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The total atomization energy and heat of formation of HCN(g)
13 September 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 259 (1996) 679-682
The total atomization energy and heat of formation of HCN(g) Jan M.L. Martin l The Institute of Chemistry, The Hebrew University of Jerusalem, Givat-Ram, Jerusalem 91904, Israel
Received 27 June 1996; in final form 31 July 1996
Abstract
An exhaustive ab initio convergence study, using coupled cluster methods and basis sets of up to [7s6p5d4f3g2h/5s4p3d2flg] quality, of the total atomization energy of HCN(g), both directly and via the isogyric reaction 2 HCN ¢, C2H2 + N2, leads to a best estimate of ~ D0=303.08+0.25 kcal/mol, in between thermochemical and spectroscopic experimental values. The best estimate for A/-/}'f= 31.04 4- 0.25 kcal/mol, or 129.9-1-1 kJ/mol.
1. Introduction Hydrogen cyanide (prussic acid, HCN) was produced by Scheele in dilute solution in 1782, and by Gay-Lussac as the pure compound in 1815. HCNbased poisons may have been known and produced since Antiquity [ 1 ] : less morbid modern uses include the production of plastics (poly-methylmethacrylate) and antioxidants (EDTA), as well as the extraction of gold and silver using NaCN. In 1979, the world production was estimated to be 0.5-0.6 million ton per year. One would expect the heat of formation and total atomization energy of such a common molecule to be precisely known. Yet the JANAF tables [2] list a heat of formation of 32.4:t:2 kcal/mol, which largely derives from two experiments, one by Berthelot [ 3] from 1881, and one by Thomsen [4] from 1886. Experiments from 1951 by Horiuchi, Yano, and Kanai (HYK) [5] suggest a value of AH 298"15 = 31.354-0.6 IOn sabbatical from: Limburgs Universitair Centrum, Department SBG, UniversitaireCampus, 3590 Diepenbeek,Belgium and University of Antwerp (UIA), Institute for Materials Science, Department of Chemistry,Universiteitsplein 1, 2610 Wilrijk, Belgium
kcal/mol, corresponding (from the thermodynamic functions in Ref. [2] ) to A/-/~f = 31.44+0.6kcal/mol, or ~ Do = 302.8 4- 0.6 kcal/mol. Recently, Morley et al. [6], in their study of the dissociation dynamics of HCN, obtained Do ( H - C N ) = 43740 4-150 cm -1 . An exhaustive ab initio calibration study by Pradhan et al. [7] confirmed definitively that the revised value [8] of D0(CN) = 7.738-1-0.02 eV is the correct one. Together, they lead to a total atomization energy ~ Do = 303.5 4- 0.6 kcal/mol, where the error bar is the RMS (root mean square) of the experimental uncertainties for the two steps. The author has shown elsewhere [9] that ab initio coupled cluster [ 10] calculations in basis sets of spdfgh quality, together with an extrapolation formula based on the known [ 11 ] (l + 1/2) -4 dependence of the electron-electron energy (l being the highest angular momentum present) and inclusion of core correlation through an additivity approximation [ 12], lead to computed ~ De values correct to 0.3 kcal/mol, on average. If a correction is introduced for triple bonds, the mean absolute error drops to 0.2 kcal/mol. It would appear that computational techniques of such accuracy are sufficient to resolve the issue of the total atomiza-
0009-2614/96/$12.00 Copyright @ 1996 Published by Elsevier Science B.V. All rights reserved. PII S0009-2614(96)00899-8
680
J.M.L. Martin/Chemical Physics Letters 259 (1996) 679-682
tion energy of HCN for once and for all.
2. M e t h o d s
All calculations were carried out using the MOLPRO 96 2 ab initio package running on a DEC Alpha TurboLaser 8200 5/300 at the Hebrew University. The CCSD(T) electron correlation method [ 13,14] - which is known to yield total energies close to the exact basis set correlation energy for molecules well described by a single reference configuration [ 15] was used in combination with the following 'correlation consistent' polarized n-tuple zeta (cc-pVnZ) basis sets due to Dunning and coworkers [ 16,17]: - cc-pVTZ, which corresponds to a [4s3p2dlf/3s2pld] contracted basis set; - cc-pVQZ, i.e. [5s4p3d2flg/4s3p2dlf] - cc-pV5Z, i.e. [6s5p4d3f2glh/5s4p3d2flg] - augt-cc-pVTZ, i.e. [5s4p3d2f/3s2pld] - augt-cc-pVQZ, i.e. [6s5p4d3f2g/4s3p2dlf] - augt-cc-pV5Z, i.e. [7s6p5d4f3g2h/5s4p3d2flg] The 'aug t' notation [ 18] means that the heavy atom basis sets have been 'augmented' [ 17] with one diffuse function of each angular momentum. The results were then extrapolated [9] from the successive basis sets using the extrapolation formulas A + B / ( l + 1/2) 4 + C/(l + 1/2) 6 (denoted Schwartz6(TQ5)), A + B / ( l + 1/2) a (denoted Schwartzce(TQ5) ), and A + B / ( l + 1/2) 4 (denoted Schwartz4(Q5) ). As is reflected in the acronyms, the former two formulas require three points to determine all parameters, while two are sufficient for the latter formula. Only the valence electrons were correlated: the core correlation contribution to the total atomization energy was determined elsewhere [ 12] to be 1.67 kcal/mol. In a variational calculation from a C C S D ( T ) / [ 4 s 3p2dlf/4s2pld] potential energy surface, Bentley et al. [ 19] found the anharmonic zero-point energy to be 10.21 kcal/mol. After correcting for the difference between their harmonic frequencies and the experimental ones [20], this is reduced to 10.09 kcal/mol. 2 MOLPRO 96 is an ab initio MO package by H.J. Wemer and EJ. Knowles, with contributions from J. AlmlOf, R.D. Amos, M.J.O. Deegan, S.T. Elbert, C. Hampel, W. Meyer, K.A. Peterson, R.M. Pitzer, A.J. Stone, ER. Taylor, and R. Lindh.
The CCSD(T)/cc-pVTZ and CCSD(T)/ccpVQZ calculations were carried out from fully optimized geometries: all further calculations used the CCSD(T)/cc-pVQZ geometry.
3. R e s u l t s a n d d i s c u s s i o n
All pertinent results can be found in Table 1. Using the 'regular' basis sets, we find extrapolated values of ~ De, after core correlation correction, of 312.96, 312.94, and 312.95 kcal/mol using Schwartz4(Q5), Schwartza(TQ5), and Schwartz6(TQ5) extrapolations, respectively. Using the 'augmented' basis sets, the corresponding values are 312.95, 312.79, and 312.85 kcal/mol, respectively. It is thus seen that all six estimates agree well: the ones with the augmented basis set should normally be the more accurate. It was found elsewhere [9] that, while single and double bonds are essentially converged at this level, triple bonds go up another 0.3 kcal/mol when proceeding from Schwartz6(TQ5) to Schwartz6(Q56), i.e. including calculations with a [7s6p5d4f3g2hli/6s5p4d3f2glh] basis set. For Schwartzce, the corresponding value is 0.4 kcal/mol. Adding this as a correction term to the present resuits, we find best energies of 313.19 (Schwartza) and 313.15 (Schwartz6) kcal/mol. After subtracting the zero-point energies, these are found to correspond to ~ Do values of 303.11 and 303.07 kcal/mol. After subtracting the spin-orbit splitting [2] of 0.09 kcal/mol in C(3p), we end up with final values of 303.02 and 302.98 kcal/mol, well within the upper half of the error bar on the HYK value, and within the lower half of the error bar on the spectroscopically derived value. As a check on our results, we may consider the isogyric reaction 2H-C=N
I ¢¢,H-C-C-H+IN--N
I
Not surprisingly, we see in Table 1 that the reaction energy (in the hypothetical motionless state) of this reaction converges much more rapidly that the ~ De of HCN. From the known ~ Do values of 388.904-0.24 kcal/mol (HCCH [21]) and 225.064-0.03 kcal/mol (N2), respectively, and the anharmonic zero-point energies of 16.46 and 3.36 kcal/mol derived from the
681
J.M.L. Martin~Chemical Physics Letters 259 (1996) 679-682 Table 1 Computed (CCSD(T) with basis set indicated) and experimental atomization and reaction energies (keal/mol) HCN
HCCH
NN
2HCN~C2H2+N2
cc-pVTZ cc-pVQZ cc-pV5Z Schwartz4( Q5 ) Schwartzot ( TQ5 ) Schwartz6 ( TQ5 )
301.36 307.67 309.67 311.29 311.27 311.28
393.26 399.43 401.31 402.83 402.72 402.77
216.45 227.87 225.12 226.95 227.25 227.08
-7.00 -6.97 -7.10 -7.20 - 7.43 - 7.29
aug-cc-pVTZ aug-cc-pVQZ aug-cc-pV5Z Schwartz4 ( Q5 ) Schwartza(TQ5) Schwartz6 ( TQ5 )
302.68 308.30 309.94 311.28 311.12 311.18
394.27 399.94 401.50 402.77 402.53 402.62
217.99 223.80 225.59 227.05 226.98 227.01
-6.90 -7.15 -7.21 -7.27 -7.27 - 7.27
core corr. spin-orbit a best ~ De exp. ZPE
1.67 -0.084 313.07
2.44 -0.17 405.19 405.36(24) [21] 16.46 [22]
0.85 0.00 228.16 228.42(3) 12] 3.36 [23]
+0.05 0.00 -7.22
10.09
+0.36
a Difference between average energies of atomic states and energies of lowest spin-orbit components. Vanishes for H (2S) and N (4S): for C(3p) equals (0.0+3 × 16.40+5 ×43.40)/9=29.6 c m - 1=0.084 kcal/mol. See e.g. Ref. [ 24].
respective spectroscopic constants [22,23], we then find ~ De values of 405.36-4-0.24 and 228.42+0.03 kcal/mol, respectively. Combining this with our best reacti on energy of - 7.22 kcal / mol ( obtained from the 'augmented' basis set results irrespective of extrapolation formula), we obtain ~ De = 313.28 5:0.12 kcal/mol, in excellent agreement with our best direct calculation. The corresponding ~ Do value is then 303.19±0.12 kcal/moi. It therefore appears established that the true value of ~ D0(HCN) lies somewhere in between the HYK and spectroscopic values. The average of both values, ~ D 0 = 303.15 kcal/mol, is in excellent agreement with the average of our three best calculated values, 303.10 kcal/mol. Considering the spread in our calculated values and small uncertainties in the zero-point energy used, we estimate an error bar of about 0.25 kcal/mol. Using the heats of formation of the atoms from the JANAF tables - which add up to 334.14-t-0.11 kcal/mol - this finally leads to a best estimate of A/-/~f = 31.04 + 0.25 kcal/mol, or 129.94-1 kJ/mol.
4. Conclusion An exhaustive ab initio convergence study of the total atomization energy of H C N ( g ) , both directly and via the isogyric reaction 2HCN ¢:~ C2H2 + N2, leads to a best estimate of ~ D0=303.10+0.25 kcal/mol, in between thermochemical and spectroscopic experimental values. The best estimate for A/-/~f = 31.04 + 0.25 kcal/mol, or 129.9+1 kJ/mol.
Acknowledgement The author is a Senior Research Associate ('Onderzoeksleider') of the National Science Foundation of Belgium (NFWO/FNRS) and acknowledges a Golda Meir Fellowship from the Hebrew University. He thanks Professor Joel Liebman for helpful discussions. Generous allowances of computer time were provided by the Institute of Chemistry (Hebrew University).
682
J.M.L. Martin~Chemical Physics Letters 259 (1996) 679-682
References [ 1 ] Kirk-Othmer encyclopedia of chemical technology, 3rd Ed. (Wiley, New York, 1979). [2] M.W. Chase Jr., C.A. Davies, J.R. Downey Jr., D.J. Frurip, R.A. McDonald and A.N. Syverud, JANAF thermochemical tables, 3rd Ed., J. Phys. Chem. Ref. Data 14, Suppl. 1 (1985). [3] J. Berthelot, Ann. Chim. Phys. 23 (1881) 252, [4] J. Thomsen, "Thermochemische Untersuchungen", (Barth, Leipzig, 1886). [5] J. Horiuchi, T. Yano and K. Kanai, Shokubai 7 (1951) 8 (quoted in Ref. [2]). [6] G.P. Morley, I.R. Lambert, M.R. Ashfold, K.N. Rosser and C.H. Western, J. Chem. Phys. 97 (1992) 3157. [7] A. Pradhan, H. Partridge and C.W. Bauschlicher Jr., J. Chem. Phys. 101 (1994) 3857. [81 Y. Huang, S.A. Barts and J.E Halpem, J. Chem. Phys. 96 (1992) 425 [9] J.M.L. Martin, Chem. Phys. Lett. 260 (1996) xxx. [ 10] R.J. Bartlett, J. Phys. Chem. 93 (1989) 1697. [11] C. Schwartz, in Methods in Computational Physics 2, ed. BJ. Alder (Academic Press, New York, 1963).
[12] J.M.L. Martin, Chem. Phys. Lett. 232 (1995) 343. [131 K. Raghavachari, G.W. Trucks, J.A. Pople and M. HeadGordon, Chem. Phys. Lett. 157 (1989) 479. [14] C. Hampel, K.A. Peterson and H.-J. Werner, Chem. Phys. Lett. 190 (1992) 1. [15] T.J. Lee and G.E. Scuseria, in: Quantum mechanical electronic structure calculations with chemical accuracy, ed. S.R. Langhoff (Kluwer, Dordrecht, 1995). [16] T.H. Dunning Jr., J. Chem. Phys. 90 (1989) 1007. [ 17] R.A. Kendall, T.H. Dunning Jr. and R.J. Harrison, J. Chem. Phys. 96 (1992) 6796 [18] J.E. Del Bene, J. Phys. Chem. 97 (1993) 107. [19] J.A. Bentley, J.M. Bowman, B. Gazdy, T.J. Lee, and C.E. Dateo, Chem. Phys. Lett. 198 (1992) 563. [20] A.M. Smith, S.L. Coy, W. Klemperer and K.K. Lehmann, J. Mol. Spectrosc. 134 (1989) 134. [21] D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey, K.L. Chumey and R.L. Nuttall, J. Phys. Chem. Ref. Data 11, Suppl. 2 (1982). [22] G. Strey and I.M. Mills, J. Mol. Spectrosc. 59 (1976) 103. [23] K.P. Huber and G. Herzberg, Constants of diatomic molecules (Van Nostrand Reinhold, New York, 1979). [24] T.J. Lee, J.M.L. Martin, C.E. Dateo and P.R. Taylor, J. Phys. Chem. 99 (1995) 15858.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 259 (1996) 679-682
The total atomization energy and heat of formation of HCN(g) Jan M.L. Martin l The Institute of Chemistry, The Hebrew University of Jerusalem, Givat-Ram, Jerusalem 91904, Israel
Received 27 June 1996; in final form 31 July 1996
Abstract
An exhaustive ab initio convergence study, using coupled cluster methods and basis sets of up to [7s6p5d4f3g2h/5s4p3d2flg] quality, of the total atomization energy of HCN(g), both directly and via the isogyric reaction 2 HCN ¢, C2H2 + N2, leads to a best estimate of ~ D0=303.08+0.25 kcal/mol, in between thermochemical and spectroscopic experimental values. The best estimate for A/-/}'f= 31.04 4- 0.25 kcal/mol, or 129.9-1-1 kJ/mol.
1. Introduction Hydrogen cyanide (prussic acid, HCN) was produced by Scheele in dilute solution in 1782, and by Gay-Lussac as the pure compound in 1815. HCNbased poisons may have been known and produced since Antiquity [ 1 ] : less morbid modern uses include the production of plastics (poly-methylmethacrylate) and antioxidants (EDTA), as well as the extraction of gold and silver using NaCN. In 1979, the world production was estimated to be 0.5-0.6 million ton per year. One would expect the heat of formation and total atomization energy of such a common molecule to be precisely known. Yet the JANAF tables [2] list a heat of formation of 32.4:t:2 kcal/mol, which largely derives from two experiments, one by Berthelot [ 3] from 1881, and one by Thomsen [4] from 1886. Experiments from 1951 by Horiuchi, Yano, and Kanai (HYK) [5] suggest a value of AH 298"15 = 31.354-0.6 IOn sabbatical from: Limburgs Universitair Centrum, Department SBG, UniversitaireCampus, 3590 Diepenbeek,Belgium and University of Antwerp (UIA), Institute for Materials Science, Department of Chemistry,Universiteitsplein 1, 2610 Wilrijk, Belgium
kcal/mol, corresponding (from the thermodynamic functions in Ref. [2] ) to A/-/~f = 31.44+0.6kcal/mol, or ~ Do = 302.8 4- 0.6 kcal/mol. Recently, Morley et al. [6], in their study of the dissociation dynamics of HCN, obtained Do ( H - C N ) = 43740 4-150 cm -1 . An exhaustive ab initio calibration study by Pradhan et al. [7] confirmed definitively that the revised value [8] of D0(CN) = 7.738-1-0.02 eV is the correct one. Together, they lead to a total atomization energy ~ Do = 303.5 4- 0.6 kcal/mol, where the error bar is the RMS (root mean square) of the experimental uncertainties for the two steps. The author has shown elsewhere [9] that ab initio coupled cluster [ 10] calculations in basis sets of spdfgh quality, together with an extrapolation formula based on the known [ 11 ] (l + 1/2) -4 dependence of the electron-electron energy (l being the highest angular momentum present) and inclusion of core correlation through an additivity approximation [ 12], lead to computed ~ De values correct to 0.3 kcal/mol, on average. If a correction is introduced for triple bonds, the mean absolute error drops to 0.2 kcal/mol. It would appear that computational techniques of such accuracy are sufficient to resolve the issue of the total atomiza-
0009-2614/96/$12.00 Copyright @ 1996 Published by Elsevier Science B.V. All rights reserved. PII S0009-2614(96)00899-8
680
J.M.L. Martin/Chemical Physics Letters 259 (1996) 679-682
tion energy of HCN for once and for all.
2. M e t h o d s
All calculations were carried out using the MOLPRO 96 2 ab initio package running on a DEC Alpha TurboLaser 8200 5/300 at the Hebrew University. The CCSD(T) electron correlation method [ 13,14] - which is known to yield total energies close to the exact basis set correlation energy for molecules well described by a single reference configuration [ 15] was used in combination with the following 'correlation consistent' polarized n-tuple zeta (cc-pVnZ) basis sets due to Dunning and coworkers [ 16,17]: - cc-pVTZ, which corresponds to a [4s3p2dlf/3s2pld] contracted basis set; - cc-pVQZ, i.e. [5s4p3d2flg/4s3p2dlf] - cc-pV5Z, i.e. [6s5p4d3f2glh/5s4p3d2flg] - augt-cc-pVTZ, i.e. [5s4p3d2f/3s2pld] - augt-cc-pVQZ, i.e. [6s5p4d3f2g/4s3p2dlf] - augt-cc-pV5Z, i.e. [7s6p5d4f3g2h/5s4p3d2flg] The 'aug t' notation [ 18] means that the heavy atom basis sets have been 'augmented' [ 17] with one diffuse function of each angular momentum. The results were then extrapolated [9] from the successive basis sets using the extrapolation formulas A + B / ( l + 1/2) 4 + C/(l + 1/2) 6 (denoted Schwartz6(TQ5)), A + B / ( l + 1/2) a (denoted Schwartzce(TQ5) ), and A + B / ( l + 1/2) 4 (denoted Schwartz4(Q5) ). As is reflected in the acronyms, the former two formulas require three points to determine all parameters, while two are sufficient for the latter formula. Only the valence electrons were correlated: the core correlation contribution to the total atomization energy was determined elsewhere [ 12] to be 1.67 kcal/mol. In a variational calculation from a C C S D ( T ) / [ 4 s 3p2dlf/4s2pld] potential energy surface, Bentley et al. [ 19] found the anharmonic zero-point energy to be 10.21 kcal/mol. After correcting for the difference between their harmonic frequencies and the experimental ones [20], this is reduced to 10.09 kcal/mol. 2 MOLPRO 96 is an ab initio MO package by H.J. Wemer and EJ. Knowles, with contributions from J. AlmlOf, R.D. Amos, M.J.O. Deegan, S.T. Elbert, C. Hampel, W. Meyer, K.A. Peterson, R.M. Pitzer, A.J. Stone, ER. Taylor, and R. Lindh.
The CCSD(T)/cc-pVTZ and CCSD(T)/ccpVQZ calculations were carried out from fully optimized geometries: all further calculations used the CCSD(T)/cc-pVQZ geometry.
3. R e s u l t s a n d d i s c u s s i o n
All pertinent results can be found in Table 1. Using the 'regular' basis sets, we find extrapolated values of ~ De, after core correlation correction, of 312.96, 312.94, and 312.95 kcal/mol using Schwartz4(Q5), Schwartza(TQ5), and Schwartz6(TQ5) extrapolations, respectively. Using the 'augmented' basis sets, the corresponding values are 312.95, 312.79, and 312.85 kcal/mol, respectively. It is thus seen that all six estimates agree well: the ones with the augmented basis set should normally be the more accurate. It was found elsewhere [9] that, while single and double bonds are essentially converged at this level, triple bonds go up another 0.3 kcal/mol when proceeding from Schwartz6(TQ5) to Schwartz6(Q56), i.e. including calculations with a [7s6p5d4f3g2hli/6s5p4d3f2glh] basis set. For Schwartzce, the corresponding value is 0.4 kcal/mol. Adding this as a correction term to the present resuits, we find best energies of 313.19 (Schwartza) and 313.15 (Schwartz6) kcal/mol. After subtracting the zero-point energies, these are found to correspond to ~ Do values of 303.11 and 303.07 kcal/mol. After subtracting the spin-orbit splitting [2] of 0.09 kcal/mol in C(3p), we end up with final values of 303.02 and 302.98 kcal/mol, well within the upper half of the error bar on the HYK value, and within the lower half of the error bar on the spectroscopically derived value. As a check on our results, we may consider the isogyric reaction 2H-C=N
I ¢¢,H-C-C-H+IN--N
I
Not surprisingly, we see in Table 1 that the reaction energy (in the hypothetical motionless state) of this reaction converges much more rapidly that the ~ De of HCN. From the known ~ Do values of 388.904-0.24 kcal/mol (HCCH [21]) and 225.064-0.03 kcal/mol (N2), respectively, and the anharmonic zero-point energies of 16.46 and 3.36 kcal/mol derived from the
681
J.M.L. Martin~Chemical Physics Letters 259 (1996) 679-682 Table 1 Computed (CCSD(T) with basis set indicated) and experimental atomization and reaction energies (keal/mol) HCN
HCCH
NN
2HCN~C2H2+N2
cc-pVTZ cc-pVQZ cc-pV5Z Schwartz4( Q5 ) Schwartzot ( TQ5 ) Schwartz6 ( TQ5 )
301.36 307.67 309.67 311.29 311.27 311.28
393.26 399.43 401.31 402.83 402.72 402.77
216.45 227.87 225.12 226.95 227.25 227.08
-7.00 -6.97 -7.10 -7.20 - 7.43 - 7.29
aug-cc-pVTZ aug-cc-pVQZ aug-cc-pV5Z Schwartz4 ( Q5 ) Schwartza(TQ5) Schwartz6 ( TQ5 )
302.68 308.30 309.94 311.28 311.12 311.18
394.27 399.94 401.50 402.77 402.53 402.62
217.99 223.80 225.59 227.05 226.98 227.01
-6.90 -7.15 -7.21 -7.27 -7.27 - 7.27
core corr. spin-orbit a best ~ De exp. ZPE
1.67 -0.084 313.07
2.44 -0.17 405.19 405.36(24) [21] 16.46 [22]
0.85 0.00 228.16 228.42(3) 12] 3.36 [23]
+0.05 0.00 -7.22
10.09
+0.36
a Difference between average energies of atomic states and energies of lowest spin-orbit components. Vanishes for H (2S) and N (4S): for C(3p) equals (0.0+3 × 16.40+5 ×43.40)/9=29.6 c m - 1=0.084 kcal/mol. See e.g. Ref. [ 24].
respective spectroscopic constants [22,23], we then find ~ De values of 405.36-4-0.24 and 228.42+0.03 kcal/mol, respectively. Combining this with our best reacti on energy of - 7.22 kcal / mol ( obtained from the 'augmented' basis set results irrespective of extrapolation formula), we obtain ~ De = 313.28 5:0.12 kcal/mol, in excellent agreement with our best direct calculation. The corresponding ~ Do value is then 303.19±0.12 kcal/moi. It therefore appears established that the true value of ~ D0(HCN) lies somewhere in between the HYK and spectroscopic values. The average of both values, ~ D 0 = 303.15 kcal/mol, is in excellent agreement with the average of our three best calculated values, 303.10 kcal/mol. Considering the spread in our calculated values and small uncertainties in the zero-point energy used, we estimate an error bar of about 0.25 kcal/mol. Using the heats of formation of the atoms from the JANAF tables - which add up to 334.14-t-0.11 kcal/mol - this finally leads to a best estimate of A/-/~f = 31.04 + 0.25 kcal/mol, or 129.94-1 kJ/mol.
4. Conclusion An exhaustive ab initio convergence study of the total atomization energy of H C N ( g ) , both directly and via the isogyric reaction 2HCN ¢:~ C2H2 + N2, leads to a best estimate of ~ D0=303.10+0.25 kcal/mol, in between thermochemical and spectroscopic experimental values. The best estimate for A/-/~f = 31.04 + 0.25 kcal/mol, or 129.9+1 kJ/mol.
Acknowledgement The author is a Senior Research Associate ('Onderzoeksleider') of the National Science Foundation of Belgium (NFWO/FNRS) and acknowledges a Golda Meir Fellowship from the Hebrew University. He thanks Professor Joel Liebman for helpful discussions. Generous allowances of computer time were provided by the Institute of Chemistry (Hebrew University).
682
J.M.L. Martin~Chemical Physics Letters 259 (1996) 679-682
References [ 1 ] Kirk-Othmer encyclopedia of chemical technology, 3rd Ed. (Wiley, New York, 1979). [2] M.W. Chase Jr., C.A. Davies, J.R. Downey Jr., D.J. Frurip, R.A. McDonald and A.N. Syverud, JANAF thermochemical tables, 3rd Ed., J. Phys. Chem. Ref. Data 14, Suppl. 1 (1985). [3] J. Berthelot, Ann. Chim. Phys. 23 (1881) 252, [4] J. Thomsen, "Thermochemische Untersuchungen", (Barth, Leipzig, 1886). [5] J. Horiuchi, T. Yano and K. Kanai, Shokubai 7 (1951) 8 (quoted in Ref. [2]). [6] G.P. Morley, I.R. Lambert, M.R. Ashfold, K.N. Rosser and C.H. Western, J. Chem. Phys. 97 (1992) 3157. [7] A. Pradhan, H. Partridge and C.W. Bauschlicher Jr., J. Chem. Phys. 101 (1994) 3857. [81 Y. Huang, S.A. Barts and J.E Halpem, J. Chem. Phys. 96 (1992) 425 [9] J.M.L. Martin, Chem. Phys. Lett. 260 (1996) xxx. [ 10] R.J. Bartlett, J. Phys. Chem. 93 (1989) 1697. [11] C. Schwartz, in Methods in Computational Physics 2, ed. BJ. Alder (Academic Press, New York, 1963).
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