J . Phys. Chem. 1988, 92, 2757-2762
Electronic and Geometric Structures of the Scandium Cations ScH', ScCH,', and ScCH'
2757
ScCH,',
Aileen E. Alvarado-Swaisgood and James F. Harrison* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1 322 (Received: September 19, 1986)
The electronic and geometric structures of the scandium cations ScH', ScCH3+,ScCH2+,and ScCH' have been studied by ab initio MCSCF and CI techniques. Sc' forms a single bond to CH3 and double bonds to both CH2 and CH. A detailed analysis of the electron distribution in the metal-ligand bond is presented. The results are compared with experiment and with previous theoretical results on the corresponding Cr' compounds.
Introduction The experimental study of the gas-phase bimolecular reactions of transition-metal cations with organic molecules is resulting in significant insights' into the mechanism of these important reactions. In addition, the thermochemical data extracted from the experiments2are providing an invaluable data base that will allow the systemization of the observed chemistry, prediction of possible new chemistries, and bench marks for theoretical studies. To date, these experiments have not been able to provide structural information on the various reaction products. The purpose of this study is to provide such structural information via a uniformly accurate theoretical study of the electronic and geometric structures of the titled molecules. We anticipate that this and related studies3s4will result in our understanding the nature of the transition metal-main group element bond sufficiently well so as to permit a qualitatively correct prediction of the bonding in more complex transition-metal-containing systems. The absolute energies and calculated bond energies for the various molecules to be discussed are collected in Table I, the fragment energies in Table 11, and the experimental bond energies in Table 111. The technical details are collected in the Appendix. Results and Discussion Sc', ScW, and ScCH,+. The ground state of Sc' is of 3D symmetry and arises from the 4s3d config~ration.~The companion 'D is 0.302 eV above the 3D, with the lowest state arising from the 3d2 configuration (3F symmetry) being 0.596 eV above the ground state. Figure 1 compares our calculated separations with these experimental results. As a 2SH atom approaches a Sc' in the 3D state, we may form doublet and quartet states of 2+,II,and A symmetry. If the Sc+ is in the ,F state, the molecular states are of 2-, II, A, and symmetry. As shown p r e v i o ~ s l y ,the ~ ~ ground state is of 2A symmetry and (with the argon core on Sc suppressed) is represented by a function of the form (ah + ha) da(@ - Pa). where at large separation u is a Sc' 4s orbital and h is a hydrogen (1) Representative publications of the groups currently active in this area include the following: (a) Jacobson, D. B.; Freiser, B. S.J. Am. Chem. Soc. 1985, 207, 5870. (b) McElvany, S. W.; Allison, J. Organometallics 1986, 5, 1219. (c) Aristov, N.; Armentrout, P. B. J. Am. Chem. SOC.1984,106,4065. (d) Mandich, M. L.; Halle, L. F.; Beauchamp, J. L. J . Am. Chem. Soc. 1984, 106, 4403. (2) (a) Elkind, J. L.; Armentrout, P. B. fnorg. Chem. 1986,25, 1078. (b) Tolbert, M. A.; Beauchamp, J. L. J. Am. Chem. SOC.1984, 106, 8117. (3) (a) Alvarado-Swaisgood, A. E.; Allison, J.; Harrison, J. F. J . Phys. Chem. 1985, 89, 2517. (b) Alvarado-Swaisgood, A. E.; Harrison, J. F. J. Phys. Chem. 1985,89, 5198. (c) Mavridis, A.; Alvarado-Swaisgood, A. E.; Harrison, J. F. J. Phys. Chem. 1986, 90, 2584. (d) Harrison, J. F. J. Phys. Chem. 1986, 90, 3313. (4) Carter, E. A,; Gcddard, W. A. J. Phys. Chem. 1984, 88, 1484. Schilling, B. J., Gcddard, W. A,, 111; Beauchamp, J. L. J. Am. Chem. SOC. 1986, 108, 582. ( 5 ) Moore, C. E. Azomic Energy Leuels; National Bureau of Standards Reference Data Series, NSRDS-NBS Circular No. 35; 1971, Vol. 2.
0022-3654/88/2092-2757$01.50/0
1s function. If one represents this state by an MCSCF function consisting of all configuration state functions ( C S F S )of ~ A symmetry arising from three valence orbitals of Z symmetry and one A symmetry, one has, in C,,, nine CSFs. The bond energy versus R calculated from this MCSCF function is shown in Figure 2 along with the C I results generated from all single and double excitations from this nine-configuration reference space (MCSCF+1+2). Curves are shown for two levels of basis sets: the spd basis described in the Appendix and the more extensive spdPbbasis. AU of the calculations reported in subsequent sections of this article use the spd basis. Although the energy scale on the figures is in millihartrees, we will use kilocalories per mole in the text. The conversion factor is 1 millihartree = 0.6275 kcal/mol. The spdf results are included to give an indication of the sensitivity of the De to the Sc' basis. The De calculated with the spd CI function is 50.7 kcal/mol, while the spdf CI gives 54.8 kcal/mol, which when corrected for zero-point motion with use of the calculated o values (1559 and 1595 cm-'), results in Do values of 48.5 and 52.6 kcal/mol, both in reasonable agreement with the experimental value' of -55 kcal/mol. To determine the atomic orbital composition of the bonding orbitals, we carried out a population analysis of the natural orbitals of the MCSCF wave function, and the results are shown in Figure 3. At !arge separation we have one electron in both the H 1s orbital and the Sc 4s orbital. As the distance is decreased, the Sc 4s character drops while the Sc 3d, and 4p, occupations increase until at equilibrium we have 1.10 electrons "on" H with the remaiping 0.9 valence electrons distributed as 4s0 45, 3d: 29, and 4p> 16. The d6 electron is, of course, localized on Sc+. When CH, approaches Sc+it can bond via its singly occupied pmorbital, and the resulting molecular state will have 2A" symmetry with the odd electron being hosted by the (essentially) d, orbital on Sc+. An MCSCF function (similar to ScH+) was constructed as a function of the Sc-C separation, and the resulting potential curve is shown in Figure 4, along with the MCSCF+ 1+2 result. Wote that, as one would expect, the De for ScCH3+(41 kcal/mol), is similar to that computed for ScH+ (51 kcal/mol). The experimental valuezb for Do is 65 f 5 kcal/mol, suggesting that the C H 3 group is more strongly bound than H. Given the demonstrated sensitivity of De to basis set size and the probability that our basis set underestimates the polarizability of CH,, it is not unreasonable that we disagree with the experimental order. We are studying the question of whether CH3 or H bonds more strongly to an earlier transition-metal cation and will report the results in a future publication. The MCSCF+1+2 optimized geometry for ScCH3+corresponds to a Sc-C distance of 2.25 8, and a H C H angle of 108.9' (at a fixed C-H distance of 1.07 8,). The population of the various bonding orbitals as a function of the Sc-C bond length is presented in Figure 5 . Note that the Sc-C bond is slightly ionic, with Sc hosting 0.8 electrons and CH, the remaining 1.2 electrons; the distribution of the 0.8 electrons (6) Lischka, H.; Shepard, R.; Brown, F. B.; Shavitt, I. Inr. J . Quantum Chem. Symp. 1981, 15, 91.
0 1988 American Chemical Society
2758 The Journal of Physical Chemistry, Vol. 92, No. 10, 1988
Alvarado-Swaisgood and Harrison
TABLE I: Calculated Energies, Bond Strengths, and Geometric Parameters for Various Scandium Cations MCSCF
MCSCF+1+2
molecule ScH+('A)
R(Sc-R)/B(CH,)" 1.862 10.58
CSFs 9 9
E , au -760.098 71 -760.027 77
ScC H 3+( 'A")
2.270 / 109.2 10.58/ 120.0
4 4
-799.145 81 -799.091 96
ScCH,+('A,)
2.004/112.0 10.53/ 128.8
10 IO
-798.539 32 -798.454 67
SCCH~+(~A,)
2.286/112.36 10.58/128.8
7 7
-798.499 88 -798.454 67
ScCH+('II)
1.973 10.58
17 17
-797.91880 -797.81235
SCCH+(~II)
2.185 10.58
6 6
-797.883 36 -797.812 35
De, kcal/mol
R(Sc-R)/O(CH2)" 1.834 10.58
CSFs 601 601
E , au -760.109 16 -760.028 34
2.246/ 108.9 10.58/ 120.0
3153 3153
-799.15661 -799.09253
1.997/ 1 12.0 10.58/128.8
3660 3660
-798.570 14 -798.461 81
2.263/111.36 10.58/128.8
4683 4683
-798.529 15 -798.461 81
1.940 10.58
12099 12099
-797.982 68 -797.829 63
2.177 10.58
7270 7270
-797.939 48 -797.829 63
44.5
50.7
33.7
40.1
53.1
68.0
28.4
42.3
66Ab
96.0
44.6b
" Bond lengths are in angstroms; angles are in degrees.
~
SC+(~D) Sc+('D) CH,('A') CH'('B,) CH(4Z-) CH('Il)
68.9
Values for De for ScCH' are relative to the 42-C H fragment,
TABLE 11: Fragment Enernies' ~~
MCSCF+ 1+2 -759.529 06 -759.51874 -39.563 48 -38.932 15 -38.300 18 -38.31240
MCSCF -759.528 48 -759.51 1 38 -39.563 48 -38.925 58 -38.283 93 -38.27056
De, kcal/mol
TABLE 111: Experimental Bond Energies molecule D o , kcal/mol ScH+ ScCH,+ ScCH,'
54 4;'a 5 5 65 52b 9778
*
22b
~
i
"Energies in atomic units (hartree).
100-
t
3F(3dZ)
G 5 9 6 e v [G92ev I S C F ) . 0 7 5 e v I C I ~
7
[G 47ev ISCF), 0 28 eu (CI] 0 3 0 2 e v
L
3D(453d)
Figure 1. Experimental and calculated separation between low-lying states of Sc+.
-10-
I
-20-
-30-
R(Sc-H)(au)
Figure 3. Electron population of selected atomic orbitals of u symmetry from the bonding natural orbitals for the MCSCF wave function of 'A ScH+. MCSCF(spd1
on Sc is 4s0.423d:284p,0.08 and is similar to the distribution in ScH+.
ScCH2+. The doubly bonded state of this molecule arises when ) to the two valence the two valence electrons on S C + ( ~ Dcouple electrons on CH&B,). The wave function has the schematic form
MCSCF ( s p d f l
'Al
-
[uCH,USc
5 00
R(Sc-H)
IO00
(au)
Figure 2. Potential energy curve for the lowest 'A state of ScH' for the spd and spdf basis sets in both the MCSCF and MCSCF+l+Z calculations. The energy units are millihartrees (mH), and 1 millihartree = 0.6257 kcal/mol.
+ uScuCH21 [rCH2rSc + r S ~ r C H 2 1 ( a @ -
- Pa)
where at large separation uCH2and rCH2 are the p, and px orbitals of free CH#BI), while uScand rscare the 4s and 3d, orbitals of SC(~D).An MCSCF function that separates to the correct SCF products consists of 10 CSFs, while the singles and doubles C I (MCSCF+1+2) from this reference space consists of 3660 CSFs. The energy as a function of Sc-C separation is shown in Figure 6 for both wave functions. The Sc-C bond length changes from 2.004 8,at the MCSCF to 1.998 8, at the CI level, while the HCH
The Journal of Physical Chemistry, Vol. 92, No. 10, 1988 2759
Structures of Scandium Cations
r
R (SC-C)0 . u 20 40 I
80
60
100
n. 0-
I E
40-
I
r
"/
40
60
80
100
N
MCSCF
\ v /,MCSCF+
40
I
Y
6
.
20
20
-2"
.
2.0
ScCH: (IA,)
0
60-
1
0
n
Fixed CHI Geometry
x
It2
60
t
Figure 4. Binding energy of ScCH3' in the 2A" state as a function of S c C H 3 ' separation. The CH3 geometry was constrained to be tetrahedral with a C-H bond length of 1.070 A.
10
'..i
Figure 7. Potential energy curves for the 'Al state of ScCH,'.
The CH2
group is constrained as in the IAl calculation.
11 L
08
2
il
3
3
06-
a
Fixed CH,
0
a 04
Geometry
t
R(Sc-CH,)
0.u
Figure 5. Electron population of selected atomic orbitals from the bonding natural orbitals for the MCSCF wave function of 2A'fScCH3'. R (SC- C)o u 40
20 O
80
60
100
r
R(Sc-C) o u
Figure 8. Electron population of selected atomic orbitals from the bonding natural orbitals for the 'A, MCSCF function of ScCH2+.
mental data on the geometric parameters with which to compare. The triplet state that correlates to the ground-state products, maintains a bond in the u system, and triplet couples the A electrons has the schematic representation 'AI
t MCSCF ScCH2+ ( ' A i l
Figure 6. Potential energy curves for the 'A, state of ScCH2'. The CH2 group is constrained to have a C-H bond length of 1.075 A and an H C H angle of 1 2 8 . 8 O .
angle remains at 112' in both calculations. The primary effect of the C I is to increase the calculated De by 15 kcal/mol to 68 kcallmol. This is to be compared with the experimental value of 97 kcal/mol from Armentrout et al.7a There are no experi-
-
+ "Sc"CH21ACH~"Sc(aP
[UCH~USC
- Palaa
An MCSCF function of this form that separates to the S C F products consists of 7 CSFs, while the corresponding singles and doubles CI from this reference space contains 4683 CSFs. The energy of this 3AIstate as a function of Sc-C separation is shown in Figure 7. The optimized Sc-C bond length changes from 2.287 8, at the MCSCF level to 2.264 8,at the CI level, with the H C H angle going from 112.4' to 113.4'. The increased bond length of this state relative to the 'Al state is consistent with the loss of the A component of the ScC double bond. The calculated De for the 3Al state is 42 kcal/mol, which is 26 kcal/mol less than the 'Al De and reflects the strength of the ScC A bond. The electron distribution in selected valence orbitals is plotted as a function of internuclear separation in Figures 8 and 9. Although these plots correspond to the HCH angle of 128.8', they faithfully track the overall distance dependence of the orbital occupancies. At the optimal H C H angle the Sc orbitals have the in the 'Al state and 4s0384p,,""3d; 21 occupancies 4s0224p,0043d243 (7) (a) Elkind, J. L.; Aristov, N.; Georgiadis, R.; Sunderlin, L.; Armentrout, P. B., private communication. (b) Lie, G. c.;Hinze, J. J . Chem. Phys. 1972, 57, 625; 1973, 59, 1872.
2160
The Journal of Physical Chemistry, Vol. 92, No. 10, 1988
Alvarado-Swaisgood and Harrison
L
R (Sc-C) o u 0
20
P 40
n 60
04
I E
-$
02
I
80
x
ow 5 00
10 00
IO0
R(Sc-C)o u
I
I
Figure 9. Electron population of selected atomic orbitals from the bonding natural orbitals for the 'A, MCSCF function of ScCH2'.
in the 3A1 state. The corresponding carbon occupancies are ~ s O . ~ ' Z ! P ~ % and 2s0.422p2s9.Interestingly, the total Sc and carbon u populations in both states are essentially identical, although the electron distribution among the valence obitals differs between the two states. Since the 7~ distribution in both states has one electron on Sc and one on carbon, there has been a net loss of 0.3 electrons from Sc to carbon in both states. Note that this transfer of electrons from Sc to the carbene po orbital is consistent with the calculated in situ H C H bond angle being less than free 3B, but greater than CH2('A,). ScCH'. The ground state of C H is of 211symmetry and has the schematic representation
I20
I40
I60
Figure 10. Potential energy curves for the 211 state of ScCH'. The C-H distance is taken as 1.082 A.
dUf-i b bH - u The 48-lies 17 k ~ a l / m o l 'above ~ the 2rI and has the representation
$&-
O)t,
H
With suppression of the Ar core on Sc, the carbon 1s orbital, and CH bond orbital, the primary component of the wave function would have the form 2nx
[bCHuSc
+ bScuCHI [rycHayQ
+ ayQa)cH17rXCH(aP - Pa)(aP - P a b
Asymptotically, the usc would be a 4s orbital, the ayQa 3d,, orbital, gCH a pg (sp hybrid on C), and aXCH a p, orbital on C. The MCSCF function that separates to the S C F products (3D + 421-) consists of 17 CSFs while the corresponding C I (MCSCF+ 1+2) contains 12099 CSFs. The distance dependence of energy of the 211 state is shown for both functions in Figure 10, while the electron population of various orbitals is shown in Figure 11. The calculated dissociation energy is 96.3 kcal/mol, and the bond length is 1.940 A. If we reference this Deto the ground 'II state of C H using our calculated 2rI-42-separation
&
2
3
,
,
,
9
10
;,Req
I
If the S C + ( ~ Dforms ) a double bond with the carbyne group, we anticipate the unpaired electron will be in a 7~ orbital localized primarily on carbon:
,
01
4
5
R(Sc-C)
6
7
8
~d
Figure 11. Electron population of selected atomic orbitals from the bonding natural orbitals for the *II MCSCF function of ScCH'.
of 10 kcal/mol, we predict a Deof 86 kcal/mol. This double bond is substantially stronger and somewhat shorter than that in ScCH2+('Al) (68 kcal/mol and 1.998 A). If one breaks the a bond in this 211state by triplet coupling the a electrons 4n
IUCHuSc
+ u S c u C H I a y c ~ a ~ s 7 r x c ~ ( C Y-P pa)aaa
the resulting state is bound by 69.4 kcal/mol and has a bond length of 2.177 A. Breaking the a bond costs 27 kcal/mol and increases the bond length by 0.24 A. Recall that breaking the a bond in ScCH2+costs 26 kcal/mol and increases the bond length by 0.17 h;. The larger bond energy in the carbyne relative to the carbene is therefore due to the u bond. Looking at the electron distribution in Figures 8 and 11, we see that the carbyne po orbital hosts 1.5 electrons and is essentially 50% 2s, while the carbene pr orbital contains 1.3 electrons and is 25% 2s. The stronger bond is the more ionic and the one in which the carbon bonding orbital is essentially an sp hybrid. There are several other low-lying states associated with the 3D + 42-asymptote. For example, if we keep the u bond intact but
-
-
The Journal of Physical Chemistry, Vol. 92, No. 10, 1988 2761
Structures of Scandium Cations
-
SC+(~D)+CH(~C-)
ScCH+
I 69
75
44
j2n196/l 9401 MCSCF
MCSCF+1+2
Figure 12. Bond energy and Sc-C bond lengths in various states of ScCH' that dissociate to SC(~D) + CH(42-). 50 7154)
('A)
25 I(59)
('E+)
.Sc-H
rt
I834 A
I636
6 8 ( 71 1H , ('A,) S~=C)112~
f
I 998
aH'
sc-
f
iiia
LH
96 3 (99!
('n)
Sc=C-H
rt
'
210155)
C'j
2 264
38 7199) /H C r = C ) 117' H '. I92 A
42145) (%)
'CrJH
1946
PE,)
Cr-C/)
I17O
t LH
206
7081148)
(3r-)C r2e C - H I77H
Figure 13. Comparison of the calculated bond energies and geometric parameters for various Sc and Cr compounds.
intetgral as 17 kcal/mol, the exchange energy loss in forming one, two, or three bonds is approximately 34, 60, and 77 kcal/mol. This is interpreted as the amount of energy Cr+ must use to randomize the spins of its high-spin coupled electrons in anticipation of forming one, two, or three bonds by using only d, orbitals. The Sc' ion in the 3D state requires 3 kcal/mol to prepare for either a single or double bond. When the calculated De values are augmented by the estimated exchange energy loss, the numbers in parentheses in Figure 13 are obtained. These very simple "corrections" result in "derived" bond energies that are more in line with conventional bond length/bond strength expectations. Note in particular that the Cr=CH+ derived bond strength is -3 times as large as that of Cr-CH3+ and suitably larger than that of Sc=CH+. While this simple model brings some order to the calculated bond strengths, it must be emphasized that the calculated and not the "derived" bond energies should be compared with experiment. Furthermore, the model is based on SCF concepts and presumes that the ground electronic configuration of the transition-metal ion is dominant not only asymptotically but also at Re.This clearly is not the case as the various population analysis plots demonstrate. Nonetheless, we believe the model accounts in large measure for why the positive ions of the early transition-series elements have such different De values for the same ligand.
Conclusions The preceding discussion suggests that Sc+ forms a single bond with CH, while forming double bonds with both CHI and CH. The calculated Sc-C bond lengths 2.245 (SC-CH~+(~A"), 1.998 are consistent with (Sc-CH,'('A,)), and 1.940 A (SCCH+(~JI)) this observation. The Sc-CH3+ bond energy is calculated to be less than that of Sc-H'. This may be due to an inadequate representation of the C H 3 polarizability. The ScC double bond in ScCH' has a calculated strength of 96 kcal/mol, which is substantially larger than that calculated for the formally similar bond in ScCH2+(68 kcal/mol). There are two reasons for this: (a) The above De for ScCH' is referenced to the 42-state of CH, which in this study is 10 kcal/mol above the ground ,II; (b) the C H bonding orbital is essentially an sp hybrid, while the CH, orbital has more sp2 character. This is an unusual situation in which the carbon hybridization in ScCH+ is appropriate for a triple bond but the metal can support, at most, a double bond. The Sc contribution to the u bond decreases as we go from CH3 (38%) to CH, (35%) to C H (25%), Le., the bond becomes more ionic. Along this series the percent d, in the Sc bonding orbital varies as CH3 (36%), CH, (62%), and CH (57%). Note that the multiple bonds have the larger d, character, an observation made in the study of the corresponding Cr compounds. In the series ScH+(,A), ScCH3'(,A"), ScCH,+('A,), and ScCH('II) the computed bond strengths are all less than the experimental values (see Table 111). The x bond in ScCH,' and ScCH' has considerable open shell character (at equilibrium the two natural orbitals representing this bond have occupation 1.0). The associated GVB orbitals have a small overlap. All of the calculations reported are size-consistent to within 1 or 2 millihartrees, at both the MCSCF and MCSCF+l+2 level.
singlet couple the Sc 3d8orbital to the carbyne pr orbital, we have a 2A state. Alternatively, we may singlet couple the Sc 3d, electron state. We optimized to the carbyne pz orbital and obtain a the geometry for these states, and the corresponding quartets and the results are summarized in Figure 12. Note that when we go from a 2JI to a 4JI the energy increases, but in going from a 2A to a 4A or a 2Z- to a 4Z-the energy decreases. This is because the higher multiplicity in the JI state can be achieved only at the expense of a chemical bond between orbitals of the same symmetry (there is also a substantial increase in the bond length). In the A and 2- cases the higher multiplicity is lower in energy because the bond is between orbitals of different symmetry (and, therefore, very weak), and one recovers an exchange interaction that more than compensates for the lost bond energy. Consistent with this we note that the bond lengths in the A and Z- symmetries are considerably less sensitive than the JI to the multiplicity. Comparison with the Corresponding Cr Compounds. In Figure 13 we summarize the structural and energetic results discussed in this article along with those reported previously for the analogous Cr compounds. Note that the Sc-ligand bond length and Acknowledgment. We are indebted to the Argonne National bond energy are always larger than the corresponding Cr comLaboratory Theoretical Chemistry Group for providing the pound. For the metal-H, -CH3, and -CH2bond lengths the large QUEST- 164 electronic structure codes used in this study. During Sc-ligand bond length is a reflection of the larger ionic radius the course of this work A. E. A.-S held a Dow Chemical Co. of Sc'. Of course, the metal-CH bond is longer in ScCH+ than Summer Fellowship and a College of Natural Science Deans in CrCH+ because the former is a double bond while the latter Fellowship. This work was supported in part by N S F Grant is a triple bond. It is interesting that although the Cr compounds CHE8519752. have the shorter bond they have the smaller bond energies. This Appendix is quite different from the structure/energy systematics familiar from molecules composed entirely of main group elements and Basis Sets, Molecular Codes, and Total Energies. The spd reflects the importance of the exchange energy loss c o n ~ e p t ~ ~ , basis ~ set for Sc used in this study consists of 14s,11p,6d functions in transition-metal systems. If we estimate a Cr+ d-d exchange constructed by augmenting Wachters'* 14s,9p,5d basis with two
-
2762
J. Phys. Chem. 1988, 92, 2162-2166
additional diffuse p functions9 and an extra d as recommended by Hay.l0 The Sc functions were contracted to 5s,4p,3d following Raffenetti." The spdf set consists of the spd set with a set of f functions with a = 1. The basis for carbon was 9s,5p,ld set consisting of Duijneveldt'sIz 9s,Sp basis augmented by a d set with exponent 0.85. The hydrogen basis was a 4s,lp set consisting of Huzinaga'sI3 4s set augmented by a p set with exponent a = 1. The expanded H s set used in the spdf calculation on ScH+consists of the Huzinaga 4s set contracted to 3s and augmented with two p sets with exponents a = 1.73 and 0.43. The carbon and hydrogen basis was contracted to 3s,2p,ld and 2s,lp as recommended by Raffenetti." (8) Wachters, A. J. H. J . Chem. Phys. 1970, 52, 1033. (9) Dunning, T. H., Jr., private communication. (10) Hay, P. J. J . Chem. Phys. 1977,66,4377. (11) Raffenetti, R. C. J . Chem. Phys. 1973, 58, 4452. (12) Duijneveldt, F. B. IBM Research Laboratory, San Jose, CA, 1971; IBM Technical Research Report No. RJ-945. (13) Huzinaga, S.J . Chem. Phys. 1965, 42, 1293.
All calculations were done on a FPS-164 jointly supported by the Michigan State University Chemistry Department and the Office of the Provost by using the Argonne National Laboratory collection of QUEST-164 codes. In particular, the integrals were calculated with the program ARGOd4 written by Pitzer; the SCF and MCSCF calculations used the GvB16415program by Bair and ~~ and related utility codes written by Shepard. the U E X P program The configuration interaction calculations were done with the program U C I ~(and its related utility codes) written by Lischka et al. Registry NO. SCH", 83018-00-2; SCCH~', 93349-1 1-2; ScCHZ', 113748-51-9; SCCH", 101653-84-3. (14) The ARGOS integral program was developed by R. M. Pitzer, Ohio State University. (15) The GvB164 program was written by R. Bair, Argonne National Laboratory. (16) A description of the UEXP program is given in the following: Shepard, R.; Simons, J.; Shavitt, I. J . Chem. Phys. 1982, 76, 543.
The n,-3s Rydberg State of Acetone: Absorption Spectroscopy of Jet-Cooled (CH3)2C0and (CD3)2C0 Geoffrey A. Gaines, D. J. Donaldson, S. J. Strickler, and Veronica Vaida* Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-021 5 (Received: September 18, 1987; In Final Form: December 1 , 1987)
The direct absorption spectra of jet-cooled acetone and acetone-d6have been measured from 5 1 000 to 54 000 cm-'. Cooling in a molecular jet allows the resolution of several previously unreported vibronic bands and, consequently, a revised vibrational analysis. Sequence bands of the methyl torsional motion are also resolved; the frequencies of the two torsional vibrations in the Rydberg state are deduced to be 138 and 214 cm-]. A single-methyl rotor calculation yields a barrier to the hindered rotation of 1140 cm-l.
Introduction Extensive information has been obtained in the past few years about the dissociation dynamics of many diatomic, triatomic, and "triatomic like" polyatomic molecules. Little information is available about more complex polyatomics. Recently, acetone has been gaining attention as an intermediatesized molecule with interesting excited-state The exploration of photodissociation processes requires a detailed knowledge of the potential energy surfaces that control the reaction; such information can, in principle, be obtained form high-resolution optical spectra. In practice, efficient photochemical reactions can shorten the excited-state lifetime so much that fluorescence and ionization techniques for measuring the spectrum of the excited state become impossible. For this reason, we developed the technique of direct UV absorption by supersonically cooled samples5and have applied it to the study of the spectroscopy of dissociative molecule^.^^ (1) Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; Wiley: New York, 1966. (2) Lee, E. K. C.; Lewis, R. S. Adv. Photochem. 1980, 12, 1. (3) Donaldson, D. J.; Leone, S. R. J . Chem. Phys. 1986, 85, 817. (4) Solomon, J.; Jonah, C.; Chandra, P.; Bersohn, R. J. Chem. Phys. 1971, 55, 1908. (5) Vaida, V. Ace. Chem. Res. 1986, 19, 114. (6) McCarthy, M. I.; Vaida, V. J. Phys. Chem. 1986, 90, 6759. (7) Vaida, V. NATO A S I Ser., C 1987, 200, 253. (8) Vaida, V.; McCarthy, M. I.; Engelking, P. C.; Rosmus, P. Werner, H. 3.; Botschwina, P. J. Chem. Phys. 1987, 86, 6669. (9) Donaldson, D. J.; Vaida, V.; Naaman, R. J . Chem. Phys. 1987, 87,
2522.
0022-3654/88/2092-2762.$01.50/0
In the present report, we employ this method to obtain the electronic spectra and thus structural information concerning the no-3s state of (CH3)zC0and (CDJZCO. This band of acetone involves a Rydberg excitation, as demonstrated both by the effect of high pressure on the spectrumlo and by ab initio calculations." The polarization of the 5 1 000-cm-I band has been shown by an electric field studylZ to be perpendicular to the CO bond, in agreement with the assignment of a no-3s promotion. The electronic spectrum of acetone has been investigated previously at room temperature.I3J4 The spectrum of the no-3s Rydberg state of acetone and its interpretation are reexamined here. The elimination of inhomogeneous effects has allowed the observation of additional, previously unresolved, vibronic bands. New excited-state fundamental vibrational modes and low-frequency sequence bands are obtained. Our examination of the (CH3)2C0 and (CD3)2C0 spectra under different expansion conditions and therefore different temperatures leads to a revised picture of this excited electronic state.
Experimental Section The principle of the technique of direct absorption in supersonic jets and the apparatus employed here are both fully described in previous report^.^ In the present experiment, the acetone sample (10) Robin, M. B.; Kuebler, N . A. J . Mol. Spectrosc. 1970, 33, 274. (1 1) Hess, B.; Bruna, P. J.; Buenker, R. J.; Peyerimhoff,S. D. Chem. Phys. 1976, 18, 267. (12) Scott, J. D.; Russell, B. R. J. Chem. Phys. 1975, 63, 3243. (13) Lawson, M.; Duncan, A. B. F. J . Chem. Phys. 1944, 12, 329. (14) Ho, H.f Nogata, Y.; Matsuzaki, S.;f Kuboyama, A. Bull. Chem. SOC. Jpn. 1969, 42, 2453.
0 1988 American Chemical Society
