The Journal of
Physical Chemistry
0 Copyright 1994 by the American Chemical Society
VOLUME 98, NUMBER 50, DECEMBER 15,1994
LETTERS New Theoretical and Experimental Proton Affinities for Methyl Halides and Diazomethane: A Revision of the Methyl Cation Affinity Scale Mikhail N. Glukhovtsev,’P-c Jan E. Szulejko,ld T.B. McMahon:’ld James W.Gauld,lB Anthony P. Scott,lS Brian J. Smith,le Addy Pross:llbJ and Leo Radom*Ja Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia; School of Chemistry, University of Sydney, Sydney, NSW 2006, Australia; Department of Chemistry and Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3GI; and Biomolecular Research Institute, Parkville, VIC 3052, Australia Received: August 8, 1994; In Final Form: October 31, 1994@
High-level ab initio calculations and variable-temperature proton-transfer equilibrium constant measurements have been used to obtain new thermochemical data for protonated halogenomethanes (CH3XH+, X = F, C1, Br, and I) and protonated diazomethane (CH3NNf). Proton affinities of CH3X and CHzNN and methyl cation affinities of HX and NZ have been derived. The theoretical and experimental results are in good agreement with one another but in several cases are in conflict with currently accepted experimental proton and methyl cation affhities. Experimental and theoretical methyl cation affinities are presented for a variety of molecules, leading to the proposal of a new methyl cation affinity scale.
Introduction In the course of other we recently calculated the proton affinities of the halogenomethanes (CH3X, X = F, C1, Br, and I) and diazomethane (CHzNN). We used the G2 theoretical procedure which was introduced in 1991 by Pople and co-workers4 for the purpose of obtaining reliable theoretical thermochemical predictions. Whereas G2 theory was found in previous studies to consistently predict proton affinities to within 10 kI m0l-l,49~we found several instances for the halogenomethanes and diazomethane in which there were substantially greater differences between the currently accepted experimental proton affinities6 and our calculated values. We also found similar large differences between theory and experiment for related methyl cation affinities. In order to assess such discrepancies, new variable-temperature measurements of protontransfer and methyl cation-transfer equilibrium constants have @Abstractpublished in Advance ACS Absrrucrs, December 1, 1994.
0022-365419412098-13099$04.50/0
been carried out7 to obtain new experimental proton affinities and methyl cation affinities. A brief report of both the theoretical and experimental results is presented in this Letter. Full details will be reported e l ~ e w h e r e . ~ -We ~ , ~find ~ * that the new experimental and theoretical results generally agree well with one another. Our results are used to propose a new methyl cation affinity scale.
Theoretical and Experimental Procedures Standard ab initio molecular orbital calculations9 were performed with the GAUSSIAN 92 and GAUSSIAN ~ D F Tseries of programs.1° Energies were obtained at the G2 level of t h e ~ r y . ~ JThis ~ J ~corresponds effectively to calculations at the QCISD(T)/6-31I+G(3df,2p) level on MP216-3 1G(d) optimized geometries, incorporating scaled HFl6-3 1G(d) zero-point energies and a so-called higher-level correction. Enthalpy temperature corrections were derived using the scaled vibrational frequencies and standard statistical thermodynamics methods.
0 1994 American Chemical Society
13100 J. Phys. Chem., Vol. 98, No. SO, 1994
Letters
TABLE 1: HPMS Thermochemical Data for AH (kJ mol-') and AS (J mol-' K-') BI Bz -AH -As (a) Proton-Transfer Equilibria (Eq 3) ((2%Bath Gas) cs2 CHsI 8.4 5.4 CH3I CW5 4.6 15.9 c6F6 CH3Cl 7.1 21.8 c 6 F 6 CH3Br 18.8 14.6 (b) Methyl-Cation-Transfer Equilibria (Eq 4) (Nz Bath Gas) Nz CH3Cl 67.8 30.5 HBr CH3Cl 24.3 13.8
TABLE 3: Calculated and Experimental Proton Affities (PAzs, kJ mol-') PA298 molecule LBLHLM" G2b experimentC CHsF 605 597.2 597.1 CH3C1 682 649.8 650.6 CH3Br 693 662.9 662.3 CH3I 715 691.1 683.1 CH2NN 858 883.7 a Experimental values from ref 6. Theoretical values obtained in the present work; see refs 2 and 3 for further details. Experimental values obtained in the present work; see refs 7 and 8 for further details.
TABLE 2: FT-ICR Proton Affinity Bracketing of CHlF HBl' Bz proton transfer observed? CH3FHf NzO no CH3FH' co no CH3FH' C2H6 yes, slow CZH7' CH3F yes, very slow HCO' CH3F yes, fast NzOH' CH3F yes, fast
TABLE 4: Calculated and Experimental Methyl Cation Affinities (MCA~B,kJ mol-')
~
~~~~
MCAm molecule
Proton affiities (PAS)and methyl cation affiities (MCAs) were calculated as the negative of the enthalpy changes at 298 K for reactions 1 and 2, respectively:
All energies are quoted in kJ mol-' (1 kcal mol-' = 4.184 kJ mol-'). The proton-transfer (eq 3) and methyl cation-transfer (eq 4) equilibria involving pairs of bases B1 and BZ
+ B, e HB; + B, CH3Blf + B, == CH,B; + B, HB,'
(3) (4)
were investigated using a pulsed ionization high-pressure mass spectrometer (HPMS) configured around a VG 70-70 doublefocusing spectrometer whose geometry has been reversed to that of a B-E instrument. The apparatus and its capabilities have been described in detail p r e v i o u ~ l y . The ~ ~ ~ion ~ source temperature ranged from 295 to 610 K. The HPMS proton-transfer equilibrium experiments were conducted in a bath gas of methane. The methyl cationexchange equilibria were studied in a bath gas of nitrogen in which methyl chloride was present at the 100-1OOO ppm level. The methyl chloride served as the methyl source for the CH3B1+ ions in reaction 4, CHsN*+ being produced, for example, via reactions 5 and 6:
N, - eN,"
---,
-
4-CH3C1
N,'+
CH,N;
+ C1'
(6)
Only proton-transfer bracketing experiments were carried out for methyl fluoride. These were performed under low-pressure conditions on a Briiker Spectrospin FT-ICR, the occurrence or nonoccurrence of proton transfer to or from a reference base of known proton affinity being noted. Experimental proton affinities and methyl cation affinities were derived from the results presented in Tables 1 and 2 using
HF NZ HCl HBr HI HzO CH30H HzS NH3
LBLHLM"
G2b
experimentC
143 191 234 257 290 284 349 350 436 482
123.2 179.4 199.4 222.5 262.3 277.0 336.7 337.5 437.7 487.3
134.9 184.2 203.6 227.7 257.4 283.3 339.0 341.w
441.6 489.7 CH3NZ Experimental values derived from data in ref 6. Theoretical values obtained in the present work; see refs 2, 3, and 5 for further details. Experimental values obtained in the present work, unless otherwise specified; see refs 7 and 8 for further details. See ref 14. previously reported reference proton affinities8 and literature AHf 298 values6 for H+, CH3+, and relevant neutral molecules.
Discussion
Proton Affinities. Calculated G2 proton affinities for CH3F, CH3C1, CH3Br, CH3I, and CHzNz are compared with values from the Lias et al. compendium (LBLHLM)6 in Table 3. The agreement is generally poor, with differences between theory and experiment ranging from about -30 to f 3 0 kJ mol-'. This contrasts with the results of our recent extensive study of G2 proton affinities where very good agreement with experimental values was ~ b s e r v e d .In ~ order to ascertain whether theory or experiment is at fault for the present systems, new proton affinity measurements were carried out, and the results are shown as the right-hand column in Table 3. The agreement between the new experimental numbers and the G2 results is very good, the largest discrepancy being 8 kJ mol-l in the case of CH3I and less than 1 kJ mol-' for the other systems. We conclude that the LBLHLM proton affiities for CH3C1, CH3Br, CH3I, CHZNZ, and possibly CH3F need to be reexamined. Methyl Cation AfRnities. Calculated methyl cation affhities for 10 molecules are compared with experimental values in Table 4. The LBLHLM compendium provides the most comprehensive recent collection of relevant experimental data and is therefore used as the initial basis for comparison. Agreement with LBLHLM, however, is again poor, with a mean absolute difference between theory and experiment of 17 kJ mol-' and a largest difference of 35 kJ mol-l. Seven out of the 10 comparisons show differences greater than the 10 kJ mol-' expected accuracy of G2 theory. On the other hand, there is generally good agreement between the theoretical methyl cation affinities and the new experimental values, shown in the right-hand column of Table 4.14 The mean absolute difference is 5 kJ mol-', with a largest discrepancy of 12 kJ mol-'.
J. Phys. Chem., Vol. 98, No. 50, 1994 13101
Letters
TABLE 5: Calculated and Experimental Relative Methyl Cation Affinities (Relative MCAm, kJ mo1-Y relative MCA298 molecule
G2
experiment
HF
-56.2
-49.3
Nz HC1 HBr HI
0
0
20.0 43.1 82.9
19.4 43.5 73.2
relative MCAz98 molecule
HzO CH30H H2S NH3 CH3NHz
G2
experiment
97.6 157.3 158.1 258.3 307.9
99.1 154.8 156.8 257.4 305.5
Evaluated from data in Table 4.
There is a greater difference between G2 theory and experiment for methyl cation affinities than for proton affinities. The theoretical MCAs are systematically smaller than the experimental MCAs by about 3-4 kJ mol-'. The reason for this result is readily apparent. The experimental MCAs are derived for the most part from experimental PAS utilizing literature values of the heats of formation of CH3+ and relevant neutral molecules. There is a difference of 3.7 kJ mol-' between the G2 (1089.6 kJ m01-l)'~ and experimental (1093.3 k J A H f 2 9 8 values for CH3+, and this contributes directly to the difference between the theoretical and experimental MCAs. Indeed, relative methyl cation affinities, listed in Table 5, remove this particular deficiency, and there is significantly better agreement between theory and experiment, the mean absolute difference being reduced to less than 3 kJ mol-'. It would be desirable to obtain confirmatory evidence for the currently accepted experimental A H r 298 for CH3+ to eliminate this small but significant source of uncertainty. Two cases which show larger discrepancies than the remainder are the methyl cation affinities for HF and HI. In the former case, this may be associated with a significant uncertainty in the experimental AHf 298 value for CH3F while in the latter case the discrepancy may be associated with the greater difficulties in theoretically describing iodine-containing molecules compared with lower-row homologues. A New Methyl Cation Affinity Scale. Methyl cation affiity scales in the literat~re'~.'~ are currently anchored to the methyl cation affinity (MCA) for N2, this having been thought to be "the only base for which a good literature value for the MCA is available". An initial value of 217 kJ mol-' and a subsequent value of 202 kI mol-' have been used. The LBLHLM value for the MCA of N2 is 191 kJ mol-'. Previous theoretical calculations by Datal8 and by Glaser19 and co-workers have suggested that these values should be revised further downward. A similar conclusion is reached on the basis of our G2 calculated value2"of 179.4 kJ mol-'. Indeed, our new experimental results give a value for the methyl cation affinity of NZ of 184.2 kJ mol-'. This implies a significant error for molecules on the old scales whose MCAs depended on that of N2 as a reference. The values determined in the present work (listed in Table 4) represent an improved methyl cation affinity scale.
Concluding Remarks The new theoretical and experimental proton affinities and methyl cation affinities reported in this paper show substantial differences from current literature values but are in reasonably good agreement with one another. Our results suggest that a
revision of the existing methyl cation affinity scales would be appropriate, and the beginnings of such a new scale are presented.
Acknowledgment. We gratefully acknowledge useful discussions with Professor John Holmes, a generous allocation of time on the Fujitsu VP-2200 supercomputer of the Australian National University Supercomputing Facility, the financial support of the Natural Sciences and Engineering Research Council of Canada, the award (to M.N.G.) of an ANU Visiting Fellowship, and the award (to A.P.) of an Australian Research Council Senior Research Fellowship. References and Notes (1) (a) Australian National University. (b) University of Sydney. (c) Permanent address: Institute of Physical and Organic Chemistry, Rostov University, Rostov on Don, 344104, Russia. (d) University of Waterloo. (e) Biomolecular Research Institute. (f) Permanent address: Ben-Gurion University of the Negev, Beer Sheva, Israel. (2) (a) Glukhovtsev, M. N.; Gauld, J. W.; Pross, A.; Radom, L., to be published. (b) Glukhovtsev, M. N.; Pross, A.; Radom, L., to be published. (3) Scott, A. P.; Russell, A. J.; Radom, L., to be published. (4) Curtiss, L. A.; Raghavachari, K.; Trucks, G. W.; Pople, J. A. J. Chem. Phys. 1991, 94,7221. (5) S&th, B. J.; Radom, L. J. Am. Chem. SOC. 1993, 115, 4885. (6) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G.J . Phys. Chem. Re$ Data, Suppl. 1988, 17. We exclusively use the literature printed version of this data base. There are occasional small differences from the computer version. (7) Szulejko, J. E.; McMahon, T. B., to be published. (8) See also: Szulejko, J. E.; McMahon, T. B. J . Am. Chem. SOC.1993, 115, 7839. (9) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Wong, M. W.; Foresman, J. B.; Robb, M. A.; Head-Gordon, M.; Replogle, E. $4.; Gomperts, R.; Andres, J. L.; Raghavachari, K. ; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN d ~ mRevision , F.3; Gaussian Inc.: Pittsburgh, PA, 1993. (1 1) The bromine basis sets used in the G2 calculations are described in: McGrath, M. P.; Radom, L. J. Chem. Phys. 1991, 94, 511. Note that the splitting factor for the three d functions in the basis set that corresponds to 6-311+G(3df,2p) is 3 rather than the 4 used for first- and second-row atoms. See: McGrath, M. P.; Curtiss, L. A.; Radom, L., to be published. (12) The iodine basis sets used in the G2 calculations are described in: Glukhovtsev, M. N.; Pross, A.; McGrath, M. P.; Radom, L., to be published. Note that an effective-core potential (Bergner, A.; Dolg, M.; Kuchie, W.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 80, 1431) was used for iodine and that the splitting factors for the two and three d functions in the basis sets that correspond to 6-311+G(2df,p) and 6-31 l+G(3df,2p), respectively, are 1.5 and 2 rather than the 2 and 4 used for fust- and second-row atoms. (13) Szulejko, J. E.; McMahon, T. B. Int. J . Mass Spectrom. Ion Processes 1991, 109, 279. (14) The experimental methyl cation affinity of H2S (341.0 kJ mol-I) was evaluated using proton-transfer energies between CH3SH and c6H6 from Meot-Ner et al. (Meot-Ner, M.; Sieck, L. W. J . Am. Chem. SOC.1991, 113, 4448) and the proton affinity of C& from ref 8. (15) Wong, M. W.; Radom, L., to be published. (16) McMahon, T. B.; Kebarle, P. Can. J. Chem. 1985, 63, 3160. (17) McMahon, T. B.; Heinis, T.; Hovey, J. K.; Kebarle, P. J . Am. Chem. SOC. 1988, 110, 7591. (18) Ikuta, S. J . Chem. Phys. 1989, 91, 1376. (19) (a) Glaser, R.; Choy, G. S. C.; Hall, M. K. J.Am. Chem.SOC.1991, 113, 1109. (b) Glaser, R.; Choy, G. S. K. J . Phys. Chem. 1991,95, 7682. (c) Glaser, R.; Horan, C. J.; Choy, G. S. C.; Harris, B. L. J. Phys. Chem. 1992,96,3689. (d) Horan, C. J.; Glaser, R. J . Phys. Chem. 1994,98, 3989. (20) We note that the 298 K G2 methyl cation affinity for Nz reported in ref 18d is greater than that reported in the present paper by about 5 kJ mol-'. This appears to be largely due to their neglect (for reasons that are not specified) of two low-frequency vibrations for CHsN*+ in evaluating both the zero-point energy and enthalpy temperature corrections.
