J . Phys. Chem. 1990, 94, 8041-8044
8041
Ab Initio Proton Affinities of HCOOH, CH3COOH, CH,OH, and CPH,OH Ellak I. von Nagy-Felsobuki**tand Katsumi Kimura Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: June 27, 1989)
Ab initio molecular orbital calculations using the STO-3G, STO-3G**, 3-21G, and 6-3 1G** basis sets have been performed for the series HCOOH, CH3COOH, CH30H, and C2H50H. Structural calculations of the neutrals and predictions of the structure of the respective cations are discussed and reported. Relative proton affinities have been calculated. Only at the 6-31G**//6-31G** Hartree-Fock level of theory are the relative proton affinities and trend for this simple organic acid/base series in agreement with experiment. Of the other models, the STO-3G//STO-3G model is the most effective.
Introduction Ab initio molecular orbital theory has achieved considerable success in the computation of molecular properties.' In particular, Pople and co-workers24 have calculated absolute proton affinities of AH, and AH,- molecules to a high level of accuracy. Using a series of extended basis sets within the fourth-order MerllerPlesset (MP4) ansatz, they concluded that their calculated proton affinities are within f 2 kcal mol-' of the absolute values. Furthermore, Dixon et aL5 have calculated absolute proton affinities for Li,, Na,, LiH, and N a H using an extended basis set within a CI-SD ansatz. Hence they have provided anchor points in the 240-280 kcal mol-' range. Moreover, recently Dixon and Lias6 have reviewed the discrepancy between theory and experiment of absolute gas-phase proton affinities. For electron-dense molecules with many degrees of freedom, more tractable molecular orbital theories are sought in order to calculate relative proton affinities and therefore trends within a series of molecules. Typically, proton affinities are calculated at the Hartree-Fock (HF) level either at standard geometries or including complete geometry optimization, using minimal and/or split valence basis set^.^,^ For example, Silvesto et aL8 have found that for an extended series of substituted methylamines, XCH2NH2, the minimal STO-3G calculations give closest agreement with experiment averaged over all substituents, whereas for alkyl and fluoroalkyl substituents the 6-31G*//6-31G* calculations lead to the lowest mean deviation. Simple organic acids and bases such as HCOOH, CH3COOH, C H 3 0 H , and C 2 H 5 0 H are electron-sparse enough (and the number of degrees of freedom is small enough) to enable relative proton affinities to be calculated at the Hartree-Fock level by use of a variety of different quality basis sets (such as STO-3G, STO-3G**, 3-21G, and 6-31G**), thereby testing the severity of using tractable models. Furthermore, it is well-known that in the gas phase carboxylic acids are more acidic than aliphatic alcohols?,1° making this series even more demanding. For example, the experimental gas-phase proton affinities for HCOOH, CH3COOH, CH,OH, and C 2 H 5 0 Hare 178.8, 190.2, 181.9, and 188.3 kcal mol-', respectively, with carboxylic acid having the largest proton a f f i n i t ~ . ~ Thus far, only the H F proton affinity of HCOOH"-I3 and that of CH30H'4,'5have been reported. The problem centres on the requirement of a 3N - 6 geometry optimization of both the neutral and the protonated species. For the neutral species, there are numerous references in the literature of partially optimized and fully optimized geometries of HCOOH, CH3COOH, C H 3 0 H , and C 2 H 5 0 H using a large variety of different basis sets.16 However, more pertinent to this study are the reported STO-3G, STO-3G**, 3-21G, and 6-31G** partially and fully optimized geometries of HCOOH,"-'3,17J8 C H3COOH,'3919C H3O H. 14~1520-22 and C2H5OHZ1as well as the respective experimental geometries.23-26 For the protonated species, there have been few analogous studies using these basis sets,'4J5,27-29although the optimized geometries would be extremely helpful for mass IMS Visiting Associate Professor. On leave from Department of Chemistry, University of Newcastle, Newcastle, 2308 NSW, Australia.
0022-3654/90/2094-8041$02.50/0
TABLE I: Total Energies - of the Stationary Points for Neutral and Protonated Moleculeso 3-2 1 G 6-3 1G** molecule STO-3G STO-3G** HCOOH -186.217 88' -186.36458 -187.70020 -188.77057' (HCOOH)' -186.59881 -186.75953 -188.0029Id -189.077 60 (CH3COOH) -224.81020' -224.978 78 -226.534 2 6 -227.822 171 (CH3COOH)H' -225.21 125 -225.391 64 -226.853 29 -228.145 26 CHiOH -113.549 198 -113.62573 -1 14.398 02 -1 15.046 71* (CHjOH)H+ -1 13.92960' -114.00662 -1 14.724 92 -115,35744' (CZHSOH) -152.13268 -152.23203 -153.222 6d -154.090 16' (CIHSOH)H+ -152.52282 -152.62289 -153.55661 -1 54.408 59 OAll energies in hartrees. bSee also ref 1 1 and 17. 'See also ref 13 and 18. dSee also ref 27. CSeealso refs 13 and 19. /See als ref 19. gSee also ref 20. hSee also ref 14, 15, 21 and 22. 'See also ref 29. jSee also ref 14 and 15. 'See also ref 21.
spectroscopic investigations of the hydrogen-bonded cluster^.^^^^' Recently, the proton affinities of some water clusters have been (1) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab initio Molecular Orbital Theory; Wiley: New York, 1986. (2) Pople, J. A.; Luke, B. T.; Frisch, M. J.; Binkley, J. S. J . Phys. Chem. 1985, 89, 2198. (3) Curtiss, L. A,; Pople, J. A. J . Phys. Chem. 1988, 92, 894. (4) Pople, J. A.; Schleyer, P. v. R.; Kaneti, J.; Spitznagel, G. W. Chem. Phys. Lett. 1988, 145, 359. (5) Dixon, D. A,; Cole, J. L.; Komornicki, A. J. Phys. Chem. 1988, 92, 2134. ( 6 ) Dixon, D. A.; Lias, S. G. In Molecular Structure and Energetics, Vol. 2; Liebman, J . F.. Greenberg, A., Eds.; VCH: New York, 1987. (7) Taagepera, M.; Hehre, W. J.; Topsom, R. D.; Taft, R. W. J . Am. Chem. SOC.1976, 98, 7438. (8) Silvestro, T.; Topsom, R. D.; Bock, C. W.; Taft, R. W. J . Mol. Srrucr. (Theochem) 1989, 184, 33. (9) Lias, S. G.; Liebman, J. F.; Levin, R. D. J . Phys. Chem. Ref. Datu 1984, 13, 695. (IO) Siggel, M. R.; Thomas, T. D. J . Am. Chem. SOC.1986, 108,4363. ( 1 1 ) Del Bene, J. E. Chem. Phys. Lett. 1978, 55, 235. (12) Sreerama, N.; Vishveshwara, S. J . Mol. Strucr. 1989, 194, 61. (13) Nguyen, M. T.; Hegarty, A. F. J. Am. Chem. SOC.1984,106, 1552. (14) Del Bene, J. E.; Frisch, M. J.; Raghavachari, K.; Pople, J. A. J . Chem. Phys. 1982, 86, 1529. (15) Nobes, R. H.; Radom, L. Org. Muss. Specrrom. 1982, 17, 340. (16) Ohno, K.; Morokuma, K.; Hosoya, H . J . Mol. Struct. (Theochem) 1988, 51, Suppl. 7; see also the previous supplements in the same journal. (17) Chang, Y.-T.; Yamaguchi, Y.; Miller, W. H.; Schaeffer 111, H. F. J . Am. Chem. Soc. 1987, 109, 7245. (18) Dory, M.; Delhalle, J.; Fripiat, J. G.; Andre, J.-M. Inr. J . Quantum Chem., Quanrum Biol. Symp. 1987, 14, 85. (19) Ruelle, P. Chem. Phys. 1986, 110, 263. (20) Del Bene, J. J. Chem. Phys. 1971, 55, 4633. (21) Nishi, N.; Koga, K.; Ohshima, C.; Yamamoto, K.;Nagashima, U.; Nagami, K. J. Am. Chem. SOC.1988, 110, 5246. (22) Latajka, Z.; Ratajczak, H.; Murto. J.; Orville-Thomas, W. J. J. Mol. Struct. (Theochem) 1989, 194, 45. (23) Davis, R. W.; Robiette, A. G.; Gerry, M. C. L.; Bjarnov, E.; Winnewisser, G.J . Mol. Spectroc. 1980, 81, 93. (24) van Eijck, B. P.; van Opheusden, J.; van Schaik, M. M. M.; van Zoeren, E. J. Mol. Spectrosc. 1981, 86, 465. (25) Gerry, M. C. L.; Lees, R. M.; Winnewisser, G. J . Mol. Spectrosc. 1976, 61, 231. (26) Sasada, Y.; Takano, M.; Satoh, T. J . Mol. Specfrosc. 1971, 38, 33. (27) Ha, T.-K.; Nguyen, M. T. J. Phys. Chem. 1984,88,4295. (28) DeFrees, D. J.; McLean, A. D. Chem. Phys. Lett. 1986. 131, 403.
0 1990 American Chemical Society
8042 The Journal of Physical Chemistry, Vol. 94, No. 21, 1990
determined from photoionization efficiency curves measured by synchrotron radiation.32 Therefore the present study will provide basic information for future calculations of proton affinities of molecules studies here. Hence proton affinity calculations of HCOOH, CH3COOH, C H 3 0 H ,and C 2 H 5 0 H(denoted as the R H series) using a variety of basis sets are timely. Furthermore, such a study adds to an ongoing extensive body of work on the effectiveness of a b initio molecular orbital calculations in reproducing experimental equilibrium geometries and, moreover, in reproducing experimental energies for isodemic equilibria.'-*
Molecular Structure The geometries for R H and RH2+molecules were calculated by using the Berny gradient optimization package33within the GAUSSIAN 82 suite of programs.34 Full 3N - 6 optimizations were performed for both neutrals and cations using the internal STO3G, 3-21G, and 6-31G** basis sets. The STO-3G** basis set was constructed from the internal STO-3G basis, adding the standard p polarization functions on the hydrogens and 6d polarization functions on the heavier atoms.] Although optimized geometries of many of the neutrals have been reported for some basis sets J ~ - *not ~ all geometrical coordinates studied in this w ~ r k , ' ' - ~ ~often were reported. Hence for those molecules a full 3.N - 6 optimization was repeated. Table I gives the total energies at stationary points for the neutrals and respective cations using the variety of basis sets. Figure 1 compares the experimental geometries (where possible) with the 6-31G** optimized geometries. All experimental ge~ m e t r i e s in ~ ~Figure - ~ ~ l have been determined from microwave spectra. HCOOH and (HCOOH)H+. All basis sets calculate a planar structure for HCOOH and essentially reproduce the experimental structure.23 Perhaps the most noticeable error is the 3-21Gpredicted H-0-C bond angle. STO-3G,11-'7STO-3G**, and 6-312G**13si8calculate this angle as 105', 104', and 109', respectively, which compares favorably with the experimental determination of 106°.23 However, the 3-21G calculation gives this angle as 1 13'. This illustrates a common failure of double-l basis sets at the H F level in predicting accurate angles about the oxygen a t ~ m . ] ~The , ~more ~ extensive 6-31G** basis set significantly reduces this error. All basis sets predict protonated formic acid to be planar, with a cis-trans configuration (which is consistent ~ ~calwith previous calculation^^^^^^^^^). Ha and N g ~ y e nhave culated that at the 3-21G H F level the energy differences between the cis-trans, cis-cis, and trans-trans conformers are 5.2 kcal mol-' (the cis-cis and trans-trans differ by only 20 phartrees at this level of the theory). CH3COOH and ( CH3COOH)H+. All calculations essentially give the CH3COOH experimental geometryZ4with the carbonyl, hydroxyl, and one of the C-H bonds of the methyl group all being coplanar. It is interesting to note that the addition of the polarization functions lengthens the C-C bond (STO-3G, l S 3 7 A;I3,l9STO-3G**. 1.552 A; 3-21G, 1.498 A;" 6-31G**, 1.501 AI9) but decreases both C - 0 bonds (STO-3G, 1.216 and 1.392 A;",19STO-3G**, 1.177 and 1.364 A: 3-21G. 1.202 and 1.360 A;I96-31G**. 1.187and 1.331 AI9). Italsoshouldbenoted that thc cis-trans difference relative to the single C-0 bond is 5.9 kcal mol-' 37 compared with a calculated value of 8 kcal mol-' for the (29) Williams. I . J . Mol. Struct. (Theochem) 1983, 105, 105. (30) Iraqi, M.; Lifshitz, C . Int. J . Mass Spectrom. Ion Process. 1989,88, 45. ( 3 1 ) Nishi. N . Personal communication, 1989.
(32) Shiromaru, H.; Shinohara, H.; Wasida, N.; Yoo, H.-S.; Kimura. K. Chem. Phys. Lett. 1987, 1 4 1 , 7. (33) Schlegel. H. B. J . Comput. Chem. 1982, 3, 214. (34) Binkley, J. S.; Frisch, M. J.; DeFrees, D. J.; Raghavachari, K.; Whiteside, R. A.; Schlegel. H . B.; Fluder, F. M.; Pople, J. A. GAUSSIAN 82 Release A; Department of Chemistry, Carnegie-Mellon University: Pittsburgh, PA, 1981 (a library program of Computer Centre of the Institute for Molecular Science, Okazaki, 1985). (35) Boggs, J. E.; Cordell, F. R. J . Mol. Srruct. 1981, 76. 329. (36) Hopkinson. A . C.; Yates. K.; Csizmadia, I . G.J . Chem. Phys. 1974. 52, 1784.
von Nagy-Felsobuki and Kimura TABLE 11: Comparison of Experimental with 6-31G**//6-31G** Proton Affinities" molecule PA: HCOOH 192.7 CH3COOH 202.7 CH3OH 195.0 CZHSOH 199.8
AZPEC
PA,,,
7.6 7.4 7.4 7.7
ISd
186.6 196.8 189.1 193.6
exptl'
b'
178.8 190.2 181.9 188.3
0.0 1.2 0.6 2.5
"All energy entries a r e in kcal mol-'. See also ref 14 and 15. *See eq 2 and subsequent discussion.
