The Journal of
Physical Chemistry VOLUME 97, NUMBER 14, APRIL 8,1993
0 Copyright 1993 by the American Chemical Society
LETTERS Heat of Formation of CHzOH John C . Traeger' Department of Chemistry, La Trobe University, Bundoora, Victoria 3083, Australia
John L. Holmes Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K l N 6N5 Received: December 29, 1992; In Final Form: February 18, 1993
Threshold photoionization mass spectrometry has been used to measure the appearance energy for the formation
of CH*OH+ from methanol. A value of 708.5 f 0.8 kJ mol-' is obtained for the 298 K cationic heat of formation, which results in a corresponding heat of formation for the hydroxymethyl radical of -18.9 f 1.0 kJ mol-I. The discrepancy between this value and the significantly higher heat of formation obtained from a kinetic study of the reaction of CHzOH with HBr and HI is discussed and partly resolved. An absolute proton affinity for formaldehyde of 712.9 f 1.0 kJ mol-' is derived. Introduction
+
In a recent kinetic study of the reactions between CHzOH HBr and CHzOH HI, Seetula and Gutman' proposed a value of -8.9 f 1.8 kJ mol-' for the hydroxymethyl radical heat of formation at 298 K. This is significantly higher than previous determinations made from both kinetic and spectroscopic studies,2-6as well as several high-level ab initio molecular orbital c a l ~ u l a t i o n s . ~Because - ~ ~ of the importance of AH0r,298(CH2OH) in kinetic modeling studies involving the oxidation of methanol, it is vital that any revision be supported by other independent experiments. Seetula and Gutman did indicate possible sources of the discrepancy between their experimental value and those obtained from theother investigations. However, they were unable to satisfactorily explain why the photoionization value of Ruscic and Berkowitz6 should be 16.6 kJ mol-I lower. In an attempt to resolve this problem, we have remeasured the photoionization appearance energy (AE) for CHIOH+formation from methanol and carefully evaluated the auxiliary thermochemical data required to obtain an accurate AHor.2ss(CH20H).
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Experimental Section The microcomputer-controlled photoionization mass spectrometer used in this study has been described elsewherein detail. I I The photon source used was the hydrogen pseudocontinuum with 0022-365419312097-3453304.00/0
the band-pass of the Seya-Namioka monochromator fixed at 0.125 nm. Atomic emission lines were used to calibrate the absolute energy scale to an accuracy of better than 0.003 eV. All experiments were performed at 297 K, with sample pressures in the range 10-6-10-5Torr. There was no evidence of processes due to collision-induceddissociationsoccurring in the prethreshold region. The methanol used was of research grade purity.
Results and Discussion Ruscic and Berkowitz6 have used photoionization mass spectrometry to accurately measure the ionization energy (IE) for the CDzOH radical. Their valueof 7.54f 0.006eV is in excellent agreement with an earlier photoelectron spectroscopic study by Dyke.I2 A detailed Franck-Condon analysis showed that the weak, prethreshold spectral structure was consistent with hot band excitation rather than a lower energy adiabatic transitiona6 The corresponding IE for the undeuterated CH2OH radical was found to be 0.01 eV higher.6.12 Since the threshold m / r 31 cation formed from ionized methanol has the hydroxymethyl ~ t r u c t u r e , ~it~ .should '~ be possible to combine the AE for its formation with the CH2OH IE to obtain the radical heat of formation. Ruscic and Berkowitz6 used the 0 K corrected photoionization AE of 1 1.67 f 0.03 eV, measured by Refaey and Chupka," to calculate AHor.~(CH2OH+) I719.6 f 2.9 kJ mol-' and hence obtained LWOr.o(CH20 1993 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 97, No. 14, 1993
3454 I
t
methanol m/z 31 AE =
11.3
11.4
11.5
11.6
11.7
11.8
119
Photon Energy lev Figure 1. Threshold photoionization efficiency curve for CH?OH+ fragment ions produced from methanol. OH) I -8.8 f 2.9 kJ mol-'. This was converted to the corresponding 298 K value of AHof.298(CH20H)I-15.5 f 2.9 kJ mol-' using the theoretically calculated vibrational frequencies of CH20H+.' The cited upper limit reflects the possibility of excess energy involved in the fragmentation process, although this is not predicted by high-level G2 ab initio molecular orbital calculations.14 Furthermore, both the absenceof any competitive lower energy dissociation channel and the simple bond cleavage involved with H loss should provide a true thermochemical threshold free from any competitive or kinetic shift. Unlike methanol, the production of CH20H+via methyl loss from ionized ethanol is not the lowest energy fragmentation process observed in the mass ~pectrometerl~ (H loss occurs at photon energies -0.4 eV lower); the desired AE is subject to a competitive shift, which is apparent from the poorly defined photoionization onset, and thus cannot be used to derive an accurate cationic heat of formation. The threshold photoionization efficiency (PIE) curve for CH2OH+ produced from ionized methanol in this study is shown in Figure 1. Because the theoretical threshold law for this fragmentation process is a linear function of excess energy,l5 we have carried out a linear least-squares analysis of the postthreshold data. This yielded an AE of 11.578 f 0.007 eV and includes the 0.0003-eV absolute error in the photon energy. A 298 K cationic heat of formation may then be obtained from the expression M0fJ98(CH20H+) = AE,,q)+ M0f,298(CH30H) M0f,,98(H) + M c o r where AE,,, is the above experimental AE obtained from a postthreshold linear extrapolation of the PIE curve and AH,,,, the thermal energy correction term, is simply given by H029g Hoofor the CH2OH+ cation,15which has been calculated to be 10.21 kJ m01-I.'~ A small correction of +0.65 kJ mol-) (0.0625 nm at 11.58 eV) was also included in AH,,, to compensate for the 0.125-nm bandwidth of the monochromator. From AHor.298(CH30H)= -201.5 f 0.3 kJ mol-I,l6 M0f,298(H) = 218.0 kJ AE,,, = 11 17.1 f 0.7 kJ mol-' ( 1 eV = 96.4846 kJmol-'),I7andAH,,,= 10.87kJ mol-', weobtainAHof.29g(CH2OH+) = 708.5 f 0.8 kJ mol-'. Using the above H0298 - Hoo value for CH20H+ and a corresponding value of 11.31 kJ mol-' for C H 2 0 H (obtained from statistical mechanical calculations17 with experimental vibrational frequencies for the radical Is), the IE for CH20H becomes 727.4 f 0.6 kJ mol-' at 298 K. Thus, the 298 K heat of formation for CHIOH is derived to be -18.9 f 1.0 kJ mol-', in close agreement with the value of Ruscic and Berkowitz.6 In fact, the only difference between the two values is the slightly different AE used in each thermochemical calculation. The AEo of 1 1.67 f 0.03 eV obtained by Refaey and Chupka' may be converted to an equivalent AE,,, of 11.61 f 0.03 eV
(using H029g - f f 0 o = 1 1.43 kJ mol-' for CH30H19and assuming that all internal energy associated with the precursor molecule is effective in dissociation,15 i.e., AH* = 6.2 kJ mol-'), which is within the experimental error of the present value of 11.578 f 0.007 eV. Refaey and Chupka did not however publish a detailed threshold region of their PIE curve for CH20H+ from methanol, so it is difficult to know how their A& was derived. Because the adiabatic IE for methanol measured here as 10.85 f 0.02 eV20 is in excellent agreement with their value of 10.84 f 0.02 eV,I3 it appears unlikely that the AE difference is due to a discrepancy between the two respective photon energy scale calibrations. The present value for AHof,29g(CH20H)is very close to the -17.6 f 8 kJ mol-' reported by Tsang4 following a shock tube investigation of the thermal stability of some branched alcohols. It is also within the experimental error range of -24 f 8 kJ mol-' obtained by Holmes and LossingSfrom a monoenergeticelectron impact study of free radical heats of formation. However, there is a significant discrepancy between the value of -8.9 f 1.8 kJ mol-' obtained recently by Seetula and Gutman.' In their kinetic study, Seetula and Gutman derived the heat of formation of CHzOH by two third law calculations and one second law calculation. Two reactions were investigated:
+ HBr C H 3 0 H + Br C H 2 0 H + HI * CH,OH + I
(1)
CH20H
(2)
For reaction 1 an equilibrium constant at 349 K was obtained by combining kl with k-' for the reverse reaction, recalculated from the methanol photobromination data of Buckley and Whittlee2 The derived AC0349(1) was then converted to AG029g(1) = -32.0 f 1.3 kJ mol-' and combined with AS0298(1) = -39.3 f 3.0 J K-I mol-' to obtain AH029g(1) = 4 3 . 7 f 1.6 kJ mol-', which leads to AHof.298(CH20H) = -9.1 f 1.7 kJ mol-'. In calculating ASo2g8(l),Seetula and Gutman used values for S0298(HBr)and S029e(Br)from the JANAF Thermochemical Tables,I7together with SOjw(CH3OH) and So3m(CH20H) from a kinetic compilationofTsang.2' However, this last value (255.55 J K-l mol-') was obtained froma calculation which used estimated vibrational frequencies for the CH2OH radical, one of which was assumed to be a free rotor. The experimentally measured torsional frequency is 420 cm-l,l8 which is supported by two different theoretical calculations.1g-22A recalculation of S029g(CH20H) using the experimental vibrational frequenciesIg leads to the significantly lower value of 240.2 J K-I mol-'. If we take the above experimental value of AG0349(1) = -29.9 f 1.3 kJ mol-' and combine it with AS0349(l)= -26.0 f 0.3 J K-' mol-I (see Table I), we obtain m 3 4 9 ( 1 ) = -39.0 f 1.3 kJ mol-'. This becomes AHoz~8(1)= -38.5 f 1.3 kJ mol-', using the enthalpy data from Table I, resulting in AHor,29g(CH20H)= -14.7 f 1.4 kJ mol-'. Because of insufficient information for the reverse of reaction 1, Seetula and Gutman were only able to carry out a second law calculation for reaction 2. They combined their experimental data with that reported by Cruickshank and Benson3 for the iodination of methanol between 547 and 630 K to obtain values of ASoS&) = 4 3 . 7 J K-'mol-' and AHosg6(2) = -113.4 kJ mol-'. These were converted to 298 K using tabulated heat capacities for the reactants and products,21yielding the corresponding values of AS0*&) = -40.3 f 18.6 J K-I mol-' and AH0298(2) =-112.0* 7.5 kJ mol-'. Thederived thermochemical properties for the hydroxymethyl radical were AHor,29g(CH20H) = -8.7 7.6 kJ mol-' and S029g(CH20H) = 254 f 19 J K-' mol 1. If the experimental value of AGOs8&) = -87.8 7.8 kJ mol is combined with the calculated M0586(2) = -33.3 f 0.3 J K I mol I (Table I), a reaction enthalpy of AHosx6(2)= -107.4 f 7.8 kJ mol I results. This is well within the error limits of the value of -1 13.4 f 7.5 kJ mol-' obtained from the difference in
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Letters
The Journal of Physical Chemistry, Vol. 97, No. 14, 1993 3455
TABLE I: Supplementary Thermochemical Data' AHo,:un (kJ mol
I)
Br
I H H+ S O M (J K
mol
I)
(J K I mol
I)
SOO,(J K I mol
I)
S O W
I
- H0o (kJ mol I)
HOzyli
H o UY - H O o (kJ mol I) How,- H O o (kJ mol I)
Br I CHzOH Br CHzOH
I CHzOH Br
I CHzOH CHIOH Br CHzOH
I CHzOH
111.9 106.8 218.0 1530.0 175.0 180.8 240.2 f 0.z' 178.3 247.9 f 0.2 194.9 277.7 f 0.2' 6.20 6.20 11.31' 1 1.43" 7.26 13.79' 12.19 27.54'
H Br HI CHiOH
-36.4 26.4 -201.5 f 0.3" HCHO -108.6 f 0 . 9 HBr 198.7 HI 206.6 CHiOH 239.7 f 0.2" H Br 203.3 CH3OH 246.9 f 0.2" HI 226.5 CHiOH 276.0 f 0.2" H Br 8.65 HI 8.66 CHIOH+ 10.21* HBr CH,OH HI CH30H
10.13 13.87" 17.13 28.07"
Molecular Data Used for C H l O H Calculations' molecular mass = 31.035 g mol-' symmetry number = 1 ground-state quantum weight = 2 product of moments of inertiaf ( I J & ) = 3.746 X g3 cm6 vibrational frequenciesh (cm-I): 3650,3084,f 2960,f 1459, 1334, 1183, 1048,569,420
"
From ref I7 unless specified. Reference 16. ' This work. Reference 19. I' Reference 14. /Based on statistical mechanical equations from ref 17. Reference 22. Reference 18.
variable temperature selected ion flow tube study of the PA for H2Ol7obtained a value of 690.8 f 2.9 kJ mol-'. Use of this value as a reference standard would then lower the Lias et al. PA to 712 kJ mol-],in accord with both the present experimental value and the high-level theoretical calculation. Any increase in our CH20H+ heat of formation, which a higher CHzOH heat of formation would require, only lowers the calculated PA for formaldehyde and further adds to the divergence from the Lias et al. value.
Conclusions Dissociative photoionization mass spectrometry has been used to measure the appearance energy for CH20H+ formed from methanol. A 298 K cationic heat of formation of 708.5 f 0.8 kJ mol-' is derived, which corresponds to an absolute proton affinity for formaldehyde of 7 12.9 f 1.OkJ mol-l. This is shown to be in excellent agreement with other experimental and theoretical values and, when combined with the ionization energy for the hydroxymethyl radical, results in a value of AHor.298(CH20H) = -18.9 f 1.0 kJ mol-'. An explanation for the relatively high value obtained from a recent kinetic study of the reaction of CH2OH with HBr and HI is provided, although a 3-4 kJ mol-' discrepancy still remains. Acknowledgment. We thank Professor Leo Radom for communicating the results of his a b initio proton affinity calculations prior to publication.
f
Arrhenius activation energies for the forward and reverse reactions.' AHoSg6(2)may be converted to 298 K using data from Table I, leading to M029g(2) = -105.3 f 7.8 kJ mol-' and Mor.298(CH20H) = -15.8 f 7.8 kJ mol-l; the corresponding values obtained by Seetula and Gutman were -1 12.6 f 7.9 and -8.1 f 8.0 kJ mol-', respectively. As previously noted,' the larger error associated with the CH20H heat of formation derived from the kinetic study of reaction 2 is largely due to the uncertainty in the Arrhenius parameters for the reverse reaction.' Thepresentresult ofM0f,29s(CH20H)=-18.9 f 1.0 kJ mol-' is lower than both of the two redetermined values of -14.7 f 1.4 and -15.8 f 7.8 kJ mol-', based on the kinetics for reactions of C H 2 0 H with HBr and HI, and is just outside the combined uncertainty limits of the two measurement^.^^ However, it is difficult to rationalize a systematic error of 3 or 4 kJ mol-' in the threshold photoionization value because this would require either a higher measured AE for CH20H+ or a lower IE for CH2OH. As discussed above, it is most unlikely that the adiabatic IE is any lower than the recent measurement of Ruscic and Berkowitz.6 Moreover, any kinetic shift or excess energy associated with the fragmentation products following photoionization of methanol would only result in an overestimation of the thermochemical AE, ;.e., the opposite of the required change. The heat of formation for CH20H+ obtained in this study can be used to obtain an absolute proton affinity (PA) for formaldehyde. From the known heats of formation for H+ and HCHO (Table I) a value of 712.9 f 1.0 kJ mol-' is derived. This is in excellent agreement with a recent G2 a b initio calculation of 71 1.8 kJ mol-' Z4 but is lower than the 718 kJ mol-' adopted by Lias et in their extensive compilation of evaluated gas-phase basicities and proton affinities. However, Lias et al. based their value on PA(H20) = 697 f 8 kJ mol-', obtained from an earlier photoionization study of the water dimer,26 whereas a recent
References and Notes ( I ) Seetula, J. A.; Gutman, D. J. Phys. Chem. 1992, 96, 5405. (2) Buckley, E.; Whittle, E. Trans. Faraday Soc. 1962, 58. 536. (3) Cruickshank, F. R.; Benson, S . W. J . Phys. Chem. 1969, 73, 733. (4) Tsang, W. Int. J . Chem. Kinet. 1976, 8, 173. (5) Holmes, J. L.; Lossing, F. P. Int. J. Mass Spectrom. Ion Processes 1984, 58, 1 1 3. (6) Ruscic, B.; Berkowitz, J. J. Chem. Phys. 1991. 95, 4033. (7) Curtiss, L. A.; Kock, L. D.; Pople, J. A. J. Chem. Phys. 1991, 95,
4044. (8) Sana, M.; Leroy, G.J. Mol. Sfruct. 1991, 226, 307. (9) Bauschlicher, C. W.; Langhoff, S. R.; Walch, S. P. J. Chem. Phys. 1992, 96, 450. (IO) Pardo, L.; Banfelder, J. R.;Osman, R. J. A m . Chem.Soc. 1992,114, 2382. ( I I ) Traeger, J. C. Inf.J . Mass Specfrom. Ion Processes 1984,58, 259. (12) Dyke, J. M. J. Chem. Soc., Faraday Trans 2 1987, 83. 69. (13) Refaey, K. M. A.; Chupka. W. A. J. Chem. Phys. 1968,48, 5205. (14) Ma, N. L.; Smith, B. J.; Pople, J. A.; Radom, L. J. A m . Chem. SOC. 1991, 113, 7903. (15) Traeger, J. C.; McLoughlin, R. G.J. A m . Chem. Soc. 1981, 103, 3647. (16) Pedley, J. B.; Naylor, R. D.; Kirby, S. P. Thermochemical Data of Organic Compounds, 2nd ed.; Chapman and Hall: New York. 1986. ( I 7) Chase, M. W.; Davies,C. A.; D0wney.J. R.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. J. Phys. Chem. Ref. Data 1985.14. Suppl 1. (JANAF Thermochemical Tables, 3rd ed.). ( I 8) Jacox, M. E. Chem. Phys. 1981, 59, 2 13. (19) Chen, S. S.;Wilhoit, R. C.; Zwolinski, 8.J. J . Phys. Chem. Ref. Data 1977, 6 , 105. (20) The larger error reflects the uncertainty associated with assignment of the adiabatic transition. (21) Tsang, W. J . Phys. Chem. Ref Data 1987, 16, 471. (22) Saebs, S.; Radom, L.; Schaefer I l l , H. F. J. Chem. Phys. 1983, 78, 845. (23) See also: D6bC. S.2. Phys. Chem. (Munich) 1992, 175, 123. (24) Smith, B. J.; Radom, L. J . A m . Chem. SOC. Submitted. ( 2 5 ) Lias, S. G.;Liebman, J. F.; Levin, R. D. J. Phys. Chem. Ref. Data 1984. 13, 695. (26) Ng.C. Y.;Trevor,D. J.;Tiedemann. P. W.;Ceyer,S.T.; Kronebusch. P. L.; Mahan. B. H.; Lee, Y. T. J . Chem. Phys. 1977, 67. 4235. (27) McIntosh, B. J.; Adams. N. G.;Smith, D. Chem. Phys. Left. 1988, 148,,142.
