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The structure and properties of carbonyl compounds
219 Journal of Molecular
Srrucmwe, 18 (1973)
@ Eisevier Scientific Publishing
THE
STRUCTURE
J. T. GLEGEORN Department
(Received
AND
AND
F. W.
of Chemistry,
6 February
Company,
219-225 Amsterdam
PROPERTIES
- Printed in The Netherlands
OF
CARBONYL
COMPOUNDS
MCCONKEY
University
of Luncaster,
Bail&g,
Lancaster
(England)
1973)
ABSTRACT
The (planar) minimum energy configurations of cis and trans glyoxal, and of parabenzoquinone, have been determined using the MIND0/2 method. Good agreement with the experimental structures is obtained. Other molecular properties for these systems are reported_
INTRODUCTION
It is now possible to calculate molecular structures by minimisation of the total energy function with respect to variation of the geometric parameters. Whilst this is a lengthy process in the “ab initio” field the use of semi-empirical MO theories makes such a procedure not overly time consuming. In particular of heats of formation, the MIND0/2 methodI* ’ is parametrised togivegoodvalues and may therefore be preferred over other simple semi-empirical methods. The MINDO/2 method has been applied to the structure determination of cis and trans glyoxal, and of parabenzoquinone, and the molecular properties of the equilibrium geometries calculated.
METHODS
The M tND0/2 programme was obtained by modification of the “Quantum version of MINDO/l. A geometry was chosen Chemistry Program Exchange” as a &arting point, and displacements along symmetry coordinates provided the data for the minimisation method. The method used was the- one dimensionai search method -of-Powell3 (using_no derivatives of the energy function), which was found to be preferable-over the sirppiex method4. The convergence criterion-
220 used within-the MENDO/
method was 10-’ eV, which was used in conjunction with geometric accuracy of 0.0001 A in bond lengths and 0.001 radians in bond angles. With parabenzoquinone the geometry optimisation was done in stages, with gradual refinement of the geometric accuracy required. The optimisation sequence was fully computerised, requiring only an-initial geometry. A very large number of function evaluations was required, but computing time was minimised by storing the eigenvector matrix of the SCF solution of the previous calculation, and using that as an initial solution for the next geometry. TypicalIy this reduced a 15 iteration SCF calculation to one involving 5, or even less, iterations.
RESULTS
The structures are best presented as a series of diagrams, with the bond distances and angles written in the appropriate places (structures 1 to 3). These structures may be compared with the experimental structures, obtained in the gas phase for the glyoxals 5*6 but in the crystalline state for parabenzoquinone’ (structures 4 to 6).
No accurate gas-phase structure of parabenzoquinone has been reported, since the electron diffraction study of Swingle 8. Trotter’s7 structure (naturally) includes no C-H data. Only two geometric variables were reported for cis glyoxal. The calculated and observed energies and heats of formation are entered in Table 1; the charge distributions are displayed in Table 2. The only molecule of the three to have a permanent dipole moment is cis glyoxal; a value of- 4r30 D was calculked using the above zero differential overlao (ZDO) charge distribution.
221
0
0
(6)
TABLE 1 ENERGY
DATA
Compound
Total c;alence shell energy
tram glyoxal cis glyoxal parabenzoquinone
(ev)
-912.9866 -913.0022 - 1457.6687
A HrO 298 (kcal moleCalc. Obs.
-58.52 -58.89 -51.98
‘)
-50.66&0.19 (ref. 9) -28.3 (ref. 10) -44.65 (ref. 11)
TABLE 2 CHARGE
DISTRIBUTIONS
Compound
Atom
o=
Jn?
Total
charge
charge
net charge
MINDOj2”
Ab initio’ 2
CO.516 -0.483 -0.032
-!-0.217 -0.477 to.260
trans glyoxal
C 0 H
2.799 5.168 1.032
0.685 1.315 -
cis glyoxal
C 0 H
2.780 5.145 1.075
0.704
+0.516
f-O.216
1.296 -
-0.441 -0.075
- 0.452 f0.236
0 C1 CZ H
5.168 2.729 3.083 0.969
1.357 0.696 0.973 -
-0.525 to.575 -0.056 f-o.03 1
parabenzoquinoneb
&‘Charges computed on the ZDO bask b_The numbering of the carbons is indicated in structure (6)
-
222 TABLE ENERGY
3 LEVELS
OF THE TWO
GLYOXAL
Truns glyoxal Symmetry
SPECIES
Cis glyoxal
Energy (e V)
Symmetry
-41.187
-41.206
-37.957
-37.506 -25.201 -18.192
-23.292 -20.454 - 15.959 -15.185 -
TABLE
4
ENERGY
LEVELS
Energy (e V)
-17.332 - 15.160 - 13.946 - 23.938
14.748 13.501
-
13.373
-
12.262 10.610 1.264
-
13.556 12.082 10.592 1.261
f
0.076
+
0.127
OF PARABENZOQUINON~
Symmetry
Energy
4u, 463,
(G) (0)
5u* 2&, 563. 26*,
(e V)
Symnzmy
Energy
-41.679 - 39.228
462” 162,
(a) Gd
-14.152 -14.011
(0)
-33.479
361,
(0)
-13.125
(c) (0) Co)
-30.866 -23.424 -21.691
763” ga, I&,
(G) (0) (-q)
- 12.672 - 11.829 -11.475
6%
t=)
-20.228
261,
@)
-11.162
lb*.
CZ)
-
15.362
562”
(fl, n)
-10.576
362,
(0)
-
15.326
461,
(g, n)
-
7rr,
CO)
-
15.273
262,
w*)
-
1.869
6b,,
(0)
-
15.238
la,
(;G*)
-
0.068
a The 1s orbitals of carbon and one bl, orbital. Hence
(ev)
9.897
and oxygen give rise to three a, and three bsu orbitak, and one bzu the sequence formed from the valence shell orbitals only starts with
the 40. orbital.
The eigenvalues of the Fock matrix, together with their symmetry classifications, are displayed in Tables 3 and 4. The symmetry subscripts are according to the tables of Jaf% and 0rchin13.
DISCUS!XON (A)
Parabenzoquinone (PBQ)
The calculated strixture of -PBQ is in good agreement with the crystal structure of Trotter’, which structure is in turn contained among the possible
223
(7) Acrolein
structures determined by Swingle’. The bond localisation is well accounted for. Accurate experimental data on the C-H bond length are unfortunately lacking. There appears to be a remarkable agreement in the bond lengths for acroleir? and those for PBQ (see structure 7), as remarked by Cyvin et al.14. However the ring angle at the carbonyl group in PBQ is calculated to be some 6” larger than observed. This may partly account for the discrepancy in the heat of formation, as an underestimation of the interaction between the two “vinyl” groups. The experimental assignment of the photoelectron spectrum15 is still uncertain, although the position of the b,, lone pair orbital seems firm. The use of the petiuoro effect was unable to establish the relative ordering of the remaining n orbital (b2J, and the b,, and big x orbitais. These workersI described four ionisations (at 10.11, 10.41, 11.06 and 11.25 eV) as being from the n and 7~orbitals; these values certainly are well reproduced by MlNDO/2 (9.90, 10.58, 11.16 and 11.47 eV) although the experimental ionisation energies are by no means unambiguously determined. This theoretical ordering of the upper orbitals (see Table 4) is reproduced by the CNDO/CI calculations of Stevenson”. Agreement with respect to both ener,T and ordering is poor below these, however. The symmetries of the lowest antibonding z* orbitals give direct products with the two n orbital symmetry species which correspond with the observed spectral state symmetriesl’, ‘A, and lBjg (n* + n), the uppermost n orbital giving rise to the lower energy ‘BBg state. Similarly the symmetries involved in the four possible IL* t z transitions (arising from the two highest rc levels, and the two lowest X* levels) give rise to those states which are experimentally observed (ref. 18) (lBl,, and ‘B1,). So whilst the ordering of the top four orbitals may be stiil an open question, the symmetries of those orbitals seem to be clear. The calculated heat of formation appears to be too negative .by some 24 kcal mole-‘, by comparison with the gas phase data of Magnus” (-28.3 kcal mole- ‘)_ More reliable data exist for the solid phase’ ’ (-44.65 kcal mole-‘), which again indicates that the MIND0/2 value is much too negative. (B) Tram and cis glyoxal The calculated structures differ but little in their bond lengths, although , some angular changes are apparent. Both structures show the lengtheningof the
224
C-H bond characteristic of aldehyde groups* but do not show the long carboncarbon bond length determined experimentally for the trans isomer’. Although some extension has taken place over the normal sp2-sp2 bond length5 (1.463) as in butadiene. The estimated value for the cis C-C bond length6 (1.505) is more closely reproduced, as is the CC0 bond angle (talc. 125.6”, expt. 123-9”). Both structures are for the planar molecules, although Currie and Ramsay have suggested that the cis form need not in fact be planarlg. MINDO/2 overestimates the stability of the cis form, which is predicted to be 0.37 kcal more stable than the trans form. Experimentally1q the energy difference is 3.2kO.3 kcal mole-‘, with the trans isomer more stable. Ab initio calculations’ 2*2o perform better in this respect, although the difference is overestimated. The photoelectron spectrum of trans glyoxal” shows bands at ca. 10.6 (10.3-11.0, broad), 12.19 (sharp), and 13.85 eV (sharp). Turner et al.” assign the ionisations as from the n (7a,), n (6b,) and z (lb,) orbitals respectively, using the formaldehyde spectrum to separate the rr-ionisation from the Ione pair ionisations. The appearance of the two “lone pair combinations” above the rr orbitals is confirmed by this work, although both the ab initio calculations*2* 2o give the b, r orbital above the second n, G orbital. The visible spectrum of trans glyoxal shows a band at 455.0 nm, which has been assigned A, symmetry, since it is allowed” (g t, u). This symmetry is expected on the basis of promotion from the highest n, cr orbital to the lowest rr* orbital. The other X* c n transition (giving a B, state) is forbidden (g +I+g)_ These observations are not inconsistent with the present level sequence. Cis glyoxal has recently been detected spectroscopically’g through an absorption at 487.5 nm, which transition has been assigned as n* t n, with an upper state of B1 symmetry (in C,, symmetry). This too is in accord with the level sequence calculated. The computed atomic charge densities of both glyoxals agree well with the ab initio results, if the difference between the hydrogen charges (MIND0/2ab initio) is added to rhe carbon charge in each case. The absence of polarisation functions in the basis set of Pincelli et al. l2 leads to an overestimation of the electron withdrawing power of the carbon atoms, resulting in too large a charge separation, so that some justification for the above somewhat arbitrary procedure may be adduced. Correction of the MIND0/2 charge distribution to an overlapping basis gives little change in the magnitudes involved, and did not seem to be worth while. The calculated dipoIe moment of cis glyoxal(4.3 D) is close to the experimental value (4.8t0.2 D), as determined from the Stark effect on the microwave spectrum6. Note that the 0.1 A added to C-H removed.
l
bond leqths1*2
in the MINDO/Z
method
has already been
225 ACKNOWLEDGEMENTS
We thank the University of Lancaster for a grant to F. W. McC., and the staff of the computing centres at Lancaster and at the Regional Centre, Manchester.
REFERENCES 1 M. J. S. DEWAR AND E. HASELBACH, J. Amer. Chem. Sot., 92 (1970) 590. 2 N. BODOR, M. J. S. DEWAR, A. HARGET AND E. HASELBACH,J. Amer. Chem. Sot., 92 (1970) 3854. 3 M. J. POWELL, Computer J., 7 (1964) 155. 4 J. A. NELDER AND R. MEAD, Computer J., 7 (1964) 308. 5 K. KUCHITSU, T. FUKWAMA AND Y. MORINO, J. Mol. Structure, 1 (1967) 463; 4 (1969) 41. 6 J. R. DURIG, C. C. TONG AND Y. S. Lr, J. Chem. Phys., 57 (1972) 4425. 7 J. TRO~I-ER, Actu Crystailogr., I3 (1960) 86. 8 S. M. SHINGLE, J. Amer. CJlem. Sue.. 76 (1954) 1409. 9 R. A. FLETCHERAND G. PILCHER, Trans. Faraday Sue., 66 (1970) 794. IO A. MAGNUS, Z. ?%ys. Chem. (Frankfurt am Main), 9 (1956) 141. I I G. PILCHER AND L. E. SV~~ON, J. Chem. So-, (1956) 2695. I2 IJ. PINCELLI, B. CADIOLI AND D. J. DAVID, J. Mol. Structure, 9 (1971) 173. 13 H. H. JAFFEAND M. ORCHIN, Symmetry in Chemistry, Wiley, London, 1965. I4 H. H. JENSEN, G. HACEN AND S. J. CYVIN, J. Mol. Structure, 4 (1969) 5 I _ I5 D. W. TURNER, C. BAKER, A. D. BAKER AND C. R. BRUNDLE, MolecrrlarPhotoelectron Spectroscopy, Wiley, London, 1970. I6 C. R. BRUNDLE, M. B. ROBIN AND N. A. KUEBLER, J. Amer. Gem. Sot., 94 (1972) 1466. I7 P. E. STEVENSON, J. Phys. Chem., 76 (1972) 2424. 18 H. P. TROMMSDORF, J. Chem. Phys_, 56 (1972) 5358. I9 G. N. CURRIE AND D. A. RAMSAY, Can. J. Phys_, 49 (1971) 3 17. 20 T. K. HA, J. Mol. Structure, I2 (1972) I71_ 3 I J. PALDUS AND D. A. RAMSAY, Can. J. Ph_vs.. 45 (1967) 1389.
Srrucmwe, 18 (1973)
@ Eisevier Scientific Publishing
THE
STRUCTURE
J. T. GLEGEORN Department
(Received
AND
AND
F. W.
of Chemistry,
6 February
Company,
219-225 Amsterdam
PROPERTIES
- Printed in The Netherlands
OF
CARBONYL
COMPOUNDS
MCCONKEY
University
of Luncaster,
Bail&g,
Lancaster
(England)
1973)
ABSTRACT
The (planar) minimum energy configurations of cis and trans glyoxal, and of parabenzoquinone, have been determined using the MIND0/2 method. Good agreement with the experimental structures is obtained. Other molecular properties for these systems are reported_
INTRODUCTION
It is now possible to calculate molecular structures by minimisation of the total energy function with respect to variation of the geometric parameters. Whilst this is a lengthy process in the “ab initio” field the use of semi-empirical MO theories makes such a procedure not overly time consuming. In particular of heats of formation, the MIND0/2 methodI* ’ is parametrised togivegoodvalues and may therefore be preferred over other simple semi-empirical methods. The MINDO/2 method has been applied to the structure determination of cis and trans glyoxal, and of parabenzoquinone, and the molecular properties of the equilibrium geometries calculated.
METHODS
The M tND0/2 programme was obtained by modification of the “Quantum version of MINDO/l. A geometry was chosen Chemistry Program Exchange” as a &arting point, and displacements along symmetry coordinates provided the data for the minimisation method. The method used was the- one dimensionai search method -of-Powell3 (using_no derivatives of the energy function), which was found to be preferable-over the sirppiex method4. The convergence criterion-
220 used within-the MENDO/
method was 10-’ eV, which was used in conjunction with geometric accuracy of 0.0001 A in bond lengths and 0.001 radians in bond angles. With parabenzoquinone the geometry optimisation was done in stages, with gradual refinement of the geometric accuracy required. The optimisation sequence was fully computerised, requiring only an-initial geometry. A very large number of function evaluations was required, but computing time was minimised by storing the eigenvector matrix of the SCF solution of the previous calculation, and using that as an initial solution for the next geometry. TypicalIy this reduced a 15 iteration SCF calculation to one involving 5, or even less, iterations.
RESULTS
The structures are best presented as a series of diagrams, with the bond distances and angles written in the appropriate places (structures 1 to 3). These structures may be compared with the experimental structures, obtained in the gas phase for the glyoxals 5*6 but in the crystalline state for parabenzoquinone’ (structures 4 to 6).
No accurate gas-phase structure of parabenzoquinone has been reported, since the electron diffraction study of Swingle 8. Trotter’s7 structure (naturally) includes no C-H data. Only two geometric variables were reported for cis glyoxal. The calculated and observed energies and heats of formation are entered in Table 1; the charge distributions are displayed in Table 2. The only molecule of the three to have a permanent dipole moment is cis glyoxal; a value of- 4r30 D was calculked using the above zero differential overlao (ZDO) charge distribution.
221
0
0
(6)
TABLE 1 ENERGY
DATA
Compound
Total c;alence shell energy
tram glyoxal cis glyoxal parabenzoquinone
(ev)
-912.9866 -913.0022 - 1457.6687
A HrO 298 (kcal moleCalc. Obs.
-58.52 -58.89 -51.98
‘)
-50.66&0.19 (ref. 9) -28.3 (ref. 10) -44.65 (ref. 11)
TABLE 2 CHARGE
DISTRIBUTIONS
Compound
Atom
o=
Jn?
Total
charge
charge
net charge
MINDOj2”
Ab initio’ 2
CO.516 -0.483 -0.032
-!-0.217 -0.477 to.260
trans glyoxal
C 0 H
2.799 5.168 1.032
0.685 1.315 -
cis glyoxal
C 0 H
2.780 5.145 1.075
0.704
+0.516
f-O.216
1.296 -
-0.441 -0.075
- 0.452 f0.236
0 C1 CZ H
5.168 2.729 3.083 0.969
1.357 0.696 0.973 -
-0.525 to.575 -0.056 f-o.03 1
parabenzoquinoneb
&‘Charges computed on the ZDO bask b_The numbering of the carbons is indicated in structure (6)
-
222 TABLE ENERGY
3 LEVELS
OF THE TWO
GLYOXAL
Truns glyoxal Symmetry
SPECIES
Cis glyoxal
Energy (e V)
Symmetry
-41.187
-41.206
-37.957
-37.506 -25.201 -18.192
-23.292 -20.454 - 15.959 -15.185 -
TABLE
4
ENERGY
LEVELS
Energy (e V)
-17.332 - 15.160 - 13.946 - 23.938
14.748 13.501
-
13.373
-
12.262 10.610 1.264
-
13.556 12.082 10.592 1.261
f
0.076
+
0.127
OF PARABENZOQUINON~
Symmetry
Energy
4u, 463,
(G) (0)
5u* 2&, 563. 26*,
(e V)
Symnzmy
Energy
-41.679 - 39.228
462” 162,
(a) Gd
-14.152 -14.011
(0)
-33.479
361,
(0)
-13.125
(c) (0) Co)
-30.866 -23.424 -21.691
763” ga, I&,
(G) (0) (-q)
- 12.672 - 11.829 -11.475
6%
t=)
-20.228
261,
@)
-11.162
lb*.
CZ)
-
15.362
562”
(fl, n)
-10.576
362,
(0)
-
15.326
461,
(g, n)
-
7rr,
CO)
-
15.273
262,
w*)
-
1.869
6b,,
(0)
-
15.238
la,
(;G*)
-
0.068
a The 1s orbitals of carbon and one bl, orbital. Hence
(ev)
9.897
and oxygen give rise to three a, and three bsu orbitak, and one bzu the sequence formed from the valence shell orbitals only starts with
the 40. orbital.
The eigenvalues of the Fock matrix, together with their symmetry classifications, are displayed in Tables 3 and 4. The symmetry subscripts are according to the tables of Jaf% and 0rchin13.
DISCUS!XON (A)
Parabenzoquinone (PBQ)
The calculated strixture of -PBQ is in good agreement with the crystal structure of Trotter’, which structure is in turn contained among the possible
223
(7) Acrolein
structures determined by Swingle’. The bond localisation is well accounted for. Accurate experimental data on the C-H bond length are unfortunately lacking. There appears to be a remarkable agreement in the bond lengths for acroleir? and those for PBQ (see structure 7), as remarked by Cyvin et al.14. However the ring angle at the carbonyl group in PBQ is calculated to be some 6” larger than observed. This may partly account for the discrepancy in the heat of formation, as an underestimation of the interaction between the two “vinyl” groups. The experimental assignment of the photoelectron spectrum15 is still uncertain, although the position of the b,, lone pair orbital seems firm. The use of the petiuoro effect was unable to establish the relative ordering of the remaining n orbital (b2J, and the b,, and big x orbitais. These workersI described four ionisations (at 10.11, 10.41, 11.06 and 11.25 eV) as being from the n and 7~orbitals; these values certainly are well reproduced by MlNDO/2 (9.90, 10.58, 11.16 and 11.47 eV) although the experimental ionisation energies are by no means unambiguously determined. This theoretical ordering of the upper orbitals (see Table 4) is reproduced by the CNDO/CI calculations of Stevenson”. Agreement with respect to both ener,T and ordering is poor below these, however. The symmetries of the lowest antibonding z* orbitals give direct products with the two n orbital symmetry species which correspond with the observed spectral state symmetriesl’, ‘A, and lBjg (n* + n), the uppermost n orbital giving rise to the lower energy ‘BBg state. Similarly the symmetries involved in the four possible IL* t z transitions (arising from the two highest rc levels, and the two lowest X* levels) give rise to those states which are experimentally observed (ref. 18) (lBl,, and ‘B1,). So whilst the ordering of the top four orbitals may be stiil an open question, the symmetries of those orbitals seem to be clear. The calculated heat of formation appears to be too negative .by some 24 kcal mole-‘, by comparison with the gas phase data of Magnus” (-28.3 kcal mole- ‘)_ More reliable data exist for the solid phase’ ’ (-44.65 kcal mole-‘), which again indicates that the MIND0/2 value is much too negative. (B) Tram and cis glyoxal The calculated structures differ but little in their bond lengths, although , some angular changes are apparent. Both structures show the lengtheningof the
224
C-H bond characteristic of aldehyde groups* but do not show the long carboncarbon bond length determined experimentally for the trans isomer’. Although some extension has taken place over the normal sp2-sp2 bond length5 (1.463) as in butadiene. The estimated value for the cis C-C bond length6 (1.505) is more closely reproduced, as is the CC0 bond angle (talc. 125.6”, expt. 123-9”). Both structures are for the planar molecules, although Currie and Ramsay have suggested that the cis form need not in fact be planarlg. MINDO/2 overestimates the stability of the cis form, which is predicted to be 0.37 kcal more stable than the trans form. Experimentally1q the energy difference is 3.2kO.3 kcal mole-‘, with the trans isomer more stable. Ab initio calculations’ 2*2o perform better in this respect, although the difference is overestimated. The photoelectron spectrum of trans glyoxal” shows bands at ca. 10.6 (10.3-11.0, broad), 12.19 (sharp), and 13.85 eV (sharp). Turner et al.” assign the ionisations as from the n (7a,), n (6b,) and z (lb,) orbitals respectively, using the formaldehyde spectrum to separate the rr-ionisation from the Ione pair ionisations. The appearance of the two “lone pair combinations” above the rr orbitals is confirmed by this work, although both the ab initio calculations*2* 2o give the b, r orbital above the second n, G orbital. The visible spectrum of trans glyoxal shows a band at 455.0 nm, which has been assigned A, symmetry, since it is allowed” (g t, u). This symmetry is expected on the basis of promotion from the highest n, cr orbital to the lowest rr* orbital. The other X* c n transition (giving a B, state) is forbidden (g +I+g)_ These observations are not inconsistent with the present level sequence. Cis glyoxal has recently been detected spectroscopically’g through an absorption at 487.5 nm, which transition has been assigned as n* t n, with an upper state of B1 symmetry (in C,, symmetry). This too is in accord with the level sequence calculated. The computed atomic charge densities of both glyoxals agree well with the ab initio results, if the difference between the hydrogen charges (MIND0/2ab initio) is added to rhe carbon charge in each case. The absence of polarisation functions in the basis set of Pincelli et al. l2 leads to an overestimation of the electron withdrawing power of the carbon atoms, resulting in too large a charge separation, so that some justification for the above somewhat arbitrary procedure may be adduced. Correction of the MIND0/2 charge distribution to an overlapping basis gives little change in the magnitudes involved, and did not seem to be worth while. The calculated dipoIe moment of cis glyoxal(4.3 D) is close to the experimental value (4.8t0.2 D), as determined from the Stark effect on the microwave spectrum6. Note that the 0.1 A added to C-H removed.
l
bond leqths1*2
in the MINDO/Z
method
has already been
225 ACKNOWLEDGEMENTS
We thank the University of Lancaster for a grant to F. W. McC., and the staff of the computing centres at Lancaster and at the Regional Centre, Manchester.
REFERENCES 1 M. J. S. DEWAR AND E. HASELBACH, J. Amer. Chem. Sot., 92 (1970) 590. 2 N. BODOR, M. J. S. DEWAR, A. HARGET AND E. HASELBACH,J. Amer. Chem. Sot., 92 (1970) 3854. 3 M. J. POWELL, Computer J., 7 (1964) 155. 4 J. A. NELDER AND R. MEAD, Computer J., 7 (1964) 308. 5 K. KUCHITSU, T. FUKWAMA AND Y. MORINO, J. Mol. Structure, 1 (1967) 463; 4 (1969) 41. 6 J. R. DURIG, C. C. TONG AND Y. S. Lr, J. Chem. Phys., 57 (1972) 4425. 7 J. TRO~I-ER, Actu Crystailogr., I3 (1960) 86. 8 S. M. SHINGLE, J. Amer. CJlem. Sue.. 76 (1954) 1409. 9 R. A. FLETCHERAND G. PILCHER, Trans. Faraday Sue., 66 (1970) 794. IO A. MAGNUS, Z. ?%ys. Chem. (Frankfurt am Main), 9 (1956) 141. I I G. PILCHER AND L. E. SV~~ON, J. Chem. So-, (1956) 2695. I2 IJ. PINCELLI, B. CADIOLI AND D. J. DAVID, J. Mol. Structure, 9 (1971) 173. 13 H. H. JAFFEAND M. ORCHIN, Symmetry in Chemistry, Wiley, London, 1965. I4 H. H. JENSEN, G. HACEN AND S. J. CYVIN, J. Mol. Structure, 4 (1969) 5 I _ I5 D. W. TURNER, C. BAKER, A. D. BAKER AND C. R. BRUNDLE, MolecrrlarPhotoelectron Spectroscopy, Wiley, London, 1970. I6 C. R. BRUNDLE, M. B. ROBIN AND N. A. KUEBLER, J. Amer. Gem. Sot., 94 (1972) 1466. I7 P. E. STEVENSON, J. Phys. Chem., 76 (1972) 2424. 18 H. P. TROMMSDORF, J. Chem. Phys_, 56 (1972) 5358. I9 G. N. CURRIE AND D. A. RAMSAY, Can. J. Phys_, 49 (1971) 3 17. 20 T. K. HA, J. Mol. Structure, I2 (1972) I71_ 3 I J. PALDUS AND D. A. RAMSAY, Can. J. Ph_vs.. 45 (1967) 1389.