polarized signals from the re- agents by tile RF field eventually affects the total

mm

Fig 4. PMR spectra detected during the photolysis of cycle-

hepranone (VII) in CDC13 (0.05 hi) (a) without any satura- tion, (b) saturation of p protons in (VII), (cl saturation of Q protons in (VII).

Volume 96, number 1 CHEMICAL PHYSICS LETTERS 25 March 1983

hu (c@&c=o F

solvent SH

(IX)

2(C6H,),COH -+ C6H,-C-C-C6H, I I

OH OH

Scheme 3.

intensity of the polarized signals from the reaction products. This condition is fulfilled in the reaction of cycloheptanone decomposition which to a great ex- tent is reversible_

The use of RF labelling for studying the intermolec- ular hydrogen abstraction is exemplified in the well- known process of benzophenone photolysis [9] (see scheme 3). The benzophenone photolysis was realized in ChDI2 with admixtures of trimethyltinhydride (XI)_ A successive RF pumping of proton signals from bemophenone, solvent and (CH&SnH has demonstrated the ketyl radical to abstract mainly a hydride proton of tin hydride. Indeed, the signal in- tensity of(X) hydroxyl protons reduces only under saturation of hydride protons in (CH,),SnH.

References

[I] S. For&n and R.A. Hoffman, J. Chem. Phyr 39 (1963) 2892

[ 21 A.L. Buchachenko, R.Z. Sagdeev and KM. Salikhov, hfagnetic and spin effects in chemical reactions (Nauka. Novosibirsk. 1978), in Russian_

[3] F-hi. Lewis and hf.!% Matheson, J. Am. Chem. Sot. 71 (1949) 741_

[4] G. Hammond. J. Sen and C. Boozer, J. Am. Chem. Sot. 77 (1955) 3244.

[ 51 M-G. Voronkov, S.Kh. Khangazheev. RG. Mirskov and V.I. Rakhlin, Zh. Org. Khim. SO(1980) 1426. in Russian.

[6] M-G. Voronkov, V.I. Rakhlin, S.Kh. Khangazheev. R.G. Mirskov and AS. Dneprovskii. Dokl. _4kad_ Nauk SSSR 259 (1981) 1364. in Russian.

[ 71 R-S. Srinivasan. in: Advances in photochemistry. Vol. 1. eds. W.A. Noyes, G. Hammond, J.N. Pitts Jr_ (Wiley- Interscience. New York, 1963) pp_ 83-l 13.

IS] R. Kaptein. R. Freeman and H-D-W. Hill, Chem. Phyr Letters 26 (1974) 104.

[ 91 H-G-0. Becker, ed_. Introduction into the photochemistry

of organic compounds (Khimia, Leningrad, 1976). in

Russian.

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