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Theoretical study of electron impact mass spectrometry. I. Ab initio MO study of the fragmentation of n-butane
International
JournaC
of Mass
Spectrometry
and Ion
Physics,
52 (1983)
139-
139.
148
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THEORETICAL STUDY OF ELECTRON IMPACT MASS SPECTROh$ETRY. I. Ab initio MO STUDY OF THE FRAGMENTATION OF n-BUTANE
TAKAE
TAKEUCHI
Department Nara
and MASAO
of Chemistry,
YAMAMOTO
Faculty of Science, Nara
Women’s
University,
Kitauoyanishi
- machi,
630 (Japan)
KICHISUKE
NISHIMOTO
and HIDETSUGU
TANAKA
Department of Chemistry, Faculty of Science, Osaka City University, Sumiyoshi- ku, Osaka 558
(Japan) KOZO
HIROTA
I - iO- 13, Sakushindai,
Chiba
281 (Japan)
(Received 15 March 1982)
ABSTRACT The fragmentation mechanism of n-butane by low-energy electron bombardment has been studied by means of the ab initio MO method. Optimized geometries of possible n-butane cation conformers, reaction intermediates and fragments have been calculated using the energy gradient technique. The results suggest that the fragmentation to C, + C, is more favorable than that to C, C C,, when the electron impact energy is at most only a few eV above the ionization threshold. The base peak at m/z 43 has been calculated to be due to the 2-propyl cation. In the course of fragmentation to C, + C, proton tunneling is expected.
INTRODUCTION
Generally, mass spectra of organic compounds by electron impact are quantitatively different according to the ionizing voltage. Theoretical analysis of the fragmentation has been done by the following methods. Rosenstock et al. [l] proposed the quasi-equilibrium theory (QET) for the interpretation of mass spectra. QET can explain the energy dependence of mass spectra and is recognized as being useful for the investigation of fragmentation mechanisms at low-energy electron impact (5 25 ev). Thompson [2] adopted MO theory based on the equivalent MO method for the fragmentation of n-octane. Fueki et al. [3) applied Thompson’s idea to some organic compounds. 0020-7381/83,‘$03.00
0 1983 Elsevier Science Publishers B.V.
140
There is another approach for the study of chemical reaction, in which the potential curve is calculated and possible reaction mechanisms are discussed. This approach provides a clear chemical picture. The present study is based on the MO calculation for potential curves of molecular cation species produced by electron bombardment. Fragmentation at low-energy bombardment proceeds via a minimum energy path with a small or no barrier. Recent rapid progress in computer technology makes it possible to predict quantitatively the geometry of medium-sized molecular species. Usually the lifetime of a molecular cation is very short so that the experimental determination of its structure is practically impossible. Accordingly, the ab initio MO method is valuable in obtaining the geometry of a molecular cation or its geometrical deformation. In the present study, the fragmentation of n-butane is considered, which is the simplest hydrocarbon including two different kinds of C-C bonds. Recently, detailed experimental work on the fragmentation of n-butane has been carried out [4-73. Sunner and Szabo [8] analyzed charge-transfer mass spectra of butane by QET calculations. For comparison, we have measured the mass spectra and total-ion abundance in detail. CALCULATION
Optimized geometries of possible n-butane cation conformers, reaction intermediates and fragments have been calculated with the STO-3G minimal basis set using the energy gradient technique. The energies have been calculated by the 4-3 1G basis set using geometries from the STO-3G method. Potential
curve
In the calculation of the potential curve, the following assumptions have been made. The potential curve is drawn against the C-C bond distance since it is scission of this bond that is of interest here. In the initial stage of reaction, the change in C-C bond distance is very small, so the other parameters of the geometry such as C-H bond distances and the remaining C-C bond distances and bond angles are assumed to be unchanged. When the C-C bond distance becomes larger than 2.0 A, the geometry is optimized with the STO-3G minimal basis set at a given C-C bond distance. For comparison, the energy has also been calculated with the 4-3 1G basis set.
141
The fragmentation process can be described as follows M : M+‘* (1)
(2)
M+’
(1) Ionization
A++
B - (2) Fragmentation
where M+‘* is the molecular cation radical at the Franck-Condon state. In the present study, the following four fragmentation schemes have been assumed +. --I+ .CH3 I
Scheme
n-C,H,,
-
CHICH&H2----
CH3
-
CH&H,CH,+ I-2
I-1
Scheme
II
n-C4Hx)
+*
CHsCH2----CHgH3
-
II -
SC
he
m’e
Ill
“-c&plo
+-
1’. _
n-
C4H,;
CHqc&r---CH;l+*
-
,CH2CHx
II -2
II -3
&+-CH2 l
aCH3
2 111- 1
IV’
+
lc,/
‘Cd
Scheme
l-3
CH3CH2+
1
CH,----CH2----CH;1+.
-
+
Ill’-1
III-3
Ill -2
‘T;dcu3
III’ -2
+
(=l-3)
.CH3
,,f’-3
(= l-3)
The number of carbon atoms in n-butane used here is as follows
H-
Ei’ C (l>-
B C (2)-
B C (3)-
P C (4)-H
A
A
IL
A
Calculations were carried out using the IMSPAK
program.
EXPERIMENTAL
The mass spectra of n-butane were measured precisely at an ionizing voltage (Ve) of 8-25 V with an interval voltage of 1 V. A Hitachi RMU-6M mass spectrometer was used. Total-ion abundance was measured by a total ion monitor. RESULTS
AND
DISCUSSION
Mass spectra
The total-ion ionization efficiency curve and mass spectra of n-butane are shown in Fig. 1. Numerical data are given in Table 1. It can be seen that the
142
i0
i0
6’0
m/z
Ve /
V
Fig. 1. Total-ion ionization efficiency curve and mass spectra of n-butane.
TABLE
1
Mass spectra (2%)
m/z
and relative total ionization (Xi) of n-butane Ionizing voItage (V) 10
27 28 29 30 39 40 41 42 43 44 55 56 57 58 59
2.6 0.7
11.2 50.2 2.1 2.8 1.7 27.4 1.4 100.1
total Zi a (lO-lo
A)
0.030
15
20
25
3.1 8.4 10.7 0.2 0.2 0.2 8.2 6.9 46.6 1.6 0.7 1.8 2.7 8.2 0.5
5.4 9.5 12.7 0.3 1.3 0.3 9.8 6.0 42.0 1.6 0.6 I.4 2.0 7.1 0.2
loo.0
100.0
100.2
1.1
3.0
4.1
1.0 7.7 6.3 0.2
4.8 7.7 55.3 2.3 0.2 1.5 2.1 IO.5 0.4
a Nominal current from the total ion monitor.
143
ion at m/z
43 is produced in the greatest abundance, especially at lower
voltage. Total energy of each species The calculated total energies of reactants, reaction intermediates and fragments. are summarized in Table 2, where some are compared with the values obtained by Hehre [9]. Geometries The calculated geometries of the reaction intermediates and fragments are shown in Fig. 2. For comparison, the optimized structures of trans and gauche forms of n-butane are also shown Characteristic features of geometries of assumed reaction intkmediates are as follows. (al Geometry of I-l. When the C(3)-C(4) bond distance is lengthened by 0.2 A, the optimized geometry of I-l is obtained. It is a characteristic feature of the geometry of the intermediate I-l that only the C(3)-C(4)-I-I and H-C(4)-H bond angles and the C(3)-C(4) bond length vary, the other bond angles and lengths remaining almost unchanged. (b) Geometry of II-l. Lengthening the C(2)-C(3) bond distance by 0.1 A, the optimized geometry of II-1 is obtained. Lengthening the C(3)-C(4) bond distance by Oil A gave a rather curious optimized geometry which is the same as that of II-l, that is, the C(2)-C(3) bond has been lengthened greatly. (c) Geometries of III-1 and III’- 1. Calculation shows that 111-l and III’- 1 are not intermediates but transient species. Therefore, geometry optimization was not done. Possible reaction paths The results in Table 1 show that at a bombardment voltage of 12 V, the ion at m/z 43 (C,Hq) forms the base peak, that is, C(3)-C(4) or C(l)-C(2) bond cleavage occurs overwhelmingly. Energy variations due to C-C bond lengthening have been calculated for Schemes I and II. They are shown in Fig. 3. From this figure, the course-of reaction in each scheme is expected to be as follows. Scheme I (simple bond scission). When the C(3)-C(4) bond distance is greatly lengthened by electron bombardment, C,HT + - CH, fragmentation occurs via the reaction intermediate I-1. Fragmentation to C,H, - + CHZ is unfavorable (see Table 2). At low-energy bombardment this scheme produces molecular cation I- 1. Scheme II (simple bond scission). When the C(2)-C(3) bond distance is
144
155.46669 155.46483 155.09273 155.08627 155.12712 155.14032 115.99597 - 39.07700 - 38.77948 - 116.24288 - 77.40806 - 77.66299 - 116.02766 - 115.99131
-
- 116.05031 - 116.01389
- 7746353
- 38.83894
- 116.00294
- 155.48306 - 155.47079
157.07131 157.0695 1 156.67452 156.66946 156.71258 156.74111 117.18151 - 39.49989 -39.17120 - 117.46251 -78.19491 - 78.48527 - 117.20887 - 117.I8114
-
- 116.02765 - 115.99130
- 77.40806
- 38.77948
STO-3G
4-31G
STO-3G
DECI a
Hehre [9]
Present work
Total energy (a.u.)
- 117.20864 - 117.18109
- 78.19852 b
-39.17512 b
4-31G
a DECI indicates the CI calculation including the doubly excited configurations from 5 HOMO’s to 5 LUMO’s, which are constructed from the MO’s calculated by the STO-3G minimal basis set. Energy has been calculated at optimum STO-3G geometry. b The geometry was optimized with 4-31G.
n-ButaneQrans) n-Butanqgauche) n-Butane cation(trans) n-Butane cation(gauche) I-l II- 1 I-2 I-3 CH,+ CH,CH,CH,II-2 II-3 III-2 III-2
Species
Calculated total energies of reactants, reaction intermediates, and fragments
TABLE 2
E
146
(a) STO-3G -155.07. ii \ L509t w
.
iii b + -t55.11-
1 1.5 , * 1.6
1.7 . 1.8 - 1.9 - 2.0 ' 2.1 - 2.2
co
R(C-C) / A
1,_,*1 1.5
rl
1.6
1.7 '1.9 * 1.9
2.0' 2.1 -2.2
m
R(C-C) / d Fig. 3. Calculated potential curves for Schemes I and II (see text); (a), STO-3G; (b), 4-31G. R(C-C) indicates the C(3)-c(4) bond distance for Scheme I and the C(2)-C(3) bond distance for Scheme II. (O), Scheme I; (I), Scheme II; (), truns; ( - - - - -->, gauche.
lengthened or the C(3)-C(4) bond distance is slightly lengthened by electron bombardment, C, H&’ gr*ves the stable molecular cation II-l. Therefore,. during low-energy bombardment the fragmentation pattern of Scheme II hardly occurs. In this scheme the molecular cation II-I is produced. To explain C, + C, fragmentation at low energy, a reaction scheme which is more favorable than Scheme II must be considered. Schemes III and III’ (fragmentation with bending-mode control). Since C, + C, fragmentation occurs overwhelmingly and the 2-propyl cation has been calculated to be the most stable C,HT species, it may be expected that the reaction ‘will proceed via three-membered ring formation resulting 2,3H-shift. There are two possibilities depicted in Schemes III and III’, respecl**
-
CHJCH2CH2CJ +--
+
H /=qcH 3
+ 3
H (PI
.‘=‘-‘a
147
tively. However, preliminary calculations for these schenies show that the intermediate III’-1 is more stable than III- 1. Therefore, Scheme III’ was examined, Scheme III being responsible_ for CH, + C,Hz’ fragmentation. First, the energy change was calculated starting from the molecular cation ((R) -+ (T), see diagram). This produced the curve seen to the left in Fig; 4. Next, the energy change was calculated starting from the isolated (CH,),CH + - CH, system (P) ((P) -+ (T), see diagram). For each R(C(3)-C(4)), the geometry has been optimized. This produced the curve seen to the right in Fig. 4. It can be seen that there is a rather high energy barrier. To the left of the barrier, the geometry is that+of CH,CI-12~H2- - -CH,, whilst to the right of this barrier, it is that of CH,CHCH,- -CH,. Therefore, proton tunneling at this energy barrier is to be expected. Proton tunneling has a large isotope effect. The mass spectra of deuterium-labeled n-butane using 80 V electrons were reported [ 101,but those at low-energy electron impac’t have not yet been reported. Mass spectrometry ‘of deuterium-labeled n-butane at low-energy bombardment is in progress.
R(C(31-C(4))
/’
6
&156.68k 15 % z +
-156.70-
.,:5.
.
y2:o’
.
.
.2:5.
.
R(C(3)-C.(L) Fig. 4. Calculated potential (-----), gauche.
-3:o‘
1 1
.
.
3f5.
.
-
ilof
bp
d
curve for Scheme III’;
{a), STO-3G;
(b), 4-31G.
(-
), trans;
I48 ACKNOWLEDGMENTS
The authors thank the Computer Center of the Institute for Molecular Science, for the use of the HITAC M-200H Computer and the Library Program IMSPAK written by Prof. K. Morokuma and co-workers (IMS) and would like to express their gratitude to the Data Processing Center of Kyoto University, for its generous permission to use the FACOM M200 Computer_ REFERENCES I H.M. Rosenstock, M.B. Wallenstein, AL. Wahrhaftig and H. Eyring, Proc. Natl. Acad. Sci. U.S.A., 38 (1952) 667. 2 R. Thompson, Conf. on Mass Spectrometry, Institute of Petroleum, London, 1953, p. 154. 3 (a) K. Fueki and K. Hirota, Nippon Kagaku Zasshi, 80 (1959) 1202; (b) K. Fueki, J. Phys. Chem., 68 (1964) 2656; (c) M. Hatada and K. Hirota, Z. Phys. Chem., N-F., 44 ( 1965) 328. 4 G.D. Flesch and H.J. Svec, J. Chem. Sot., Faraday Trans. 2, 69 (1973) I 187. 5 P. Wolkoff and J.L. Holmes, J. Am. Chem. Sot., 100 (1978) 7346. 6 J-L. Holmes and P. Wolkoff, J. Chem. Sot., Chem. Commun., (1979) 544. 7 A. Lavanchy, R. Houriet and T. GBumann, Org. Mass Spectrom., 14 (1979) 79. 8 J. Sunner and 1. Szabo, Int. J. Mass Spectrom. Ion Phys., 25 (1977) 241. 9 W.J. Hehre, in H.F. Schaefer III (Ed.), Application of Electronic Structure Theory, Modem Theoretical Chemistry, Vol. 4, Plenum Press, London, 1977, chapt. 7. 10 W.H. McFadden and A.L. Wahrhaftig, J. Am. Chern. Sot., 78 (1956) 1572.
JournaC
of Mass
Spectrometry
and Ion
Physics,
52 (1983)
139-
139.
148
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THEORETICAL STUDY OF ELECTRON IMPACT MASS SPECTROh$ETRY. I. Ab initio MO STUDY OF THE FRAGMENTATION OF n-BUTANE
TAKAE
TAKEUCHI
Department Nara
and MASAO
of Chemistry,
YAMAMOTO
Faculty of Science, Nara
Women’s
University,
Kitauoyanishi
- machi,
630 (Japan)
KICHISUKE
NISHIMOTO
and HIDETSUGU
TANAKA
Department of Chemistry, Faculty of Science, Osaka City University, Sumiyoshi- ku, Osaka 558
(Japan) KOZO
HIROTA
I - iO- 13, Sakushindai,
Chiba
281 (Japan)
(Received 15 March 1982)
ABSTRACT The fragmentation mechanism of n-butane by low-energy electron bombardment has been studied by means of the ab initio MO method. Optimized geometries of possible n-butane cation conformers, reaction intermediates and fragments have been calculated using the energy gradient technique. The results suggest that the fragmentation to C, + C, is more favorable than that to C, C C,, when the electron impact energy is at most only a few eV above the ionization threshold. The base peak at m/z 43 has been calculated to be due to the 2-propyl cation. In the course of fragmentation to C, + C, proton tunneling is expected.
INTRODUCTION
Generally, mass spectra of organic compounds by electron impact are quantitatively different according to the ionizing voltage. Theoretical analysis of the fragmentation has been done by the following methods. Rosenstock et al. [l] proposed the quasi-equilibrium theory (QET) for the interpretation of mass spectra. QET can explain the energy dependence of mass spectra and is recognized as being useful for the investigation of fragmentation mechanisms at low-energy electron impact (5 25 ev). Thompson [2] adopted MO theory based on the equivalent MO method for the fragmentation of n-octane. Fueki et al. [3) applied Thompson’s idea to some organic compounds. 0020-7381/83,‘$03.00
0 1983 Elsevier Science Publishers B.V.
140
There is another approach for the study of chemical reaction, in which the potential curve is calculated and possible reaction mechanisms are discussed. This approach provides a clear chemical picture. The present study is based on the MO calculation for potential curves of molecular cation species produced by electron bombardment. Fragmentation at low-energy bombardment proceeds via a minimum energy path with a small or no barrier. Recent rapid progress in computer technology makes it possible to predict quantitatively the geometry of medium-sized molecular species. Usually the lifetime of a molecular cation is very short so that the experimental determination of its structure is practically impossible. Accordingly, the ab initio MO method is valuable in obtaining the geometry of a molecular cation or its geometrical deformation. In the present study, the fragmentation of n-butane is considered, which is the simplest hydrocarbon including two different kinds of C-C bonds. Recently, detailed experimental work on the fragmentation of n-butane has been carried out [4-73. Sunner and Szabo [8] analyzed charge-transfer mass spectra of butane by QET calculations. For comparison, we have measured the mass spectra and total-ion abundance in detail. CALCULATION
Optimized geometries of possible n-butane cation conformers, reaction intermediates and fragments have been calculated with the STO-3G minimal basis set using the energy gradient technique. The energies have been calculated by the 4-3 1G basis set using geometries from the STO-3G method. Potential
curve
In the calculation of the potential curve, the following assumptions have been made. The potential curve is drawn against the C-C bond distance since it is scission of this bond that is of interest here. In the initial stage of reaction, the change in C-C bond distance is very small, so the other parameters of the geometry such as C-H bond distances and the remaining C-C bond distances and bond angles are assumed to be unchanged. When the C-C bond distance becomes larger than 2.0 A, the geometry is optimized with the STO-3G minimal basis set at a given C-C bond distance. For comparison, the energy has also been calculated with the 4-3 1G basis set.
141
The fragmentation process can be described as follows M : M+‘* (1)
(2)
M+’
(1) Ionization
A++
B - (2) Fragmentation
where M+‘* is the molecular cation radical at the Franck-Condon state. In the present study, the following four fragmentation schemes have been assumed +. --I+ .CH3 I
Scheme
n-C,H,,
-
CHICH&H2----
CH3
-
CH&H,CH,+ I-2
I-1
Scheme
II
n-C4Hx)
+*
CHsCH2----CHgH3
-
II -
SC
he
m’e
Ill
“-c&plo
+-
1’. _
n-
C4H,;
CHqc&r---CH;l+*
-
,CH2CHx
II -2
II -3
&+-CH2 l
aCH3
2 111- 1
IV’
+
lc,/
‘Cd
Scheme
l-3
CH3CH2+
1
CH,----CH2----CH;1+.
-
+
Ill’-1
III-3
Ill -2
‘T;dcu3
III’ -2
+
(=l-3)
.CH3
,,f’-3
(= l-3)
The number of carbon atoms in n-butane used here is as follows
H-
Ei’ C (l>-
B C (2)-
B C (3)-
P C (4)-H
A
A
IL
A
Calculations were carried out using the IMSPAK
program.
EXPERIMENTAL
The mass spectra of n-butane were measured precisely at an ionizing voltage (Ve) of 8-25 V with an interval voltage of 1 V. A Hitachi RMU-6M mass spectrometer was used. Total-ion abundance was measured by a total ion monitor. RESULTS
AND
DISCUSSION
Mass spectra
The total-ion ionization efficiency curve and mass spectra of n-butane are shown in Fig. 1. Numerical data are given in Table 1. It can be seen that the
142
i0
i0
6’0
m/z
Ve /
V
Fig. 1. Total-ion ionization efficiency curve and mass spectra of n-butane.
TABLE
1
Mass spectra (2%)
m/z
and relative total ionization (Xi) of n-butane Ionizing voItage (V) 10
27 28 29 30 39 40 41 42 43 44 55 56 57 58 59
2.6 0.7
11.2 50.2 2.1 2.8 1.7 27.4 1.4 100.1
total Zi a (lO-lo
A)
0.030
15
20
25
3.1 8.4 10.7 0.2 0.2 0.2 8.2 6.9 46.6 1.6 0.7 1.8 2.7 8.2 0.5
5.4 9.5 12.7 0.3 1.3 0.3 9.8 6.0 42.0 1.6 0.6 I.4 2.0 7.1 0.2
loo.0
100.0
100.2
1.1
3.0
4.1
1.0 7.7 6.3 0.2
4.8 7.7 55.3 2.3 0.2 1.5 2.1 IO.5 0.4
a Nominal current from the total ion monitor.
143
ion at m/z
43 is produced in the greatest abundance, especially at lower
voltage. Total energy of each species The calculated total energies of reactants, reaction intermediates and fragments. are summarized in Table 2, where some are compared with the values obtained by Hehre [9]. Geometries The calculated geometries of the reaction intermediates and fragments are shown in Fig. 2. For comparison, the optimized structures of trans and gauche forms of n-butane are also shown Characteristic features of geometries of assumed reaction intkmediates are as follows. (al Geometry of I-l. When the C(3)-C(4) bond distance is lengthened by 0.2 A, the optimized geometry of I-l is obtained. It is a characteristic feature of the geometry of the intermediate I-l that only the C(3)-C(4)-I-I and H-C(4)-H bond angles and the C(3)-C(4) bond length vary, the other bond angles and lengths remaining almost unchanged. (b) Geometry of II-l. Lengthening the C(2)-C(3) bond distance by 0.1 A, the optimized geometry of II-1 is obtained. Lengthening the C(3)-C(4) bond distance by Oil A gave a rather curious optimized geometry which is the same as that of II-l, that is, the C(2)-C(3) bond has been lengthened greatly. (c) Geometries of III-1 and III’- 1. Calculation shows that 111-l and III’- 1 are not intermediates but transient species. Therefore, geometry optimization was not done. Possible reaction paths The results in Table 1 show that at a bombardment voltage of 12 V, the ion at m/z 43 (C,Hq) forms the base peak, that is, C(3)-C(4) or C(l)-C(2) bond cleavage occurs overwhelmingly. Energy variations due to C-C bond lengthening have been calculated for Schemes I and II. They are shown in Fig. 3. From this figure, the course-of reaction in each scheme is expected to be as follows. Scheme I (simple bond scission). When the C(3)-C(4) bond distance is greatly lengthened by electron bombardment, C,HT + - CH, fragmentation occurs via the reaction intermediate I-1. Fragmentation to C,H, - + CHZ is unfavorable (see Table 2). At low-energy bombardment this scheme produces molecular cation I- 1. Scheme II (simple bond scission). When the C(2)-C(3) bond distance is
144
155.46669 155.46483 155.09273 155.08627 155.12712 155.14032 115.99597 - 39.07700 - 38.77948 - 116.24288 - 77.40806 - 77.66299 - 116.02766 - 115.99131
-
- 116.05031 - 116.01389
- 7746353
- 38.83894
- 116.00294
- 155.48306 - 155.47079
157.07131 157.0695 1 156.67452 156.66946 156.71258 156.74111 117.18151 - 39.49989 -39.17120 - 117.46251 -78.19491 - 78.48527 - 117.20887 - 117.I8114
-
- 116.02765 - 115.99130
- 77.40806
- 38.77948
STO-3G
4-31G
STO-3G
DECI a
Hehre [9]
Present work
Total energy (a.u.)
- 117.20864 - 117.18109
- 78.19852 b
-39.17512 b
4-31G
a DECI indicates the CI calculation including the doubly excited configurations from 5 HOMO’s to 5 LUMO’s, which are constructed from the MO’s calculated by the STO-3G minimal basis set. Energy has been calculated at optimum STO-3G geometry. b The geometry was optimized with 4-31G.
n-ButaneQrans) n-Butanqgauche) n-Butane cation(trans) n-Butane cation(gauche) I-l II- 1 I-2 I-3 CH,+ CH,CH,CH,II-2 II-3 III-2 III-2
Species
Calculated total energies of reactants, reaction intermediates, and fragments
TABLE 2
E
146
(a) STO-3G -155.07. ii \ L509t w
.
iii b + -t55.11-
1 1.5 , * 1.6
1.7 . 1.8 - 1.9 - 2.0 ' 2.1 - 2.2
co
R(C-C) / A
1,_,*1 1.5
rl
1.6
1.7 '1.9 * 1.9
2.0' 2.1 -2.2
m
R(C-C) / d Fig. 3. Calculated potential curves for Schemes I and II (see text); (a), STO-3G; (b), 4-31G. R(C-C) indicates the C(3)-c(4) bond distance for Scheme I and the C(2)-C(3) bond distance for Scheme II. (O), Scheme I; (I), Scheme II; (), truns; ( - - - - -->, gauche.
lengthened or the C(3)-C(4) bond distance is slightly lengthened by electron bombardment, C, H&’ gr*ves the stable molecular cation II-l. Therefore,. during low-energy bombardment the fragmentation pattern of Scheme II hardly occurs. In this scheme the molecular cation II-I is produced. To explain C, + C, fragmentation at low energy, a reaction scheme which is more favorable than Scheme II must be considered. Schemes III and III’ (fragmentation with bending-mode control). Since C, + C, fragmentation occurs overwhelmingly and the 2-propyl cation has been calculated to be the most stable C,HT species, it may be expected that the reaction ‘will proceed via three-membered ring formation resulting 2,3H-shift. There are two possibilities depicted in Schemes III and III’, respecl**
-
CHJCH2CH2CJ +--
+
H /=qcH 3
+ 3
H (PI
.‘=‘-‘a
147
tively. However, preliminary calculations for these schenies show that the intermediate III’-1 is more stable than III- 1. Therefore, Scheme III’ was examined, Scheme III being responsible_ for CH, + C,Hz’ fragmentation. First, the energy change was calculated starting from the molecular cation ((R) -+ (T), see diagram). This produced the curve seen to the left in Fig; 4. Next, the energy change was calculated starting from the isolated (CH,),CH + - CH, system (P) ((P) -+ (T), see diagram). For each R(C(3)-C(4)), the geometry has been optimized. This produced the curve seen to the right in Fig. 4. It can be seen that there is a rather high energy barrier. To the left of the barrier, the geometry is that+of CH,CI-12~H2- - -CH,, whilst to the right of this barrier, it is that of CH,CHCH,- -CH,. Therefore, proton tunneling at this energy barrier is to be expected. Proton tunneling has a large isotope effect. The mass spectra of deuterium-labeled n-butane using 80 V electrons were reported [ 101,but those at low-energy electron impac’t have not yet been reported. Mass spectrometry ‘of deuterium-labeled n-butane at low-energy bombardment is in progress.
R(C(31-C(4))
/’
6
&156.68k 15 % z +
-156.70-
.,:5.
.
y2:o’
.
.
.2:5.
.
R(C(3)-C.(L) Fig. 4. Calculated potential (-----), gauche.
-3:o‘
1 1
.
.
3f5.
.
-
ilof
bp
d
curve for Scheme III’;
{a), STO-3G;
(b), 4-31G.
(-
), trans;
I48 ACKNOWLEDGMENTS
The authors thank the Computer Center of the Institute for Molecular Science, for the use of the HITAC M-200H Computer and the Library Program IMSPAK written by Prof. K. Morokuma and co-workers (IMS) and would like to express their gratitude to the Data Processing Center of Kyoto University, for its generous permission to use the FACOM M200 Computer_ REFERENCES I H.M. Rosenstock, M.B. Wallenstein, AL. Wahrhaftig and H. Eyring, Proc. Natl. Acad. Sci. U.S.A., 38 (1952) 667. 2 R. Thompson, Conf. on Mass Spectrometry, Institute of Petroleum, London, 1953, p. 154. 3 (a) K. Fueki and K. Hirota, Nippon Kagaku Zasshi, 80 (1959) 1202; (b) K. Fueki, J. Phys. Chem., 68 (1964) 2656; (c) M. Hatada and K. Hirota, Z. Phys. Chem., N-F., 44 ( 1965) 328. 4 G.D. Flesch and H.J. Svec, J. Chem. Sot., Faraday Trans. 2, 69 (1973) I 187. 5 P. Wolkoff and J.L. Holmes, J. Am. Chem. Sot., 100 (1978) 7346. 6 J-L. Holmes and P. Wolkoff, J. Chem. Sot., Chem. Commun., (1979) 544. 7 A. Lavanchy, R. Houriet and T. GBumann, Org. Mass Spectrom., 14 (1979) 79. 8 J. Sunner and 1. Szabo, Int. J. Mass Spectrom. Ion Phys., 25 (1977) 241. 9 W.J. Hehre, in H.F. Schaefer III (Ed.), Application of Electronic Structure Theory, Modem Theoretical Chemistry, Vol. 4, Plenum Press, London, 1977, chapt. 7. 10 W.H. McFadden and A.L. Wahrhaftig, J. Am. Chern. Sot., 78 (1956) 1572.