Study of ions with excess kinetic energy

Zntcrmdonal Journal of Mass Spccrromet~ and Zon Phpics, 20 (1976) 7-18 Q EIscvkr Scicnli6c Publishing Company, Amsterdam - Printed in Tk Ncthcrkuxis

STUDY

OF IONS WITH EXCESS KTNETIC ENERGY

II_ MASS SPECTRA OF EXCESS-KINETIC-ENERGY IONS OF ORGANIC COMPOUNDS CONTAINING THE C2H50 OR CHO, GROUP

41- TSUCHLYA ASD Ye ADACHI Facdty of Engineering, Unicersiry of Tokyo (Jupan)
AISTRACT

The mass spectra of excess-kinetic-ener_q ions (KE ions) formed under electron impact have been studied by using a conventional mass spectrometer with a modified ion source_ Direct cleavages are favored over rearrangementprocesses for KE-ion formations- Therefore the correlation between the structure of a compound and the mass spectrum of its KE ions is simpler than that for ordinary massspectra-Several aliphatic compounds containing the CIHSO or CH02 groups have been measuredand the characteristicsof KE ions and of their massspectra are described in relation to the determination of the structure of organic compounds.

Combination of GC and MS is a most useful analytical method, especially for a small amount of sample consisting of several compounds_it is, however, not easy to interpret the mass spectrum of an unknown compound, because the fragmentation mechanism is complicated, owin g mainly to rearrangements and the existence of various kinds of ions with the same mass, It is known that rearrangements are favored over direct cleavages for metastable ions [I] and also for molecular ions formed by low-voltage electron imp&t [2]_ This indicates that rearrangements are probably slow processes and the precursors of rearrangement ions have little excess internal energy_ In contrast, direct cleavages are probably rapid processes and their precursors have large exazss internal energy- It has also been shown [3] that direct cleavage is a faster process than a rearrangementFragment ions with excess kinetic energy (so-calkd “ICE ions”) have long been _known [4] to exist in the mass spectra produced by electron impact and many

8

studies have been done on the kinetic-ener_gymeasurements and formation mechanisms of these ions_ However7 no attempts have been made to apply the mass spectra of these KE ions to the determination of the structure of organic compounds_ Appearance potentials of KE ions of organic compounds are generally CL 30eV [S], which is more than IO-20eV higher than those of ordinary ions with thermal energy_ Therefore it is likely that ICE ions are formed mainly from direct deavzges of highly excited parent mokcuks and it is easy to interpret the KE-ion mass spectrum, It was verified in a previous paFr [6] that primary alcohols gave intense peaks at &Z/z31 corresponding to the CHzOHf ion, but ethers produced either no peaks or onIy small peaks at M/z 3 I through rearrangements_ of KE ions are thought to be The main processes responsibIefortheforation

M** would be h,i_&Iyexcited and M? in eqn_ (I ) would have e~eess internal energy, part of which would be partitioned between two fmgments A* and B as their translational energy_ Even if M ‘* in eqn. (2) has no excess internai energy, the coulombic repulsive force of two positive charges in M” is enough to produce KE ions, A* and BIG_Thus it can be said, in both cases, that the molecular ions have no excess kinetic enerw but the two fragments have excess kinetic energy, inversely proportional to their masses_ In the decomposition of a fragment ion, however, the veiocity off particks does not change- so that the kinetic energy of A+ and of its daughter ion and neutral fragment are proportional to their respective masses In fact, the fragment ions havin,= a mass greater than half the molecular weight of the parent molecuie are generally considerably less abundant, as we have shown in our previous papers- This property of KE ions can be explained in part by means of the above mechanisms_ This paper will describe the characteristics of ICE ions and their application to the determination of the structure of organic compounds, particularly for fragments with mass 4% EXPEFUMENTAL It has been observed that the mass spectra of KE ions (named KE mass spectra) are obtained quite easily using a conventional mass spectrometer with a modified ion source, as reported in the previous paper IS]_ The ion sources used B&S1 were buiit for studying neutral fi-agxnents,but not for KE ions- The new ion source shown schematicaliy in Fig_ I was desigued to improve the detection sensitivity and reproducibility for the KE mass spectrum and was installed in a Hitachi RMU-6 sector type mass spectrometer_ One electrode (repeller-21, Fig_ I) which acts as an ion fiIter was placed just outside the ionization-chamber_

Repeller pozentid

The potentials of repeller-21 and of repeller-11 and -12 have significant

9

_____-__--_ -REPELLER

l-

LFOCUS SLIT H LEXIT SLIT --==--

ARALYSER

Fig- I_ Sckmatic

si&

-

l

ANALYSER and

end

views

of the modified

ion source

effectson ion abundances, thus they have to be adjusted to get the best focus for obtaining

both KE and ordinary

mass spectra The peak-heights of a11ions decrease

as the repeller-21 potential increases_ It is necessary to reduce the rearrangement

ions, but not necessary to eliminate all ions with thermal ene%y. Thus the standard method of adjusting the repeller potentials is as foliowsr (I) adjust all repelier potentials to ger the best focus for the molecular ions under normal conditions; (2) increase the repeller-21 potential to reduce the M’ peak height to about l/IO’ of its value under normal conditions; (3) mcasurc the KE mass spectrum of the sample by magnetic sunning and compare it with its standard spectrum (detector sensitivity must bc increased IOO-fold, because the abundance of KE ions is generally only a few percent of that for thermal ions)_ This method is an easy and good way to establish the optimum operational conditions and once they are established, it is only necessary to set the repeller-21 voltage to its optimum vaiue for obtaining a KE mass spectrum or an ordinary mass spectrum_ The first step of the above procedure is sufkient for ordinary mass spectra_ Benzene is used as the standard material and its KE mass spectrum is shown in Table I_ instead of benzene, any compound giving an intense molecular ion peak can be used as the standard, because all molecular ions should have no excess kinetic energy- The nornina voltage applied to repeller-21 depends on the geometry of the ion source, ion draw-out voltage, the repeller-I 1 and -12 voltages and so on. Although an accurate measurement of energy has not yet been done, it is thought that ions whose translational energies arc higher than about 0.9 eV are measured as ICE mass spectrumAs reported previously [6]_ electron ener_gy has an effect on the KE mass spectrum pattern- Lower-enerB electrons produce Iess abundant ions, incIuding some undesirable rearrangement ions, and higher-energy ekctrons produce a Iess characteristic mass spectrum owing to more extensivk decompositions. Therefore

10 TABLE

I

REPRODUCIBILITY

is

OF 7XE

175 209 100 97 187 139 17 65 47 4

36 27 37 58 39 49 50 ?G 63

RE MAP

178 208 100 98 183 139 13 56 39 3

179 220 100 106 18.5 159 17 64 52 4

SFECXRtJ%S

FOR

-

(61

Aremge of (a) % 5,: PC?

184 2cH 100 96 179 147 12 53 34 0

1771 2 7 2122 100 IoO& 3 2 1854 146f: 12 162 2 5 622 461 7 I 4f

15_5&0_6 18.6kO.3 S-7&03 8.8rf;O2 16s2$0.5 12_7&0-7 1_4&0_2 5.4po.3 4.0 50.4 0.3 2 0.05

= (a) Results by new ion source at ditk-cnt date- (b) RcsuIu by old fan s0ut-u~ b Pattern co&Seat based on the Al/= 27 peak (M/z 27 = I OO)_

both

u&g

KE and ordinary mass spectrum reported in this paper were obtained by the same energy, 70 eV, of electrons

the

RESULTS AND DISCUSSION

Reproducibikry Tables 1 and 2 show the KE mass spectra of benzene and methoxy-2propanol respectively. The reproducibility of these spectra seems to be similar to -l-ABLE2 REPUODUClBILlTY

I5 I8 19 26 27 29 30 31 39 43 44 45

OF THE RE iUdS

529527 13i 1 17% 3 36& 4 100 471 f31 134* 5 622 5 50* 2 116_; ,8 43* z 184&11

26.OiO_4 O-620-I Q820~I 1.8&O-2 4_9-_Lo2 233.1j;O.6 6.6&O-2 3.1 f0-2 z44po.i 5.7f9.2 Z’I f0.1 9.0*02

SPEcfR’rfy

815 20 25 55 100 540 150 65 55 100 45 195

FDR METHOXY-?-PROPASOL*

29-7 o-7 0.9 2.0 3.6 19-7 55 24 20 3.6 1.6 7-I

a (al RcsuUsby new ion source, average of nine mcamrcmcms. b ~-a=t=a1tbascdontheM/t27peaIc.

(b) Results by .oldiOn fottt’CC-

11

that of ordinary mass spectra. In both tabks the relative abundances of KE ions are expressed by the pattern coefficients based on the peak height at M/z 27 and by the percenta_to the total ionization (%xg2) of KE ions. It has been found [6] that the peaks at M/z 27 from the CtHS, C,H, and/or C,H, groups appear in the KE mass spectra of many organic compounds w5:h certain intensities and such pattern coefficients are very useful to indicate the characteristics of various functional groups. Results obtained at different dates by the use of the new ion source are shown in (a) of Tables 1 and 2, and those obtained with the double-beam mass spectrometer 16, SJ are shown in (b) of these tables. In the case of (b), there are two ionization chambers through which ions formed must travei before entering the ion accelerating field, so that ions with small kinetic enerq like C5H3+ were not detected. ln the new ion source there is only one ionization chamber, so that even ions with small kinetic enezy can get out of the chamber, thereby providing better ion detection sensitivity-about 3-10 times greater, on average. The difference in the reIative intensities of peaks at M/z 15 in Table 2 indicates that CH3+ ions probabIy have large kinetic energy. it can be said, however, that the experimental conditions have less effit on KE mass spectra than on ordinary mass

spectra. KE nuzss spectrum -slnrcmre Accord&

offragment

to McLafYerty [9] there are several different formulae for the

fragments of M/z 45, which are CHOL, CHsSi, C?HsO, CzHIN, etc. These formulae can be analysed easily by a high resolution mass spectrometer but it is not easy to identify their structures, since there are several different structures even for

P-C

130

27

100

30

Jsll 13

0

i0

Fig. 2. Mass

i

spectra of Z-propo.xy ethanol.

Fig-

3- Mass

specmx

of 2,3-butanedioi.

one formula; for instance, there are five structures for the CLHSO group l993and four of these are shown in this paper, The characteristic peaks for identifying these structures are few in ordinary mass spectra but there are certain peaks corresponding to each fmgment in tae KE mass spectra Three examples of ICE mass spectra are shown in Fig. 24compared with each ordinary mass spectrum_ These show that the terminat groups of a compound, such as CH30, CH,OH, CH&O, CH,CHOH, give the abundant KE ions. The intense peak at M/z 31 is very useful for identifying a primary alcohol, because ail of 23 primary alcohols m.ezxsured, including benzyl alcohol, gave this intense peak

13 whose pattern coefficients are more than 100and no other compound _Fve such an intense peak at M/z 31, The peak at M/z 19 is also us&u1 to identify the hydroxyl group, since this peak appears in the KE mass spectra of most alcohols- Other compounds without an OH moouprareiy give this peak The only exception among over 100 compounds measured was pa.raIdehyde, as shown in Table 3_ It is interesting that compounds con’&ning the COOH group aIso give this peak, as shown in Table 4, For ordinary mass spectra, peaks invoiving the OH group appear irregularly_ The characteristic peaks for the ethoxy soup are those at M/z 29 and M/z 27 (&H,‘) produced from the ethyl group_ For both ethyl and ethoxy groups, the pattern coefficients of MIX 29 peak are generally from 50 to 110 and % ‘& of the M/z 27 peak is generally from IO to 30. Other peaks involving rhe ethoxy group are small.

TABLE

3

CEURICKERISYICSOf 45 ION

fRAGXE?cl

CH,CH=OH.

GROUPS

CH,CHOH

AXD CHJCHO,

IhYOLVIXG

MB=

dfi= for KE muss s~wcn-zP (27) = JW (ISI

(19)

c-w

(30

120 30 105 220

6 1 2 -, ;

73 74 130 270

3-S

/-‘-CH=CH,OH

:

10

HOCH&H--CH&HrOH

26

-.--_ (43)

W5)

-CH=CH=OH iso-C,H~-CH~CH=OH n-C,H,0-CH2CH20H iso-&H,O-CH=CH~OH CH~OCHKZH&CH=CH=CH CzHsO-C&CH=OH

530 160

23 9s 83 37

4 7 17 5

9 45 70 6 25

‘:

550

28

13

2

15

210

slo

51

8

5

110

5

45

7

30

38

100

630

10

510

4s

103

52

loo

130

200

30 19

ii0 120

350 16

130 120

63 f30

330

-

180

350

9-B -

1r

CH,:HOH n-C,H,CH(CHs)OH CHJOC)OH CH,CH(OH)-CH(CH,)OH

94 100

Cl&HO-

C% CH,(CH,ICH&HCH,

41

4

130

14

470

14

470

10s

ml

CH,CHO-CHCHx &H-&

290

5

&H, * Pawxn coctikients based on peak height of _&f/z27 ion. c Pattern cocfiicicnts of ordinary mass spcarum-

100

I4

~HARACXERISR~S OF FRAGTbm GROUPSCH,CH=O,

CH,OCHr

AxD COOH,

I?c~OtVtXG

df/=

45 ioscornpurrdr

cH,cff~on-CA-&OCH,CH, C=HsOCH_XH=CH, CzH~OCH2CH_POCH~CH~ C2H,CO-OCH&H, n-CSH~CO-OCE=CHs n-&H, &D-OCHzCH, CH,CCH2CO-OCHlCH~

51

-

110

73 31 35 49 ‘)T w

-

110

75 51 60 120 61

f, CH&TOCH~CH~-OCHICHB

ss

-

SS

29

r, -CH,C(OCHICH&-OCH=CH~

64

-

89

cH~oc&n-C,H,-CH20CH, CH,COCH=-CH=OCH,

110 500

-

97 350

58

6 CH~OCH,CH,OCH,-CH=OCH, CH,OCH=(CH=OCH&-CHzOCH, HOCH:CH(OH)_CHIOCH, CH,CH(OH)_CHzOCHJ HOCHICH20CH2-CHrOCHJ

160 120 490 530 120

19 17 3

320 230 13’70 470 360

74

51

420

33

73 Isjo 62 570

73 250 116 81

650 250 IS0 125

27

I40

6

37

II

35

2

70

If IO

8 3 2 4 3

6 IS 40 12 20 22 5

240

IO

IO

78

310

28

53

IO

7 690

43 210

100 52

13 23

56 2 9 13 7

24

29 24 17 16 14 63

6

100

-coon n-C,HrCOOH n\--CH,CH,-COOH “\__/ m Pattern cOefiicicnls

fmgment

A

ofordinary

group

ma

8

7

125

20

7

x0

20

spectrum-

iike CH,CHOH,

CH,CHO,

CH,OCH,,

in which a

methyl is attached to a secondary or tertiary carbon or to an oxygen atom, gives an intense peak at M/z 15 whose pattern coefficient is more than 100, as shown in Tables

3 and 4_ Thus the peak at M/z 15 may be characteristic

for such methyl-

containing groups (named side-methyl)_ Most compounds 29 and 15 corresponding

containing to HCO+

more than IO& Thus these couid

CH,OCH,

groups_

the methoxy group give intense peaks at M/z and CH3+

whose pattern coefficients are both be the characteristic peals for methoxy and

15

Stabilityof ion McLafferZypostulated [IO J three factors as playing major roles in the formation of an abundant ion in ordinary mass spectra_ Theseare: (a) the relative stabilities of the various bonds in the decomposing ion;
and

CH3G6, respectively, Thus the peaks at Mj_T 45 are small, being smaller than the peaks at M/z 43 for both fngments-

I6 In contrast, since CH,CH-GH, CH&CHz and HOC& ions are thems&es the stable oxonium ions, these M/z 45 ions are abundant in KE mass spectra. Some differences can be seen in these abundances, probably owing to ion stability and the preference for hydrogen elimination- In the case of CH,CHOH, the ion could not have a symmetrical structure and it would be easy to lose Hz (or 2H) to produce CH,-C-GH. so the intensities of the tv;-opeaks are comparable- In the cases of CHBOCHz and COOH, both ions wouId have a symmetrical structure and there is no process which can lose hydrogen to form an oxonium ion_ Thus the peak intensity ratio of M”z 45 to M/z 43 are both greater than unity. Although only two compounds containing the COOH group are shown in Table 4, all COOH groups would give an intense peak at M/z 45 because of its stability- The fact that a11the seven methyl esters measured gave large peaks at M/z 59, which must be CH@CG (or CH&-C-O). a homolog of HOCO*, supports this assumption_ In ordinary mass spectra. some compounds containing the CH3CH0 group give an intense peak at Mlz 45 corresponding to the stable CH,CHOH’ ion which is formed by the rearrangement of a hydrogen atom from another part of the compound_ it is expected that this ion peak would be smali in the KE mass spectrum because of less probability for rearrangements- This is the case for isopropyl ether but not for _paraldehyde, as shown in Table 3. The reason for this is considered to be as follows- The first decomposition of the molecular ion of paraldehyde must be a direct cleavage process- Since there are- however, three CHJZHO groups in a molecule of paraidehyde, the least excited daughter ion must still contain a CH;CHO group_ In subsequent decompositions the rearrangements producing M/z 45 ions wouId occur with certain probabilities- High resolution mass spectrometry off isopropyl ether showed that two-thirds of M/z 43 ions in the KE mass spectrumare CH&O*, which is the stable oxonium ion and should be produced by the elimination of hydrogen from CH,CHO+, and the remainder are C3H,* ions_ On the contrary? the dominant ion at M/z 43 in the ordinary mass spectrum are C,H,+ eons_ The peak intensity ratio of M/z 45 to M/z 43 is 0-I for this ether and 0.2 for paraIdehyde_ These u..lues are smaller than those of CH,CHOH group which are greater than ca_ O-5_ Thus it can be said that the probability of direct cleavage forming the M/z 43 ion is still far larger than that of rearrangement forming M[z 45 ion and this ratio is useful to understand which structure is invoived, CH&HOH or CH&HO_

Since the hydroxonium ion, H30’, IS - stable, compounds containing the OH group give M/z 19 ions which would be produced by a rearrangement in the subsequent decomposition of some daughter ions_ Molecular w-eightdependence

It has been observed that there is a tendency to increase the mass of the largest KE ion, according to the molecular weight of a compound- The KE ion

-17 with the largest mass we have observed is M/z 149 ion from di-n-butyl phthaiate (mol wt_ = 2X3), the heaviest compound measured_ This is one of the strong features of KE ions, giving us the chance to utilize these spectra for compounds with large molecuhu weight- Of course, it is necesszy to solve the problem of characteristic peak overlapping which results from the presence of multi-functional ZFoupsIt has also been observed that there is a tendency for increase in the relative intensity of a fragment peak with a Iarger mass. according to the moIecuIar weight For example, the pattern coefficient of M/z 45 ion corresponding to CH,OCH,+ shown in Table 4, increased from 210 to 680, according to the molecular weight of 11s (mol. wt. of CH,COOCH,CH,0CH3) to 222 compounds from (CH,0CHI(CH20CHz)&H,0CH,). I n contrast, the pattern coefficient of M/z showed a decrease from 480 to 230 for the same 29 correspondin,Q to CH,09 compounds_ In the case of n-C,H,CH20CHs, the reason why the pattern coeflicient of M/z 45 is less than 100 is that the value 45 is greater than half the molecular weight (SS).

TABLE 5

-CH$H=OH

0

CH~dHOH

0

CH,dHO-

CH,CH:O-

0

0

0

0

0

0

x

CH,OCHI-

3-c

-COOH

0

0

< 0.5 0.5-I .3

-a

0

0

0

0

< o-5

18 Structure determination Table 5 summarizes the characteristics of the fr;l_ementgroups discussed above_ The significant characteristic peaks are those at &Z/z 31 and 19 for the CH,CH20H group, those at Mjz 45,29 and I5 for the CH,0CH2 group, and those at M/z 45 and 19 for the COON group. Those at M/z 45,43, I5 and 19 are also usefui to identify the CH&HOH group. The difference in peak-intensity ratios shown in Table 5 may be used to distinguish CH&HOH from CH3CHO_ The peak patterns shown in Fig 5 give other information about the fi-agment structure_ in ordinary mass spectra. the peaks at M/r 45 are observed for these compounds, as shown in TabIes 3 and 4_ Since the KE mass spectrum is closely related to the ordinary mass spectrum, it is heIpfu1to interpret ordinary spectra. It can be inferred that the information obtained from KE mass spectra about the presence of functional groups is similar to that from iR and NMR spectra. In addition, both the KE and ordinary mass spectra of a compound can be measured in 10 s using a

conventional mass spectrometerOne of the advantages of KE mass spectra is the regularity in relative ion abundances corresponding to each fra_gment group- In other words, the relative abundance of some fragment ion is rather insensitive to the structure of other parts

of a compound_ The absence of characteristic peaks indicates the absence of such a fmpent group- This is a very useful property for computerizing mass spectral data and for computer identifications without a reference library, Information about KE ions is usefui not only for determining the structure but ako for understanding the nature of ions and the fragmentation mechanism_

REFEREKCES

1 I- Howe. in D_ H- William (Ed_)_ &zss Specrromer~w~VoL 2, llu Ckmicsl 2 3 4

5 6 7 8 9 10

Society, London. 1971, Ch- 2; and rrrs- cited therein_ M- Tstdiya. S- Matsuhiraand H_ Kamada., Jup- rim& 14 (1965) 465; and rcfs. cited in rcf- Ip- J- Derrick. A- hi- Falick and A- t_ Burlingame, J- Atner_ C/etx &c., 94 (1972) 6794. W- Blukncy. PZt>x Rrr.. 35 (1930) I ISOT- Tshchiya, BuU- Chem- S&c- Jap--) 36 (1963) 1290M- Tsuchiya and Y- Horii, Ltiars Specrmsc (Tok_ro). 22 (1974) 79J- Appeil and J_ DUNP, IM_ I_ Afpff Speceom_ Ion Phys_, 10 (1972) 237, M- Ts~cfii_ya, F_ J_ Preston and H_ J_ Sk-cc,Proc- 26th Amu_ Co& Mass SpcrromAC&d Top., 1965. p- 6t F- vJ- M~LNerty~ Mars Spewat Correiarions, (Adrances in Chcm&