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Theoretical study of electron impact mass spectrometry. II. ab initio MO study of the fragmentation of ionized 1-propanol
International Journal of Mass Spectrometry
Elsevier
Science
Publishers
and Zon Processes,
B.V., Amsterdam
64 (1985)
33-48
33
- Printed in The Netherlands
THEORETICAL STUDY OF ELECTRON IMPACT MASS SPECTROMETRY. II. AB INITIO MO STUDY OF THE FRAGMENTATION OF IONIZED l-PROPANOL
TAKAE
TAKEUCHI,
SHOKO
UENO,
MASAO
of Chemistry, Faculty of Science,
Department
YAMAMOTO
Nara Women 3 Uniuersdty, Kitauoyanishi - machi,
Nara 630 (Japan)
TOSHIO
MATSUSHITA
Department
of Chemistry,
and KICHISUKE
NISHIMOTO
*
Faculty of Science, Osaka City University, Sumiyoshi - ky
Osaka $58
tJapW (First
received
9 July 1984;
in final form 15 October
1984)
ABSTRACT In order to elucidate qualitatively the fragmentation mechanism of 1-propanol following low energy electron bombardment, the potential energy curves have been calculated using ab initio MO methods (4-31C//STO-3G). The present study indicates that H,O elimination proceeds via the formation of a five-membered ring intermediate/transition state. A hydrogen atom of the methyl group is shifted to form H,O, which is in line with the deuterium labelling experiment. In the simple bond cleavage process, it is mainly the CW-Ca bond which is expected to be broken.
INTRODUCTION
In a previous paper [I], the fragmentation mechanism of n-butane following low energy bombardment was studied by means of the ab initio MO method with STO-3G//4-31G basis sets. The calculated potential energy curves for the assumed reaction schemes suggested that the fragmentation to C&I,+ was more favorable than that to C,Hf, when the electron impact energy was at most only a few eV above the ionization threshold. The base peak at m/z 43 was calculated to be due to the 2-propyl cation. In the course of the fragmentation to CJI T, proton tunneling was expected to occur.
* To whom correspondence 0168-1176/85/%03.30
should be addressed. 0 1985 Elsevier Science Publishers
B-V.
34 The next goal was to elucidate the fragmentation mechanism of ionized I-propanol, which is isoelectronic with ionized n-butane. l-Alcohols have received much attention [2] since McLafferty [3] proposed a rearrangement mechanism for H,O elimination. It is of interest to discover why H,O elimination occurs easily from ionized l-propanol, while the corresponding CH, elimination from ionized n-butane occurs to a small extent only. This may be due to the effect of the heteroatom, oxygen, on the reaction mechanism. Recently, low energy EI mass spectra [4] of CH,CH,CH,OH and CH,CH,CD,OH have been reported. In order to further clarify the fragmentation scheme, the EI mass spectra of CD,CH,CH,OH, CH,CD,CH,OH and CH,CH ,CD,OH were measured. In this paper, the potential energy curves for the possible fragmentation processes of I-propanol +* as calculated by the ab initio MO method are reported and the fragmentation mechanism at low energy bombardment is discussed. EXPERIMENTAL
The EI mass spectra were measured using a Hitachi RMU-6M mass spectrometer operating at a pressure of ca. 10e7 torr. Ionization was achieved by electron impact using electron beams whose energies were changed from 8 to 25 eV at intervals of 1 eV. Total ion abundance was measured by a total ion monitor. High resolution mass spectra have also been measured. An unlabelled propanol was available commercially and was used without further purification. Labelled compounds were synthesized by the routes shown. CH,(COOC,H,),
0)
+ C,H,ONa
“2” CD,CH(COOK),
(2)
CH,CH(COOH),
CD,1
-+ CD,CH(COOC,H,), 78°C (x) Li:Hd CD&H,CH,OH
H,S04CD,CH,COOH D,O CH,CD(COOD),
+
CH,CD,COOD
100°C
(Y) LiAIH, ---,
(3)
CH,CD,CH,OH
(CH,CH,COj,O
LiAID,
4;c
CH,CH,CD,OH
These routes are exemplified
by the following
procedures.
35
Step I. (3,3,3-*H,)Propan-I-or. (3,3,3-2H,)Propan-l-ol was prepared by reduction with anhydrous LiAlH, of (3,3,3-*H,)propanoic acid, which was made from diethyl malonate via the diester (x) by malonic ester reaction. Step 2. (2,2-2H2)Propan-I-oZ. Quantities of 2.5 g of methylmalonic acid and 2.3. ml D,O were heated to boiling with stirring and careful exclusion of moisture. As soon as hydrolysis began, the heat was removed and a further 10, ml D,O was added dropwise at such a rate that the mixture was kept boiling. After the addition, the mixture was boiled for a further 10 min, during which propionic acid-d, (y) was prepared. (2,2- 2H,)Propan-l-ol was prepared by reduction with LiAlH, of propionic acid-d, (y). Step 3. (I,1 -*H,)Propan-I -OZ.Propionic acid anhydride (0.65 g) was added to a stirred suspension of 0.6 g LiAlD, in 10 ml of dry diethyl ether: the mixture was stirred for 1 h after which 3 ml of 6% NaOH solution was added. All labelled compounds were purified by gas chromatography. EXPERIMENTAL
RESULTS
The observed EI mass spectra are given in Table 1. These data show that H,O loss is dominant at 10 eV. On increasing the ionizing energy, the intensity of the peak at m/z 31 increases. A metastable peak (m* 60 * 42; calcd. 29.4, obsd. 29.4) for H,O loss from M+’ was observed at low energy. EI mass spectra of ionized 2H-labelled l-propanols are summarized in Tables 2-4. From the present data, H,O elimination occurs mostly via y-hydrogen atom transfer to an OH group. The peak at m/z 31 is due to P-cleavage. Table 1 shows a significant peak at m/z 59. This peak is due to hydrogen loss from the molecular ion. The high intensity peaks at m/z 60 due to CH,CH,CD20H++‘ (Table 2), m/z 61 due to CH,CD,CH,OH+’ (Table 3), and m/z 62 due to CD$H2CH,0H+’ (Table 4) indicate that a hydrogen bound to the a-carbon has been eliminated. This result might be due to the stability of the product ion, i.e. CH,CH,CH=OH+. It should be noted that CH,CHz, CHC and CH,CH,OH+ have not been observed at low ionizing energy. CALCULATION
The ab initio MO calculation was carried out in the following manner; the optimized geometries of molecular cation conformers, reaction intermediates and products were calculated with the STO-3G minimal basis set using the IMSPAK program [5]. Single-point energies have been calculated using the 4-31G basis set with the STO-3G optimized geometries. Although the
36
STO-3G minimal basis set is not sufficient for calculation of the potential energy curve, it has an advantage that the qualitative chemical picture, such as frontier electron densities of the HOMO and LUMO, is comparable with that obtained when more sophisticated basis sets, for example 4-31G or 6-31G*, are being used. Our purpose is to understand, first of all, the reaction mechanism at a qualitative level. When we compare the results of ab
TABLE
1
Mass spectra of I-propanol a*b Ion
Energy of ionizing electrons (eV) 10
15 26 27 28
0.1
CO C2H4 CHO
30
CH20
C,H,
C2H6
31 32 33 37 38 39 40
9.5
CH,O
%H,O
(, CH,O)
C,H, CH,O
2.6
0.1
C,H3 c20
GH,
49.1
C2H30
0.1 1.9
0.2 4.3
23.8 0.1 1.3
14.5 0.4 0.9
1.2 0.1 0.2 25.5 23.7 0.9
1.3 0.3
C2H40 GH,
45 57 58 59 60 61
0.3 4.1 2.7 2.9 2.2 4.1 1.4 0.2 100.0 0.8 2.6 1.4
1.5 0.1 0.6 7.3
C2H20
CgH7. 13CC,H, 44
1.4 0.1 1.5 0.6 0.1 100.0 0.4 1.9 0.9
2.2 1.7 2.1 0.4 3.8 0.7 0.1 100.0 0.4 1.8 0.7
C3H2
W-b
43
70
C3H
GH4
41 42
25
20
0.6
C% GH, C*H,
29
15
C,H,O C,H,O C,H,O C,H,O W-W’
‘3CC,H,0
2.8 22.7 100.0 5.7
15.1 11.9 0.2
11.5 1.5 1.3 0.1 0.2 2.0 0.9 0.2 12.1 10.8 0.5
a Abundances relative to the base pe& = 100%. b Values at m/z 32, 43, 60 and 61 contribute to the isotopic atoms (13C, “0,
2.9 4.5 9.0 6.5 5.2 7.0 6.6 2.3 0.4 100.0 2.3 3.1 1.5 0.9 1.7 5.9 0.5 1.2 7.6 0.9 9.0 3.1 1.5 0.3 0.4 2.5 1.4 0.2 9.9 9.5 0.5 etc.)_
37 TABLE
2
Mass spectra of CH,CH,CD,OH m/z
Ion
a Energy of ionizing electrons (ev) 10
13 14 15 25 26 27 28
CH, CH, C,H C,H, GH, co
30
GH,, CH,O,
31
C,H,, C,H,D, CH,O, CHDO
32
C,H,D, CH,DO
45 46 47 56 57 58 59 60 61 62 63
70
0.5
GH, CHO
44
25
?O
CH
29
33 34 35 37 38 39 40 41 42 43
15
C,H,D CD0
(1 GHD,) C,H,Dz
0.7 14.2
0.3
5.6
3.7 0.1 5.4 0.3 0.5 4.6
5.9 13.1 1.7 1.7 8.7
16.3 2.3 21.3 5.3 3.7 12.2
10.1 100.0 4.5 2.0
14.9 100.0 5.3 2.5
22.0 100.0 6.6 3.2
C,H,D,
C, H,D, CHD,O 13CHD,0 CH,D,O
10.7 (, CH2D,0)
C,H GH,
V-5
0.5
C3H6
2.3
0.5 1.2 5.6
3.4
7.0
90.2
59.9
38.3
3.6
5.5
4.9
2.3
3.2
0.7 2.5 2.8 4.1 0.7 10.8 1.8 39.9 2.0 5.5 0.2 4.2
1.6 13.2 0.7 21.7 0.7
0.2 0.7 2.1 14.4 0.9 23.9 0.9
C3H3 C3H.s
C2H30 C,H,D
C,H,DO GH,D, C, HD,O ‘3CCZHqDZ,
C3H,D,
C2H2D20 C,HJ’,O C3H-40 C,H,O
C,H,DO C,H,DO C,H,DO ‘3CC2H,D0 C3H6D20
“3CCzH,D20
9.8 100.0 7.8
a Abundances relative to the base peak = 100%.
1.9 15.9 1.0 30.5 1.3
0.2 0.7 1.7 0.4 4.3 12.3 0.8 10.4 2.5 10.8 7.6 2.4 6.1 0.6 0.1 10.6 100.0 2.8 1.4 0.4 0.9 2.4 3.9 2.8 4.3 0.9 5.2 1.5 15.8 1.3 2.7 0.2 1.8 0.1 0.1 0.4 0.9 5.6 0.4 10.1 0.5
38
TABLE
3
Mass spectra of CH,CD,CH,OH
m/z
Ion
a Energy of ionizing electrons (ev> 10
13 14 15 26 27 28
0.1
C2H2 C,H,,
0.2 1.3
0.1 0.3 0.6 2.4
42.7
5.4
5.6
64.0
13.6
1.9
1.3 0.2 1.1
9.4 0.4 1.0 0.3 1.4
0.1 0.1 0.6 2.7 16.7 10.9 0.6
0.1 0.1 0.5 2.1 12.1 7.6 0.3
C,H,D
C,H,, C,H,D, CH,O, CD0
C,HD,
30
C,H,, CH,O
C,H,D,
31
C,H,D,
C,H,D, C,H,D, 13CH,0 C,H,D C2%P2
0.3 0.1 1.6 0.3 4.3 2.2 3.8 2.0 3.8 100.0 4.5 9.5 1.8 0.4
C3H C3H2 C3H3 C3H.4 C3H5 C3f-b C2H30 C,H,D
46 47 56 57 58 59 60 61 62 63
0.2 0.1 0.6 0.7 1.2 100.0 1.6 5.7 1.0 0.3
0.7 0.1 2.1 0.8 2.2 1.3 2.5 100.0 3.4 7.9 1.5 0.3
C,HD
co
CHO
45
C,H,DO GHw 1 C,H,D, C, H, 0, C, HD,O 13CC2H,D2, C,H,D, C*H,DO C,H,D,O
C,H,O, C,H,DO 17.6 C,H,O, C,H,DO, C,HD,O (C,H,O, 1 C3H,D0, C31-I,D,0 C,H,DO C,H,D,O, C,H,DO 19.7 C,H,D,O 28.5 C,H,D,O 100.0 ‘3CCIH6D20 3.8
aAbundances
70 0.1
CH, CH,
C2&,
44
25
CH
29
32 33 34 37 38 39 40 41 42 43
20
15
relativeto the base peak =lOO%.
0.4 0.8 1.0 3.0 0.3 6.1 0.6 8.1 1.1 0.4 0.5 0.1 0.1 0.3 0.5 1.8 10.5 6.8 0.3
0.7 1.7 1.5 6.3 0.7 10.4 7.3 7.7 3.0 5.2 100.0 5.7 10.8 2.0 0.4 0.4 0.8 2.0 2.6 1.7 3.4 6.3 1.2 8.1 1.3 1.1 0.5 1.5 0.1 0.1 0.4 0.7 1.7 10.3 7.3 0.3
39 TABLE
4
Mass spectra of CD&H,CH,OH
a*b Energy of ionizing electrons {ev)
Ion
10 13 14 15 16 17 18 25 26 27 28 29
CD, CHD, CD, C,H C,H* C,H, co CHO
30 31
C,H,, CH,O
32
C,H,D, ‘%HsO,
44 45 46 56 57 58 59 60 61 62 63 64
25
20
CH CH,, CD CHD
C,H,, G&D CH,O, CD0
33 37 38 39 40 41 42 43
15
C,H,D,
C,H,D,
CzH,D,, CH,DO,
G&D,, C,H C,H, C,H, C,H, W-I, C,H,,
1.5 100.0
C,HD, CD,0
C,H,D,
0.1 2.2 0.2 2.2 100.0
2.0 3.3 1.8 3.9 1.2 3.5 100.0 4.1 10.8 3.9 3.3
7.9 4.2 2.7
10.5 1.4
2*0 10.2 4.1 3.0
3.4 7.9 1.7 3.1 2.0
0.6 4.2 7.4 1.8 3.2 2.0
23.3 22.1 1.2
17.6 13.7 1.0
14.5 12.1 1.0
C,H,D
C,H,O C,HsD C,H,D, C,HsO CsHsDz, C,H,D, C,H,O C,H,O, C,H,O, CsH,O, C,H,O, C,H,DO
44.2 ‘Y&D,
C,H,DO C,H,DO C,H,DO C,H,DO,
GwP2O C,H,D,O
‘3CC,H,D30
C,H,D,O 42.9 loo.0 4.0
a Abundances relative to the base peak = 100%. b The mixture of CD,CH,CH,OH and CD,HCH,CH20W
(CD,CH,CH,OH
70 0.2 0.9 2.1 0.8 0.4 0.7 0.2 1.6 5.2 6.5 7.1 7.9 2.5 4.6 loo.0 2.7 11.7 5.7 2.9 0.2 0.2 1.0 2.4 2.5 1.2 1.9 4.0 7.1 0.5 2.2 1.6 0.1 0.1 0.1 0.6 0.3 0.1 12.8 11.6 0.4 2 70%).
40 TABLE
5
Comparison
of calculated
CW (G,)
CH;(&,)
UHF
results using various basib sets
Basis set
Calculated total energy (a-u.)
Calculated C-H distance (A)
STO-3G ’ 3-21G a 4-31G 6-31G* a STO-3G a 3-21G a 4-31G 6-31G* a
-
1.120 1.078 1.076 1.078 1.078 1.072 1.070 1.073
38.77948 39.009r3 39.17513 39.23064 39.07671 39.34261 39.50497 39.55899
a Ref. 7.
initio calculations for the simple species, CHZ and CHJ (Table 5), we see that the essential features of the geometries and the relative energies are not so different. Therefore, we might be able to obtain a qualitatively reasonable result for fragmentation mechanisms of simple molecular cations. More rigorous calculation, employing the large CI method, of the potential energy curves for the fragmentation of ionized n-butane, 1-propanol and n-propylamine are in progress. Assumed reaction schemes McLafferty [3] proposed that fragmentation of ionized alcohol might proceed via a cyclic transition state involving H,O loss. In our previous paper [l], it was shown that there are two kinds of fragmentation schemes for n-butane+-. One is a simple bond cleavage process. The other is a bending mode controlled concerted process. Therefore, in this paper, the following two kinds of fragmentation schemes (Schemes 1 and 2) are examined. The potential energy curve for each scheme was calculated. Firstly, an important geometrical deformation was selected and the geometry optimization was performed using the energy gradient technique [5]. In Scheme 1, the saddle point for five-membered ring formation was calculated by means of
-:’ ,,,A
CH,O,<
+.-
CH 3CH2CH2Ot?
/
=y CH
/s”2----*<; =y
2
Scheme
1, Rearrangement
via five-membered
ring formation.
-
C,Hz
+.+ H,O
41
(a)
ccl-cp
cleavage
7 P a CH3 CHzCHl
CH,CH,1Ok-?+’
-
CH3CH2 -----
CH,Ol-?+’
7 PQ CH,CH,CH,O,~+~
CH2 OH1+
\
cteavage
(b) Cp-C7
+
f
CH,CH,Og’ CH, ----
-
CH2CH20~+’ CHZCHZ Ot?+
Scheme 2. Simple bond cleavage.
the McIver-Komornicki algorithm [5,6]. In Scheme 2, the bond distance associated with the corresponding bond cleavage was taken as the reaction coordinate. RESULTS
AND
DISCUSSION
The fragmentation at low energy electron bombardment should proceed to the most stable reaction products, provided there exists no or at most, a small energy barrier for the reaction path. In Table 6, the calculated total energies of the Franck-Condon and equilibrium states of the reactants are given. In Table 7, the calculated total energies of the products are summarized. Figure 1 shows an exploded view of an energy diagram relative to the total energy of the gauche form of ionized l-propanol of equilibrium geometry, This figure shows that the main product at the low energy bombardment is indeed due to H 2O elimination. The optimized geometries of the reactants and products are shown in Fig. 2. Comparing the neutral molecule [Fig. 2(a)] with the molecular cation at the equilibrium state (Fig.
TABLE 6 Calculated UHF total energies for the neutral molecule and molecular cation of I-propanol z+b Form
STO-3G ffWP2S
gauche
M+‘*
M(neutral)
- 190.71267 - 190.71326
M+
4-31G
STO-3G
4-31G
STO-3G
4-31G
- 192.82851 - 192.82887
- 190.37413 - 190.45931
- 192.48434 - 192.48495
- 190.47045 - 190.47170
- 192.49550 - 192.49704
a M+‘* and M+. represent the molecnlar cations at the Franck-Condon states, respectively. b Total energies are given in a-u.
and equilibrium
42 TABLE 7 Cakulated UHF total energies a of fragments STO-3G CH,OH CHfOH
+ (planar) . (planar)
C,H,OH+(q;) CH&HOH+
(C,)
C,H,OH’(C,) H*O C,H;(C,) C&(C,> CH;(C,,) CHf (&,,I c-C,H,+‘V%J C,H,+’ cc,)
-
4-31G
112.70702 112.91357 151.29846 151.32320 151.48822 - 74.96590 - 77.66300 - 77.40806 - 39.07701 - 38.77948 - 11538241’ - 115.40747
’ b b
-
113.97488 114.24050 152.93634 152.98957 153.21725 - 75.90333 - 78.48527 - 78.19491 - 39.49989 - 39.17120 - 116.56930 = - 116.60104 c
b b b b b =
a Energies are given in a.u. b Ref. 7. c Ref. 8.
2(b)], only C,-0 and O-H distances are lengthened, while the other bond lengths and bond angles remain almost unchanged. This may be due to local ionization, that is, one of the lone-pair electrons of the oxygen atom is removed. The geometries of some fragments have been published in our previous paper [l] and other literature [7].
&--
C H’+CH 252
OR’
&IQ
Fig. 1. Exploded view of an energy diagram relative to the UHF total energy of the gauche form of ionized I-propanol, as calculated by the 4-31G basis set using STO-3G geometry.
43
(&neutral
molecules
Tmns
(b) molecular
CatiOnS
..
Tram
H,l+
H5
1.113 120.5
113.
5” 0.999
Cl"'"
2
119.41
1.515 107.80 1.09 4c Ha & '%
C
110.56
(C5)
1.088 H,
Fig. 2. Optimized geometries as calculated by the STO-3G basis set. Bond lengths in A; bond angles in degrees. (a) Neutral molecules (trans and gauche forms); (b) molecular cations (trans and gauche forms) of equilibrium conformation; (c) fragments.
4.125
/
H+0.108 (a) bans
form
Fig. 3. Net charge on each atom in ionized form; (b) gauche form.
(b) gauche
1-propanol
form
of equilibrium
geometry.
(a)
Tram
Fig. 4. Potential energy curve for H,O elimination form as calculated by 4-31G basis set with STO-3G
IT,0
from ionized geometries.
l-propanol
in the gauche
elimination
The gauche form of 1-propanol is somewhat more stable than the truns form (see Table 6). At low energy bombardment, the rearrangement reaction might start from the gauche form. In Fig. 3, the charge distribution of ionized l-propanol in the equilibrium geometry is shown. As seen from this figure, the oxygen has a positive net charge, while C, has a negative net charge. These two atoms attract each other by Coulombic force. This
TABLE
8
Variation of net charge transition state
in the course
of Reaction
Scheme
1 in the neighborhood
Net charge
I-b WC,,) WC,, > WC,n) I-&n 1 WC,,) WC,,)
WC, -+ 0) * See Fig. 4.
Before saddle point
Saddle point
After saddle point
@I*
(cl *
Cd) *
+ 0.09 + 0.03 - 0.10 - 0.18 + 0.38 + 0.15 + 0.15 -+ 0.11 -I-0.11 + 0.08 +0.11 + 0.08
+ + + + + + + + +
- 0.24 + 0.05 -0.11 -0.13 +0.36 +0.13 +0.13 +0.12 +0.12 +0.14 +0.14 + 0.29
0.06 0.03 0.11 0.12 0.35 0.15 0.14 0.13 0.12 0.13 0.14 0.12
of the
P
96O'ot 960-O& Ht ._
-
uw!sod
-
le
_-
(PI
W~OO’L =(ceaJJ(9)
IP)
-
.__
._
.._...
__-._.
uor)!sod
_ -....
(e)
46
Coulomb attraction might help to bring the hydrogen (H,) towards the oxygen. It is therefore expected that H,O elimination proceeds preferentially via five-membered transition states. Assuming this rearrangement scheme, we have calculated the potential curve for Scheme 1, optimizing the geometry by changing the C,-H, distance. The calculated potential energy curve for this reaction scheme is shown in Fig. 4. In order to examine the changes in atomic net charges and geometries during the reaction course, these quantities at the saddle point (c) and at the neighborhood points (b) and (d) have been calculated. It is interesting to see how the net charges develop in the neighborhood of the transition state. The results summarized in Table 8 show that a charge transfer occurs in the course of the reaction given in Scheme 1 in the neighborhood of the transition state. This suggests that the driving force of the reaction in the (a)-(c) region must be a charge transfer interaction. As seen from Fig. 4, there is a local energy minimum at region (e). In Fig. 5 the calculated geometries and the atomic net charges of (b)-(f) are shown. As for the path from (c) to the products, a small amount of energy is required from the stable intermediate (e) to the products. This energy may be supplied by the excess vibrational energy, which is delivered by the stablization from the saddle point (c) to the energy minimum (e). Simple bond cleavage Figures 6 and 7 show the potential energy curves of Ccr-Cfi and C,--C, bond cleavage reactions. As can be seen from these figures, Ca-Ca cleavage
z ‘i
1
50CHi+ C21140t?’
jfj
40-
I
<30-
C2Hj+CH2d+
ai g
zo10
3 % ._ v
; O -10
TJ a
-2o-
M+‘* A id’ /
1.4 1.5
1.6
R(C&p
1.7
1.8
1.9
2.0
or Cp-Cr>
I
I
8
A
/ Fig. 6. Potential energy curves for simple bond cleavage from the trans form of ionized l-propanol calculated using the 4-31G basis set and STO-3G optimal geometries. Energy is at the optimized structure_ The reaction plotted relative to the trans form of I-propanol+’ coordinate represents the interatomic A, C,-C, cleavage.
distance of Cm-Cs or Cs-C,
bond. 0, Ca-Cp
cleavage;
47
#:;;I , , , 1.4
1.5
1.6
(
,
1.7
1.8
,
,
1.9 2.0
R (Cd- Q or Q-Q>
I ’ _ O”
/ A
Fig_ 7. Potential energy curves for simple bond cleavage from the gauche form of ionized 1-propanol calculated using the 4-31G basis set and STO-3G optimal geometries. Energy is plotted relative to the gauche form of 1-propanol+’ at the optimized structure. The reaction coordinate represents the interatomic distance of the Cm+ or C,-C, bond. 0, C--C, cleavage; A, Cs--C, cleavage.
occurs more easily than the C,& cleavage. The structure of CH30+ is the protonated aldehyde which is shown in Fig. 3(c). It is expected that C,-C, cleavage occurs easily due to the stability of this ion. On the other hand, in the case of n-butane, rupture of the middle C-C bond is less favorable than that of the terminal C-C bond, because C,H,+ is not so stable. CONCLUSION
A possible reaction mechanism of H,O elimination of l-propanol was examined by the ab initio MO calculation. The other types of fragmentation mechanisms of 1-propanol, i.e. simple bond cleavages, were also examined. In line with experimental results, the present calculations indicate that H,O elimination proceeds preferentially via a five-membered ring transition state. A small energy barrier (ca. 25 kcal mol -I)’ was found. In the H 2O elimination reaction, a hydrogen atom of YCH3 is shifted to the hydroxyl group to eventually form H,O. This result agrees with the deuterium labelling experiment. The most favorable simple bond cleavage is expected to be the Ca-C, bond fission, which also explains the experimental results quite well. ACKNOWLEDGEMENTS
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). The authors would also like to express their gratitude to the Data Processing
48
Center of Kyoto University for its generous permission to use the FACOM M200 computer. REFERENCES 1 T. Takeuchi, M.
2
3 4 5
6 7
8
Y amamoto, K. Nishimoto, H. Tanaka and K. Hirota, Int. J. Mass Spectrom. Ion Phys., 52 (1983) 139. H. Budzikiewicz, C. Djerassi and D.H. Williams, Mass Spectrometry cf. Organic Compounds, Holden-Day, San Francisco, CA, 1967, Chap. 2. F.W. McLafferty, Interpretation of Mass Spectra, University Science Books, Mill Valley, CA, 3rd edn., 1980, p. 189. F.W. McLafferty, Mass Spectrometry of Organic Ions, Academic Press, New York, 1963, p_ 331. M.N. Danchevskaya and S.N. Torbin, Int. J. Mass Spectrom. Ion Processes, 56 (1984) 251. K. Morokuma, S. Kato, K. Kitaura, I. Ohmine, S. Sakai and S. Obara, IMSPAK, IMS Computer Center Program Library, The Institute for Mdecular Science, Okazaki, Aichi, Japan, 1980, Program No. 372. J.W. Mclver and A. Komornicki, Chem. Phys. Lett., 10 (1971) 303. J.W. McIver and A. Komornicki, J. Am. Chem. Sot., 94 (1972) 2625. R.A. Whiteside, M.J. Fricsh, J.S. Binkiey, D.J. DeFrees, H.B. Schiegel, K. Raghavachari and J.A. Pople, Carnegie-MelIon Quantum Chemistry Archive, Carnegie-Mellon University, Pittsburgh, PA, 2nd edn., 1981. A.M.P. Nicholas, R.J. Boyd and D.R. Arnold, Can. J. Chem., 60 (1982) 3011.
Elsevier
Science
Publishers
and Zon Processes,
B.V., Amsterdam
64 (1985)
33-48
33
- Printed in The Netherlands
THEORETICAL STUDY OF ELECTRON IMPACT MASS SPECTROMETRY. II. AB INITIO MO STUDY OF THE FRAGMENTATION OF IONIZED l-PROPANOL
TAKAE
TAKEUCHI,
SHOKO
UENO,
MASAO
of Chemistry, Faculty of Science,
Department
YAMAMOTO
Nara Women 3 Uniuersdty, Kitauoyanishi - machi,
Nara 630 (Japan)
TOSHIO
MATSUSHITA
Department
of Chemistry,
and KICHISUKE
NISHIMOTO
*
Faculty of Science, Osaka City University, Sumiyoshi - ky
Osaka $58
tJapW (First
received
9 July 1984;
in final form 15 October
1984)
ABSTRACT In order to elucidate qualitatively the fragmentation mechanism of 1-propanol following low energy electron bombardment, the potential energy curves have been calculated using ab initio MO methods (4-31C//STO-3G). The present study indicates that H,O elimination proceeds via the formation of a five-membered ring intermediate/transition state. A hydrogen atom of the methyl group is shifted to form H,O, which is in line with the deuterium labelling experiment. In the simple bond cleavage process, it is mainly the CW-Ca bond which is expected to be broken.
INTRODUCTION
In a previous paper [I], the fragmentation mechanism of n-butane following low energy bombardment was studied by means of the ab initio MO method with STO-3G//4-31G basis sets. The calculated potential energy curves for the assumed reaction schemes suggested that the fragmentation to C&I,+ was more favorable than that to C,Hf, when the electron impact energy was at most only a few eV above the ionization threshold. The base peak at m/z 43 was calculated to be due to the 2-propyl cation. In the course of the fragmentation to CJI T, proton tunneling was expected to occur.
* To whom correspondence 0168-1176/85/%03.30
should be addressed. 0 1985 Elsevier Science Publishers
B-V.
34 The next goal was to elucidate the fragmentation mechanism of ionized I-propanol, which is isoelectronic with ionized n-butane. l-Alcohols have received much attention [2] since McLafferty [3] proposed a rearrangement mechanism for H,O elimination. It is of interest to discover why H,O elimination occurs easily from ionized l-propanol, while the corresponding CH, elimination from ionized n-butane occurs to a small extent only. This may be due to the effect of the heteroatom, oxygen, on the reaction mechanism. Recently, low energy EI mass spectra [4] of CH,CH,CH,OH and CH,CH,CD,OH have been reported. In order to further clarify the fragmentation scheme, the EI mass spectra of CD,CH,CH,OH, CH,CD,CH,OH and CH,CH ,CD,OH were measured. In this paper, the potential energy curves for the possible fragmentation processes of I-propanol +* as calculated by the ab initio MO method are reported and the fragmentation mechanism at low energy bombardment is discussed. EXPERIMENTAL
The EI mass spectra were measured using a Hitachi RMU-6M mass spectrometer operating at a pressure of ca. 10e7 torr. Ionization was achieved by electron impact using electron beams whose energies were changed from 8 to 25 eV at intervals of 1 eV. Total ion abundance was measured by a total ion monitor. High resolution mass spectra have also been measured. An unlabelled propanol was available commercially and was used without further purification. Labelled compounds were synthesized by the routes shown. CH,(COOC,H,),
0)
+ C,H,ONa
“2” CD,CH(COOK),
(2)
CH,CH(COOH),
CD,1
-+ CD,CH(COOC,H,), 78°C (x) Li:Hd CD&H,CH,OH
H,S04CD,CH,COOH D,O CH,CD(COOD),
+
CH,CD,COOD
100°C
(Y) LiAIH, ---,
(3)
CH,CD,CH,OH
(CH,CH,COj,O
LiAID,
4;c
CH,CH,CD,OH
These routes are exemplified
by the following
procedures.
35
Step I. (3,3,3-*H,)Propan-I-or. (3,3,3-2H,)Propan-l-ol was prepared by reduction with anhydrous LiAlH, of (3,3,3-*H,)propanoic acid, which was made from diethyl malonate via the diester (x) by malonic ester reaction. Step 2. (2,2-2H2)Propan-I-oZ. Quantities of 2.5 g of methylmalonic acid and 2.3. ml D,O were heated to boiling with stirring and careful exclusion of moisture. As soon as hydrolysis began, the heat was removed and a further 10, ml D,O was added dropwise at such a rate that the mixture was kept boiling. After the addition, the mixture was boiled for a further 10 min, during which propionic acid-d, (y) was prepared. (2,2- 2H,)Propan-l-ol was prepared by reduction with LiAlH, of propionic acid-d, (y). Step 3. (I,1 -*H,)Propan-I -OZ.Propionic acid anhydride (0.65 g) was added to a stirred suspension of 0.6 g LiAlD, in 10 ml of dry diethyl ether: the mixture was stirred for 1 h after which 3 ml of 6% NaOH solution was added. All labelled compounds were purified by gas chromatography. EXPERIMENTAL
RESULTS
The observed EI mass spectra are given in Table 1. These data show that H,O loss is dominant at 10 eV. On increasing the ionizing energy, the intensity of the peak at m/z 31 increases. A metastable peak (m* 60 * 42; calcd. 29.4, obsd. 29.4) for H,O loss from M+’ was observed at low energy. EI mass spectra of ionized 2H-labelled l-propanols are summarized in Tables 2-4. From the present data, H,O elimination occurs mostly via y-hydrogen atom transfer to an OH group. The peak at m/z 31 is due to P-cleavage. Table 1 shows a significant peak at m/z 59. This peak is due to hydrogen loss from the molecular ion. The high intensity peaks at m/z 60 due to CH,CH,CD20H++‘ (Table 2), m/z 61 due to CH,CD,CH,OH+’ (Table 3), and m/z 62 due to CD$H2CH,0H+’ (Table 4) indicate that a hydrogen bound to the a-carbon has been eliminated. This result might be due to the stability of the product ion, i.e. CH,CH,CH=OH+. It should be noted that CH,CHz, CHC and CH,CH,OH+ have not been observed at low ionizing energy. CALCULATION
The ab initio MO calculation was carried out in the following manner; the optimized geometries of molecular cation conformers, reaction intermediates and products were calculated with the STO-3G minimal basis set using the IMSPAK program [5]. Single-point energies have been calculated using the 4-31G basis set with the STO-3G optimized geometries. Although the
36
STO-3G minimal basis set is not sufficient for calculation of the potential energy curve, it has an advantage that the qualitative chemical picture, such as frontier electron densities of the HOMO and LUMO, is comparable with that obtained when more sophisticated basis sets, for example 4-31G or 6-31G*, are being used. Our purpose is to understand, first of all, the reaction mechanism at a qualitative level. When we compare the results of ab
TABLE
1
Mass spectra of I-propanol a*b Ion
Energy of ionizing electrons (eV) 10
15 26 27 28
0.1
CO C2H4 CHO
30
CH20
C,H,
C2H6
31 32 33 37 38 39 40
9.5
CH,O
%H,O
(, CH,O)
C,H, CH,O
2.6
0.1
C,H3 c20
GH,
49.1
C2H30
0.1 1.9
0.2 4.3
23.8 0.1 1.3
14.5 0.4 0.9
1.2 0.1 0.2 25.5 23.7 0.9
1.3 0.3
C2H40 GH,
45 57 58 59 60 61
0.3 4.1 2.7 2.9 2.2 4.1 1.4 0.2 100.0 0.8 2.6 1.4
1.5 0.1 0.6 7.3
C2H20
CgH7. 13CC,H, 44
1.4 0.1 1.5 0.6 0.1 100.0 0.4 1.9 0.9
2.2 1.7 2.1 0.4 3.8 0.7 0.1 100.0 0.4 1.8 0.7
C3H2
W-b
43
70
C3H
GH4
41 42
25
20
0.6
C% GH, C*H,
29
15
C,H,O C,H,O C,H,O C,H,O W-W’
‘3CC,H,0
2.8 22.7 100.0 5.7
15.1 11.9 0.2
11.5 1.5 1.3 0.1 0.2 2.0 0.9 0.2 12.1 10.8 0.5
a Abundances relative to the base pe& = 100%. b Values at m/z 32, 43, 60 and 61 contribute to the isotopic atoms (13C, “0,
2.9 4.5 9.0 6.5 5.2 7.0 6.6 2.3 0.4 100.0 2.3 3.1 1.5 0.9 1.7 5.9 0.5 1.2 7.6 0.9 9.0 3.1 1.5 0.3 0.4 2.5 1.4 0.2 9.9 9.5 0.5 etc.)_
37 TABLE
2
Mass spectra of CH,CH,CD,OH m/z
Ion
a Energy of ionizing electrons (ev) 10
13 14 15 25 26 27 28
CH, CH, C,H C,H, GH, co
30
GH,, CH,O,
31
C,H,, C,H,D, CH,O, CHDO
32
C,H,D, CH,DO
45 46 47 56 57 58 59 60 61 62 63
70
0.5
GH, CHO
44
25
?O
CH
29
33 34 35 37 38 39 40 41 42 43
15
C,H,D CD0
(1 GHD,) C,H,Dz
0.7 14.2
0.3
5.6
3.7 0.1 5.4 0.3 0.5 4.6
5.9 13.1 1.7 1.7 8.7
16.3 2.3 21.3 5.3 3.7 12.2
10.1 100.0 4.5 2.0
14.9 100.0 5.3 2.5
22.0 100.0 6.6 3.2
C,H,D,
C, H,D, CHD,O 13CHD,0 CH,D,O
10.7 (, CH2D,0)
C,H GH,
V-5
0.5
C3H6
2.3
0.5 1.2 5.6
3.4
7.0
90.2
59.9
38.3
3.6
5.5
4.9
2.3
3.2
0.7 2.5 2.8 4.1 0.7 10.8 1.8 39.9 2.0 5.5 0.2 4.2
1.6 13.2 0.7 21.7 0.7
0.2 0.7 2.1 14.4 0.9 23.9 0.9
C3H3 C3H.s
C2H30 C,H,D
C,H,DO GH,D, C, HD,O ‘3CCZHqDZ,
C3H,D,
C2H2D20 C,HJ’,O C3H-40 C,H,O
C,H,DO C,H,DO C,H,DO ‘3CC2H,D0 C3H6D20
“3CCzH,D20
9.8 100.0 7.8
a Abundances relative to the base peak = 100%.
1.9 15.9 1.0 30.5 1.3
0.2 0.7 1.7 0.4 4.3 12.3 0.8 10.4 2.5 10.8 7.6 2.4 6.1 0.6 0.1 10.6 100.0 2.8 1.4 0.4 0.9 2.4 3.9 2.8 4.3 0.9 5.2 1.5 15.8 1.3 2.7 0.2 1.8 0.1 0.1 0.4 0.9 5.6 0.4 10.1 0.5
38
TABLE
3
Mass spectra of CH,CD,CH,OH
m/z
Ion
a Energy of ionizing electrons (ev> 10
13 14 15 26 27 28
0.1
C2H2 C,H,,
0.2 1.3
0.1 0.3 0.6 2.4
42.7
5.4
5.6
64.0
13.6
1.9
1.3 0.2 1.1
9.4 0.4 1.0 0.3 1.4
0.1 0.1 0.6 2.7 16.7 10.9 0.6
0.1 0.1 0.5 2.1 12.1 7.6 0.3
C,H,D
C,H,, C,H,D, CH,O, CD0
C,HD,
30
C,H,, CH,O
C,H,D,
31
C,H,D,
C,H,D, C,H,D, 13CH,0 C,H,D C2%P2
0.3 0.1 1.6 0.3 4.3 2.2 3.8 2.0 3.8 100.0 4.5 9.5 1.8 0.4
C3H C3H2 C3H3 C3H.4 C3H5 C3f-b C2H30 C,H,D
46 47 56 57 58 59 60 61 62 63
0.2 0.1 0.6 0.7 1.2 100.0 1.6 5.7 1.0 0.3
0.7 0.1 2.1 0.8 2.2 1.3 2.5 100.0 3.4 7.9 1.5 0.3
C,HD
co
CHO
45
C,H,DO GHw 1 C,H,D, C, H, 0, C, HD,O 13CC2H,D2, C,H,D, C*H,DO C,H,D,O
C,H,O, C,H,DO 17.6 C,H,O, C,H,DO, C,HD,O (C,H,O, 1 C3H,D0, C31-I,D,0 C,H,DO C,H,D,O, C,H,DO 19.7 C,H,D,O 28.5 C,H,D,O 100.0 ‘3CCIH6D20 3.8
aAbundances
70 0.1
CH, CH,
C2&,
44
25
CH
29
32 33 34 37 38 39 40 41 42 43
20
15
relativeto the base peak =lOO%.
0.4 0.8 1.0 3.0 0.3 6.1 0.6 8.1 1.1 0.4 0.5 0.1 0.1 0.3 0.5 1.8 10.5 6.8 0.3
0.7 1.7 1.5 6.3 0.7 10.4 7.3 7.7 3.0 5.2 100.0 5.7 10.8 2.0 0.4 0.4 0.8 2.0 2.6 1.7 3.4 6.3 1.2 8.1 1.3 1.1 0.5 1.5 0.1 0.1 0.4 0.7 1.7 10.3 7.3 0.3
39 TABLE
4
Mass spectra of CD&H,CH,OH
a*b Energy of ionizing electrons {ev)
Ion
10 13 14 15 16 17 18 25 26 27 28 29
CD, CHD, CD, C,H C,H* C,H, co CHO
30 31
C,H,, CH,O
32
C,H,D, ‘%HsO,
44 45 46 56 57 58 59 60 61 62 63 64
25
20
CH CH,, CD CHD
C,H,, G&D CH,O, CD0
33 37 38 39 40 41 42 43
15
C,H,D,
C,H,D,
CzH,D,, CH,DO,
G&D,, C,H C,H, C,H, C,H, W-I, C,H,,
1.5 100.0
C,HD, CD,0
C,H,D,
0.1 2.2 0.2 2.2 100.0
2.0 3.3 1.8 3.9 1.2 3.5 100.0 4.1 10.8 3.9 3.3
7.9 4.2 2.7
10.5 1.4
2*0 10.2 4.1 3.0
3.4 7.9 1.7 3.1 2.0
0.6 4.2 7.4 1.8 3.2 2.0
23.3 22.1 1.2
17.6 13.7 1.0
14.5 12.1 1.0
C,H,D
C,H,O C,HsD C,H,D, C,HsO CsHsDz, C,H,D, C,H,O C,H,O, C,H,O, CsH,O, C,H,O, C,H,DO
44.2 ‘Y&D,
C,H,DO C,H,DO C,H,DO C,H,DO,
GwP2O C,H,D,O
‘3CC,H,D30
C,H,D,O 42.9 loo.0 4.0
a Abundances relative to the base peak = 100%. b The mixture of CD,CH,CH,OH and CD,HCH,CH20W
(CD,CH,CH,OH
70 0.2 0.9 2.1 0.8 0.4 0.7 0.2 1.6 5.2 6.5 7.1 7.9 2.5 4.6 loo.0 2.7 11.7 5.7 2.9 0.2 0.2 1.0 2.4 2.5 1.2 1.9 4.0 7.1 0.5 2.2 1.6 0.1 0.1 0.1 0.6 0.3 0.1 12.8 11.6 0.4 2 70%).
40 TABLE
5
Comparison
of calculated
CW (G,)
CH;(&,)
UHF
results using various basib sets
Basis set
Calculated total energy (a-u.)
Calculated C-H distance (A)
STO-3G ’ 3-21G a 4-31G 6-31G* a STO-3G a 3-21G a 4-31G 6-31G* a
-
1.120 1.078 1.076 1.078 1.078 1.072 1.070 1.073
38.77948 39.009r3 39.17513 39.23064 39.07671 39.34261 39.50497 39.55899
a Ref. 7.
initio calculations for the simple species, CHZ and CHJ (Table 5), we see that the essential features of the geometries and the relative energies are not so different. Therefore, we might be able to obtain a qualitatively reasonable result for fragmentation mechanisms of simple molecular cations. More rigorous calculation, employing the large CI method, of the potential energy curves for the fragmentation of ionized n-butane, 1-propanol and n-propylamine are in progress. Assumed reaction schemes McLafferty [3] proposed that fragmentation of ionized alcohol might proceed via a cyclic transition state involving H,O loss. In our previous paper [l], it was shown that there are two kinds of fragmentation schemes for n-butane+-. One is a simple bond cleavage process. The other is a bending mode controlled concerted process. Therefore, in this paper, the following two kinds of fragmentation schemes (Schemes 1 and 2) are examined. The potential energy curve for each scheme was calculated. Firstly, an important geometrical deformation was selected and the geometry optimization was performed using the energy gradient technique [5]. In Scheme 1, the saddle point for five-membered ring formation was calculated by means of
-:’ ,,,A
CH,O,<
+.-
CH 3CH2CH2Ot?
/
=y CH
/s”2----*<; =y
2
Scheme
1, Rearrangement
via five-membered
ring formation.
-
C,Hz
+.+ H,O
41
(a)
ccl-cp
cleavage
7 P a CH3 CHzCHl
CH,CH,1Ok-?+’
-
CH3CH2 -----
CH,Ol-?+’
7 PQ CH,CH,CH,O,~+~
CH2 OH1+
\
cteavage
(b) Cp-C7
+
f
CH,CH,Og’ CH, ----
-
CH2CH20~+’ CHZCHZ Ot?+
Scheme 2. Simple bond cleavage.
the McIver-Komornicki algorithm [5,6]. In Scheme 2, the bond distance associated with the corresponding bond cleavage was taken as the reaction coordinate. RESULTS
AND
DISCUSSION
The fragmentation at low energy electron bombardment should proceed to the most stable reaction products, provided there exists no or at most, a small energy barrier for the reaction path. In Table 6, the calculated total energies of the Franck-Condon and equilibrium states of the reactants are given. In Table 7, the calculated total energies of the products are summarized. Figure 1 shows an exploded view of an energy diagram relative to the total energy of the gauche form of ionized l-propanol of equilibrium geometry, This figure shows that the main product at the low energy bombardment is indeed due to H 2O elimination. The optimized geometries of the reactants and products are shown in Fig. 2. Comparing the neutral molecule [Fig. 2(a)] with the molecular cation at the equilibrium state (Fig.
TABLE 6 Calculated UHF total energies for the neutral molecule and molecular cation of I-propanol z+b Form
STO-3G ffWP2S
gauche
M+‘*
M(neutral)
- 190.71267 - 190.71326
M+
4-31G
STO-3G
4-31G
STO-3G
4-31G
- 192.82851 - 192.82887
- 190.37413 - 190.45931
- 192.48434 - 192.48495
- 190.47045 - 190.47170
- 192.49550 - 192.49704
a M+‘* and M+. represent the molecnlar cations at the Franck-Condon states, respectively. b Total energies are given in a-u.
and equilibrium
42 TABLE 7 Cakulated UHF total energies a of fragments STO-3G CH,OH CHfOH
+ (planar) . (planar)
C,H,OH+(q;) CH&HOH+
(C,)
C,H,OH’(C,) H*O C,H;(C,) C&(C,> CH;(C,,) CHf (&,,I c-C,H,+‘V%J C,H,+’ cc,)
-
4-31G
112.70702 112.91357 151.29846 151.32320 151.48822 - 74.96590 - 77.66300 - 77.40806 - 39.07701 - 38.77948 - 11538241’ - 115.40747
’ b b
-
113.97488 114.24050 152.93634 152.98957 153.21725 - 75.90333 - 78.48527 - 78.19491 - 39.49989 - 39.17120 - 116.56930 = - 116.60104 c
b b b b b =
a Energies are given in a.u. b Ref. 7. c Ref. 8.
2(b)], only C,-0 and O-H distances are lengthened, while the other bond lengths and bond angles remain almost unchanged. This may be due to local ionization, that is, one of the lone-pair electrons of the oxygen atom is removed. The geometries of some fragments have been published in our previous paper [l] and other literature [7].
&--
C H’+CH 252
OR’
&IQ
Fig. 1. Exploded view of an energy diagram relative to the UHF total energy of the gauche form of ionized I-propanol, as calculated by the 4-31G basis set using STO-3G geometry.
43
(&neutral
molecules
Tmns
(b) molecular
CatiOnS
..
Tram
H,l+
H5
1.113 120.5
113.
5” 0.999
Cl"'"
2
119.41
1.515 107.80 1.09 4c Ha & '%
C
110.56
(C5)
1.088 H,
Fig. 2. Optimized geometries as calculated by the STO-3G basis set. Bond lengths in A; bond angles in degrees. (a) Neutral molecules (trans and gauche forms); (b) molecular cations (trans and gauche forms) of equilibrium conformation; (c) fragments.
4.125
/
H+0.108 (a) bans
form
Fig. 3. Net charge on each atom in ionized form; (b) gauche form.
(b) gauche
1-propanol
form
of equilibrium
geometry.
(a)
Tram
Fig. 4. Potential energy curve for H,O elimination form as calculated by 4-31G basis set with STO-3G
IT,0
from ionized geometries.
l-propanol
in the gauche
elimination
The gauche form of 1-propanol is somewhat more stable than the truns form (see Table 6). At low energy bombardment, the rearrangement reaction might start from the gauche form. In Fig. 3, the charge distribution of ionized l-propanol in the equilibrium geometry is shown. As seen from this figure, the oxygen has a positive net charge, while C, has a negative net charge. These two atoms attract each other by Coulombic force. This
TABLE
8
Variation of net charge transition state
in the course
of Reaction
Scheme
1 in the neighborhood
Net charge
I-b WC,,) WC,, > WC,n) I-&n 1 WC,,) WC,,)
WC, -+ 0) * See Fig. 4.
Before saddle point
Saddle point
After saddle point
@I*
(cl *
Cd) *
+ 0.09 + 0.03 - 0.10 - 0.18 + 0.38 + 0.15 + 0.15 -+ 0.11 -I-0.11 + 0.08 +0.11 + 0.08
+ + + + + + + + +
- 0.24 + 0.05 -0.11 -0.13 +0.36 +0.13 +0.13 +0.12 +0.12 +0.14 +0.14 + 0.29
0.06 0.03 0.11 0.12 0.35 0.15 0.14 0.13 0.12 0.13 0.14 0.12
of the
P
96O'ot 960-O& Ht ._
-
uw!sod
-
le
_-
(PI
W~OO’L =(ceaJJ(9)
IP)
-
.__
._
.._...
__-._.
uor)!sod
_ -....
(e)
46
Coulomb attraction might help to bring the hydrogen (H,) towards the oxygen. It is therefore expected that H,O elimination proceeds preferentially via five-membered transition states. Assuming this rearrangement scheme, we have calculated the potential curve for Scheme 1, optimizing the geometry by changing the C,-H, distance. The calculated potential energy curve for this reaction scheme is shown in Fig. 4. In order to examine the changes in atomic net charges and geometries during the reaction course, these quantities at the saddle point (c) and at the neighborhood points (b) and (d) have been calculated. It is interesting to see how the net charges develop in the neighborhood of the transition state. The results summarized in Table 8 show that a charge transfer occurs in the course of the reaction given in Scheme 1 in the neighborhood of the transition state. This suggests that the driving force of the reaction in the (a)-(c) region must be a charge transfer interaction. As seen from Fig. 4, there is a local energy minimum at region (e). In Fig. 5 the calculated geometries and the atomic net charges of (b)-(f) are shown. As for the path from (c) to the products, a small amount of energy is required from the stable intermediate (e) to the products. This energy may be supplied by the excess vibrational energy, which is delivered by the stablization from the saddle point (c) to the energy minimum (e). Simple bond cleavage Figures 6 and 7 show the potential energy curves of Ccr-Cfi and C,--C, bond cleavage reactions. As can be seen from these figures, Ca-Ca cleavage
z ‘i
1
50CHi+ C21140t?’
jfj
40-
I
<30-
C2Hj+CH2d+
ai g
zo10
3 % ._ v
; O -10
TJ a
-2o-
M+‘* A id’ /
1.4 1.5
1.6
R(C&p
1.7
1.8
1.9
2.0
or Cp-Cr>
I
I
8
A
/ Fig. 6. Potential energy curves for simple bond cleavage from the trans form of ionized l-propanol calculated using the 4-31G basis set and STO-3G optimal geometries. Energy is at the optimized structure_ The reaction plotted relative to the trans form of I-propanol+’ coordinate represents the interatomic A, C,-C, cleavage.
distance of Cm-Cs or Cs-C,
bond. 0, Ca-Cp
cleavage;
47
#:;;I , , , 1.4
1.5
1.6
(
,
1.7
1.8
,
,
1.9 2.0
R (Cd- Q or Q-Q>
I ’ _ O”
/ A
Fig_ 7. Potential energy curves for simple bond cleavage from the gauche form of ionized 1-propanol calculated using the 4-31G basis set and STO-3G optimal geometries. Energy is plotted relative to the gauche form of 1-propanol+’ at the optimized structure. The reaction coordinate represents the interatomic distance of the Cm+ or C,-C, bond. 0, C--C, cleavage; A, Cs--C, cleavage.
occurs more easily than the C,& cleavage. The structure of CH30+ is the protonated aldehyde which is shown in Fig. 3(c). It is expected that C,-C, cleavage occurs easily due to the stability of this ion. On the other hand, in the case of n-butane, rupture of the middle C-C bond is less favorable than that of the terminal C-C bond, because C,H,+ is not so stable. CONCLUSION
A possible reaction mechanism of H,O elimination of l-propanol was examined by the ab initio MO calculation. The other types of fragmentation mechanisms of 1-propanol, i.e. simple bond cleavages, were also examined. In line with experimental results, the present calculations indicate that H,O elimination proceeds preferentially via a five-membered ring transition state. A small energy barrier (ca. 25 kcal mol -I)’ was found. In the H 2O elimination reaction, a hydrogen atom of YCH3 is shifted to the hydroxyl group to eventually form H,O. This result agrees with the deuterium labelling experiment. The most favorable simple bond cleavage is expected to be the Ca-C, bond fission, which also explains the experimental results quite well. ACKNOWLEDGEMENTS
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). The authors would also like to express their gratitude to the Data Processing
48
Center of Kyoto University for its generous permission to use the FACOM M200 computer. REFERENCES 1 T. Takeuchi, M.
2
3 4 5
6 7
8
Y amamoto, K. Nishimoto, H. Tanaka and K. Hirota, Int. J. Mass Spectrom. Ion Phys., 52 (1983) 139. H. Budzikiewicz, C. Djerassi and D.H. Williams, Mass Spectrometry cf. Organic Compounds, Holden-Day, San Francisco, CA, 1967, Chap. 2. F.W. McLafferty, Interpretation of Mass Spectra, University Science Books, Mill Valley, CA, 3rd edn., 1980, p. 189. F.W. McLafferty, Mass Spectrometry of Organic Ions, Academic Press, New York, 1963, p_ 331. M.N. Danchevskaya and S.N. Torbin, Int. J. Mass Spectrom. Ion Processes, 56 (1984) 251. K. Morokuma, S. Kato, K. Kitaura, I. Ohmine, S. Sakai and S. Obara, IMSPAK, IMS Computer Center Program Library, The Institute for Mdecular Science, Okazaki, Aichi, Japan, 1980, Program No. 372. J.W. Mclver and A. Komornicki, Chem. Phys. Lett., 10 (1971) 303. J.W. McIver and A. Komornicki, J. Am. Chem. Sot., 94 (1972) 2625. R.A. Whiteside, M.J. Fricsh, J.S. Binkiey, D.J. DeFrees, H.B. Schiegel, K. Raghavachari and J.A. Pople, Carnegie-MelIon Quantum Chemistry Archive, Carnegie-Mellon University, Pittsburgh, PA, 2nd edn., 1981. A.M.P. Nicholas, R.J. Boyd and D.R. Arnold, Can. J. Chem., 60 (1982) 3011.