Vibrational spectra and structure of cyclopropane-d1 and dicyclopropyl ketone

Spectroch~m~o Acra, Vol.

&A,

No

0584 8539/8X .$3.W+O.O0

II, p,,. 1109-I 115, 1988.

fi: 1988 Pergamon Press

Printed in Great Bntain.

Vibrational spectra and structure of cyclopropane-d, ketone

plc

and dicyclopropyl

A. 0. DIALLO* and D. N. WATERS? *Laboratoire d’Infrarouge Associe au CNRS, Universite de Paris-Sud, Batiment 350, 91405 Orsay Cedex, France; and TDepartment of Chemistry, Brunel University, Uxbridge, Middx, UBX 3PH, England (Receioetl 15 June 1987; infina~form

1 March

1988; accepted

21 March

1988)

Abstract-Infrared spectra have been obtained for cyclopropane-d,, C,H,D (CP-d,), in the vapor phase and for dicyclopropyl ketone (C,H,), CO (DCPK) in the liquid state. Raman spectra of the latter compound in the liquid and solid phases have also been recorded. By comparing the experimental with the theoretical spectra of CP-d, it has been possible to assign most of the fundamental frequencies of the molecule. Additionably, empirical spectra-structure correlations from the literature for cyclopropyl derivatives containing the COX moiety (X = halogen) have been used to investigate the conformational preference of DCPK. The experimental results confirm that the molecular configuration with the carbonyl group cis to the planes of the two rings is the predominant species in the liquid state, though evidence for a second conformer, which we tentatively identify as the cis-trans conformer, is presented.

INTRODUCTION Substituted cyclopropanes form a remarkable group of compounds on which a considerable amount of work has been done by physical chemists in recent years. The electronic interaction between the cyclopropyl ring and a variety of substituents has been scrutinized and discussed in terms of current theories [l-9]. Many molecules in which the conformations are dependent on internal rotation about the ringsubstituent bonds have been studied structurally by vibrational and microwave spectroscopy [l&21] and by other methods [22,23]. The most interesting cases arise when the substituent groups bonded to the ring possess electrons with orbitals of n-symmetry because of the possibility of conjugative effects in the same molecule. In many examples of this kind an important result has been the recognition of two types of preferred conformations. These two conformations are termed bisected and symmetrical to specify the geometrical arrangements of the cyclopropyl ring and the adjacent z system [24, 251. In early vibrational analysis of DCPK, NEASE and WURREY [20] concluded that the compound occurs as a single conformation in which the planes of the cyclopropyl rings are cis to the double bond of the carbonyl group. SCHRUMPF and ALSHUTH [21] have recently reinvestigated the vibrational spectra of DCPK. They interpreted their results as showing a conformational equilibrium, in the liquid phase, between the predominant cis-cis species, referred to above, and a second higher energy conformer called the gauche--gauche form. In this paper we re-examine the vibrational spectra of DCPK. Although the conformation of the predominant liquid-phase species does not appear to be in doubt, we report new evidence to question the earlier conclusions regarding the minor conformer. Another problem much studied by i.r. and Raman spectroscopists concerns the assignments of the vibra-

tion frequencies of cyclopropane derivatives [26-311. Such studies have resulted in a satisfactory understanding of the important features of the cyclopropane spectra, although quantitative descriptions explaining all the experimental observations are still missing. However, cyclopropane itself has been the subject of a number of experimental and theoretical investigations [32-341. In addition, the molecular vibrations of three symmetrically deuterated modifications, namely cyclopropane-d,, cyclopropane1,1,2,2-d, and cyclopropane-1,1-d, have been analyzed thoroughly [30, 321. The vibrational spectra of partly fluorinated cyclopropanes have also been discussed in considerable detail [35]. We wish to add to this series the measurement and interpretation of the i.r. spectrum of the unsymmetrically deuterated species C,H,D, observed in the gas phase. EXPERIMENTAL DCPK was obtained commercially and was distilled before use. CP-d, was prepared by two methods. In the first method D was exchanged for Br by heating at 80°C for several hours a solution of C,H,Br (Fluka product) and excess of D,O in dimethyl sulfoxide in the presence of granular Zn. The solution was prepared in a dry atmosphere and the experiment was carried out in a glass flask provided with a tap. The mixture was then cooled and the CP-d, liberated in the reaction was placed in an evacuated absorption gas cell. The second method involved the reduction of C,H,Br by metallic Zn in the presence of heavy acetic acid, CH,COOD [36]. A similar experiment in which CH,COOH was used in place of CH,COOD yielded cyclopropane, C,H,. The purity of the CP-d, sample was checked by recording the i.r. spectrum of the gas and comparing it with the spectra of C,H,Br and CjH, as shown in Fig. 1. The observed i.r. frequencies of CP-d, are listed in Table 1. The i.r. spectra of liquid DCPK were measured with a Perkin-Elmer model 283 spectrometer (Laboratoire d’hrfrarouge, UniversitC Paris-Sud) and with a FTIR Model 1710 spectrometer (Department of Chemistry, Brunel University). The Raman spectrum was obtained at Brunel University with a Spex “Ramalab” instrument, with samples

1109

A. 0. DIALLOand D. N. WATERS

m-1 IO

Fig. 1. Infrared

spectra of gaseous bromocyclopropane, CsHsBr cyclopropane-d,, C,H,D (C).

maintained at various temperatures in an Oxford Instruments Model DN70 cryostat. Our spectra agreed well in regard to their principal features with those which have been previously published [20,21] and so are not reproduced here. Figure 2, however, shows the Raman spectrum in the region of the carbonyl group frequency for the sample in the liquid and solid phases.

(A), cyclopropane,

C,H,

(B) and

system are distributed as follows: 12a’ +9a”. The magnitudes and orientations of the principal moments of inertia were estimated using the known structural

RESULTSAND DRXXJSSION

Assignment of the fundamental bands of CP-d, When the present study was nearly complete we found that KEEPORTS and EGGERS[30] had already calculated the fundamental frequencies of the CP-dr

molecule. The calculations were done in an attempt to assign bands suspected to arise from deuterated impurities in the commercially available cyclopropane1,1-d; CP-dr, C,H5D, belongs to point group C,, the symmetry plane of the molecule being normal to the plane of the three-membered ring and containing the CHD group. The 21 i.r. active fundamentals of the

(b) I

1720

I

I

cm-1

I

I

1640

Fig. 2. Raman spectrum, 1640-1720 cm-‘, of dicyclopropyl ketone (DCPK) at (a) 290 K (liquid), (b) 240 K (solid).

1111

Vibrational spectra of cyclopropanes

Table

1 - Infrared

--I

absorption

Expected fundamental

cm

3120 R

frequencies

of cyclopropene-d,

--I cm

, CP-d, vapor

EXpWted fundamental

1330 R

3095 Q V.B.

"t* vL3S "+

3075 P

1308 P

3045 R

1210 R

3028 P V.S.

"3, VA.

v

1317 Q II?

1205 Q V.W.

”

e

7

1195 P

3020 sh

2970 sh

1125 R

2940 R 2930 Q ".S.

1115 Q w.

2915 P

1110 P

2890 R

1102 w,

2865 Q s.

1075 sh.

2850 P

1055 sh.

2840 sh.

1045 V.8.

2510 R

1035 R

” 17

2490 Q v.".

1029 Q v.8.

2%4P

1025 P

” I*

“P

2292 R "

2270 Q 9.

.

1015 sh.

2252 P

2030 R

882 R

2080 Q m.

863 Q

2060 P

850 Q V.S.

1953 V.W.

835 P

1415 R

810 sh.

Y

1875 P

704 V.W.

Y

1788 R

680 w.

Y

10

,v

LO

II

1890 Q m. 12

1775 Q IT.. 1760 P

670 V.Y.

1462 R 1455 Q s.

"5

655 Y.

1445 P

144OR

9

"

IS

1430 P

parameters of cyciopropane [37]. The axes of least and greatest moments of inertia (A and C respectively) lie in the molecular symmetry plane. Accordingly,

under the conditions used here, the spectrum of the gaseous CP-d, molecule would be expected to show A/C hybrid bands for the in-plane (a’) modes together

1112

A. 0. DIALLOand D. N. WATERS

with pure type E bands for the out-of plane (a”) vibrations. There appears to be no basis for improving the earlier normal mode frequencies of CP-d, calculated by KEEPORTS and EGGERS using the force field of cyclopropane [30,33]. Since the same force constants have proved successful in interpreting the vibrational spectra of partly deuterated cyclopropane molecules of C,, symmetry [30] the data reported by these authors were used in the present investigation for the sake of internal consistency. Our contribution is to add the needed i.r. spectrum of the gaseous CP-d, for correlation with theory. The assignments of the fundamental bands are given in Table 2, which also gives qualitative descriptions of the vibrations. As can be seen from this table the correspondence between observed and calculated frequencies is satisfactory considering the experimental errors in the measurements. It should be realized, however, that for a molecule with the symmetry of CP-d, (C,) a number of vibrations of the same species may be considerably mixed. Consequently, the descriptions given in the table are useful only for comparison with appropriate fundamentals reported for other cyclopropyl compounds. In some regions of the spectrum supporting evidence for the assignments was obtained by comparing the observed band contours with those predicted for Table

2. Assignments

of the fundamental B-dl,

Species -~mode

the C, structure of the molecule. It is clear from Fig. 1 and Table 1 that most of the (1’absorptions which are well resolved show PQR envelopes expected for A,C or A/C type bands. The absence of a constant difference between the values of the P-R separations is indicative of hybrid bands. At least two bands of approximately B-type shape assignable to a” vibrations also definitely appear in the gas phase spectrum. These are at N 1440-1430 cm- ’ (R and P branches) and at 882-863 cm-’ and 850-835 cm-’ (RQ and QP branches respectively). The band type could not be established for many of the assigned a” fundamentals. The CH stretching vibrations The structure of the representation formed by the motions involving mainly the CH(D) stretchings is 4a’ + Za”. For CP-d, only two CH, stretching bands can be recognized with certainty. The highest frequency band of the spectrum presents a typical PQR envelope with the Q branch located at 3095 cm-‘. This absorption can therefore be identified as CHI stretching vibrations of species a’. Under low pressure conditions the second band is characterized by an absorption minimum at 3036 cm-‘. This minimum is probably the band center of the CH2 stretching modes of species a”. The vibrationa

of cyclopropane-d,,

vapor.

a'

Approximate

observed

descriptions

cm-l

Species Calculated -1

3o mode

Approximate descriptions

a"

Observed

Calculated3o -1

cm-'

1

C?$stretch

3119

13

CIitatretch

2

CHzstretch

3071

14

Mzatretch

3036

3025

3

CH stretch

3033

15

CIiz def

1435

1431

4

CD stretch

2270

2257

16

CHz

3101

1186

twist

5

CHrdef

1455

1464

17

CHz wag

1102

1107

6

CHbend

1317

1320

18

CHbend

1045

1056

1205

1192

19

ring

7 ring

breathing

def

941

8

uiztwist

1115

1115

20

Cli~rock

856

848

9

atwag

1029

1031

21

CDbend

655

659

10

ring

def

11

CI$rock

810

806

12

CDbend

704

697

872

Vibrational

spectra

latter absorption is accompanied by a shoulder near 3010 cm-’ which has been tentatively assigned to the a-CH stretching vibration, v3, of species a’. The only band in the CH stretching region which remains unassigned occurs at ca 2930cm-’ with a distinctly PQR contour. It is probably an overtone or combination of lower frequency CH deformation. In addition to showing the CH stretching frequencies CP-d, shows an absorption band having its Q branch at 2270cm- ‘. It is assigned to the C-D stretching mode, vqr of species a’. Hydrogen bending vibrations

Two deformation frequencies arising from the scissor-like bending of the CH2 bonds are expected for CP-d,, one of species a’ and one of species a”. The prediction is confirmed and both bands occur at characteristic frequencies. The present work locates the Q branch of the a’ vibration, vs. at 1455 cm-’ and locates the band center (between the P and R branches) of the a” vibration, vL5, at 1435 cm-r. In addition to the CH, deformation modes there are the vibrations in which the CH, groups move as rigid units. These are not as characteristic as the CH, deformations. Their vibrational motions may be described approximately (in order of decreasing frequencies) as the twistings, waggings and rockings. The structure formed by these modes is 3a’ + 3~“. The a’ vibration va, vg and vll are easily located from the shapes of the observed bands. The Q branches observed at 1115, 1029 and 810 cm-’ are taken as the fundamentals in question. With the exception of the asymmetric CH, rocking ,I vibration, vzO, the other a modes, v16 and vi, are assigned with less confidence than the corresponding a’ modes. The assignments are made with the aid of normal frequency calculations (see Table 2). Four further fundamentals of bending type, two of species a’ and two of species a” are expected to correspond approximately to the in-plane and out-ofplane motions, respectively, of the single hydrogen atom and of the deuterium atom. The in-plane CH bending, vg, of species a’ has been assigned to the band having a Q branch at 1317 cm- l. The corresponding

cis-cis

(C 2v)

Fig. 3. The possible

cis-trans conformations

of cyclopropanes

1113

asymmetric vibration, vis, has been assigned to the weak fundamental occurring at 1045 cm- ‘. The C-D bending vibrations, vi2 and vzl, are expected to be lower in frequency than the CH bending modes referred to above. Their assignments have been made on the basis of the calculated spectrum (Table 2). The ring vibrations

These are the vibrations involving mainly the motions of the carbon atoms of the ring. Two are of species a’ and one is of species a”. The a’ vibrations may be described roughly as ring breathing and symmetric ring deformation. The a” mode is referred to as asymetric ring deformation. The ring breathing mode, v,, is assigned to the Q branch of a weak band at 1205 cm-i. No extra absorption bands are available for assignments to the ring deformations vlo and vi9. Molecular

conformation

of DCPK

Several molecular configurations are theoretically possible for DCPK, (C,H,),CO, resulting from internal rotation of the cyclopropane rings about their respective bonds to the carbonyl group. However, in earlier spectroscopic studies of related molecules containing the COX moiety (X = halogen, H, CHJ, only the conformations in which the plane of the threemembered ring and the double bond (of the carbonyl group) adopt the cis and trans positions have been shown to be favored [lo, 11, 15, 163. The abundance ratios of the isomers which are present depend on the particular compound and on the physical state. On this basis the conformations of DCPK considered to be the most likely to occur would be the cis-cis, cis-trans and trans-trans, which differ in the location of the double bond with respect to the planes of the two cyclopropyl rings as shown in Fig. 3. For other possible molecular configurations see Refs [20] and

WI.

The trans-trans conformer is expected to be energetically unstable because of the pronounced steric hindrance between the rings. Group frequencies and

(Cs)

of dicyciopropyl

trans-trans ketone,

DCPK,

(C,H,),CO..

(Czv)

1114

A. 0. DIALLOand D. N. WATERS

depolarization ratios were used by earlier workers to assign the strongest bands of the spectra to the cis-cis

isomer of DCPK [20, 21-J. Although the vibrational assignments reported by a given investigator seem to be reasonable there are several significant differences between these assignments. For example, the symmetric ring deformation mode (A, in C,, symmetry) is assigned either at 874 cm-’ [20] or at 919 cm-’ [21]. The A, and B, C-C stretches are assigned at 1183 and 983 cm-’ respectively [20], or at 756 and 1013 cm-‘, respectively L-213.A band at 846 cm- 1 is assigned [20] to a CH, rock, or [21] to the B, ring deformation. Because these frequencies are expected to be conformationally sensitive, the absence of an agreed assignment renders their use as criteria for establishing conformation at best equivocal. There is general agreement that the in-phase and out-of-phase ring breathing modes of DCPK occur at 1203 cm-’ (Raman strong, polarized; i-r. very weak) and at 1220 cm-’ (Raman very weak; i.r. medium strong). These observations confirm the cis-cis conformer to be the predominant liquid phase species. The carbonyl stretching mode, at cu 1680 cm- ‘, is also unambiguously assigned. In the i.r. spectrum of the liquid phase a strong band occurs at 1685 cm - 1with a shoulder at approximately 1672 cm- ‘. The intensity of the shoulder is strongly reduced in the i.r. spectrum of the solid [21]. The Raman spectrum of the liquid also shows two components, at 1683 and 1674 cm-‘, but only a single, symmetrical, band at 1680 cm-’ in the solid phase at 240 K (Fig. 2). Although the Raman spectra of both liquid and solid phases were recorded by NEASE and WURREY [20], these authors were unable to discern any change in the contour of the carbonyl feature on freezing. The appearance of two carbonyl bands in the liquid phase and the strong attenuation, or disappearance, of one of these on freezing provides clearer evidence than any yet adduced for the occurrence of two conformers

of DCPK. The Raman intensity of the minor band component relative to the major component, estimated by graphical resolution, is ca 0.5. There is no reason to expect that the intrinsic intensities (i.e. intensities per mole) of the two bands should be the same, so the observed relative intensities should not be taken as a direct measure of the relative abundances of the two forms. The more stable (undoubtedly the cis-cis) conformer has its CO frequency at 1683 cm-’ in the liquid phase. This is consistent with a “conjugative” interaction between the CO group and the ring-carbon p orbitals and serves to rule out any molecular conformation of DCPK in which the CO group and rings are not conjugated. The second conformer has its CO frequency

at 1674 cm-‘. This value suggests an even stronger conjugative interaction than is shown by the cis-cis structure. Data for C,H,COF [lSa] and C3H,COCl [16] have shown that the conformer in which the CO group is oriented trans to the CBH, moiety has a lower CO frequency (by some 5 cm- 1 for these mole-

cules) than that in which the two groups are mutually cis. There appears to be an argument, therefore, that the second conformer of DCPK has at least one of its rings oriented trans to the CO group. Since the trans-trans structure is excluded on steric grounds, the data are consistent with the hypothesis that the second conformer has the cis-trans orientation (Fig. 3). Such a conclusion is at variance with the conclusion reached by SCHRUMPF and ALSHUTH [Zl]. These authors proposed a gauche-gauche structure, of C, symmetry, as the second liquid-phase conformer. Part of their argument in support of this conformer lay in the fact that certain Raman lines attributed to this species were substantially depolarized. In our view this is a weak argument, since for all the point groups having contention for the second structure (gauche-gauche, CI; gauche-gauche’, C,; c&gauche, Cl; trans-gauche, Cl; and cis-trans, C,) all Raman lines should exhibit some degree of polarization. More positively, however, we should expect a gauche-gauche structure to achieve a smaller p-orbital overlap between the CO and ring carbons in the WALSH structural model [S], and thus to show a higher CO frequency, than the predominant cis-cis

isomer. This is not observed and therefore the minor conformer of DCPK probably has the cis-trans arrangements of the carbonyl group and the rings as suggested here. Acknowledgement-It is the authors’ pleasure to acknowledge the correspondence they have had concerning this subject with Professor S. F. A. KETTLE. REFERENCES [l] A. T. PERRETTA and V. W. LAURIE,J. Chem. Phys. 62, 2469 (1975). R. PEARSON JR, A. CHOPLIN and V. W. LAURIE, J. Chem. Phvs. 62. 4859 (1975). A. -SKA~&KE,ti. FL&D and J. E. BOOGS,J. molec. Struct. 40, 263 (1977). A. SKANCKE, J. molec. Struct. 42, 235 (1977). C. A. DEAKYNE, L. C. ALLENand V. W. LAURIE,J. Am. them. Sot. 99, 1343 (1977). A. SKANCKE and J. E. B-s, Acta Chem. Stand. 32A, 893 (1978); J. molec. Struct. SO, 173 (1978); ibid. 51, 263 (1979). R. N. NANDY,J. V. TIETZ,JONGIN c71 M. D. HARMONY, CHOE,S. J. GET~Yand S. W. STALEY, J. Am. them. SOC. 10!5,3947 (1983). A. D. WALSH,Trans. Faraday Sot. 45, 179 (1949). :;; R. HOFFMANN, Tetrahedron Lett. 33,2907 (1970). JR and J. T. MILLER Cl01 J. E. KATON,W. R. FEAIRHELLER JR. J. Chem. Phvs. 49. 823 119681. and R. k. SC~WENDEMAN, J. Chem. Cl11 H.‘N. VOLLTRA~ER Phys. !34,260,268 (1971). C. 0. BRITTand J. E. BOOGS,J. Gem. El21 L. A. DINSMORE, Phys. 54,915 (1971). Cl31 A. R. MOCHEL,C. 0. BRIITand J. E. Booos, J. Chem. Phys. ,sS, 3221 (1973). D. NORBURY and J. CHERIDAN, .I. Cl41 J. N. MACDONALD, Chem. Phys. Faraday Trans. II 14, 1365 (1978). 1151 I. R. DURIG,H. D. BISTand T. S. LITTLE:(a) J. Chem. Phys. 77, 4884 (1982); (b). J. molec. Struct. 116, 345 (1984). II161 J. R. DURIG,H. D. BIST,S. V. SAARI,J. A. SMUIOOTER

Vibrational

spectra

SMITH and T. S. LITTLE, .I. molec. Struct. 99,217 (1983). 1171 P. M. GREEN, C. J. WURREY and V. F. KALASINSKY, SDectrochim. Acta 42A, 141 (1986).

of cyclopropanes

1115

LEAHIIY, M. D. WEAKLEY, Y. Y. YEH and C. J. WURREY, Specrrochim. Acta 42A, 149 (1986). [I91 A. 0. DIALLO, NGUYEN-VAN-THANH and I. ROSSI, Spectrochim. Acta 43A, 415 (1987). 1201 A. B. NEASE and C. J. WURREY, J. phys. Chem. 83,2135

W. G. ROTHSCHILD, J. Chem. Phys. 44, 3875 (1966). J. BRACKENand D. GUTHAIS, J. phys. Chem. 79, 2139 (1975). C. J. WURREY, Y. Y. YEH, R. KRISHNAMOURTHI, R. J. m BERRY, J. E. DEWI~ and V. F. KALANSKI, J. phys. Chem. 88, 4059 (1984). c301 D. D. KEEPORTS and D. F. EGGERS JR, Spectrochim. Acta 4OA, 7 (1984). c311 G. SCHRUMPF and H. DUNKER, Spectrochim. Acta 42A,

1211 G. SCHRUMPF and T. ALSHUTH. Spectrochim. Acta 43A,

1321 A.

[221 L. S. BARTELL and J. P. GUILLORY, J. Chem. Phys 43,

c331 C.

ll81 v’. F. KALASINSKY, J. L. POOL, N. S. EYMANN, J. T.

(1979).

939 (1987).

c231

647 (1965). M. J. AROMEY, K. E. CALDERBANK and H. J. STOOT-

MAN, J. them. Sot. Perkin Trans. 2 1365 (1973). H. COWLEY and T. H. FURTSCH. J. Am. them. Sot. 91,

~241 A.

39 (1969). M. J. COLLINS, C. 0. BRITT and J. E. BOGGS, J. Chem. Phys. 56,4262 (1972). 1261 H. HART and 0. E. CURTIS JR, J. Am. them. Sot. 78, 112

PI

(1956).

s*(A)

IA.:,,

4

[27] [ZS]

785

(1986).

W. BAKER and R. C. LORD, J. Chem. Phys. 23, 1636

(1955).

E. BLOM and C. ALTONA, Molec. Phys. 31, 1377 (1976). 1341 D. D. KEEPORTS, J. molec. Strut. 140, 57 (1986). 1351 N. C. CRAIG, T. N. HUCHAO, E. CUELLAR, D. E. HENDRIKSEN and J. W. KEOPKE, J. phys. Chem. 79, 2270 (1975). C361 D. 0. SHISSLER, S. 0. THOMPSON and J. TURKEVICH, Disc. Faraday Sot. 10, 46 (1951). c371 P. N. SKANCKE, Thesis, Norwegian Technical Univer-

sity Trondheim,

Norway

(1960).