JOURNAL OF MASS SPECTROMETRY, VOL. 31, 1191-1192 (1996)

JMS Letters

Dear Sir,

Collision-induced Fragmentation of Ionized 3-phenyl-1-butyn-3-01 Mediated by an Intermediate Proton-bound Complex

In recent decades, reaction intermediates with loose structures, such as ion-neutral complexes, have increasingly been report- ed in the interpretation of unimolecular fragmentations of organic ions in the gas phase.'-4 A proton-bound complex is a special type of ion-neutral c ~ m p l e x , ~ - ' ~ in which two neutral partners are loosely held together through a proton. In this letter we report an unusual proton-bound [C. . -H+. . .C] intermediatei3 in the collision-induced frag- mentation of 3-phenyl-1-butyn-3-01 upon electron impact, which is different from that of the structural analogues 1- phenyl-2-alkylcyclobutanols'4 and 1-phenylalkan-1-oh,' reported previously.

The electron impact (El) mass spectrum of 3-phenylbut-l- yn-3-01 is shown in Fig. 1. The main fragment ions are [M -CH,]+ at m/z 131, [M-H,O]'' at m/z 128 and [M - CH, - GO]' at mjz 103. In order to study the fragmenta- tion mechanism, collision-induced dissociation (CID) spectra of this compound and its counterpart deuterated on the hydroxyl group were obtained. The CID spectra of the molec- ular ions (Fig. 2) show no difference except for the mass shifts of some fragment ions due to isotope labelling. In contrast, the CID spectra of both the [M - CI-13]+ ions (Fig. 3) and [M - CH, - CO]' ions (Fig. 4) show striking differences; m/z 77 peak in each of the spectra of non-deuterated parent ions is divided into two peaks at mjz 17 and m/z 78 with almost identical intensities in the spectra of the selected ions bearing a deuteron. These different spectra obtained with the labelled compound could not be interpreted by only simple bond cleavage and hydrogen transfer. Therefore, a fragmenta- tion pathway mediated by an intermediate proton-bound complex is proposed as shown in Scheme 1.

The molecular ion of 0-deuterated 3-phenyl-1-butyn-3-01 starts its fragmentation with the elimination of a methyl radical followed by 1,Cdeuteron transfer from the hydroxyl group to the benzene ring. The resulting carbon-centred cation would expel a carbon monoxide molecule to form an intermediate proton-bound complex of benzyne and acetylene. The further fragmentation pathway of a proton-bound inter- mediate should be determined by the proton afinity (PA) of the two neutral molecules. The P A of benzyne is 891 kJ m o l ~ ', much higher than that of acetylene (641 kJ mol- '),lh

which means that benzene would capture the shared proton and then give rise to a phenyl cation. In the precursor ion this

Figure 1. El mass spectrum of 3-phenyl-I -butyn-3-01.

I x10

53

Figure 2. CID spectra of the molecular ions from (a) 3-phenyl-l- butyn-3-01 (m/z 146) and (b) 0-deuterated 3-phenyl-1 -butyn-3- 01 (m/z 147).

53 F I

x5.9

c

Figure 3. CID spectra of [M - CH,]+ ions from (a) 3-phenyl-l- butyn-3-01 (m/z 131) and (b) 0-deuterated 3-phenyl-1 -butyn-3- 01 (m/z 132).

CCC 1076-5174/96/101191-02 0 1996 by John Wiley & Sons, Ltd.

Received 10 June 19% Accepted 10 July 1996

1192 JMS LETTERS

Figure 4. CID spectra of [M - CH, - CO]' ions from (a) 3- phenyl-I -butyn-3-ol (m/z 103) and (b) 0-deuterated 3-phenyl- 1 -butyn-3-ol (m/z 104).

complex, H and D atoms on the sp3 hybrid carbon are chemi- cally identical, and therefore the proton- and deuteron-bound complexes would have similar abundances if an isotope effect is ignored, and the final product ions of rn/z 77 and m/z 78 appear with similar abundances.

It 0

mh 147 mh132 mh 1 32

mh 104 mh 104

1 mh 77

Scheme 1.

aD

All mass spectrometric experiments were performed on a VG Quattro triple-stage quadrupole mass spectrometer with a direct insertion probe and a source temperature of 200 "C. The ionization energy was 70 eV. CID spectra were obtained using argon as the collision gas at a collision energy of 20 eV, and the pressure of argon was adjusted to reduce the main beam intensity by about 50% of its original value.

3-Phenyl-1-butyn-3-01 (m.p. 53--54 "C) was synthesized17 from methyl phenyl ketone by the reaction with potassium acetylide in dimethyl sulphoxide solution, and purified by fractional distillation under reduced pressure and rec- rystallization from CH,OH-H,O. The deuterated sample was prepared by classical exchange within the inlet system, intro- ducing a slurry of the sample with an excess of CD,OD.

Yours

YI-MIN SHE, YA-PING TU, FENG-RUT SONG AND SHU-YING LIUt Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China

t Author to whom correspondence should be addressed.

References 1. P. Longevialle, Org. M a s s Spectrom. 29,l (1 994). 2. R. D. Bowen,Acc. Chem. Res. 24,364 (1 991). 3. H.-F. Grutzrnacher, Int. J . M a s s Spectrom. Ion Processes

4. T. H. Morton, Org. Mass Spectrom. 27,353 (1 992). 5. J.-P. Morizur, E. Chapon and D. Berthomieu, Org. M a s s .

6. R. D. Bowen and 0 . H. Williams. J. Am. Chern. SOC. 102,

7. D. Kuck, Mass Spectrum. Rev. 9,187 (1990). 8. T. Cairns, E. G. Siegmund and J. J. Stamp, Org. M a s s

Spectrom. 21,161 (1 986). 9. M. Vairamani, M. S. Rajeev and G. K. V. Rao, Org. M a s s

Spectrom .27,57 (1 992). 10. J. M. H. Pakarinen, P. Vainiotalo, T. A. Pakkanen and H. I.

Kenttamaa, J. Am. Chern. SOC. 11 5,12431 (1 993). 11 . P. Meyrant, R. Flammang, A. Maquestiau, E. E. Kingston, J. H.

Beynon and J. G. Liehr, Org. M a s s Spectrom. 20,479 (1985). 12. P. C. Burgers, K. J. van den Berg, H. Visser and J. K. Terlouw,

Int. J. M a s s Spectrom. Ion Processes 101.83 (1 990). 13. J. Tortajada, D. Berthornieu, J.-P. Morizur and H.-E. Audier, J .

Am. Chem.Soc. 114,10874 (1992). 14. J.-F. Rontani and C. Aubert, Org. M a s s Spectrom. 28, 795

(1993). 15. H. Budzikiewicz, G. Drabner and Ch. Hamrnes, Org. M a s s

Spectrom. 28. 1326 (1 993). 16. S. G. Lias, J. F. Liebman and R. D. Levin, J. fhys. Chem. Ref.

Data 13,695 (1 984). 17. J. H. Blumenthal, US Pal . 2996552 (1961).

118/119,825 (1992).

Spectrom. 26,1119 (1991).

2752 (1980).