Nitric Oxide Chemical Ionization Mass Spectra of Olefins Donald F. Hunt' and T. Michael Harvey Depatiment of Chemistry, University of Virginia, Charlottesville, Va. 2290 1
Electron bombardment of nltrlc oxlde .at 1 Torr affords reagent ions at m/e 30 ( NOf) and 60 (NO*NO)+. These Ions, in turn, react wlth internal oleflns and dlenes to produce spectra contalnlng Ions corresponding to (M NO)+, Mf, and (M l)+. For most samples, these Ions carry ca. 7090% of the total Ion current. Termlnai olefins undergo reactlons with NO+ to form the same three ions plus a novel serles of fragments derlved from the Markownlkoff addltlon of NO+ to the olefin linkage. Ions characterlstlc of sample molecular welght constltute 30-60% of the total sample ion current In the CI( NO) spectra of terminal olefins. Mechanlsms, energetics, and analytlcal appllcatlons of the lonmolecule chemistry of NO+ with alkenes are discussed.
-
+
-
A H : -35 kcol-mol;'
7 + CH5+
--O
+
+ CHI + H*
A H = - 2 6 kcal-mole
A H = - 1 8 kcol-mole
In this report, we describe the results of our investigation on the utility of nitric oxide as a reagent gas for obtaining chemical ionization (CI) mass spectra of olefins. This work was undertaken to further our understanding of the gas phase ion molecule chemistry of the nitrosonium ion, (NO+), and as part of an effort to develop a sensitive analytical mass spectrometric technique suitable for the analysis of unsaturated hydrocarbons. Conventional 70-eV electron impact (EI) mass spectra of olefins are less than satisfactory for analytical purposes, since most of the sample ion current is carried by structurally insignificant ions at the low mass end of the spectrum ( I , 2 ) . In general, E1 spectra of olefins exhibit a set of odd electron ions and a set of even electron ions having formulas corresponding to CnHzn+ and C,H2,+1+, respectively. The intensities of the peaks due to these ions decrease rapidly with increasing n,and the disparity between the abundance of the low- and high-mass members of these ion families increases with increasing sample molecular weight. In the case of 1-tetradecene,over 90% of the ion current is carried by fragment ions containing three to seven carbon atoms ( I , 2 ) . The intensity of the molecular ion also diminishes rapidly with increasing molecular weight under E1 conditions. The percent total ion current carried by the molecular ion M*, in the spectra of 1-hexene and l-hexadecene is 4.3 and 0.2596, respectively (2). The extensive fragmentation observed under E1 conditions can be traced to the excess internal energy (1-8 eV) ( 3 ) imparted to the sample molecule during the ionization process. The low activation energies required for both hydrogen and carbon scrambling in the molecular ion and for fragmentation involving simultaneous electron pair formation and cleavage in the transition state, also facilitate ion decompositions. Enhancement of molecular ion intensity and suppression of fragmentation can be partially achieved by operating with low electron voltages but sample sensitivity drops by as much as two orders of magnitude under these conditions. Chemical ionization of olefins with strong Briisted acid reactant ions such as CH5+ and C2H5+ is also somewhat unsatisfactory for most analytical purposes ( 4 ) . As shown in Equations 1-5, the ion-molecule reactions of CHb+ with olefins are all highly exothermic. Author to whom all correspondence should be addressed. 2136
(3) I
-I
Enthalpy changes for these reactions are calculated using the group equivalent method of Franklin (5, 6) and values of 221, 208, and 197 kcal mol-l for l-CH2+, 2-CH+, and 3-C+, respectively. These numbers, in turn, are obtained from recent determinations of the heats of formation for the isomeric butyl cations (7, 8). A value of 221 kcal mol-l is used for the heat of formation of CH5+ (9). The group equivalent assignment for the +CH2--CH--CH--CH2group, 209 kcal mol-l, is estimated from the known heat of formation of the 1-methyl allyl cation (7). The energy liberated in Reactions 1-5 represents the maximum internal energy possessed by the sample ions at the moment of their creation and prior to deactivating collisions with neutral methane molecules. Consequently, these ions possess sufficient internal energy to overcome the activation barrier (8) for the endothermic hydride migration (6-12 kcal mol-l), alkyl migration (18-20 kcal mol-'), and @-fissionprocesses shown in Equation 6.
+ -4
&A4
+ &4-
t
A H =+20 k c a l - m o l e '
Extensive fragmentation of the M f 1 ions results. Like the situation obtained under E1 conditions, ions characteristic of molecular weight (M f 1) decrease in abundance with increasing carbon content of the sample ( 4 ) . In CI(CH4) spectra, the fraction of the ion current carried by the M f 1 ions is 15 and 3% for 1-hexene and 1-eicosene, respectively (4).
The above results suggest that it should be possible to enhance the abundance of ions in the molecular weight region of olefin CI spectra by reducing the exothermicity associated with the initial ion-molecule reaction. Previously, we have shown that the ion-molecule reactions of NO+ with saturated hydrocarbons are either endothermic or slightly exothermic (IO). Consequently, very little sample fragmentation was observed. Thermodynamic considerations sug-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
gested that a similar situation would prevail in the ion-molecule reactions of NO+ with olefins. We, therefore, initiated research to explore this possibility. Mechanisms, energetics, and analytical applications of NO+-olefin ion molecule chemistry are discussed in this paper.
EXPERIMENTAL Nitric oxide (99.97%), ethylene (99.5%), propene (99%), and 1butene (99%) employed in this study were purchased from Matheson Gas Products, Inc., East Rutherford, N.J. Unsaturated hydrocarbon samples were obtained from Chemical Samples Co., Columbus, Ohio, and were of the highest purity available (99.5-99.9%). CI mass spectra were :recorded a t a resolution of 2000 on an AEIMS-902 double focusing mass spectrometer equipped with a dual CI/EI source manufactured by Scientific Research Instruments Corp., Baltimore, Md. ( 1 1 ) . Primary ionization of the CI reagent gas was accomplished using either a Townsend discharge (12) or a 1000-eV beam of electrons from a heated rhenium filament. In the filament mode of operation, pressures for the reagent gas and sample were maintained a t 0.5-1.0 Torr and Torr, respectively. Filament lifetime in the presence of nitric oxide a t 1 Torr is about 6 hr. Other operating parameters include an accelerating potential of 8000 V, an analyzer pressure of 3 X lo-' Torr, and a source temperature of 100-200 "(2. Spectra recorded with the Townsend discharge source were obtained at a nitric oxide pressure of 1 Torr and the discharge operating in the anode mode a t a voltage between 700-750 V and a current of 20-40 pA (12). Sample introduction was accomplished by one of three procedures. Gases and highly volatile liquids were metered through a fine needle valve into the reagent gas stream as it passed through an all-glass unheated inlet system. Higher boiling liquids were introduced from a capillxy vial through a stainless steel needle valve and a six-inch piece of glass-lined stainless steel tubing screwed directly into the source block. Solids were added via a direct insertion probe heated independently of the source. At no time was the sample ion current permitted to exceed 10%of the reagent ion current in either the filament. or Townsend discharge modes of operation.
RESULTS AND DISCUSSION Electron bombardment of nitric oxide at 1 Torr affords a spectrum in which !30% of the ion current is carried by the nitrosonium ion, NO+. The remaining 10% of the ion current is divided about equally between the adduct ions, (NO.NO)+ and (NCbH20)+ at mle 60 and 48, respectively. As shown in Equations 7 and 8, a third body collision with a neutral nitric oxide molecule is required to dissipate the energy liberated in the formation of these adduct ions (13, 14). XO'
+
NO' + NO + H,O
NO- H,O*
2NO
+
f
NO*
NO* NO* + NO*
C H = -18.5 kcal mol-'
( 7)
(8)
In the presence of organic sample, the nitrosonium ion functions as a gas phase electrophile, hydride abstractor, hydroxide abstractor, and one-electron oxidizing agent (15).Previous studies have shown that nitric oxide chemical ionization is a useful technique for identifying functional groups in organic compounds (161, for enhancing the molecular ion abundance in spectra of trimethylsilyl ether derivatives of biological compounds (17, 18) and for the analysis of saturated hydrocarbons (10). Analytically useful CI spectra are also obtained from olefins when nitric oxide is employed as the reagent gas. As shown in Tables I and 11, the CI(N0) spectra of internal olefins and dienes exhibit ions corresponding to M+, (M l)+,and (M NO)+. For most samples, these ions constitute 70-90% of the total sample ion current. Terminal olefins (Table 111) undergo ion-molecule reactions with NO+ to form these same three ions plus a novel series of fragments containing th.e NO group a t mle 72,86,100, and 114. For terminal olefins, 30-60% of the total sample ion current
+
is carried by ions characteristic of sample molecular weight. As indicated earlier, the abundance of ions characteristic of sample molecular weight in the E1 and CI(CH4) spectra of long chain olefins seldom exceeds 0.3 and 3.0%, respectively. To understand the reduced fragmentation observed when nitric oxide replaces methane as the CI reagent, it is necessary only to compare the energetics of the ion-molecule reactions responsible for the initial ionization of the olefin samples. (Factors responsible for the differences observed in CI(CH4) and E1 spectra have been discussed previously (19).) Heats of formation required for this comparison are obtained using the group equivalent method of Franklin (5, 6). Since a group equivalent assignment for the -NO group is not available in the literature, a value of +23 kcal mol-I was estimated from the heats of formation of C ~ H B N O(+9 kcal mol-l), i-C3H7NO (+2 kcal mol-'), and t-C4H,NO (-7 kcal mol-l) (20,21).The CH3-, -CH2and CH= units were assigned the usual values of -10, -5, and -1 kcal mol-', respectively (6). I t should be noted that the group equivalent method of estimating heats of formation of gaseous ions fails to take into account either the stabilizing influence of polarizable groups adjacent to the charged center (22) or the effect of internal solvation of the cation by the hydrocarbon chain (23-25). Consequently, the calculated enthalpy changes may be high by as much as 10 kcal mol-l. Even though the absolute values determined by the group equivalent method are subject to considerable error, the method is quite useful for making qualitative predictions on the behavior of ion molecule reactions. Although it is likely that electronic levels of NO+ other than the ground state are populated during bombardment with 100-eV electrons, we assume that most of the ions in these states are collisionally relaxed to the ground state configuration (AHf = 235 kcal mol-l) prior to suffering ion-molecule reactions with organic sample molecules. If this were not the case, then the exothermicity of the reactions outlined below would be more than 100 kcal mol-1 higher than indicated and extensive fragmentation of sample would occur. As shown in Tables 1-111, this is not the experimentally observed result. Vibrationally excited NO+ should also be present in the reactant ion plasma. In a photoionization study of NO+-alkane ion-molecule reactions, Searles and Sieck observed no difference in the reactivity of ground state NO+ and NO+ with as much as 14 kcal mol-I extra internal energy ( u = 2) (26). Similar behavior is expected in the present study. (M - 1)+ Ions. As shown in Equation 9, abstraction of an allylic hydrogen from 1-octene is exothermic by 7 kcal mol-l. Removal of any other hydrogen bonded to a secondary carbon is slightly endothermic (Equation 10).
-+
N
7 + NO+
J
-t H N O
A
(91
A H = -7 k c o l - m o l e '
&+ AH
(I01
HNO -1
= + 2 kcal-mole
Previous work has shown that the nitrosonium ion will not abstract hydrogen from a primary carbon in saturated alkanes (AH = +20 kcal mol-l) (10). Based on the heat of formation of the 2-propenyl cation (27) (AHf = 233 kcall mole) an endothermic reaction of similar magnitude would be expected for abstraction of a vinyl hydrogen by NO+. I t is clear from the above calculations that very few of the M - 1 ions in CI(N0) spectra will have sufficient internal energy to overcome the activation barrier (20-40 kcal mol-')
ANALYTICAL CHEMISTRY, VOL. 4 7 , W O . 13, NOVEMBER 1975
2137
Table I. Nitric Oxide Chemical Ionization Mass Spectra of Internal Olefins Ox Sample ion current
Mol Compound
No.
wt
TCC)
2 -Me -2 -C, 2- Me- 2-C, 2-Me- 3-C 2,3- Me, - 2 -C 4,4-Me2- 2-C, cis-2-ce trans- 4 - C 2 -Me- 2 - C 2,3- Me,- 2 - C 2, 5-Mez-3-C, 2,4,4-Me3-2-C, trans-2-C1 0 trans- 3 - c 1 0 k)")'n44-Ci, trans- 5-C, 4-Pr-3-C, I-Me-1-cyclo-C,
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
70 84 84 84 98 112 112 112 112 112 112 140 140 140 140 140 96
100 100 80 80 100 180 150 150 150 150 160 180 170 80 80 100 100
,
(M)+
24.3
.. , , . , 42.2 ... .,, ... 9.2 ... .., ... , , . ... ... 1.7 15.4
...
(M-l)&
( M + NO)+
5.9 65.4 75.2 13.3 9.9 58.8 92.6 84.0 89.3 80.0 43.3 43.3 55.2 83.3 84.8 64.5 82.0
65.8 19.6
Other, m/e (%)
98(3.4) 112(13.1), 43(2.0) 99(21.7)
...
. .. ... lOO(11.8) .. ... ... . ..
44.5 90.1 29.4 7.4 6.7 10.7 20.0 33.8 18.6 13.8 16.7 13.5 20.5 12.3
*
85(18.2) 128(2.5), 114(6.1), 100(6.1), 97(5.6), 83(13.0) 168(8.3), 114(1.7), 111(5.0), 97(5.5), 83(6.3)
... . ..
iii(5.7)
Table 11.Nitric Oxide Chemical Ionization Mass Spectra of Dienes -4 Sample ion current Mo 1 Compound
1,3-c4 2,3-Me,- 1,3-C4 1,3-c8 1,4-c, 2 ,5-Me2- 1,5-c6 2,5- Me, - 2,4 -c 6
NO.
Wt
T0C)
(M)-
18 19 20 21 22 23
54 82 110 110 110 110
140 100 100 100 110 120
10.2
-k NO'
--P
wNo f /? A H = t 4 k c o l - mol;'
+
7 + N O *
+ e +
-0
(13)
A H = +I3 kcal- mole I
In general this reaction plays a minor role in CI(N0) spectra of most olefins. In contrast, attack of CH5+ on the hydrocarbon backbone accounts for a significant fraction of the fragment ions observed in CI(CH4) spectra of alkanes (28) and alkenes ( 4 ) (Equations 5 and 6). (M)+ Ions. The energy released when an ion is neutralized on collision with an electron is defined as the recombination energy. Einolf and Munson have determined this quantity to be 8.3 eV for NO+ (16). The corresponding ion2138
89.8 100 14.1 8.2 5 .O
10.4 58.5 55.7
14.1 6.4 6.7 100.0
(Ill
( M + NO)&
... ...
...
for fragmentation to smaller alkyl and alkenyl ions by the p-fission pathway so prevalent in CI(CH4) spectra of olefins (Equation 6) ( 4 ) . Electrophilic Attack of NO+ at S a t u r a t e d Carbon. As shown in Equations 11, 12, and 13, electrophilic attack of NO+ on a carbon-carbon single bond is endothermic for terminal olefins but slightly exothermic for internal and branched chain isomers.
-
(M - 1)*
I
.
...
.
Other, m / e ( % )
84 (61.4) 98(12.3), 84(12.8) 9 5(20.6)
ization potential of nitric oxide is 9.25 eV (6). Electron transfer (charge-exchange) between NO+ and organic substrates becomes exothermic when the ionization potential of the sample is less than the recombination energy of the nitrosonium ion (Equation 15). NO' NO'
+
e
-
+ R,C=CR,
NO +
AH = -8.3 eV +
[R,C=CR,]'
+
(14) (15)
NO
For alkenes, this is only the case when the olefin linkage is tri- or tetrasubstituted. Dienes also fulfill the necessary criteria. Accordingly, abundant M+ ions in the CI(N0) spectra are found for dienes in Table I1 and compounds 1, 4,8, 16, and 46 in Tables I and 111. For internal olefins, formation of an M+ ion is diagnostic for additional branching a t one or both of the olefin carbons. Electrophilic Addition. Electrophilic addition of NO+ to the olefin linkage is a major reaction pathway for all olefins except ethylene. The latter molecule is unreactive toward NO+ in the gas phase. As shown in Equations 16 and 17, electrophilic addition of NO+ to an olefin is exothermic by 24 kcal mol-I for the pathway leading to a secondary carbonium ion and by 6 kcal mol-l for the antiMarkownikoff addition to form a primary carbonium ion. Generation of a tertiary ion on addition of NO+ to a trisubstituted olefin is exothermic by 36 kcal mol-I (Equation 18).
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
M
+NO+-
+
o
A H = -24 k c o l - m o l e
(16) -i
Table 111. Nitric Oxide Chemical Ionization Mass Spectra of Terminal Olefins % Sample ion currenta
Mol Compound
ho
wt
28 42 56
n-C13H26 n-Ci4H28
24 25 26 27 26 29 30 31 32 33 34 35 36
t2-c 16H32 n-C18H36
C2H4
( M - l)'(M + NO)'
84 98 112 126 140 154 168 182 196
95.0 77.2 76.9 19.0 12.7 24.2 31.6 30.7 41.3 45.2 45.5 42.9
3'1
224
1.0
52.1
38
252
2.4
61.0
38
308
2.5
71.6
3,3-Me2-C4b 40 2- Me- C, 42 2-Me-C7 42 3-Me-C, 43 4-Me-C,b 44 6- Me-C 7 b 4'3 2-Et-c~ 46 3,3-Me2-CBb 47 2,4,4-Me3-Csb 48 49 4-Et-CBC 3,7-Me,-C8 50 5-Et-Cgb 51
84 84 112 112 112 112 112 112 112 140 140 154
38.5 54.1 43.5 15.4 11.8 11.1 31.6 14.2 25.1 7.0 11.2 7.7
n-C,H, n-C 0 n-C gH 12 n-C ,Hi4
?-c BH
16
n- C ,H 18 n-c 1OH:! o b n-c ilH22 1ZH24
E-
2 ZH44
70
86
100
114
Other, m i e (%)
No reaction
5.0 16.2 11.5 10.0 7.0 5.5 2.1 1.2 1.0 1.5 1.0 1.7
C3H6
72
............ ............ ............ 5.5 50.0 . . . . . . 4.5 2.3 1.5 1.0
..
,
... ... .,,
.
41.0 34.2 22.1 19.3 12.4 10.8 9.1 8.6
18.4 25.6 22.1 20.5 18.6 15.8 14.5 12.9
4.4 14.2 17.5 16.5 19.8 15.9 12.9
5.2
10.4
10.4
3.0
7.3
8.5
1.4
4.3
5.2
.. , . . ,
, ,
69(6.5) 75(11.5) 81(3.2), 75(4.5), 69(7.5) 95(7.0), 83(4.9), 75(2.9) 109(2.4), 97(2.7), 75(1.8), 69(1.0) 123(1.6), 104(6.3), 75(0.7), 69(2.2) 137(1.1), 128(1.5), 97(1.1), 69(1.5) 156(1.2), 151(1,0), 142(1.0), 128(4.1) 165(0.5), 142(1.3), 128(3.5) 170(1.3), 156(2.3), 142(3.2), 128(5.5) 212(1.0), 198(2.1), 184(1.0), 170(2.1), 156(3.0), 142(4.3), 128(6.9) 212(3.6), 198(1.0), 184(3.1), 170(2.1), 156(1.6) 142(4.2), 128(4.2) 268(1.2), 254(1.2), 226(0.6), 212(2.4), 198(1.8), 184(1.8), 170(1,2), 156(1,2), 142(1,8), 128(3.O) 324(0.4), 310(0.7), 296(0.7), 282(0.5), 268(0.7), 254(0.9), 240(1.0), 226(1.0), 212(1,1), 198(1.1) 184(0.9), 170(0.8), 156(1.0), 142(2.1), 128(1.9)
Branched Chain Terminal Olefins
......... ............... 35.6 , , , , . . 13.0 3.9 30.4 . . . 15.4 17.5 4.6 18.8 . . 6.4 53.8 9.1 42.9 . , . 11.6 12.4 . . , 38.9
2.3
I
57.5 22.6 17.3 28.0 44.8 30.7
............ 5.6 12.4 4.5 3.4 . . . 2.6 . . . . . . .., , . . , ..
4.0 6.7 7.1
2.3 11.2 8.6
33.3 4.5 6.7
97(6.2), 75(2.3), 69(5.4) gg(45.8) 128(3.0) 97(17.5) 97(2.1) 109(11.1), 57(10.7) 112(10.9) 97(21.4) 97(2.3), 85(1.7), 57(48.5) 128(11.7), 125(2.1), 142(13.3) 137(2.2), 128(3.5), 125(8.9), 69(6.7) 156(6.1), 142(6.1), 128(21.5), 111(1.5), 97(2.8), 83(1.2)
...............
52 168 30.5 69.5 2-Me-C Source block temperature is: a 100 "C; * 150 " C ; 190 "C.
Subsequent decomposition of these ions by the P-fission pathway is only a minor process for most of the samples ex48, is amined. The spectrum of 2,4,4-trimethyl-l-pentene, an exception to the generalization and contains an ion a t mle 57 resulting from the steps shown in Equation 19. Formation of the tertiary ion provides the driving force for this particular transformation.
The failure of NO+ to react with ethylene and the difference in the enthalpy changes calculated for Equations 17 and 18 suggest that, electrophilic addition of NO+ to terminal olefins may occur preferentially in the Markownikoff sense. Support for this suggestion appears in the CI(N0)
spectrum of 3,3-dimethyl-l-butene, 40. In analogy to the situation outlined in Equation 19, addition of NO+ at C-2 of 40 should lead to formation of an abundant ion a t mle 57 (Equation 20). This is not observed. Instead we note the occurrence of an abundant M - 1 ion. Since abstraction of primary hydrogen by NO+ is energetically forbidden (IO), it seems reasonable to assume that the M - 1 ion results from addition of NO+ to C-1 of the terminal olefin followed by a 1,2 migration of a methyl group and 1,2 elimination of HNO (Equation 21).
A
(20)
-I A H - -14 kcal-mole
A H - -15 kcol-mole-'
The enthalpy change for this reaction (Equation 21) is about the same as that calculated for the sequence in Equation 20 and yet only the former pathway is traversed under our experimental conditions. This result is consis-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
2139
R
86
Et
3
100
Me
4
114
H
5
Figure 1. Suggested mechanlsm for production of m/e 72, 86, 100, and 114 In CI(N0) spectrum of 1-octene
tent with the idea that the direction of addition of NO+ to the double bond determines the nature of the products obtained in the ion-molecule reaction. For terminal olefins, electrophilic addition of NO+ results in the formation of a novel series of fragment ions having the general formula (CnHznNO)+,where n 2 3. High resolution analysis using the peak matching technique and an M 1 ion from an appropriate alkane as the internal standard was employed to confirm the composition of these fragments. As shown in Table 11, most of the ion current carried by this family of ions is concentrated under signals due to members containing 4-6 carbon atoms (mle 86,100, and 114). The CI(N0) spectra of 1-hexene contains ions a t mle 72 and 86 (mle 114 = M NO+), while that of 1-heptene shows fragments a t mle 72, 86, and 100 (mle 128 = M NO+). Spectra of 1-octene and higher terminal olefins show strong signals at either mle 72, 86, 100, and 114 or a t the last three of these rnle values. It should also be noted that the fragmentation responsible for the production of these ion types is either absent or plays a minor role in the CI(N0) spectra of internal olefins. To account for the above experimental observations, we suggest that the mechanism shown in Figure 1 is operative. According to this description, electrophilic addition of NO+ to the olefin linkage is followed by a series of rapid hydride migrations and delocalization of charge onto the nitroso group by preferential (but probably not exclusive) formation of 5, 6, 7, and 8-membered heterocyclic rings. Migration of hydrogen from carbon to nitrogen via a sixmembered cyclic transition state with expulsion of olefin affords the observed ions. We suggest that, for internal olefins, hydride migration to place the positive charge a t the carbon bearing the nitroso group competes favorably with the above process. Formation of the expected fragment ions is therefore suppressed (Equation 22).
-
+
+
Support for the proposed mechanism was obtained in experiments with branched chain terminal olefins. Introduction of an alkyl group a t C-2 of the 1-olefin should favor location of the charge a t that site and therefore reduce the likelihood of fragmentation via the proposed cyclic intermediates. As shown in Table 111, this is indeed the observed result. Production of an ion a t mle 86, 100, and 114 is either suppressed or completely inhibited in the spectra of compounds 42,46, and 52. In contrast to the above situation, placement of alkyl groups on one of the next four carbons (C-3 through C-6) should favor location of the positive charge on the resulting tertiary sites and therefore facilitate fragmentation through a particular cyclic intermediate. Consistent with this interpretation is the finding that the abundance of ions derived from the five-membered heterocyclic intermediate is enhanced threefold on introduction of a methyl group a t C-3 of 1-heptene, 43. 2140
Compounds 44 and 49, which contain an alkyl group at C-4, fragment preferentially through a six-membered heterocyclic intermediate. In the case of the latter compound, this intermediate can decompose with loss of either ethylene or 1-butene to produce ions a t mle 114 and 142, respectively. A similar option involving a seven-membered heterocyclic intermediate is open to compound 51 and abundant ions a t mle 128 and 142 are observed. Although ions at m/e 86, 100, and 114 account for most of the fragment ion current observed in the CI(N0) spectra of terminal olefins, it is clear from Table I11 that small but significant peaks are also observed at mle values corresponding to higher homologs in the series. Extensive hydride migration and/or alkyl migration followed by a p-fission reaction could account for the formation of these ions if the charge were always retained on the part of the molecule containing the nitroso group (Equation 23). Charge retention on the other part of the molecule (Equation 24) seems equally probable however, but the expected series of ions, (CnHzn+l)+,is absent from the CI(N0) spectra. Accordingly, intervention of the &fission pathway seems unlikely as an explanation for the experimental results.
Aside from suggesting the involvement of macrocyclic intermediates analogous to the structures shown in Figure 1, we have no explanation for the occurrence of (CnHZnNO)+ ions at m/e values greater than 114 for straight chain olefins. The origin of the abundant ion at mle 99 in the spectra of compounds 3 and 41 also remains a mystery. Dienes. As shown in Table 11, CI(N0) spectra of dienes closely resemble those obtained for simple olefins. Oneelectron oxidation, hydride abstraction, electrophilic addition of NO+, and fragmentation proceeding through heterocyclic intermediates are all observed for this class of carbon compounds. Ions due to fragmentation via a six-membered heterocyclic intermediate are particularly prominent in the CI(N0) spectrum of 1,3-octadiene since localization of positive charge at C-4 is energetically favorable and the resulting fragment ion is highly conjugated (Equation 25).
Effect of Chain Length. As a final observation, we note that the abundance of (M NO)+ ions in the CI(N0) spectra of terminal olefins exhibits a strong dependence on chain length. As shown in Table 111, increasing the number of carbon atoms in the alkene from 6 to 22 increases the total sample ion current carried by the M 30f ions from 19 to 72%. A corresponding decrease in the abundance of M - 1 and (CnH2,NO)+ fragment ions is also observed. This trend can be rationalized in part by assuming that an increase in chain length increases the probability of charge migration beyond the preferred sites for ring closure to the heterocyclic ionic intermediates in Figure 1. We suggest that these heterocyclic ions are required to facilitate fragmentation and therefore expect a decrease in the abundance of fragment ions and an increase in the abundance of the precursor M NO+ ions.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
+
+
+
In contrast to the above situation, just the opposite trend is noted in the abundance of ions produced by the corresponding electrophilic addition reaction in CI(CH4) spectra ( 4 ) . Here, ions such as the (M l)+ (Equation 1) and (CnH2n+l)+(Equation 6) fragments decrease in abundance from 70 to 40% as the chain length is increased from 6 to 20 carbon atoms. Alkenyl ions (CnH2n-1)+ resulting from attack of reagent ions on the saturated hydrocarbon chain (Equations 2-5) show a corresponding increase in abundance from 24 to 4806. Field suggests that these results reflect the relative directing ability of a double bond and saturated alkane moieties on the course of ion molecule reactions involving electrophilic reagent ions ( 4 ) . A single double bond was shown to exert about the same influence as 14 methylene units in a hydrocarbon chain. In an earlier study, we found that reactions of NO+ with straight chain alkanes (alkyl and/or hydride abstraction and subsequent &fission) are either endothermic or slightly exothermic (10). Accordingly, it is not surprising that the double bond plays such an influential role in the ion-molecule chemistry of NO+ and olefins.
+
LITERATURE CITED (1) A. H. Budzlkiewlcz, C DJerassl,and D. H. Williams, "Mass Spectrometry of Organic Compounds," Holden-Day, inc., 1967, p 55. (2) Catalog of Mass Spectra Data, Amerlcan Petroleum Institute, Research ProJect44, Carnegle Institute of Technology, Plttsburgh, Pa. (3) D. H. Willlams and I. Howe, "Prlnclples of Organic Mass Spectrometry", McQraw-HIII Book Co., New York, 1972, p 24. (4) F. H. Field, J. Am. Chem. Soc., 00, 5649 (1968). (5) J. L. Franklin, J. Chem. Phys., 21, 2029 (1953). (6) J. L. Franklln, J. Q. Dlllard, H. M. Rosenstock, J. T. Herron, K. Draxl, and F. H. Field, Net. Stand, Ref.Data Ser., Net. Bur. Stand., No. 28, (1969).
(7)
F. P. Lossing and 0. P. Semeluk, Can. J. Chem., 48, 955 (1970).
(6)J. A. Popie, J. Radon, and P. v. R. Schleyer, J. Am. Chem. SOC.,04, 5935 (1972). (9) Q. W. A. Miine,and M. J. Lacey, "Modern ionlzation Technlques In Mass Spectromet CrIt. Rev. Anal. Cham., 1074, 45. (10) D. Hunt andr: M. Harvey, Anal. Chem., 47, 1965 (1975). (11) D. Beggs, M. L. Vestal, H. M. Fales, and G. W. A. Mllne, Rev. Scl. Instrum., 42, 1572 (1971). (12) D. F. Hunt, C. N. McEwen, and T. M. Harvey, Anal. Chem., 47, 1730 (1975). (13) L. J. Puckett and M. W. Teague, J. Chem. Phys., 54, 2564 (1971). (14) M. A. French, L. P. Hllls, and P. Kebarle, Can. J. Chem., 51, 456 (1973). (15) D. F. Hunt and J. F. Ryan, J. Chem. SOC.,Chem. Commun., 620 (1972). (16) N. Einoif and B. Munson, Int. J. Mass Spectrom. /on Phys., 0, 141 (1972). (17) B. Jelus, B. Munson, and C. Fenselau, Anal. Chem., 48, 729 (1974). (16) B. Jelus, 6 . Munson, and C. Fenselau, Blomed. Mass Spectrom., 1, 96 (1974). (19) F. H. Field, "MTP internatlonai Revlew of Science, Physlcal Chemlstry, Serles one," Vol. 5, A. Maccoll, Ed., Butterworths, London, 1972, pp 133-185. (20) S. Benson, "Thermochemlcal Kinetics,'' John Wiley and Sons, New York, 1968. (21) S. W. Benson, F. R. Crulckshank, D. M. Golden, Q . R. Haugen, H. E. O'Neal, A. S. Rodger, R. Shew, and R . Welsh, Chem. Rev., 80, 279 (1969). (22) J. I. Brauman and L. K. Blair, J. Am. Chem. SOC.,03, 806 (1971). (23) I. Dzidic and J. A. McCloskey, J. Am. Chem. Soc., 03, 4955 (1971). (24) T. L. Morton and J. L. Beauchamp, J. Am. Chem. Soc., 04, 3671 (1972). (25) R. Yamdagni and P. Kebarle, J. Am. Chem. SOC.,05, 3504 (1973). (26) S. K. Searles and L. W. Sieck, J. Chem. Phys., 53, 794 (1970). (27) L. Radom, P. C. Hariharln, J. A. Pople, and R. v. R . Schleyer, J. Am. Chem. Soc., 03, 6531 (1973). (28) F. H. Fleld, M. S. B. Munson, and D. A. Becker, Adv. Chem. Ser., No. 58, Amerlcan Chemical Society, Washington, D.C. (1966).
RECEIVED for review May 30, 1975. Accepted July 30, 1975. The authors gratefully acknowledge support for this research from the National Science Foundation, the Chevron Research Company, and the U.S. Army Research Office, Durham.
Effect of Doping Gases on Microwave-Induced Emissive Spectrometric Detectors for Gas Chromatography F. A. Serravallo and T. H. Rlsby' Department of Chemistry, The Pennsylvania State Unlverslty, Unlverslty Park, Pa. 16802
The effect of doplng gases on a reduced pressure ( 1 Torr) mlcrowave-Induced hellum plasma has been studled. The results show that the quantlty of oxygen added as a doplng gas Is crltlcal In maxlmlzlng the selectlvlty and sensltlvlty of the microwave-Induced emlsslve spectrometric detector for the determlnatlon of chromlum. The addltlon of approxlmately 3 % 0 2 maxlmlzes emlsslon from atomlc chromlum and slmultaneously quenches molecular emlsslons. Posslble mechanlsms of fragmentatlon and excltatlon are glven.
The development of the microwave-induced emissive spectrometric detector (MIESD) (1) for gas chromatography has been followed by its application for sensitive and selective organic analyses (2-12). Compounds analyzed have usually contained a heteroatom (such as sulfur, phosphorus, or a halogen) and it is the atomic emission from these atoms which can be monitored selectively. Alternatively, this detector can be operated in the nonselective mode, usually with an increase in sensitivity, by monitoring the atomic carbon emission (11. Also, methods have been Author to whom
all correspondence should b e addressed.
developed for determining interelement ratios (12, 13) using atomic emissions, while molecular emissions have been used to determine the extent of deuterium labeling in hydrocarbons (14). Recently, the MIESD has been employed for the selective determination of inorganics. Bache and Lisk (15) used a reduced pressure helium plasma for the analysis of organomercury compounds recovered from fish. Dagnall et al. (16) employed an atmospheric pressure argon plasma for the determination of a number of metal chelates. Talmi and Andren (17) determined selenium in environmental samples, also using an atmospheric argon plasma. Recent work in our laboratories (18)involving the use of a reduced pressure helium plasma for the selective detection of chromium P-diketonates has shown that the presence of trace quantities of oxygen (10) is important in obtaining atomic chromium emission instead of molecular emission. The results of this work indicate a need for an understanding of the mechanism of fragmentation and excitation in microwave discharges in order to have a basis for optimizing the MIESD. For this reason, the effect of oxygen, nitrogen, argon, and hydrogen on the spectral characteristics of a reduced pressure helium plasma has been studied.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
2141
