J. Phys. Chem. 1987, 91, 17-19
17
Determinatlon of the C-H Bond Dissociation Energies of Ethylene and Acetylene by Observation of the Threshold Energies of Ht Formation by Synchrotron Radiation Haruo Shiromaru, Yohji Achiba,+ Katsumi Kimura,* and Yuan T. Lee* Institute for Molecular Science, Okazaki 444, Japan (Received: September 4, 1986)
To determine the C-H bond dissociation energies of ethylene and acetylene (R-H), we have measured the threshold energies of H+ formation using synchrotron radiation in the wavelength region 58-70 nm. Subtracting the ionization potential of the hydrogen atom (1 3.598 eV) from the observed threshold energies, we have deduced values of 5.06 f 0.05 and 5.75 i 0.05 eV for the C-H bond dissociation energies Do(R-H) of ethylene and acetylene, respectively.
Introduction The C-H bond dissociation energies D,(R-H) of ethylene (C2H4) and acetylene (C2H2)are among some of the most important quantities in chemistry which need to be accurately determined. Acetylene is especially important in the area of combustion and has received a great deal of attention in recent years. So far, several different values have been reported for the C-H bond dissociation energies of ethylene and acetylene, showing some scatters to a considerable extent as summarized in Table I.'-' In most of these studies,'" the C-H bond dissociation energies Do(R-H) of given hydrocarbons (RH) have been determined from the measurements of the threshold energy Eth(R+) of the R+ formation in the dissociative ionization process R-H
+ hv
-
R+
+ H + e-
(1)
and the ionization potential of the radical Zth(R), by using the relationship DO(R-H) = Eth(R+) - Ith(R)
(2) The energy diagram relevant to R+ formation is schematically shown in Figure 1. For acetylene molecule, Botter et al. (BDWR)' and Dibeler et al. (DWM)2have earlier obtained Eth(C2H+)= 17.22 and 17.36 eV, respectively, which are similar to each other. Recently, Ono and Ng (ON)3 have deduced a value of 16.79 eV for Eth(C2H+) from their photoionization experiments, indicating that the onset of the photoionization efficiency curves of C2H+ has a weak tail toward lower energy. This value of Eth(C2H+)is smaller than the earlier values by 0.4-0.5 eV. Such significant deviations in Eth(C2H+)among the different studies are quite disturbing, and it makes accurate assessment of Do(R-H) more difficult. Furthermore, different values (1 l .6, l l .96, and l l .5 l eV) for Zh(C2H) have been reported by Wyatt and Stafford (WS),4 Okabe and Dibeler (OD),S and Miller and Berkowitz (MB).6 These differences in Zth(C2H)also add to the uncertainties in accurate evaluation of Do,as seen from Table I. Very recently, Wodtke and Lee (WL)' have obtained Do(CzH-H) = 5.72 eV from measurements of the maximum translational energy release in the process C2H2+ hu (193 nm) C2H H (3)
-
+
suggesting that the ionic species detected by Ono and Ng3 may be CzH+ produced from the ion pair formation (R+ H-), since the difference in Eth(C2H+)between BDWRl-DWM2 and ON3 is nearly equal to the electron affinity of H atom. Concerning the ethylene molecule, three different values (1 3.80, 13.25, and 13.22 e v ) for Eth(C2H3+)have been reported by Botter et al. (BDWR),8 Chupka et al. (CBR)? and Stockbauer et al. (SI).'O For Ith(C2H3),the most reliable value may be the value
+
Present address: Department of Chemistry, Tokyo Metropolitan University, 2- 1-1 Fukazawa, Setagaya-ku, Tokyo 158, Japan. *On leave from Lawrence Berkeley Laboratory and the Department of Chemistry, University of California, Berkeley, CA 94720, as IMS visiting professor (March-June, 1986).
0022-3654/87/2091-0017$01.50/0
TABLE I: C-H Bond Dissociation Energies (in eV) of Ethylene and Acetylene
Dn(R-H
authors Ethylene BDWR (ref 8) CBR (ref 9)
4.85 4.30 4.27 4.7 1 5.06 f 0.05
SI (ref 10)
WHDL (ref 13) this work
5.26-5.1 1" 5.40-5.85" 4.83-5.28" 5.72 5.12 f 0.23b 5.75 f 0.05
Acetylene BDWR (ref 1) DWM (ref 2) ON (ref 3) WL (ref 7) ref 16
this work
"The uncertainties of the Do values are due to the variation in the fth(C2H)literature values. Derived by combining the electron affinity of C2H (2.94 f 0.10 eV, ref 16) with the gas-phase acidity of C2H2 (16.39 f 0.13 eV, ref 17).
of 8.95 eV reported earlier by Lossing,'l giving rise to Do(C2H3-H) = 4.27-4.85 eV. Therefore, the Do values again show some scatters, as seen from Table I. In the present work, using synchrotron radiation we have deduced the C-H bond dissociation energy Do(R-H) from another relationship, namely D,(R-H) = Eth(H+) - Z(H)
(4)
by measuring the threshold energy of H+ formation in the process RH
+ hv-,
R
+ H+ + e-
(5)
This process (5) is also shown schematically in Figure l . The advantage of such a determination of D,(R-H) is the following. (1) Since the ionization potential of the H atom is accurately known to be 13.598 eV, the derivation of Dois straightforward from the observed threshold energy of the H+ formation, as can be seen from Figure 1. (2) Compared with a conventional vacuum-UV light source, a synchrotron radiation source is much more (1) Botter, R.; Dibeler, V. H.; Walker, J. A.; Rosenstock, H. M. J . Chem. Phys. 1966, 44, 1271. (2) Dibeler, V. H.; Walker, J. A.; McCulloh, K. E. J . Chem. Phys. 1973, 59, 2264. (3) Ono, Y.; Ng, C. Y. J . Chem. Phys. 1981, 74, 6985. (4) Wyatt, J. R.; Stafford, F. E. J. Phys. Chem. 1972, 76, 1913. (5) Okabe, H.; Dibeler, V. H. J . Chem. Phys. 1973, 59, 2430. (6) Berkowitz, J. Photoabsorption, Photoionization, and Photoelectron Spectroscopy; Academic: New York, 1979; pp 271-290. (7) Wcdtke, A. M.; Lee, Y. T. J . Phys. Chem. 1985, 89, 4744. (8) Botter, R.; Dibeler, V. H.; Walker, J. A,; Rosenstock, H. M. J . Chem. Phys. 1966, 45, 1298. (9) Chupka, W. A.; Berkowitz, J.; Refaey, K. M. A. J . Chem. Phys. 1969, 50, 1938. (10) Stockbauer, R.; Inghram, M. G. J. Chem. Phys. 1975, 62, 4862. (11) Lowing, F. P. J. Can. Chem. 1971, 49, 357.
0 1 9 8 7 American Chemical Society
18
The Journal of Physical Chemistry, Vol. 91, No. I , 1987
Letters
n
.-cC v)
3
e
U
W
C
Do( R- H)
0 0
c
r
0
lnlll
I
\
Figure 1. Schematic diagram of the energy levels relevant to evaluation of the C-H bond dissociation energy Do(R-H), where Ithis the threshold ionization potential, I(H) is the ionization potential of the H atom, Et,, is the threshold energy of ion formation, and E, is the electron affinity.
suitable to perform photoionization experiments in the wavelength region shorter than about 70 nm. The helium Hopfield continuum is relatively weak in this region, and the cross section of H+ formation is usually smaller than the other fragmentation channels (process 1 ). The conventional vacuum-UV source, however, has the advantage that, in general, there is no problem related to higher order radiation, which the synchrotron radiation source obviously has. In the present paper we report the first measurements of E,,(H+) for ethylene and acetylene using synchrotron radiation and D,(R-H) derived from these measurements.
Experimental Section Photoionization measurements were performed in the energy region 58-70 nm, by using synchrotron radiation from the beam line BL2-B2 of the UVSOR facility (750-MeV electron storage ring) a t this Institute.'* The circulation current of electrons in the ring during the present measurements was 20-60 mA. The whole apparatus used here consists of a 1-m Seya-Namioka monochromator and a molecular-beam photoion-photoelectron apparatus. Synchrotron radiation is focused on the entrance slit of the monochromator by three prefocusing mirrors, and then further focused on the ionization region by a toroidal mirror after the monochromator. The main chamber which includes the ionization region is separated from the chamber of a molecular-beam source, by using a skimmer. The ionization chamber and the molecular beam source are evacuated separately by two turbo molecular pumps (1000 and 1500 L/s) and by an oil diffusion pump (5000 L/s), respectively. Four differential pumping arrangements are used between the monochromator and the molecular-beam apparatus to keep the monochromator below 2 X lo4 Torr. The instrumental setup will be described elsewhere in more detail. A gaseous sample was introduced through a nozzle as a free jet to produce collision-free conditions under which secondary ion-molecule reactions of primary photoions are made negligible. In the present experiments, cluster formation was negligibly small. A quadrupole mass filter (ULVAC Model MSQ-400) with a Channeltron was used for the detection of H+ formation. In order to eliminate the second-order radiation from the diffractive grating of the monochromator, helium gas was introduced into the monochromator at a pressure of less than 0.1 Torr. For both (12) (a) Watanabe, M.; Uchida, A.; Matsudo, 0.;Sakai, K.; Takami, K.; Katayama, T.; Yoshida, K.; Kihara, M. IEEE Tram. Nucl. Sa'. 1981, NS-28, 3175. (b) Koyano, I.; Achiba, Y.; Inokuchi, H.; Ishiguro, E.; Kato, R.; Kimura, K.; Seki, K.; Shobatake, K.; Tabayashi, K.; Takagi, Y.; Tanaka, K.; Uchida, A.; Watanabe, M. Nucl. Instr. Methods 1982, 195, 213.
C
-
.-0
60
64
68
Wavelength (nm)
-
Figure 2. The efficiency curves of the H+ formation in the process C2H, hv C2H, H + e-. These efficiency curves were observed by using synchrotron radiation through the helium filter gas at three different pressures: (a) 0 Torr, (b) 0.050 Torr, and (c) 0.098 Torr. The background signals due to second-order radiation are shown by solid lines. The onset of H + formation is indicated by an arrow (1).
+
+
;I
+
.-..
60
-
64
68
Wavelength (nm)
+
Figure 3. The efficiency curve of H + formation in the process C2Ht hu C 2 H H+ + e-. This efficiency curve was observed by using synchrotron radiation through the helium filter gas at 0.1 Torr. The background signal due to second-order radiation is indicated by a solid line. The onset of H+formation ishdicated by an arrow (1).
+
ethylene and acetylene, efficiency curves for H+ formation were obtained as a function of wavelength at an interval of 0.05 nm. Data were accumulated from 5-10 s at each point and averaged after four scans.
Results and Discussion 1 ne HT etticiency curves ot ethylene and acetylene obtained are shown in Figures 2 and 3, respectively. The increasing
Letters
The Journal of Physical Chemistry, Vol. 91, No. I, 1987 19
backgrounds with the wavelength are due to the ionization by residual second-order radiation from the monochromator which contains twice the energy. In other words, the signals due to the second-order radiation still remain to some extent even after He is used to absorb these short wavelength photons. In Figure 2 the background signals due to the second-order radiation are indicated by solid lines. The H+ efficiency curves obtained at different pressures of the helium filter gas are compared in Figure 2: (a) 0 Torr, (b) 0.050 Torr, and (c) 0.098 TOK.The background signals seem not to affect the positions of the onsets. From the efficiency curves shown in Figures 2 and 3, we have obtained
that the 18.66-eV observed threshold is due to the decay of the fifth ionic state (1 2B1,) of ethylene and the real threshold might be slightly lower, since the threshold energy of this ionic state is located at 18.8 eV.15 If so,the value of '18.66eV may be regarded as the upper limit of H+ formation of process 5. For acetylene, there are many vibronic levels near the threshold of H+ formation. The nearest ionic state of acetylene is the 1 'Zr: ionic state located that is about 1.0 eV lower than the at 18.4 eV (thre~hold),'~ 19.35-eV observed threshold energy. Therefore, acetylene is the favorable case in the determination of the adiabatic energy of H+ formation. The onset of H+ formation would also be affected by the ionpair formation
Eth(H+) = 18.66 f 0.05 eV for ethylene
RH E,,(H+) = 19.35 f 0.05 eV for acetylene The corresponding dissociation energies deduced from eq 4 are
Do = 5.06 f 0.05 eV for ethylene
Do = 5.75 f 0.05 eV for acetylene compared with the literature values in Table I. The value of 5.06 eV obtained here for the C-H bond dissociation energy of ethylene is considerably larger than the value of 4.30 (CBR)9 and 4.27 eV (S1).Io Very recently, Wodtke et al.13J4have obtained a value of 4.71 eV from the analysis of the photofragment translational spectra of C2H3Brand the translational energy release in the F + C2D4 C2D3 DF(u=4) reaction. For acetylene, on the other hand, the value of 5.75 eV deduced here is in fairly good agreement with the values of 5.71 (BDWR),I 5.85 (DWM),' and 5.72 eV (WL).' In general, the appearance energy observed for a specific ion in photoionization is regarded as an upper limit of the adiabatic energy of producing the ion, aside from the effect of ion pair formation. If there exist optically allowed rovibronic levels around the adiabatic energy, and if the internal energy of the ion is transferred to vibrational modes associated with the dissociation, then the appearance energy should correspond to the adiabatic energy. (In the case of methane, for example, the process CH4 CH, H+ e- has a threshold at 18.072 eV which is beyond the (1 t2)-l band, and the H+ formation does not begin to appear significantly until the threshold of the higher (2al)-' band is attainede6) In the present work, the observed thresholds of H+ formation for ethylene and acetylene have been assigned to process 5. However, in the case of ethylene there might be the possibility
-
-
+
+
+
(13) Wodtke, A. M.; Hintsa, E. J.; Dubourg, I.; Lee,Y. T., to be submitted for publication. (14) Parson, J. M.; Lee, Y. T. J . Chem. Phys. 1972, 56, 4658.
+ hu
-
R-
+ H+
(6)
if the ion-pair formation of process 6 were to be important in the threshold region of process 5. The threshold energy of the H+ formation in process 6, however, should be much lower than that in process 5 for acetylene and ethylene. The electron affinity of the C2H radical is reported to be 2.94 eV.I6 Although no data have so far been reported on the electron affinity of the C2H3 radical, this is considered to be smaller than that of the C2H radical. Furthermore, it is expected that the cross section of process 6 is very small compared with that of process 5. Thus, H+ signals due to ion-pair formation would appear as a spectral tail toward the longer wavelength. The efficiency curves of H+ formation are considered to be free from the effect of ion-pair formation. The most important aspects in the present work are that the C-H bond dissociation energies of ethylene and acetylene are determined from the threshold energies of H+ formation from these compounds through reaction 5. The synchrotron radiation technique of measuring the threshold energy of H+ formation provides a useful method for the determination of the C-H bond dissociation energies of various organic molecules. It should be mentioned here that the use of synchrotron radiation has problems related to higher order radiation. The synchrotron radiation has especially a great intensity advantage, but it is often required to remove higher order radiation. The effect of higher order radiation largely depends on the optical materials such as the mirrors and gratings used. In this sense, the effective removal of higher order radiation is an important task in synchrotron radiation experiments. (15) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S . Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules; Halsted: New York, 1982. (16) Janousek, B. K.; Brauman, J. I.; Simons, J. J . Chem. Phys. 1979, 71, 2057. (17) Mackay, G. I.; Bohme, D. K. In!. J. Mass Spectrum. Ion. Phys. 1978, 26, 327.
