Evidence for a triple-bond muonium adduct

Volume

116, number

EVIDENCE D.A.

2,3

FOR A TRIPLE-BOND

GEESON,

Deporrmenr

CHEhUf.XL

of

M.C.R

Chenusny,

E. RODUNER,

3 May 1985

PHYSICS LElTERS

MUONIUM

ADDUCT

*

SYMONS

Vnrversrty of Letcesler.

Lewesrer.

VI’

H. FISCHER

and S.F.J. COX Rutherford Appleton

Recewed

Laboratory.

17 December

1984.

Chrlton. Oxfordrhrre.

UK

m final form 17 February

1985

The rsotop~cally substituted phenyl-vmyl rackals. PhC =CHMu and PhC =CDMu have been IdenuFied m a pSR study of hquid phenyl acetylene and phenyl-d,-acetylene Formal ad&bon of muomum at the triple bond is tbereby estabhshed The pomt of addluon LS dete-cd to be the termm al carbon atom The hyperfiie couphng constants of the muon and @-proton show Isotope effects comparable ~th those III eyclohexadienyl radxals The structural mformation IS consistent wth that obtamed from ESR studies of the radxal PhC =CH,. Three other radicals are also observed. these mmor prcxlucts are tenlatwely assIgned as three of the four possible nng adducts, MuH(C, H,)C=CH.

1. lritroduction

Following implantation of energetic positive muons m unsaturated organic materials, free radicals may be generated in which muonium is incorporated in lihe P-position with respect to the unpaired electron [ 121 _ Muonium (Mu = ,u+e-) is the light isotope of hydrogen which has the proton nucleus replaced by the positive muon. The evolution of the muon spin polarisation in these species may be monitored by the technique which has become known as muon spin rotation, or @R. The technique has certain analogies with magnetic resonance methods [2,3] but relies on the nature of the muon radioactive decay and benefits greatly in sensitivity from the powerful “nuclear” techniques of single-particle counting. AU the muonated ralcals which have been identi* Expenments

performed at the Scbweuensc Nukkuforschung, Vtigeo. Swtzeriand.

186

hes Ins.~~tut fir

fied with certainty to date are derivatives of double bonds or conjugated double bonds [1,2,4]. They are neutral species, in which one muon, and one electron, have been added to the host molecule. Formally, therefore, they may be described as muonium adducts (whatever the actual mechanism of their formation may be) [5,6] Triple-bond muomum adducts would be of similar interest in chemical physics. As for the double-bond adducts, pSR studies provide information on the mech-

anism of radical formation, on the physical properties of the labelled molecules, as well as on their subsequent chemical history. The unprecedented isotopic mass ratio (iWMU:MH = $ : 1), makes muonation a significantly more sensitive probe of molecular conformadon and dynamics than the more common isotopic substitutions, e.g. deuteration. The relative addition rates of muonium to double and triple bonds, and to more complicated conjugated systems, would be of especial interest as little is known of the corresponding selectivity for hydrogen addition. 0 009-2614/85/s (North-Holland

03.30 0 Elsevier Science Publishers Physics Publishing Division)

B.V.

Volume 116, number 2.3

3 May 1985

CHEMICAL PHYSICS LETTERS

Identification of a muonated radical derived from a triple bond would therefore open a substantial new field in the studies and applications of this unusual isotopic substitution. We present here a comparative @R study of phenyl acetyiene and its selectively deuterated counterpart, phenyldl-acetylene, designed to determine whether the muonated radical previously observed in this material [4] corresponds to a triplebond adduct or a ring adduct. Of interest are the relatrve probabilities of addition to the ring and to the triple bond and the precise position adopted by the muonium (in each case), as well as the comparison of structure and hyperfine coupling with the corresponding hydrogen adducts.

2. The pSR spectrum and its assignment The formation of four muonated radicals in liquid phenyl acetylene is demonstrated in the JLSR spectrum, figs. la and 1b. This spectrum is recorded in a field sufficiently high to decouple the electronic and nuclear spins (i.e. the electroruc Zeeman interaction dominates the electron-muon and electron-proton hyperfine interactions; this is the Paschen-Back regime). Each radical is then characterised by a pair of spectral lines which correspond essentially to transitions of the muon spin alone [2,3] (the hyperfme field experienced by the muon reinforces the external field in radicals formed with electron spin-up, opposes it in those formed spindown). The major radical product evident in fig. la is labelled, RI; this is the radical detected by Roduner et al. [4]. Three minor products are also detected in this high-statistics study (the spectrum represents 60 X IO6 recorded muon decays). These are labelled R2, R3 and l$ in fig. 1b, which is the same spectrum printed with the vertical scale expanded. The isotropic muon-electron hyperfiie coupling constants determined for each radical, from the frequencies of the corresponding spectral componeh, are listed in table 1. In the assignment of the pSR spectrum, we consider all the possible muonium adducts of phenyl acetylene. In particular, both possible directions of addition to the triple bond are considered (structures I and IT), and all possible points of addition to the ring. Rapid inversion of isomers is to be expected at ambient temperature for the case of bent structures [7] (as in II,

I

1

I

250

3m Frequency/MHz

350

Fig. 1 _&R SpeCbd of (a, b) phenyl acetyiene at 0.3 T, (c) phenyl acetylene at 68.5 mT and (d) pheny!rll+uzetylene at 685 mT,aU at ambient temperature Fourier power (i e. spectral intensity) is in arbitrary units. The vertical scale has been expanded X 10 between (a) and @). which are othe:v:ise identiml.

and possibly in I): PhC=CH -+ (I) PhC=CHMu

or

(II) PhMuC=CH,

PhCH=CH, --f (III) PhCH-CH2Mu. The point of addition which will be favoured cannot, in this case, be predicted with certainty. Phenyl acetylene may be contrasted here with styrene, where the muoniurn addition occurs at the terminal carbon atom of the olefhic bond yielding structure III [2,5,8]. Thus whereas anti-Markovnikov addition to styrene would block delocalisation of the singly occupied orbital (the SOMO) onto the ring, this conjugation is preserved in structure II. 187

Volume

11.6. number

CHEMICAL

2.3

Table 1 Structure and hyperfiie

PHYSICS

LETTERS

coupling constants of radicals derived by muoniurn

addition

3 May 1985

to phenyl

acetylene,

and their comparison

with corresponding hterature values for slrmlar radicals Structure

Reduced hyperfine coupling amstant tn G a) I

Phi-+

Iv CzCH H Mu Ph

X=Y=H X=Mu,Y X=Mu,Y

=H =D

ortho meta

52 6 * O.l(Mu)

para

47.8 * 0.1. (Mu) I

ortho

52.0 f 0.1 (Mu)

meta

57.7 f O.l(Mu) 47.3 * O.l(hlu)

57.9

va

V

415 f 03(2HI 47.1 f O.i(hIu)/44.7 47.2 f 0.1 mu)

X=Y=H X=Mu,Y X=Mu,Y

=K =D

Temperature QI

f 1.2(H) 1

-L 0.1 guru)

110

1121

295

RI ~ this work

29.5

R3

ths

work

2

295

47.85 f 05(2H) 57.7 * O.l(Mu)/52 58.4 i 0.1 (Mu)

Ref.

* 2(H)

295

141

1131 1141 1141

a) Couphng constants are expressed in equwalent proton couplings for comparlson, i.e. measured values are reduced m the ratlo pH/flx for a nudeus x wth ma@wic moment px2.1. Assignment

of radical RI

Selective deuteration can assist the assignment in the manner used for @3R spectra by Roduner et al. [4] _At Iower fields, where the nucIear and electronic spins are not completely decoupled, the @R spectral lines are split by proton multiplicity. The spectrum of fig. Ic shows this to be the case for phenyl acetylene, the doublet splitting indicating that a single proton dominates- This splitting is absent in the comparable ,xSR spectrum of phenyl-dl-acetylene (fig. Id). The phenyl protons are not substituted in this material (Uus was established by monitoring the proton NMR spectrum during deuteration); we conclude that the major radical R, cannot be a ring adduct, and must be I or II. The magrutude of the spectral splitting in fig. 1 c, together with the absolute line positions, detemune the proton-electron coupling responsible to be 44.7 + 1.2 G and 40 have the same sign as the muonelectron coupling. The muon-electron coupling itself, allowing for the different magnetic moments of muon and proton, is equivalent to a proton coupling

188

of 47.1 + 0.1 G. (We call this the reduced muon coupling, writing it 47.1 G*.) These data may be compared with the various proton couplings in the vinyl radical, CH+H, which are known from low-temperalure ESR studies @-protons 34.2 and 68.5 G, a-proton 15.7 G) [9]. We conclude that the anti-Markovnikov structure for the triple-bond adduct may be excluded, since the ar-proton in structure II should have a small coupling, inconsistent with our measured splitting. This conclusion remains valid even allowing for the different conditions of the pSR and ESR measurements. (Our results were obtained at 295 K at which temperature radical II is expected to undergo rapid inversion; this, however, leaves the a-proton coupling unchanged.) Structure I(IvIu,H) is thus confirmed, in which muoniurn is bound to the terminal carbon. The simulation of the lower doublet of fig. lc, close to 180 MHz, is shown in fig. 2. In addtion to the dominan t S-proton coupling (44.7 G), this simulation includes equal couplings to three ring protons (6.1 G each) so that each resolved line actually comprises an unresolved quartet. The different spacings of the quartet lines are noteworthy. This accounts for

CHEMICAL

Volume 116, number 2,3

PHYSICS LETTERS

3. Discussion

a9

_

-z-+

I

I

3.1. Selectivity

0.18

L 180

of the addition

The spectral intensities representing the ring adducts in figs. 1 a and 1b reflect their lower yields relative to that of the triple-bond adduct. They give an in185

Frequency/MHz

Fig. 2. slnlulatioln 01Pthe lower doublet of fig. 1c. The resolved splittmg (~2 MHz) is due to a &proton with couphng 445 G. The smaller unresolved splittmgs are due to 3 ring protons with couplmgs of 6.1 G; these splittings are iess in the higher-frequency group so its amplitude appears greater. The upper doublet of fig. Ic, centxed at 241 MHz IS the mn-ror -ge.

different widtt of the resolved lines (I.e. their npparently different intensity). The upper doublet of fig. lc, close to 241 MHz, is the mirror image. This inlcation of ring-proton coupling, and of its approximate magnitude, is significant in the discussion of structure given below (section 3.3). 2.2. Assignmen fi of radicals R,. R3 and R4 We identify

3 May 1985

the three minor radicals visible in the

spectrum of fig. 1b as ring adducts. Splitting of then spectral lines due to ring protons accounts for their dlsappearance below noise level in figs. 1c and 1d and excludes the possibility that any of these radicals is the ipso adduct. The muon coupling constants are hsted in table 1. We tentatively assign R2 = IV (para), R3 = IV (ortho) and % = IV (meta) as their relative coupling constants follow closely the pattern observed in the su.bstituted cyclohexadienyl radicals MuH(C6H4)X with strongly conjugating substituents [4] ; the corresponding values for X = Ph are also shown in table 1. These may be compared with the muon cupling in unsubstituted cyclohexadienyl MuH(C6H5). When the muon is meta to the substituent (X = CSCH or Ph) there is little change since the substituent is close to a nodal surface. However, for the ortho and para derivatives there is an appreciable reduction because the electron spin is delocalised onto the substituent.

dication of the selectiwty of formal muonium addition to aromatic and acetylinic bonds, which we hope

to quantify in future experiments. The clear preference for addition at the acetylinic bond is especially interesting. Rate constants have been reported for the addition of atomic hydrogen to benzene and to acetylene separately in aqueous solution: KH(benzene) = 1.1 X 109 M-l s-l and K”(acetylene) = 1.1 X lo* M-l s-l [lo]. Although these values, for the isolated aromatic and triple bonds, are not directly applicable to intramolecular competition, preferential addition to the ring of phenyl acetylene might reasonably have been predicted. Ring adducts have not been detected in fiR studies of styrene. However, the &SR spectra which revealed the formation of the muonated phenyl ethyl radical (structure III) were recorded at much lower statistics. (Typic&y only 20 X IO6 recorded muon decays [S], as against 60 X 106 in the present work.) flR measurements of the relative rate constants for addition to benzene and to styrene in binary mixtures have been reported [5,1 1] ; it would be of interest to carry Oirt similar competitive studies using phenyl acetylene. Amongst the ring adducts themselves, it is also noteworthy that ortho and para addition is clearly favoured over meta addition. This presumably represents the relative stability of the conformers. The SOMO (again as a resdt of its nodal plane close to the meta position) is able to delocalise more effectively over the substltuent when this is at the ortho or para position. 3.2. Isotope

efiects in radical I

Comparison

of the hyperfiie

coupling

constants

for I(Mu,H) and I(H,HJ, given in table I, reveals two

isotope effects: (i) The adjusted muon coupling (47 G*) is signifcantiy greater than the proton coupling in I(H,H) (41.5 G). (ii) The coupling of the wzszhstihffed proton in Imu,I-I) (44.7 G) is also greater than the proton coupl@ in I(H,H) (415 G). IS9

CHEMICAL PHYSICS LETTERS

Volume 116. number 2.3

These values are not directly comparable, since the ESR measurements on I(H,H) were made at a lower temperature [ 121. However, the muon-electron coupling in I(MuJI) vanes approximately linearly from 469 G* at 350 K to 473 G* at 240 K. Thus, it would appear from extrapolation that the isotope effect at comparable temperatures is even more pronounced _ The corresponding isotope effects in the cyclohexadienyl radicals [ 13,141 (structure V) are also reported 111table 1. (We choose these for comparison, rather than radicals of the form X-CH,Mu, since in neither I nor V does the muon have any rotational freedom.) The proportional difference (i) between muon hyperfme coupling in I(Mu,H) and proton hyperfiie coupling in I(H,H) is comparable (allowing for the possibly different temperature dependences) with the primary isotope effect between V(Mu,H) and V(H,H). Likeme the proportional increase (A) in the hyperfine coupling of the unsubstituted proton, on going from IQ&I-I) to 1QLluJ-I); is similar to the increase on going from V(H,H) to V(Mu,I-Q. The absence of any significant difference in muon coupling between I(Mu,H) and I(Mu,D) is surprising, however. This latter result contrasts with the situation in cycIohexadieny1, where a small but significant increase in muon coupling is found on going from VWJ,W to VWuP) 1141.

3 May 1985

@R linewidths increase with temperature, with an Arrhenius dependence, suggesting instead the onset of a chemical reaction. In the linear structure, the onentation of the ring is of interest. The two extreme structures are depicted in I(c) and I(d). In the planar structure I(c) ?rconjugation between ring and double bond is preserved but the SOMO is non-bonding and cannot delocalise onto the ring. Since hyperfme coupling to three nearly equivalent ring protons is demonstrated explicitly in the ESR spectrum of I(HJ-l) [12,16], and since the same values give a satisfactory simulation of the JAR linewidths for I(Mu,H) (fig. 2), structure I(c) can be eliminated. In the perpendicular configuration I(d) the SOMO is n to the ring, which allows its delocahsation, but the conjugation of ring and double bond is lost. This structure, I(d), accommodates the experimental results. Furthermore it is predicted by INDO calculations to have an energy lower than that of I(c) by 10 kcal M-l [ 161. We note however that an intermediate configuration, having a twisted structure, is also consistent with the results. This compromise, depicted in l(e), would permit a degree of n-conjugation between the ring and the double bond, as well as the stabilization of the SOMO, but the angle of twist cannot be determined from present data. I(U)

I(c)

-c=c 3.3. Structure

Ph-C=C

A

v

of radical I

/H(Mu)

/ H( Mu)

'-H

‘H

The structure of I(H,H) has been shown to be “linear” at the radical centre (Ia) rather than <(bent” (Ib) by low-temperature ESR spectroscopy [ 12,151. It could be argued that isotopic substitution of a methylene proton will not alter this significantly despite the inherent loss of symmetry. Certainly there is no evidence in the @R spectrum for two conformers. Except in the extreme hmit of rapid inversion, the @R linewidth of a time-averaged spectrum would be expected to narrow with increasing temperature. No such motional narrowing was observed. In fact, the 190

.

0

I(e)

,H(Mu) N

H(D)

yu 9 <90”

@b H

4. Conclusions In view of the sirrukuity of some of the muon coupling constants, especially between radicals I(H,Mu) and IV(paraj, selective deuteration is a crucial test in the positive identification of the triple-bond adduct IQ-&Mu). The relatively low yield of ring adducts is

Volume I1 6. number 2,3

CHEMICAL PHYSICS LETTERS

especially interesting; from such information as is available for the rate constants of hydrogen addition to aromatic and triple bonds the contrary might have been expected. Conf”umation of muonium addition, by whatever mechanism, to 3 triple bond opens up a field of study of this selectivity with respect to double and triple bonds and more complicated conjugated systems. For the radical I(H,h9u) Itself, the structural information from the PSR study is consistent with that available from ESR studies of I(I4J-i). The structure is linear at the radical centre, presumably because this maximises delocalisation of the SOMO. (Bending, which introduces s-p hybridisation of the SOMO, would reduce such delocalisation.) Of especial interest are the isotope effects, both in the muon-electron coupling, and in that of the unsubstituted P-proton. These deserve a thorough theoretical interpretation. A similar study of the muonium adduct of acetyJene itself would be valuable, especially with regard to the different P-proton couplings in the static lowtemperature structure, and the inversion rate at high temperature. Would the cis or trans isomer of the muonium-substituted vinyl radical be preferred? How would the substitution affect the inversion rate? Attempts to answer these questions are in progress.

with the NMR measurements, and W. Strub (Zurich) for assistance with some of the ySR experiments. References [ I] E. Roduner, W. Stxub. P. Burkhard, J Hochman,

[2] 131 [4]

[5 ] 161 171 181 191 [IO]

Ill] [I21 [13] 1141 [15]

Acknowledgement

[ 161

We thank M. Robinson (Lelcester)

for assistance

3 May 1985

P-W. Percival, H. Fscher, M. Amos and B.C. Webster, Chem Phys. 67 (1982) 275. A. H&G. fUen. G. Sbrling and M.C R. Symons. J. Chem 8oc Faraday Trans. I78 (1982) 2959. E. Roduner and H. Fischer, Chem. Phys 54 (1981) 261. E. Roduner,C.A. Brinkman and P.W.F. Louwrrer. Chem. Phys. 88 (1984) 143 S FJ Cox, A. Hill and R. de Renu, J. Chem. Sot. Faraday Trans I 78 (1982) 2975. E. Roduner, Hyperfie InteractIons 17-19 (1984) 785. R.W. Fessenden and R.H. Schuler, J. Chem. Phys. 39 (1963) 2147. J&l. Stadlbauer, B.W. Ng, D.C. Walker, Y.C. Jean and Y. Ito, Can. J. Chem. 59 (1981) 3261. E.L. Cochra~~. F.I. Atin and V A. Bowers, J. Chem. Phys.40 (1963) 213. M. Aubar. Fahataziz and A B. Ross, NSRDS-NBS No 5 1 (Natl. Bur. Std., Washmgton. 1975). B.W. Ng, J&I. Stadlbauer. Y. Ito. Y. thyake and DC. Walker. Hyperfme Interadons 17-l 9 (1984) 821. J.E. Bennett and J.A Howard, Chem Phys. Letters 9 (1971) 470. M B. Yim and D E. Wood, J. Am. Cbem. Sot. 97 (1975) 1004. E. Roduner, G A. Rrinkman and P.W.F. Loutier, Chern. Phys. 73 (1982) 117. G.W. Nedson and M.C.R. Symons, J. Chem. Sot. Perkin II (1973) 1405. L. BonazzoIa. S. Fenistein and R. Marx, Mol. whys. 22 (1971) 689.

191