J. Phys. Chem. 1988, 92, 7053-7057
7053
ARTICLES Potential Energy Surfaces for the Reaction Si
+ H20
Shogo Sakai,t Mark S. Gordon,* Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105
and Kenneth D. Jordan Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: December 3, 1987; In Final Form: June 13, 1988) The potential energy surfaces for the reactions of silicon atom (IDand 3P)with the water molecule were calculated by ab initio self-consistentfield (SCF) methods. The transition states for the insertion of silicon atom into the 0-H bond of water were also calculated by the multiconfigurational SCF method. It is found that the crossing point of the singlet and triplet surfaces along the insertion reaction paths is near the transition state for the triplet rearrangement from the S O H 2complex to HSiOH. The potential energy surfaces for the 1,2-hydrogen migration HSiOH H2Si0 in the singlet and triplet states and the hydrogen (H,) elimination from HSiOH and HzSiO were also investigated.
-
Introduction The chemical reaction mechanisms for reactions of atomic species (M) with molecules in the gas phase have recently received considerable experimentalld and attention. In experimental studies, the molecular complexes between metal atoms ( M = B, Al, Si, Sc, Ti, and V) and R H type molecules including CH4, H 2 0 , HF, NH,, and H2S have been reported. Most of these complexes rearrange to form the insertion products R-M-H, with or without appreciable energy barriers. The interactions of silicon atoms and water molecules in a solid argon matrix (15 K) have been studied by Ismail and co-~orkers.~ They reported that the silicon-water adduct (Si:OH2) was formed initially. Subsequently, the insertion product HSiOH was produced spontaneously, whereas the deuterium-substituted products were obtained only upon photolysis (A >400 nm). Ismail et al. proposed the following reaction mechanism for the deuteriumsubstituted species: Si(3P) H 2 0 Si:OH2(3A2) HSiOH(3A") HSiOH('A') SiO('Z+) + H2('Zg+)
+
-
-- -
This suggests that the singlet-triplet spin-orbit coupling-induced intersystem crossing occurs at the HSiOH compound and that tunneling may play an important role in the overall mechanism. A theoretical study of the reaction of silicon atom and water was performed by Tachibana and co-workers." They investigated the %OH2 complex, HSiOH, and the insertion reaction transition states for the singlet and triplet at the MP4/6-31G(d,p)// MP2/6-3 1G(d) computational level. The tunneling reaction path between %OH2 and HSiOH in the triplet state was analyzed with the aid of the intrinsic reaction coordinate (IRC) path at the UHF/6-3 1G(d) computational level; however, there has been no theoretical search for the singlet-triplet intersystem crossing point. In the present work, we present the potential energy surfaces for the reactions of Si('D, and 3P) with H20. Of particular interest is the location of the crossing point of the singlet and triplet surfaces for the insertion reaction paths of atomic silicon into the H-O bond of water to obtain HSiOH. Reaction mechanisms from HSiOH to other possible products are also considered. Theoretical Approach The basis sets used here were the split-valence 3-21G set1*and the split-valence plus polarization 6-31G(d) and 6-3 lG(d,p) sets.lg All equilibrium and transition-state geometries were determined with analytical energy gradientsm at the Hartree-Fock (HF) level with the 6-31G(d) basis set. Restricted (RHF) and unrestricted 'Present address: Osaka University of Art, Department of Planning Art, Kanan, Osaka 585, Japan.
0022-3654/88/2092-7053$01.50/0
(UHF) wave functions were used for closed shells and triplet states, respectively. Complex Hartree-Fock (CHF) wave functions were chosen as the simplest approach for the description of the lowest energy open-shell singlets. This is analagous to the use of U H F wave functions to describe low-lying triplets. The force constant matrix and thereby the vibrational frequencies were obtained with analytically calculated energy second derivatives.2' To obtain improved energy comparisons, we performed additional calculations at the HF-optimized structures, with electron correlation (excluding inner shells) incorporated through second-, third-, and fourth-order Mraller-Plesset perturbation theory (MP2, MP3, and MP4 (SDQ and SDTQ)).Z2 Results thus obtained are (1) Meier, P. F.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. SOC.1978, 100, 2108. (2) Hauge, R. H.; Meier, P. F.; Margrave, J. L. Ber. Bumen-Ges. Phys. Chem. 1978,82, 102. (3) Hauge, R. H.; Kauffman, J. W.; Margrave, J. L. J. Am. Chem. SOC. 1980, 102, 6005. (4) Thiel, P. A,; Hoffman, F. J.; Weinberg, W. H. J. Chem. Phys. 1981, 75, 5556. (5) Ismail, Z. K.; Hauge, R. H.; Fredin, L.; Kauffman, J. W.; Margrave, J. L. J. Chem. Phys. 1982, 77, 1617. (6) Ismail, Z. K.; Fredin, L.; Hauge, R. H.; Margrave, J. L. J. Chem. Phys. 1982, 77, 1626. (7) Nicely, V. A.; Dye, J. L. J. Chem. Phys. 1970, 52, 4795. (8) Trenary, M.; Schaeffer 111, H. F.; Kollman, P. J. Am. Chem. SOC. 1977, 99, 3855; J . Chem. Phys. 1978,68, 4047. (9) Curtiss, L. A.; Frurip, D. J. Chem. Phys. Lerr. 1980, 75, 69. (10) Dill, J. D.; Schleyeri P. R.; Binkley, J. S.; Pople, J. A. J. Am. Chem. SOC.1977, 99, 6159. (11) Kurtz, H. A.; Jordan, K. D. J. Am. Chem. SOC.1980, 102, 1177. (12) Holloway, S.; Bennemann, K. H. Surf.Sei. 1980, 101, 327. (13) Bentley, J. J. Am. Chem. SOC.1982, 104, 2754. (14) Sakai, S.; Jordan, K. D. J. Phys. Chem. 1983,87, 2293. (15) Sakai, S.; Jordan, K. D. Chem. Phys. Lett. 1986, 130, 103. (16) Ahmed, S. N.; McKee, M. L.; Shevlin, P. B. J. Am. Chem. Soc. 1983, 105, 3942. (17) Tachibana, A.; Koizumi, M.; Teramae, H.; Yamabe., T. J. Am. Chem. SOC.1987, 109, 1383. (18) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J . Am. Chem. SOC.1980, 10.2, 1980. Gordon, M. S.; Binkley, J. S.; Pople, J. A.; Pietro, W. J.; Hehre, W. J. J. Am. Chem. SOC.1982, 104, 2997. (19) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209. Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654. (20) Komornicki, A.; Ishida, K., Morokuma, K.; Ditchfield, R.; Conrad, M. Chem. Phys. Lett. 1977, 45, 595. (21) Pople, J. A.; Binkley, J. S.; Seeger, R.In?. J. Quantum. Chem. 1975, 9, 229. Pople, J. A.; Krishnan, R.; Schegel, H. B.; Binkley, J. S. Ibid. 1979, 513, 225.
0 1988 American Chemical Society
Sakai et al.
7054 The Journal of Physical Chemistry, Vol. 92, No. 25, 1988
SINGLET STATE
50
TRIPLET STATE n 113.2
124.6
TS-S3
G
si
n
TS-51
Si
COMPLEX -S
> (3-10 -
a W
z w-20
\
\
\-I
I
122.4
2.287
COMPLEX-T
&HOH* Si)= 109.4 TS-T1
+(HOH*Sl;=107.3 TS-SI
'
//
H
COMPLEX-1
-
W
L
116.1
2-30 -I w
a
-
1.515 1.661
-40
-
-50
-
H
HS-ST
P
1.654 119.3 y 9 , ,
98.0
1.533
-60 HS-ST
0.947
Q[HOSiH)=90.4 HS-T
H
H
HS-SC
Figure 1. MP3/6-3lG(d)//HF/6-3lG(d) correlation diagram for the Si + H 2 0 reaction network. S and T refer to singlet and triplet, respectively. COMPLEX = Si:OH2; HS = HSiOH; SO = H,SiO; TS = transition state.
denoted MPn/6-31G(d)//HF/6-31G(d). To study the crossing region of the singlet and triplet energy surfaces for the insertion of atomic silicon into an H-0 bond of water, we calculated the transition states by the multiconfigurational (MC) self-consistent field (SCF) method with the 3-21G basis set. The reaction paths for the insertion reaction were calculated at this level by determining the intrinsic reaction coordinate (IRC).23 The IRC and localized orbital calculations were performed with the program GAMESZ4 All other calculations were carried out with GAUSSIANBZ."
Results and Discussion The correlation diagram for the reactions of atomic silicon with water is shown in Figure 1. The total energies in the figure were calculated at the MP3/6-31G(d)//HF/6-31G(d) level. The HF/6-31G(d) structures for all species are illustrated in Figure 2. Analytical force fields for all species in Figure 2 were calculated, and the transition-state species have only one imaginary frequency corresponding to motion along the reaction coordinate. The other species have all real frequencies and are therefore potential energy minima. The energy of Si('D) was obtained by using the complex R H F (CHF) method.26 Because the Morller-Plesset energy for a ID
_. 1.719
/
so-s
SO-T
H
1.510
CHF wave function c a n be obtained only at t h e MP2 level in t h e (22) Mdler, C.; Plesset, M. S. Phys. Reu. 1934, 46, 618. Binkley, J. S.; Pople, J. A. Int. J . Quantum Chem. 1975, 9, 229. Pople, J. A,; Binkley, J. S.; Seeger, R. Ibid. 1976, 510, 1. Krishnan, R.; Pople, J. A. Ibid. 1978, 14, 91. Krishnan, R.; Frisch, M. J.; Pople, J. A. J . Chem. Phys. 1980, 72, 4244. (23) Fukui, K. J . Phys. Chem. 1970,74,4161. Ishida, K.; Morokuma, K.; Komornicki, A. J . Chem. Phys. 1977, 66,2153. Schmidt, M. W.; Gordon, M. S.;Dupuis, M. J . Am. Chem. Soc. 1985, 2585. (24) (a) Dupuis, M.; Spangler, D.; Wendoloski, J. J. NRCC Software Catalog, 1980, 1 , Program QGO1. (b) Schmidt, M.W.; Boatz, J. A.; Baldridge, K. K.; Koseki, S.; Gordon, M. S.; Elbert, S. T.; Lam, B. T. QCPE 1987, 1, 115. (25) Binkley, J. S.; Frisch, M. J.; DeFrees, D. J.; Ragavachari, K.; 1987, Whiteside, R. A.; Schlegel, H. B.; Fluder, E. M.; Pople, J. A. GAUSSIANBZ; Carnegie-Mellon University, Pittsburgh, PA, 1983. (26) Seeger, R.; Pople, J. A. J . Chem. Phys. 1977, 66, 3045.
1.565
TS-S4 Figure 2. HF/6-31G(d) stationary point geometries for the Si + H 2 0 reaction network. Bond lengths are given in angstroms, angles in degrees. The angles @ are dihedral angles.
current version of GAUSSIANSZ,the total MP3 energy for 'Dsilicon was estimated from the equation EMPJ(lD)
= EhfP3(3P) + (EhfPZ('D) - EhiP2(3P))
(1)
Si
+ H 2 0 Potential Energy Surfaces
The Journal of Physical Chemistry, Vol. 92, No. 25, 1988 7055
TABLE I: Relative Energies' for the Insertion Reaction with HF/6-31G(d) Optimized Geometries
'4 HF/6-31G(d) MP3/6-31G(d) MP4(SDTQ)/6-31G(d,p)
8.7 14.4b
'ET 35.0 14.9b
1 l.gb
12.g6
'E,
'Ec
'ET
3Ep
EO
El
E2
E3
101.9 90.9 97.1
10.0 12.6
42.4
26.7
27.9 23.g6
20.6
12.8
31.6
57.3 45.0 42.2
24.0 37.5 39.5
33.5
23.9b 23.96
8.3 5.4
25.9b
'The units are kcal/mol. bThese values are from the MP2 calculation: see text. This assumes that the MP3 and MP2 singlet-triplet splittings are approximately the same. Si H20 SI':0H2Complex HSiOH. To check the accuracy of the HF calculation for the Si:OH2 complex, the hydrogen-transfer transition state, and product HSiOH on the singlet surface, we calculated the energies of these three species at the 6-31G(d) geometries by the C H F method. The use of complex wave functions reduces the total energies by 12.2, 7.3, 0.3, and 0.0 kcal/mol for Si atom, the Si:OH2 complex, the transition state, and trans-HSiOH, respectively. The R H F calculation (using real orbitals) for Si('D) is not meaningful, but the C H F approach treats this state correctly. In the Si:OH2 complex, the 7.3 kcal/mol error at the R H F level is about 60% of the error in Si('D), so the C H F wave function provides a significant, albeit approximate, improvement over the R H F treatment. The minimum energy distance of the S i 4 bond for singlet Si:OH2 (with the internal OH2structure fixed) lengthens by about 0.14 8, when the C H F method is used-a difference of 6%. At the transition state, the error in the R H F energy is only 2.4% of the Si('D) error. It is assumed that the difference in the geometry is negligible there. Therefore, in the following discussion, we use the R H F method for the singlet Si-H20 reaction systems except for the total energies of Si('D) and the Si:OH2('AI) addition complex. The singlet ('Al) and triplet (3A") addition complexes are denoted COMPLEX-S and COMPLEX-T, respectively in Figure 2. COMPLEX-S is a weakly bound Si:OH2 complex, with an Si0 bond length 0.5 A longer than that in SiHJOH at the same level of theory.27 This weak bond arises from a charge transfer from the lone pair (al) occupied orbital of water into the empty p (al) orbital of silicon, and the back charge transfer from the occupied b2 orbital of silicon into the b2 unoccupied orbital of water. In COMPLEX-T, the back charge transfer is weaker than that in COMPLEX-S, leading to a longer Si-0 bond distance in the triplet. COMPLEX-T has a pyramidal structure (C, symmetry), with a very small inversion barrier (1.5 kcal/mol at the MP3/6-3 lG(d)//HF/6-3 1G(d) computational level), while COMPLEX-S is planar with C2, symmetry. Closed-shell hydroxysilylene (HS) has two planar structures that are minima on the potential energy surface. These are denoted HS-ST (s-trans) and HS-SC (s-cis) in Figure 2. The nonplanar triplet is HS-T. The transition states for the rearrangement of COMPLEX-S and COMPLEX-T to the hydroxysilylenes are denoted TS-SI and TS-T1 for the singlet and triplet states, respectively. Both transition states possess only CI symmetry. TS-S1 has been tracked to HS-ST by following the IRC. The transition state TS-SI is seen to be earlier than that for the triplet state (TS-TI), on the basis of the oxygen to migrating H (H*) distance. The geometries of TS-SI and TS-TI are similar, except for the Si-0 and 0-H* bond lengths and the HOH* bond angle. For the singlet state, the energies of the cis and trans structures (HS-ST and HS-SC) are predicted to be very close, with the trans isomer slightly lower (0.6 kcal/mol) at the MP3/6-31G(d)// HF/6-3 1G(d) computational level. The rotational barrier for trans-to-cis isomerization is 11.5 kcal/mol at the same level. This barrier is less than half that predicted for HCOH based on a configuration interaction (CI) calculation2*(27.5 kcal/mol) with a double-{ basis set. In the triplet state, the HSiO plane is approximately perpendicular to the SiOH plane. From Figure 1, the singlet and triplet surfaces appear to cross just after the 1,2-hydrogen migration transition states leading to
+
-
-
(27) Kudo, T.; Nagase, S.J . Phys. Chem. 1984, 88, 2833. (28) Goddard, J. D.; Schaeffer 111, H. F. J . Chem. Phys. 1979, 70, 5 1 17.
Sl:OH,
\
b
O
H
)
'IHSIOHI
Figure 3. Schematic for the Si + H20 reaction network and definitions of energy differences in Table I.
HSiOH are passed. On the singlet surface, this hydrogen migration is strongly exothermic (76.0 and 84.3 kcal/mol with MP3/6-3 lG(d)//HF/6-3 1G(d) and MP4/6-3 1G(d,p)//HF/63 1G(d), respectively). The relative energies for the insertion of singlet and triplet silicon into an H - O bond of water are listed in Table I, with the energy differences defined in Figure 3. The latter are generally very sensitive to the inclusion of correlation for both singlet and triplet states. The predicted singlet-triplet splitting in Si atom (Eo= 23.9 kcal/mol) is larger than the experimental value of 18.0 kcal/m01~~ and is very similar to the previous MRD-CI value of 22.1 kcal/m01.~~ To study the energy profile for the insertion of silicon into an H-0 bond of water more carefully, we calculated the transition states of the insertion reaction with MC-SCF wave functions and the 3-21G basis set. The four orbitals included in the active space were the two highest occupied and two lowest unoccupied S C F orbitals. Within this space, all configurations arising from distribution of the four electrons among the four orbitals have been included. These four orbitals are the four valence orbitals in atomic silicon. This results in 20 and 15 configurations for the singlet and triplet, respectively. The singlet-triplet separation (E2 = 6.7 kcal/mol) at the transition states is in good agreement with those (E, = 8.3 and 5.4 kcal/mol) given by the MP3/6-31G(d) and MP4(SDTQ)/6-31G(d,p) methods (see Table I). The saddle-point geometries for the insertion reactions for the singlet and triplet states at various computational levels are shown in Figure 4. The longest &Si bond distances are those calculated with correlated wave functions (MC-SCF/3-21G and MP2/631G(d)). The singlet is predicted to be almost planar at the HF/3-2 1G level, whereas all other computational levels, including HF/6-3 1G(d) (Figure 2), predict a rather nonplanar structure. The singlet vs triplet structural diffkrences are similar at the MP2/3-21G, MP2/6-31G(d), and MC-SCF/3-21G levels. In the triplet state, there is less variation in the S i 4 bond distance, HOH bond angle, and dihedral angle as a function of the computational level. It is of interest to determine the location of the crossing of the singlet and triplet surfaces and therefore the relative geometries of the singlet and triplet states. The most important geometry difference between the singlet and triplet transition states is the 0-H* (active hydrogen) bond distance. The differences in other structural parameters are much smaller. (29) Moore, C. E. Circ. U.S. National Bur. Szand. 1949, No. 467, Vol: 1. (30) Lewerenz, M.; Bruna, P. J.; Peyerimhoff, S . D. Mol. Phys. 1983,49, 1.
7056
Sakai et al.
The Journal of Physical Chemistry, Vol. 92, No. 25, 1988
TRIPLET STATE
SINGLET STATE H F I 3-21G
Q)
e 2
H*
-r
/ H i 3 9 1
;.-?oy>
62.4 f
Si
1.87L
5
-365.14
-5
-365.16
c
W
Single1 S t a t e
t
H*
H*
-365.12
a
3.974
4 (HOH*Si)= 118.0 MP2/3-21G
-365.10
L L
120
110
130
140
150
6 (degrees) +(HOH*Si)=111.6 H*
2I :$
Si
7.6
1.937
--7H 0.973
9 ( HOH*Si)= 114.3 MP2/6-31G(d) H*
H*
Si
LL;l14,8 -H
2.014
0.982
+( HOH*Si)= 1 0 4 . 0
$’( HOH*Si)= 106.2
Figure 4. Saddle-point geometries for the singlet and triplet hydrogen
migration reactions at various computational methods. Bond lengths are in angstroms, angles in degrees. The angles @ are dihedral angles.
I
TS
Figure 6. Potential energy curves of the singlet and triplet states for the change of the angle HSiO.
2) shows that the HF/6-31G(d) Si-H, Si-0, and 0-H bond lengths and SiOH bond angle for triplet HSiOH (HS-T) are similar to those for the singlet state (HS-ST and HS-SC), whereas the HSiO bond angle and HSiOH dihedral angle are rather different for the two states. A singlet-triplet surface crossing could not be detected when the dihedral angle was used as the independent parameter (with the other parameters being held at the HS-T geometry). This suggests that the change in the dihedral angle is not important for the singlet-triplet surface crossing. The total MP4(SDQ) energies of the singlet and triplet states as a function of the change in the HSiO angle are shown in Figure 6. Again, the remaining parameters used are those of the triplet-state HSiOH (HS-T). From the figure, the two structures cross at an HSiO angle of about 140’. At this structure, the triplet energy is only 6 kcal/mol above that of its equilibrium geometry. Therefore we cannot discount the possibility that the singlet-triplet surface crossing occurs near the HSiOH compound. However, the surface crossing would occur at or near the transition states of the insertion reaction without requiring exam energy; therefore, it is more likely to occur there. Clearly, spin-orbit coupling matrix elements are needed to more quantitatively distinguish between these two possibilities. Next consider the following reaction paths from HSiOH in Figure 1, using MP3/6-3lG(d)//6-3lG(d) energies: H2SiO(’Al)
(2)
HSiOH(3A”)
H2Si0(3A’t)
(3)
HSiOH(’A’) I 1.0
,
, 1.1
1
,
1.2
1.3
.
1.4
0-H
,
,
I
,
,
1.5
1.6
1.7
1.8
1.9
,
dlstance t i )
Figure 5. Potential energy curves for the insertion reactions obtained from the respective intrinsic reaction coordinates, as a function of the OH* distance.
The reaction paths for the insertion reactions were calculated by the IRC method with MC-SCF/3-21G wave functions, and the resulting potential energy curves as a function of 0-H bond distance are shown in Figure 5 . It is found that the crossing point of the singlet and triplet surfaces is near the transition state of the triplet surface. The geometry parameters for the crossing point are 1.40 A for 0-H*, 1.90-1.91 A for Si-0, and 1.65-1.72 A for Si-H*. Other geometry parameters (0-H bond, H*OH bond angle, and dihedral angle) a t the crossing point are almost the same as those for the singlet and triplet states. The crossing point occurs before the triplet transition state and after the singlet transition state. The energy at the crossing point is about 1.9 kcal/mol below the triplet transition state. If a physical surface hopping to the singlet surface from the triplet surface occurs at this crossing point, the singlet HSiOH compound could be produced from the triplet reactants (Si(3P) + H20) without the triplet activation energy. If this is correct, this would be the lowest energy path for the reaction (Si(3P) + HzO HSiOH(’A’)). Other Alternatives. Now consider the possibility of a singlet-triplet surface crossing at the HSiOH compound. A comparison of the structures of singlet and triplet HSiOH (Figure
-
-- ++
HSiOH(IAI)
H2
SiO(’Z+)
(4)
H, SiO(lZ+) (5) Reactions 2 and 3 (HSiOH H2SiO). The transition states for the hydrogen 1,Zmigration from HSiOH (HS-ST or HS-SC and HS-T) to silanone, H2Si0 (SO4 and SO-T), are TS-S2 and TS-T2 for the singlet and triplet states, respectively. Both transition states have CI symmetry. A localized molecular orbital (LMO) a n a l y s i ~shows ~ ~ . ~that ~ in the singlet state the migrating hydrogen behaves like a hydride ion (H-) along the reaction path. That is, the migrating hydrogen interacts with the vacant p(a”) orbital of the silicon of HSiOH. The activation energy is 69.5 kcal/mol from cis-HSiOH, and the reaction is 4.7 kcal/mol endothermic. This high activation energy is similar to that obtained earlier33but very different from the results of the reaction HCOH HzCO (the barrierz8is calculated to be about 36 kcal/mol with a CI wave function). The heat of reaction for HCOH HzCO is about 53 kcal/mol exothermic. This large difference in the heat of reaction between silicon and carbon comes from the difference in double-bond stabilization energy of Si=O vs C=O and from the stability of silylenes relative to carbenes. In the triplet state, the LMO analysis shows that the migrating hydrogen behaves like a hydrogen radical along the reaction path and correlates with HZSiO(’Al)
-+
-
-
(31) Ha, T.; Nguyen, M.; Hendrickx, M.; Vanquickenborne, L.G. Chem. Phys. Lett. 1983,96, 267. (32) Sakai, S.; Morokuma, K. J . Phys. Chem. 1987, 91, 3661. (33) Tachibana, A.; Fueno, H.; Tamabe, T. J . Am. Chem. Soc. 1986,108, 4346.
J. Phys. Chem. 1988, 92, 7057-7059 the 3A” state of H2Si0. The 3A” state (SO-T) in H2Si0 is 8 kcal/mol lower than the 3A’ state. The activation energy is 49.1 kcal/mol, and the reaction is 19.3 kcal/mol endothermic. Reactions 4 and 5. Reactions 4 and 5 correspond to the elimination of H2from cis-HSiOH (HS-SC) and H2Si0 (SO-S), and the respective transition states are TS-S4 and TS-S3. The activation energy for reaction 5 is a very large 91.6 kcal/mol, because of orbital symmetry requirements. Similarly, the barrier height for the reaction HzCO H2 CO is about 94 kcal/mol.26 Reaction 5 is 3.7 kcal/mol exothermic. In reaction 4, the activation energy is 53.4 kcal/mol, and the heat of reaction is almost zero (0.4 kcal/mol exothermic). In the reaction HSiOH H2 SiO, the following two reaction channels are possible:
-
+
HSiOH
+
- - + - + Hz
H2Si0
cis-HSiOH
H2
-
Si0
Si0
(6) (7)
According to the results presented here, the direct reaction 7 through the transition state (TS-S4) is more favorable than reaction 6.
Conclusions Atomic silicon in both its ID and 3Pstates is found to react with water to form Si:OH2 without a barrier. Hydroxylsilylene is subsequently produced via an early (late) transition state for the singlet (triplet) reaction. The crossing point of the singlet and
7057
triplet surfaces is between the transition states of the singlet and triplet. Therefore, surface hopping to the singlet occurs near the transition states of the migration reaction, but we cannot discount the possibility of the occurrence of a surface hopping at hydroxylsilylene, 6 kcal/mol above the triplet energy at the equilibrium geometry. The 1,2-hydrogen migrations from hydroxylsilylene to silanone occur by an ”ionic hydrogen migration mechanism” for the singlet state and a “radical hydrogen migration mechanism” for the triplet state: on the basis of an LMO analysis, the migrating hydrogen behaves like an anion along the reaction path in the singlet state and like a hydrogen atom along the reaction path in the triplet state. The activation energy for the elimination of H2 from silanone is very high, so the reaction channel cis-HSiOH Hz + Si0 is more favorable than HSiOH HzSiO Hz SiO.
- - +-
Acknowledgment. This work was supported by grants from the donors of the Petroleum Research fund, administered by the American Chemical Society, the National Science Foundation (CHE86-40771 and CHE8411293), and the Air Force Office of Scientific Research (87-0049). The computer time was made available by the North Dakota State University Computer Center and the Computer Center of the Institute for Molecular Science (IMS), and they are gratefully acknowledged. Registry No. Si, 7440-21-3; H20, 7732-18-5.
Failed Attempt To Observe Phosphorescent Exclmers of I-Bromonaphthalene Suzanne Beckham, Tony M. Wright, and Merlyn D. Schuh*,’ Department of Chemistry, Davidson College, Davidson, North Carolina 28036 (Received: March 18, 1988: In Final Form: June 20, 1988)
Measurements of phosphorescence lifetimes and time-resolved phosphorescence spectra suggest the presence of a single phosphorescent species in aqueous solution, which is indicated to be monomeric 1-bromonaphthalene. Implications for the absence of phosphorescent excimers of 1-bromonaphthalene,in view of the reported excimeric phosphorescenceof naphthalene in solution, are discussed.
Introduction During the past 15-20 years interest has been shown in observing the phosphorescence of excimers of aromatic hydrocarbons in solution, and such emission has been alleged in intramolecular” and intermolecular”’3 systems. However, attempts recently reported by Nickel and PrietoI4 to reproduce some of the experimental results in ref 2-13 strongly indicate that excimeric phosphorescence of aromatic hydrocarbons in soluton has not been observed. In particular, Nickel and Prieto attributed the excimeric phosphorescence reported in fluid media for naphthalene and 1,n-di-a-naphthylalkanes to the sensitized phosphorescence of biacetyl-like impurities. Locke and Lim15 recently responded to (1) Author to whom correspondence should be addressed. (2) Subudhi, P. C.; Lim, E. C. J. Chem. Phys. 1975,63, 5491. (3) Subudhi, P. C.;Lim, E. C. Chem. Phys. Lett. 1976, 44, 479. (4) Okajima, S.; Subudhi, P. C.; Lim, E. C. J. Chem. Phys. 1977,67,4611. ( 5 ) Subudhi, P. C.; Lim, E. C. Chem. Phys. Lett. 1978, 56, 59. (6) Subudhi, P. C.; Lim, E. C. Chem. Phys. Lerr. 1978,58, 62. (7) Webster, D.; Baugher, J. F.; Lim, B. T.; Lim, E. C. Chem. Phys. Lett. 1981, 77, 294. (8) Lim, B. T.; Lim, E. C. J . Chem. Phys. 1983, 78, 5262. (9) Takemura, T.; Baba, H.; Shindo, Y . Chem. Lett. 1974, 1091. Shindo, H. J. Am. Chem. SOC. (10) Takemura, T.; Aikawa, M.; Baba, H.; 1976, 98, 2205. ( I t ) Aikawa, M.;Takemura, T.; Baba, H.Bull. Chem. SOC.Jpn. 1976, 49, 437. (12) Takemura, T.; Aikawa, M.; Baba, H. J . Lumin. 1976, 22/13, 819. (13) Yamamoto, K.;Takemura, T.; Baba, H. Bull. Chem. SOC.Jpn. 1978, 51, 729.
0022-3654/88/2092-7057$01.50/0
Nickel and Prieto in ref 14 with additional results in an attempt to eliminate biacetyl or biacetyl-like molecules as a possible source for their reported emission, and they reasserted the assignment of the emission to excimer phosphorescence of naphthalene and its derivatives in isooctane. In spite of the results reported in ref 15, it is still unsettling that the experiments of Nickel and Prieto did not reproduce the excimeric phosphorescence of aromatic hydrocarbons. In view of the present controversy the experiments described here were undertaken in order to provide further information and to determine whether excimeric phosphorescence could be observed under experimental conditions that strongly favor excimer formation. It was felt that the hydrophobicity of 1-bromonaphthalene would favor the formation of molecular aggregates of these molecules in aqueous solution. The existence of such aggregates should be expected to reduce the solvent barrier to formation of excimers and provide an optimal solvent environment for excimer formation. Furthermore, the bromine heavy-atom effect was expected to enhance the phosphorescence in 1-bromonaphthalene over that observable in naphthalene. The conclusion of our experiments is that if excimeric phosphorescence occurs in 1-bromonaphthalene, it is weaker than that reported in naphthalene* and is too weak for us to detect. However, since excimeric phosphorescence should be more fa~~~
(14) Nickel, B.; Prieto, M. F. R. Z . Phys. Chem. (Munich) 1986,150, 31. (15) Locke, R.J.; Lim, E. C. Chem. Phys. Lett. 1987, 138, 489.
0 1988 American Chemical Society
