Joumal of Molecular Structure, 120 (1986) X-20 2TiilEOCBEM Elsevier SciencePublishers B.V., Amsterdam - printed in The Netherlam+

AB INITIO INVESTIGATION OF THE EFFECT OF GEOMETRY OPTIMIZATION AND

CONFIGURATION INTERACTION ON THE RELATIVE ENERGIES AND STRUCTURES

OF THE GROUND AND LOWEST EXCITED STATES OF THIOFORMALDEHYDE*

GEORGE R. DE MARE

Laboratoire de Chimie Physique Moleculaire, Faculte des Sciences,

CP160, Universite Libre de Bruxelles, 50 av- F.-D, Roosevelt,

B-1050 Brussels, Belgium

ABSTRACT

Analytic gradient (force) methods with the 3-21G basis set have been used to optimize completely the structures of the ground ('Al) and four low-lying excited (3Az, 'AZ, 3A1 and 'Al) states of thio- formaldehyde within the restricted-Hartree-Fock (RHF) formalism. In addition the geometric parameters of the molecule have also been determined for the ground and triplet states at the configuration interaction (CISD) level of theory.

INTRODUCTION

Although thioformaldehyde was prepared and positively identified

less than fifteen years ago by Johnson and Powell (ref.l), an

impressive amount of data on its ground and lowest excited states

has been generated in the intervening period. The intense interest

in the molecule, a prototype for C=S containing compounds, is evi-

denced by the number of recent experimental studies (ref.2) and is

outlined by Clouthier and Ramsay (ref.3) in their review on the

spectroscopy of CH2S and CH20.

In contrast to the experimental activity surrounding CH2S,

theoretical investigations have been rather limited in number.

The study reported by Bruna et al. (ref.4) in 1974 is the most

comprehensive. They investigated the orbital and transition ener-

gies as a function of the C-S bond length and the HCH out-of-plane

(OOP) bending angle at both the SCF and CI levels. More recently

Goddard (ref.5) compared the experimental ground and 3Az

state

geometries with those predicted by SCF optimizations with basis

sets ranging from minimal (STO-3G) to double 5 plus both diffuse

* Presented in part at the XIth International Conference on Photo-

chemistry, University of Maryland, Aug_ 21-26, 1983.

d1661280/85/$03.30 Q1985 ElsevierSciencePubHahersB.V.

16

and polarization functions. For the 3 A2 state, the optimizations

were all performed within the unrestricted-Hartree-Fock (UHF)

approximation and only those with either the STO-3G or with the

largest basis sets gave "reasonable" ( % 5 pm too long ('ref.5))

C-S bond lengths. The split-valence and double 5 basis sets yield-

ed very long C-S bonds: this seems to be a general difficulty and

not one which is specific to CH2S (ref.6) _

The present study was undertaken as an extension to our investi-

gations on C,H,O-containing species and also in an attempt to de-

termine the predictive limitations of recent restricted-Hartree-

Fock (RHF) computations on the structures and stabilities of C-

and S-protonated thioformaldehyde (ref.7).

COMPUTATIONAL METHOD

The analytical gradient methods used for the SCF optimizations

of the geometry at the minima and maxima in the potential energy

surfaces are given in reference 8 where the references to the 3-21G

basis and the MONSTERGAUSS program (used for all computations in

this work) can also bs found.

Direct CI calculations were first performed at the 3-21G/RHF

optimized geometries and include all single and double excitations

within the Hartree-Fock interacting space (ref.9). Due to program

and computer limitations, no excitations were permitted from the

three inner core orbitals, leaving 23 active orbitals out of the 26

basis functions.. The Davidson correction for the effect of higher

excitations (ref.10) was applied in all cases; these results are

denoted CISDQ whereas the uncorrected results are labelled CISD.

The CI geometry optimizations are based on obtaining the minimum

CISD energy as a flunction of all parameters (within C2v or C,

symmetry) by successive iterations involving R(C-S), LOOP, R(C-H)

and LHCH, in that order.

RESULTS AND DISCUSSION

Some of the results are given in Tables 1-3. Emphasis will be

given on the structures and energies. Although the 3-21G basis has

been found to give reasonable dipole moments for C,H,O-containing

compounds, the values obtained for CH2S are much too large.

C-H bond length (R(C-H))

As can be seen in Table 1, all of the SCF optimized values for

R(C-H) are smaller than the experimental ones. The largest differ-

ence ( 'L 2 pm) is observed for the ground state. As expected

17

TABLE 1

SCF optimized and experimental values for the angles (") and bond

lengths (pm) in thioformaldehyde.

State LOOP R(C-S) R(C-H) LHCH Reference

1 Al&round) 0.0

0.0

3A2 0.0

26-l

18.1

16.0

11.9

0.0 182.0 106-9 123.1 0-W

24.6 182.7 107.1 120.7 (TW

0.0 168.2 107.7 120.7 (13;exp.I

3 Al

0.0

26.4

'Al 0.0

25.1

163.81 107.32 116.49 (5;TwI

161.08 109.25 116.87 (12;exp.I

179.9 106.9 122.9

180.7 107.2 120-l

180.2 107.1 121.7

168.3 108.2 119.6

168.2 108.1 119.5

189.8 106.8 123.6 (TW)

190.7 107.0 120.9 (TWI

190.8 106.8 123.9 (Twl

191.5 107.0 121.6 UW

('J='W

('J-WI

(5;UHF)

(1l;exp.I

(13;exp.I

TW= This work.

R(C-H) increases with optimization at the CISD level (see Table 2).

The increase is remarkably constant however,

lying in the range 1.20 - 1.32 pm. The CISD

length is thus still 0.7 pm shorter than the

the ground state. For the3A2 state the CISD

for R(C-H) are essentially equal.

C-S bond length (R(C-S))

The SCF optimized value for R(C-S) in the ground state of thio-

formaldehyde is 2.8 pm or 1.7 3 longer than the experimental value.

When the usual reserves considering experimental error and compari-

son of computed and experimental bond lengths are taken into account

the five increases

optimized C-H bond

experimental one for

and experimental values

18

TABLE 2

CISD relative energies (kJ mol -1)a and CISD optimized values for

the angles (") and bond lengths (pm) in thioformaldehyde.

STATE ENERGYa LOOP R(C-S) R(C-H) LHCH

1 A, (ground) 0.0 0.0 166-8 108.6 116.8

3A2 151.67 0.0 180.3 108.2 122.7

151.44 19.8 180.7 108.3 127.2

3 Al 206.94 0.0 193.0 108.1 123.4

206.88 13.4 193.2 108.2 122.4

a. Relative to -434.500937 hartree and computed with the CISD/3-21G optimized geometries.

this agreement is quite satisfactory. Optimization at the CISD

level causes R(C-S) to increase by 3 pm however and one would have

preferred a value slightly less than the experimental at the SCF

level.

For the 3 A2 and 'A2 states, the experimental R(C-S) values are

almost equal and are just over 7 pm longer than the distance in

the ground state. The SCF optimized values are much longer, how-

ever, being 12 and 14 pm longer than the experimental ones in the 3 A2 and 'A2 states, respectively. The UHF-SCF/3-21G result obtain-

ed by Goddard (ref.5) for R(C-S) in the 3 A2 state of thioformalde-

hyde is thus shown to apply to the RIB?-SCF/3-21G optimization.

Surprisingly, CISD optimization of R(C-S) in the 3 A2 state causes

it to increase by only 0.5 pm over the SCF optimized value.

For the 3 A, and 'A, (lowest excited) states there seems to be

no experimental structural data available. The present SCF opti-

mizations indicate that R(C-S) should be about 9 pm longer than

those in the 3 A2 and 'A2 states. Considering the disagreement be-

tween the experimental and computed R(C-S) values for the A2 states,

"reasonable" values for R(C-S) in the open-shell A, states should

lie in the range 175 f 5 pm. (Note that these values are smaller

than that, 216.9 pm, used by Burton et al. (ref.14) for their ex-

tensive MRD CI calculations with a large AC basis (77 functions

19

TABLE 3

Energies relative to that computed for the planar ground state.

The superscript 1 refers to the SCF optimized geometry, the super-

script 2 to the CISD optimized one. (Energies in kJ mol-'-1

STATE LOOP SCF' CISD' CISDQ' SCF2 CISD2 CISDQ'

3A2 0.0 71-9 150.05 163.57 70.2 151.67 166.58

26.1a 70.5 150.05 163.63 69.1 151.45 166.42

'A2 0.0 85.4

24.6 83.9

3 A1 0.0 117.5 206.2 221.4 116.8 206.9 222.9

26.5b 115.9 206.6 222.1 116.2 206.9 223.0

'Al 0.0 126.1 -

25.1 125.0

a and b, The angle is 19.8 and 13.4", respectively, for the CISD optimized geometri_es_

including polarization, bond and Rydberg functions) on the 'Al (ie.

B(lr * IT*)) excited state.) In contrast to the 3 A2 results,

R(C-S) in the 3 A, state increases at the CISD level by .% 1.5 %

over the SCF optimized value.

HCB out-of-plane angle (LOOP) and LHCH

In agreement with experiment, SCF optimizations predict that

the ground state of thioformaldehyde is planar and the 3A2 state

non-planar. LOOP is predicted to be 26.1" in the latter, compared

to experimental values of 16.0 and 11.9O. The agreement is impro-

ved slightly by CISD optimization which reduces the predicted

angle to 19.8O. For the 'A2

state, experiment yields a planar

structure whereas the SCF optimization predicts a shallow minimum

'( % 1 kJ mol-') at 24.6O. For the 3 A, state the SCF optimization

predicts a minimum for an OOP angle of 26.5O. Optimization at the

CISD level reduces the angle to 13.4".

The HCH angle is almost the same in the SCF and CISD optimized

20

structures. The optimized values, in agreement with experiment,

open up by a -few degrees in going from the ground to one of the

excited states.

Relative energies

The relative energies for the CISD optimizations (Table 3) show

very small differences between the planar and non-planar structures

in both the 3 A2 and 3A, states. The relative CISD energies are

almost twice the relative SCF energies_ It is

with the 3-21G basis, optimization at the CISD

effect on the CISD and CISDQ energies compared

ed with the CI on the SCF optimized geometry.

also apparent that

level has very little

to the values obtain-

REFERENCES

1

2

3

4

9

10

11

12

13

14

D.R. Johnson and F.X. Powell, 169 (1970) 679-680.

For examples sse (a) M. Kawasaki, K. Kasatani, Y. Ogawa and H. Sato, Chem. Physics, 74 (1983) 83-88; (b) T. Suzuki, S. Saito and E. Hirota, J. Chem. Phys., 79 (1983) 1641-1657; (c) R.N. Dixon and M.R. Gunson, Chem. Phys. Letters, 104 (1984) 418-423; (d) A.E. Bruno and R.P. Steer, J. Chem. Phys., 78 (1983) 6660- 6665.

D.J. Clouthier and D.A. Ramsay, Ann. Rev. Phys. Chem. 34 (1483) 31-58.

P.J. Bruna, S.D. Peyerimhoff, R.J. Buenker and P. Rosmus, Chem. Physics, 3 (1974) 35-53.

J.D. Goddard, Can. J. Chem., 59 (1981) 3200-3203.

O.P. Strausz, private communication.

G.R. De Mare, Bull. Chem. Sot. Belges, 92 (1983) 553-554.

M.R. Peterson, G.R. De Mare, 1-G. Csizmadia and 0-P. Strausz, J. Mol. Struct. Theochem, 92 (1983) 239-253:

N.C. Handy, J.D. Goddard and H.F. Schaefer III, J. Chem. Phys., 71 (1979) 426-435.

E.R. Davidson, in R. Daudel and B. Pullman (Eds.), The World of Quantum Chemistry, Reidel, Dordrecht, Holland (1974).

R.H. Judge, D.C_ Moule and G.W. King, J. Mol. Spectrosc., 81 (1980) 37-59.

D.R. Johnson, F.X. Powell and W.H. Kirchhoff, J. Mol. Spectrosc., 39 (1971) 136-145.

P. Jensen and P.R. Bunker, J. Mol. Spectrosc., 95 (1982) 92-100.

P.G. Burton, S.D. Peyerimhoff and R.J. Buenker, Chem. Physics, 73 (1982) 83-98.