Struct. Chem., Vol. 1, pp. 61-72

ISSN 1040-0400

Conformational Stability, Barriers to Internal Rotation, Normal Coordinate Analysis, and Vibrational Assignment of Chloroacetyl Bromide J. R. Durig* and H. V. Phan 1 D e p a r t m e n t of C h e m i s t r y , U n i v e r s i t y of South Carolina, Columbia, S o u t h Carolina 2 9 2 0 8 ,

The infrared spectra (3200 to 30 cm -1) of gaseous and solid chloroacetyl bromide, CH2CIC(O)Br, and the Raman spectra (3200 to 10 cm 1) of the gas, liquid (with depolarization data), and solid have been recorded. From the observed asymmetric torsional transitions, the potential function governing internal rotation of the CH2CI moiety has been determined with the following coefficients: V1 = 336 • 11, V2 = 73 • 10, V3 = 757 + 7, V4 = 103 • 3, and V6 = 5 • 2 cm -1. This potential function is consistent with s-trans to gauche and gauche to gauche barriers of 963 • 11 and 709 • t 2 cm-1, respectively, and enthalpy difference of 373 • 24 cm -1 with the dihedral angle of the gauche rotamer being 115 ~ The enthalpy difference has been determined experimentally from the studies of the Raman spectra at different temperatures to be 359 • 6 8 c m -1 (1.03 • 0.19kcalmo1-1) and 507 • 2 4 c m 1 (1.45 • 0 . 0 7 k c a l m o l 1) f o r t h e gas and liquid, respectively, with the s-trans conformer being the more stable conformer in the gas and liquid and the only one present in the annealed solid. A complete assignment of the vibrational fundamentals is proposed from spectral data obtained for the gas, liquid, and solid. The assignment is supported by a normal coordinate calculation utilizing a modified valence force field to obtain the frequencies for the normal vibrations and the potential energy distribution. The results are discussed and compared to the corresponding quantities for some similar molecules.

*To whom correspondence should be addressed. 1Taken in part from the thesis of H. V. Phan, which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree.

Manuscript received 12/9/88; accepted 1/12/89. 9 1989 VCH Publishers, Inc.

USA

INTRODUCTION Rotational isomerism in chloroacetyl bromide, CH2C1C(O)Br, has been the subject of a previeusly published vibrational study [1] in which the authors proposed assignments of the fundamental modes for two conformations. These authors utilized the Raman spectrum of the liquid in conjunction with the infrared spectra of all three phases, but the asymmetric potential function governing the interconversion of the two conformers was not determined since the asymmetric torsional fundamentals were not observed in the gas phase. H o w e v e r , from a study of the relative intensity of the infrared bands at variable temperatures, the enthalpy difference was determined to be 1.86 + 0.27 kcal tool- ~for the gas with the s-trans conformer (chlorine atom s-trans to the bromine atom) being the more stable form. Similarly, for the liquid, the s-trans conformer was assumed to be the more stable form and the only one present in the spectrum of the solid. The high-energy rotamer was calculated with a Urey-Bradley potential function to have the staggered conformation with the azimuthal angle close to 180 ~. Based on the results of our earlier studies of chloroacetyl fluoride [2] and chloroacetyl chloride [3], there appear to be inconsistencies in the assignments for the normal modes in this previously published study [1]. Hence, we have carried out this spectroscopic investigation in order to determine the asymmetric torsional potential function from which the barriers to internal rotation can be obtained, to determine the relative stability of the observed conformations in the gas and liquid phases, and to propose a complete assignment of the fundamental vibrations. The Raman spectra of the gas and solid have not been previously reported.

61

62

J. R. Durig and H. V. Phan cell [4]. The study of the Raman spectrum of the liquid at variable temperatures and the Raman spectrum of the solid (Figure 1C) were obtained with the sample sealed in a glass capillary housed in a Cryogenic Technologies, Inc. cryostat connected to a model DTC-5000 cryogenic temperature controller by Lake Shore Cryogenics, Inc. The midinfrared spectra of the gas and annealed solid (Figure 2) were recorded using a Digilab model FTS-14C Fourier transform interferometer equipped with a Ge/KBr beamsplitter and a TGS detector. The spectrum of the gas (Figure 2A) was obtained with the sample contained in a 12-cm cell equipped with CsI windows. The spectrum of the annealed solid (Figure 2B) was obtained by depositing the sample onto a CsI plate cooled by boiling liquid nitrogen and housed in a cell fitted with CsI windows. The far-infrared spectrum of the gas (Figures 3A

EXPERIMENTAL The sample of chloroacetyl bromide was synthesized from sodium chloroacetate by reacting the salt with PBr3 under reduced pressure. Purification was carried out with a low-temperature vacuum fractionation column. The pure chloroacetyl bromide was stored under vacuum and in a slush of n-propanol and Dry Ice. The Raman spectra (Figure 1) were recorded on a Cary model 82 spectrophotometer equipped with a Spectra-Physics model 171 argon ion laser operating on the 5145-.& line. The spectrum of the gas phase (Figure 1A) obtained at 62~ as well as the study of the Raman spectrum of the gas at different temperatures, were carried out by using a standard Cary multipass accessory. The spectrum of the liquid (Figure 1B) was recorded with the sample sealed in a spherical

A !

i

I

I

~

t

l

r

B

I

,

3000

I st,

i

i

1500 1000 W a v e n u m b e r (crn'l)

i

i

~

i

500

,

i

0

Figure 1. Raman spectra of chloroacetyl bromide: (A) gas, (B) liquid, and (C) annealed solid.

Conformational Stability of Chloroacetyl Bromide

63

A

Figure 2. Midinfrared spectra of chloroacetyl

F ' ' / ~ 3000 2000

I

1500

i

i

~

i

i

1000

i--

I

500

WAVENUMBER (cm-1)

bromide: (A) gas and (B) annealed solid.

and 4) was recorded on a Nicolet model 200 SVX Fourier transform interferometer equipped with a vacuum bench and a liquid-helium-cooled Ge bolometer containing a wedged sapphire filter and a polyethylene window. The sample was contained in a 10-cm cell fitted with polyethylene windows. The resolution was 0.1 c m - 1 and the 25-/xm Mylar beamsplitter was used. The far-infrared spectrum of the annealed solid (Figure 3B) was obtained on a Digilab model FTS-15B Fourier transform interferometer equipped with a 6.25-txm Mylar beamsplitter. The sample was condensed onto a silicon plate, which was cooled by boiling liquid nitrogen, and the silicon plate was housed in a cell fitted with polyethylene windows.

of temperature T, then the value of AH can be determined from a plot of - I n K against 1/T. In order to determine the magnitude of the enthalpy difference between the gauche and the s-trans conformers of chloroacetyl bromide, we carried out studies of the Raman lines occurring at 690 and 528 c m - i in the gas and the corresponding lines at 683 and 526 cm-1 in the liquid at different temperatures. The data (Table 1 and Figures 5 and 6) were fitted to Equation 1 and values of 359 + 68 c m - 1 (1.03 _+ 0.19 kcal mol i) and 507 -+ 24 c m - I (1.45 _ 0.07 kcal mol-1) were obtained for AH for chloroacetyl bromide in the gas and liquid phases, respectively.

TORSIONAL POTENTIAL FUNCTION

CONFORMA TIONAL ENTHALPY DIFFERENCE The enthalpy difference AH between two stable forms of a molecule is related to the population according to the equation - I n K = ( A H / R T ) - (AS~R)

i

(Eq. 1)

where K is the relative population of the two forms, and AS is the entropy change. If we assume that the intensity of the Raman line is proportional to the population, then K can be taken as the ratio of the Raman lines assigned to the same fundamental of the two rotamers. If it is assumed further that AH is not a function

The far-infrared spectrum of gaseous chloroacetyl bromide (Figure 4) reveals a series of Q branches starting from 80.15 cm 1 and falling to lower frequency. Based on the results of our recent studies of chloroacetyl fluoride [2] and chloroacetyl chloride [31[ we presently assign these to the asymmetric torsional transitions of the s-trans conformer. For the high-energy form, weak features occurring at 53.84 and 52.18 c m - i are observed. The frequencies of these features are reasonable since the asymmetric torsional fundamental of the high-energy form of chloroacetyl fluoride [2] and chloroacetyl chloride [3] was observed at 47.80 and 50.70 cm-1, respectively.

64

J. R. Durig and H. V. Phan equation for F(~b) by assuming them to be small periodic functions of the general type: B(qS) = a + b cos ~b + c sin ~b

'

I

i

I

I

'

200

100

I

I

I

'

B

(Eq. 4)

Structural parameters were taken from the results of ab initio calculations of chloroacetyl chloride [3], and those from the electron diffraction experiment of bromoacetyl bromide [5]. Initial values for the potential coefficients were taken from the study of chloroacetyl chloride [3]. With the assignment of the asymmetric torsional transitions for both the s-trans and gauche forms (Table 2), the kinetic terms, and the value of 2~H experimentally determined, the potential function for chloroacetyl bromide was calculated. This potential function (Figure 7) is consistent with the s-trans to gauche and gauche to gauche barriers of 963 _+ 11 cm- 1 and 709 _+ 12 cm -1, respectively, with the enthalpy difference calculated to be 373 +_ 24 c m - i . The observed and calculated torsional frequencies are listed in Table 2, whereas the calculated potential coefficients and the " F numbers" are given in Table 3. An alternative assignment of the 1 ~ 0 and 2 +- 1 torsional transitions arising from the gauche form to those occurring at 44.48 and 43.04 cm- 1 was also considered. However, this assignment gave a calculated gauche to gauche barrier much smaller than the corresponding barrier calculated for chloroacetyl chloride. Therefore, we rejected this alternative assignment since one expects the gauche to gauche barrier to be larger for the bromide.

VIBRATIONAL ASSIGNMENT

I

300

WAVENUMBER (cm -1)

Figure 3. Far-infrared spectra of chloroacetyl bromide: (A) gas and (B) annealed solid.

The potential energy V(qS) for the asymmetric torsional motion is a function of the angle of internal rotation ~b and can be represented by a cosine series: V(qS) = ~

V,(1 - cos i4~)

(Eq. 2)

i=1

The internal rotational constant F, or the kinetic term, is also a function of the angle of internal rotation and can be represented by a Fourier series: 6

F(~b) = F0 + ~2 Ficosio5

(Eq. 3)

i=1

The relaxation of the structural parameters, B(q~), during the internal rotation can be incorporated into the

The assignment of the two C - - H stretching modes is straightforward (see Table 4) and in agreement with those previously proposed [1]. For the CH2 deformation of both conformers a weak band of B-type contour is observed at 1408 cm 1 in the infrared spectrum of the gas. Also in this spectrum, weak features occurring at 1281 and 1267 cm -1 are observed and these are presently assigned to the CH2 wag of the s-trans and the gauche conformer, respectively. The remaining two CH2 bending vibrations of the s-trans rotamer are A" fundamentals and should give rise to infrared bands of C-type contour. The CH2 twist of this rotamer can be assigned to the weak C-type band observed at 1176 cm-1, whereas the CH2 rock is found at 894 cm-1. The assignment of these two modes is based on "group frequencies," the assignments made for the corresponding fundamentals for chloroacetyl fluoride [2] and chloroacetyl chloride [3], and the depolarization ratios of the corresponding lines in the Raman spectrum of the liquid. For the corresponding modes of the gauche

Conformational Stability of Chloroacetyl Bromide

65

/

Figure 4. Far-infrared spectrum of gaseous chloroacetyl bromide in the region of the asymmetric torsional mode.

I

I

90

70

Table 1. Temperatures and Intensity Ratios [1(528)/1(690) Gas and 1(526)/1(683) Liquid] for the Conformational Studies of Gaseous and Liquid Chloroacetyl Bromide Phase

Liquid

K

-In K

24 34 44 59 62 70 80

3.36 3.26 3.15 3.01 2.98 2.91 2.83

0.761 0.813 0.858 0.884 0.919 0.972 1.018

0.273 0.207 0.153 0.123 0.084 0.028 -0.018

- 113 - 103 -93 - 83 - 73 - 63 -53 - 43 - 33 - 23 - 13 - 8 17 22

6.24 5.88 5.55 5.26 5.00 4.76 4.54 4.34 4.16 4.00 3.84 3.77 3.45 3.39

0.062 0.074 0.096 0.124 0.140 0.176 0.191 0.231 0.261 0.280 0.316 0.377 0.458 0.509

2.781 2.604 2.343 2.088 1.966 1.737 1.656 1.465 1.343 1.273 1.152 0.976 0.781 0.675

T (~

1000 x (1/T)K ~

30

WAVENUMBER (cm -1)

conformer no spectral features are observed. Therefore, they are either too weak to be observed or they are degenerate with those of the s-trans form. With the exception of the C H 2 deformation, the present assignment of all of the CH2 bending fundamentals of both

Gas

50

conformations is in disagreement with that given earlier [1 ]. The assignment of the C~---O stretching fundamental of the two conformers is clear with the observation of the two intense B-type bands at 1837 (s-trans) and 1809 cm-~ (gauche). Although the C - - C stretching mode of the s-trans form is readily assigned to the relatively intense infrared band at 951 cm -l, the assignment of this mode for the gauche form is less certain. If reasonable structural parameters are assumed for this rotamer where they are transferred from those from similar molecules it can be shown that the dipole moment change for the C - - C stretch would be more along the a principal axis than along the other two axes. Hence, an A-type infrared band would be expected for this fundamental for the gauche conformer. Based on this argument, we have assigned the A-type band observed at 1023 cm ~to the C - - C stretch of the gauche rotamer. The alternative assignment is to the B-type infrared band at 974 cm -1. The C--C1 stretching fundamentals of both conformers are presently assigned to the intense Raman line observed at 781 cm -I. It is worth noting that in chloroacetyl chloride [3], the corresponding modes of the two rotamers were also found to occur as degenerate. In contrast, the C - - B r stretching fundamental of the s-trans and the gauche form is assigned in the Raman spectrum of the gas to the lines of medium intensity at 690 and 528 c m - l, respectively. The present assignment of the C--C1 and C - - B r

66

J. R. Durig and H. V. Phan

0.30

/

0.20

v e" i

0.10

/

/

0.00

, 2.8

I

,

3.0

I

I

3.2 l I T x 10 3

stretches of the gauche conformer does not agree with that given earlier by Khan and Jonathan [1]. Based on the earlier studies of fluoroacetyl bromide [6], we assign the shoulder at 518 cm -1 in the Raman spectrum of the liquid to the C(O)Br wag of the high-energy form. The corresponding mode of the s-trans rotamer, which has Cs symmetry, gives rise to the depolarized feature at 460 c m - 1 in the Raman spectrum of the liquid. Also based on our other studies of haloacetyl halides [2,3,6-9], we assign the weak features observed at 370, 320, and 155 cm- 1in the Raman spectrum of the gas to the C(O)Br deformation, C(O)Br rock, and CCC1 bend, respectively, of the gauche conformer. These fundamental vibrations of the s-trans rotamer are observed at 354, 244, and 189 cm -l, respectively, in the Raman spectrum of the gas. The fundamental frequencies are compared to those for chloroacetyl fluoride and chloroacetyl chloride in Table 5.

N O R M A L C O O R D I N A T E A N A L YSIS A normal coordinate analysis has been carried out for chloroacetyl bromide based on the more stable s-trans

I 3.4

Figure 5. Temperature dependence of the ratio of gauche and s-trans conformers of chloroacety! bromide in the gas phase.

conformation by using the Wilson FG matrix method [10]. The 16 symmetry coordinates were defined in terms of the changes of the internal coordinates (Figure 8) as given in Table 6. Initial force constants were taken from the studies of chloroethanol [11], fluoroacetyl bromide [6], and propenoyl bromide [12]. A computer program, written by Schachtschneider [13], was used to iterate the force constants until the best fit between the calculated and the observed fundamental frequencies was obtained. An excellent fit (0.28%) was achieved with the resulting modified valence force field given in Table 7. The corresponding potential energy distribution among the symmetry coordinates for the s-trans conformer is given in Table 8. This force field was then utilized to calculate the fundamental frequencies of the less stable gauche form. These frequencies (Table 8) were reproduced with an average of 4.04%, which appears to be reasonable since no further adjustments were carried out. The potential energy distribution for this conformer is also given in Table 8. While the magnitude of the force constant presently determined for the C - C - H bending coordinate seems to be small compared to the corresponding quantity calculated in the study of fluoroacetyl bromide [6],

Conformational Stability of Chloroacetyl Bromide

67

3.0

v c

2.0

//

1.0

Figure 6. Temperature dependence of the ratio of gauche and trans conformer of chloroacetyl bromide in the liquid phase.

!

3.0

a

1

1 4.0

I

, , , I , , -5.0,

, t6.0,

|

Ip

1/1" x 10 3

the other force constants appear to be in good agreement. For example, our value of 1.047 mdyn A i for the C-C-C1 bending force constant is in excellent agreement with the value of 1.053 mdyn A - i reported for chloroethanol [11]. It is interesting to note that the values of the C-C=O, C - C - B r , and O--C-Br bending force constants (0.632, 2.453, and 0.701 mdyn ~ - 1 , Table 2. Observed Torsional Transitions (cm -1) of Gaseous Chloroacetyl Bromide Conformer

Transition

Observed

Obs.-Catc.a

Trans

1 ~-- 0 2 ~-- 1 3 *-- 2 4 +- 3 5 ~ 4 6 ~ 5 7 ~ 6 8 ~ 7 9 ~ 8 1-7- ~-- 0 -+ 2-T ~-- 1 _+

80.15 77.44 74.85 72.37 69.94 67.48 65.00 (63.00) b 59.92 53.84 52.18

0.39 0.03 -0.18 -0.24 -0.20 -0.14 -0.02

Gauche

!

0.38 0.30 -0.32

aCalculated using the potential coefficients and "F numbers" given in Table 3. bNot used in the calculations.

respectively) compare satisfactorily with the corresponding force constants for propenoyl bromide [12] (0.590, 2.090, and 0.960 mdyn .~-~, respectively). Additionally, the same trend is observed for the C(O)Br wagging and the C - - B r stretching force constants for these two molecules. Within the A" symmetry block of the s-trans conformer, the potential energy distribution initially calculated indicated extensive coupling between the CH2 twisting and rocking fundamentals. The subsequent inclusion of the interaction (0.059 mdyn ,~-~) constant between the C - C - H and H-C-C1 bending coordinates reduced the coupling and gave a reasonable potential energy distribution among the vibrations in this symmetry block. The interaction also improved the fit of the fundamentals in the A' symmetry block as well, particularly the CH2 deformation and the CH2 wag, as expected. The other interactions (Table 7) were utilized mainly to improve the fit for the frequencies for the skeletal vibrations. For the gauche rotamer, the resulting force constants predict the order of the assignment of the fundamental vibrations, and the potential energy distri-

68

J. R. Durig and H. V. Phan

1000'

70!

500"

373

O, -180

180

0

DIHEDRAL ANGLE, q~

bution among the symmetry coordinates is surprisingly pure. However, the force constants were not adjusted, although they are expected to be somewhat different because of the changes in angles that must result from the internal rotation from the s-trans to the gauche conformation. Nevertheless the changes in the force constants would be expected to lead to significantly different potential energy distributions among the normal vibrations.

DISCUSSION

From the Raman and infrared spectral data presently obtained, it is obvious that chloroacetyl bromide Table 3. Comparison of Potential Coefficients and Barriers to Interconversion for CH2CIC(O)X (X = F, CI, Br) a s Determined from Far-Infrared Spectral Data V~ V2 V3 V4 V6 s - t r a n s / g a u c h e b arrier g a u c h e / g a u c h e barrier E n t h a l p y difference Dihedral angle

(Cl-C-CO) (~ Torsional fundamentals s-trans g auche

X = F"

X = CI b

X = Br "

350 _+ 12 306 + 6 420 + 1 44 _+ 1 2 -+1 796 245 525 -+ 19 122

438 _+ 16 278 _+ 8 557 _+ 1 67 + 2 6-+1 944 408 587 -+ 25 118

336 -+ 73 -+ 757 -+ 103 -+ 5+2 963 -+ 709 -+ 373 -+ 115

11 l0 7 3

86.5

82.7

80.2

47.8

50.7

53.8

11 12 24

"Taken from Ref. [2]. ~'Taken from Ref. [3]. 'Parameters as determined from the assignment given in Table 2, and Fo = 0.625143, F~ = 0.004611, Fz = 0.183226, F3 = -0.036273, F4 = 0.038282, F5 = -0.012764, and F6 = 0.009482 cm -1.

Figure 7. A s y m m e t r i c torsional potential function for c h lo r o acety l bromide as d e t e r m i n e d from the far-infrared spectral data. The dihedral angle of 0 ~ c o r r e s p o n d s to the s-trans conformer.

exists in two isomeric forms in the gas and liquid with the s-trans conformer being the more stable form. Additionally, this form is the only conformer remaining in the annealed solid. The observed spectral data for the high-energy rotamer is consistent with this rotamer having the gauche conformation. The enthalpy difference for the gas has been determined to be 359 +- 68 cm-I (1.03 + 0.19 kcal mol-~). This value for chloroacetyl bromide seems to be out of line with the corresponding quantities determined previously for chloroacetyl fluoride [2] and chloride [3], and small compared to the value of 650 _+ 94 cm -1 (1.86 + 0.27 kcal mol-J) obtained from the study [1] of the temperature dependences of the infrared bands. For the liquid, a value of 507 + 24 cm 1 (1.45 _+ 0.07 kcal mol l) was determined. The values for the gas and liquid, determined in the present study, indicate greater relative stability of the s-trans conformer in the liquid, which implies that the s-trans form has a larger dipole moment than the gauche form [14]. As can be seen in Table 5, the fundamental frequencies of the s-trans rotamer reflect the effects of substitution of halides of heavier masses. For the gauche form, the observed changes in the frequencies compared to those for the corresponding modes for the s-trans conformer can be attributed, at least in part, to the changes in the angles for the heavy atoms around both carbon atoms as well as to steric hindrance between the two halogen atoms. The vibrational assignment made for both the s-trans and the gauche conformers should be highly reliable for two reasons. First, data were obtained for all three physical phases. Second, we have previously studied the lighter chloroacetyl halides [2,3] and utilized ab initio calculations so that trends in the observed fundamental frequencies

Conformational Stability of Chloroacetyl Bromide

69

Table 4. Observed a Infrared and Rarnan Frequencies (crn- 1) and Vibrational Assignment for Chloroacetyl Bromide Infrared Gas

Rel. int.

Raman Rel. int.

Solid

3004Q, C 2968 R 2963 Q , A 2961P

vw

2986

m

w

2942

m

1909 1875

bd, sh bd, sh

2901 1914 1883

bd, vw m w

vs

1809 1802

s vs

1840 R 1837min, B 1816 R 1809min, B 1806 P 1734 1412 R 1408min, B 1404 P

128l 1271R 1267min, B 1263 P 1181Q 1176 Q, C

1110 1026 R 1023Q, A 1019 P 997 976 R 974min, B 969 P

2966

1835

m

vw

Assignment

Rel. int. & depol.

Solid

2987

m, dp

2986

2947

vs, p

1913

Liquid

1816

Rel. int.

~b

Approximate description

m

Pll

CH2 antisymmetric s t r e t c h

2942

vs

t.q

CH2 symmetric s t r e t c h

vw, p

1912

bd, vw

bd, m, p

1816 1805 1800

sh m m

sh, s

bd, vw

W

sh, w

1776 1726 1628

w vw vw

1385

s

1337

w

1276

w

1768

w w

1173

vw

bd, vw

1138 1096

vw vw

2~

u2

C=O

stretch

u'

C~O

stretch

vw

2 ~13 ~5 + /"6 ~12 -~ P14

1410

1278

bd, vw

w

1394

1275

m, dp

1386

m

w, p

1339 1280 1278

vw w w

w

1172

w, dp

1175 1172

w w

P3

CH2 deformation P13 -c P14

u4

CH2 wag

U'

CH2 wag

~2

CHz twist P9 + ul3 PJo -}- Pl3

1017

m

v'

vw, p

C - - C stretch

sh, m 972

m

956 R 951 min, B 948 P 934 Q, A 928 P 914 Q, C 894Q, C 867 824 780 Q, C

sh vw bd, vw bd, vw m

724 712

sh, w sh, m

s

957

vs

953

w

w

PI3 -[- Pl5

957

w, p

970 960 958

sh w w

v5

vs

674 Q , A 667 P 642

sh

C - - C stretch I)7+/2 9

sh

690 Q, C

909 887

m m

914

775

m

781

s

w

722 708

vw vw

714 686 680

vs vs

690

vw

m

914 892

w, p sh, dp

910 888

w vw

773

s, p

776 773 756

vs s vw

699 690

s w

2U14

1,~3

CH2 rock

~'6

u~ + u;o C - - C I stretch 2u8 Impurity b~9 -}- ~14

683

m, p

v7

C - - B r stretch Impurity Impurity

vw 633 611

593 R

Gas

Rel. int.

vw vw

604

vw, p

us + /~9

70

J. R. Durig and H. V. Phan Table 4. (continued) Infrared Rel. int.

Gas 584 Q, A/C 531Q

w

458max

bd, vw

355 Q 318max 247 R 244Q, A/B 240 P 190 R 184min, B 163 R 155 min, B 80 Q, C 54 Q

Raman Rel. int.

Solid

528

vw 499 456 436 431

vw vw vw vw

m bd, vw

364

s

s

248 245

w vw vw vw

Rel. int.

Gas

Liquid

m

Assignment

Rel. int. & depol.

Rel. int.

Solid

526 518

m, p sh, p

458

vw, dp

458

w

vib

v' u'

A p p r o x i m a t e description

u~4

C - - B r stretch C(O)Br wag 2p9 C(O)Br wag

u~ v'

Impurity C(O)Br deformation C(O)Br rock

370 354

sh, w s

384 358 323

sh, vw s, p sh, p

377 364

vw vs

vs s

244

vs

247

vs, p

252 246

s sh, m

u9

C(O)Br rock

200

vw

189

vw

189

vw, p

203

vw

u~o

CCC1 bend

155

vw

167

vs

164 115

vw, p vw, dp

164

vw

v' u~5 u'

CCCI bend A s y m m e t r i c torsion A s y m m e t r i c torsion

88

w

93 88 76 70 51 47 43 38 32

m sh vw m sh m s s s

Lattice m o d e s

"Abbreviations used: s, strong; m, moderate; w, weak; v, very; sh, shoulder; bd, broad; p, polarized; dp, depolarized; max, maxima; min, minimum; P, Q, and R refer to vibrational-rotational branches; A, B, and C refer to infrared band contours. hu' refers to assignments made for the gauche conformer.

Table 5. Comparisons of the Observed Fundamental Frequencies ~ of the Chloroacetyl Ha#des CHzCICOF h

CHaC1COCI r

CHRC1COBr d

Fundamental

s-trans

gauche

s-trans

gauche

s-trans

gauche

CH2 antisymrnetric stretch CH2 s y m m e t r i c stretch C = O stretch CH2 deformation CH2 wag CH2 twist C - - X stretch CH2 rock C - - C stretch C - - C I stretch C(O)X deformation C(O)X wag C(O)X rock CCCI bend A s y m m e t r i c torsion

3010 2969 1888 1417 1315 1188 1097 926 882 808 634 526 389 214 87

3040 2969 1860 1417 1282 1132 1234 938 851 794 668 552 405 204 49

3014 2965 1835 1413 1283 1186 724 901 992 793 447 477 299 200 83

3014 2965 1802 1427 1268 1186 565 918 1043 793 447 626 357 174 51

3004 2963 1837 1408 1281 1176 690 894 951 781 354 458 244 184 80

3004 2963 1809 1408 1267 1176 528 894 1023 781 354 5182 323 e 155 54

~Raman and/or infrared frequencies for the gas unless otherwise noted. bTaken from Ref. [2]. ~Taken from Ref. [3]. dThis study. eObserved in the Raman spectrum of the liquid.

Conformational Stability of Chloroacetyl Bromide

71

Table 7. Valence Force Constants for CH2CICOBr

CI

Force constants

~

KA Ke KB Kr Kr He H~ H, Hp

< ...I~/~H/_../B

Ho

Br

H,o He H, Fe~ Fr

Figure 8. Internal coordinates for chloroacetyl bromide.

could be established and utilized. The presently proposed vibrational assignment differs significantly from that given earlier [1], particularly for the normal modes for the gauche conformer for which we have reassigned about 50% of the fundamental frequencies. A comparison of the potential function calculated for the three chloroacetyl halides shows that the s-trans to gauche barriers increase from 796 cm-~ for CHzCICOF to 944 c m - ~ for CH2C1COC1 and finally to 963 c m - ~ for CH2C1COBr. The increase in this barrier can be attributed to the increased steric interaction between the carbonyl halogen atom and the hydrogen atoms in the transition state. Since the bromine atom is only slightly larger than the chlorine, the s-trans to gauche barrier is calculated to increase only slightly from CHzC1COC1 and CH2C1COBr. On the other hand, this barrier increases significantly from CH2C1COF and CHzC1COC1. At the transition state between the two gauche wells (the two halogen atoms eclipse each other),

Fpo Ft3o

Description

Value (mdyn A-~)"

C - - C I stretch C - - C stretch C - - H stretch C = O stretch C - - B r stretch CCC1 bend C C H bend HCCI bend H C H bend CCO bend C C B r bend O C B r bend OCBr wag C--C/CCH CCH/HCCI C--C/CCO CCCI/CCO

2.968 4.487 4.851 11.020 2.897 1.047 0.610 0.721 0.445 0.632 2.453 0.701 0.347 0.300 0.059 1.151 0.50~

OThe bending coordinates are weighted by 1 A.

repulsive interactions increase and the gauche to gauche barrier increases dramatically. The increase in this barrier as one proceeds from chloroacetyl chloride to chloroacetyl bromide has a significant effect on the fundamental frequencies and the spacing of the torsional transitions of the gauche conformer. Although the chloroacetyl halides exist as an equilibrium between the more stable s-trans conformer and the high-energy conformer, which is the gauche rotamer, the equilibrium for the corresponding a~dehyde, chloroacetaldehyde (CHzC1CHO), is one between the more stable s-cis and the high-energy s-trans conformer [15-17]. There is a small "hump" at the s-cis conformation but it is probably below the first energy level for the asymmetric torsion [ 15,16,18]. igor

Table 6. Symmetry Coordinates Used in the Normal Coordinate Calculations for

Chloroacetyl Bromide Approximate description CH2 symmetric stretch C ~ O stretch CH2 deformation CH2 w a g C - - C stretch C - - C 1 stretch C - - B r stretch C(O)Br deformation C(O)Br rock CCC1 bend

Redundancy Redundancy CHz antisymmetric stretch CHz twist CH2 r o c k C(O)Br wag

Symmetry coordinate" $1 Sz Ss $4

= (~kBI Ac ~kB2)

= AY = [(6 m + 2)Ap - (6 I/2 - 2)Aft = (Ao-~ + Ao-z - Aej - AE2)

Ao-t -- Ao-2 -- AE~ -- AE,]

$5 = AP $6 = AA $7 = AT Ss = (A0 + Ao~ - 2A0) $9 = (A0 - A~o) S~0 = [(6 '/2 - 2)Ap - (6 m + 2)A/3 + Agl + A~r2 + AEI -~- AI~2] SIR = (A0 + AW + A~h) SzR = (A/3 + Ao5 + A~: + Ae~ + AE2 + Ap) S~l = (2tB~ - zkB:) Sl2 = (AO-I -- AO'2 -- A e l -7 A~2) Sis = (Ao5 - A~2 + Ael -- Ae2) $I4 = A"O

"Not normalized. For the s-trans conformer, S~ through Sl0 belong to the A' symmetry block and SH through S~4

belong to the A" symmetry block.

72

J. R. Durig and H. V. Phan Table 8. Comparison of Observed and Calculated Frequencies (cm -1) of s-Trans and Gauche Conformers of Chloroacetyl Bromide s-Trans Fundamental

Obs."

Calc.

2963 1837 CH2 deformation 1408 CH2 wag 1281 C - - C stretch 951 C--C1 stretch 781 C - - B r stretch 690 C(O)Br deformation 355 C(O)Br rock 244 CCCI bend 189 A" CHz antisymmetric stretch 3004 CHz twist 1176 CH2 rock 894 C(O)Br wag 458 aRaman and/or infrared frequencies from the gas. hPED = potential energy distribution. cThe symmetry coordinates are defined as in Table 6,

2947 1839 1408 1282 953 786 692 354 242 189 3019 1177 891 458

A'

CH2 symmetric stretch C - - O stretch

Gauche PED b'C

99S~ 79S2 80S3, 15S4 69S4, 18S3 69S5, 10S2

32S6, 33S9, 13S~0 35S7, 20S5, 20S8, 16S9 82Ss 58S9, 11S7, 22S~o 34S1o, 18S5, 15S8, 24S9 100S. 100S~2 92SI3

92Sj4

the corresponding haloacetone, chloroacetone [CHzCIC(O)CH3], the equilibrium has been considered [19] to be between the more stable s-trans conformer and the high-energy gauche rotamer, but there really has not been any experimental evidence on the structure for the high-energy conformer of this molecule. Therefore, a determination of the structural parameters for both conformers of chloroacetone for comparison with those for the haloacetyl halides is very desirable.

ACKNOWLEDGMENT The authors gratefully acknowledge partial financial support of this study by the National Science Foundation by Grant CHE-83-11279.

REFERENCES 1. Khan, A. Y.; Jonathan, N. J. Chem. Phys. 1969, 50, 1801. 2. Durig, J. R.; Zhao, W.; Lewis, D.; Little, T. S. J. Chem. Phys. 1988, 89, 1285.

Obs. ~

Calc.

PED ~,~

2963 1809 1408 1267 1023 781 528 354 321 155 3004 1176 894 518

2941 1836 1413 1290 990 751 666 385 299 170 3027 1169 927 522

99S~ 82S2 88S3 77S4, 12S3 51Ss, 11S7, 1089, 13S~3 64S6, 16S~o 34S7, 17S8, 27S9, 10S13 75S8 37S9, 16S7, 29S8 35Slo, 16S5, 13S8, 26S9 99S~ 95Siz 60Si3, 14S2,13S5 54S14, 19S6, 12S10

3. Durig, J. R.; Zhao, W.; Lewis, D.; Little, T. S. Chem. Phys. 1988, 128, 353. 4. Furic, K.; Durig, J. R. Appl. Spectrosc. 1988, 42, 175. 5. Steinnes, O.; Shen, Q.; Hagen, K. J. Mol. Struct. 1980, 66, 181. 6. Durig, J. R.; Phan, H. V.; Little, T. S. Chem. Phys. 1989, 126, in press. 7. Durig, J. R.; Phan, H. V.; Little, T. S. J. Mol. Struct. 1989, 180, in press, 8. Durig, J. R.; Phan, H. V.; Hardin, J. A.; Berry, R. J.; Little, T. S. J. Mol. Struct. 1989, 180, in press. 9. Durig, J. R.; Phan, H. V.; Hardin, J. A.; Little, T. S. J. Chem. Phys. 1989, 90, in press. 10. Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations; McGraw-Hill: New York, 1945. 11. Giguere, P. A.; Schneider, M. Can. J. Chem. 1972, 50, 152. 12. Durig, J. R.; Berry, R. J.; Groner, P. J. Chem. Phys. 1987, 87, 63O3. 13. Schachtschneider, J. H. Technical Reports Nos. 231 and 57; Shell Development Co.: Houston, TX, 1964 and 1965. 14. Shimanouchi, T. Structure of Molecules and Internal Rotation; Academic Press: New York, 1954. 15. Ford, R. G. J. Chem. Phys. 1976, 65, 354. 16. Malloy, T. B., Jr.; Carreira, L. A. J. Chem. Phys. 1977, 66, 4246. 17. Dyngeseth, S.; Sehei, H.; Hagen, K. J. Mol. Struct. 1983, 102, 45. 18. Durig, J. R.; Phan, H. V.; Little, T. S.; van der Veken, B. J. J. Mol. Struct. Theochem. 1989, 185, in press. 19. Pawelka, Z. J. Mol. Struct. 1988, 172, 15, and references cited therein.