6504
J. Phys. Chem. 1996, 100, 6504-6508
Gas-Phase Acidity of CF3OH L. Gregory Huey,*,† Edward J. Dunlea,† and Carleton J. Howard Aeronomy Laboratory, NOAA, Boulder, Colorado 80303 ReceiVed: October 16, 1995; In Final Form: January 25, 1996X
Ion-molecule reactions of CF3O-, CF3O, and CF3OH were studied using the flowing afterglow technique to evaluate the thermochemistry of CF3OH and related compounds. The gas-phase acidity of CF3OH was measured to be ∆rH°acid ) 329.8 ( 2.0 kcal mol-1 by ion-molecule reaction bracketing. A limit on the electron affinity of the CF3O radical of g89.2 kcal mol-1 was also determined. The CF3O-H bond strength derived from these results is 124.7 ( 3.6 kcal mol-1, which is consistent with recent ab initio calculations of the heats of formation of CF3O and CF3OH.
Introduction CF3OH is a product of the atmospheric degradation of hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) which contain a CF3 group.1 CF3OH is formed in the atmosphere by hydrogen abstraction reactions of CF3O radicals with CH4 and other organic species.1-6 The reaction of CF3O with H2O has also been suggested to be a source of CF3OH in the atmosphere.3
CF3O + H2O f CF3OH + OH
(1)
Lovejoy et al.7 demonstrated that reactive uptake of CF3OH onto water droplets is efficient. Consequently, CF3OH has a relatively short lifetime (∼2 days) in the troposphere. However, an efficient loss mechanism has not been established for CF3OH in the stratosphere.7-9 A possible stratospheric loss mechanism for CF3OH is the reverse of reaction 1, i.e., reaction with OH. However, both the kinetics3,6 and thermodynamics10-15 for reaction 1 are uncertain. Consequently, it is impossible to predict which direction is favored for reaction 1 under atmospheric conditions. The energetics for reaction 1 are uncertain because the heats of formation of CF3O and CF3OH are not well established. Schneider and Wallington11 calculated ∆fH°298(CF3O) ) -150.4 ( 2.0 and ∆fH°298(CF3OH) ) -217.7 ( 2.0 kcal mol-1 using ab initio techniques. This gives a CF3O-H bond strength (DH°298(CF3O-H)) of 119.4 ( 2.8 kcal mol-1 and ∆rH°298 ) -0.2 ( 2.8 kcal mol-1 for reaction 1. Montgomery et al.12 and Sana et al.15 also used ab initio methods to calculate ∆fH°298(CF3OH) ) -217.7 and -217.4 kcal mol-1, respectively. Batt and Walsh10 evaluated studies of the kinetics of the pyrolysis of CF3OOCF3 to determine ∆fH°298(CF3O) ) -157 ( 1.5 kcal mol-1. They also used group additivity to estimate ∆fH°298(CF3OH) ) -213.5 ( 2.0 kcal mol-1. This gives a CF3O-H bond energy of 108.6 ( 2.5 kcal mol-1 and ∆rH°298 ) 10.6 ( 2.5 kcal mol-1 for reaction 1. Benson13 also used group additivity to determine ∆fH°298(CF3OH) ) -215 ( 1.0 kcal mol-1 and DH°298(CF3O-H) ) 110 ( 1.8 kcal mol-1. Thus, group additivity and ab initio calculations of the thermochemical properties of CF3OH are in marked disagreement. The CF3O-H bond strength (∆rH°298 for reaction 5) can be determined from the following thermodynamic cycle: * Corresponding author. † Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309. X Abstract published in AdVance ACS Abstracts, March 15, 1996.
0022-3654/96/20100-6504$12.00/0
CF3OH f CF3O- + H+
(2)
CF3O- f CF3O + e-
(3)
H+ + e- f H
(4)
CF3OH f CF3O + H
(5)
∆rH°298 for reaction 2 is equal to ∆rH°acid(CF3OH) for which a value of 347.6 ( 2.0 kcal mol-1 was reported by Taft and coworkers.16 ∆rH°298 for reaction 3 is the electron affinity (EA) of CF3O. ∆rH°298 for reaction 4 is the negative of the ionization potential of the hydrogen atom (313.6 kcal mol-1).17 The EA of CF3O has not been directly measured but can be derived from the following thermodynamic cycle:
CF3O- f CF2O + F-
(6)
F- f F + e-
(7)
F + CF2O f CF3O
(8)
CF3O- f CF3O + e-
(9)
∆rH°298 for reaction 6 is the fluoride affinity (FA) of CF2O. ∆rH°298 for reaction 7 is 78.42 kcal mol-1, the electron affinity of the fluorine atom.18 ∆rH°298 for reaction 8 is the negative of the F-CF2O bond energy. The FA of CF2O was reported by Larson and McMahon19 to be 42.6 ( 2.0 kcal mol-1. They measured the FAs of CF2O and many other compounds using ion cyclotron resonance fluoride-exchange equilibrium techniques. From these measurements they constructed a scale of relative FAs. The relative scale was converted to an absolute scale using the known absolute FAs of a few compounds. However, recently it has been shown that this absolute scale is in error by ∼8 kcal mol-1 for compounds such as CF2O, HF, and SO2 due to inaccuracies in the thermochemistry of the anchor points.20-22 The value for the FA of SO2 recommended in the NIST Negative Ion Database17 is 53 ( 2.5 kcal mol-1 from an absolute measurement by Squires.21 Larson and McMahon19 measured the FA of CF2O relative to SO2 to be -1.2 ( 1 kcal mol-1. This gives a value of 51.8 ( 2.7 kcal mol-1 for the absolute FA of CF2O. © 1996 American Chemical Society
Gas-Phase Acidity of CF3OH
J. Phys. Chem., Vol. 100, No. 16, 1996 6505
The F-CF2O bond strength can be derived from the following thermochemical cycle as suggested by Schneider and Wallington:23
TABLE 1: Gas Phase Acidities of the Compounds Used in this Study
∆rH°298(kcal mol-1) CF3O + CF3O f CF3OOCF3
-46.8 ( 0.524
(10)
CF3OOCF3
f CF3OF + CF2O
24.5 ( 0.725
(11)
CF3OF
f CF3O + F
44.0 ( 0.510
(12) a
CF3O
f CF2O + F
21.7 ( 0.9
(13)
mol-1
A value of 108.5 ( 3.0 kcal is calculated for the EA of CF3O using this F-CF2O bond strength. A CF3O-H bond energy of 142.5 ( 3.6 kcal mol-1 is calculated using the derived EA of CF3O and the value of ∆rH°acid(CF3OH) ) 347.6 ( 2.0 kcal mol-1 reported by Taft et al.16 However, the derived CF3O-H bond strength is in poor agreement with the work of both Schneider et al.14 and Batt and Walsh10 and is probably too large to be physically reasonable. In the present work, a new measurement of ∆rG°acid(CF3OH) and a lower limit for the electron affinity of CF3O are reported. A value for ∆rH°acid(CF3OH) is derived and is used to determine a value for the CF3O-H bond strength. Experimental Section I. CF3OH Gas-Phase Acidity. The gas-phase acidity of CF3OH was measured by ion-molecule reaction bracketing in a flowing afterglow apparatus. Reaction rate coefficients were measured for the proton-transfer reactions of CF3OH and CF3Owith a series of bases (X-) and conjugate acids (HX) (reactions 14 and 15) for which ∆rH°acid and ∆rG°acid are known:
X- + CF3OH f CF3O- + HX
(14)
CF3O- + HX f X- + CF3OH
(15)
CF3O- also reacted with all of the acids studied by fluoride transfer (reaction 16):16,26
CF3O- + HX f X‚HF + CF2O
(16)
The fluoride-transfer reactions of CF3O- may also directly produce X- by reaction 17. This possibility complicates the
CF3O- + HX f X- + HF + CF2O
(17)
identification of the proton-transfer channel for the reactions of CF3O- with HX. However, unambiguous results are obtained from the reactions of CF3O- with HX which do not form X-. For these cases proton abstraction by CF3O- from HX is slow. This indicates that HX is a weaker acid than CF3OH because exothermic proton-transfer reactions of this type are usually fast.27 (a) CF3O- + HX Reactions. The rate coefficients for the reactions of CF3O- with the series of protic acids in Table 1 were measured by the flowing-afterglow technique, which has been described thoroughly elsewhere.28 The apparatus used for these measurements has been described in detail previously and is only briefly discussed here.29 The measurements were carried out in a 7.30 cm i.d. stainless steel flow tube using helium carrier gas at a flow rate of ∼100 STP cm3 s-1 (STP ) 1 atm, 273 K)
acid
∆G°acida (kcal mol-1)
∆H°acida (kcal mol-1)
HCO2H HONO HCl CH3C(O)CO2H CF2HCO2H CCl2HCO2H HNO3 CF3CO2H HI
338.3 ( 2.0 332.6 ( 2.1 328.0 ( 0.2 326.5 ( 2.8 323.8 ( 2.0 321.9 ( 2.0 317.8 ( 0.2 317.4 ( 2.0 309.3 ( 0.2
345.4 ( 2.2 340.3 ( 2.0 333.4 ( 0.1 333.6 ( 2.9 331.0 ( 2.2 328.4 ( 2.1 324.5 ( 0.2 323.9 ( 2.9 314.4 ( 0.1
All acidity values are taken from ref 17.
and a flow tube pressure of ∼0.4 Torr. CF3O- was made by electron attachment to CF3OOCF3 which was added just downstream of the ionization source.26 The ionization source was a heated, thoriated-iridium filament biased at -60 V. The filament heating current was regulated to produce a constant emission current of ∼7.5 µA. Two reactant addition ports were spaced along the flow tube axis to give ion-molecule reaction distances of 55 and 80 cm. The decrease in the CF3O- signal was measured as a function of the neutral reactant concentration, and the reaction rate coefficients were calculated by the standard method.28 The apparent reaction rate coefficients for the proton transfer channel of the reactions of CF3O- with HX (reaction 15) were derived from the total rate coefficient for the reaction and the measured branching ratios of the X- and X-‚HF products. The branching ratios were measured at low resolution and low HX concentrations to minimize the effects of mass discrimination and secondary reactions, respectively. All of the carboxylic acids were obtained from Aldrich at the highest available purity. [CH3C(O)CO2H] was determined from the measured flow rate of manometrically prepared dilute mixtures (∼1%) in UHP helium. The concentrations of the other carboxylic acids were derived from their flow rates which were measured by the pressure rate of change of the pure compound in a calibrated volume. The rate coefficients for HNO3, HCl, and HI are taken from previous work performed in this laboratory.26 (b) X- + CF3OH Reactions. The ions used in this study were synthesized in the flow tube in the following manner. Cl-, F-, and I- were made by dissociative electron attachment to CCl4, NF3, and CF3I, respectively. SF6- was prepared by electron attachment to SF6. NO2- was formed by the chargetransfer reaction of SF6- with NO2.29 NO3- was formed by the reaction of SF6- with a mixture of O3 and NO2. SF6- charge transfers to both O3 and NO2 to form O3- and NO2-.29 Both the reaction of NO2- with O3 and the reaction of O3- with NO2 produce NO3-.30 The carboxylate ions (RCO2-) were formed by proton-transfer reactions of RCO2H with either Cl- or F-. Due to the difficulty in preparing and handling CF3OH, it was made in situ in a 2.23 cm i.d. × 120 cm long, Teflonlined, Pyrex flow reactor attached to the flowing-afterglow apparatus. The flow reactor/flowing afterglow experimental setup is shown in Figure 1 and has been described thoroughly elsewhere.31 CF3OH was made by the following reaction sequence.8 ∆
CF3OOCF3 + M 98 2CF3O + M
(18)
CF3O + i-C4H10 f CF3OH + C4H9
(19)
CF3O radicals were produced by passing ∼1 STP cm3 s-1 of a dilute mixture of CF3OOCF3 in UHP He through a hot (∼750
6506 J. Phys. Chem., Vol. 100, No. 16, 1996
Huey et al. TABLE 2: Proton-Transfer Reactions of X- with CF3OH X
k (10-10 cm3 molecule-1 s-1)
HCO2 NO2 Cl CH3C(O)CO2 CF2HCO2 CCl2HCO2 NO3 CF3CO2 I
>2.0 >4.0 >6.0 2 × 10-10 cm3 molecule-1 s-1 (Table 4). The rate coefficient for the reverse reaction (k26) was not measured because of the difficulties in preparing NO3 without a significant HNO3 or N2O5 impurity:
CF3O- + NO3 f NO3- + CF3O
f X- + H+
H+ + F- f HF
of reactions show a break in reactivity between HCl and pyruvic acid (CH3C(O)CO2H) on the gas-phase acidity scale (see Tables 2 and 3). This is strong evidence that the observed X- product channels are due to proton transfer (reaction 15) and that the gas-phase acidity of CF3OH is near that of pyruvic acid. For this reason, the X- channels of the CF3O- reactions are assigned as proton transfer and a lower limit of ∆rG°acid(CF3OH) g321.4 kcal mol-1 is obtained from the CF2HCO2H/CF2HCO2- results. This gives a value of ∆rG°acid(CF3OH) ) 323.0 ( 1.6 kcal mol-1. A value of ∆rH°acid(CF3OH) ) 329.8 ( 2.0 kcal mol-1 was derived using the measured value for ∆rG°acid and ∆rS°acid ) 22.9 ( 4 cal mol-1 K-1. ∆rS°acid was obtained from a statistical mechanical calculation utilizing the geometry and vibrational frequencies of CF3OH calculated by Francisco.34 The vibrational frequencies and geometry of CF3O- were assumed to be the same as CF3O and were also obtained from Francisco.34 All of the anions studied in this work were found to charge transfer rapidly to CF3O (Table 4). Therefore, only a lower limit could be placed upon the electron affinity of CF3O. The best limit that can be obtained for the EA of CF3O is based upon its charge-transfer reaction with NO3- (EA (NO3) )3.937 ( 0.014 eV).35
NO3- + CF3O f CF3O- + NO3
CF3O- f CF2O + F-
(∆rG°acid(HX) 322) ( 2.7 (17)
∆rG°298 for reaction 17 is 324.7 kcal mol-1 was not formed via reaction 17. The X- channels observed by Taft et al.16 were possibly due to an experimental artifact such as a nonthermal distribution of ion energies. Our lower limit for ∆rG°acid(CF3OH) may be complicated by X- produced by reaction 17 or by collisional dissociation of X-‚HF clusters in the ion sampling region of the flowing afterglow. However, we found that the measured branching ratios of the X- to X-‚HF product channels were insensitive to the magnitude of the ion extraction voltages used in these experiments. This indicates that our observed X- channels were not due to collisional dissociation. These observations coupled with the consistency of the results of the CF3O- and CF3OH reactions support the assigned lower limit for ∆rG°acid(CF3OH). Wiberg36 used ab initio techniques to calculate ∆rH°acid(CF3OH) ) 327 kcal mol-1 which is in good agreement with our value of 329.8 ( 2.0 kcal mol-1. Wiberg36 also calculated ∆rH°acid values for several other alcohols such as CH3OH for which ∆rH°acid is known. The calculated values reported by Wiberg are within 4 kcal mol-1 of the known values. This further supports our measured value for ∆rH°acid(CF3OH) and suggests that the data of Taft et al.16 are incorrect. A value for DH°298(CF3O-H) of 124.7 ( 3.6 kcal mol-1 is derived from our value for ∆rH°acid(CF3OH) and EA (CF3O) ) 108.5 ( 3.0 kcal mol-1. This is incompatible with the values of DH°298(CF3O-H) obtained by Batt and Walsh10 (108.6 ( 2.5 kcal mol-1) and Benson13 (110 ( 1.8 kcal mol-1) using group additivity. However, our value for the CF3O-H bond energy is consistent with the value of 119.4 ( 2.8 kcal mol-1 derived from the calculations by Schneider and Wallington.11 Thus our data support the values of ∆fH°298 for CF3O and CF3OH calculated by several groups using ab initio techniques. Our data also indicate that reaction 1 is exothermic by more than
CF3O + H2O f CF3OH + OH
(1)
2.0 kcal mol-1. This supports the suggestion of Wallington et al.3 that reaction 1 may be an important loss mechanism for CF3O in the atmosphere. An upper limit for k-1 the rate coefficient for the reverse of reaction 1 can be derived from the upper limit for k1 of 121.1 kcal mol-1 . We estimate ∆rS°298 = 0 cal mol-1 K-1 for reaction 1 which gives ∆rG°298 < -1.9 kcal mol-1 and k-1 < 4.0 × 10-18 cm3 molecule-1 s-1 at 298 K. This indicates that reaction with OH radicals will not be an important loss mechanism for CF3OH in the atmosphere. Recently, Schneider and Wallington23 demonstrated that a revised value for ∆fH°298(CF2O) eliminates the discrepancies between the values for ∆fH°298(CF3O) derived by Batt and Walsh10 and those calculated by ab initio procedures.11,12,15 The revised heat of formation of CF2O is based on ab initio calculations by Montgomery et al.12 and Schneider and Wallington23 of ∆fH°298(CF2O) ) -144.8 ( 1 and -145.3 ( 1.7, respectively. Batt and Walsh derived ∆fH°298(CF3O) using the value for ∆fH°298(CF2O) ) -151.7 ( 2 kcal mol-1 reported by Wartenberg and Riteris.37 For this reason, it is interesting to derive a CF3O-H bond strength using an EA derived for CF3O from the thermodynamic cycle involving reactions 2-6 and the ab initio values for ∆fH°298(CF2O) and ∆fH°298 (CF3O). The EA of CF3O derived by this procedure is 106.1 ( 3.8 kcal mol-1 which coupled with our value for ∆rH°acid gives a CF3O-H bond strength of 122.3 ( 4.3 kcal mol-1. This value for DH°298(CF3O-H) is in agreement with the value of 124.7 ( 3.6 kcal mol-1 derived from our value of ∆rH°acid(CF3OH) and other experimental thermochemical data. Thus our data are consistent with the revised heat of formation of CF2O suggested by Wallington and Schneider.23 However, it should be noted that the CF3O-H bond energy derived in this work cannot be reconciled with the group additivity estimates of this quantity by Batt and Walsh10 and by Benson.13 The measured lower limit for the electron affinity of CF3O of >89.2 kcal mol-1 is consistent with the value of 108.5 ( 2.9 kcal mol-1 derived in the introduction of this work. This supports the method used for the derivation of the CF3O-H bond energy. It is interesting to note that because the EA of CF3O is so large, it can not be measured by standard continuouswave laser photodetachment techniques. However, the pulsed laser photodetachment technique used by Weaver et al.35 for the measurement of the EA of NO3 might be a viable method for an accurate measurement of the electron affinity of CF3O. An experiment of this type would provide a critical test of the thermodynamic parameters derived in this work. Acknowledgment. This work was supported in part by NOAA Climate and Global Change. We thank AFEAS and Prof. Darryl DesMarteau for providing the CF3OOCF3. We gratefully acknowledge Prof. J. S. Francisco for providing some of the geometrical parameters for CF3O and CF3OH. We are grateful to Prof. R. R. Squires and Prof. T. B. McMahon for useful discussions concerning the fluoride affinity of CF2O. We thank P. W. Villalta, E. R. Lovejoy, and Prof. V. M. Bierbaum for critically reading the manuscript. We also gratefully acknowledge a reviewer for pointing out ref 36. References and Notes (1) Atkinson, R.; Cox, R. A.; Lesclaux, R.; Niki, H.; Zellner, R. Scientific Assesment of Stratospheric Ozone: 1989; WMO Report No. 20, 1989, 2, 159.
Huey et al. (2) Bevilacqua, T. J.; Hanson, D. R.; Howard, C. J. J. Phys. Chem. 1993, 97, 3750. Barone, S. B.; Turnipseed, A. A.; Ravishankara, A. R. J. Phys. Chem. 1994, 98, 4602. Jensen, N. R.; Hanson, D. R.; Howard, C. J. J Phys. Chem. 1994, 98, 8574. Turnipseed, A. A.; Barone, S. B.; Ravishankara, A. R. J. Phys. Chem. 1994, 98, 4594. (3) Wallington, T. J.; Hurley, M. D.; Schneider, W. F.; Sehested, J.; Nielsen, O. J. J. Phys. Chem. 1993, 97, 7606. (4) Chen, J.; Zhu, T.; Niki, H.; Mains, G. J. Geophys. Res. Lett. 1992, 19, 2215. (5) Ravishankara, A. R.; Turnipseed, A. A.; Jensen, N. R.; Barone, S. B.; Mills, M.; Howard, C. J.; Solomon, S. Science 1994, 263, 71. (6) Turnipseed, A. A.; Barone, S. B.; Jensen, N. R.; Hanson, D. R.; Howard, C. J.; Ravishankara, A. R. J. Phys. Chem. 1995, 99, 6000. (7) Lovejoy, E. R.; Huey, L. G.; Hanson, D. R. J. Geophys. Res. 1995, 100, 18, 775. (8) Huey, L. G.; Hanson, D. R.; Lovejoy, E. R. J. Geophys. Res. 1995, 100, 18, 771. (9) Wallington, T. J.; Schneider, W. F. EnViron. Sci. Technol. 1994, 28, 1198. (10) Batt, L.; Walsh, R. Int. J. Chem. Kinet. 1982, 14, 933. Batt, L.; Walsh, R. Int. J. Chem. Kinet. 1983, 15, 605. (11) Schneider, W. F.; Wallington, T. J. J. Phys. Chem. 1993, 97, 12783. (12) Montgomery, J. A.; Michels, H. H.; Francisco, J. S. Chem. Phys. Lett. 1994, 220, 391. (13) Benson, S. W. J. Phys. Chem. 1994, 98, 2216. (14) Schneider, W. F.; Wallington, T. J.; Hurley, M. D. J. Phys. Chem. 1994, 98, 2217. (15) Sana , M.; Leroy, G.; Peters, D.; Willante, C. J. Mol. Struct. (THEOCHEM) 1988, 164, 249. (16) Taft, R. W.; Koppel, I. A.; Topsom, R. D.; Anvia, F. J. Am. Chem. Soc. 1990, 112, 2047. (17) Bartmess, J. E. NIST NegatiVe Ion Energetics Database, Version 3.0, Standard Reference Database 19B; National Institute of Standards and Technology, 1993. (18) Blondel, C.; Cacciani, P.; Delsart, C.; Trainham, R. Phys. ReV. A 1989, 40, 3689. (19) Larson, J. W.; McMahon, T. B. J. Am. Chem. Soc. 1983, 105, 2944. (20) Squires, R. R.; McMahon, T. B., private communication. (21) Squires, R. R. Int. J. Mass Spetrom. Ion Processes 1992, 117, 565. (22) Wenthold, P. G.; Squires, R. R. J. Phys. Chem. 1995, 99, 2002. (23) Schneider, W. F.; Wallington, T. J. J. Phys. Chem. 1994, 98, 7448. (24) Kennedy, R. C.; Levy, J. B. J. Phys. Chem. 1972, 76, 3480. Descamps, B.; Forst, W. Can. J. Chem. 1975, 53, 1442. (25) Levy, J. B.; Kennedy, R. C. J. Am. Chem. Soc. 1972, 94, 3302. (26) Huey, L. G.; Villalta, P. V.; Dunlea, E. J.; Hanson, D. R.; Howard, C. J. J. Phys. Chem. 1996, 100, 190. (27) Bartmess, J. E., McIver, R. T. Gas Phase Ion Chemistry; Academic Press; New York, 1979; Vol. 2, Chapter 11. (28) Ferguson, E. E.; Fehsenfeld, F. C.; Schmeltekopf, A. L. AdV. At. Mol. Phys. 1969, 5, 1. (29) Huey, L. G.; Hanson, D. R.; Howard, C. J. J. Phys. Chem. 1995, 99, 5001. (30) Ikezoe, Y.; Matsuoka, S.; Takebe, M.; Viggiano, A. A. Gas Phase Ion-Molecule Reaction Rate Constants through 1986; Maruzen Company, Ltd.: Tokyo, 1987. (31) Villalta, P. W.; Huey, L. G.; Howard, C. J. J. Phys. Chem. 1995, 99, 12, 829. (32) Jensen, N. R.; Hanson, D. R., Howard, C. J. J. Phys. Chem. 1994, 98, 8574. (33) Streit, G. E. J. Chem. Phys. 1982, 77, 826. (34) Francisco, J. S. Chem. Phys. 1991, 150, 19; Francisco, J. S., private communication. (35) Weaver, A.; Arnold, S. E., Bradforth, S. E.; Neumark, D. M. J. Chem. Phys. 1991, 94, 1740. (36) Wiberg, K. B. J. Am. Chem. Soc. 1990, 112, 3379. (37) Wartenberg, H.; Riteris, G. Z. Anorg. Allg. Chem. 1949, 248, 356.
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