J. Phys. Chem. 1989, 93, 7095-7098 Conclusions A series of bichromophoric compounds was designed incorporating N,N-diethylaniline and four arenes. The TICT fluorescence from these compounds correlates with the Ed of the arenes. This study supports the radical cation/radical anion description of the TICT state. The consistently lower than predicted pe values may
7095
be due to fluorescence from nonorthogonal conformations on the TICT surface. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the USC Faculty Research and Innovation Fund for their support of this research.
Mass Spectrometric Study of Negative Ions from Unsaturated Carbonyls J. Hacaloglu, A. Gokmen, S. Suzer,* Department of Chemistry, Middle East Technical University, Ankara, Turkey
E. Illenberger, and H. Baumgartel Institut fur Physikalishe Chemie, Freie Universitat Berlin, Berlin, FRG (Received: February 20, 1989)
Negative ion formation from some unsaturated carbonyl derivatives is investigated as a function of electron energy by using a monochromatic (0.2 eV) and low-energy (0-10 eV) electron beam and a mass spectrometer. Negative ions at low energies are formed via a dissociative attachment process. Appearance potentials of certain negative ions can be correlated with the electron-releasingor -withdrawingpower of the substituents and/or with bond strengths. For example, the appearance potentials of C6H3CO- and C2H3CO- both shift to higher energies as the C-X borid is weakened through the electron-withdrawing power of X in the order X = CI > OH > H > CH3. The appearance potentials are also used to estimate the bond dissociation energies not available in the literature. Accordingly, C6H5-COX bond energies can be calculated as 3.9, 3.7, and 3.0 eV for X = H, OH, and C1, respectively, by using the measured appearance potentials of C6HS-ion from these molecules and the published bond energy of 4.033 eV for acetophenone (X = CH,).
Introduction Molecules containing carbonyl groups have always been interesting from the various structural and spectroscopic aspects. The influence of unsaturated substituents on the oxygen electron density of the C O group was investigated by infrared and photoelectron spectroscopies.Id It was determined that the C=O frequency decreases considerably due to the unsaturated substituent. Both C6HSand C2H3 groups destabilize the no molecular orbitals as indicated by the shift of ionization energies of those molecular orbitals to lower values.'-5 It was also noted that interaction of a electrons of C6H5 and C2H3 with cox increases the ionization energies of the a molecular orbitals. Mass spectroscopic investigations of the negative ionization processes of the carbonyl compounds were carried out at normal conditions (low pressure, 70 eV) or at increased source pressures (- 1 mbar).7-9 Recently, we have studied negative ionization processes of some acetyl derivatives with very low energy (0-10 eV) and monochromatic (fwhm = 0.2 eV) electrons.1° In this (1) AI-Jallo, H. N.; Jalhoom, M. G. Spectrochim. Acta, Part A 1975,31A, 265. (2) Katrib, A.; Rabalais, J. W. J. Phys. Chem. 1973, 77, 2358. (3) Egdell, R.;Green, J. C.; Rao, C. N. R. Chem. Phys. Left. 1975,33, 600. (4) Meeks, J.; Wahlborg, A.; McGlynn, S.P. J. Electron Spectrosc. 1981, 22, 43. ( 5 ) Gan, T. H.; Livett, M. K.; Peel, J. B. J . Chem. SOC.,Faraday Trans. 2 1984,80, 1281. (6) van Dam, H. M.; Oskam, A. J. Electron Spectrosc. 1978, 13, 273. (7) Budzikiewicz, H. Angew. Chem., Int. Ed. Engl. 1981, 20, 624. (8) Aplin, R. T.: Budzikiewicz, H.; Djerassi, C. J. Am. Chem. SOC.1965, 87, 3180. (9) DeSouze, B. C.; Green, J. M. J. Chem. Phys. 1967, 46, 1421. (10) Hacaloglu, J.; Gokmen, A.; Suzer, S . J . Phys. Chem. 1989,93, 3418.
0022-3654/89/2093-7095$01 .50/0
work we report negative ionization processes of some unsaturated carbonyls, mainly C&COX (X = C1, OH, H , or CH3) and C2H3COX (X = C1 or OH), with the same technique. Experimental Section The negative ionization processes of the unsaturated carbonyls were studied by using a quadrupole mass filter (QMF) and a Trochoidal electron monochromator"-'* (TEM) as the electron source. A monochromatic electron beam was produced (0.2 eV at fwhm) by the TEM. The ions obtained by the interaction of the electron beam with the sample were analyzed by the Q M F as a function of electron energy. A time-of-flight system was also used to determine the excess energy imparted to the fragments during or after ionization proces~es.'~-'~The details of the experimental setup have been described p r e v i o u ~ l y . ' ~The ' ~ experiments associated with C2H3COXcompounds were carried out at the Institut fur Physikalische Chemie,I3J4 Freie Universitat Berlin, whereas experiments associated with C6H~COX compounds were carried out at the Chemistry Department of Middle East Technical University. Thus, the ion yields (given in arbitrary units) cannot be directly compared for ions from C,&COX and C2H3COX. Results and Discussion Low-energy electron attachment to unsaturated carbonyls C6H5COX (X = C1, OH, H, or CH3) and C2H3COX (X = c1 ( 1 1) Hacaloglu, J.: Suzer, S.; Oster, T.; Illenberger, E. Chem. Phys. Lett. 1988, 153. (12) Stamatovic, A,; Schultz, G. J. Rev. Sci. Instrum. 1968, 39, 1732; 1970, 41, 423. (13) Illenberger, E. Chem. Phys. Lett. 1981, 80, 153. (14) Illenberger, E. Eer. Bunsen-Ges. Phys. Chem. 1982.86, 247. (15) Harland, P. W.; Franklin, J. L. J. Chem. Phys. 1974, 61, 1621.
0 1989 American Chemical Society
7096 The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
Hacaloglu et al.
TABLE I: Appearance Potentials (AP, eV) and Relative Interrsities (RI) of Negative Ions from C&COX, Where X = Cl, OH, H, or CH3 C6H5COCI C6H5COOH C 6H 5C 0H C&@CH, ion APQ RI AP RI AP RI AP RI 0-
8.0
2
0.0 2.7 5.3 4.4 5.0
1000 400 200 1 1
7.6 0.6 1.8
1 5 0.5
C6H,COH,C6H5COO-
6.8 7.8
6.0
1.3 1.9
1.5
500 Wb
50
1000 100
6.6 7.6
1000 W
7.0 7.2
50 25
7.4 1.5 8.2
25 500 50
7.0 8.4 6.8
500
6.4 8.4 1.5 6.8 1.6 2.1 8.9 0.6
50 50 600 25 1000 100
W
25
W W
30
"Experimental uncertainty is 0.2 eV. *Weak.
h
CHJCO-
7:
X3
ii
I
X=CH
' I
X=H
I
4
I ,
Figure 2. Formation of C6H5CO-from various C6HSCOXmolecules. /
I
I ! !,
I
1
,
1
,
I 0'
0
u)
8.0
electron energy l e v ) +
Figure 1. Observed negative ions from acetophenone, C6HSCOCH3,in the 0-10-eV electron energy range. The electron energy is calibrated by taking the appearance potential of SF6- ions as 0 eV, and the energy distribution is measured to be less than 0.2 eV at fwhm again measured On SF6-.
or OH) shows two distinct resonances in the range 0-10 eV. Furthermore, time-of-flight measurements indicate that none of the ions from these molecules have significant translational energy. Thus, from the energy considerations it is clear that most of the ions formed at low energies arise from simple dissociative attachment processes yielding one ionic and one neutral fragment, whereas multiple fragmentation occurs at relatively high energies, producing more than one neutral fragment. The 0-, c z o - , Y - , and Y c o - ions (Y = C6H5 or CzH3) are the common ions observed from these molecules, among which only Y- and YCO- are due to simple fragmentation processes.
The results and discussions of them will be presented for C6H5COXand CzH3COX separately, and in the last part they will be compared together. A . C6HSCOX. The experimentally determined appearance potentials of the observed ions from C6HsCOX are given in Table I. The low-energy negative ions from acetophenone, C6HSCOCH3, are plotted in Figure 1. ( i ) Formation of C6H5CG. Of the common ions observed, benzoyl anion, C6H5CO-, is one of the weak ones (Figure 2), and its formation at low energies is associated with the reaction C6HsCOX
+ e-
-+
C ~ H J C O X * - C ~ H S C O+ - x -+
where X = C1, OH, or CH3. The appearance potential of c6H5CO- shifts to lower energy in the order X = CH3 > O H > C1 (AP(C6H5CO-) = 2.1, 1.9, and 1.8 eV for X = CH3, OH, and C1, respectively). The electron-releasing effect of the X group decreases from CH3 to C1. Furthermore, it is known16 that when the COX group is bound to an unsaturated group, its conjugation with the unsaturated group is more extensive than the X substituent having lone pairs such as O H and C1. Generally, both a molecular orbitals of C6H5and no molecular orbitals of co are (16) Patai, S. The Chemistry of Acylholides; Interscience Publishers: London, 1972. (17) Rabalais, J. W.; Cotton, R. J. J . Electron Spectrosc. 1972173, I , 83.
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7097
Negative Ions from Unsaturated Carbonyls
X.5
r)
TABLE I 1 Appearance Potentials (AP) and Relative Intensities (RI) of Negative Ions from C2H3COOHand C2H$XKI
11
C2H3COOH RI
ion
CH2-
7.2 4.0 6.2 7.4 5.5 2.2 5.0
0-
OHC2HC2H3-
c1c20-
0.2 1.6 1.6 2.8 0.0 2.5
HC2OC2H3COC2H3COO-
x5
'I! , I
.
I
10
30
50
70
90
electron energy lev)+
Figure 3. Formation of C6Hr from various C6HSCOXmolecules.
stabilized with increasing electronegativity of X.z" Therefore, it can be expected that the C-X borid of C6HSCOXmust be stronger when the X group has an electron-releasing power like CH3. The observed shift in appearance potentials confirms that the C-X bond dissociation energy decreases in the order X = CH3 > O H > C1. The benzoyl anion from benzaldehyde is observed at relatively high energy ( A P ( C ~ H @ - / C ~ H S C H O ) = 8.2 e v ) and must necessarily be from an electronically excited state of the molecule. The high-energy resonance from acetophenone may lead to a multiple fragmentation reaction producing more than one neutral fragment or may be due to fragmentation from an electronically excited state like in the case of benzaldehyde. (ii) Formation of C a s - . The formation of phenyl anion at low electron energy is associated with the reaction C ~ H S C O X+ e-
+
C&sCOX*-
-
C6H5-
+ cox
where X = Cl, OH, H, or CH3. The energy of the fragmentation process can be given as AH,,,,, = AP(C,H,-)
6 7 49 1 25
300
= D(C6H5-COX) - EA(C6Hs)
+ E*
where AP is the appearance potential, D is the dissociation energy, EA is the electron affinity, and E* is the excess energy. The ionization probability curves of C6H5- ion are given in Figure 3. It can be seen from the figure that the appearance potential of C6Hs- anion shifts to lower energy in the order X = C H 3 > H > O H > C1 (AP(C,H,-) = 1.6, 1.5, 1.3, and 0.6 eV for X = CH3, H, O H , and C1, respectively). The observed trend indicates a decreasing interaction between C6H5and the COX group in the order X = CH3 > H > O H > C1. Photoelectron spectroscopic
6.5
Wb
2.8
6
0.0 2.6 0.3 6.0 5.4 2.4
1000
60 1 15 1.5 1.5 1000
W
0.1 W
0.4
91 0.0
'Experimental uncertainty is 0.2 eV.
x-CI
I
AP,eV
CZHICOCI-
X:OH x25
f'
C2H3COCI RI
AP,"eV
0.05
Weak.
studies reveal more or less a constant value (9.6 eV) for the benzene 7r molecular orbitals in these molecules with no observable splitting, indicating a very small resonance interaction with X s u b s t i t u t i ~ n . ~Hence, * ~ ~ ~the observed variations in the appearance potentials of the C6H< ion from these molecules cannot be correlated with photoelectron data. However, the net dipole moment of the C6H5-COX bond given in the literaturel6,l8decreases in the order X = CH3 > H > O H > C1, in agreement with our experimental results. Only for acetophenone, the C6HS-COX bond dissociation energy is given in the 1iteraturelgas 4.033 eV. The excess energy imparted to the fragmentation process can be approximated to zero since no thermal energy distribution is observed in the time-of-flight analysis. The electron affinity of C6HScan now be calculated from the energy balance equation as 2.4 eV. This value is high compared with the literature value,*O 1.1 eV. The values we calculate using our experimental appearance potentials are constantly 1-2 eV higher than the other experimentally determined v a l ~ e s . ~ O -This ' ~ discrepancy must be due to the low Franck-Condon factors governing the dissociative attachment process. Although we cannot obtain reliable absolute values for electron affinities or bond energies using our data, we can obtain much more reliable relative values. For example, taking the available bond energy of acetophenone as our reference, C6H5COX bond dissociation energies can be estimated as 3.9 eV for X = H , 3.7 eV for X = OH, and 3.0 eV for X = C1 which are not available in the literature. The resonances a t relatively high energy region (Figure 3) observed for X = CH3 and O H may be due to multiple fragmentation reactions producing more than one neutral fragment or may be due to fragmentation from an electronically excited state. B. C2H3COX. The experimentally determined appearance potentials of the ions observed from C2H3COX where X = C1 or O H are given in Table 11. All the common ions 0-,CzO-, HC20-, C2H3-, and C2H3CO- except C2H3- and CzH3CO- are due to multiple fragmentation processes yielding more than one neutral fragment. ( i ) Formation of CpY3CCr. C2H3CO-anion formation (Figure 4) is associated with the reaction
+ e-
C2H3COX
-
C2H3COX*-
-
CzH3CO- + X
where X = C1 or OH. It can be seen from the figure that the (18) Zlatkis, A.; Breitmaier, E.; Gunther, J. A Concise Introduction Io Organic Chemistry; McGraw-Hill: Kogakusha, LTP, 1973. (19) Weast, R. C., Ed. Handbook of Chemistry and Physics, 67th ed.; CRC Press: Boca Raton, FL, 1986-1987. (20) Illenberger, E.; Comita, P. B.; Brauman, J. I.; Fenzlaff, P.; Martin, H.; Heinrick, N.; Koch, W.; Frenking, G. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 1026, and references cited therein.
7098 The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
Hacaloglu et al.
d
,I
'\
I'!
z
I '
,-,-
-a L
I
,
M
.
, L;:-
.._,.-,
,
,-
LO 0.0 electron energy(eVI+
L/
1 0.0
.+-.-#-. j/
L.,
.
1.0 e.0 electron energy(eV) j
*.--.
11) electron energy
80 lev)+
Figure 4. Formation of C2H3CO-and C2H< from C2H3COOHand CZH3COCI.
Figure 5. Formation of C6H5COO-from C6H5COOHand C2H3COOfrom C2H3COOH.
appearance potential of C2H3CO-shifts to lower energy in the order X = OH > C1 like in the case of C6HSCO-. The appearance potential of C2H3CO-is 2.8 eV for X = OH and 2.4 eV for X = C1. The overall electron-withdrawing power of C2H3C0in acryloic acid is greater than that of acryloyl chloride due to the high electronegativity of chlorine. Thus, it is expected that the C2H$O-X bond dissociation energy is greater in C2H3COOH as indicated by the observed shift in appearance potentials in the order OH > C1. (ii) Formation of C2H3-. The ion yield CUNW for CzH< anion from C2H3COXwhere X = CI or OH are also given in Figure 4. Formation of C2H3- can be associated with the reaction
give a shorter bond length for Y = CzH3,indicating a stronger bond. Another common ion due to simple fragmentation processes is the YCOO- anion from the unsaturated carboxylic acids under investigation (Figure 5). The reaction associated with the fragmentation process can be given as
+
-
C2H3COX e .+ C2H3COX*-
C2H3- + COX
The appearance potential of C2H< from acryloic acid is lower than that of acryloyl chloride. The electron-withdrawing power of the COCl group is greater than that of COOH. From photoelectron studies2 it was observed that both C=C a and C=O no molecular orbitals are stabilized with increasing electronwithdrawing power of the X substituent, indicating that the C2H3 group releases electrons to the carbonyl group more efficiently. Our experimental results are in agreement with this trend, and the C2H3-COCl bond is stronger than the C2H3-COOH bond. c. c6H5coXand C2H3COX. In general, the appearance potentials of common ions resulting from simple fragmentation processes of unsaturated carbonyls YCOX, where Y = C6H5or C2H3, mainly YCO- and Y-, increase in the order Y = C2H3 > CsHs (Tables I and 11). This trend can be explained by the greater electron-withdrawing power of C2H3C0than that of C6HSC0 in the case of YCO- anion. Moreover, it was given in the literature that the YCO-X bond length in acrylic acid is shorter than that in benzoic acid,21 indicating a greater interaction with the X substituent in the case of acrylic acid. Our experimental observations are in parallel with this general behavior. However, our experimental results show a reversal for the Y- anions although the electron-releasing power of C6Hsis greater than that of CzH3. This reversal could be due to the differences in the electron affinities of C6H5 and C2H3. Data related with Y-COOH bond 1engths2l are in agreement with our experimental findings and ~~~~
~~
~
(21) Patai,S . The Chemistry of Carboxylic Acids and Esters; Interscience
Publishers: London, 1969.
YCOOH
+ e-
-
YOOCH*-
-
YCOO-
+H
The appearance potential of C2H3COO-is considerably lower than that of C6HsCOO- (AP(C2H3CO-)= 0.0 eV, AP(C&COO- = 1.6 eV). As the electron-withdrawing power of C2H3C0is greater than that of C6HSC0,one would expect the interaction between OH groups to be greater in the case of C2H3COOH also. The release of electrons from the OH oxygen will in turn increase electron-withdrawing power of hydroxyl oxygen toward hydrogen. Thus, 0-H bond dissociation energy in acrylic acid should be greater than in benzoic acid, in agreement with the observed acid in aqueous solutions. The observed reversal dissociation con~tants'~ of these appearance potentials can only be explained by proposing that at 1.6 eV the C6H5COO-anion is formed from an electronically excited state. Similarly, the second resonance for C,H3COO- at 2.3 eV can be attributed to fragmentation from an electronically excited state of C2H3COOH,too.
Conclusion At low electron energies, the main negative ionization process for unsaturated carbonyl derivatives is the dissociative electron capture, as was reported for the acetyl derivatives.I0 The appearance potentials of negative ions from different molecules can best be correlated with bond dissociation energies or bond lengths which in turn can be correlated with electron releasing or withdrawing power of the substituents. The observed appearance potentials can be used to estimate relative bond energies as well. Taking the literature value of 4.033 eV for the C6H5-COCH3 molecule, the C6H5-COX bond energies can be calculated as 3.9, 3.7, and 3.0 eV for X = H, OH, and C1, respectively, which are not available in the literature. Acknowledgment. We gratefully acknowledge the generous funds provided by the Volkswagen-Stiftung of the FRG. Registry NO. C~HSCOCI, 98-88-4; C~HSCOOH, 65-85-0; C,HSCOH, 100-52-7; CsHSCOCH3, 98-86-2; C2H3COC1, 8 14-68-6; C2H3COOH, 79-10-7.
