2 PHYSICAL METHODS Part (i) Mass Spectroscopy By John M. Wilson (Department of Chemistry The University of Manchester Manchester 13) DURING 1968 there has been a further increase in the number of papers on this topic in the chemical literature. A large number of these are concerned mostly with either simple descriptions of spectra or with necessarily speculative inter- pretations of fragmentation patterns of specific groups of organic compounds for reasons of space these cannot all be discussed here. The popular acceptance of orbital symmetry correlations in the explanation of photochemical and pyrolytic reaction mechanisms has led to their appli- cation to mass spectral problems. The early results look promising particularly with regard to problems of skeletal rearrangements.There is also a greater interest in a more detailed treatment of the kinetic aspects of mass spectro- metry which this reporter welcomes. General Methods of Interpretation.-A general approach involving correlation of molecular orbitals of the decomposing ion and the products has been sugges- ted.' According to this approach the butadiene ion should be formed in an excited state by the retro-Diels-Alder reaction on the cyclohexene molecular ion. The disadvantages of this approach are that it assumes a concerted process and that as yet only 7c orbitals are considered for ionisation. Decomposition of ions of the type (1)to produce (3) can go by a single-step process with elimina- tion of hydrogen when X = CH:CH (stilbene) or by a successive elimination of two hydrogen atoms when X = NH (diphenylamine).If this.difference is the r 1+. (1) (2) CO,Me R0- (3) (4) ' R.C. Dougherty J. Arner. Chern. SOC.,1968,90 pp. 5780 5788. John M. Wilson result of cis-and trans-arrangements of the hydrogen atoms in (2)then we have dis- and con-rotatory ring closures respectively which can only result from the excited states of the molecular ions.2 A different MO approach to mass spectra is used in the calculation of fragmentation probabilities in cyclic hydrocarbon^.^ In this case the authors make the assumption that the majority offragmentations take place within one vibration after impact a rather bold generalisation but the results are in accord with measured spectra.Much more detailed calculations have been carried out on the molecular ion of eth~lamine.~ The calculated potential-energy surface shows that it is much easier for the C-C bond vibra- tion to be transformed into a reaction co-ordinate than for the C-N.4 Previous studies of mass spectra which involved Hammett relationships have been criticised mainly on two grounds. If a molecular ion decomposes by metastable transitions the abundance measured at the collector may not be accurately ‘representative of ion concentrations in the ion source.’ The second objection is that the substituent affects not only the activation energy of the decomposition process but also the internal energy distribution in the decom- posing ion.6 Such effects can be seen in a study of the mass spectra of 4-phenyl-butyrates of the general type (4).’ The loss of a methoxy-radical from the mole- cular ion is decelerated by electron-donating substituents because of the in- creasing number of low-energy electronic states of the molecular ion below the fragmentation threshold.Such criticism does not imply any great restrictions on the value of such studies but shows that one must take all these factors into consideration in the interpreation of data.8 The spectra of ring-substituted anisoles exhibit two important primary processes the elimination of a methyl radical which is accelerated by electron-releasing substituents and the loss of formaldehyde which is decelerated by such sub~tituents.~ The results to- gether with metastable-ion data are in agreement with the proposal of the four-membered ring transition-state (5) for the latter process.The approach of Johnstone has been to use in the Hammett-plot activation energies rather R. A. W. Johnstone and S. D. Ward J. Chem. SOC.(C),1968,1805. K. Hiroto and Y. Niwa J. Phys. Chem. 1968,72 5. J. C. Lorquet A. J. Lorquet and J. C. Leclerc in ‘Advances in Mass Spectrometry’ vol. 4 ed. E. Kendrick Inst. Petroleum London 1968. I. Howe and D. H. Williams Chem. Comm. 1968,220;J. Chem. SOC.(B) 1968,1213. R. G. Cooks R. S. Ward I. Howe and D. H. Williams Chem. Comm. 1968 837. ’ D. G. I. Kingston and H. P. Tannenbaum Chem. Comm. 1968,444. * F. W. McLafferty Chem. Comm. 1968 957. F. W. McLafferty and M. M. Bursey J. Org. Chem. 1968,33 124.Mass Spectrometry than ion abundances.'O These are measured by taking the difference between the appearance potential of the fragment ion and the ionisation potential of the molecule and have the advantage that there are no complications due to the kinetics of the process and that since the method is comparative the usual errors in absolute appearance potential measurements are avoided. The plot for the process (6) -+ (7) gives different slopes for meta-and para-substituents suggesting that an excited state of the molecular ion is involved. There has also been some more detailed study of metastable-ion igtensities. Although in general the shape and intensity of peaks corresponding to decom- positions of the same ion from different sources are similar there are small differences which can be interpreted.The peak due to the transition C6H5CO+ -+ C6H5+ in the spectra of a series of substituted benzophenones has an in- tensity which varies with the internal energy distribution in the molecular ion.'' The same metastable peak in the spectra of compounds PhCOR with a wide variety of R-groups is similarly affected,12 particularly by the number of vibrational degrees of freedom in R reflecting its ability to remove excess energy from the molecular ion in the decomposition. The ions C12HloO+' in the mass spectra of diphenyl ether and diphenyl carbonate decompose at different rates. In the latter case the average internal energy of the molecular ion will be greater and the carbon dioxide molecule eliminated cannot remove much of the excess of energy.The ion formed therefore decomposes faster than the molecular ion of diphenyl ether although there is also the possibility that an intermediate (8) of different structure may be involved.' Similar behaviour lo R. A. W. Johnstone and D. W. Payling Chem. Comm. 1968. 601. 'I M. L. Gross and F. W. McLafferty Chem. Comm. 1968 254. R. G. Cooks and D. H. Williams Chem. Comm. 1968 627. l3 D. H. Williams. S. W. Tam. and R. G. Cooks. J. her. Chem. SOC.. 1968.90.2150. 10 John M. Wilson is found in the mass spectra of phenol phenetole and tropolone where a C,H,O'+' ion is found which is considered to have the phenol structure. The ion from tropolone decomposes faster than the molecular ion of phenol; in this case the neutral fragment is carbon monoxide.In the case of phenetole where the neutral fragment is ethylene the C6H,0+' ion decomposes more slowly than phenol because there is a high probability of the excess of vibrational energy being removed by eth~1ene.I~ The controversy centring round the concept of a fixed charge location con- tinues. Compound (9) does not undergo typical ketone fragmentations when R = NH,.15 as reported last year. This was taken as an indication that the positive charge was located in the arylamine ring and did not migrate to the other ring. In the compound (lo),however fragmentations of the acyl group can be observed when R = NH, and the behaviour of (9) is explained in terms of more favourable competing processes e.g.fission of the benzylic bond.16 It is reported however that the ion (11) in the mass spectra of some amino- steroids does not undergo elimination of acetic acid.I7 The implication here is that the charge cannot migrate from ring A to ring D. Fragmentation and Rearrangement Processes.-Last year's report of random hydrogen migrations in aromatic molecules' has been followed by similar studies of deuteriated systems. In furan and thiophen where elimination of C2H2 from the molecular ion can be observed scrambling of hydrogen and and deuterium atoms in the ion is incomplete i.e. fragmentation can compete with the randomisation proce~s.'~ In the elimination of HCN from the mole- cular ion of a-and P-cyanonaphthalenes randomisation over the two rings appears to be complete.20 In the mass spectrum of 2-deuteriothiazole elimina- tion of DCN takes place and is 98 % specific.In the case of the analogous benzo- thiazole the same process is 92% specific but the corresponding metastable ions show a random loss of HCN and DCN.20 In the spectra of tris-(2,4,6- trideuteriopheny1)phosphine and its oxide there are completely specific losses of a deuterium atom.21a There is therefore a hydrogen randomisation process which can take place in all aromatic systems but in order to be observed it must be able to compete kinetically with the fragmentation process which is being observed. In the mass spectrum of diphenylmethanol there is in addition a slow process of hydrogen exchange between the two aromatic Hydrogen scrambling processes can also be observed in the spectra of some ali- phatic compounds.The production of acyl ions from aliphatic ketones is a specific single-bond cleavage at high electron energies.22 In the spectrum of l4 D. H. Williams R. G. Cooks I. Howe J. Amer. Chem. SOC.,1968,90 6759. l5 T. J. Wachs and F. W. McLafferty. J. Amer. Chem. SOC.,1967,89 5054. l6 A. Mandelbaum and K. Biemann J. Amer. Chem. SOC. 1968,90,2975. I' H. Bruderer W. Richter and W. Vetter Helu. Chim. Acta 1967 50 1917. l* J. M. Wilson Ann. Reports (B),1967 64 59. l9 D. H. Williams R. G. Cooks J. Ronayne and S. W. Tam Tetrahedron Letters 1968 1777. R.G. Cooks I. Howe S. W. Tam and D. H. Williams J. Amer. Chem. SOC. 1968,90,4064. (a) D. H. Williams R. S. Ward and R. G.Cooks J. Amer. Chem. SOC. 1968 966; (b) D. H. Williams R. S. Ward and R. G. Cooks J. Chem. SOC.(B) 1968 522 22 M. Kraft and G. Spiteller Annalen 1968,712 28. Mass Spectrometry [2,2,4,4-2H4]octan-3-one the C6H ion contains two deuterium atoms at 70 ev but at 12 ev analogous ions containing three and four deuterium atoms appear.23 Such randomisation has been confirmed in the spectra of other deuteri- ated ketones.24 Random rearrangements are also common in the mass spectra of olefins. A study of ionisation potentials and appearance potentials of the isomeric methylcyclohexenes and methylenecyclohexane shows that although ground states of the molecular ions reflect the differences in structure most of the fragments appear to be formed from a common intermediate.25 An examination of the spectra of various deuteriated derivatives of 3-phenyl- propanol shows that prior to fragmentation an exchange of hydrogen atoms takes place between the hydroxy-group and the aromatic ring.26 An interesting long-range hydrogen migration takes place in the mass spectrum of the steroidal alkaloid (12).27One of the fragmentation processes leads to an ion for which the structure (13 ;R = H) has been suggested and this ion decomposes further by loss of amonia.If the compound is fully deuteriated on nitrogen the ion (13; R = 2H) is formed and this decomposes by loss of N2H3.The author suggests that this ion is formed after a process involving transfer of hydrogen atoms between two ions. The reason for this suggestion is the conceptual diffi- culty involved in envisaging a specific hydrogen transfer from the C-3 to the C-20 amino-group.Whether such an interionic process is allowed by simple electrostatic considerations is open to question. The question of ring expansion in aromatic systems continues to provoke discussion. The mass spectrum of [l,a-13C2] toluene has provided us with further information on the tropylium ion. In the decomposition C,H,+ + C5H5+the decomposing ion appears to have the 13Catoms randomly mixed with the I2C atoms.28 The simple picture in which the a-carbon atom is in- serted between C-1 and C-2 is not valid. This is in agreement with the original work of Meyerson on 2H labelled compounds.29 By using a method which 23 W. Carpenter A. M.Duftield and C. Djerassi J. Amer. Chem. SOC.,1968 90,160. 24 A. N. H. Yeo R. G. Cooks and D. H. Williams Chem. Comm. 1968 1269. 25 R. E. Winters and J. H. Collins J. Amer. Chem. SOC.,1968,90 1235. 26 N. M. M. Nibbering and Th. J. deBoer Tetrahedron 1968 1415. 27 P. Longevialle Chem. Comm. 1968 545. ’* K. L. Rinehart jun. A. C. Buchholtz G. F. Vanlear and H. L. Cantrill J. Amer. Chem. SOC. 1968,90,2983. 29 H. M. Grubb and S. Meyerson in ‘Mass Spectrometry of Organic Ions’ ed F. W. McLafferty Academic Press New York 1963. 12 John M. Wilson involves the examination of substituent effects at different electron energies,30" Brown has suggested that substituted toluenes may initially form a benzyl ion which later rearranges to tropyli~m.~~' He comes to a similar conclusion from an examination of Hammett plots of formation of substituted benzyl ions from substituted benzyl phenyl ethers.31 From deuterium labelling results on diphenylmethane it would appear that the C7H7+ ion produced from this molecule has a tropylium-ion structure and is formed without extensive randomisation of the hydrogen atoms taking place.In this respect it differs from the corresponding ion formed from biben~yl.~~ Djerassi has examined a series of N- and C-methyl derivatives of isoquinoline pyrrole and indole. The prominent process in the spectra of all these compounds is loss of a hydrogen atom from the methyl group followed by elimination of HCN.33 By I3C labelling he has shown that in the N-methyl compounds the + Me SiOSiMe N (14) (151 [M -HI + ion has not undergone ring expansion [e.g.(14) -+(15)]. The results in the C-methyl series are not unambiguous but are consistent with ring expan- sion. The elimination of HCN from the molecular ion of aniline takes place from an ion in which the carbon skeleton is the same as in the parent mole- ~~le.~~*~~ The same elimination takes place from the C6H6N+ fragment ion from aniline and the 13C labelling evidence suggests that this may have the azatropylium structure although the rearrangement may not be ~omplete.~ The same is true of the C,H,N+ ion from ~ulphanilimide.~~ Skeletal rearrangements of various types continue to be reported in large numbers. A number of trimethylsilyl ethers of glycols undergo rearrangements which produce the ion (16).36*37 It can be formed by long-range migrations as in the case of the trimethylsilyl ether of de~ane-l,lO-diol.~~ Other long-range migration processes have been observed in the spectra of trimethylsilyl ethers of aliphatic hydroxy-ester~.~~ Nevertheless these derivatives provide useful structural information.The simplest approach so far to the solution of the problem of the determination of double-bond positions in unsaturated hydro- carbons and esters is the process which entails oxidation with osmium tetroxide followed by trimethylsilylation of the glycol. In the spectrum of the bistrimethyl- silyl ether there are two abundant fragment ions which are formed by simple 30 P. Brown J. Amer. Chem. SOC.,1968,90 (a) p.4459; (b)p. 4461. 31 P. Brown J. Amer. Chem. SOC. 1968,90 2694. 32 S. Meyerson H. Hart and L. C. Leitch J. Amer. Chem. Soc. 1968,90 3419. 33 M. Marx and C. Djerassi J. Amer. Chem. SOC.,1968 90 678. 34 A. V. Robertson M. Marx and C. Djerassi Chem. Comm. 1968,414. 3s K. L. Rinehart jun. A. C. Buchholtz and G. E. VanLear J. Amer. Chem. SOC.,1968,90 1073. 36 J. Diekman J. B. Thomson and C. Djerassi J. Org. Chem. 1968,33,2271. 37 J. A. McCloskey R. N. Stillwell and A. M. Lawson Analyt. Chem. 1968,40,233. 38 W. J. Richter and A. L. Burlinghame Chem. Comm. 1968 1159. Mass Spectrometry 13 fission of the bond between the two oxygen-bearing carbon atoms.37*39 Interpretation of such spectra may sometimes be simplified by the use of deuterium labelling in the trimethylsilyl t.RL CO-N-C0-R3 I RZ (19) (17) Elimination of carbon dioxide from substituted maleimides is expected to proceed by initial rearrangement of the imide (17) to the isoimide (18). Calcula- tions suggest that an electronically excited state of the molecular ion may be in~olved.~' The interconversion is not thermal.41 Acyclic imides of the general formula (19) undergo carbon monoxide elimination from their molecular ions.42 A large number of skeletal rearrangements of ions can be described by the general scheme (20) +(21) 4(22).43 In this scheme the three-atom zc Z--c &z=c I +* II +.I or A-B A=B A~BA%B (20) (211 (22) group must have a p, orbital system and the migrating group Z must have either a pn orbital or a vacant p-orbital.A typical example is (6)+ (7). Some other migrations appear to take place by five-membered ring transition-states. Examples are the transition (23) 4 (24) detected by "0 labelling44 and the +S/CH2 \c + S=CH2 I %-I dI CH,-0 'R R (25) (26) 39 P. Capella and C. M. Zorzut A'nalyt. Chem. 1968,40 1458. 'O T. W. Bentley and R. A. W. Johnstone J. Chem. SOC.(C),1968,2354. '' C. M. Anderson R. N. Warrener and C. S. Barnes Chem. Cornm. 1968 166. '' C. Nolde S.-0. Lawesson J. H. Bowie and R. G. Cooks,Tetrahedron 1968 1051. 43 T. W. Bentley R. A. W. Johnstone and D. W. Payling Chem. Comm. 1968 1154. "T.H.Kinstle. 0.L. Chapman. and M. Sung J. her. Chem. Soc. 1968.90 1227. John M. Wilson elimination of carbon monoxide from ions in the spectrum of thioglycollic esters (25) -+ (26).45 Aryl migrations are found in the spectra of nitrone~~~ and azine~.~~ A number of heterocyclic compounds which have two phenyl groups on adjacent carbon atoms exhibit the ion C13H9' in their spectra.It is considered that formation of a C-C bond between the two benzene rings takes place prior to fragmentation in a manner analogous to (2).48In cyclic ~ulphones~~ the migration of a group from sulphur to oxygen takes place as was found with simple diary1 s~lphones.~' Other unusual rearrangements include the loss of hydrogen followed by nitrous oxide from dibenzylnitros- amine5' and the elimination of carbon dioxide from the molecular ion of (27).52 The structure of the ion produced by loss of carbon monoxide from 2-pyrone is still in doubt but it appears certain that those C4H40+' ions which have enough energy to decompose further do not have the furan structure.53 The molecular ions of enol acetates decompose by elimination of keten to produce ions of the same molecular weight as the parent ketones.54 Examination of further decompositions shows that they cannot have the same structure and it is suggested that the enols are formed.The C4R4+*ion formed from the molecular ion of substituted tetraphenylcyclopentadienones(28) appears to have a distorted tetrahedral rather than square symmetry.55 In an exhaustive study of the mass spectra of methyl esters of long-chain monocarboxylic acids using both I3Cand 2Hlabelling Dinh-Nguyen has shown that ions ofthe general formula CH,OCO[CH,],+ are formed by two processes partly by direct fission of the C-C bonds but mostly by a skeletal rearrangement pro- c~ss.~~ In the spectra of methyl esters of dicarboxylic acids ions of the general formula CH,OCO[CH,]~ decompose by loss of keten probably through an intermediate such as (29).5' 45 J.0.Madsen S.-0. Lawesson J. H. Bowie and R. G. Cooks Chem. Comm. 1968,698. 46 T. H. Kinstle and J. G. Stam Chem. Comm. 1968 185. 47 S. E. Schappele R. D. Grigsby E. D. Mitchell D. W. Miller and G. R. Waller,J. Amer. Chem. Soc. 1968,90 3521. 48 J. H. Bowie P. F. Donaghue H. J. Rodda and B. K. Simons Tetrahedron 1968,3965. 49 D. S. Weinberg C. Stafford and M. W. Scoggins Tetrahedron 1968,24 5409.50 S. Meyerson H. Drews and E. K. Fields Analyt. Chern.. 1964,36 1294. 51 T. Axenrod and G. W. A. Milne Chem. Comm. 1968,67. 52 J. Dekker and D. P. Venter J. Amer. Chem. Soc. 1968,90 1225. 53 W. H. Pirkle and M. Dines J. Amer. Chem. Soc. 1968 90,2318. 54 D. G. B. Boocock and E. S. Waight J. Chem. Soc (B) 1968,258. 55 M. M. Bursey D. Rieke T. A. Elwood and L. R. Dusold J. Amer. Chem. Soc. 1968,90 1557. 56 N. Dinh-Nguyen Arkzv Kemi 1968,28,289. 57 I. Howe and D. H. Williams J. Chem. SOC.(C) 1968,202. Mass Spectrometry 15 Ion intensities are difficult to correlate with structure in monoterpene mass spectra and a study of metastable ions in this series also suggests that such compounds may often decompose uia common intermediate^.^ * Theoretical calculations on C3H + ions indicate that the edge-protonated cyclopropane ion is stable with respect to C,HZ' + Applications of Various Techniques.-The problem of the determination of molecular formulae of compounds which have unstable molecular ions has been attacked by the use of three different techniques.The metastable-ion focussing technique has been used with some success.6o It has been possible to detect the transition CCl," + CCL; but in some cases it proved impossible to detect transitions of very unstable molecular ions. Negative-ion spectra have also been used. A report on such spectra for some sulphur compounds shows that some of these have stable negative molecular ions but those with acidic functions tend to have only the [M -HI -ion.61 The compound (30)is repor- ted to have a stable negative molecular ion.62 It has been shown that in the Me Me C.a:]' Me Me (31) (30) H-2u-H Me Me (33) (3 4) presence of a moderate pressure of a non-reacting gas e.g nitrogen there will be an increase in the flux of low-energy secondary electrons in the ion source which will increase the probability of electron attachment processes.63 The most successful method for dealing with the above problem has been field-ion mass spe~trometry.~~ Good spectra have been obtained for terpene~,~ but the most impressive results are the spectra of a series of unstable compounds all of which exhibited molecular ions the most spectacular being pentaery- thritol tetranitrate.66 Improvements in field-ionisation techniques have made H.C. Hill R. I. Reed and M. T. Robert-Lopes J. Chem. SOC. (C) 1968,93. 59 J. D. Petke and J. L. Whitten J. Amer. Chem. SOC.,1968 90 3338. 6o L. A. Shadoff Analyt. Chem. 1967,39 1902. 61 R. Mayer P. Rosmus M. von Ardenne K. Steinfelder and R. Tummler Z. Naturforsch. 1967 22B,1291. 62 R. S. Gohlke J. Amer. Chem. SOC. 1968 90 2713. 63 R. C. Dougherty and C. R. Weisenberger J. Amer. Chem. SOC. 1968,90 6570. 64 H. D. Beckey Internat. J. Mass Spectrometry Ion Phys. 1968 1 93. " H. D. Beckey and H. Hey Org. Mass Spectrometry 1968 1,49. 66 C. Brunee G. Kappus and K.-H. Maurer Z. Analyt. Chem. 1967,232 17. 16 John M. Wilson it possible to obtain high-resolution measurements on unstable ions,67 and to obtain sensitivities sufficiently reproducible for paraffin wax analysix6 Field-induced fragmentation is producing surprising results.The fragment ions produced from but-1-ene and but-2-ene are quite different.69 The technique of analysing the 14C content of fragment ions by measuring the radioactivity of photographic plates from a mass photograph7' has been applied to the problem of the fragmentation of salicyclic acid.7 The ion (3 1) decomposes by elimination of carbon monoxide from C-2 and not from the original carbonyl group. The structure of the product ion may be (32). A tech-nique for obtaining pure metastable mass spectra has been described in which kinetic-energy selection takes place between the magnetic analyser and the collector.72 By using such a system it has been possible to detect differences between the spectra of cis-and trans-but-2-ene.Williams has noted that since molecules containing functional groups with low ionisation potentials e.g. p-or rn-anisyl often show abundant molecular ions it should be possible to use such groups in the preparation of suitable derivatives for the determination of molecular weight^.^ Although in the interpretation of the spectra of saturated hydrocarbons the intensity of even- electron alkyl ions is usually considered it has been shown that odd-electron CnH2"+' ions are useful in the analysis of the spectra of multiply branched hydrocarbon^.'^ Although many examples of stereochemical effects on mass spectra have been reported the first case has been recorded of a difference in spectra between two compounds in which the stereochemical difference is due to the presence of deuterium atoms.The ratio [M-2HC1] [M -HCl] + [M -2HCl] is significantly greater for (33) than (34).75The mass spectra of cis-and trans-cyclic sulphites and carbonates are different at high temperatures but are quite similar at low temperatures. In these cases the pyrolytic processes are affected by stereochemistry but not the electron impact-induced decomposition^.^^ 67 E. M. Chait T. W. Shannon J. W. Amy and F. W. McLafferty Analyt. Chem. 1968,40 743. 68 W. L. Mead Analyt. Chem. 1968,40 743. 69 M. J. Weiss and D. A. Hutchinson J. Chern. Phys. 1968,48,4386. 70 H. Knoppel and W. Beyrich Tetrahedron Letters 1967 291. 71 J.L. Occolowitz Chem. Comm. 1968 1226. '' N. R. Daly A. McCormick and R. E. Powell Org. Mass. Spec. 1968 1 167. 73 A. N. H. Yeo and D. H. Williams J. Chem. SOC.,1968,2660. l4 E. D. McCarthy J. Han and M. Calvin Analyt. Chem. 1968 40,1475. l5 M. M. Green J. Amer. Chem. SOC.,1968,90 3872. 76 P. Brown and C. Djerassi. Tetrahedron. 1968,24,2949.