J. Phys. Chem. 1983, 87, 3305-3310
3305
Charge Stripping of Hydrocarbon Positive Ions M. Rabrenovib, C. J. Proctor,+ 1.Ast,* C. 0. Herbert, A. 0. Brenton, and J. H. Beynon' Royal Society Research Unit, Unlverslty College of Swansea, Slngleton Park, Swansea SA2 8PP, U.K. (Recelved October 4, 1982)
-
The energetics of the charge-stripping process can be studied by measuring the translational energy loss which accompanies the charge-strippingreaction: m+ + G m2++ G + e- (1).The minimum amount of translational energy lost (Qmin) is a direct measure of the energy necessary to remove the second electron from an ion m+ in a collision with a neutral gas (G). Assuming that both singly and doubly charged ions are in their ground states, Q- can be related to the difference between single- and double-ionization energies: Qmin= IE(m+-+m2+) (2). In the work reported here, the technique has been used to study a number of singly charged hydrocarbon ions, ranging in carbon number from C1 to Cg, in order to determine their Qminvalues. Trends in the value of Qminwith respect to the number of carbon and hydrogen atoms have been noted.
Introduction Charge stripping is an established technique for measuring the difference between single- and double-ionization energies.' It has been shown that the method is equally applicable to molecular and fragment ion^.^?^ From the energetics point of view, the charge-stripping reaction m+ G --m2+ G e(1) takes place as an isolated system in which the translational energy of ml+ provides the only available source of energy to drive the reaction. Therefore, the minimum translational energy loss (Qmin)corresponds to the ionization energy of ml+. Qmin= IE(m+ m2+) (2) The translational energy loss can be measured by passing the product m12+ions through an electric sector. Ions mI2+will be detected at an electric sector voltage Ev = ( E - Qmin)/2 (3) where E represents the voltage required to transmit stable ml+ ions from the source. Interference from these singly charged ions does not occur since they are detected at voltage E. The charge-stripping peaks are very narrow and exhibit sharp and well-defined onsets, which enables more precise measurement of their position than is possible with the threshold extrapolation procedure of the conventional appearance energy measurements for doubly charged ions. Therefore, the charge-stripping method overcomes the main difficulty associated with the electron impact threshold measurement technique. It has been shown that doubly charged ions can be formed by the charge-stripping process even for species which do not form doubly charged ions in the ion source. Thus, for example, CH42+ions were formed from CH4+and the corresponding ionization energy measured, in spite of the fact that no trace of CH42+can be detected in the mass spectrum of methane.4 The charge-stripping method can only give a value for the difference between first and second ionization energies of a given species and, thus, accepted values for first ionization energies are required. In the absence of such data, values of Qmincan still be considered significant when a series of related ions are studied. Relatively little information is available in the literature concerning the ionization energies of singly charged ions
+
+ +
-
or the structures of doubly charged ions. In previous studies, the charge-stripping method has been applied to mono- and disubstituted benzenes,2 m e t h ~ n e , rare ~ - ~ gas ions! and halocarbon i0ns.I In most cases, where literature values were available, the agreement between the charge-stripping method and other methods of doubleionization-energy determination was satisfactory. In the present study, we are reporting Qminvalues for 68 hydrocarbon ions obtained from different precursor molecules. Experimental Section All experiments were performed on a reversed geometry VG Micromass ZAB-2F double-focusingmass spectrometer.8 Singly charged positive ions, formed in the electron-impact source, are mass selected before interaction with a nitrogen target gas in the collision cell located in the second field-free region (2FFR). Charge-stripping peaks are obtained by scanning the electric sector voltage between values of 0.5E and 0.495E. The displacement of the peak from 0.5E reflects the translational energy loss in the charge-stripping process. The value of Qminis obtained from the high-energy side of the charge-stripping peak by extrapolation to the base line. Measurement of the peak onset in this way eliminates additional translational energy loss from any other process which may occur.' The process C7H8+. C,H:+ in toluene gives an intense and Well-defined charge-stripping signal. The value of Q= 15.7 eV9 for this process was used as a standard to calibrate the energy scale in all measurements. The experimental method used can be understood by reference to Figure 1. Toluene was first inserted into the ion source and the energy spectrum plotted for both the main beam of C7H8+.ions (A+) and the charge-stripped peak (A2+). The high-energy sides of both these peaks were extrapo-
-
(1)R. G. Cooks, T. Ast, and J. H. Beynon, Int. J. Mass Spectrom. Ion Phys., 11, 490 (1973). (2)T. Ast, C.J. Porter, C. J. Proctor, and J. H. Beynon, Bull. SOC. Chin., Beograd, 46, 135 (1981). (3)C.J. Proctor, C. J. Porter, T. Ast, P. D. Bolton, and J. H. Beynon, Org. Mass Spectrom., 16, 454 (1981). (4) T. Ast. C. J. Porter, C. J. Proctor, and J. H. Beynon, Chem. Phys. Lett., 78,439 (1981). ( 5 ) A. M. Hanner and T. F. Moran, Org. Mass Spectrom., 16, 512 (1981). ( 6 ) T.Ast, J. H. Beynon, and R. G. Cooks, J. Am. Chem. SOC.,94,6611 (1972). --,\--
+Present address: Baker Laboratory of Chemistry, Cornel1 University, Ithaca, New York, 14853. Permanent address: Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4/11, Belgrade, Yugoslavia.
*
0022-36541831 2087-3305$0 7.5010
(7)C.J. Proctor, C. J. Porter, T. Ast, and J. H. Beynon, Int. J.Mass Spectrom. Ion Phys., 41,251 (1982). (8)R.P. Morgan, J. H. Beynon, R. H. Bateman, and B. N. Green, Int. J.Mass Spectrum. Ion Phys., 28, 171 (1978). (9)F. H. Dorman and J. D. Morrison, J . Chem. Phys., 35,575 (1961).
0 1983 American Chemical Society
RabrenoviE et al.
3306 The Journal of Physical Chemistry, Voi. 87, No. 17, 1983 TABLE I: Values of Qmin (eV) from Charge-Stripping Experiments Ho
H,
H 2
H3
H4
HS
C,
22.0 (24.0a)
C;
20.3 18.8 18.0 17.0 16.4
22.8 20.4 19.9 18.0 17.8 16.7 16.4 15.4 15.7
19.6 20.5 17.1 17.8 16.2 16.2 15.0 14.8 14.7
18.9 20.1 18.9 17.5 16.9 15.4 15.2 15.6 14.1
17.9 17.8 17.3 16.5 15.6 15.0 14.7 14.4 13.9
19.lC 18.3 17.3 16.3 15.7 15.7 14.8 14.9
C,
c, Cs C6 C, C8
C, a
15.5
Value taken from ref 11. A*+
d495 E
For details see Table 111. A+
~
Ea' Illustratlon of the method used to determine Q,,,,,, from a chargastripping spectrum. For an explanation of the symbols used, see text. EAZ+
Flguro 1.
lated to the base line, as shown in Figure 1, to give energies E A + and EA%.A correction factor, 6 eV, was then calculated for the energy scale, according to the relationship 15.7 = E A + - ~ E A+z + 6 These measurements were then repeated on the ion of mass m for which the value of Qmh was required. Using the same method of measuring the high-energy sides of the main beam and charge-stripped peaks, we found values of E,+ and Emz+.Qminis then given by Qmin. = E,+ - 2Em2++ 6 Finally, toluene was inserted again and the value of 6 checked to ensure that it had not changed during the measurements. Values of 6 may change over a period of weeks depending on such factors as the state of cleanliness of the electric sector plates or the way in which the controls have been tuned. Normally, 6 lies within the range -2 to +2 V. By deliberately altering the tuning controls so that 6 assumes values as high as 7 eV, we have checked that, using the above method, the actual value of 6 does not affect the values of Qminfound. Generally, no correction for the widths of the main beam and charge-stripped peaks is necessary since they are constant for the various ions, the width of the chargestripped peak being some 20% greater than that of the main beam. A more broadened charge-stripped peak than this is evidence of the existence of electronically excited states of either m+ or m2+and has been used to detect such states in previous measurements. Standard experimental conditions involved bombardment with 70-eV electrons, a trap current of 100 PA, and an indicated sample pressure of -1 X lo4 torr. An accelerating voltage of 5 kV was used. Nitrogen collision gas was employed at a pressure of 5 X lo4 torr as indicated on an ion gauge located near the diffusion pump in the 2FFR. It is estimated that the actual pressure in the collision cell is higher than the indicated pressure by a factor of -IO3.
H6
H,
H8 c, C,
18.2
18.4
16.0 16.4 14.7 15.4 14.6
16.9 16.1 15.7 15.7 14.9
16.2 15.2 14.3-15.7' 15.5 15.0
c; c4 c, c, C, C8
c,
For details see Table V.
All spectra were recorded and stored on a PDP8e computer system interfaced to the spectrometer, which permits a peak to be signal averaged by successive scans.1° For charge-stripping experiments, a stable ion beam is essential, especially when signal averaging during long periods is required and this is achieved by the use of a "beamlocking" circuit.8 In favorable cases Q- can be measured to within f0.2 eV. More usually, a number of factors can combine to increase the error. These can include (a) small chargestripping cross sections for some ions resulting in poor singal-to-noise ratios and (b) collision-induced decomposition (CID) of the type
bll+
-
[m1/21+ + m,/2
(4)
which occur in addition to charge-stripping and tend to obscure the charge-stripping peak. In such circumstances, the Qminvalues are estimated to be, at worst, reliable to *0.5 eV. Interference from CID reactions of type 4 may sometimes be overcome by selecting ions containing a 13Cisotope for study. For example, if the ion ml+ has the composition c6+, then, if the ion [C, 13C]+is studied, the charge-stripped ion will still appear at an energy-to-charge ratio of about 0.5 but fragmentation according to eq 4 can only give ions to energy-to-charge ratio 36/73 or 37/13 and, thus, the interference is made less severe. However, in the case of hydrocarbons there is likely to be another ion present, namely, [C6H]+,and this cannot be separated from [C, 13C]+. This difficulty can be overcome by using fully halogenated compounds such as C6F6 to produce the C6+ ion. Results and Discussion In excess of 30 different precursor molecules (hydrocarbons and halogenated hydrocarbons) were examined in order to obtain Qminvalues for the [C,H,]+ ions listed in Table I. The Q- values span a wide range between 13.9 and 22.8 eV. These are, in all cases, higher than the corresponding single-ionization energies of the neutral hydrocarbon species. Each value of Q- given is the average value for at least three compounds (listed in Table 11) measured several times. Different precursor molecules were used in order to determine whether different isomeric structures of the singly charged ions might affect the measured value of Qmin. However, only in the case of C7H8+-ions were significantly different Qmin values obtained showing that isomeric ions corresponding to this formula were being formed. The ionization energies for C,H8 isomers are listed in Table 111. Values of Qminfor the ions increase in the sequence cycloheptatriene, bicyclo[2.2.11 hepta-2,5-diene, toluene. The ionization energies of the corresponding (10)P. D. Bolton, G. W. Trott, R. P. Morgan, A. G. Brenton, and J. H. Beynon, Int. J. Mass Spectrom. Ion Phys., 29, 179 (1979).
Charge Stripping of Hydrocarbon Positive Ions
The Journal of Physical Chemistry, Vol. 87,No. 17, 1983
3307
TABLE 11: Precursor Molecules Examined in Order To Obtain Qmin Values Listed in Table Ia Ho
C,
9, 10, 16, 18 10, 15, 21 9, 10, 1 5
9, 10, 16, 18, 21 9, 10, 1 5
H3 1 5-7,9 9-13, 15, 18,19 9, 16, 18, 21 9, 10, 1 5
C,
27
9, 10, 23
9, 10. 23
c, c,
9, 20, 21 26, 29
C,
HI 1, 2 5-7 9-13
1-3
c 2
C3 9-13 C,
C,
H2
1, 4 5-7, 9, 10 9-13
9, 10, 18
9, 10, 23
H4 1 5-8 9, 11-13, 18 9, 16, 20, 21 9, 10, 1 5 9, 10. 23
H,
9, 20, 21, 9, 20, 21, 9, 15, 29 26, 29 26, 29 26, 29-31, 26, 29, 3 1 15, 29-31, 34 35 15, 34, 35 15, 34, 35 15, 20, 34, 15, 34, 35 15, 34, 3 5 35 9, 20, 21, 26, 29 29-31
H6
5-8 9, 11, 1 2 16, 18, 1 9 9, 16, 18, 21 9, 15, 21, 22 9 , 1 0 , 21, 23, 25 9, 15, 21, 29, 30 29-31, 35 15, 20, 34, 35
H,
H,
9 , 1 1 , 12, 16-18
9, 11, 12, 14, 16, 18
9, 21, 22
19, 21, 23, 24 21, 23, 25, 26 9, 21, 22, 26, 28, 32, 3 3 15, 20, 25, 32 15, 20, 25 34,35
10, 21, 23 26 9, 15, 29, 31 20, 29, 31, 34 15, 34, 35
9, 23, 24 23, 26 9, 21, 22 15, 29, 3 1 15, 34, 35
(1)methane, ( 2 ) dibromomethane, ( 3 ) carbon tetrachloride, ( 4 ) isobutane, ( 5 ) ethane, ( 6 ) iodoethane, ( 7 ) bromoethane, (8) chloroethane, ( 9 ) toluene, (10) benzene, (11)propane, (12) 1-iodopropane, (13) 1-bromopropane, (14) l-chloropropane, (15) indene, (16) butane, (17) bromocyclopentane, (18) hexane, (19) limonene, (20) propylbenzene, (21) 1,3,5-~ycloheptatriene, (22) bicyclo[ 2.2.l]hepta-2,5-diene, (23) 1,5-cyclooctadiene, (24) Decalin (perhydronaphthalene), (25) cumene (isopropylbenzene), (26) ethylbenzene, (27) hexafluorobenzene, (28) 2-fluorotoluene, (29) p xylene, (30) m-xylene, (31) o-xylene, (32) butylbenzene, (33) pentylbenzene, (34) naphthalene, (35) methylnaphthalene. TABLE 111: Ionization Energies and Qmin Values of C,H, Isomers
TABLE IV: Ionization Energies and Qmin Values of Carbon Species neutral species
cycloheptatriene bicyclo[ 2.2.11hepta-2,5diene toluene
8.20
f
0.05
14.3 t 0.3
22.5
8.42
f
0.02
15.2
23.6
8.82
f
0.02
15.7'
t
0.2
24.5
Values taken from ref 11. = Qmin + IE(m+m+). Error limits are not given for toluene because that value has been used t o calibrate the energy scale throughout all measurements.
neutral compounds follow the same trend. Values of IE(m-+m+) were taken from the literature1' and doubleionization energies have been calculated from
+ Qmin
(5)
Except for the C7H8isomers, there does not seem to be a simple relationship between the IE(m-m+) and IE(m+-+ m2+)values. I t is interesting to note that differences observed in the ionization energies of C7H8+.ions are lost in C7H7+ions. As shown by the results in Tables I and 11, C7H7+ions from all seven different precursors studied gave the same value for IE(C7H7+-C7H72+-). Ionization energies and Qminvalues for carbon species containing three to six carbon atoms are given in Table IV. The energies required to remove the first electron from C3 and (26 molecules differ by only 0.1 eV while the energies required to remove a second electron from the corresponding singly charged ions show a difference of more than 3 eV. It seems, therefore, to be the case that single-ionization energies are not as sensitive to the size of the neutral species as are the ionization energies of the corresponding singly charged ions. This can be explained on the basis of electrostatic repulsion between the two positive charges: it becomes less and less difficult to ac~~~
~
(11)H. M.Rosenstock, K. Draxl, B. W. Steiner, and J. T. Herron, 'Energetics of Gaseous Ions", National Bureau of Standards, Washington, DC, 1977. (12)J. Drowart, R. P. Burns, G. DeMaria, and M. G. Inghram, J. Chem. Phys., 31, 1131 (1959).
12.6 f 0.6 12.6 12.5 f 1 12.5 f 0.3
Qmin,
eV
20.3 t 18.8 f 18.0 i 17.0 t
0.4 0.2 0.2 0.2
IE(m+m"), eV
32.9 31.4 CS 30.5 C6 29.5 Values obtained by using the electron-impact technique (ref 12). = Qmin + IE(m+m+). c3
c,
a
IE(m-+m2+)= IE(m+m+)
IE(m+m+),a eV
TABLE V: Appearance Energies and Qmin Values of C,H,+. Ions Formed from Ethane and Halogenated Ethane neutral AE( C,H, +),a sample fragment eV Qmin,b eV 19.1 t 0.4 C,H, H. 12.66 t 0.05 C,H,Cl C1. 11.83 i. 0.06 19.1 i 0.3 C,H,Br Br. 11.15 19.1 f 0.3 I. 11.0 i. 0.3 19.1 t 0.3 CAI Values taken from ref 11. = IE(C2H,++C,Hs2'~).
commodate the two charges in the same ion as the structure is extended in going from C3 to C6. It has been shown2that, in general, for polyatomic ions only the ground state of m+ is involved in the chargestripping process. Therefore, it is possible to obtain reliable values of IE(m-m2+) for C3to c6 molecules. In the case of the monatomic species C+., however, the experimental Qminvalue of 22.0 eV, given in Table I, is lower than the corresponding literature value of 24.0 eV." The reaction C+. + G C2++ G + e-, where G represents the neutral collision gas molecules, has been studied13and has shown the presence of a long-lived excited state of the C+. ions. Because of the existence of this excited state, the Qvalue for C+. ioiis is not included in Table IV. The charge-stripping reaction of C,+. ions could not be studied due to interference from the decomposition reaction C2+-
-
- c+.+ c.
Literature values of the appearance energies and Qmin values of C2H5+ions formed from ethane and halogenated ethanes are compared in Table V. (13)C. J. Porter, C. J. Proctor, T. Ast, and J. H. Beynon, Int. J . Mass Spectrom. Zon Phys., 41,265 (1982).
3308
RabrenoviE et al.
The Journal of Physical Chemistty, Vol. 87, No. 17, 1983 Qmin (eV)
Qmin (eV)
1
c2
201
I
0
,
I
I
1
2
3
4
,
I
5
6
7
8
HYDROGEN NUMBER
Figure 2. Dependence of Q,,, on the number of hydrogen atoms in odd-carbon-number ions.
Appearance energies are dependent on the strength of the bond being broken in order to obtain C2H5+ ions from the parent molecule. The Q- value for stable C2H5+ ions can be seen to be independent of the precursor molecule because it depends only on the structure of the C2H5+ ions themselves. Trends in the values of Qmin, listed in Table I, with respect to the number of carbon and hydrogen atoms, have been noted. The dependence of Q- values on the number of hydrogen atoms in odd-carbon-number ions (Figure 2) shows the existence of minima at even hydrogen numbers. This might suggest that the energy required to remove an unpaired electron from ions is less than that required to remove a paired electron. However, the same situation does not occur for ions of even carbon number with increasing number of hydrogen atoms (Figure 3). A single, sharp minimum is exhibited for each set of ions when four hydrogen atoms are present. It is interesting to compare this behavior with the abundances of doubly charged ions in the mass spectra of hydrocarbons. A number of studies have ~ h o w n ' ~ -that '~ C,H22+and C,H62+ ions are very stable doubly charged species and that these ions are prominent (often forming the base peaks) in the doubly charged ion mass spectra of all aromatic and aliphatic hydrocarbons. Ions C6HZ+and C7HZ+are also present in significant abundance in the mass spectra of higher alkanes." However, ions of the general composition C,H,2+ were of minor importance in all doubly charged ion mass spectra of hydrocarbons studied. The significance of the persistant minima for ions C,H42+ in Figures 2 and 3 still has to be rationalized. With the exception of C2H2+-,and C4H2+.,for a constant number of hydrogen atoms Q- is found to decrease as the number of carbon atoms increases (Figure 4). The experimental values for two series C,H2 and C,H4 (Figure (14) T. Ast, J. H. Beynon, and R. G. Cooks, Org. Muss Spectrom., 6, 749 - -- (1972). .-,-
I
1
,
I
I
I
I
I
I
0
1
2
3
4
5
6
7
8
HYDROGEN NUMBER Figure 3. Dependence of O M on the number of hydrogen atoms in even-carbon-number ions. Qmin (eV)
Omin ( e V )
221
2018-
'I
16
16-
1
2
3
4
5
6
7
8
9
1
2
CARBON NUMBER
1
2
3
L
5
6
CARBON NUMBER
3
4
5
6
7
8
9
7
8
9
CARBON NUMBER
7
8
9
1
2
3
L
5
6
CARBON NUMBER
Flgure 4. Dependence of O,,,,, on , the number of carbon atoms for ions containing (a) no hydrogen, (b) one hydrogen atom, (c) two hydrogen atoms, and (d) four hydrogen atoms: (-) experimental results and (- - -) values calculated by using the MIND013 method.
\--
(15) H. Sakurai, A. Tatematau, and H. Nakata, Bull. Chem. S O ~Jpn., . 49, 2800 (1976).
E. Bostwick, and T. F. Moran,
4, c and d) are compared with theoretical values obtained by using the MIND0/3 method.18 In each case, although
(17) B. E. Jones, L. E. Abbey, H. L. Chatham, A. W. Hanner, L. A. Teleshefsky, E. M. Burgess, and T. F. Moran, Org. Mass Spectrom., 17, 10 (1982).
(18) T. W. Bentley and C. A. Wellington, private communication, 1981.
(16) B. P. Mathur, E. H. Burgess, D. Org. Muss Spectrom., 16, 92 (1981).
Charge Stripping of Hydrocarbon Positive Ions (I1
lbl
The Journal of Physical Chemistry, Vol. 87,No. 17, 1983
3309
(Cl
7,
115
b 116
i
Oi97E
O'LBBE
bA€
I 4
O'WE
01500 E
Figure 5. Schematic representation of part of a MIKE spectrum around the electric sector voltage value of 0.5€, showlng chargestripping peaks for three ions of different Q~ values: (a) 15.0, (b) 17.5, and (c) 20.0 eV. Peak d represents the peak due to the fragmentation process m+ (m/2)+ m/2.
-
+
the measured Qminvalues are higher than the calculated values, the variations with increasing carbon number can be seen to be predictable. A method has been developed for plotting mass spectra which records all ions undergoing charge tripping.'^.^^ The high-energy ion-molecule reactions that occur when a collision gas is introduced into the analyzer region are of two types that lead to a halving of the kinetic-energyto-charge ratio of the reactant ions: (a) charge-stripping reactions (eq 1) in which the charge on the ions is doubled and (b) fragmentation reactions (eq 4) in which the mass of the ion is halved. If the slit following the electric sector is sufficiently wide, a mass spectrum representing the relative abundance of all the products of such reactions can be obtained by scanning the magnetic field after setting the electric sector voltage to approximately E/2 (where E is the normal value correspondingto tranmission of stable ions). These spectra are therefore termed E/2 mass spectra by analogy with the 2E mass spectra which record ions whose kinetic-energy-to-charge ratios are doubled in charge-exchange reactions.21 For reactions of type 1 (charge stripping), the observed peak occurs at an electric sector value (E - Qmin)/2. For reactions of type 4 (fragmentation), the observed peak occurs much closer to the value E/2, but the peak is much broader. In order to study reactions 1 with the minimum interference from reactions 4, the electric sector voltage should be set to (E - Q,3/2 and the exit slit closed. However, all ions do not have the same Qmh and, thus, in order to see all charge-stripped ions (for which Q- spreads over the range 13.9-22.8 eV with a mean value 18.3 eV) the electric sector is set to a value (E - 18.3)/2 and the exit slit opened to transmit a range of energies of (f4.5 eV)/2. For 5-keV ions, this corresponds to a setting at (5000 18.3)/2 = 0.4982E and a range of ion energies to be transmitted of f2.25 eV. Consider Figure 5, which shows schematically the shapes and positions of peaks in a MIKE spectrum coming from three different ions for which Qmh values are (a) 15.0, (b) 17.5, and (c) 20.0 eV. With the electric sector set to the calculated "optimum" value of 0.4982E, major parts of peaks b and c will be transmitted, but part of the peak a will be lost. With the setting at a higher value, e.g., 49903, peak c will be completely lost and (19)R. G. Cooks, J. H. Beynon, R. M. Caprioli, and G . R. Lester, 'Metastable Ions", Elsevier, Amsterdam, 1973,p 139. (20) D. L. Kemp, J. H. Beynon, and R. G . Cooks, Org. Mass Spectrom., 11, 867 (1976). (21)J. H.Beynon, A. Mathias, and A. E. Williams, Org. Mass Spectrom., 5, 303 (1971).
A 20
40
4-
60
80
100
120
mfe
115
a 116
74
63
20
40
60
1 80
100
120
d e
Figure 6. Mass spectra of indene obtained by scanning the magnet and setting the ESA at (a) 0.4982E and (b) 0.4985E.
peak b severely truncated. This shows that there is not a single optimum value of electric sector setting which would be appropriate for all compounds; even for a single compound, some discrimination against certain peaks is to be expected at any setting. Therefore, in reporting E/2 mass spectra, it is always essential to specify the ESA setting as well as the range of energies transmitted, if the results are to be meaningful and comparable. As an example, the charge-stripping mass spectrum of indene is plotted at two values of the electric sector setting. Figure 6b shows the spectrum obtained with an electric sector voltage set at 0.4985E, which correspondsto the Q- value of -15 eV. The ions with lowest ionization energies are favored (the highest masses). Figure 6a illustrates the spectrum obtained with the electric sector voltage set at 0.7982E (Qminvalue of 18.3 eV). The differences in two spectra are quite marked. The large span of IE(m+-.m2+) values for the various ions formed from a given compound will also affect the appearance of doubly charged ion mass spectra plotted by the charge-exchange method. There, we follow the process
+
m2+ N
-
m+ + N+
(6)
where N represents an atom or molecule of the neutral collision gas. By setting the electric sector voltage to the
J. Phys. Chem. 1983,87,3310-3319
3310
value of 2E, and by scanning the magnetic sector current, one obtains a mass spectrum reflecting all doubly charged ions formed in the ion source which have undergone reaction 6 and become singly charged ions.17 For reaction 6 to proceed from left to right, the collision gas has to be ionized; therefore, the recombination energy of m2+has to be equal to or greater than the ionization energy of N. The recombination energy of m2+is the same quantity Qminwhich was obtained from the charge-stripping process described in this study. In order for all m2+ ions to be recorded, their Qmin values should exceed the value IE(N-N+); it follows that the best collision gas in this respect will be the one with lowest ionization energy. The use of He (IE = 24.48 eV) or Ne = 21.56 eV) as collision gases will result in heavily distorted 2E mass spectra. On the other hand, nitrogen (IE = 15.58 eV) is a fairly good choice, although some discrimination against m2+ions with smallest Q- values (usually the molecule ion region) might be encountered. A procedure adopted by some authors whereby they employ the same compound as the sample and collision gas seems to be a good practice in this respect. Conclusions Charge stripping of hydrocarbon gaseous ions, using nitrogen collision gas, was used to measure minimum energy needed to ionize the ions in a vertical transition. Ions from C1 to C9 possessing between zero and eight hydrogen
atoms have been studied. The dependence of Qminupon precursor ion structure has been observed for C7H8+isomeric ions. Trends in the value of Qminwith respect to the numbers of carbon and hydrogen atoms have been noted. Peak abundances in E / 2 spectra and 2E spectra were related to Qmin values. Registry No. Methane cation radical, 20741-88-2; dibromomethane cation radical, 69423-11-6;carbon tetrachloridecation radical, 69050-45-9;isobutane cation radical, 34479-71-5;ethane cation radical, 34488-65-8;iodomethane cation radical, 12538-72-6; bromoethane cation radical, 84893-53-8; chloroethane cation radical, 56339-90-3;toluene cation radical, 34504-47-7;benzene cation radical, 34504-50-2;propane cation radical, 34479-70-4; 1-iodopropanecation radical, 79240-91-8;1-bromopropanecation radical, 34475-58-6;1-chloropropanecation radical, 86101-40-8; indene cation radical, 42949-13-3;butane cation radical, 3447972-6; bromccyclopentane cation radical, 86101-41-9;hexane cation radical, 34478-20-1; limonene cation radical, 83861-57-8;propylbenzene cation radical, 53649-54-0;1,3,5cycloheptatrienecation radical, 34488-67-0;bicyclo[2.2.11 hepta-2J-diene cation radical, 41153-22-4;1,5-cyclooctadienecation radical, 84847-91-6;decalin cation radical, 86161-25-3;cumene cation radical, 68199-09-7; ethylbenzenecation radcial, 39600-67-4;hexafluorobenzenecation radical, 34528-23-9; 2-fluorotoluene cation radical, 58436-60-5; p-xylene cation radical, 34510-22-0; m-xylene cation radical, 34510-21-9; o-xylene cation radical, 34510-20-8; butylbenzene cation radical, 53649-55-1;pentylbenzene cation radical, 7924597-9;naphthalene cation radical, 34512-27-1;methylnaphthalene cation radical, 86161-26-4.
Electron Spin Resonance Study of Conformational and Dynamic Characteristics of Molecules Trapped within the Channels of Thiourea-Cyclohexane Inclusion Compounds Eva Melrovltch Isotope Department, The Weizrnann Institute of Science, 76 100 Rehovot, Israel (Received:April 1, 1982; In Final Form: January 19, 1983)
ESR spectra of spin-probe-doped thiourea-cyclohexane channel-type inclusion compounds were recorded as a function of temperature and orientation in the magnetic field. Large and nearly cylindrically shaped spin probes, such as stearamide (4-(octadecanoylamino)-2,2,6,6-tetramethylpiperidinyl-l-oxy) and cholestane (4',4'-dimethylspuo[3a,5cr-cholestane-3,2'-oxazolidin]-3'-yloxy),are highly ordered within the channels. Stearamide was found to be immobile up to 80 "C (above which decomposition occurs) whereas cholestane reorients at a rate of 3.4 X lo7s-l at 80 "C which drops to lo5 s-l upon lowering the temperature to -58 "C, with an activation energy of 6.7 kcal/mol. A second conformer, with magnetic parameters different from those of the common species but with similar orientation of the N-0 bond, was detected at approximately -40 "C. We associate it tentatively with oxmlidine ring pucker. Smaller and nearly spherically symmetric spin probes such as Tempyo (2,2,5,5-tetramethyl-3-cbamidopyrrolidinyl1-oxy)and Tan01 (4-hydroxy-2,2,6,6-tetramethylpiperidinyl1-oxy) are also entrappable within the inclusion channels wherein they reorient isotropically. A motionally narrowed triplet with unusually large isotropic hyperfiie constants and very narrow lines is observed at room temperature, suggesting conformational changes imposed by the restrictive surrounding.
I. Introduction Inclusion compounds are formed as a result of nonchemical-type interactions between the guest and the host molecules, determined mainly by structural and physical characteristics such as size, shape, conformation, polarity, as well as by motional properties of both guest and h0st.l These materials are used to store oxidizable, flammable, (1)(a) S. G. Frank, J. Pharmacol. Sci., 64, 1585 (1975);(b) D. D. McNicol, J. J. McKendrick, and D. R. Wilson, Chem. SOC. Reu., 7, 65 (1978).
volatile, and poisonous guests., to separate steric conformers according to their ability to become trapped within cavities or channels, to perform chemical reactions such as polymerization processes, etc. Besides their use in industry, inclusion compounds are very rewarding in pure research as well, serving as models for stereospecific and regiospecific reactions, for specific enzyme-substrate interactions, for locally ordered media such as liquid crystals, etc. While structural properties have been studied by a variety of physical methods and related to the above-mentioned functions, the dynamic characteristics have been investi-
0022-3654/83/2087-3310$01.50/0@ 1983 American Chemical Society
