Spectra and structure of small ring compounds

Journal of Molecular Structure, 52 (1979) 27-37 o Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

SPECTRA AND STRUCTURE OF SMALL RING COMPOUNDS XL.* Infrared and Raman spectra of cyclobutyhnethylether J. R. DURIG and G. A. GUIRGIS** Department of Chemistry, 29208 (U.S.A.)

University

of South

Mississippi

State

Carolina, Columbia,

South Carolina

V. F. KALASINSKY Department of Chemistry, 39762 (U.S.A.)

University,

Mississippi

State, Mississippi

(Received 7 August 1978)

ABSTRACT The infrared spectra (50-4000 cm-‘) of gaseous and solid cyclobutylmethylether and cyclobutylmethyl-d,-ether and the Raman spectra (25-4000 cm-‘) of gaseous, liquid and solid cyclobutyhnethylether and cyclobutyhnethyld,-ether have been recorded. Depolarization values were measured for both the gaseous and liquid states. Mc& of the 42 fundamental vibrations have been assigned and support for only one molecular configuration (gauche) is presented. The interesting methyl and methoxy torsions were observed at 195 and 101 cm-‘, respectively, for the “light” molecule (155 and 92 cm-’ for C,H,OCD,) and the three-fold barrier to internal rotation of the methyl group was calculated to be 3100 5 50 cal mole-‘. The asymmetric potential function could not be determined because of the limited number of observed transitions. INTRODUCTION

Four-membered ring molecules have one low frequency ring puckering mode and the nature of the potential governing this vibration determines the conformation of the ring. In the early studies [l],far infrared and microwave data were used to determine whether the ring was planar or puckered and the form of the potential function. More recently laser-Raman spectroscopy of molecules in the gas phase has been applied to the study of these anharmanic vibrations [2] . In a number of cases the differences in selection rules between the infrared and Raman transitions have been extremely useful in that they frequently provide a check on the assignments to specific energy levels. Additionally, Raman spectroscopy has recently been applied to the study of internal rotations and has been shown to be extremely useful in assigning the torsional transitions of molecules which contain asymmetric *For part XXXIX, see J. Chem. Phys., 69 (1978) 3714. **Taken in part from the thesis of G. A. Guirgis which will be submitted in partial fulfillment of the Ph.D. degree.

28

internal rotors [ 3-71. Such studies have provided information as to the most stable conformer along with the enthalpy differences between the conformers. Also we have recently shown that Raman spectroscopy can be used to obtain the barriers to internal rotation of methyl rotors [8,9] . The cyclobutylmethylether molecule is particularly interesting since it contains all three of these anharmonic vibrations. Furthermore, the potential functions governing the methoxy torsion and the ring puckering vibration are both expected to be asymmetric. Our interest in these anharmonic vibrations has prompted our investigation of the infrared and Raman spectra of cyclobutylmethylether and cyclobutylmethylðer and the results are reported herein. EXPERIMENTAL

The cyclobutylmethylether and cyclobutylmethyld,-ether used in this study were prepared by the method of Dauben et al. [lo] and were purified by low temperature fractionation using a vacuum sublimation column [ 1 l] . Raman spectra were recorded using a Cary Model 82 Raman spectrophotometer equipped with a Coherent Radiation Model 53 argon-ion laser or a Spectra Physics model 171 argon-ion laser operating on the 5145 W line. The power was estimated to be from 1.5 to 2.0 W at the sample for the liquid or solid phases, whereas for the gas phase studies the maximum power of nearly 4 W was used. The instrument was calibrated with mercury and neon emission lines. The spectrum of the gas was obtained with a standard Cary multipass cell with suitable modification to allow the cell to be sealed [12]. Samples were held at their room temperature vapor pressure (-25 torr). Spectra of the solid phase samples were obtained using a low temperature cell in which the sample holder is a solid brass plate held at an angle of 15” from the normal. Samples were sublimed onto the brass plate held at - -196°C by boiling nitrogen and then annealed until the spectrum showed no changes. The spectra of the liquid phase samples were obtained from samples sealed in glass capillaries. Typical spectra are shown in Figs. land2. The infrared spectra in the 4000-200 cm-’ region were recorded on a Perkin-Elmer model 621 spectrophotometer with an extended source which was purged with dry air and calibrated as described in the literature [13] . For the gas phase studies, the compounds were contained in a 25 cm glass cell equipped with CsI windows. For studies of the solid phase, a vacuum cold cell similar to that described previously [ 141 with outer windows and sample substrate prepared from CsI was used. Representative spectra are shown in Figs. 3 and 4. The far infrared spectra (20-400 cm- ‘) were obtained with a Digilab model FTS15B interferometer using 6.25,12.5 or 25.0 /.LMylar beam splitters. During operation the spectrometer was flushed with dry nitrogen. The gaseous sample was contained in a 12 cm cell equipped with poly-

29

I.!....!....!....!....!....!....1

3ooo2500~l5oolooo500 WAVENUMBER (CM-l)

Fig. 1. Raman spectra of cyclobutylmethyl-d,ether: (A) gas, spectral = 3 cm-‘, (B) liquid, SBW = 3 cm-l, (C) solid, SBW = 2 cm*

band width (SBW)

ethylene windows and the spectrum of the evacuated cell was used as a reference single beam spectrum. Trace amounts of water in the sample were removed by distillation from fresh LiAlH4, but it was nearly impossible to obtain far infrared spectra of the gaseous phase which showed no interfering bands from the pure rotational water spectrum. RESULTS AND ASSIGNMENT

The s-tram cyclobutylmethylether molecule has a plane of symmetry bisecting the four-membered ring and passing through the oxygen atom and can therefore be classified as belonging to the C, point group. On the other hand, if the methyl group carbon atom is off the plane of symmetry then the molecule belongs to the C1 point group. The ring puckering mode arises from the bending motion of the cyclobutyl ring, which can exist in two puckered conformers, i.e. in one conformer the oxygen will be axial to the ring and in the other equatorial. Thus, there is the possibility of the existence of four conformers for cyclobutylmethylether, since both the axial and the

30

Fig. 2. Raman spectra of cyclobutyhnethyl-d,ether: SBW = 3 cm-‘, (C) solid, SBW = 2 cm-‘.

(A) gas, SBW = 3 cm-‘, (B) liquid,

equatorial conformers can exist with the methyl group in the s-tram or the gauche position relative to the a-hydrogen. However, the point group of the molecule does not depend on the conformation of the ring but only on the orientation of the methoxy group. For the s-tram (C,) conformer the 42 normal modes of vibration span the irreducible representations of 25A’ + 17A”. The A’ bands should be polarized in the Raman effect and have A or C or A/C hybrid bandcontour in the infrared spectrum of the gas. The A” modes will be depolarized in the Raman effect and have B-type band contour in the infrared spectrum of the gas. The gauche (C,) molecule has 42 vibrational modes (symmetry species A) which should be all polarized in the Raman effect and show A, B and C or hybrid band contours in the infrared spectrum of the gas. The Raman spectra of both isotopes of cyclobutylmethylether showed only one depolarized band in the liquid phase at approximately 1450 cm- ‘. The spectra of the gas phase showed no B-type band contour and the lack of depolarized Raman bands and the infrared B-type band for the gas phase indicate that the molecules exist predominantly in the gauche form. Furthermore, no bands in the spectrum of the liquid were noted to be absent in the spectrum of the solid, which indicates no high energy conformer was present

31

&

----. i B

4ooo3ooo

2000

l!500

woo

500

WAVENUMBER (CM? Fig. 3. Infrared spectra of cyclobutyhnethyl-d,ether:

(A) gas, (B) solid.

WAVENUMBER K3l-l) Fig. 4. Infrared spectra of cyclobutylmethyl-d&her:

(A) gas, (B) solid.

32

It is felt that the single depolarized Raman band observed must be due to accidental depolarization of a gauche A vibration and not to the presence of the s-tram conformer. Hence the following assignment of the infrared and Raman spectra are made on the basis of the presence of the gauche conformer only. The vibrational assignments for cyclobutylmethylether and cyclobutylmethyl-d,-ether are given in Tables 1 and 2, respectively. Previous investigations [ 15-191 for a large number of four-membered ring molecules facilitate the assignments of all the vibrational modes associated with the ring portion of the molecule. Selective deuteration of the ring positions has established the assignment [ 161 of most of the Cl& motions. This molecule differs from previously studied cyclobutyl compounds because of the presence of a methoxy group which gives rise to methyl and methoxy torsions, ring-O-C bending modes, and the C-O and methyl motions. In the study [ 161 of cyclobutanol both the (Yand p positions were selectively deuterated and it was shown that the -r-CH, stretching modes gave the highest frequencies in this region. Also it was shown that the ol-CH stretch gives a relatively strong band slightly above 2900 cm- *. Utilizing these data, we have proposed assignments for the carbon-hydrogen stretching motions of the “ring hydrogens” in cyclobutylmethylether. The CHJ stretching modes were assigned on the basis of the gas phase Raman bands which disappear with deuteration of the CH3 group. There is some ambiguity in the assignment of the fl-CH2 stretching motions. There are both in-phase and out-of-phase symmetric and antisymmetric fl-CH, stretches but it is not possible to distinguish among these motions. Also, it should be pointed out that the bands in the liquid and solid phases are very broad in this region and it appears that several modes coalesce into one band. For the “heavy” molecule the CD3 stretching vibrations are clearly observed in the infrared spectrum of the gas at 2248, 2189 and 2062 cm-’ with the latter two bands having strong distinct Q branches. The CH2 and CHJ deformations fall in the region 1430-1470 cm-’ and the (w-CHbend gives a distinct strong infrared band at 1358 cm- ‘. This mode gave a very strong infrared band in cyclobutanol [16]. The next strongest infrared band appears at 1136 cm- ’ and it must be associated with the C-0 bond and is thus assigned to the C-O-C antisymmetric stretch. The symmetric motion is assigned to the Raman line at 1031 cm- ‘. The assignment of the CH2 wagging and twisting modes follows directly from those given for the corresponding motions in cyclobutanol. The CH3 rocking modes are assigned to bands at 1228 and 1160 cm- * (frequencies from the infrared spectrum of the solid). The ring deformation can be readily assigned on the basis of previous assignments [ 15-191 for these motions in other four-membered rings. For example, the ring breathing mode appears around 950 cm-’ as a strong Raman line and for cyclobutylmethylether there is a strong Raman line at 949 cm-‘. Similarly, three of the other four ring deformations can be assigned

33 TABLE 1 Observed fundamental frequencies (cm-‘) A

2995 2988 2974 2966 2955 2949 2936 2929 2806 2831 1467 1462 1456 1450 1444 1433 1357 1240 1229 1212 1196 1165 1162 1136 1126 -

s “a ““a w s s sh s s “a ah sh m m ah ah w VW VW VW w MY VW m m

1043 1031 94s 905 -

VW m s m

833 VW 751 610 448 439 261 -

m w m m VW

107 VW 100 VW

Assignments

Infrared

Raman Gas

for cyclobutylmethyletherdOa

Liquid

Solid

2983 s 2983 s 2971 vs -

2983 2983 2974 2965 2943 2943 2935 2920 2871 2822 1467 1467 1446 1444 1444 1433 1356 122s 1229 1215 1196 1165 1163 1128 1122 1097 1043 1024 946 907 894 857 833 787 751 611 453 442 272 -

2943 vs 2943 “S 2935 ah 2918 “s 2873 s 2823 “s 1467 sh 1462 sh 1445 m 1444 sh 1444 ah 1435 sh 1355 w 1228 w 1228 w 1215 VW 1193 w 1165 VW 1162 VW 1128 sh 1121 m 1097 VW 1042 “w 1025 m 943 s 906 m 894 sb 852 VW 833 VW 783 VW 751 m 615~ 453 sh 443 m 266 w -

Solid

GaS s s “S ah “.? “S ah s s s m sh m sh ah sh w w w VW w VW VW ab m VW VW m s m sh VW VW w m w ah m w

2994

“S

2986 8 2976 “s 2970 sh 2946 “a 2930 2865 2832 1469 1462 1452 1445 1445 -

m s s m m m sh sh

1358 1244 1226 1218 1198 1165 1163 1136 1130 -

s m sh VW w w w s sh

1042 1025 943 903 -

ah ah s m

830 VW 751 612 455 442 263 193 106 100

m w w w w w VW VW

2986 s 2986 s 2976 s 2940 2940 2940 2920 2874 2824 1467 1462 1450 1444 1444 -

s s s m s s m m m sh sh

1355 1239 1228 1214 1196 1160 1160 1126 1120 1096 1042 1024 946 906 696 659 834 786 752 612 458 445 276 210 172 140

m m m VW m m.b m.b s sh m m m s m VW VW w VW m m w m w VW VW w

CH, antisymmetric stretch yCH, antisymmetric stretch ,y-CH, symmetric stretch &CH, antisymmetric stretch CH, antisymmetric stretch @X-I, symmetric stretch &CH, antisymmetric stretch c&I-I stretch @CH, symmetric stretch CH, symmetric stretch CH, antisymmetric deformation r-CH, deformation CH, antisymmetric deformation &CH, deformation (in phase) CH, symmetric deformation P-CH, deformation (out of phase) LY-CHbend y-CH, wagging CH, rocking &CH, wagging (out of phase) P-CH, WaeginB ,y-CH, twist CH, rocking O-C stretch Ring deformation @CH, twist (out of phase) P_CH,twist (in phase) C-O stretch Ring breathing Ring deformation Ring deformation o-CH bend (out of plane) &CH, rocking (in phase) &CH, rocking (out of phase) Ring deformation y-CH, rocking Ring-O< bend C-O-C bend Ring-O--C bend Methyl torsion Ring puckering Methoxy torsion

aAbbreviations used: m, medium; s, strong; w, weak;v, very; sh, shoulder; b, broad.

to pronounced Raman lines at 1126,905 and 751 cm-‘. The other ring deformation is assigned tentatively to a rather weak infrared band at 896 cm-‘. The CH2 rocking modes are usually rather weak and these motions are assigned to bands at 834,786 and 612 cm-‘. The c&H bend perpendicular to the ring plane is assigned to the infrared band at 859 cm- ’ . The three C-C-C bending motions can be assigned to Raman bands at 448,439 and 261 cm-‘. The only modes remaining to be assigned are the methyl and methoxy torsions and the ring puckering vibration. From previous experiences [15-191 with similar four-membered ring compounds we expect the ring puckering

34 TABLE 2 Observed fundamental frequencies (cm-‘) A

Ramall

Infrared

GaS

Liquid

Solid

2250 m 298.9 VI 2977 ws 2962 w 2188 w 2945 “S 2934 sh 2925 s 2885 s 2061 vs 1064 VW 1463 sh 1064 VW 1450 m 1165 VW 1436 m 1373 w -

2245 2983 2969 2960 2188 2938 2935 2914 2874 2055 1058 1462 1058 1443 1158 1433 1368 1240 996 1220 1197 1158 965 1158 1127 -

m s vs sh m vs sb s s vs w sb w m w sh w VW m VW w w m w m

1036 1025 936 904 895 863 825 -

w m m m sh VW VW

2245 2983 2970 2955 2189 2939 2935 2914 2869 2056 1058 1463 1058 1444 1165 1436 1370 1239 995 1217 1197 1165 967 1155 1127 1098 1035 1024 938 907 896 863 830 786 745 601 450 424 255 186 -

1000 m 1196 1165 968 1153 1136 -

VW w m w m

1037 1031 936 905 895 -

VW m m m sh

825 792 744 605 443 421 242 -

VW VW vw VW w m VW

105 VW 90 VW

for cyclobutylmethyl~,-ethe~

743 603 443 423 250 -

m w w m w

Gas m s vs sh m vs sb s s s w m w m w sh w VW m w w w m w m VW m m w m m VW vw VW m m w m w VW

ASSignmentSb Solid 2246 2980 2980 2954 2190 2945 -

m s s m m s

2922 2870 2057 1052 1462 1052 1444 1154 1432 1368 1238 995 1220 -

s s s m m m m “S w m s m m

s m sh vs

1160 966 1154 1126 1096 -

m.b m vs vs sh

1025 936 903 895 836 -

sh w w sh

744 610 445 241 155 106 92

m w m

1024 935 906 894 861 824 785 744 602 450 426

m w w m VW VW VW m w m VW

2248 2978 2978 2957 2189 2945 2930 2930 2887 -

m vs vs sh m “S s s s

1050 1462 1050 1444 1162 1437 1370 1235 1000 1223 -

sh sh sb m “8 w s s m m

1162 966 1159 1136 -

VW

w w VW w

CD, antisymmetric stretch Y-CH, antisymmetric stretch y-CH, symmetric stretch P-CH, antisymmetric stretch CD, antisymmetric stretch P-CH, symmetric stretch P-CH, antisymmetric stretch orCH stretch @-CH, symmetric stretch CD, symmetric stretch CD, antisymmetric deformation ‘yCH, deformation CD, antisymmetric deformation @CH, deformation (in phase) CD, symmetric deformation P-CH, deformation (out of phase) a-CH bend YCH, wagping CD, rocking @-CH, wagging (out of phase) P-CH, wagging y-CH, twisting CD 3 rocking O-C stretch Ring deformation &CH, twist (out of phase) ~~~,~e;ti(” phase) Ring breathing Ring deformation Ring deformation a-CH bend (out of plane) /3-CH, rocking (in phase) fl-CH, rocking (out of phase) Ring deformation y_CH, rocking Ring-O-C bend C-O-C bend Ring-O-C bend Methyl torsion Ring puckering Methoxy torsion

aFor abbreviations used see Table 1. bThe normal modes are numbered the same as those for the “light” compound. mode to be in the region 120-160 cm-‘. The torsion8 associated with the methoxy group were observed in ethylmethylether [20] at 202 cm-’ (methyl torsion) and 115 cm- ’ (methoxy torsion) and the corresponding mode8 in cyclobutylmethylether should have similar frequencies. In the “light” compound Q branches were observed in the infrared spectrum as an apparent series which started at 193 cm-’ and the excited states fell at lower frequencies. A pair of bands was observed in the spectrum of the heavy compound at 159 and 155 cm-’ (see Fig. 5). On the basis of this isotopic shift

35

I I

I I

300

I I

I I

200

1 I

I

100

WAVENUMBER (CM-l) Fig. 5. Low frequency infrared (effective resolution 0.5 cm-‘): ether gas, (B) cyclobutylmethyl-d,-ether gas.

(A) cyclobutylmethyl-d,-

bands were assigned to the -CHJ and -CD3 torsions, respectively. This isotopic shift indicates that the torsion is slightly coupled with the other low frequency vibrations. By using the frequencies listed in Table 3, the potential function of the methyl group was calculated. An assumed gauche structure was used to calculate the reduced internal rotational constant, F, for the torsion. The reduced internal rotational constant is defined as F(cm-‘) = h2/8n21r, where 1, = 1, (1 - CJ*, (I, /Ii)) and 1, is the moment of inertia of the internal top, 1i is the i-th principal moment of inertia of the entire molecule, and hi is the cosine of the angle between the axis of the internal top and the i-th principal axis of inertia of the molecule. It is assumed that the methyl potential can be represented by a Fourier cosine series in the internal rotational angle, V(a) = (l/2) V, (l-cos n (Y),with only the V, and V, retained because of the three-fold top symmetry. The periodic potential barrier was calculated for the light. compound to be 1083 cm- ’ (-3.1 kcal moIB ‘). The frequencies, assignments and calculated potential constants are listed in Table 3. these

36 TABLE 3 Methyl torsional

frequencies

(cm-’ ) and barriers (cm-‘)

t+C%

d9CD,

Transition

Observed

I+0 2+-l 3~2

190.9 i88.9 182.6

Coefficient v, V,

Observed -

Calculated

Observed -

Transition

Observed

0.3 -0.8 0.5

I+0 2+1

169 156

-1.5 1.5

Value

Dispersion

Coefficient

Value

Dispersion

1088 -89

10.9 5.8

V, -

1094 -

20 -

Calculated

Examination of Fig. 5 8hOW8 that both compounds have pronounced absorption in their infrared spectra near 100 cm- ‘. Polarized Raman lines were also observed at approximately the 8ame frequencies. For the “light” compound the strongest Q branches are at 102 and 100 cm-’ with weaker ones at 111, 109,107,104 and 98 cm-‘. In the CD3 compound it appears that the 102 and 100 cm-’ bands shift to 93 and 92 cm-‘, respectively. Thus, these Q branches are assigned to the methoxy torsion. The 107 and 104 cm- ’ bands appear to shift only to 106 and 103 cm-’ and we have taken these as the ring puckering modes. It appears that the methoxy torsion and the ring puckering vibration fall inthe same region. Since no transitions could be assigned in the tram well it was not possible to obtain the potential function for the methoxy torsion. Similarly it was not possible to obtain the ring puckering potential function since a confident assignment could not be given for the various observed transitions. CONCLUSION

The infrared and Raman studies of cyclobutylmethylether were undertaken due to an interest in the molecular conformation and barrier height8 for the internal rotation and ring puckering motion. The Raman spectrum of cyclobutylmethylether showed the absence of depolarized bands which indicates the predominant conformation for the molecule is the gauche conformer and therefore the molecule has C1 symmetry. This conclusion is in agreement with the microwave study of cyclopropylmethylether [21] which demonstrated that this molecule also exists only in the gauche form. The barrier of internal rotation for the methyl group was calculated to be 1083 cm-’ (3.1 kcal mole-‘). The rotational barrier of the methyl group in cyclobutylmethylether is thus substantially higher than the corresponding barrier of 796 cm-’ (2276 cal mold ‘) in cyclopropylmethylether. The reason for this difference is probably because of the geometry of the ring as well as a difference in the distances to the p hydrogen8 in the two compounds. The C-O barrier in methylethylether was found [20] to be 914 cm-’ (2.61 kcal mole-‘) whereas the barrier in dimethylether was found

37

[22] to be 943 cm-’ (2.70 kcal mole-‘). Thus, the methyl barrier in cyclobutyhnethylether is high but not unreasonably large. The methoxy torsions were found between 115 and 100 cm-’ in methylethylether so the value (100 cm- ‘) for this mode in cyclobutylmethylether is quite reasonable. The ring puckering vibration (107 cm- ‘) seems rather low compared to the frequency for this normal mode in other four-membered rings [15-191. It is possible that the first few transitions at the bottom of the well were not observed. Further information on these latter anharmonic vibrations could possibly be obtained from a microwave study of this molecule. ACKNOWLEDGEMENT

The authors gratefully acknowledge the financial support of this study by the National Science Foundation by Grant CHE-76-23542. REFERENCES 1 For a review of the earlier studies in this area, see C. S. Blackwell and R. C. Lord, Vibrational Spectra and Structure, Vol. 1, J. R. Durig (Ed.), Marcel Dekker, New York, N.Y., 1972, Chapter 1. 2 C. J. Wurrey, J. R. Durig and L. A. Carreira,VibrationaI Spectra and Structure, Vol. 5, J. R. Durig (Ed.), EIsevier, Amsterdam, 1976, Chapter 4. 3 J. R. Durig, W. E. Bucy, C. J. Wurrey and L. A. Carreira, J. Phys. Chem., 79 (1975) 988. 4 J. R. Durig and A. W. Cox Jr., J. Chem. Phys., 63 (1975) 2303. 5 J. R. Durig and Y. S. Li, J. Chem. Phys., 63 (1975) 4110. 6 J. R. Durig and W. E. Bucy, J. Mol. Spectrosc., 64 (1977) 474. 7 E. C. Tuazon and W. G. Fateley, J. Raman Spectrosc., 4 (1976) 227. 8 J. R. Durig, W. E. Bucy, L. A. Carreira and C. J. Wurrey, J. Chem. Phys., 60 (1974) 1754. 9 J. R. Durig, W. E. Bucy and C. J. Wurrey, J. Chem. Phys., 60 (1974) 3293. 10 W. G. Dauben, J. H. Smith and J. Saltiel, J. Org. Chem., 34 (1969) 261. 11 J. Dobson and R. Schaeffer, Inorg. Chem., 9 (1970) 2183. 12 L. A. Carreira, R. 0. Carter and J. R. Durig, J. Chem. Phys., 59 (1973) 812. 13 R. N. Jones and A. Nadeau, Spectrochim. Acta, 20 (1964) 1175. 14 F. G. Baglin, S. F. Bush and J. R. Durig, J. Chem. Phys., 47 (1967) 2104. 15 J. R. Durig and A. C. Morrissey, J. Chem. Phys., 46 (1967) 4854. 16 J. R. Durig and W. H. Green, Spectrochim. Acta Part A, 25 (1969) 378. 17 J. R. Durig, J. N. Willis Jr. and W. H. Green, J. Chem. Phys., 54 (1971) 1547. 18 R. 0. Carter, J. E. Katon and F. F. Bentley, Appl. Spectrosc., 26 (1972) 378. 19 V. F. Kalasinsky, G. A. Guirgis and J. R. Durig, J. Mol. Struct., 39 (1977) 51. 20 J. R. Durig and D. A. C. Compton, J. Chem. Phys., in press. 21 R. E. Penn and J. E. Boggs, J. Chem. Phys., 59 (1973) 4208. 22 P. Groner and J. R. Durig, J. Chem. Phys., 66 (1977) 1856.