Infrared and nuclear magnetic resonance studies of rotational isomerism in chlorinated propanes—II

Bpectrochimica A&a,Vol.278. pp.1683to 1578.Persamon Press 1971.Printed inNorthern Ireland

Infrared

and nuclear magnetic resonance studies

of

rotational isomerism in chlorinated propanes-II Co&~ti~ of mokculfx3 with one terminal CB, or CCI,group A. B. DEMPSTER, K. PRICE and N. SHEPPARD School of Chemical Scienoes,University of East Anglia, Norwich, NOR MC, U.K. (Received 20 October 1970) Abstr&---Detailed infrared spectra are presented for eight chlorinated propanes. The compounds studied were CH,ClCHCICH,, CHCI,CHClCHs, CClsCH2CHrCl, CHC12CC12CHs, CClsCH,CHCl,, CClsCHC1CH2Cl, CCl,CHClCHCl,, CClsCCl,CHCl,. Each contains one terminal CHs or CCls group. The number of rotational isomers present in the liquid phase and their conformations are discussed. Where appropriate, nuclear magnetic resonance vicinal coupling constants are also used to aid in the ‘determinationof isomer conformations. For each compound an assignment is given of the bands in the infraredspectra to the fundamental vibrations of the dominant rotational isomeric form; where possible, assignments are also indicated for other isomers. Raman frequenciespreviously reported in the literature have been taken into aocount in making vibrational assignments. 1. INTRODUCTION IN A PREVIOUS paper [l] we briefly discussed the evidence for rotational isomerism in some chlorinated propanes. We directed particular attention to the importance of parallel [l : 3] chlorine-chlorine interactions in determining which of the possible rotational isomers are energetically more frtvoured. In this paper we demonstrate the considerable advantage of combining infrared and nuclear magnetic resonance (NMR) spectroscopy to establish the number and conformations of rotational isomers present in the liquid phase. Detailed infrared data and vicinal NMR inter-proton coupling constants are given for eight chloropropanes. Assignments have been made of the fundamental vibration frequencies. Each of the molecules discussed contains either Ccl, or CH, as one termirml group. Consequently, the number of possible staggered rotational isomers is reduced from the maximum of nine for unsymmetrical chlorinated propsnes to not more than three. In addition, for chloropropanes in which the central carbon atom is also symmetrically substituted (i.e. Ccl, or CH,) only two spectroscopically distinguishable forms are present, since in these cases two of the three possible isomers form a mirror-image pair. As discussed in our previous paper [l], the number of rotational isomers present in the liquid phase of a chlorinated propane may often be deduced from the number of bands in the 6(CH) deformation region (15OO-1100 cm-l) or the Y(C-CI) frequency region (830-600 cm-l) as observed in the infrared spectrum. The 6(CH) region is simpler and more readily interpreted for heavily chlorinated propanes and the Y(C-Cl) region for the lightly chlorinated ones. The latter region is also of assistance in assigning conformations to the observed isomers. Further information about conformations has also been obtained by variable temperature NMR studies and by measurements of vicinal inter-proton coupling constants.

[l] A. B.

DEMFSTER, K. PRICE and N. SHEPPARD, Spectrochim. Acta 25A, 1381 (1969).

1663

1564

A. B. DEMPSTER,K. PRICE and N. SHEPPARD

2. EXPERIMENTAL of 1,1,2,2-tetrachloropropane, Samples 1,1,1,2,3-penttlchloropropsne rend 1,1,1,2,3,3-hexachloropropane were kindly donated to us by Professor Dr. H. GERDIN~ on completion of Raman work by himself and Dr. HARES [2]. The sample of 1,1,1,3-tetrachloropropane was donated by Dr. J. L. Wood of I.C.I. Plastics Division and that of 1,1,1,3,3-pentachloropropsne by Professor A. M. Whaley of the University of South Carolina. The other compounds were obtained from commercial sources (K. & K. Laboratories and Aldrich Chemicals Ltd.). The infrared spectra were recorded on a Perk&Elmer 125 diffraction-grating spectrometer, using cells with sodium chloride or potassium bromide windows, and a path length of about 0.025 mm for the pure liquids. To obtain low temperature spectra of the heavier molecules, a Resesrch and Industrial Instrument Co. variable temperature cell was used with the sample held as a thin film between sodium chloride I, 1,2-trichloropropone

1,1,1,2,3-pemachloropropane

Fig. 1. The infrared spectra of some liquid chloropropenes(a) CHCl,X?HCWH,, (b) CHCl,*CCI,~CHS,(c) CCl,+CH,.CHCl,, (d) CCls-CHCIXXX&l,(e) CCl&HCl-CHCl, (0 denotes bands due to impurity). [ 21 H. G. H-C), Raman Andy& of PolychloroCompounde,Thesis, University of Amskdam (1955); H. G. HARINU and H. GERDINCJ, Rec. Trav. C&m. 74, 841 (1955).

Infrared and NMR studies of rotational isomerismin chlorinrttedpropanes-

1666

discs. For lighter molecules the vapour was deposited on a cold potassium bromide disc in another simple glass low-temperature cell. Raman spectra were obtained using the 6328 A line of a 60 mW He/Ne laser and a Spex 1401 double monochrom&or. The NMR spectra were recorded as 5 per cent by volume solutions in carbon tetrachloride on 40 MC/Sand 60 MC/SPerkin-Elmer spectrometers, and at 100 MC/S using a Varian HA100 instrument. Tetramethylsilsne was added as an internal reference. Samples of the chloropropanes were mostly small and were therefore used without further purification. In a few cases samples did contain small amounts of impurities consisting of other closely related chloropropanes. These other molecules could usually be unambiguously identified from the NMR spectrum, and allowance for them is made in the interpretation of the NMR and infrared spectra. 3. REHJLTS AND DISCUSSION FOR INDMDUAL MOLECULES The infrared spectra discussed in this paper are of the liquid chloropropsnes recorded in the 3000 cm-l region for the Y(C-H) vibrations, and in the 150% 500 cm-l region which covers the CH deformation modes, CH, scissors, wagging, twisting and rocking modes, methyl rocking modes and Y(C-C) and ~(C-41) bondstretching frequencies. Where obtainable, details of the spectrum of the crystalline solid or the amorphous solid are alao given. At room temperature all the samples were liquids except for the heptachloropropane. The infrared spectra of the liquid chloroproprtnes for which crystalline spectra were not obtained are shown together in Fig. 1. 3 (a) 1, Z- Dichloropopane Descriptions of the C-Cl stretching modes, abbreviated infrared data and discussions of the three rotational isomers of this molecule were given in the first paper of this series [l]. The staggered rotational isomers are illustrated below.

I

II

m

The infrared spectra of the liquid and crystalline solid compound are shown in Fig. 2. In the liquid phase 17 bands of appreciable intensity are found between 1500 and 1100 cm-l; in addition, five C-Cl stretching bands are found at 739, 721, 671, 663 cm-l (the latter being observed only in the low temperature spectrum of the glass owing to reduction of the width of the band at 670 cm-l) and at 621 cm-l. This complexity of both spectral regions indictttes that each of the three possible rotational isomers is present in the liquid. On cry&all&&ion a notable reduction in the number of bands is seen, only fifteen bands being observed beween 1500 and 600 cm-l compared to the fourteen

1666

A. B. DEBPSTER, K. PRICE and N. SHEPPARD

Liquid

r

1

Cryst. -l-L 5000

ioo

Kl

Fig. 2. The infra-red spectrum of 1,2-dichloropropane (a) in the liquid state and (b) in the crystalline solid state.

expected for a single form of the molecule. A schematic assignment of these bands has been made in Table 1 and is seen to be consistent with Form II with tram C-Cl bonds. This has been identified previously as the most stable isomer by MIZUS~ and S~OUCHI [3] who estimated the energy differences between the three forms. Also in Table 1 group-frequency assignments are attempted for some of the fundamental vibrations of the other two isomers although, except for the C-Cl stretching modes, it is not possible to associate the vibrational frequencies with the particular conformations of the non-trans forms. 3(b) 1,1,2-Trichloropropane The complex infrared spectrum of this compound suggests that again each of the three possible forms is present in the liquid state. The observed infrared [3] S. MIZUSEIMAand T.

SHIMANOUOHI,

.hxm.

Enzyrnd.28, 1 (1961).

Infrared and NMR studies of rotational isomerism in chlorinatedpropanes--II

1567

Table 1. Infrared and Reman frequenciesand schematic assignmentsfor liquid and crystalline 1,2-dichloropropane

Assignmentof Liquid ix. 3020 w

2974 ah 2960 w 2860 VW 1452 s 1446 m 1437 m 1427 m 1379 s 1341 w 1330 w 1324 m 1281 sh 1275 s 1261 m 1252 m 1232 s 1190s 1173 w 1123 w 1113 m 1064 m 1014 8 998 w 940 VW 913 m 906 m 887 w 877 w 864 w 736 s 723 w 665 s 617 s 525 m

Form II

Crystal i.r.

2996 m 2981 m 2970 w 2960 m 2932 2864 w 1457 s 1436 s 1422 w 1376 s 1341 m

3014 (2, dp) 2998 (4, dp)

Assignment of other two isomerat

VW,) (=4

2959 (10, p) 2936 (8, P) 2874 (1) 1452 (6, dp) 1435 (6, dp) 1385 (2, p?) 1341 (2-3, dp)

CH,w CH,W 1289 w

1281 (7, p)

1127m

1236 (2-3, dp) 1195 (2, dp?) 1179 (l-2, dp?) 1121 (4, p)

1073 m 1013 8

1072 (4, P) 1014 (4-5, p)

1237 s 1191 m

907 m

CH,w

SCH (CHCI)

E SCH

CH,tw CH,tw CH*r CH,r v(=) v(C--c)

910 (4. dp) 868 (4, dp)

860 w 736 m

739 (30, P)

CH,r v@-JJ)

Px

v(CC1)

SxIi

#--cl) 663 s

671 (7, p) 621 (15, P) 528 (3-4, P?)

I Px

v(CC1) III PH v(c-cl) I & III sm skeletal bend

l Ramen data from Ref. [2]. vs, very strong; s. strong; ms, medium strong; m, medium; w, weak; VW, very weak; (p, polar&d Raman line; p, highly polarised; dp, depolarised Raman line); for the Raman data the numbers in braoketa denote the visually observed intensities. C&w, CH, wag; CH,tw, CH, twist; C&r, CH, rock; CH,r,methyl rock; (c--C), skeletal stretabing mode. m It is not possible to assign bends explicitly to either Form I or III except for the v(C-Cl) modes.

frequencies are given in Table 2, along with a general assignment of regions of the spectrum to the various types of vibration. The three possible all-staggered conformations of the molecule are illustrated below.

1668

A. B.

DEMPSTER,

K.

PRICE

and N. SHEPPARD

Table 2. The infrared and l&man frequenciesand schematic assignmentsfor liquid 1,1,2-trie~oroprop%ne Liquid ix. 3004 sh 2996 m 2960 VW 2942 w 2880 w 1448 8 1382 B 1329 In 1306 w 1277 m 1261 w 1242 m 1228 w 1210 8 1197 w 1121 w 1106 w 1081 m 1057 1015 s 989 m 929 m 910 w 894 m 768 WI 748 VW 732 w 706 s 680 * 617 m 640 m 632

Assignment

RWlll%IA*

2996 (7, dp) 2966 (2, P) 2943 (8, P)

v(CH)

1451 (5, dp) 1386 (1) 1332 (O-l)

VW%) (4 *VW (ad v(CHs) W 26(C~J (4 6C% f-1 GCEF, 63 6CH (CHCX)

1261 (2, dp) 1243 (2, dp)

60H (CHCl,) 6CH (CHCI)

1213 (2, dp)

SCH (CHCI,)

1117 (I. p?)

CH,r

1076 (0)

v(C-C)

1021 (O-1, p) 990 (O-l)

VP--C)

898 (3, dp) 769 (10, P)

CHG.

vw-clt

(C-Cl) v(c-cl) v (C-41) Form III v(C--Cl) v(C--Cl) SHE Form II skeletsl def Y

736 (l-2, p) 709 (5, P) 682 (5, P) 619 P-5, P) 540 (3, P)

* Reman data from Ref. [Z]. See footnote to Table 1 for symbols and abbreviations.

Between 1330 and 1100 cm-l eight bands are seen, which may be associated with CH bending vibrations; only four CR bends are expected for a single form. In the G-Cl stretching region six, possibly seven, bands are observed, whereas a single form would have only three such fundamentals. The C-41 stretching region was examined in cyclohexane and methyl cyanide solutions. It was found that the band at 617 cm-1 increased in intensity relative to the band at 706 cm-1 in the latter solvent. The former band was hence assigned to the most polar form of the molecule, i.e. Form II. From its frequency it is satisfactorily assigned to the Snu mode. The carbon-chloride stretching modes are observed at 768, 748, 732, 706, 680 and 617 cm-1 with Reman counterparts at 769, 736, 709, 682, and 619 cm-l. The NMR vi&ml coupling constant between the two methine protons has a low value (3.57 Hz in s, 2% by volume solution in carbon tetrachloride, and 3.22 Hz at a similaf concentration in methyl cyanide) indicating that these vicinal CH bonds are p~dom~antly gable with respect to each other. This shows that Form I is not an abundant isomer. From the number of Y(C--Cl) infrared frequencies Forms II and III are evidently both present in considerable quantity. It is perhaps somewhat surprising that Form I is not a relatively stable isomer

Infrared and NMR studies of rotational isomerism in chlorinatedpropanes-

1669

for the steric considerations do not appear unfavourable. However, the additional stabilization of isomer II with its larger dipole is understandable in the liquid phase. 3(c) 1,1,1,3-Tetrachbropropane The infrared frequencies and their assignments for this molecule are shown in Table 3. Six CH deformation bands (two each for CH, sym. bend, CH, wag and CH, twist) and four C-Cl stretching modes are expected for a single form of this molecule. In the spectrum of the liquid (Fig. 3) only six strong bands are seen in the deformation region indicating that only one of the two possible distinguishable forms is predominant. Also, on crystallisation little change is observed in the spectrum apart from a general sharpening of the bands and the appearance of some crystal splittings in the CH deformation region. One notable observation is that the band at 956 cm-l disappears suddenly over a few degrees as the sample crystallises. This is thought to be a weak band from a second rotational isomer. The C-Cl stretching vibrations of the CCl, group are highly coupled but can be identified by comparison with data for model compounds. l,l,l-trichloroethane [4] gives a doubly degenerate Y(C-Cl) frequency at 713 cm-l and a third at 524 cm-l, l,l, 1-trichloropropane [5] is less symmetrical and gives three separate frequencies of 529, 697 and 776 cm-l. The bands at 568, 702 and 796 cm-l observed in 1,1,1,3tetrachloropropane are therefore schematically assigned to the symmetric and two asymmetric vibrations respectively of the CCl, groups. Table 3. Infrared and Raman frequenciesand schematic assignmentsfor liquid and crysta.lline1,1,1,3-tetraohloropropane Liquid ix.

2976 m 2940 1450 1429 1342 1281 1264 1172 1068 1026 956 878 824 794 740 729 700 690

w m m m w m m m a m w s B eh 8 B ah

664 m 666 m

Crystal ix. 3020 m 2976 m 2964 w 2938 w 1461 m 1430 m 1344 m 1288 w 1258 m 1172111 1068 m 1030 m 881 827 796 742 720 702 693 670 666 668 653

VW 8 s w 8 8 w vw m m w

RaIlUUl

2979 (3, a) 2944 1464 1433 1346 1287 1260 1180 1071 1030

(3, P) (1. dp) (3, dp) (0) (1, dp) (0, p) (0) (3, p) (2, dp)

830 (5, p) 799(3, dp) 743 (3. p) 725 (10, P) 706 (5, ?p)

668 (1,

p)

572 (22, P)

Assignment v(CHJ as (CH,Cl) WJQ = WH,) 8 (cH,W v(CHJ s G&Isaic~,~l) &H, CH,w CH,w (CH,Cl) CH.tw CH,tw (CH,CI) v(CC) Y(c--c) 2nd isomer

W-Cl)

Y(CcI,) as

CH,CI I Y(CCl) Px? 2nd isomer Y (C-Cl) Px @-Cl) v(CcI,) as

Y

(O-41) PH of

v(C-Cl)

less stable isomer Y(CC&) s

For symbols and abbreviations BBBfootnotes to Table 1.

[4] D. C. S~~ITH,G. M. BROWN,J. R. NIELSEN, R. M. SMITEand C. Y. LIAN~, J. C&m. P&p. so, 473 (1952). [6] R. G. GASANOV,Opt. &e&y $33,294 (1967).

1670

A. B. DEMPSTER,K. P&ICE and N. SHEPPARD

Liquid

cm-’

Fig. 3. The infrared spectrum of 1,1,1,3-t&ra&loropropme (a) in the liquid stata and (b) in the crystalline solid state.

The fourth &Cl stretching mode from the chlorine of the CH,Cl group is not expected to be highly coupled to the others, and should conform basically to the relationships of vcclto structure as discussed in the previous paper. Hence we should expect either a Pu or a Px type vibration. The strong band at 720 cm-l is assigned to this vibration of the CH,Cl group in the crystalline state and corresponds to the P, mode. Of the two spectroscopically distinguishable forms of the molecule depicted in the diagrams, Form I is the isomer with this Px vibration, and this must be the stable form in the liquid as expected on steric grounds. Analysis of the NMR spectrum of the AA’BB’ type gives vicinal coupling constants of JAB = 10.9 Hz and JAB, = 4-9 Hz at room temperature which are also consistent with predominantly bans and gauckz coupling constants respectively expected for Form I. These values would not be consistent with the averages expected from the mirror-image pair II. However, 4.9 Hz is a little larger than

Infraredand NMR studies of rotational isomerism in chlorinated propene~--II

1571

IT

I

expected for a pure gauche coupling constant, and small changes of the coupling constants with temperature indicate the possibility of the presence of Form II in low concentration. The conformation of the second form involves a parallel (1: 3) chlorine-chlorine interaction, and is hence expected to be of high energy. If present at all, it is likely to be distorted from an all-staggered conilguration to relieve the strain. The assignment of the CH, angle bending and *(C-C) bond stretching frequencies suggested in Table 3 are in good general agreement with those of the tram form of 1,2dichloroethane [6]. 3(d) 1,1,2,2-Tetrachloropropane There are only two spectroscopically distinguishable staggered forms of this molecule as shown below. From Fig. 1 it is clear that there are more bands than

H&Hcl& I

II

expected for one form in the CH, rocking and particularly the C-Cl stretching regions of the infrared spectrum. The bands of the two forms above 1200 cm-1 (CH and CH, deformation modes) must be partially superimposed on each other. The infrared frequencies and a schematic assignment are given in Table 4. This compound was also examined in dilute cyclohexane and methyl cyanide solutions. It was found that the bands at 678 and 696 cm-l increased in intensity in the latter solvent, with respect to those at 640, 713 and 726 cm-l; the former bands may therefore be associated with the more polar form II. A schematic assignment of these C-Cl stretching modes to the two forms is shown in Table 4. The two S,u modes of Form I are expected to be coupled and split. The bands at 640 and 713 cm-l are assigned to these modes, since they have a splitting of 73 cm-l and a mean of 676 cm-1 which is near the frequency expected for the S,, mode of a monohalogenohydrocarbon: -The remaining two Pximodes of Form I are assigned to the bands at 726 and 790 cm-l with a mean value of 758 cm-l. For Form II, the more polar form, the bands at 578 and 696 cm-l are assigned to the k&u and [s] S. MIZUSEIMA, J. Chem. Phye. 21, 2196 (1963).

1672

A. Table

B.

DEMPSTIDR, K.

4. Infrared

assignm&ts Li&id

PRICE

and Raman for liquid

md N.

frequencies

SHEPPARD and schematio

1,1,2,2-tetrachloropropane

Raman*

i.r.

3000 m 2984 sh 2940 w 1443 m 13818 1260 m 1212 m 1166~ 1151 m 1128 w

3003 2976 2941 1445 1336 1267 1214 1176 1159

1074 1067 1013 929 900 790 767 726 713 696 665 640 678 667

1076 (3, p?) 1062 (3-4, p 1)

* m VW m w “8 m w “8 “8 VW vs m VW

kssignment t v(CH)(CHCl,) v(CHJ BB +=a) 8 NH, a~ SCH, 8 SCH(CHC1,) 6CH (CHCl,) CH8r CH,r

(6, dp) (1, p) (6,p) (7, dp) (1, p) (3-4, p) (6, dp) (1, p?) (1, p?)

P@--C) Y(C-C)

706 (2-3, dp)

CH,r CHJ Y(C--cI) Y(c-Cl) Y(C-cI) Y(c--cl) *(c-Cl)

I Px II Px I Px I SXH II SXH

643 (4-6 p) 533 (9, P)

v(CC1) @-cl)

I SXH II SEE

931 904 797 771 730

(l-2, p) (4, p) (12, dp) (4, p) (5, p)

* Raman data from Ref. [2]. t I, Wand isomer, II, gouchs isomer. For symbols and abbreviations BW footnotes to Table 1.

Sxu modes, and one of the two remaining Px modes to the band at 767 cm-l. The remaining Px mode expected for Form II has not been identified and probably overlaps one of the P, modes associated with Form I. Since considerable coupling must occur between the vCGlvibrations, the above scheme only shows an approximate relationship to the correlations between frequency and structure observed for the monochlorohydrocarbons. 3(e) 1,1,1,3,3-Pentachloropropane The two spectroscopically distinguishable staggered forms of this molecule shown below each involve parallel (1: 3) chlorine-chlorine interactions. If an all-

I

La

staggered coniiguration of the bonds is maintained, Form I (Table 1) would have two such interactions and hence is likely to be least stable. A small amount (approx 2 per cent) of 1,1,1,2,3-pentachloropropane was present in the liquid state as an impurity in this sample: If the bands from this compound

In&red and NMR etudieaof rotational iaomerimnin cblorincttedpropanea-

1673

are subtracted, the infrared spectrum of the liquid has four strong bands in the range 1600-400 cm-l and five strong bands in the C.-Cl stretching region, as shown in Fig. 1. This data suggests that only a single form is present in any quantity. The observed infrared frequencies are given in Table 6 with a schematic assignment of bands to the normal modes of vibration. Comparison with l,l, l-trichloropropane, and l,l, 1,3tetrachloropropane suggests the assignment of the bands at 673, 712 and 828 cm-l to the Ccl, group. The remaining two bands are well assigned to Px and PH type y(C-Cl) vibrations using the notation of SHIPM~~N [7]; this is consistent with Form II (or a conformation close to this) being the stable isomer of the liquid. Table 6. In&wed and Ramau frequenciesand schematic assignmentafor liquid 1,1,1,3,3-pentachloropropane Liquid i.r.

Assignment

RIImlUl

3000 m 2980 sh 2940 w 1417 8 1343 m

3005 (1, P) 2984 (1, dp) 2945 (3, p) 1420 (3,P) 1360 (1) 1300 (1)

1248 VW 1218m 12018 1067 m 1024 B 970 va 828 VB 730 m 712 vs 671 v8 673 8

1208 (1,dp) 1070 (2, ?p) 1028 (2, dp) 972 (2. dp) 834 (8. p) 737 (12, Tp) 720 (13, P) 973 (9, p) 580 (20, P)

v(CH)(CHCI,) ~a (C-H) (CH,) ~a (C-H) (CH,) 6CH, (s) CH, wag -

CH, tw? N-C) NC-C) CH,r? v(C-cl)(ccr,) B v(C-Cl)(ccl,) es Y(U) Px(CHCJ) N-CWH~,~) w (c-c1) (C%)

For symbols and abbreviationa see footnotes to Table 1.

The vicinal coupling constant obtained from the AX, spectrum is 5.25 Hz at room temperature. Form I is expected to give rise to a smaller (gaucise) coupling constant; so the observed coupling constant is more consistent with the value expected from averaged trans and gauche coupling constants of Form II and its mirror image. Thus infrared and NlWR and steric considerations all point to Form II as the preferred staggered isomer. However, consideration of the steric factors involved using models suggests that some distortion of the carbon-carbon chain will probably take place to relieve the parallel (1: 3) chlorin+chlorine interaction, i.e. the stable form will probably have a conformation that deviates to some degree from a staggered form. 3(f) 1,1,1,2,3-Pentachlmopropane The only sample of this compound available contained a considerable quantity of 1,1,2,2,3-pentachloropropane, which was easily identified from the NMR spectrum. The NMR spectrum of 1,1,1,2,3-pentachloropropane occurs as an ABC spin system, computer analysis of which gave values of 2.16 and 9.47 Hz for the two vicinal coupling constants. 173 J. J. Sm, 6

V. L.

FOLT

and S. KRnna, Sp&ochim.

Acta

18, 1603 (1962).

A. EL mm,

1674

K.

Para

and N. SH~EPP~RD

The i&s& spectrum of the liquid was compared with that available for a pure sample of 1,1,2,2,3-pentachloropropane, and these impurity bands are marked in the spectrum shown in Fig. 1. The frequencies of the infrared spectrum and an assignment of the fundamentals of 1,1,1,2,3-pentachloropropane are given in Table 6. Table 6. Infrared and Raman frequenciesand schematic assignmentsfor liquid 1,1,1,2,3-pentachloropropane Liquid i. r. 3024 w 2964 m 2856 w 1433 s 1319 m 1269 w 1249 m 1209 m 1168m 1069 m 1013 * 938 m 880 VW 836 8 790 “8 734 vs 695 vs 600 vs

Assignment

R-* 3026 (2, dp) 2996 (1-2) 2968 (7, P)

v(CH,) m

1436 (2, dp) 1400 (0) 1327 (0) 1261 (2, dp)

SCH,8

1207 (2, dp?)

GCH(CHC1) CH, tw v(C-C) v(C-C) CH,r

v(CH,) 8 + v(CH)

6CH(CHCl) CH, w

1062 (1. dp) 1016 (2, p?) 934 (1, p?) 839 794 745 697 607

*(C-Cl) @--Cl) @--Cl) @-Cl) v(C-Cl)

(3-4, P) (6, P) (4-k P) (3, P) (5, P)

VCCI, (Ls Px VcCl, an SXH YCCl, B

* Reman data from Ref. 2. For symbols and abbreviations see footnotes to Table 1.

In the infrared spectrum five bands for the main compound are seen in the CH deformation region, and just five in the C-Cl stretching region. This information suggests that just one form is present in the liquid state. This interpretation is oonflrmed by the NMR coupling constants, which correspond to a trane and gauche type. It may be deduced that Form II of the three possible all-stsggered conformations shown below, which would have two gauclie coupling constants, is not the

I

It

III

stable form. A choice between Form I and Form III is made on steric grounds. Form I does not involve a parallel (1: 3) chlorine-chlorine interaction, and should be the more stable conformation. 3(g) 1,1,1,2,3,3-HexachJoropropa~e This molecule has already been discussed in the first paper of this series [l] and thus will be only briefly referred to here. The infrared spectrum of the liquid is

I&wed

and NMR studies of rotational isomerism in oblorinatedpropenes--

1676

shown in Fig. 1, and the assignment of the fundamental infrared frequencies given in Table 7. The presence of four sharp bands in the region 1500-1100 cm-l corresponding to four CH deformation modes, and two in the C-C stretching region indicate that just one form is present in quantity. The three possible all-staggered forms are shown below. The very small value of the vicinal coupling constant (l-4 Hz) suggests that

I

m

II

Form III which has a trana arrangement of the vicinal hydrogen atoms is certainly not present (cf. 1,1,2-trichloropropane). As previously outlined, Form II is chosen over I on steric grounds, and again probably a distortion occurs from the all staggered conformation to relieve the steric strain of the remaining parallel (1:3) chlorinechlorine interaction. 3.8. 1,1,1,2,2,3,3-Hepifu&kw~~ne This compound is a solid which melts just above room temperature. The liquid and crystal infrared spectra obtained near room temperature are shown in Fig. 4. Table 7. Inkwed and Raman frequenciesand schematic aasignmentafor liquid 1,1,1,2,3,3-hexaohloropropane ABaignment

Liquid i.r.

F&man*

3008 8 2964 s 1442 w 1330 m 1276 m 1262 VW 1217 s 1207 w 1183 w 1946 VW 998 s 959 xv 944w 920 w 849 vs 827 e 793 w 782 vs 741 vs 703 w 670 8 610 VB 662 s

3993 (4, P) 2954 (4. p)

v(CH)(CHW v(CH)(CHcI)

1306 (2, Tp) 1275 (2, rp)

mlw=w GCH(CHG1,) inm. &k(CHCl) K=(CH$)

1212 (4, P) 1062 (2, lp) 1991 (4, p)

W-Q) v(c--c)

854 (4. p) 828 (6. p) 779 (3. p) 744 (6. p) 671 (&P) 912 (4. P) 566 (4. P)

* Ramen d&a from Ref. 2. For symbola and abbreviations we footnotes to Table 1.

1676

A. B. I~WPSTER, K.

hIOE

and N.

&EPPAlZD

I

Liquid

l-L

r

rl

Cryst.

&ii c-i T---d+-

cm-’

f

Y I )O

Fig. 4. The infraredspectrum of 1,1,1,2,2,3,3-heptachloropropane (a) in the liquid state and (b) in the crystalline solid state.

No indication was found of more than one rotational isomer. The two possible spectroscopically distinguishable forms are again shown below.

Infmred

and NMR

stud&~ of rotetior&

lb77

isomerism in chlorineted propenes-II

From the limited experimental information, detaila concerning the configuration of the isomer cannot be deduced. However, from the sterio arguments previously outlined, it is likely that Form II would be preferred over Form I, but that distortion would occur to relieve the strain of the rem&ing parallel (1:3) chlorins chlorine interaction. The infrared bands are listed in Table 8 and separately assigned to the fundamental frequencies of the molecule. Table 8. Infrared and Raman frequencies and schematic assignments liquid and crystalline 1,1,1,2,2,3,3-heptachloropropane Liquid ix. 3010 m 1261 vw 1211 w 1022 w 966 w 870 m 828 m 790 m 772 s 742 8

Cry&al

Rsman*

ix.

Assignment

3006 (3, P) 1266 (3, dp) 1212 (3, dp)

3020 m 1260 w 1212 m 1029 m 966 m 870 m 829 m 790 m 768 s 742 m 738 m

1023 (4, 967 (7, 872 (8, 830 (6, 796 (3, 768 (4, 744 (2,

for

p)

P) dp) p)

P) P) P)

NW K=%J SCH(CHCLJ 6CH(CHCb) y(c--c) y(c-c) V(CC1.J as Y(CC1)

Px

+---w Px Y(CC1.J 89 ~(C-CU

sxx

722 (1, P)

665 m 578 m

692 vw 668 s 678 f3

674 (6, p) 580 (8,~)

* Raman data from Ref. [2]. For eymbols and sbbreviationa

N-m &cl? 4cqJ *

see footnotes to Table 1.

4. CONCLUSIONS In the chloroethanes the maximum number of spectroscopically distinguishable isomers possible for a given molecule are all present in the liquid phase [S]. The molecules discussed in this paper fall into two classes. The Srst three molecules discussed each contain a terminal CH, group. For this class all of the possible sllstaggered distinguishable isomers are observed, with the exception of Form I for 1,1,2-trichloropropane. The second set contsins the latter five molecules, each with a terminal CCl, group. For none of the members of this class is more than one spectroscopically distinguishable isomer desnitely observed in the liquid, although there appears to be some evidence for a second isomer in the case of l,l,l,t-tetrachloropropsne. As explained in our earlier paper, the steric interaction between chloride atoms on carbon-chlorine bonds which are parallel to each other and on the same side of the carbon skeleton is large. It is this ‘parallel (1: 3)’ chlorinechlorine interaction which effectively determines the rotational isomeric situation in the chloropropanes and reduces the number of possible staggered isomers to the one observed, in cases where there is a terminal Ccl, group present. [8] G. bI.JZN,P. N. BRIER and G. LANE, Tram.

Faraday Sot. a,

824 (1967).

1678

A. B. DEMPS~,

K. PRIM and N. S~PPARD

In a subsequent paper the ideas developed so far will be extended to rather more unsymmetrioally substituted chloropropanes where, in principle, rotationalisomerism can occur about each C-C bond giving rise to four or more possible rotational isomers. E’urther systematic discussion of the group frequencies in the vibrational spectra of the molecules presented here will be reserved until the end of the next paper. However, except for uncertainties in assignments of coupled K-Cl vibrations and possibly CH, rocking modes, the assignments given form part of a con&tent pattern. Achledgemntu-We me very grateful to Prof. Dr. H. CEIWIN~ for the donation of many of the compounds studied, and to the Hydrocarbon Research Group of the Institute of Petroleum and to the Mini&-y of Te&nology for fiuancial support of thie work. We thank Mr. D. H. CEENEBY for measurements of the Raman spectra of 1,1,1,3-tetrachloroproopropane and 1,1,1,3,3pantaohloroprop8ne.