Theoretical study of the thermochemistry of molecules in the silicon

J . Phys. Chem. 1991, 95, 1410-1419

1410

Theoretical Study of the Thermochemistry of Molecules in the Si-N-H-F

System

Carl F. Melius* Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551 -0969

and Pauline Ho* Division I 126, Sandia National Laboratories, Albuquerque, New Mexico 87185-5800 (Received: April 30, 1990; In Final Form: July I I , 1990)

Downloaded by RYERSON UNIV on September 9, 2015 | http://pubs.acs.org Publication Date: February 1, 1991 | doi: 10.1021/j100156a070

A self-consistent set of heats of formation for over 30 molecules in the Si-N-H-F system is obtained from a combination of ab initio electronic structure calculations and empirical corrections. The molecules studied include the HSiNH,,, and F,,SiNH,,, species, some of the H,,Si(NH2),, F,,Si(NH2),,,, and H,N(SiH3),,,(SiF3)/ species, and several transition states for the decomposition of silylamine.

1. Introduction Silicon nitride thin films are used in as well as for protective coating^.^*^ Many of the techniques currently used for the production of such films, such as chemical vapor deposition (CVD) or plasma-assisted CVD processes, involve gas-phase reactions. Some techniques used to produce powders for forming bulk silicon nitride ceramics for structural applications also involve gaseous reactionss Understanding and modeling the chemistry of such materials processing methods can lead to faster process optimization and scaleup. Recently, detailed computer models of such chemically reacting flow systems have been constructed that require knowledge of the energetics of the chemical system in the form of kinetic parameters for many reactions and thermochemical data for all species c ~ n s i d e r e d . ~Although ,~ a wide variety of chemical systems have been used for the production of silicon nitride materials, the reactions of silane and halogenated silanes with ammonia are frequently used.8 Thus, thermochemical information on species in the Si-N-H and Si-N-H-F systems are important in understanding silicon nitride production techniques. Unfortunately, very little information is available in the literature on the thermochemical properties of these species. Previously, we have used ab initio electronic structure calculations, combined with empirical bond additivity corrections (BACs), to obtain heats of formation for Si,H, ( n = I , 2; m 5 2n 2 ) , SiH,CI, ( n m 5 4 ) , and SiH,F, ( n m 5 4 ) spec i e ~ , ~ as - ~ well ' as numerous molecules made up of first-row elements.'* In this paper, we extend our methods to molecules in the Si-N-H-F system.

+

+

+

11. Theoretical Methods and Results The theoretical methods used in this work have been described in detail but we present a general description of ( I ) Kapoor, V. J.; Stein, H. J. Silicon Nitride Thin Insulating Films; Symposium Proceedings; The Electrochemical Society, Inc.: Pennington, NJ, 1983; VOI. 83-8. (2) Belyi, V . I.;Vasilyeva, L. L.; Ginovker, A. S.; Gritsenko, V. A.; Repinsky, S. M.: Sinitsa, S. p.; Smirnova, T. p.; Edelman, F. L. Silicon Nitride in Elecfronics; Materials Science Monographs; Elsevier: Amsterdam, 1988; Vol 34. (3) Galasso, F. S.; Veltri, R. D.; Croft, W. J. Am. Ceram. Soc. Bull. 1978, 57. 453. (4) Gebhardt, J. J.: Tanzilli, R. A,; Harris, T. A. J. Elecfrochem. Soc. 1976, 123, 1578. ( 5 ) See, for example: Ho, P.; Buss, R. J.; Loehman, R. E. J . Mafer. Res. 1989, 4, 873 and references therein. (6) Coltrin. M. E.; Kee, R. J.; Evans, G . H. J. Electrochem. SOC.1989, 136, 819 and references therein. (7) Kushner, M. J . J. Appl. Phys. 1988,63, 2532 and references therein. (8) Kingon. A. 1.; Lutz, L. J.; Davis, R. F. J. Am. Ceram. Soc. 1983.66. 551.

(9) Ho, P.; Coltrin, M. E.; Binkley, J. S.; Melius, C. F. J. Phys. Chem. 1985. 89, 4647. Revised values for heats of formation are given in ref I 1. (IO) Ho, P.; Coltrin, M. E.; Binkley, J. S.; Melius, C. F. J. Phys. Chem. 1986, 90. 3399. Revised values for heats of formation are given in ref 1 1. ( I I ) Ho, P.; Melius, C. F. J. Phys. Chem. 1990. 94, 5120.

(12) Mclius, C. F. Thermochemistry of Hydrocarbon Intermediates in Combustion: Application of the BAC-MP4 Method. I n Springer- Verlag D F V L R Lecture Nofes; Springer-Verlag: Berlin, 1991.

TABLE I: Parameters for BACs for BAC-MP4 (SDTQ) Level of Theory bond

A'

Si-H Si-F Si-N N-H N-F

92.79 260.65 847.99 70.08 170.05

ref species

SiH,

SiF, d NH, e

ref ab

species

2.0 2.0 2.5 2.0 2.0

C

c

d C C

'In kcal mol-'. .k'.'Based on related classes of compounds. "Same as Si-C, reference compound methylsilane (see text). Based on overall fit to several compounds containing N-F bonds. See section 1II.C.

the methods here. Electronic structure calculations were done using GAUSSIAN86.13 Equilibrium geometries and harmonic vibrational frequencies were obtained at the HF/6-31G* level of theory (restricted Hartree-Fock theory,I4 RHF, for the closed-shell molecules, and unrestricted Hartree-Fock theory,I5 UHF, for the open-shell molecules, using the 6-31G* basis setI6si7). This level of theory provides sufficiently accurate equilibrium geometries but does not provide total energies suitable for determining reaction energies involving the breaking of covalent bonds. Vibrational frequencies calculated at this level of theory are known to be systematically larger than experimental valuesI8so each calculated frequency has been scaled by dividing it by 1.12. To determine atomization enthalpies, the effects of electron correlation were included in the calculation by performing MP4(SDTQ)/ 6-3 1G**//H F/6-3 1G* (Mdler-Plesset perturbation theoryie2I with single, double, triple, and quadruple substitutions using the 6-3 IG** basis seti6.l7)single-point calculations at the HF/6-31G* geometries. Empirical BACs are used to account for systematic errors in the ab initio calculations that result primarily from basis-set truncation. The BACs are described in detail elsewhere,'i~i2 but we present the empirically derived functional form of the BAC here also. The BACs depend mainly on the bond type and bond (13) Frisch, M. J.; Binkley, J. S.; Schlegel, H. B.; Raghavachari, K.; Melius, C. F.; Martin, R. L.; Stewart, J. J. P.; Bobrowicz, F. W.; Rohlfing, C. M.; Kahn, L. R.; DeFrees, D. J.; Seeger, R.; Whiteside, R. A.; Fox, D. J.; Nuder, E. M.; Topiol, S.; Pople, J. A. GAUSSIAN 86; Carnegie-Mellon Quantum Chemistry Publishing Unit: Carnegie-Mellon University, Pittsburgh, PA, 1986. (14) Roothan, C. C. J. Reu. Mod. Phys. 1951, 23,69. ( 1 5 ) Pople, J. A,: Nesbet, R. K. J. Chem. Phys. 1954, 22, 571. (16) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (17) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654. (18) Pople, J. A.; Schlegel, H. B.; K,ishnan, R.; DeFrees, D. J.; Binkley, J . S.: Frisch, M. J.; Whiteside, R. A. In!. J. Quonf. Chem. 1981, SIS, 269. (19) Pople, J. A.; Binkley, J. S.; Seeger, R. I n f . J. Quanfum Chem. 1976, SIO. I .

(20) Krishnan, R.; Pople, J. A. Inf. J. Quanfum Chem. 1978, 14, 91. (21) Krishnan, R.; Frisch, M. J.; Pople, J. A. J. Chem. Phys. 1980, 72, 4244.

0022-3654/91/2095-1410%02.50/0 0 1991 American Chemical Society

Thermochemistry of the Si-N-H-F

System

The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 1411

length, but there are additional corrections for neighboring atoms. For the example of the chemical bond between atoms Xj and Xj in the molecule Xk-Xj-Xj, the BAC has the form BAC(Xi-Xj) =A,gkj., whereAj = Ai, eXp(-a,,Rj ), gkjj = (I-hjkhi.), and hjk = and aijare empiricaf parameters Bk exp(-aik(Rik - I .4 that depend on the bond type Xi-Xj. The values given in Table I are obtained either from reference compounds that have wellestablished experimental heats of formation or from analogous correction parameters observed in other classes of compounds where many reference species were available. In the case of compounds containing Si-N bonds, however, no good reference species was available, so we have used values for Si-C bonds based on methylsilane. R , is the bond distance (in angstroms) determined from the H F geometry optimization. The parameter B k , used in the neighboring-atom correction factor gkij,depends on the type of atom k . Values of 0.0,0.33, and 0.20 for BH,E,, and EN,respectively, were obtained from previous work on molecules composed of first-row elements." Bsi was taken to be 0.2, based on overall trends observed in other atoms, but the results reported in this paper are not sensitive to the value of this parameter. In the case of a chemical bond between atoms Xi and Xj in the molecule X,-Xi-X-Xi, the added heavy atom leads to an additional factor and the BAC has the form BAC(Xi-Xj) =hjgk,jg,. There is an additional BAC term for spin. For open-shell molecules, the unrestricted Hartree-Fock (UHF) method used leads to spin contamination errors which are removed by using the Maller-Plesset projection approximations of SchlegeLZ2 The BAC (spins2) is defined to be the difference between the calculated energy for the MP3 level of theory using the UHF wave function (E(UMP3)) and the projected UMP3 energy (E(PUMP3)).22 For closed-shell molecules that are UHF unstable (e.g., SiH2), we use a correction BAC (SpinuHF-1) defined as KUHF-lS(S I ) , where S is the spin obtained from the UHF/6-31G** calculation and KUHF-1 has a value of 10.0 kcal mol-', based on the heat of formation of Oj. This value of KUHF-I also gives reasonable heats of formation for singlet methylene and silylene." The sum of these bond additivity correction (BAC) terms are combined with the MP4 (SDTQ) electronic energy and the unscaled zero-point energy to obtain the heat of formation at 0 K. From the heats of formation at 0 K, we use statistical-mechanics equations involving the calculated geometries and scaled frequencies to determine entropies, heat capacities, enthalpies, and free energies at other temperatures.12 We have applied our calculational procedure to a series of molecules in the Si-N-H-F system, including radicals. Although some information on N-H-F compounds is given in ref 12, we include data on these species in this paper also. Table I1 lists the resulting heats of formation at 0 K for various levels of theory. The computed electronic energies as well as Z matrices, moments of inertia, and scaled vibration frequencies are available as supplementary material (see the paragraph at the end of the paper). Table 111 lists the BACs at the MP4 (SDTQ) level of theory for each of thc species. Table IV gives our calculated heats of formation at 298 K, along with uncertainty estimates. These uncertainty estimates indicate the extent to which our calculational methods provide consistency with other (lower) levels of theory. The uncertainties in Table IV result from a systematic, although ad hoc, method"*I2 that involves applying the BAC procedure to calculations at the MP3 and MP4 (SDQ) levels of theory. The errors in the BAC-MP4 heats of formation are estimated from the differences between the BAC-MP4, BAC-MP3, and BAC-MP4 (SDQ) values, plus a term for BAC (spin) and an inherent uncertainty term of 1.O kcal mol-'. (Note that we use the notation BAC-MP4 to refer specifically to the BAC-MP4 (SDTQ) results.) In contrast, our initial papers on silicon compound^^.'^ gave uncertainties for the calculated heats of formation that were based on our overall experience*I2 in comparing our results with experimental values, which indicates that the calculated values are generally within 2-3 kcal mol-' of the literature values. In the


A)].

+

(22) Schlegel, H. B. J. Chem. Phys. 1986,84,4530.

TABLE 11: Calculated AHOXO) for Si-N-H-F Compounds at Various Levels of Theory (kcal mol-') species H,SiNH2 H3SiNH H2SiNH2 H,SiN H2SiNH HSiNH2 H2SiN(2A') H2SiN(*A ,) HSiNH SiNH, HSiN SIN H SiN('n) SiN (%+) H,SiNHSiH, H3SiNSiH3 H2Si(NH2)2 HSi(N H2)3 HSi(NHJ2 H2SiNH3 H3SiNH2 H2SiNH H2" H3SiNH2 SiH, + NH," H3SiNH2 H2' HSiNH, NH3 N H2 NH F,SiNH, F,SiNH F2SiNH2 F3SiN F,SiNH FSiNH2 F2SiN FSiNH FSiN F,SiNHSiH, F,SiNSiH, FPi(NHh FSi(NH2), FSi(NH2), WNHh Si(NH2)3 N F3 N F2 NF NHlF NHF2 NHF HF

-+ -+

BAC-

BAC-

MP4 37.2 89.0 70.8 123.5 79.5 64.0 182.7 176.8 121.9 81.4 125.4 67.3 151.8 143.9 52.0 105.6 38.4 37.2 75.3 71.9 124.2

MP2 -8.1 55.4 29.8 100.5 42.9 30.2 153.4 145.3 87.1 50.3 94.6 40.4 135.5 119.3 -9.1 55.3 -33.0 -59.6 8.2 31.9 74.5

MP3

SDQ

SDTO

-8. I 54.0 30.6 98.8 46.2 29.8 152.0 144.1 89.0 51.1 115.0 44.0 133.6 121.2 -9.0 52.9 -33.4 -60.4 8.9 31.3 79.0

-8.7 53.3 30.0 98.0 43.8 28.4 149.5 141.6 85.7 49.8 102.6 40.6 130.3 116.6 -10.4 51.4 -34.7 -62.1 7.6 30.6 77.8

-7.7 54.2 30.7 98.7 43.2 28.5 150.4 142.8 86.0 50.0 93. I 38.9 130.4 11 5.2 -8.7 53.6 -32.4 -58.8 9.5 30.1 77.7

110.1

66.8

69.3

68.2

66.5

101.7

56.2

56.0

55.5

55.5

19.1 65.9 96.8 -254.7 -197.5 -1 14.2 -158.9 -98.1 -37.1 -25.8 21.0 88.1 -240.0 -182.0 -161.6 -63.5 -20.6 35.3 77.0 -0.4 30.1 67.6 24.7 17.0 54.3 -51.7

-9.2 47.9 88.2 -3 16.0 -247.2 -168.1 -197.7 -146.8 -77.9 -62.1 -10.8 62.7 -31 6.8 -248.2 -244.5 -166.6 -94.4 -86.5 -15.5 -18.9 13.5 54.9 -4.7 -8.8 34.0 -65.3

-9.2 46.6 87.0 -317.1 -249.0 -165.6 -199.3 -145.1 -76.5 -61.5 -10.7 60.4 -3 17.4 -251.3 -245.9 -168.0 -92.8 -87.9 -14.8 -25.7 10.4 55.5 -4.7 -12.0 33.8 -65.3

-9.2 46.6 86.9 -3 17.3 -249.4 -166.4 -200.1 -146.4 -78.1 -63. I -13.0 55.7 -318.4 -252.5 -246.7 -169.4 -94.1 -90. I -16.6 -26.2 8.9 54.1 -4.7 -12.2 32.9 -65.3

-9.2 46.8 87.0 -315.3 -247.9 -165.3 -198.9 -145.5 -78.1 -62.5 -12.9 54.4 -315.9 -249.4 -243.2 -165.2 -91.9 -85.6 -13.8 -26.5 8.5 53.9 -4.7 -12.2 32.7 -65.3

BAC-MP4

'Transition state for' the indicated reaction.

case of molecules containing the Si-N bond treated in this paper, however, there are no experimental data available for comparison and no good reference species was available for the Si-N bond. Thus, molecules containing this bond could have additional systematic errors due to the Si-N BACs. An examination of the Si-N BACs gives some insight into these possible errors. The BACs for the Si-N bond were assumed to be the same as those for Si-C bonds, obtained by using methylsilane as a reference species with R = I .8880 A. An exponent of 2.5 was used-smaller than the exponents 2.7 for Si-Si bonds or 2.8 for C-N bonds but larger than the 2.0 used for Si-F or Si-H bonds. Using exponents of 2.0 or 2.8 (retaining methylsilane as the reference species) would alter the BACs for Si-N bonds by 1-2 kcal mol-', with relatively larger changes for shorter bonds. Thus, the lack of a good reference species could contribute a systematic error t o the heats of formation of -2 kcal mol-' per Si-N bond. The tables include two entries for H2SiN, a 2A' and a ZAl state. I n this case, the nonplanar 2A' geometry was lower in total energy

Melius and Ho

1412 The Journal of Physical Chemistry, Vol. 95. No. 3, 1991 0

(I

h

71-

0

E

-

-100

0

-200

-E

-100

5

-200

0 0

a,..

'

0

Y

SIFnNH,

1-

' -

0

r

-300 -400

-300

0

'

-400 0

1

2

3

4

n Figure 3. Calculated heats of formation of the SiF,NH2 species as a function of n. 60 (L c. 40 7-

-E


Q 0 Y

$-

.

SiH,NH,

-

',*_ _....._ . 20

-

0 -

-20

SiCI,H4, systems?," Figures 2 and 3 show the heats of formation of the SiH,NH2 and SiF,NH2 species as a function of n. These heats of formation do not vary linearly with n, as was observed for SiH,, SiCI,, and SiF, species. This nonlinearity can be explained by the change at SiXY from s2p2hybridization of the silicon to sp3 hybridization. The trends in the heats of formation discussed above are reflected in the bond dissociation enthalpies (BDEs). Table VI lists calculated BDEs for Si-H, Si-F, N-H, and Si-N bonds. For successive dissociation of Si-H, Si-F, and N-H bonds, these systems exhibit a fairly regular pattern of alternating high and low BDEs. The low BDEs correlate with the formation of silylenes or species with shorter Si-N bonds. Analogous variations in successive BDEs are observed for the SiH,, SiF,, and SiCI, species.9-' I For Si-H bonds, the BDEs for H-SiH,, H-SiH2NH2, H-SiH(NH,),, and H-Si(NH2)3 are 91 .3,91.5,95.0, and 98.2 kcal mol-', respectively, indicating that NH2-for-H substitution on the Si has a minor effect on the Si-H BDE. For Si-F bonds, the BDEs for F-SiF,, F-SiF2NH2, F-SiF(NH2)2, and F-Si(NH2), are 167.5, 169.5, 170.7, and 170.8 kcal mol-', respectively, indicating that Si-F BDEs are insensitive to NH2-for-F substitution. N-H bonds are somewhat more sensitive to substituents. The BDEs for H-NH2, H-NHSiH3, and H-N(SiHJ2 are 109.1, 114.8, and 115.3 kcal mol-', respectively. The first SiH,-for-H substitution on the N is accompanied by a 5 kcal mol-' change in BDE, but the second substitution has little effect. The BDE for H-NHSiF, is 120.3 kcal mol-', showing that, compared to NH,, SiF,-for-H substitution on the N increases the BDE by more than IO kcal mol-'. A subsequent SiH,-for-H substitution on the N (forming F3SiNHSiH3)leads to almost no change in the N-H BDE (120.3 vs 119.4 kcal mol-'). We have not calculated the BDE for I-1-N(SiF3)2 because of computational limitations. However, from the observation that the second SiH,-for-H substitution on the N had little effect on the N-H BDE, we estimate a BDE for H-N(SiF3)2 of 120 kcal mol-', close to the BDE for H-NHSiF,. The Si-N BDEs are quite sensitive to silyl substitutions. The BDEs for H3Si-NH2 and H3Si-NHSiH, are 104.9 and 113.1 kcal mol-', while the BDEs for F3Si-NH2 and F,Si-NHSiH, are 126.5 and 134.1 kcal mol-', respectively. SiH3-for-H substitution on the N thus increases the Si-N BDE by -8 kcal mol-' in both cases. SiF,-for-SiH, substitution increases the Si-N BDE by -21 kcal mol-' for both the silylamine and the disilylamine. The BDE for H3Si-NHSiF3 is 118.0 kcal mol-', which indicates that SiF3-for-H substitution on the N increases the Si-N BDE by I3 kcal mol-', - 5 kcal mol-' more than SiH,-for-H substitution. Combining the BDE for H3Si-NHSiF3 with the observation that the BDE for F,Si-NHX is 21 kcal mol-' higher than the H,SiNHX BDE (X = H, SiH,) leads to an estimated BDE for F3Si-NHSiF, of 139 kcal mol-'. Combining this estimate with the calculated heats of formation for SiF, and F,SiNH gives an estimated heat of formation of -626 kcal mol-' for F,SiNHSiF,. This result, in turn, can be used with the H-N(SiF,), BDE estimated above to obtain a heat of formation for F,SiNSiF, of -558 kcal mol-', which can then be used t o obtain a BDE for F,SiNSiF, of 121 kcal mol-'. Note that the 18 kcal mol-' difference

-

-

(23) Gordon. S.;McBride, B. P. NASA Report NASA-SP-273, 1971. (24) Kee. R. J.; Miller, J. A.; Jefferson, T. H. CHEMKIN A GenerolPurpose, Problem-Independent, Transportable, Fortran Chemical Kinetics Code Package; Sandia National Laboratories Report SAND80-8003, 1980. (25) Kee, R. J.; Rupley, F. M.;Miller, J. A. CHEMKIN-11: A Fortran Chemical Kinetics Pockoge for the Analysis of Gas-Phase Chemical Kinetics; Sandia National Laboratories Report SAND89-8009, 1989. (26) Kee. R. J.; Rupley, F. M.; Miller, J. A. The CHEMKIN Thermochemical Data Base; Sandia National Laboratories Report SAND87-8215, 1987.


I

I

d SlHz + NHJ

The Journal of Physical Chemistry, Vol. 95, No. 3, 1991

System

I

I

4

ID-

I

ID-

f

1 am-

0-

Figure 4. Calculated relative energies for silylamine and disilane de-


composition pathways at 298 K.

between the F3Si-NHSiF3 and F3Si-NSiF3 BDEs is consistent with the 18, 18, and 22 kcal mol-' differences between the H3Si-NHSiH3 and H3Si-NSiH3 BDEs, the H3Si-NHSiF3 and H3Si-NSiF3 BDEs, and the F3Si-NHSiH, and F3Si-NSiH3 BDEs, respectively. BDEs and heats of formation that have been estimated from these trends are summarized in Table VII. In contrast, N H 2 substitutions on the Si have significantly smaller effects on the Si-N BDEs. The BDEs for H2N-SiH3, H2N-SiH,NH2,H2N-SiH( NH,),, and H2N-Si( NH2)3 are 104.9, 1 1 1.4, 1 16.5, and 120.2 kcal mol-', respectively. Each NH,-for-H substitution on the Si increases the Si-N BDE by -5 kcal mol-'. However, the BDEs for H2N-SiF3, H2N-SiF2NH2, H2N-SiF(NH,),, and H2N-Si(NH2)3 are 126.5, 126.1, 121.3, and 120.2 kcal mol-', respectively. NH,-for-F substitution thus has an opposite and much smaller effect on the BDE. The small effect of NH,-for-F substitution is consistent with the fact that N H 2 and F are both electronegative groups. Note that NH2-for-H substitution and NH2-for-Fsubstitution on the Si also had relatively small effects on the Si-H and Si-F BDEs. Table VI11 gives calculated dissociation enthalpies for possible decomposition reactions for most of the Si-N-H and Si-N-H-F species considered in this paper. A few trends are noteworthy: ( I ) Decomposition reactions for the fluorides are generally more endothermic than for the analogous reactions in the hydrides. (2) In cases where such reactions are possible, 1,l-elimination of H2 from the silicon is the least endothermic channel for the hydrides whereas the analogous reaction for the fluorides, 1 ,I-elimination of F2, is the most endothermic. This reflects the fact that the Si-F bond is considerably stronger than the Si-H bond, while the F-F bond is considerably weaker than the H-H bond. (3) For the fluorides, the least endothermic reaction often involves breaking the Si-N bond; this is never the case for the hydrides. (4) As discussed by Luke et al.27328carbon and silicon behave quite differently in these systems. HCN is lower in energy than HNC, while HSiN is higher than HNSi. Similarly, H2CNH is lower in energy than HCNH2, while H2SiNH is higher than HSiNH,. B. Silylamine Decomposition. In this section, we discuss the energetics of various pathways for silylamine decomposition. Figure 4 summarizes our calculated results for decomposition channels of silylamine and, for comparison, disilane.l0," The silylamine results are similar to those obtained by Raghavachari et al.29and Truong and Gordon,3o although there are some differences in the relative energies that are discussed in more detail in section III.C. The least endothermic channels for silylamine or disilane decomposition are l ,l-H2 elimination, l ,2-H2 elimination, and silylene elimination. For silylamine decomposition, the 1 ,l-HZ elimination is significantly less endothermic than silylene elimi(27) Luke, B. T.; Pople, J . A.; Krogh-Jespersen, M.-B.; Apeloig, Y.; Chandrasekhar, J.; Schleyer, P. v. R . J . Am. Chem. SOC.1986, 108, 260. (28) Luke, B. T.; Pople, J. A.; Krogh-Jespersen, M.-B.; Apeloig, Y.; Karni, M.; Chandrasekhar, J.; Schleyer, P. v. R. J . Am. Chem. Soc. 1986, 108, 270. (29) Raghavachari, K.; Chandrasekhar, J.; Gordon, M. S.; Dykema, K. J. J . Am. Chem. SOC.1984, 106. 5853. (30) Truong, T. N.;Gordon, M. S. J . Am. Chem. SOC.1986, 108, 1775.

3413

nation (37.8 and 65.3 kcal mol-', respectively). In contrast, I,l-H2 elimination from disilane is 1.9 kcal mol-' more endothermic than silylene elimination. The calculated activation energies for 1 ,I-H2 elimination and silylene elimination from silylamine are 63.2 and 74.1 kcal mol-', respectively. These values are significantly larger than the endothermicities, but the less-endothermic reaction still has the smaller activation energy. These results indicate that the reverse silylene-insertion reactions have significant barriers. The insertion of silylene into ammonia has a calculated barrier of 8.9 kcal mol-', while the insertion of aminosilylene into H2 has a calculated barrier of 25.4 kcal mol-'. In contrast, the transition states for 1, I-H2 elimination and silylene elimination from disilane were found to be slightly below the products in energy, indicating that silylene insertion into silane and silylsilylene insertion into H2 have negligible barriers. For both silylamine and disilane decomposition, the 1,2-H2 elimination channel has such a large barrier that it is kinetically unimportant even though it has a low endothermicity. The SiH2 + NH3 reaction can also involve a species with a dative bond between the SiHz and the NH3. Our calculations place this H2SiNH3species 27.4 kcal mol-' below SiH2 NH,. We cannot tell how important the dative bond minimum is, although the Si-N distance is longer in the dative bond than in the transition state, so we have drawn two pathways in Figure 4. We believe that an analogous structure is unlikely to be important in the SiH2 + SiH4 reaction. We expect this reaction to be similar to SiH2+ H2, where high-level calculations3' found only a shallow minimum in the reaction pathway (3 kcal mol-' below SiH, H2) and a transition state 1.7 kcal mol-' above SiH2 + H2. The most likely channel for H3SiNH2decomposition is formation of HSiNH2 + H2. HSiNH2 in turn has a low endothermicity (1 2 kcal mol-') decomposition reaction leading to SiNH + H,. Although this reaction is a 1,2-H2elimination and thus probably has a significant barrier, the endothermicity is sufficiently low that, under conditions where the H3SiNH2can decompose, the HSiNH, is also likely to decompose. For fluorinated silylamine, we have not calculated transitionstate energies, so we can draw few mechanistic conclusions. Fluorinating the silicon in silylamine alters the energetics so that the most likely decomposition channels for hydrogenated silylamine are among the most endothermic reactions for the fluorinated compound. Thus, the decomposition channels that are important in the fluorides are unlikely to be the same as those in the hydrides and little information can be transferred between the two systems. For F3SiNH2,H F elimination is the least-endothermic reaction. This reaction is analogous to the 1,2-H2elimination in H3SiNH2, so it could have a significant barrier in addition to the endothermicity. This would raise this reaction into the same energy range as the endothermicities of the next three reactions, N H elimination, H2 elimination, and N-H bond scission. Thus, transition-state energies would be needed to determine the relative importance of the various F3SiNH2 decomposition reactions. The SiH, NH3 reaction resembles the SiF2 SiF4 insertion reaction more than the SiH2 SiH, reaction. SiF2 insertion into SiF4 has a barrier of 12 kcal mol-' (based on our calculated endothermicity" for Si2F6 SiF2 + SiF, of 33.7 kcal mol-' and the activation energy of 46.25 f 0.69 kcal mol-' obtained by Walsh and c o - w o r k e r ~ ~whereas ~), SiH2 insertion into SiH, has none. The activation energy for Si2F6 SiF2 + SiF, is similar to that for Si2H6 SiH2 + SiH4, which indicates that fluorine substitution primarily stabilizes the silylene plus silane (relative to the disilane) rather than destabilizing the transition state. C. Comparisons with Literature. Although ~ilylamines'~ and fluorine-substituted s i l y l a m i n e ~have ~ ~ been observed in mass spectrometric studies, species in the Si-N-H and Si-N-H-F systems are, in general, poorly known. Except for the smallest

+

+

+

-

-

+

+

-

-

(31) Gordon, M. S.; Gano, D. R.; Binkley, J. S.; Frisch, M. J . J . Am. Chem. Soc. 1986, 108, 2191. (32) Bains, S. K.;Noble, P. N.; Walsh, R. J . Chem. Soc., Faraday Tram. 2 1986.82. 831. (33) Wu, C.-H. J . Phys. Chem. 1987, 91, 5054. (34) Lin, S.-S. J . Electrochem. Soc. 1977, 124, 1945.

Melius and Ho

1414 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 TABLE 111: Bond Additivity Corrections for the MP4 (SDTQ) Level of Theory (kcal mol-') Si-H bond Si-N bond species length" (no.)' BAC length" (no.)' BAC H3SiNH2 1.4824 4.79 1.7243 11.38 4.85 1.4753 (2) 1.7656 10.27 4.91 I .4695 HISiNH 4.83 1.4776 (2) 4.8 1 1.7289 11.25 1.4802 (2) H2SiNH2 9.30 1.8050 4.88 1.4729 (3) HISIN 16.63 1.5727 4.85 1.4753 H2SiNH 4.98 1.4627 11.92 4.49 1.7058 1.5140 HSiNHz


H2SiN(2A') H2SiN(2A,) HSiNH SiNHz HSiN SIN H SiN(2n) SIN ( 2 S * ) H,SiNHSiH3

HSi(NH2),

1.4741 (2) 1.4645 (2) I .4888

4.87 4.96 4.72

1.4598

5.01

1.4785 I .4774 1.4730 1.4768 1.4789 1.473 1 1.4735 (4) 1.4785 (2) 1.4807 (2)

4.82 4.83 4.88 4.84 4.82 4.88 4.87 4.82 4.80

1.4782

4.83

1.6674 1.5809 1.6368 1.7084 1.5262 1S275 1.6933 1S893 1.7325 1.7330

13.12 16.29 14.17 1 1.84 18.68 18.62 12.30 15.95 1 1.07 1 1.05

1.7307 (2)

11.12

1.7162 1.7161

1 I .52 11.52

1.7217 1.7218 1.7144 1.7204 1.7233

1 1.27

11.27 11.48 1 1.40 11.32

H Si (N H 2 )

1.4885

4.73

H2SiNH1 (dative bond)

1.5139 (2)

4.49

2.0889

4.58

1.7228 1.7215 I .4760 I .4776 I .6129 1.4816 I .4849 I .6682 1.5212 I .4689

2.96 1.6134 2.92 4.85 4.83 3.59 1.9013 4.79 4.76 1.7131 3.30 4.43 4.92 Si-N bond length" (no.)/ 1.6749

15.02

HlSiNH2

-

H2SiNH + H2(

HISINHI

-

SiHz + NH]'

HlSiNH2

-

HSiNH2 + H2(

species F,SiN H2 F,SiN H F2SiNH2 FISiN F2SiNH FSiNH2 FzSiN FSiNH FSiN F,SiNHSiH, F3SiNSiHl

SiFz(N H2)*

Si-F bond length" (no.)' 1.5734 I S696 (2) I S62l 1.5679 (2) 1.5883 1.5906 1.5625 (3) 1.5605 I .5665 1.6106

9.77 9.84 9.99 9.88 10.10 IO.05 10.00 10.48 10.36 10.19

1.5790 (2) 1.6007 1s 9 7 3 1.5711 (3) 1.4732' (2) I .4705' 1.5664 I S663 1.5702 1.4718' I .4692' I .4725' 1.5863 (2)

10.30 10.39 10.46 9.82 4.87 4.90 9.89 9.90 9.83 4.89 4.9 I 4.88 9.90

BAC

N-H bond length" (no.)' 0.9980 (2)

BAC

spin correction*

9.52

1.0094

9.31

0.62 s

0.9983 (2)

9.52

0.9975

9.53

0.21 s 0.79 s 0.40 u

0.9983 0.998 1

9.52 9.52

1.0047 0.9998 (2)

9.40 9.49

0.9879

9.72

9.40 s 7.79 s 7.60 s 0.65 s 8.66 u 9.08 s 12.65 s

1.0027

9.43

0.68 s 0.9977 0.9970 (2) 0.9976 0.9976 (2) 0.9983 (2) 0.9972 (2) 0.9991 0.9964 0.9978 0.9981 1.0040 1.0045 (2) 1.5231 0.9942 0.908Id

7.31

1.3773 1.0021 1.0044 11.71 1 .0902d 0.9956 0.9950 N-H bond BAC length" (no.)' 1 1.99 0.9965 (2)

9.53 9.54 9.53 9.53 9.52 9.54 9.50 9.55 9.53 9.52 9.41 9.40 3.28 9.60 3.09 4.34 9.44 9.40 2.14 9.57 9.58 BAC

0.19 s

0.03 u 0.61 u

spin correctionb

9.55

I .7236

10.70

1.0073

9.35

0.52 s

1.6936

11.76

0.9979 0.9959

9.53 9.56

0.16 s

1.7752 1 S387

9.47 16.91

0.9920

9.64

1.6997

11.86

1.0003 0.9983

9.48 9.52

1.7624 1.6963 1.7120 1.747 1 1.6825

9.97 11.95 I1.50 10.66 1 1.68

1 .oooo

9.49

1.0024

9.44

1.6837 1.7385

1 1.64 10.89

I .6873 I .6874

11.82 11.82

0.62 s

6.21 s 2.11 s 11.77 u

0.60 s

0.9965 (2) 0.9969 (2)

9.55 9.54



System

TABLE 111 (Continued) species SiF(NH2),

Si-F bond lengthu (no.)' I S982

FSi(NH2)2

I .6046


Si(NHd4 Si(NH2h

species N H3 NH2 NH N F, N F2 NF NH2F NHF2 NHF

'In angstroms.

b ~ UHF-I .

BAC 10.01 10.1 I

Si-N bond length' (no.)' I .7094 (2) 1.6973

11.71

1.7069 1.7118

11.55 11.41

1.7217 (4) 1.7280 (2) 1.7216

11.18 11.10

N-H bond length" (no.)'

BAC

1.0025 (3) 1.0125 (2) I .0238

9.44 9.25 9.04

1.0033 (2) 1.0037 1.0133

11.28

N-F bond length' (no.)'

N-H bond length' (no.)'

BAC

0.9972 (2) 0.9968 (2) 0.9969 (2) 0.9997 0.9968 0.9977 0.9975 0.9978 (8) 1.0002 (2) 0.9971 (2) 0.9977 (2)

9.54 9.54 9.54 9.49 9.54 9.53 9.53 9.53 9.48 9.54 9.53

BAC

spin correctionb

0.19 s

0.15 s

spin correctionb 0.60 s 0.75 s

1.3279 (3) 1.3171 (2) 1.3022 1.3862 1.3535 (2) 1.3449

9.42 9.41 9.24

8.73 10.35 12.58 10.63 9.86 11.54

0.91 s 1.07 s 0.83 s

correction; s, Sz correction. CTransitionstate for the indicated reaction. dH-H bond. 'Si-H bond. fNumber of bonds.

TABLE I V Calculated AffOr(298) for Si-N-H-F Compounds at the BAC-MP4 (SDTQ) Level of Theory with Error Estimates' (kcal mol-') HsSiNH2 - I 1.5 f 1.5 F3SiNH2 -317.9 f 2.9 H,SiNH 51.3 f 1.4 F,SiNH -249.7 f 2.2 H2SiNH2 28.0 f 1.3 FzSiNH2 -167.2 f 1.5 H,SiN 96.6 f 1.3 F,SiN -200.0 f 1.6 H2SiN H 41.0 f 3.3 F2SiNH -146.9 f 1.4 HSiNH, 26.3 f 1.6 FSiNH2 -80.0 f 1.9 HzSiN(2A') 149.2 f 5.1 H2SiN(2A,) 141.4 f 4.4 F2SiN -63.1 f 3.5 HSiNH 84.8 f 5.0 FSiNH -13.6 f 2.6 SiNH2 48.7 f 1.5 93.0 f 24.3 FSiN 54.4 f 8.6 HSiN SiNH 38.4 f 5.4 SiN(211) 130.7 f 5.7 SiN(22+) 115.5 f 8.9 H,SiNHSiH3 -14.3 f 2.0 F,SiNHSiH, -320.2 f 3.2 HJSiNSiHl 48.9 f 2.5 F,SiNSiH, -252.8 f 3.8 H2Si(NHd2 -37.3 f 2.7 F2Si(NH2)2 -247.3 f 4.5 HSi(NH2), -64.9 f 3.8 FSi(NH2), -170.7 f 5.2 HSi(N H2)2 5.6 f 2.2 FSi(NH2)2 -95.5 2.5 H2SiNH,(dative bond) 26.5 f 1.7 Si(NH2)4 -92.9 f 5.1 H,SiNH2 74.0 f 1.7 Si(NH2), -18.8 f 3.2 H2SiNH + H,b H,SiNH2 62.7 f 3.5 SiH2 + NH,* H3SiNH2 51.7 f 1.2 HSiNH2 H,b NH, -11.0 f 1.0 NF, -28.0 f 1.4 NH2 46.1 f 1.1 NF2 7.9 2.2 NH 87.0 1.1 NF 53.9 f 1.9 N 113.0 NH2F -6.5 i 1.0 Si 107.4 NHF2 -13.8 f 1.0 H 52. I NHF 32.0 f 1.5 F 18.9 HF -65.3

-

BAC I I .37

*

-+

Error estimates indicate only relative applicability of the calculational methods. See text for discussion. bTransition state for the indicated rcaction.

molecules, there are no experimental data in the literature for comparison with our calculated heats of formation, although there are some experimental data o n molecular structures. The status of the Si-N-H species has recently been reviewed in ref 35 and we refer thc reader to that discussion. (35) Gmelin Handbook of Inorganic Chemistry, Silicon, Supplement Volume B4: Springer-Verlag: Berlin, 1989.

For the diatomic molecule SiN, the JANAF tables36adopted a AHOr(298) of 89 f 15 kcal mol-I, which is significantly lower than our value of I 15.5 f 8.9 kcal mol-'. The heat of formation given in the CATCH tables,37 90.89 f 10.04 kcal mol-', was derived from the JANAF value. However, we feel that the literature values for the heat of formation for SIN are suffici'ently uncertain that the discrepancy with our value is not very surprising. The JANAF value is based on estimates of Doo from spectroscopic data for the X22+ and B2Z+ states, but a good value for Doo has not been established. Although Herzberg3* included a value for Doo, the more recent compilation by Huber and H e r ~ b e r gno~ ~ longer lists a Doo value for SIN. The value of 4.5 eV (103.8 kcal mol-') originally adopted by Herzberg corresponds to a AHo,-(0) of 115.4 kcal mol-' for SiN. We note that Peterson and Woodsa recently obtained a value of 102 kcal mol-' for the SiN dissociation energy by using the CP radical as a reference molecule to correct a value from a CASSCF (complete active space self-consistent field) calculation. The experimental bond lengths39for the X2Z+ and A211states of SiN, 1 S719 and 1.6357 A, respectively, compare favorably with the 1.589 and 1.693 A from our calculations. Our calculated splitting between the X2Z+ and the A211 state, 15.2 kcal mol-', is somewhat smaller than the estimated 8000-cm-' (23 kcal mol-') value from spectroscopic data.39 The literature also contains heats of formation for the N-H-F species. Our calculated AHof for NH2 and NH, 46.1 f 1 . 1 and 87.0 f 1 .I kcal mol-', agree well with the JANAF values36of 45.5 f 1.5 and 90.0 f 4.0 kcal mol-' (NH3 was used as a reference compound and agrees by definition). For NF3, NF2, and NF, the JANAF tables give AHof of -3 1.57 f 0.27, 10.1 1 f 1.91, and 59.50 f 7.89 kcal mol-', respectively. Benson4' gives values of -3 I .4 f 1, 8.5 f 2 and 60 f 8 kcal mol-' for these species, plus a value for N H z F of -5 f 2. Our calculated values for these species are -28.0, 7.9, 53.9, and -6.5 kcal mol-'. Some of our values are higher than those in the literature, and some are lower. The lack of exact agreement for any one species is due to the fact (36) JANAF Thermochemical Tables. J . Phys. Chem. Re/. Data 1985,

14, Supplement I .

(37) Pcdley, J. B.; Iseard, B. S. CATCH Tables; University of Sussex, 1972, available from N T S N ~ ~ bAD-773468. er (38) Herzberg G.Molecular Spectra and Molecular Structure, I . Spectra New york,1950. of Diatomic Molecules; Van Nostrand (39) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure, IV. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (40) Peterson, K . A,,; Woods, R . C. J . Chem. Phys. 1989, 90, 7239. (41) Benson, S.W. Thermochemical Kinetics; Wiley: New York, 1976.


1416 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991

Melius and Ho

TABLE V: BAC-MP4 Thermochemical Parameters for Si-N-H-F Compounds at Various Temperatures (kelvin) AHo? S"b AGOP species 0 298 298 300 600 1000 1500 HSNH, -7.7 -I 1.5 65.7 0.4 13.2 31.3 54.0 H;SiNH54.2 51.3 65.5 58.5 66.5 77.6 91.1 H2SiNH2 30.7 65.8 54.4 28.0 35.1 43.0 68.7 98.7 96.6 59.5 100.9 105.8 112.9 121.5 HISiN H2SiNH 41.0 59.9 49.9 45.2 56.8 65.3 43.2 26.3 59.8 30.6 35.3 42.5 51.5 HSiNH2 28.5 149.2 59.5 148.9 148.9 149.4 HzSiN(2A') 150.4 150.1 141.4 57.2 141.8 142.7 144.9 148.0 H2SiN(2Al) 142.8 84.8 60.6 84.1 83.7 83.6 HSiN H 83.6 86.0 48.7 58.6 48.6 49.0 50.4 52.4 SiNH2 50.0 93.0 54.8 89.4 86.0 81.6 76.1 HSiN 93.1 51.7 38.4 35.8 33.3 30.6 27.3 38.9 SIN H 52.3 123.2 130.7 106.1 94.3 130.4 SiN(211) I 1 5.8 115.5 52.0 108.2 91.4 79.8 115.2 SIN (2E+) 100.9 83.5 HISIN HSiH, -8.7 - 14.3 2.8 20.9 46.0 76.5 83.7 61.3 92.2 114.1 48.9 H3SiNSiH3 53.6 74.2 74.4 34.6 -37.3 -32.4 -16.6 71.3 5.0 Wi(NH2)2 H Si(N H 2)1 -64.9 -4.6 -58.8 -35.2 82.8 36.7 87.8 75.3 5.6 37.8 9.5 21.4 H Si (N H 2 ) 60.4 88.5 H2SiNH3 (dative bond) 50.4 37.9 66.8 68.4 26.5 30.1 91.0 77.7 63.5 118.7 74.0 99.8 86.4 H3SiNH2 H2SiNH + H2C 142.3 66.5 62.7 H3SiNH2 SiH, + NH{ 62.8 108.3 89.0 75.3 132.5 64.3 55.5 51.7 77.0 63.9 H3SiNH2 HSiNHz + HZC 95.5 118.6 45.9 14.8 -9.2 -3.9 -11.0 3.7 28.9 NH3 46.4 54.6 46.8 48.3 46.1 59.5 50.8 NHl NH 43.2 82.2 87.0 85.6 87.0 84.1 79.8 80.0 FlSiNH2 -315.3 -3 17.9 -287.0 -266.4 -302.6 -241.1 F3SiN H -247.9 -249.7 82.2 -239.6 -229.9 -217.0 -201.5 77.2 -165.3 -167.2 -149.1 F2SiNH2 -1 58.3 -1 36.3 -1 20.6 73.9 F3SiN -198.9 -200.0 -I 92.2 -1 84.4 -1 74.1 -161.7 70.9 -145.5 -146.9 FzSiNH -1 34.6 -140.8 -1 26.2 -1 15.8 FSiNH, 65.8 -75.0 -78.1 -80.0 -69.6 -61.9 -52.2 70.5 F2SiN -62.5 -63.1 -59.9 -61.5 -57.8 -55.3 FSiNH 68.9 -14.1 -12.9 -13.6 -14.4 -14.5 -14.5 FSiN 54.4 54.4 50.7 63.7 47.2 42.6 37.1 FlSiNHSiH3 96.0 -3 15.9 -320.2 -299.0 -247.4 -277.0 -21 1.3 -249.4 FISiNSi HI 97.6 -252.8 -236.8 -198.5 -220.5 -171.7 82.8 -243.2 -247.3 -223.9 -168.1 -200.1 -128.6 F2Si(NH2)2 FSi( N H 2) -165.2 -170.7 88.0 -108.5 -1 40.0 -66.1 -1 3.8 79.7 FSi(N H2)2 -91.9 -95.5 -78.4 -60.9 -37.1 -7.6 -85.6 -92.9 90.0 -53.9 -14.0 39.5 105.4 Si(N H2)4 84.3 -18.8 5.8 31.0 -1 3.8 65.2 107.5 WNHA -28.0 -7.6 -26.5 61.8 6.0 22.7 -1 7.9 N Fl 59.4 15.1 8.5 7.9 11.4 20.0 26.0 N F2 NF 53.9 53.9 52.8 50.8 51.7 50.1 48.0 NH2F -4.7 -6.5 54.7 8.1 18.9 0.5 32.6 NHF2 60.1 2.7 -12.2 -13.8 -5.8 14.3 28.8 NHF 32.7 55.0 36.8 40.5 45. I 34.3 32.0 HF 41.4 -66.3 -65.3 -65.3 -66.8 -65.8 -67.2

--

In kcal mol-'.

2000

2500

78.5 106.4 85.1 132.2 75.9 62.8 153.0 153.7 85.8 56.8 73.0 26.4 85.0 70.8 110.5 139.5 109.5 140.2 118.5 1 1 5.6 167.7 158.7 143.7 43.0 64.5 77.4 -214.1 -184.2 -103.2 -147.3 -103.3 -40.4 -50.5 -12.1 34. I -171.5 -141.7 -87.5 39.9 23.8 172.5 151.5 39.1 31.9 45.9 46.2 43.1 49.7 -67.5

104.0 122.6 102.5 144.0 87.7 75.2 157.1 161.0 89.4 62.7 71.2 27.0 77.2 63.4 146.3 166.9 148.6 193.2 149.4 141.2 194.1 186.0 169.9 57.1 69.4 75.0 -1 86.2 -165.8 -84.7 -131.8 -89.5 -27.4 -44.5 -8.6 32.5 -1 29.7 -109.7 -45.5 94.3 56.1 239.8 196.2 55.3 37.7 43.7 59.6 57.2 54.4 -67.8

In cal mol-' K-I. CTransitionstate for the indicated reaction

that the BAC parameters for N-F bonds were chosen to give an acccptablc ovcrall fit to a number of the above species (and FNNF) rathcr than using a single reference compound. These N-H-F species warrant more detailed study, but they are somewhat pcriphcral to this paper and affect only a few dissociation cnthalpics in Table VIII. Our calculated heat of formation for H3SiNH2of -1 1.5 kcal mol-' agrees well with the -1 2 kcal mol-' estimated by Jolly and Bakkc4* W a l ~ lists h ~ ~derived and estimated BDEs for mcthyl-substitutcd silylamines in the 93-103 kcal mol-' range, which are comparable to our calculated value of 104.9 kcal mol-' for H3Si-NH2. Although these comparisons are exceedingly limited, they do provide a modicum of support for our results. There arc cxperimental data on molecular structures in the literature for a few other species that can be compared with our calculated gcomctrics. For SiNH in an Ar matrix, analysis of the infrarcd spcctrum by Ogilvie and CradockM yielded a Si-N (42) Jolly, W. L.; Bakke, A. A. J . Am. Chem. Soc. 1976, 98,6500. (43) Walsh, R. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z. Eds.; Wiley: 1989; p 371. (44) Ogilvie, J. F.; Cradock, S.J . Chem. Soc., Chem. Commun. 1966, 364.

bond length of -1.54 A and a N-H bond length of 1.005 A. These compare well with our calculated values of 1.527 and 0.988 A, respectively. For H3SiNHSiH3,there are structural data from electron diffraction experiments by Rankin et al.45 They obtained Si-N bond lengths of 1.725 f 0.003 A, Si-H bond lengths of 1.484 f 0.006 A, an Si-N-Si angle of 127.7 f 0.1O, and H-Si-H angles of 108.0 f I .Oo. Our calculated geometry is in good agreement with these measurements. Our Si-N bond lengths are 1.732 and 1.733 A, our Si-H bond lengths range from 1.473 to 1.479 A with an average of 1.476 A, our Si-N-Si angle is 13O.S9O, and our H-Si-H angles range from 107.0' to 1 10.0' with an average of 108.4'.

Although experimental data on the Si-N-H and Si-N-H-F species are limited, various molecules in the Si-N-H system have been the subject of other theoretical investigations. We have selected a few papers for comparison with our results; although these papers do not give heats of formation, there are generally relative energies that can be compared with our results. Reference 35 contains a more complete listing of such theoretical studies. (45) Rankin, D. W. H.; Robiette, A. G.; Shcldrick, G. M.; Sheldrick, W. 8. J.; Ellis, I . A.; Monaghan, J. J. J . Chem. SOC.A 1969, 1224.

S.; Aylett,



System

TABLE VI: Calculated Bond Dissociation Enthalpies at 298 K (kcal mol-')

bond

BDE

H-SiH2NH2 H-SiH2NH H-SiH2N H-Si( N H2),

91.5 41.7 96.9 98.2

bond (a) Si-H Bonds H-SiHNH2 H-SiHNH H-SiHN H-SiH(NH2)2

BDE

bond

BDE

50.4 95.9 3.7

H-SiNH2 H-SiNH H-SiN

74.4 5.7 74.6

F-SiNH2 F-SiNH F-SiN

147.6 70.9 80. I

95.0

F-Si F2N H F-S i F, N H F-SiFiN

169.5 121.6 155.8

F-Si( N H 2)3

170.8

(b) Si-F Bonds F-Si FNH2 F-SiFNH F-SiFN F-Si F(NH2),

H-N HSi H, H-NHSiH, H-NHSiH H-NHSi

114.8 65.0 110.5 41.8

H-NSiH, H-NSiH2 H-NSiH H-NSi

97.3 152.5 60.3 129.2

H-N HSiF, H-NHSiF, H-NHSiF

120.3 72.4 118.5

H-NSiF, H-NSiF2 H-NSiF

101.7 135.9 120.0

H-N(SiH,)2

1 1 5.3

H-N(SiH,)(SiF,)

119.4

H3Si-NH2 H2Si-NH, HSi-NH2 Si-NH,

104.9 82.8 110.7 104.7

H,Si-NH. H2Si-NH HSi-NH Si-NH

83.1 110.8 93.2 156.0

H3Si-N H2Si-N HSi-N Si-N

63.8 36.4 111.0 104.8

F,Si-NH2 F2Si-N H2 FSi-NH2

126.5 63.4 113.6

F,Si-NH F2Si-NH FSi-NH

99.3 84. I 88.2

F,Si-N F2Si-N FSi-N

75.6 26.2 46.2

H3Si-NHSiH3 H,Si-NHSiF, F,Si-NHSiH,

113.1 118.0 134.1

H,Si-NSiH3 H,Si-NSiF, F,Si-NSiH,

95.1 100.3 112.0

H2N-SiH2NH2 H2N-SiF2NH2

1 11.4 126.1

H2N-SiH(NH2), H2N-SiF(NH2)2

116.5 121.3

106.1 152.3 136.4 170.7


(c) N-H Bonds

(d) Si-N Bonds

TABLE VII: Estimated Thermochemical Values at 298 K (kcal mol-')

species

AHOr

species

BDE

F,SiN HSiF, F,SiNSiF,

-626 -558

H-N(SiF,), F,Si-N HSi F, F3Si-NSi F,

120 139 121

A 1986 paper by Gordon46reported a theoretical investigation of silylamine that primarily addressed the question of pyramidal vs planar geometry. The theoretical methods used were similar to ours and gave similar geometries and energies. Gordon's calculations showed that H3SiNH2 should be nonplanar, in agreement with our results. Our calculations indicate that the N-H bonds in H3SiNH2have a dihedral angle (the angle between the N-H bonds when projected along the Si-N bond) of 146.5O around the Si-N bond, considerably less than the 180° expected for a planar nitrogen. For H3SiNHSiH3,the dihedral angle is 175.89O, while for NH3 it is 120O. Thus, increasing silyl substitution leads to increasing planarity at the N, in agreement with experiment. Electron diffraction studies have shown that H3SiNHSiH34Sand N(SiH3)347are very close to planar, while NH3 is well-known to be pyramidal. In 1984, Raghavachari et al.29 examined silylene-insertion reactions, including the SiH2 + NH3 system. These calculations used theoretical methods very similar to ours at a slightly lower level of theory and did not use corrections comparable to our BACs. Four species are common to both papers: H3SiNH2,SiH2 + NH3, the transition state for the reaction between the two, and the H2SiNH3dative bond species. The HF/6-31G** energies are identical, and the bond lengths are identical for H3SiNH2, H2SiNH3,and the transition state. For energies relative to SiH2 NH3, they obtained -60.0 kcal mol-' for H,SiNH2, + I 3.2 kcal

+

(46) Gordon, M.S. Chem. Phys. Left. 1986, 126, 451. (47) Beaglcy, B.; Conrad, A. R. Trans. Faraday SOC.1970, 66, 2740.

H2N-Si(NH2),

120.2

mol-' for the transition state, and -25.1 kcal mol-' for the H2SiNH3dative bond species. Our corresponding values (at 0 K) are -63.8, +10.5, and -25.9 kcal mol-'. Most of the differences can be attributed to the BACs. For example, of the 2.7 kcal mol-' difference in barrier heights for SiH, insertion into NH3, 2.6 kcal mol-' is due to the fact that the BAC for the transition state is larger than the BAC for SiH2 NH3 (the BAC is subtracted from the MP4 energy). At 298 K, our calculated barrier height reduces to 8.9 kcal mol-'. In 1986, Truong and Gordon3o published a paper on the H2SiNH and HSiNH2species and their formation reactions from H3SiNH2using a level of theory comparable to ours, but a larger basis set (MP4(SDQ)/MC-31 IG**//HF/6-3IG**). As expected from differences in calculational methods, their total energies are somewhat lower than ours. For energies relative to H3SiNH2, they obtained +38.7 kcal mol-' for HSiNH2 + H2, +56.6 kcal mol-' for H,SiNH HI, +67.2 kcal mol-' for the I,I-H2elimination transition state, and +89.7 kcal mol-' for the 1,2-H2 elimination transition state. Our corresponding BAC-MP4 values at 0 K for these species are +36.3, +50.9, 63.2, and 85.4 kcal mol-', which are in fairly good agreement with the results of Truong and Gordon. At the MP4 level, our calculations yield 26.8, 42.4, 64.5, and 87.0 kcal mol-', which are in poorer agreement. Thus, the BACs appear to be appropriately correcting for the effects of basis set truncation in our calculations. In 1986, Luke et al.27.28 published an extensive set of calculations on silicon-containing compounds and their carbon analogues at the MP4(SDTQ)/6-3 IG*//HF/3-2IG1 level of theory, including several Si-N-H species. From their calculated,endothermicity for the H3SiNH2+CH3 SiH3+H3CNH2reaction and a literature value for the H3C-NH2 BDE, they obtained a BDE for the H,Si-NH2 bond of 98.7 kcal mol-'. This is somewhat smaller than our value (at 0 K ) of 103.3 kcal mol-'. The difference can be explained largely by the k c t that the BACs for the Si-N ( 1 1.4 kcal mol-') and C-N bonds (7.9 kcal mol-') do not cancel, and

+

+

-


1418 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991 TABLE VIII: Calculated Dissociation EnthalDies . (kcal . mol-') reaction AH0rxn(298) H,SiNH, H,SiNH + H 114.8 - H$iN + H2 108.0 106.7 SiH4 + N H 104.9 SiH, + NH, 91.5 H2SiNH2+ H 65.3 SiH, + NH, 52.4 H2SiNH + H2 37.8 HSiNH, + H2 97.3 H,SiNH H,SiN + H 90.1 H2SiN + H2 83.1 SiH, + N H 69.8 SiH4 + N 59.5 SiH, + NH, 41.7 H2SiNH + H 33.5 HSiNH + H2 113.4 H2SiNH2 H2SiN + H2 106.4 SiH, + N H 82.8 SiH2 + NH, 65.0 H2SiNH + H 56.8 HSiNH + H2 52.0 7SiH + NH, 50.4 HSiNH, + H 20.6 SiNH, + H2 152.5 H,SiNH H,SiN + H 119.4 SiH, + N SiH2 + N H I 10.8 SiH + NH, 96.1 95.9 HSiNH + H 52.0 HSiN + H2 SiNH + H, -2.6 125.5 HSiNH, SiH2 + N H 110.7 SiH + NH, HSiNH + H 110.5 SiNH, + H 74.4 70.1 Si + NH, HSiN + H, 66.7 12.1 SiNH + H2 H2SiN SiH + N H 36.6 SiH, + N 36.4 HSiN H 3.7 SIN + H2 -25.9 HSiNH SiH + N H 93.2 93.0 SiH, + N Si + NH, 68.6 HSiN + H 60.3 30.7 SIN + H2 SiNH + H 5.7 SiNH, SiH + N H 129.4 -Si + NH, 104.7 -SiN + H2 66.9 SiNH + H 41.8 HSiN SiH + N 1 1 1.0 101.4 -Si+NH SIN + H 74.6 SiNH SiH + N 165.6 -Si+NH 156.0 SIN + H 129.2

---------------------------

Melius and Ho

-

F,SiNH

F,SiNH2

F,SiNH

FSiNH,

F,SiN

+

-

the 3.5 kcal mol-' difference affects the calculated endothermicity for the above reaction. For SIN species with 3 H atoms, they found HSiNH2('A') to be the lowest energy form, with the H2SiNH('A') and H,SiN(,A,) forms 20.6 and 56.4 kcal mol-' higher than HSiNH2. We found the same order for these species, but our values for the relative energies (at 0 K) are 14.6 and 70.2 kcdl mol-'. For H&N, over IO kcal mol-' of the 13.8 kcal mol-' difrerencc bctween their relative energy and ours can be attributed to the BACs. For H2SiNH, however, the BACs differ by less than I kcdl mol-', so the difference must be attributed to other causes. These workers also found HSiN to be 55 kcal mol-' higher in energy than HNSi, in good agreement with our B A C - M P 4 value of 54.1 kcal mol-I at 0 K. Wlodek et ai." estimated a heat of formation for SiNH of 50.2 kcal mol-' from their calculated enthalpy of reaction for

-------------------------

reaction

FSiNH,

FSiNH

-FSiN

--

-

FSiNH, + F, F,SiNH, + F SiF, NH2F SiF, + NH, F,SiNH H F,SiN + H2 SiHF, N H F,SiNH + H F FSiNH + F2 SiF, N H F F,SiNH + F F2SiN H F F,SiN + H SiF, + N H SiHF, N SiNH, + F2 SiF NH,F SiHF, N H FSiNH, + F F2SiN H, FSiNH + H F F,SiNH + H SiF, N H 2 SiNH + F2 SiF N H F FSiNH + F FSiN + H F F2SiN H SiHF, + N SiF, N H Si + N H 2 F SiNH, + F FSiN H2 SiHF + N H FSiNH + H SiF NH, SiNH H F SIN + F, FSiN F SiF + N F SiF, N Si + N H F FSiN H SiHF + N SiF N H SiNH + F SIN + H F

+

+

+

+

+ +

+

+ +

+ +

+

+

+

+

+ + + + +

Si + N F SIN + F SiF + N

AHorxn( 298) 231.9 169.5 161.5 126.5 120.3 117.9 116.3 105.6 236.1 131.8 121.6 121.2 101.7 99.3 74.0 21 5.9 148.3 114.7 106.1 104.1 88.3 72.4 63.4 185.3 166.5 152.3 136.0 135.9 120.3 84.1 180.9 147.6 134.4 131.3 118.5 1 1 3.6 53.1 178.7 136.4 104.6 26.2 152.9 120.0 90.8 88.2 70.9 63.8

106.9 80.1 46.2

-

SiNH('Z+) Si('P) + NH(32-), which differs significantly from our value of 38.4 kcal mol-'. Our geometry and SCF and MP4(SDTQ) total energies are identical with theirs; the difference in the two values is primarily due to our use of BACs (worth 18.6 kcal mol-' for the Si-N bond) and differences in the NH heat of formation (87 vs 79 kcal mol-'). In their calculations, Peterson and Woodsm obtained a value for the SIN-H BDE of 133 kcal mol-', which compares favorably with our value of 129.2 kcal mol-'. Recently, Gordon49 has used MCSCF calculations with the 6-3 lG* basis set to study H3SiN and H2SiNH. These calculations place H3SiN(3A2) only 8.2 kcal mol-' above H2SiNH('A'), in sharp contrast with the present splitting of 55.6 kcal mol-', the 35.8 kcal mol-' of Pople and co-workers,28and the 41.1 kcal mol-' from the results of Nguyen et aLso The theoretical methods used

(48) Wlodck, S.; Rodriquez, C. F.; Lien, M . H.; Hopkinson. A. C.; Bohme.

D. K. Chem. Phys. Left. 1988, 143. 385.

(49) Gordon, M. S . Chem. Phys. Lelt. 1988, /46, 148.



System

by Gordon are sufficiently different from the other studies that the source of the disagreement is not clear. The MCSCF study also did not include HSiNH2, which is the lowest energy isomer, or H3SiNH2. Thus, it is difficult to place these results within the overall context of Si-N-H energetics. In a study of bonding in silicon nitrides, Julian and GibbsS1did H F calculations on various Si-N-H species, primarily using the STO-3G basis set, and give geometries that can be compared with our results. For H3SiNH2,they obtained bond lengths for Si-N, N-H, and Si-H of 1.738, 1.003, and 1.424 A and a Si-N-H bond angle of 122.4'. Our calculated Si-N and N-H bond lengths are 1.724 and 0.998 A, with a Si-N-H bond angle of 120.6', which agree well. Our Si-H bond lengths of 1.482 and 1.475 A, however, are somewhat longer than theirs. We can also compare geometric results for the Si(NHJ4 and H3SiNHSiH3species with those of Julian and G i b b ~ . ~For ' Si(NH2)4,they obtained a Si-N bond length of 1.722 A using the 6-31G* basis set, which is the same as our result. For H3SiNHSiH3, they report a lanar configuration at the N atom, a Si-N bond length of 1.723 and a Si-N-Si bond angle of 128.3O. These results agree fairly well with our Si-N bond lengths of 1.732 and 1.733 A, and a Si-N-Si bond angle of 130.6'.

silylamine decomposition are 1,l-H2elimination forming HSiNH2, 1,2-H2 elimination forming HzSiNH, and silylene elimination, although consideration of activation barriers reverses the order of the last two channels. These observations are similar to the results of theoretical studies by other^.^^^^^ The silylamine decomposition reactions (and the reverse silylene insertion reactions) differ from the analogous disilane reactions in that the insertion of silylene into ammonia has a calculated barrier of 8.9 kcal mol-' whereas silylene insertion into silane has a negligible barrier. For F3SiNH2,the energetic order of the reactions is not analogous to the H3SiNH2system. I,I-F2 elimination is the most endothermic decomposition reaction and H F elimination is the least endothermic reaction. The H F elimination is analogous to the 1,2-H2 elimination in H3SiNHz and could have a significant barrier in addition to the endothermicity.

IV. Summary We have obtained a self-consistent set of heats of formation for over 30 molecules in the Si-N-H-F system from a combination of a b initio electronic structure calculations and empirical corrections. The calculations provide thermochemical parameters for many species that are not otherwise available; the molecules studied include the H,SiNH, and F,SiNH, species, and some of the H,,Si(NH2),, F,,Si(NH2),, and H,N(SiH3),(SiF3), species. Although few data are available in the literature for comparison with our heats of formation, our calculations generally agree well with experimental data on molecular structures and with the results of other calculations on molecules in the Si-N-H system. The heats of formation of the Si(NHJe,H, and Si(NH2)4-nFn species vary linearly as a function of n, indicating that the successive replacement of NH2 groups with H or F atoms is accompanied by a monotonic change in stabilization. In contrast, the heats of formation of the SiH,NH2 and SiF,NH2 species do not vary linearly with n; this is reflected in the varying values for successive BDEs for Si-H, Si-F, and N-H bonds. Trends in BDEs for saturated compounds showed varying effects of substitutions. For example, the BDEs for Si-H and Si-F bonds in the H,Si(NH2)4-, and F,,Si(NH2)4, species are insensitive ton, while the BDE for the F3Si-NHSiH3 bond is 20% larger than that for the H,Si-NHSiH, bond. We have also examined the energetics of various pathways for silylamine decomposition. The least endothermic channels for

Appendix The supplementary material data are as follows: Table IX presents the Z matricesSZfor each of the molecular species obtained from the HF/6-31G* geometry optimization calculations. Molecular geometries can be obtained from these Z matrices.52 Table X gives the moments of inertia in atomic units (amu bohr2), while Table XI lists the scaled vibrational frequencies obtained at the same level of theory. Table XI1 presents the electronic energies resulting from the various perturbation-theory calculations using the 6-31G** basis set. The projected UHF (PUHF) and projected UMP2 (PUMP2) energiesz2are given for reference, although they are not used in the derivation of the BACs. Table XI11 gives polynomial coefficients for C,, H,and S as a function of temperature for the species considered in this paper. These fits are used with the CHEMKIN package of software2kz6 and are defined by C p / R = a , + a2T a 3 p a4T3 u57"


1,

(50) Nguyen, M. T.; Fad, M.; Fitzpatrick, N. J. J . Chem. Soc., Perkin Trans. 2 1987, 1289. (51) Julian, M. M.; Gibbs, G. V. J . Phys. Chem. 1985.89, 5476.

Acknowledgment. We acknowledge the contributions of J. S. Binkley to the development of the calculational methods. We also thank Fran M. Rupley for the polynomial fits to the thermochemical data. This work was supported by the U S . Department of Energy under Contract DE-AC04-76DP00789 and by DARPA through WRDC/MLBC Contract F33615-89-C-5628,

+

H a3 _ + -a22T + --T2 RT 3

S R

a3 2

+

+ -4T 3 +

- = a , In T + a2T + - p

a4

+

a6 -P +5 T

a5 + a45 T 3 + -74 + a, 4

Supplementary Material Available: Tables as described in the Appendix (23 pages). Ordering information is given on any current masthead page. (52) For an explanation of Z matrices. see, for example: Clark, T. A Handbook of Computaiional Chemistry: A Practical Guide io Chemical Siructure and Energy Calculations; Wiley: New York, 1985.

Theoretical study of the thermochemistry of molecules in the silicon

Recommend Documents