The barrier heights (including zero-point effects) for H + CH2O yields CH3O and CH3O yields CH2OH have been computed using complete active space self consistent field (CASSCF)/gradient calculations to define the stationary point geometries and harmonic frequencies and internally contracted configuration-interaction (CCI) to refine the energetics. The computed barrier heights are 5.6 kcal/mol and 30.1 kcal/mol, respectively. The former barrier height compares favorably to an experimental activation energy of 5.2 kcal/mol

Walch, Stephen P.

English

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Computed Barrier Heights for
f
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H + CH=0 +-+CHs0 _ CH=0H
Stephen P. _alch _
ELORET Institute
Palo Alto, Ca. 94303
Abstract. The barrier heights (including zero-point effects) for H + CH20 _ CH30
and CH30 --_ CH2OH have been computed using complete active space self consis-
tent field (CASSCF)/ gradient calculations to define the stationary point geome-
tries and harmonic frequencies and internally contracted configuration-interaction
(CCI) to refine the energetics. The computed barrier heights are 5.6 kcal/mol and
30.1 kcal/mol, respectively. The former barrier height compares favorably to an
experimental activation energy of 5.2 kcal/mol.
https://ntrs.nasa.gov/search.jsp?R=19930008220 2020-03-17T09:08:39+00:00Z
The CH30 radical is important in combustion and atmosphericchemistry [1]. In
addition, recent experiments have studied individual rovibrational states of CH30
above the H-CH20 dissociation threshold by stimulated emission pumping [3]. The
interpretation of these experiments depends critically on the barriers to decom-
position of CH30 to H + CH20 and rearrangment to CH2OH. Previous studies
of these barrier heights include the work of Saebo, Radom, and Schaefer [2], who
used M¢ller-Plesset perturbation theory through third order with small basis sets
up through 6-31G**. More recently, Page, Lin, He, and Choudhury [4] reported
somewhat more accurate calculations using multireference configuration-interaction
(MRCI) with polarized triple zeta basis sets. In the present paper more accurate
calculations are reported which make use of larger basis sets and extensive CI.
Two different basis sets were used in this work. For the complete active space
self consistent field (CASSCF) gradient calculations the polarized double zeta set
of Dunning and Hay [5] was used. The basis set for C and O is a (9s5p)/[3s2p]
basis augmented by a single set of 3d functions with exponents of 0.75 and 0.85
for C and O, respectively. The H basis is (4s)/[2s] augmented with a single set of
2p functions with exponent 1.00. The basis set used in the CI calculations is the
Dunning correlation consistent triple zeta double polarization basis set [6]. This
basis is [4s3p2dlf] for C and O and [3s2pld] for H and is described in detail in Ref.
6.
The calculations were carried out in C, symmetry for a wavefunction of 2A'
symmetry. From the Hessian matrix in the diagonal representation, the station-
ary points obtained in C, symmetry are the minimum for CI-I20 + H, the saddle
point for CH=O + H -+ CH30, the CH30 minimum, and the saddle point for
CH30 -+ CH2OH. For CH30 the 2A_ state corresponds to one component of the
2E state in C3, symmetry. According to the calculations of Saeb¢_ et al. [2], the
Jahn-Teller stabilization energy in this system is 0.56 kcal/mol and the Jahn-Teller
splitting is 0.12 kcal/mol, with the 2A' component lower. CH2OH is found to have
no symmetry and the use of a mirror plane of symmetry would not be appropriate.
However, the separation between CH30 and OH2 OH has been accurately computed
by Bauschlicher, Langhoff, and Walch [7] and further calculations for CH2OH were
not carried out here.
The CASSCF calculations consistedof five electronsand five orbitals. The active
electronsfor CH30 included the CO bond pair, one CH bond pair (the CH bond in
the molecular plane), and the a' O 2p like orbital. Theseorbitals correspondto the
CO _ and 7rbondsplus a Hls orbital for CH20 + H. The remaining electrons, which
are inactive, include the other two CH bond pairs, the a" O 2p like orbital and the
O ls, O 2s, and C ls like orbitals. In generating the set of reference configurations
for the subsequent internally contracted CI (CCI) calculations, no more than two
electrons were permitted in the weakly occupied CASSCF orbitals. All but the O ls
and C ls electrons were correlated in the CCI calculation. A multi-reference analog
of Davidson's correction [8] was added to the CCI energies.
The CASSCF/gradient calculations used the SIRIUS/ABACUS system of pro-
grams [9], while the CCI calculations were carried out with MOLPRO [10,111. The
calculations were carried out on the NASA Ames Cray Y-MP.
Table I gives the harmonic vibrational frequencies, rotational constants, and ge-
ometric parameters for the stationary points considered in this work. Here H_ is
the H of the spectator CH bonds, while Hb is the hydrogen which transfers during
the reaction. The geometries and frequencies in Table I are quite similar to those
obtained by Page et al. [4]. This is expected since a CASSCF wavefunction was
used in Ref. 4. However, in Ref. 4 the CO 0" bond was not correlated. For CH30
this has the expected effect that the CO bond is elongated by _ 0.03 _ compared to
the cMculations in Ref. 4. At the H-CH20 saddle point the CO bond is elongated
by _ 0.02 _ and the C-Hb bond is shorter by _ 0.05 _ as compared to Ref 4.
There are larger differences between this work and the results of SaebO et al. Here
the geometry for CH30 is similar except for the expected elongation of the C-Hb
and CO bonds. However, there are significant differences in the geometry of the
H-CH20 saddle point. In particular, the C-Hb bond length is shorter by _ 0.18 ._
• This result is consistent with the expectation that saddle point geometries will be
more sensitive to electron correlation effects than will stationary points.
Table II gives the computed energies and zero-point corrections. The energies
are from the CCI calculations, while the zero-point corrections are derived from the
CASSCF harmonic frequencies. From Table II it is seen that the relative energies are
only very slightly changed (_ 0.1 kcal/mol) in going from the [4s3p2dlf/3s2pld]
basis set to the [5s4p3d2f/4s3p2d]basis set. Table III compares the computed
barrier heights obtained in the present calculations to those obtained by Saeb¢et
al. [2] and by Page et al. [4]. Here it is seenthat the barriers obtained by Saeb¢
et al. are too large by _ 6 kcal/mol., while the barrier for H + CH20 _ CH30
obtained by Pageet al. is 2.3 kcal/mol larger than in the present calculations. Our
computed barrier height is 5.6 kcal/mol for H + CH20 _ CH30 and 5.2 kcal/mol
for D + CH20 _ CDH20. This result is in good agreementwith experimental
estimated activation energiesof 5.2 kcal/mol for H + CH_.Oand 3.9 kcal/mol for
D + H2CO [12].
It is concluded that the present calculations give energeticsfor the CH30 sys-
tem which are of chemical accuracy ( in this case the estimated error bars are
=h 1 kcal/mol). This is in contrast to results obtained by Saeb¢ et al. using Moller-
Plesset perturbation theory, where the errors in the computed barrier heights are
"--"6 k al/ 1C mo,
Acknowledgement. SPW was supported by NASA cooperative agreement NCC2-
478.
4
References
_Mailing Address: NASA Ames ResearchCenter, Moffett Field, CA 94035.
1. See references in Ref. 2.
2. S. Saeb0, L. Radom, and H.F. Schaefer Ill, J. Chem. Phys., 78, 845(1983).
3. A. Geers, J. Kappert, F. Temps, and J.W. Wiebrecht, Ber. Bunsenges Phys.
Chem., 94, 1219(1990).
4. M. Page, M.C. Lin, Y. He, and T.K. Choudhury, J. Phys. Chem., 93,
4404(1989).
5. T.H. Dunning, Jr. and P.J. Hay in Methods of Electronic Structure Theory,
H.F. Schaefer In ed., Plenum Publishing, 1977.
6. T.H. Dunning, Jr., J. Chem. Phys., 90, 1007(1989).
7. C.W. Bauschlicher, Jr., S.R. Langhoff, and S.P. Walch, J. Chem. Phys., 96,
450(1992)
8. S.R. Langhoff and E.R. Davidson, Int. J. Quantum Chem., 8, 61(1974).
9. SIRIUS is an MCSCF program written by H.J. Jensen and H. Agren and ABA-
CUS is an MCSCF derivatives program written by T. Helgaker, H.J. Jensen,
P. Jorenson, J. Olsen, and P.R. Taylor.
10. H.-J. Werner and P.J. Knowles, J. Chem. Phys., 89, 5803(1988).
11. P.J. Knowles and H.-J. Werner, Chem. Phys. Left., 145,514(1988).
12. F. Temps, private communication.
Table I. Computed stationary point harmonic frequencies (cm -1), rotational con-
stants (cm-1), bond lengths (/_), and bond angles (degrees).
CH30 CH30 --* CHeOH H-CH20 H + CH20
wl 3216 3261 3190 3174
w2 2921 2378 1657 1766
w3 1608 1556 1471 1589
w4 1471 1101 1220 1213
w5 1103 1037 605
_6 997 2254 i 1435 i
w7 3284 3391 3297 3272
us 1491 1159 1281 1304
w9 1050 1015 770
A 5.180 5.718 3.927 9.548
B 0.885 0.929 1.007 1.263
C 0.884 0.900 0.962 1.115
rC-H, 1.092 1.086 1.094 1.097
rC--Hb 1.122 1.275 1.668
rc-o 1.412 1.418 1.259 1.226
rO--Hb 1.237 2.292
L H_CH_ 111.5 119.1 117.1 117.2
H_CHb 108.2 117.1 91.2
H_CO 111.7 116.5 120.3 121.4
HbCO 105.3 54.4 102.2
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Table II1. Comparison of computed barrier heights (kcal/mol).
SaebO et al. _ Page et al. b PW c Exp.
H + CH20 _ CH30 12.5 8.0 5.6 5.2a,6.2 _
D + CH20 --* CDH_O 5.2 3.9d,4.8 e
CH30 _ CH2 OH 36.0 30.1
Reference 2.
b Reference 4.
Present work.
d Estimated activation energy from a room temperature rate constant for D +
H2CO, an assumed preexponential factor of 2 x 1013 cm 3 mo1-1 sec -1, and a zero-
point energy difference between D + H2CO and H + H2CO of _ 1.3 kcal/mol. Ref.
12.
e Estimated threshold energy from an RRKM analysis. Ref. 12.
7


Computed barrier heights for H + CH2O yields CH3O yields CH2OH

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