Molecular structure and conformation of propionyl chloride as determined by gas-phase electron diffraction

Journal of Molecular Structure, 116 (1984) 257-269 Elsevier Science Publishers B.V., Amsterdam -Printed

in The Netherlands

MOLECULAR STRUCTURE AND CONFORMATION OF PROPIONYL CHLORIDE AS DETERMINED BY GAS-PHASE ELECTRON DIFFRACTION STEINAR

DYNGESETH,

Department Trondheim

of Chemktry, (Norway)

S. HELENE Uniuersity

SCHEI and KOLB&?IRN HAGEN of Z-ondheim,

NLHT-Rosenborg,

N-7000

(Received 1 November 1983) ABSTRACT The molecular structure of propionyl chloride, CH,CH,-COCl, has been investigated by gas-phase electron diffraction at nozzle temperatures of 293, 367 and 488 K. Rotational constants from microwave spectroscopy were also included in the analysis. The molecuies exist as a mixture of two conformers with the methyl group and the chlorine atom anti (torsion angle LO = O” ) or gauche (LO =. 120” ) to each other and with the anti form the more stable. The composition (mole fraction) of the vapor with uncertainties estimated at 20 was found to be 0.77(0.10), 0.65(0.13) and 0.53(0.11) at 293, 367 and 488 K, respectively. These values correspond to an energy difference AE = E, - E, = 6 (a = 2) kJ mol* and an entropy difference AS = 7 (a = 6) J mol-’ K-l. The results at 488 K for the distance (r,) and angle (L=) parameters, with estimated uncertainties of 20, are: r(C-H) = 1.123(11) A, r(C=O) = 1.181(5) _Pl, r(C-C=) = 1.522(13) A, r(C-C-) = 1.526(15) il, r(C-Cl)= 1.800(6)&LC-C=0 = 127.1(6)‘,LCXXl= 112.0(3)“, LC-C-C = A normal coordinate calculation was made by 112.3(8)“, Lo (gauche) = 120.0(7.7)“. developing a valence force field. Usrng 29 force constants, the 24 vibrational frequencies were calculated with an average deviation from experimental values of 5 cm-l.

INTRODUCTION We have recently investigated by electron diffraction a series of molecules with general formula CH,X-COY (X = Y = Cl [I], X = Y = Br [Z] , X = Br,

Y = Cl [Z], X = Cl, Y = H [3] ) and for all observed mixtures of two conformers with different X-C-C-Y torsion angles. In the haloacetyl halides [l, 21 (X, Y = halogen) a low-energy form with X and Y anti to each other was found, while in chloroacetaldehyde [ 31 X and Y (Cl and H) were almost syn in the most stable conformer. We were also interested in investigating molecules with X or Y being a CH,-group. A methyl group and a chlorine atom have almost the same van der Waals radius, but quite different electronegativity, and it would be of interest to see the conformational effect of the different substituents. In this paper we present our results (Fig. 1) for propionyl chloride (X = CH3, Y = Cl), later we hope to present results for chloroacetone (X = Cl, Y = CH,). Microwave spectroscopy investigations 14, 51 of propionyl chloride have shown that the most stable form of this molecule has CH3 and Cl anti 0022-2860/84/$03.00

0

1984

Elsevier

Science

Publishers

B.V.

Fig. 1. Propknyl chloride. Diagram of the

anti conformer with atomic numbering.

(C-C-C-Cl torsion angle L$J = 0”), but these investigations could not

detect any second conformer. However, a vibrational spectroscopy investigation [S] observed a second conformer present both in liquid and gas phase, but could not determine accurately the torsion angle of this second form. Electron diffraction should be able to determine the conformational equilibrium in propionyl chloride and also determine bond distances, bond angles and torsion angles as well as some vibrational amplitudes. To improve the accuracy of the investigation, the observed rotational constants [4,5] were included as experimental data in our analysis. Two conformers were observed, and in order to determine energy and entropy differences between these conformers, the electron diffraction experiment was repeated at three different temperatures. EXPERIMENTAL

AND DATA REDUCTION

A commercial sample of propionyl chloride was obtained from Aldrich Chem. Co. (>97%) and was used without further purification. Electron diffraction photographs were recorded at 293,367 and 488 K with a Balzers Eldigraph KDG-2 [7,8] using a rotating sector with angular opening proportional to r3 and Kodak Electron Image plates at nominal nozzle-to-plate distances of 50 and 25 cm. The electron wavelength was determined by voltage measurements, calibrated against gaseous benzene 191. The experimental conditions are summarized in a supplemenpublication [lo]. Reduction of the data was done in the usual way [ 11,121, and a calculated background [13] was subtracted from the data for each plate to yield the experimental molecular intensity distribution in the form ST,(s). The averages of the molecular intensities are shown in Fig. 2, the data for the

259

1

‘0 Fig. 2. Propionyl chloride. Intensity curves in the form d,(s). Experimental curves are composites of all plates and all camera distances. Theoretical curves were calculated using parameter values shown in Tables 3 and 4. All curves are on the same scale. As = 0.25 X-‘.

total intensities and the backgrounds are available as a supplementary publication [lo] _ Electron scattering amplitudes and phase shifts were calculated by the use of Hertree-Fock potentials for C, 0 and Cl f14], while molecular bonded potentials were used for H [15]. FORCE

FIELD

CALCULATION

CalcuIakion of vibrational quantities were made with a valence force field. The initial valence force constants were transferred from 1-butene [IS] and acetyl chloride. The latter force field was developed to be consistent with the one for 1-butene. Thus, for acetyl chloride, a 19 parameter force field reproduced the 15 observed frequencies [17] to an average deviation of 3 cm’. From the initial force field for propionyl chloride, an improved fit to the observed anti frequencies [S] was obtained by a least squares refinement, giving an average deviation of 5 cm -l. For the gauche conformer almost all the initial force constants obtained from I-butene and acetyl chloride were used unchanged. The force field for propionyl chloride is given in Table 1, and in Table 2 are listed observed and calculated frequencies along with the potential energy distribution. These force constants were then used to calculate root-

260 TABLE

1

Valence force field for anti/gauche conformers of propionyl chloride

Stretch

Coordinate(s) involved

Value8

Type

Coordinate(s) invoived

Valuea

o=c

11.05

Out of plane

=C-cl

0.41

Stretch/stretch

C-H/C-H C-H/C-H C,-C,/C-Cl

Stretch/bend

c-C/c-C--c C-Cl/C=C-Cl c--cl/c-C--cl C-C/C-C-H C-C/C-C-H

Cl-C, G--c,

4.97 4.29 2.62 4.64 4.84

o=c-C

0.7610.87 1.26lO.92 1.2OlO.95 0.63 0.69 0.64 0.53 0.54

C-Cl C-H (CH,) C-H (CH,) Bend

O=C-Cl C-C-Cl C-C-C C-C-H C-C-H H--C-H H--C-H Torsion

(CH,) (CH,) (CH,) (CH,)

0.06 0.07

C,--C, G-c,

Bend/bend

0.06 0.08 0.78/0.7C

(CH,) (CH,)

0.22 0.53/0.61 0.50 0.22 0.32

(CH,) (CH,)

C-C-H/C--c-H (CH,, C--C common) c-C-H/c-C--H (CH,, C-C common) C-C-H/C--c-H (CH,, C-H common) C-C-H/C-C-R (tram, C-C common)

‘Units for force constants are mdyn K l for stretching and mdyn A rad* out-of-plane and torsion, with corresponding units for interaction constants.

-0.02/-O.l -0.01 0.05 0.11

for bending,

mean-square amplitudes of vibration (I), perpendicular amplitudes (K) and centrifugal distortion constants (6 r). STRUCTURE

ANALYSIS

Radial distribution (RD) curves, Fig. 3, were calculated in the normal manner horn the intensity curves shown in Fig. 2. The origin of the various peaks in the RDcurves are indicated on the figure. Approximate values for the geometrical parameters were obtained from the experimental RD-curves and from structures reported for related molecules [I, 181. The temperature dependence of the area under the peak at 4.1 A in the experimental RDcurves clearly showed the presence of more than one conformer. This peak represents the longest carbon-chlorine distance in the anti form and the amount of ati is seen to decrease with increasing temperature. Calculation of theoretical RD-curves for various combinations of conformers (anti, gauche and syn), Fig. 4, revealed that the conformational mixture consisted of anti (C-C-C-Cl torsion angle L# = 0”) and gauche (LQ = 1203, with about equal amounts of the two forms present at the highest temperature (488 K).

261

TABLE 2 Propionyl chloride. distribution in %

Observed

[ 61 and calculated

frequencies

a’

a”

Calc.

P.E.D.b

2981 2959 2897 1805 1472 1424 1385

2992 2949 2907 1805 1462 1425 1392

1339

1345

C,-H( 100) C,-H( 96) C,-H(98) O=C(79) H-C,-H( 84) H-C,-H(75) C-C,-H( 41 )C-C,-H( H-C,-H(28) C--c,-H(41)C--c,-H(29) H-C,-H( 26)

1084 1016 926 689 441 359 229

1082 1021 922 699 445 356 229

C,--c,(28)C,+X26) C-C,-H( 52) C,-C,( 4O)C-C,+( C-C1(29)O=C-C(22) C-C1(5O)O=C-C1(22)

2999 2936 1456 1261 1088 790 505 196 71

2999 2929 1457 1264 1075 792 509 197 74

C,-H(100) C,-H( 100) H--C,-H(91) C-Cz-H(86) C-C,-H(46)C<,-H(41) C-C,-H(54)C<,-H(45) w(=C<1)(87)

C-C-CI(30) C--c-C(48)C--c-cI(

t(C,-C,)(94) t(c,-C,)(94)

and potential

Approximate description

Anti Obsa

in cm4

29)

asymstr. CH, symstr. CH, symstr. CH, str. C=C asym.def. CH, def. CH, sym.def. CH,

Gauche Ol.Sa

talc.

1833

2992 2949 2906 1797 1457 1433 1385

1400

1334

wag CH,

str. 25)

25)

c-c

rock CH, str. C-C bend O=C-C str. C-Cl bend C--C-Cl bend C-C!< asymztr. CH, asymstr. CH, asymstr. CH1 twist CH, rock CH, rock CH, wag =CCl tors. C2-C3 tars. C,-C,

aObserved frequencies from gas phase. by and t denotes out-of-plane respectively. Only contributions larger than 20% are included_

energy

1127 908 576 383 257

1073 804 556

1121 1030 913 596 433 377 229 2992 2930 1460 1229 1051 792 577 187 77

wag and torsion,

Least squares refinements [19] of the structure were made at each temperature by simultaneously fitting a single theoretical intensity curve to the two average experimental curves and the three calculated rotational constants to the experimental ones. Because of the effect of vibrational averaging, the r, distances used to calculate theoretical intensity curves are inappropriate for rotational constants. We therefore defined our models in terms of the geometrically consistent rao = rz set of distances which were used to calculate rotational constants &related to the observed B. by BZ=

B. + I/2

&-===

The r, distances for the temperature of interest (7’) were generated from the rao set of the model according to [ZO]

262

Diff.

Fig. 3. Propionyl chloride. Radial distribution curves calculated from the intensity curves of Fig. 2 after multiplication by ZcZcl/(fcfcl) and with an artificialdampingconstant B = 0.002 A=_Unobserved experimental intensity data from the region 0
I(P)= -

(l’)“]

+ SrT + K” -

(p)T/r,o

The Morse function anharmoricity cor:;.rints a3 were given the diatomic molecule value [21] for bonds and otherwise assumed to be zero. Values for 1, 6r and R as well as ap”“, were calculated from the above mentioned force field. In the least squares refinements the two conformers were assumed to have the same geometry except for the C-C-C-Cl torsion angie. For the anti conformer a dynamic model was used where the anti conformer was represented by five distinct forms with different torsion angles, each weighted according to a gaussian potential function. The r.m.s. amplitude for the torsional vibration (r) was calculated from the observed frequency for the torsion, and the weights for t-he five pseudo-conformers were then calculated from 7 [12] _ Only an average value was determined for the C-43 bonds, and the two different C-C bonds were refined as an average value and a split. Among the vibrationai amplitudes .for the interatomic distances, only the value for the C-Cl bond was refined, while the others were kept constant at the values calculated from the valence force field shown in Table 1. The

263

0

“’

“I

111...,..,...,I,...(./../

“‘I i

2

“‘.I”“““’

3

‘5

’

R.A

’

Fig. 4. Propionyl chloride. Theoretical radial distribution curves for different conformers or mixture of conformers compared with the experimental curve for the 488 K data. All curves are on the same scale.

results of the final least squares refinements are listed in Tables 3 and 4. The theoretical intensity and RD-curves calculated from these results together with experimental and difference curves are shown in Figs. 2 and 3. Table 5 gives the correlation matrix for the high temperature experiments, the other two correlation matrices are similar. DISCUSSION

Molecular structure As can be seen from Table 3, the results obtained for propionyl chloride at the three different temperatures are consistent; all geometrical parameters are nearly constant. In Table 6 some of the important geometrical parameters in molecules -with the general formula CH,X-COY are shown. In all of these molecules the central carbon--carbon bond distances are almost the same. The length of the carbonyl bond seems to depend somewhat on the sub stituent Y, with a shorter carbonyl bond when Y = halogen than when Y = H. This has also been observed in other molecules Cl.81 _ r(C=O) and LC--C=O show a correlation with a larger angle when the bond distance is short.

264 TABLE Final

3 structural&b

Parameter

C-H C=O tc--Oc A(C--C)d C-Cl Lc-c-0 LCX+X LC-C-C LC-C-H, LH,-C-H, LC-C-H, LQ,e L&f rg % anti RLh RSh

results for propionyl

293 K

chloride 367

at different

temperatures

K

488 K

r&U

I

rat&I

1

rflI&

I

1.107(ii) 1.187(5) 1.524(5) -0.002(30) 1.795(5) 127.0(7 j 112.1(4 j 112.7(7) 102.0(4.1) 105.3(9.8) 109.6(2.3) 117.5(11.4) 18.0(23.7)

[0.079] lo.0381

1.103(14) l-191(7) 1.521(7) O.OlO(37) l-798(7) 126.8(g) 112.9(5) 112.7(1.1) 112.5(5.4) 105.4(10.4) 104.2(3.4) 128.4(14.4) 36.2(28.9) [ 10.71 65.2( 13.1) 0.086 0.117

[0.079] iO.0381

1.123(11) l.lSi(5) 1.524(5) 0.004(26) 1.800(S) 127-l(6) 112-O(3) 1X.3(8) 104.7(3.7) [ 105.01 109.4(2.7) 120.0(7.7) 4.4(44-O)

[0.079] [0.039]

19.6

1

76.5(9.8) 0.063 0.081

0.049(6)

0.057(9)

0.061(5)

112.41 52.9(10.8) 0.061 0.089

*Distances (ra) and root-mean-square amplitudes (I) in Angstroms, angies in degrees. bparenthesized uncertainties are 20 and include estimates of systematic error and correlation Quantities in square brackets were not refined. ?C-C) = 0.5(r(C,--C,) -!- r(C,X,)). dA(C-C) = r(C,-C,) -r(C,<,). ebb, is the gauche CCCCI torsion angle relative to 0” for the anti form. fan, is the CCXH, torsion angle relative to O” when C,-H, is anti to C-C,. % is the r.m.s. amplitude for the C-C, torsional oscillation in the anti form, calculated horn the olxerved frequency_ hR~ and Rsare the agreement factor for the long camera and short camera curves, respectively. R = ISLu~A~/(wiIi(obs_))z 1’” where A i = 1,iobs.) -Ili(calc.).

The structures of chloroacetyl chloride (X = Cl, Y = Cl) and propionyl chloride (X = CH,, Y = Cl) are very similar. Both bond distances, bond angles and the torsional angles of the two conformers are almost the same in the two molecules_ The large difference in electronegativity between CH3 and Cl therefore seems to have only a small effect on these parameters. The electron diffraction and microwave data are entirely consistent. In the final combined analysis, the three observed rotational constants agreed with the calculated values to within less than 1.5 MHz for all experiments, and even an analysis based solely on the electron diffraction data yielded results consistent with those presented in Tables 3 and 4. Energy and entropy differences of the conformers The measured variation of sample composition with temperature makes it possible to determine energy and entropy differences between the gauche and anti conformers by use of the formula

265 TABLE 4 Dependent distances and r.m.s. amplitudes of vibration for propionyl chloridea Distances

Cl-% G-G c2 -H, C, - H,

G-H,

Cl -c, c2 - 04

c, - a,

0, -Cl, g* : 1;s C: - - HI, H,.H, c, --0, C, --Cl, 0, --H, CI,..H, 0,---H, (A) c&*.-H, 0,. -‘H, Cl, - - - H, 0,.--H,, Cl, - - -HI, I c, e-0, c,--Cl, 0,--H, Cl,--H, 0,.-H, Cl, - - H, (C) 0,---H, Cl s *- .H, 0,---H, Cl, - - - H,, 0, ---HI, Cl 5---H,, I

293 K

488 K

367 K

‘a

lb

l-525(16) l-523(16) 2.161( 29) 2.057(50) 2.242(43) 2.535(10) 2.428(9) 2.755( 11) 2.608(8) 2.927(205) 3.460(38) 2.647( 155) 1.818(40) 2.857(14) 4.138(10) 3.049(40) 2.774(62) 3.058(221) 4.538(87) 3.909(59) 4.892(32) 2.580( 272) 4.366(66) 3.467(44) 3.207(84) 2.472(76) 3.643(24) 3.008(58) 2.863( 141) 3.571(171) 3.830( 194) 4.422(36) 4.045(175) 3.671(139) 2.646( 220)

0.049

0.051 0.109 0.108 0.110 0.075 0.060 0.066 0.060 0.185 0.107 0.185 0.127 0.112 0.072 0.117 0.137 0.261 0.162 0.129 0.122 0.261 0.162 0.116 0.193 0.132 0.189 0.138 0.104 0.191 0.291 0.137 0.197 0.235 0.321

ra

2b

‘a

lb

l-516(18) 1.526(21) 2.090(42) 2.183(67) 2.118(88) 2.529(16) 2.422(10) 2.762(13) 2.606(9) 2.917(266) 3.377(66) 2.497(161) l-863(48) 2.847( 22) 4.145(12) 3.146(38) 2.993(114) 3.109(281) 4.502( 120) 3.840(100) 4.812(51) 2.395(290) 4.241(67) 3.525(50) 3.101(103) 2.649(104) 3.731(25) 3.103(62) 3.081(113) 3.653(180) 3.729(316) 4.402(37) 3.877(198) 3.577(177) 2.372(210)

0.049 0.052 0.110 0.109 0.111 0.079 0.062 0.070 0.063 0.197 0.109 0.197 0.127 0.123 0.078 0.119 0.141 0.285 0.172 0.137 0.126 0.285 0.172 0.127 0.214 0.138 0.202 0.141 0.106 0.204 0.319 0.145 0.215 0.255 0.354

1.522( 13) 1.526( 15) 2.173(32) 2.102(47) 2.233(37) 2.528(13) 2.422(8) 2.753(10) 2.606(7) Z&806(362) 3.486( 26) 2.734(351) 1.847(47) 2.845( 17) 4.136(10) 3.084(26) 2.524(73) 2.847(616) 4.466( 149) 3.942(32) 4.915(27) 2.‘724(599) 4.421(148) 3.470(30) 3.166(59) 2.518(66) 3.680(24) 3.038(45) 2.924(89) 3.474(205) 3.602(431) 4.410(77) 4.140(211) 3.799( 295) 2.642(348)

0.051 0.054 0.113 0.111 0.114 0.087 0.067 0.078 0.069 0.217 0.113 0.217 0.128 0.139 0.086 0.123 0.150 0.321 0.187 0.149 0.133 0.321 0.187 0.143 0.246 0.148 0.224 0.149 0.111 0.226 0.361 0.158 0.243 0.285 0.403

“Distances (r,) and root-meansquare amplitudes (I) are in .kngstriims. Parenthesized uncertainties are 20 and include estimates of systematic error and correlation. DFor the torsion-sensitive distances in the anti form where a dynamic model was used, vibrational amplitudes were calculated without contribution from the C-C torsion.

where Ng and N, are the fractions of gauche and anti molecules and the factor 2 is the statistical weights of the two forms. Assuming AS and AE to be temperature independent in the actual temperature-interval, they can be

TABLE 6 Correlation matrix (X 100) fox propionyl chloride at 488 K

C-H 00 G-C) A(C-c) C-CI LCCO LCCCI LCCC

LCCH, LCCH, L&8 L$,O l(C-Cl) %anti

423

r1

ra

r3

Ar

r,,

L,

0.0026 0.0010 0.0009 0.0066 0.0011 0.21 0.11 0.27 2.16 0.90 2.66 14.6R 0.0012 0.036

100

-82 100

8 -2 100

4 19 22 100

-8 10 2 41 100

-10 12 3 70 50 100

L,

3; -14 -7 -20 -64 100

L,

L,

L,

8 -17 -34 10 -44 -23 36 100

-18 20 -2 37 32 16 31 -40 100

-11 -7 -22 -39 -6 26 -74 -40 -28 100

aStnndaxd deviotions from least squnres. Distances and amplitudes axe in angatxoms, angles in degrees. bLQ,I is the CCCCItorsional angle for the gaarrcheconformer. c~@, is the HCCC torsional angle.

L,

-3 6 -1s” -4 -21 9 -12 1 -3 100

L,

1,

-5 4 4 -2 4 -19 20 16 -2 -22 -14 100

-19 20

8 -5

-2

;;

3 -2 8 -8 18 12 -2 7 100

a

4 37 -16 6 7 5 -9 -56 -8 100

TABLE 6 Comparison of parameter values for some CH,X-COY

W-C) r(C-X) r(C-Y) r(C=O) Lc-c=o LC-C-Y LC-C-x

W,n

L&b Method Diet. type Ref.

molecules

CH,-COH X=H,Y=H

CH,-COCl X=H,Y=CI

CH,CI--COH X=Cl,Y =H

CH,CI-COCl X=ICI,Y=CI

CH,CH,-COCI X=CH,,Y=Cl

CH,Br--COBr XaBr,Y=Bi

CH,Br-COCI X=Br,Y=Cl

1.615(4) l-107(6) 1.128(4) 1.210(4) 124.1(3) 116.3(3) 109.0(9) 0 -

1.506(3) 1.106(G) 1.798(2) 1.187(4) 127.2( 5) 111.6(6)

1.521( 5) 1.782(4) 1.103(12) 1.206( 3) 123.3(6) 112.4(38) 110.4(3) 162 0 ED r0 3

1.519(B) 1.782(27) 1.775(25) 1.180(4) 126.5(12) 111.2(13) 112.5(19) 0 121.6(65) ED

1.522( 13) 1.526( 15) 1.800(6) 1.181(S) 127.1(6) 112.0(3) 112.3(8) 0 120.0(77) ED, MW

1.513(20) 1.915(20) 1.987(20) 1.175(13) 129.4(17) 110.7(15) 111.7(1&q 0 106.0 ED

1.619(18) 1.936( 12) 1.789(11) 1.188(g) 127.6(13) 111.3(11) 111.0(15) 0 110.0 ED

pa

ra

‘h

ra

0

ED, MW

ED, NW

“8 18

I’8 18

1

This w orlc

2

2

nLg, is the XCCY torsional angle for the low-energy conformer. Lqb, is 0” when C-X and C-Y are anti to ench other. bLq5, is the XCCY torsional angle for the high-energy conformer in those molecules where more than one form bus been observed.

Fig. 5. Propionyi chloride. Van% Hoff plot of composition bars indicate lo. Least-squares straight line.

data. The half-lengths of the

vertical

determined from a straight line fitted to the (R In R, l/T) points (Fig. 5). The best-fit (least squares) line leads to the following values with estimated standard deviations AE = E, - E, = 6 + 2 kJ rno1-l and AS (excluded the contribution from the fact that there are two enantiomeric gnu&e forms) = 7 + 6 J mol-’ K-‘. The value for AE is considerably higher than the value reported from the spectroscopic study [6] (3.5 kJ mol-‘). However, no uncertainty was reported in that work and that makes it difficult to judge how different the two values really are. For chloroacetyl chloride the values were determined to be: AE = 5 + 2 kJ mol-’ and AS = 3 -F4 J mold K-l. The change in AE as a result of substituting one of the chlorine atoms in chloroacetyl chloride with a CH,-group is therefore small, if any at all. The large uncertainties in AS prevent meaningful comparisons here. ACKNOWLEDGEMENTS

We are grateful to Siv. ing. FL Seip (University of Oslo) for help in recording the electron diffraction data and to Ms. S. Gundersen for technical assistance. Financial support horn Norges Almenvitenskapelige Forskningsmd is aclmowledged. REFERENCES 10. Steinnes, Q_ Shen and K. Hagen, J. Mol. Struct., 64 (1980) 217. 2 0. Steinnes, Q. Shen and K. Hagen, J. Mol. Struct., 66 (1980) 181. 3 S. Dyngeseth, H. Schei and K Hagen, J. Mol. Struct., 102 (1983) 45. 4 H. Kar!sson, J. Mol. Struct., 33 (1976) 227. 5 F. Mata and J. L. Alonso, J_ Mol. Struct., 56 (1979) 199. 6 S. G. Frankiss and W. Kynaston, Spectrochim. Acta, Part A, 31 (1975) 661. 7 W. Zeil, J. Haase and L. Wegmann, Z. Instrumentenkd., 74 (1966) 84. Rastiansen, G. 8 0. Graber and L. Wegmann, Balzers High Vacuum 25 (1969) 1.

Rep.,

269

9 K. Tamagawa, T. Iijima andM. Kimura, J. Mol. Struct., 30 (1976) 243. 10 Avaiiable as B.L.L.D. Supplementary Publication No. S.U.P. 26258 (10 pages)_ 11 B. Andersen, I-I. M. Seip, T. G. Strand and R. Stdlevik, Acta Chem. Stand., 23 (1969) 3224. 12 K. Hagen and K. Hedberg, J. Arn. Chem. Sot., 95 (1973) 1003. 13 L, Hedberg, Abstracts, 5th Austin Symp. on Gas Phase Molecular Structure ; Austin, TX, March 1974, p. 37. 14 T. G. Strand and R. k Bonham, J. Chem. Phys., 40 (1964) 1686. 15 R. F. Stewart, E. R. Davidson and W. T. Simpson, J. Chem. Phys., 42 (1965) 3175. 16 S. H. Schei, to be published. 17 J. A. Ramsey and J. A. Ladd, J. Chen Sot. B, (1968) 118. 18 J. H. Callomon, E. Hirota, K. Kuchitsu, W_ J. Lafferty, A. G. Maki and C. S. Pote, Structural Data of Free Polyatomic Molecules, Landolt-Biirnstein, New Series, Vol. 7, Springer-Verlag, Berlin 1976. 19 K. Hedberg and M. Iwasaki, Acta. Crystaiiogr., 17 (1964) ‘529. 20 K_ Kuchitsu and S. J. Cyvin, in S. J. Cyvin (Ed.), Moiecuiar Structure and Vibration, Elsevier, Amsterdam, 1972, Ch. 12. 21 K Kuchitsu and Y. Merino, Bull. Chem. Sot. Jpn., 38 (1965) 805.