- Email: [email protected]
Accurate D0 values for SiF and SiF+
1 May 1998
Chemical Physics Letters 287 Ž1998. 239–242
Accurate D 0 values for SiF and SiFq Alessandra Ricca, Charles W. Bauschlicher Jr. NASA Ames Research Center, Moffett Field, CA 94035, USA Received 26 September 1997; in final form 5 February 1998
Abstract Highly accurate D 0 values are determined for SiF and SiFq using the CCSDŽT. approach in conjunction with basis set extrapolation. The results include the effect of spin–orbit coupling and core–valence correlation. Our best D 0 estimates for SiF and SiFq are 141.5 and 159.7 kcalrmol, respectively, which we estimate to have an uncertainty of "1.0 kcalrmol. For SiF, the value is significantly larger than the older experiments and only slightly larger than the most recent experiment. Our value is slightly larger than previous calculations. For SiFq our best estimate is in good agreement with previous calculations and slightly smaller than the experimental value. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction SiF4 and its fragments are very interesting species because of their involvement in silicon etching reactions. While the heat of formation of SiF4 is well established, it has been found difficult to compute this quantity accurately w1x; for example the usually reliable G2 approach w2x has an error of 8.4 kcalrmol in the SiF4 atomization energy. The heats of formation of the other SiFn molecules are less well known than SiF4 and this is especially true of SiF, where most of computed results w2–5x are between 135.2 and 141.3 kcalrmol compared with the experimental value Ž128.4 kcalrmol. of Ehlert and Margrave w6x, which is the value recommended by Huber and Herzberg w7x. The computed results are also larger than the values from the NBS w8x Ž125.3. and JANAF w9x Ž130.2. tabulations. However, there is reasonable agreement between theory and the most recent experiments of Fisher, Kickel and Armentrout w10x; their 298 K value is 138.6 " 2.2 kcalrmol, which we convert to 0 K as 137.8 " 2.2 kcalrmol. The SiFnq ions are also of interest, and the two published
values for the bond energy of SiFq are in good agreement 162.6 " 1.3 w10x and 161.0 " 4 kcalrmol w11x Žconverted to 0 K by us.. The coupled cluster singles and doubles approach w12x including the effect of connected triples determined using perturbation theory w13x, CCSDŽT., is an excellent way to account for the effects of electron correlation. Using this approach in conjunction with the systematically expandable correlation-consistent basis sets w14–17x and extrapolation techniques w18,19x makes it possible to compute very accurate bond energies of small molecules. In light of the difficulty in computing the heat of formation of SiF4 and the uncertainty in the SiF bond energy, we have computed a highly accurate bond energy for SiF. We also perform analogous calculations on SiFq to compare with the literature values.
2. Methods The orbitals are optimized using the spin-restricted Hartree–Fock ŽHF. method and more exten-
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 1 6 6 - 3
240
A. Ricca, C.W. Bauschlicher Jr.r Chemical Physics Letters 287 (1998) 239–242
sive correlation is included using the restricted CCSDŽT. approach w20,21x. These calculations are performed using Molpro 96 w22x. In most calculations only the valence electrons Žthe Si 3s and 3 p and F 2 s and 2 p . are correlated. These valence calculations use the augmented-correlation-consistent polarized valence Žaug-cc-pV. sets developed by Dunning and co-workers w14–17x. We use the triple zeta ŽTZ., quadruple zeta ŽQZ., and quintuple zeta Ž5Z. sets. To compute the effect of core–valence ŽCV. correlation, the Si 2 s and 2 p and F 1 s electrons are added to the correlation treatment. Three core–valence basis sets are developed. The first is derived from the aug-cc-pVTZ set. For SiŽF., the first 10Ž5. s primitives are contracted to two functions. The second Žthird. CV basis set is derived from the aug-cc-pVQZ Žaug-cc-pV5Z. set; for Si, the first 11Ž14. s primitives are contracted to two functions, while for F the first six Žseven. s primitives are contracted to one function. The rest of the s functions and all of the p functions are uncontracted. To all three basis sets, three even-tempered tight d and two even-tempered tight f functions are added to both Si and F. A b of 2.5 is used for the d functions and a value of 3.0 is used for the f functions. These sets are denoted CVŽtz., CVŽqz., and CVŽ5z. respectively. The CV effect is computed as the difference between correlating only the valence electrons and correlating the valence plus inner-shell electrons, with both calculations performed using the CV basis set. The r e values from the analogous aug-cc-pV basis set are used in these CV calculations. Several extrapolation techniques are used: the two point Ž ry4 ., three point Ž ry4 q ry6 . and variable a Ž rya . schemes described by Martin w19x and the logarithmic convergence approach described by Feller w18x. For the neutral system, the zero-point energy is computed using the experimental w7x v e and v e X e values, while for the ion the aug-cc-pV5Z v e value is used. On the basis of the SiF results, using the CCSDŽT. v e value for SiFq should introduce essentially no error in the computed D 0 value. The effect of spin–orbit coupling on the dissociation energy is computed using experiment. For the atoms, we use the difference between the lowest m j component and the m j weighted average energy w23x. For SiF, we
assume that the spin–orbit effect is one half the separation between the 2P 1r2 and 2P 3r2 levels w24x. 3. Results and discussion The computed spectroscopic constants are summarized in Table 1. For both systems, all three properties appear to be converging with basis set improvement. For the SiF r e and v e values, the difference between the aug-cc-pV5Z set and experiment w7x is small; part of the difference is from remaining basis set incompleteness and part from core–valence correlation effects. We expect similar errors for SiFq. We do not consider the calculation of r e and v e further. For the De values, we consider several extrapolation approaches. For SiF, they are in good agreement with each other, while for SiFq the agreement is not as good. For SiF, we take the average of the 2-ptŽQZ,5Z., 3-ptŽTZ,QZ,5Z., and variable a values as our best estimate for the basis set limit valence De value Ž142.73 kcalrmol.. For SiFq we take the 3-ptŽTZ,QZ,5Z. result. It is a bit difficult to assign an error to this value, but we note that an uncertainty of "0.3 kcalrmol would include the 2-ptŽQZ,5Z. and variable a values within the error bars. The core–valence contributions to De are 0.34, 0.60, and 0.57 kcalrmol, computed using the CVŽtz., CVŽqz. and CVŽ5z. basis sets, respectively, for SiF and 0.40, 0.70, and 0.67 kcalrmol for SiFq. For SiF, the difference in the valence treatment between the aug-cc-pV and CV basis sets drops from 2.48 kcalrmol for TZ sets to 0.84 kcalrmol for the QZ sets, and to 0.18 kcalrmol for the 5Z sets. The analogous values for SiFq are slightly larger, 3.42, 1.21, and 0.25 kcalrmol. The effect of tight function is relatively small and decreasing rapidly with basis improvement, so we conclude that SiF and SiFq do not suffer from missing tight functions as found w25x for SO 2 . The decrease in the CV effect for the CVŽ5z. basis set compared with the CVŽqz. set is a result of decreasing basis set superposition error. We use the CVŽ5z. values to compute the CV effect on the De . Because the CV effect is small, it is difficult to compute accurately, but it is clear that the CVŽ5z. results are accurate to "0.3 kcalrmol. Correcting our extrapolated De values for core– valence correlation, spin–orbit effects, and zero-point
A. Ricca, C.W. Bauschlicher Jr.r Chemical Physics Letters 287 (1998) 239–242 Table 1 Computed spectroscopic constants
241
a
a
SiFq
SiF
CCSDŽT. aug-cc-pVTZ aug-cc-pVQZ aug-cc-pV5Z
re
De
ve
re
De
ve
1.625 1.613 1.608
136.50 140.35 141.64
834 851 855
1.548 1.538 1.533
154.13 158.36 159.97
1023 1042 1047
extrapolation 2-ptŽTZ,QZ. 2-ptŽQZ,5Z. 3-ptŽTZ,QZ,5Z. 3-pt variable a b 3-pt Feller
142.57 142.69 142.73 142.78 142.29
160.80 161.28 161.45 161.73 160.96
other effects CV correlation spin orbit zero point c
q0.57 y0.58 y1.22
q0.67 y0.93 y1.50
experiment w7x
1.6011
857.19
˚ the De values in kcalrmol, and the v e values are in cmy1 .b The r e values are in A, Alpha for neutral is 3.740 and for the ion is 3.184.c For SiF the value is computed using experimental v e and v e X e values, while for SiFq the aug-cc-pV5Z v e value is used.
energy, yields our best estimates for D 0 , 141.5 and 159.7 kcalrmol for SiF and SiFq, respectively. Considering the uncertainty in the effect of core–valence correlation, the difference between the extrapolation methods, the intrinsic uncertainty in the extrapolation approaches, and correction effects not accounted for Table 2 Summary of D 0 values, in kcalrmol a
SiFq
SiF JANAF w9x NBS w8x Ehlert and Margrave w6x Fisher et al. w10x G1 w2x G2 w2x IsodesmicŽG1. w4x IsodesmicŽG2. w4x BAC-MP4 w3x DC–CI w26x MP4r6-311qGŽ2df,2p. w5x ‘‘Revised’’ Žisodesmic. w5x ‘‘Best’’ w11x present work
130.2 125.3 128.4 137.8"2.2 140.4 138.6 141.3 140.4 137.4 124.5 a 135.2 b 140.0 b
a
141.5"1.0
The reported 298 K value is converted to 0 K.b The reported De is corrected to D 0 .
162.6"1.3
161.0"4 a 159.7"1.0
a
in the CCSDŽT. approach, we estimate the uncertainly in our D 0 values to be "1.0 kcalrmol. In Table 2 we compare our best estimates for D 0 with previous work. First considering SiF, our directly computed values cannot be too large and therefore they rule out the experimental value of Ehlert and Margrave w6x and the values recommended by JANAF w9x or NBS w8x. The agreement with the more recent experiment of Fisher, Kickel, and Armentrout w10x is reasonable, but we believe that the true value is slightly larger than even the upper bound of their experiment. The G1 value is in better agreement with our estimate than is the G2 value; this is fortuitous as the G2 value is usually more reliable w2x. The error in the G2 value is 2.9 kcalrmol, which is only slightly larger than the approaches target accuracy of 2 kcalrmol and is consistent with an error of 8.4 kcalrmol when four Si–F bonds are broken. Michels and Hobbs w4x improved on the G1 and G2 values by combining these approaches with an isodesmic reaction; the errors are now 0.2 and 1.1 kcalrmol, respectively. The BACMP4 result of Ho and Melius w3x is 4.1 kcalrmol smaller than our best estimate, which is disappointing since this method has been more accurate in
242
A. Ricca, C.W. Bauschlicher Jr.r Chemical Physics Letters 287 (1998) 239–242
general, but is consistent with the fact that it is difficult to describe Si–F bonds accurately. The dissociation consistent configuration interaction ŽDC– CI. value is much too small w26x; this approach uses a small basis set and restricted set of configurations. Apparently the normal cancellation of errors in this approach does not apply to SiF. Their MP4 value in a large basis set is too small as noted by Ignacio and Schlegel w5x. Therefore, they revised their value using isodesmic reactions, and this value is in much better agreement with our best estimate. For SiFq, the best value of Ignacio and Schlegel w11x is in very good agreement with our value. The experimental value of Fisher et al. is somewhat larger than our computed result, and we believe that the true value must be at the lower end of their experimental range.
4. Conclusions The D 0 values of SiF and SiFq have been computed using the CCSDŽT. approach in conjunction with large correlation consistent basis sets. The values are refined by basis set extrapolation and the inclusion of core–valence correlation and spin–orbit effects. Our best estimate for the D 0 of SiF is 141.5 " 1.0 kcalrmol. This value is much larger than the recommended values w7–9x and slightly larger than many of previous calculations and the most recent experiment w10x. The previous best estimates of Michels and Hobbs w4x and Ignacio and Schlegel w5x agree with our value to within 1.5 kcalrmol. Our best estimate for SiFq Ž159.7 " 1.0 kcalrmol. is in good agreement with the best value of Ignacio and Schlegel w11x Ž161 " 4 kcalrmol. and the experimental value Ž162.6 " 1.3 kcalrmol. of Fisher et al.
Acknowledgements AR would like to acknowledge a Swiss National Science Foundation fellowship.
References w1x J.W. Ochterski, G.A. Petersson, J.A. Montgomery, J. Chem. Phys. 104 Ž1996. 2598. w2x L.A. Curtiss, K. Raghavachari, G.W. Trucks, J.A. Pople, J. Chem. Phys. 94 Ž1991. 7221. w3x P. Ho, C.F. Melius, J. Phys. Chem. 94 Ž1990. 5120. w4x H.H. Michels, R.H. Hobbs, Chem. Phys. Lett. 207 Ž1993. 389. w5x E.W. Ignacio, H.B. Schlegel, J. Chem. Phys. 92 Ž1990. 5404. w6x T.C. Ehlert, J.L. Margrave J. Chem. Phys. 41 Ž1964. 1066. w7x K.P. Huber, G. Herzberg, Constants of Diatomic Molecules, Van Nostrand, New York, 1979. w8x D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey, K.L. Churney, R.J. Nuttall, J. Phys. Chem. Ref Data 11 Ž1982. Suppl. 2. w9x M.W. Chase, C.A. Davies, J.R. Downey, D.J. Frurip, R.A. McDonald, A.N. Syverud, J. Phys. Chem. Ref. Data 14 Ž1985. Suppl. 1. w10x E.R. Fisher, B.L. Kickel, P.B. Armentrout, J. Phys. Chem. 97 Ž1993. 10204. w11x E.W. Ignacio, H.B. Schlegel, J. Phys. Chem. 94 Ž1990. 7439. w12x R.J. Bartlett, Annu. Rev. Phys. Chem. 32 Ž1981. 359. w13x K. Raghavachari, G.W. Trucks, J.A. Pople, M. Head-Gordon, Chem. Phys. Lett. 157 Ž1989. 479. w14x T.H. Dunning, J. Chem. Phys. 90 Ž1989. 1007. w15x R.A. Kendall, T.H. Dunning, R.J. Harrison, J. Chem. Phys. 96 Ž1992. 6796. w16x D.E. Woon, T.H. Dunning, J. Chem. Phys. 98 Ž1993. 1358. w17x D.E. Woon, K.A. Peterson, T.H. Dunning, unpublished. w18x D. Feller, J. Chem. Phys. 96 Ž1992. 6104. w19x J.M.L. Martin, Chem. Phys. Lett. 259 Ž1996. 669. w20x P.J. Knowles, C. Hampel, H.-J. Werner, J. Chem. Phys. 99 Ž1993. 5219. w21x J.D. Watts, J. Gauss, R.J. Bartlett, J. Chem. Phys. 98 Ž1993. 8718. w22x MOLPRO 96 is a package of ab initio programs written by H.-J. Werner, P.J. Knowles, with contributions from J. Almlof, ¨ R.D. Amos, M.J.O. Deegan, S.T. Elbert, C. Hampel, W. Meyer, K. Peterson, R. Pitzer, A.J. Stone, P.R. Taylor. The closed shell CCSD program is described in C. Hampel, K. Peterson, H.-J. Werner, Chem. Phys. Lett. 190 Ž1992. 1. w23x C.E. Moore, Atomic energy levels, Natl. Bur. Stand. ŽUS. circ. 467 Ž1949.. w24x R.W. Martin, A.J. Merer, Can. J. Phys. 51 Ž1973. 634. w25x C.W. Bauschlicher, H. Partridge, Chem. Phys. Lett. 240 Ž1995. 533. w26x B.J. Garrison, W.A. Goddard, J. Chem. Phys. 87 Ž1987. 1307.
Chemical Physics Letters 287 Ž1998. 239–242
Accurate D 0 values for SiF and SiFq Alessandra Ricca, Charles W. Bauschlicher Jr. NASA Ames Research Center, Moffett Field, CA 94035, USA Received 26 September 1997; in final form 5 February 1998
Abstract Highly accurate D 0 values are determined for SiF and SiFq using the CCSDŽT. approach in conjunction with basis set extrapolation. The results include the effect of spin–orbit coupling and core–valence correlation. Our best D 0 estimates for SiF and SiFq are 141.5 and 159.7 kcalrmol, respectively, which we estimate to have an uncertainty of "1.0 kcalrmol. For SiF, the value is significantly larger than the older experiments and only slightly larger than the most recent experiment. Our value is slightly larger than previous calculations. For SiFq our best estimate is in good agreement with previous calculations and slightly smaller than the experimental value. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction SiF4 and its fragments are very interesting species because of their involvement in silicon etching reactions. While the heat of formation of SiF4 is well established, it has been found difficult to compute this quantity accurately w1x; for example the usually reliable G2 approach w2x has an error of 8.4 kcalrmol in the SiF4 atomization energy. The heats of formation of the other SiFn molecules are less well known than SiF4 and this is especially true of SiF, where most of computed results w2–5x are between 135.2 and 141.3 kcalrmol compared with the experimental value Ž128.4 kcalrmol. of Ehlert and Margrave w6x, which is the value recommended by Huber and Herzberg w7x. The computed results are also larger than the values from the NBS w8x Ž125.3. and JANAF w9x Ž130.2. tabulations. However, there is reasonable agreement between theory and the most recent experiments of Fisher, Kickel and Armentrout w10x; their 298 K value is 138.6 " 2.2 kcalrmol, which we convert to 0 K as 137.8 " 2.2 kcalrmol. The SiFnq ions are also of interest, and the two published
values for the bond energy of SiFq are in good agreement 162.6 " 1.3 w10x and 161.0 " 4 kcalrmol w11x Žconverted to 0 K by us.. The coupled cluster singles and doubles approach w12x including the effect of connected triples determined using perturbation theory w13x, CCSDŽT., is an excellent way to account for the effects of electron correlation. Using this approach in conjunction with the systematically expandable correlation-consistent basis sets w14–17x and extrapolation techniques w18,19x makes it possible to compute very accurate bond energies of small molecules. In light of the difficulty in computing the heat of formation of SiF4 and the uncertainty in the SiF bond energy, we have computed a highly accurate bond energy for SiF. We also perform analogous calculations on SiFq to compare with the literature values.
2. Methods The orbitals are optimized using the spin-restricted Hartree–Fock ŽHF. method and more exten-
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 1 6 6 - 3
240
A. Ricca, C.W. Bauschlicher Jr.r Chemical Physics Letters 287 (1998) 239–242
sive correlation is included using the restricted CCSDŽT. approach w20,21x. These calculations are performed using Molpro 96 w22x. In most calculations only the valence electrons Žthe Si 3s and 3 p and F 2 s and 2 p . are correlated. These valence calculations use the augmented-correlation-consistent polarized valence Žaug-cc-pV. sets developed by Dunning and co-workers w14–17x. We use the triple zeta ŽTZ., quadruple zeta ŽQZ., and quintuple zeta Ž5Z. sets. To compute the effect of core–valence ŽCV. correlation, the Si 2 s and 2 p and F 1 s electrons are added to the correlation treatment. Three core–valence basis sets are developed. The first is derived from the aug-cc-pVTZ set. For SiŽF., the first 10Ž5. s primitives are contracted to two functions. The second Žthird. CV basis set is derived from the aug-cc-pVQZ Žaug-cc-pV5Z. set; for Si, the first 11Ž14. s primitives are contracted to two functions, while for F the first six Žseven. s primitives are contracted to one function. The rest of the s functions and all of the p functions are uncontracted. To all three basis sets, three even-tempered tight d and two even-tempered tight f functions are added to both Si and F. A b of 2.5 is used for the d functions and a value of 3.0 is used for the f functions. These sets are denoted CVŽtz., CVŽqz., and CVŽ5z. respectively. The CV effect is computed as the difference between correlating only the valence electrons and correlating the valence plus inner-shell electrons, with both calculations performed using the CV basis set. The r e values from the analogous aug-cc-pV basis set are used in these CV calculations. Several extrapolation techniques are used: the two point Ž ry4 ., three point Ž ry4 q ry6 . and variable a Ž rya . schemes described by Martin w19x and the logarithmic convergence approach described by Feller w18x. For the neutral system, the zero-point energy is computed using the experimental w7x v e and v e X e values, while for the ion the aug-cc-pV5Z v e value is used. On the basis of the SiF results, using the CCSDŽT. v e value for SiFq should introduce essentially no error in the computed D 0 value. The effect of spin–orbit coupling on the dissociation energy is computed using experiment. For the atoms, we use the difference between the lowest m j component and the m j weighted average energy w23x. For SiF, we
assume that the spin–orbit effect is one half the separation between the 2P 1r2 and 2P 3r2 levels w24x. 3. Results and discussion The computed spectroscopic constants are summarized in Table 1. For both systems, all three properties appear to be converging with basis set improvement. For the SiF r e and v e values, the difference between the aug-cc-pV5Z set and experiment w7x is small; part of the difference is from remaining basis set incompleteness and part from core–valence correlation effects. We expect similar errors for SiFq. We do not consider the calculation of r e and v e further. For the De values, we consider several extrapolation approaches. For SiF, they are in good agreement with each other, while for SiFq the agreement is not as good. For SiF, we take the average of the 2-ptŽQZ,5Z., 3-ptŽTZ,QZ,5Z., and variable a values as our best estimate for the basis set limit valence De value Ž142.73 kcalrmol.. For SiFq we take the 3-ptŽTZ,QZ,5Z. result. It is a bit difficult to assign an error to this value, but we note that an uncertainty of "0.3 kcalrmol would include the 2-ptŽQZ,5Z. and variable a values within the error bars. The core–valence contributions to De are 0.34, 0.60, and 0.57 kcalrmol, computed using the CVŽtz., CVŽqz. and CVŽ5z. basis sets, respectively, for SiF and 0.40, 0.70, and 0.67 kcalrmol for SiFq. For SiF, the difference in the valence treatment between the aug-cc-pV and CV basis sets drops from 2.48 kcalrmol for TZ sets to 0.84 kcalrmol for the QZ sets, and to 0.18 kcalrmol for the 5Z sets. The analogous values for SiFq are slightly larger, 3.42, 1.21, and 0.25 kcalrmol. The effect of tight function is relatively small and decreasing rapidly with basis improvement, so we conclude that SiF and SiFq do not suffer from missing tight functions as found w25x for SO 2 . The decrease in the CV effect for the CVŽ5z. basis set compared with the CVŽqz. set is a result of decreasing basis set superposition error. We use the CVŽ5z. values to compute the CV effect on the De . Because the CV effect is small, it is difficult to compute accurately, but it is clear that the CVŽ5z. results are accurate to "0.3 kcalrmol. Correcting our extrapolated De values for core– valence correlation, spin–orbit effects, and zero-point
A. Ricca, C.W. Bauschlicher Jr.r Chemical Physics Letters 287 (1998) 239–242 Table 1 Computed spectroscopic constants
241
a
a
SiFq
SiF
CCSDŽT. aug-cc-pVTZ aug-cc-pVQZ aug-cc-pV5Z
re
De
ve
re
De
ve
1.625 1.613 1.608
136.50 140.35 141.64
834 851 855
1.548 1.538 1.533
154.13 158.36 159.97
1023 1042 1047
extrapolation 2-ptŽTZ,QZ. 2-ptŽQZ,5Z. 3-ptŽTZ,QZ,5Z. 3-pt variable a b 3-pt Feller
142.57 142.69 142.73 142.78 142.29
160.80 161.28 161.45 161.73 160.96
other effects CV correlation spin orbit zero point c
q0.57 y0.58 y1.22
q0.67 y0.93 y1.50
experiment w7x
1.6011
857.19
˚ the De values in kcalrmol, and the v e values are in cmy1 .b The r e values are in A, Alpha for neutral is 3.740 and for the ion is 3.184.c For SiF the value is computed using experimental v e and v e X e values, while for SiFq the aug-cc-pV5Z v e value is used.
energy, yields our best estimates for D 0 , 141.5 and 159.7 kcalrmol for SiF and SiFq, respectively. Considering the uncertainty in the effect of core–valence correlation, the difference between the extrapolation methods, the intrinsic uncertainty in the extrapolation approaches, and correction effects not accounted for Table 2 Summary of D 0 values, in kcalrmol a
SiFq
SiF JANAF w9x NBS w8x Ehlert and Margrave w6x Fisher et al. w10x G1 w2x G2 w2x IsodesmicŽG1. w4x IsodesmicŽG2. w4x BAC-MP4 w3x DC–CI w26x MP4r6-311qGŽ2df,2p. w5x ‘‘Revised’’ Žisodesmic. w5x ‘‘Best’’ w11x present work
130.2 125.3 128.4 137.8"2.2 140.4 138.6 141.3 140.4 137.4 124.5 a 135.2 b 140.0 b
a
141.5"1.0
The reported 298 K value is converted to 0 K.b The reported De is corrected to D 0 .
162.6"1.3
161.0"4 a 159.7"1.0
a
in the CCSDŽT. approach, we estimate the uncertainly in our D 0 values to be "1.0 kcalrmol. In Table 2 we compare our best estimates for D 0 with previous work. First considering SiF, our directly computed values cannot be too large and therefore they rule out the experimental value of Ehlert and Margrave w6x and the values recommended by JANAF w9x or NBS w8x. The agreement with the more recent experiment of Fisher, Kickel, and Armentrout w10x is reasonable, but we believe that the true value is slightly larger than even the upper bound of their experiment. The G1 value is in better agreement with our estimate than is the G2 value; this is fortuitous as the G2 value is usually more reliable w2x. The error in the G2 value is 2.9 kcalrmol, which is only slightly larger than the approaches target accuracy of 2 kcalrmol and is consistent with an error of 8.4 kcalrmol when four Si–F bonds are broken. Michels and Hobbs w4x improved on the G1 and G2 values by combining these approaches with an isodesmic reaction; the errors are now 0.2 and 1.1 kcalrmol, respectively. The BACMP4 result of Ho and Melius w3x is 4.1 kcalrmol smaller than our best estimate, which is disappointing since this method has been more accurate in
242
A. Ricca, C.W. Bauschlicher Jr.r Chemical Physics Letters 287 (1998) 239–242
general, but is consistent with the fact that it is difficult to describe Si–F bonds accurately. The dissociation consistent configuration interaction ŽDC– CI. value is much too small w26x; this approach uses a small basis set and restricted set of configurations. Apparently the normal cancellation of errors in this approach does not apply to SiF. Their MP4 value in a large basis set is too small as noted by Ignacio and Schlegel w5x. Therefore, they revised their value using isodesmic reactions, and this value is in much better agreement with our best estimate. For SiFq, the best value of Ignacio and Schlegel w11x is in very good agreement with our value. The experimental value of Fisher et al. is somewhat larger than our computed result, and we believe that the true value must be at the lower end of their experimental range.
4. Conclusions The D 0 values of SiF and SiFq have been computed using the CCSDŽT. approach in conjunction with large correlation consistent basis sets. The values are refined by basis set extrapolation and the inclusion of core–valence correlation and spin–orbit effects. Our best estimate for the D 0 of SiF is 141.5 " 1.0 kcalrmol. This value is much larger than the recommended values w7–9x and slightly larger than many of previous calculations and the most recent experiment w10x. The previous best estimates of Michels and Hobbs w4x and Ignacio and Schlegel w5x agree with our value to within 1.5 kcalrmol. Our best estimate for SiFq Ž159.7 " 1.0 kcalrmol. is in good agreement with the best value of Ignacio and Schlegel w11x Ž161 " 4 kcalrmol. and the experimental value Ž162.6 " 1.3 kcalrmol. of Fisher et al.
Acknowledgements AR would like to acknowledge a Swiss National Science Foundation fellowship.
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