739
J. Phys. Chem. 1982, 86, 739-747
torr-' exponential decays observed in both the 8- and 5-pm fluorescences. If we choose for the elementary rate constants the values k24 = 20 ms-' torr-' (800 collisions) and k4r = 50 ms-' torr-' (325 collisions), we obtain 160 and 36 ms-' torr-' as the fluorescence rise rates for the v4 state with the amplitude of the faster term twice that of the slower term. The effects of other processes which must occur in the complete energy transfer pathway will modify the values of the elementary rate constants in order to maintain agreement with the observed rate parameters. For example, deactivation of COFz(x)states must occur at a near gas kinetic rate via processes such as COFZ(2v5) + COF,(O) + ~ C O F ~ ( U ~ ) C0F2(2~3)+ COF2(O) + ~ C O F ~ ( V J C O F ~ (+ U ~~ 5 +) COFZ(0) + COFZ(v3) + C O F ~ ( V ~ )( 5 ) In addition, there is some evidence in other fluorescence emissions1° that the relaxation of vz via COFZ(v2)+ COF2 + COFz(v6) + COF,
+ 199 cm-'
(6)
may be competitive with deactivation through v4 (process 3).
Conclusions (1) The v4 asymmetric stretch mode of COF2fills from the vz symmetric stretch level via a relatively inefficient collisional mechanism which is translationally endothermic.
(2) Efficient loss of population from u4 occurs rapidly (5100 collisions) through the overtone, combination levels 2v3, 2v5, u3 v5 which are nearly resonant with v4. (3) The slow filling of v4 coupled with its rapid emptying creates a kinetic anomaly in which the filling and emptying rates both appear in the rise of fluorescence from this state. (4) The initially pumped mode u2 of COF, appears to decay via two competing mechanisms (the endothermic path through v4 and an exothermic path through 4). ( 5 ) The rate of collision-induced intermode energy transfer in COF2is quite slow (2475 collisions) for a four atom polyatomic. The features which seem to lead to this situation are the relatively wide spacing of vibrational mode frequencies and the presence of all heavy atoms in the molecule. Thus the amplitude of vibrational motions and the ability of rotations to take up energy are reduced compared to hydrogen- or deuterium-containing species. (6) The overall vibration-translation/rotation relaxation process for pure COF2 requires approximately 2800 collisions on the average.
+
Acknowledgment. We thank Marsha Lester and Mary Mandich for many fruitful discussions. This work supported by the Joint Services Electronics Program (US. Army, U.S. Navy, and U.S. Air Force) under Contract DAAG29-79-C-0079 and by the National Science Foundation under grant CHE 80-23747. Equipment support was supplied in part under Department of Energy Contract DE AS02 78ER04940.
Thermochemistry and Dfssociation Dynamics of State-Selected C,H,O,+ 1,4-Dioxane
Ions. 1.
Marla L. Fraser-MonIelro,+ LUISFraser-Montelro,$ James J. Butler, Tomas Baer, Department of Chemistty, Unlverslfy of North Carollna, Chapel Hill, North Carolina 27514
and J. Ronald Has8 Laboratory of Environmental Chemistry, National Instltufe of Environmental Health Sciences, Research Triangle Park, North Carolina 27709 (Received JuW 31, 1981; In Final Form: October 1, 1981)
The photoionization efficiency curves and appearance energies for C4H802+, C3H60+,C3H50+,C2H50+,C2H40+, C2H30+,CH30+,CH20+,CHO+, and C2H4+from 1,4-dioxanehave been obtained. Structures and heats of formation for some of the ions are proposed. Fragment ion masses 44 (C2H40+), 45 (C2H50+)and 58 (C3H60+) are the !owest energy dissociation products which appear at a photon energy of about 10.5 eV. All three ions are produced from long-lived (metastable) parent ions up to an energy of about 11.2 eV. The decay rates of internal energy selected parent ions were measured by photoion-photoelectron coincidence (PIPECO) and the results were compared to statistical theory (RRKM/QET) calculations. The three dissociation channels were found to be in competition with each other, and the data indicate that the dioxane ion does not isomerize to a more stable structure prior to dissociation. Kinetic energy release associated with the production of the m / e 58 fragment was measured as a function of the parent ion internal energy and found to be somewhat greater than that predicted by the statistical theory.
I. Introduction Upon ionization, the isomers of C4H802produce a rich assortment of fragment ions and neutrals. Some of these, such as C2H40f, C3H60+, C3H6O2+, and CH20+, are well-known and stable ions. whose amearance enereies can be readily calculated because the h&ts of formati& of the + Faculty
of Science, University of Lisbon, Portugal.
* Faculty of Science and Technology, New University of Lisbon,
Portugal.
0022-3654/82/2086-0739$01.25/0
ionic and neutral products are e~tablished.'-~However, the electron impact miss spectra of three of the C4H802 isomers, dioxane, n-butanoic acid, and ethyl acetate, are (1) H. M. Rosenstock. K. Draxl. B. W. Steiner. and J. T. Herron. J. PhYs'. Chem. Ref. Data, 6 , 1 (1977). (2) S. W. Benson in "Thermochemical Kinetics", Wiley-Interscience, New York, 1976. (3) J. B. Pedley and J. Rylance, 'SUSSEX-N.P.L. Computer Analysed Thermochemical Data: Organic and Organometallic Compounds", University of Sussex, Sussex, 1977.
0 1982 American Chemical Society
740
The Journal of Physical Chemistry, Vol. 86, No. 5, 1982
remarkably different.4 This suggests that (a) the parent C4H802+ions do not isomerize to a common structure prior to dissociating, (b) the dissociation paths of these ions are very specific, and (c) the lowest energy products are not necessarily formed. In addition McAdoo et al.5 have recently interpreted the mass spectral data of butanoic acid in terms of noninterconverting (or isolated) electronic states. An isolated electronic state is one which does not undergo a radiationless transition to another (generally lower) electronic state prior to dissociation or fluorescence. Because the only known cases of ionic isolated states in polyatomic species involve small ions such as N20,6s7 CH3Br,8CH,C1,8 C2F6: CH,NO,'O and CF3C1,11the claim of an isolated state in such a large and flexible ion as n-butanoic acid must be regarded with some caution. Although a number of electron impact12-14studies of these ions have been carried out, no mass-analyzed photoionization data on any of the C4H602molecules have been reported. Photoionization efficiency (PIE) curves (mass-analyzedion signal vs. photon energy) yield accurate appearance energies (AE) for the various fragment ions. From the AE's it is often possible to determine the structure of the ion or neutral fragmentation products. Still more detailed information can be derived from photoion-photoelectron coincidence (PIPECO) experim e n t ~ .By ~ ~this technique parent ions are prepared in selected internal energies. The ion time-of-flight (TOF) distribution, which is obtained by using zero kinetic energy electron and ion signals as starts and stops, respectively, contains dynamical information such as the dissociation rate, k(E), or the kinetic energy release distribution (KERD). In this study we have applied photoionization and PIPECO techniques to the study of the dissociation dynamics of dioxane, butanoic acid, and ethyl acetate. In addition these data have been supplemented by medium-resolution electronic impact results. 11. Experimental Section A. Photoionization Efficiency Curves. The photoionization apparatus have been described previously.16 Briefly, light from a hydrogen discharge lamp was dispersed with a 1-m normal incidence monochromator. The 200-pm slit widths resulted in a resolution of 0.2 nm (17 meV at 120 nm). The light intersected a jet of sample gas emanating from a hypodermic needle. The resulting photoions were extracted with a small electric field and focused onto the entrance of a quadrupole mass filter. The mass-selected ions, collected as a function of the photon (4) E. Stenhagen, S. Abrahamsson, and F. McLafferty, "Registry of Mass Spectral Data", Wiley-Interscience, New York, 1974. ( 5 ) (a) D. J. McAdoo, D. N. Witiak, and F. W. McLafferty, J . A m . Chem. Soc., 99, 7265 (1977); (b) D. J. McAdoo, D. N. Witiak, F. W. McLafferty, and J. D. Dill, ibid., 100,6639 (1978). (6)J. H. D.Eland, Int. J. Mass Spectrom. Ion Phys., 12,389 (1973). (7)I. Nenner, P. M. Guyon, T. Baer, and T. R. Govers, J. Chem. Phys., 72, 6587 (1980). (8)J. H.D.Eland, R. Frey, A. Kuestler, H. Schulte, and B. Brehm, Int. J. Mass Spectrom. Ion Phys., 22, 155 (1976). (9)M. G. Inghram, G. R. Hanson, and R. Stockbauer, Int. J. Mass Spectrom. Ion Phys., 33, 253 (1980). (10)G. G. Meisels, T. Hsieh, and J. P. Gilman, J. Chem. Phys., 73, 4126 (1980). (11)I. Powis, Mol. Phys., 39 311 (1980). (12)F. M. Benoit, A. G. Harrison, and F. P. Lossing, Org. Mass Spectrom., 12, 78 (1977). (13)J. S.Smith and F. W. McLafferty, Org. Mass Spectrom., 5 , 483 (1971). (14)J. E. Collin and G. CondC, Bull. Cl. Sci., Acad. R. Belg., 52, 978 (1966). (15)T.Baer in "Gas Phase Ion Chemistry", M. T. Bowers, Ed., Academic Press, New York, 1979,Chapter 5. (16) J. J. Butler and T. Baer, J. Am. Chem. SOC.,102,6764 (1980).
Fraser-Monteiro et al.
ELECTRONS
IONS
L.
i+
I
/---
+
* l V JANAL"2ER LT'CHANluFf-+
PDF l I / C ?
Figure 1. The photoion-photoelectron coincidence experiment.
energy and normalized with respect to the photon intensity, constitute a photoionization efficiency curve. Because quadrupole mass filters have discrimination factors which depend on the ion mass, we normalized the ion signals at a specific photon energy by a time-of-flight (TOF) analysis in which the mass filter was replaced by a 15-cm drift tube. There is no mass discrimination in a TOF analysis and, in addition, all ions are collected at the same time, thereby eliminating problems associated with long term drift of detector efficiencies,pressure variations, etc. On the other hand, the resolution was not as good as with the quadrupole so that the TOF analysis was used only as a calibration of the ion intensities. B. Threshold Electron Energy Analysis. As in previous studies,16the threshold, or zero kinetic energy, electrons were analyzed by a set of collimated holes with a lengthto-diameter ratio of 40 (Figure 1). The principle of this method is that initially zero energy electrons gain a velocity from the applied electric field in the direction of the collimated holes, pass through them, and are detected. On the other hand, energetic electrons generally have an initial perpendicular velocity component and therefore strike the walls of the collimated holes and are lost. Energetic electrons initially ejected toward and away from the collimated holes are also detected. However, at low energies these electrons contribute only a minor fraction of all the electrons. The overall resolution of the electron energy analyzer and photon monochromator is about 35 meV. C. Photoion-Photoelectron Coincidence. The lifetimes and kinetic energy release distributions of ions in selected internal energy states were measured by photoion-photoelectron coincidence (PIPECO). The principles and experimental aspects of PIPECO have been described previously15J6and are illustrated in Figure 1. Briefly, zero energy electrons are collected in coincidence with parent or fragment ions. The ion internal energy is then given by Eion= hv - IE, where IE is the ionization energy. The coincidence between electrons and ions is achieved by using the electron and ion signals as start and stop pulses to a time to pulse height converter, the output of which is fed to a multichannel pulse height analyzer. Long-lived, or metastable, ions dissociate along the length of the 4.5-cm acceleration region. The resulting fragment ion TOF distribution is asymmetric. This TOF distribution can then be compared to calculated distributions in which the only adjustable parameter is t,he mean ion lifetime. The other parameters such as the parent and daughter ion masses, the acceleration voltages, and distances are fixed by the experiment. In this manner ion lifetimes or unimolecular decay rates for state-selected ions can be determined. In the case of rapidly formed fragment ions, the TOF distribution is symmetric. However, kinetic energy release
The Journal of Physical Chemistry, Vol. 86, No. 5, 1982 741
Thermochemistry of C,H802+ Ions
m/e :58
m/e = 44,45
I
" W
PHOTON ENERGY (ev)
I
Flgure 2. Photoionization efficiency (PIE) curves for p dioxane(+) ( m / e 88). C~H,O+ ( m l e 58), C&,O+ ( m / e 571,C ~ H ~ O( m + / e 451, C2H40+ (m/ e 44), C2H30+ (ml e 43), CH30+ (m/ e 31), CH20+ (m l e SO), CHO' ( m l e 29), and C2H,+ ( m l e 28).
TABLE I: Fragment Appearance Energies (298 K ) of 1 ,4-Dioxanea
a
m le
fragments
58 57 45 44 43 31 30 29 28
C,H,O+ t CH,O C,H,O+ t CH,O C,H,O+ + C,H,,O C,H,O + C,H,O C , H , O t C,H,O CH,O+ + C,H,O CH,O+ + C,H,O CHO' + C,H,O C,H,+ + C,H,O, (or 2CH,O)
IE = 9.19
t
AE,,,, eV 0.10
10.56 * 11.20 * 10.46 t 10.39 i 11.96 * 11.35 i 11.41 t 11.60 f 11.90 t
0.10 0.05 0.05 0.10 0.10 0.05 0.10 0.10
0.01 eV.
manifests itself in a symmetric broadening of the TOF distribution. The latter can be inverted to a kinetic energy release distribution from which the average energy release can be obtained as well. D. Mass-Analyzed Ion Kinetic Energy Spectra (MIKES). Higher resolution mass spectra than the TOF data were obtained on a VG Micromass ZAB double focusing, reversed geometry mass spectrometer. In addition, average kinetic energy releases from mass-analyzed parent ions were measured under various draw out voltages and distances from the ionization region. These conditions were converted to dissociation at various times after ionization. By using the experimentally determined ion lifetime as a function of the internal energy (PIPECO experiment), the dissociation times were associated with an average internal energy. This was done by assuming that at a dissociation time, t , the average contribution is from parent ions with a lifetime, t. 111. Results
A. Photoionization Efficiency (PIE) Curves. The PIE curves for the parent and nine fragment ions from 9 to 13.5 eV are shown in Figure 2. The data were collected with a quadrupole mass filter, but normalized to a constant ion collection efficiency with the TOF mass analysis. The adiabatic ionization energy (IE) of dioxane was found to be 9.19 f 0.01 eV. This is considerably higher than the 1962 value of 9.13 eV obtained by Watanabe.17 An old PES value for the vertical IE is 9.43 eV.18 Onsets
12
13
TIME I4 OF FLIGHT 15 (,us) I6
17
Flgure 3. Timaof-fli ht (TOF) spectra for C3H,0+ (ml e 58), C2H,0+ ( m l e 45), and C2H40 ( m l e 44) fragment ions at three different photon
9
energies in the metastable region.
for the fragment ions appear at various energies and are listed in Table I. All the major ions produced are listed in Table I and illustrated in Figure 2. We searched carefully for H loss producing a fragment ion of mass 87. At energies below 13.5 eV this ion accounted for less than 0.1?& of the ions formed. This result is consistent with the findings of Collin and Cond614 who reported only 1%for mass 87 in an electron impact mass spectrometric study. The lack of obvious peaks in the PIE curves (except for perhaps the peak in the parent ion curve at 10.9 eV) indicates that autoionization is not a major ionization pathway. Most of the sharp structure can probably be ascribed to incomplete normalization of the ion signal with respect to the sharply structured photon signal. B. Dissociation Rates. The dioxane ion is metastable (i.e., lifetimes are in the microsecond range) in the energy region of the onsets for the fragment ions C3H60+( m / e 58), CzH50+( m / e 45), and CzH40+( m / e 44). The lifetimes as a function of the parent ion internal energy were measured by collecting fragment ion TOF distributions in coincidence with zero kinetic energy electrons. The region investigated was from 10.8 to 11.1eV, and representative TOF distributions are shown in Figure 3. Counting times ranged from 6 to 45 h. The points are the experimental results while the solid curves are the calculated TOF distributions based on a single unimolecular decay. No quadrupole mass filter was used for this study. As a result the first peak is a composite consisting of both mass 44 and 45. However, because the masses are so similar, only a small error is introduced by treating this peak as originating from a single mass ion. At least the error is small if the two ions are formed competitively from the same precursor ion. This point is discussed further in section
IV. The calculated TOF distributions are convoluted with a Gaussian function to take into account the thermal broadening of the experimental distributions. In addition, a small correction was made, described in detail previously,16to eliminate contamination of the ion internal energy by energetic electrons which are ejected in the direction of the collimated hole structures.
~~~~
(17) K. Watanabe, T. Nakayama, and J. Mottl, J. Quant. Spectroec. Radiat. Transfer, 2, 369 (1962).
(la) D. A. Sweigart and D. W. Turner, J. Am. Chem. Soc., 94, 5592
(1972).
742
The Journal of Physical Chemistry, Vol. 86, No. 5, 1982
230,
Fraser-Monteiro et al. 298
OK
I
Elements
0 c-
1050
1 I 1100 1125 PHOTON ENERGY (eV1
10.75
--a-
Flgure 5. The relationship between ion energetics at 0 and at 298 K.
_//
0 I025
y,&9
-p;OiecUle
50 k
K
1150
'I75
Flgure 4. The average kinetic energy released, ( T ) , as a function of photon energy for the dissociation of pdoxane(+) to C,H,O+ and CH,O. The two low-energy points are from a MIKES experiment. The solid curve is the result of a QET calculation assuming a CH,-CH,O-CH2+ structure for C3H,0+. The dashed line is the calculated line obtained from the empirical relation E '/0.44N.
C. Kinetic Energy Release for mle 58. Mass 58 was sufficiently well separated from other masses in the TOF spectrum that we could determine the kinetic energy released upon dissociation. This was done in two ways. Above the metastable region, but below the mass 57 onset, the kinetic energy release was determined by PIPECO with the procedure described previously. In the metastable region the TOF peaks were asymmetric, so that no energy release data could be extracted. In this energy region, the kinetic energy release was obtained from peak widths in MIKES spectra taken at several time intervals after ion formation. By converting the mean time to an energy using the ion lifetime data obtained from the coincidence results of Figures 3 and 7, we were able to extend the kinetic energy release data to energies very close to the threshold. These data are shown in Figure 4.
IV. Discussion A. Thermochemistry at 0 and 298 K. A useful property of an ion or molecule is its heat of formation, AHfo. This is a thermodynamic quantity and, at a nonzero temperature, refers to a thermal distribution of internal and translational energies. Photoionization, on the other hand, is a spectroscopic experiment and thus is associated with transitions between specific internal energy states. As experimental accuracies in measuring appearance energies have increased, it has become evident that the extraction of thermodynamic parameters from state .to state experiments requires a precise definition of the fragment ion appearance energy and a careful analysis of the thermal energy.'J9v20 In the discussion which follows the terms appearance energy (AE) and fragmentation onset are used interchangeably to denote the experimentally observed onset in the PIE curves. The AE is obtained by extrapolating the final straight line portion to the background. For the higher energy fragments the vertical scale was considerably expanded in order to detect the first hint of an onset. Yet even with this procedure the higher energy onsets are only upper limits (see following section). The dissociation energy refers to the energy of the products. This can be lower than the AE because of reverse activation energies and (19) H. M. Rosenstock in "Kinetics of Ion-Molecule Reactions", P. Ausloos, Ed., Plenum Press, New York, 1979, p 246. (20) T. Baer in "Mass Spectrometry", Vol. 6, R. A. W. Johnson, Ed., Spec. Period. Rep., The Chemical Society, London, 1981, pp 7-12.
kinetic shifts. Before deriving a heat of formation from the AE, we verified as much as possible that the AE is truly the dissociation energy. In calculating heats of formation it is convenient to derive the AH? at 0 K because at that temperature the AH? has no thermal energy contribution. The AHfoois defined in the same manner as the that is, at both temperatures the heats of formation of the elements are by definition equal to zero (see Figure 5). For a dissociative photoionization process, AB + hv A+ + B + e-, the AE of A+ can be expressed in terms of the AHfoof the various species as
-
AEo(A+)= AHfoo(A+)+ AH,Oo(B)+ AHfoo(e-)- AH,",(AB)
A t 0 K, the heat of formation of the electron, A&Oo(e-), is by definition zero. To obtain the appearance energy of A+ at 0 K from the experimental value at 298 K account has to be taken of the average thermal energy, (Eth),in AB at 298 K which is just the sum of the translational (Ek), vibrational (&b), and rotational (Erot)energies. It has been demonstrated21,22that (Evib) and (Erot)in the precursor AB are available in dissociating the molecule to A+ + B + e-. Thus in order to obtain the appearance energy, AEo, at 0 K, we must add (Evib)and (Erot)of the parent neutral to AE29S (see Figure 5). There is of course a considerable ambiguity in defining the appearance energy. The AE298shown in Figure 5 represents the onset obtained with the linear extrapolation to the baseline procedure. For small molecules with an energy deposition function (photoelectron spectrum) which does not vary greatly in the vicinity of the AE, this approach gives a meaningful onset. For larger molecules, such as are discussed in this work, the procedure becomes more arbitrary. In order to calculate the AHfo298 of a species, say A+, from AHH,O,(A+) we use the following thermodynamic cycle: elements ( 0 K )
1
AH''0
A+(O K )
AHielem)
elements ( 2 9 8 K )
1
t e-(O K )
AHiA')
-
t AH(e-)
A'(298 K ) t e-(298 K )
from which AH?Zga(A+) =
AHfoo(A+)- AH(e1em) + @(A+) + AH(e-1 A problem arises in how the electron is treated in this thermochemical cycle. In the NBS compilation of ionic heats of formation,l the electron is defined as having zero (21) R. Stockbauer and H. M. Rosenstock,Int. J. MQSSSpectrom. Ion Phys., 27, 185 (1978). (22) K. E. McCulloh and V. Dibeler, J . Chem. Phys., 64,4445 (1976).
The Journal of Physical Chemistry, Vol. 86, No. 5, 7982
Thermochemistry of C,H8O2+ Ions
1
OBSERVED ONSETS
I
(OK1
C5H60't CHzO DISSOCIATION L I M I T S (OK1
.
TABLE 11: Thermochemical Data Relevant t o the Dissociation of 1,4-Dioxane
mle
ion
neutral
88 C4H.802' C,H,O, 58 C,H,O+ CH,O 57 C3HSO' 45 CZHSO' 44 C,H,O+
I
-4
I
CH2COH CH;
9.19
4 3 C,H,O+ 31 CH,O+
1 - 2 . 9 6ev
Figure 8. The ionization potential of pdioxane and the appearance energies of its three lowest fragment ions. On the right are shown the 0 K dissociation limb of several Isomeric C,H,O+ ions H,O caiculated by Bouma et ai.25
+
energy at 0 and at 298 K.lB That is AH(e-) 0. On the other hand, the JANAF tables23treat the electron as an ordinary particle and thus AH(e-) = 5/2RT. As a result the two conventions give 298 K ionic heats of formation which differ by 6 kJ/mol. We have chosen to continue the NBS convention by setting AH(e-) = 0. Because we are dealing with gases the AH(A+) is easily calculated and is just ( E a ) + RT. In calculating AH(e1em) we need to evaluate the C d T integral from 0 to 298 K for the graphite form of Carl&. Its value is 1.050 kJ/mol.% The most tedious aspect in carrying out these calculations is the determination of (Evib).This is done by evaluating the vibrational partition function Q,. The input is the complete set of vibrational frequencies for each species. These are usually not available and must be estimated. The other energies (E,) and ( E d ) were obtained by using the classical values. Figure 6 illustrates the 0 K thermochemistry near the dissociation threshold while Table I1 summarizes the derived AHHfoo and AH;298 for the various fragments from dioxane. B. Structures and Energies of Fragments. Strictly speaking, a AHfocalculated by using the AE is only an upper limit because of the possibilities of reverse activation energies and kinetic shifts. The latter is a particular problem for onsets at higher energies because the new dissociation path must compete with all the lower energy processes. The net effect is to shift the observed onsets to an energy above the thermochemical onset. Nevertheless, in this section we propose a number of structures which are not the most stable ones. We feel this is justified by the fact that the three C4Hs02 isomers studied clearly do not isomerize to a common structure and produce very different fragment ions. This suggests that dissociation paths are determined by dynamical or mechanistic constraints rather than thermochemical ones and we therefore expect the formation of fragments which are not necessarily the most stable ones. The first three ions are m / e 58,45, and 44. Their AEo determined from the PIE curves of Figure 3 are 10.66, 10.56, and 10.49 eV, respectively. However, from the RRKM fits of the dissociation rates, we conclude that the onsets are 10.65,10.60, and 10.59 eV, respectively. These differences are not large, although the 0.1-eV discrepancy in the case of mle 44 is somewhat disquieting. We chose to accept the values obtained from the RRKM theory partly because these data (23) JANAF Thermochemical Tables, 2nd ed., Natl. Bur. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 37 (1971).
743
hHfOlps, kJ/mol AH;,, kJ/mol
* 4a
0.8' i Ba -108.8 i 0.8' 9 7 7 0 i Ba CH,O 14.6d 736 F B a C,H30 -22.6d 882 i Ba CH,CHO -165.7 i 0.4' 9874a C,H,O 17d
602.5 F 4a -285.3 i 0.8' 845 * Ba -105.0 t 0.8' s 7 8 2 i Ba 21.3c 753 * ga -15.9' 891 T 8' -155.2 i 0.4' Q 87ga -2.1' 720e
C,H,O
9
569 -316 828
i
