Vibrational spectra and assignments of 1-chloro-2-iodoethane

Vibrational Spectroscopy 16 Ž1998. 145–155

Vibrational spectra and assignments of 1-chloro-2-iodoethane Xuming Zheng, David Lee Phillips

)

Department of Chemistry, UniÕersity of Hong Kong, Pokfulam Road, Hong Kong, China Received 9 July 1997; accepted 14 February 1998

Abstract Raman and infrared absorption spectra of liquid 1-chloro-2-iodoethane have been obtained. Additional Raman spectra of 1-chloro-2-iodoethane in room temperature solutions with different solvents and at low temperature Ž77 K. in the solid state have also been obtained in order to help elucidate which vibrational modes belong to the gauche and anti conformers. Ab initio vibrational frequencies of the gauche and anti conformers have been calculated at the RHFr3-21G) level of theory by using the point group symmetries of C1 and Cs , respectively. Vibrational frequency assignments for all eighteen infrared and Raman active modes have been made for both the gauche and anti conformers. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Vibrational spectra; 1-Chloro-2-iodoethane; Gauche conformer; Anti conformer

1. Introduction Vibrational spectra Žboth IR and Raman. and vibrational assignments have been reported for many dihaloethane molecules such as 1-fluoro-2haloethanes and dichloroethane w1–16x. Several dihaloethane molecules like 1-chloro-2-fluoroethane have had extensive studies carried out that included conformational analysis, measurements of barriers to internal rotation and ab initio calculations w2–16x. However, very little work has been done for 1chloro-2-iodoethane. There has been one report on 1-chloro-2-iodoethane which presented infrared absorption and Raman vibrational spectra and an assignment of the vibrational frequencies based on symmetry arguments w1x. Because of the very limited information available at the time, this 1939 study )

Corresponding author.

mistakenly assigned the 1-chloro-2-iodoethane peaks to a anti-conformation with C 2h symmetry and a cis-conformation with C 2v symmetry w1x. This assignment of 1-chloro-2-iodoethane vibrational peaks is not consistent with later investigations of other dihaloethanes which only found anti and gauche conformations for the dihaloethane molecules w2–16x. One would also expect that the large iodine atom on 1-chloro-2-iodoethane would give rise to larger steric interactions between the two halogen atoms than in a molecule like 1-chloro-2-fluoroethane and this would make any noticeable population in a cis conformation even less likely for 1-chloro-2-iodoethane. Thus it appears that a more recent investigation of 1chloro-2-iodoethane is needed to clarify which conformations exist with noticeable populations at room temperature, the relative stabilities of these conformations and vibrational assignments of the IR and Raman peaks. In this paper we report Raman and IR

0924-2031r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 4 - 2 0 3 1 Ž 9 8 . 0 0 0 0 8 - 3

X. Zheng, D.L. Phillipsr Vibrational Spectroscopy 16 (1998) 145–155

146

spectra of liquid 1-chloro-2-iodoethane as well as Raman spectra of low temperature Ž77 K. solid state 1-chloro-2-iodoethane and room temperature 1chloro-2-iodoethane solutions with different solvents in order to elucidate the relative stabilities of the gauche and anti isomers and the assignments of the vibrational bands. Ab initio calculations of the vibrational frequencies are also reported and used to help assign the vibrational bands of the experimental Raman and IR spectra.

2. Experiment and calculations 1-Chloro-2-iodoethane Ž97%. was purchased from Lancaster and spectroscopic grade solvents Žall ) 99% purity. were purchased from Aldrich. We redistilled the 1-chloro-2-iodoethane to a purity of better than 99% as determined from NMR spectra for use in the Raman and IR experiments. Raman and IR spectra obtained with unpurified and purified 1chloro-2-iodoethane displayed no significant differences in any bands assigned to 1-chloro-2-iodoethane. The Raman spectra were taken using a Fourier-transform Raman spectrometer ŽBio-rad model Raman II. with an effective range of 100–3600 cmy1 . The

Table 1 Optimized structural parameters of 1-chloro-2-iodoethane Žbond lengths in angstroms and bond angles in degrees. Parameter

Anti

Gauche

Standarda

rŽC 1 –C 2 . rŽC 1 –I. rŽC 1 –H 4 . rŽC 1 –H 5 . rŽC 1 –Cl. rŽC 2 –H 7 . rŽC 2 –H 8 . /ŽIC 1C 2 . /ŽH 4 C 1C 2 . /ŽH 5 C 1C 2 . /ŽClC 2 C 1 . /ŽH 7 C 2 C 1 . /ŽH 8 C 2 C 1 . /ŽIC 1C 2 Cl. /ŽH 4 C 1 H 5 . /ŽH 7 C 2 H 8 .

1.5221 2.1858 1.0763 1.0763 1.8188 1.0754 1.0754 109.776 111.625 111.625 109.000 111.900 111.900 180.000 110.427 110.328

1.5248 2.1808 1.077 1.079 1.8047 1.0759 1.0794 113.374 111.371 109.724 112.105 111.949 109.441 70.337 109.955 109.718

1.536 ŽCH 3 CH 3 . 2.139 ŽCH 3 I. 1.107 ŽCH 3 CH 3 . 1.107 ŽCH 3 CH 3 . 1.784 ŽCH 3 Cl. 1.107 ŽCH 3 CH 3 . 1.107 ŽCH 3 CH 3 .

a

Fig. 1. Geometry of anti-1-chloro-2-iodoethane with most of the internal coordinates Žbond lengths and bond angles. given.

solid samples were recorded with a resolution of 1 cmy1 and the liquid and solution samples were taken with a resolution of 4 cmy1 . A glass NMR tube was used for holding the liquid and solution samples. The solid state cryogenic experiments used a sealed glass capillary tube. Liquid 1-chloro-2-iodoethane was carefully cooled and annealed using liquid nitrogen in order to prepare 77 K solid samples with minimal contamination of the less stable isomer. The infrared absorption spectra were obtained using a FourierTransform infrared spectrometer ŽBio-rad Model

109.5 ŽCH 3 CH 3 . 109.5 ŽCH 3 CH 3 . 109.5 ŽCH 3 CH 3 . 109.5 ŽCH 3 CH 3 . 109.5 ŽCH 3 CH 3 . 109.5 ŽCH 3 CH 3 .

Adopted from the Handbook of Chemistry and Physics, CRC Press, 62nd edn., 1981–1982.

Fig. 2. FT-IR and FT-Raman spectra of room temperature Ž298 K. liquid 1-chloro-2-iodoethane and FT-Raman spectrum of low temperature Ž77 K. solid state 1-chloro-2-iodoethane.

X. Zheng, D.L. Phillipsr Vibrational Spectroscopy 16 (1998) 145–155

FTS-7. with an effective range of 400–4000 cmy1 and 2 cmy1 resolution. The liquid or solution 1chloro-2-iodoethane samples were placed in KBr sample holders in order to obtain FT-IR spectra. The ab initio calculations for 1-chloro-2iodoethane were carried out at the RHFr3-21G) level of theory using the Gaussian 94 program w17x. The molecular geometry of the anti conformer was optimized by minimizing the total molecular energy within the constraint of Cs symmetry. The symmetry

147

constraint was then relaxed for the purpose of comparison. The minimum energy without the Cs symmetry constraint was at a geometry slightly different from that found for the Cs symmetry constraint. The molecular geometry of the gauche conformer was optimized by minimizing the total molecular energy without any symmetry constraint. The optimized structural parameters for both anti Žwith the Cs symmetry constraint. and gauche conformers are listed in Table 1. The parameters were used to calculate the

Table 2 Observed infrared and Raman frequencies Žcmy1 . and vibrational assignments for 1-chloro-2-iodoethane Infrared

Raman

Liquid

Liquid

Solid

ni

Descriptiona

3031 vw

3030 vw sh 3007 w dp

3031 vw 3008 w 2975 vw sh 2961 s

n 12 n 13 X X n 12 , n 13 n 1 , n 1X n2 n X2 ? ? n 3 , n X3 n4

CH 2 antisymmetric stretch CH 2 antisymmetric stretch CH 2 antisymmetric stretch, a CH 2 antisymmetric stretch a CH 2 symmetric stretch, CH 2 symmetric stretch

2973 w 2967 w sh 2940 w sh 2865 vw 2846 vw 1437 w sh 1432 m 1412 w sh 1293 w 1273 w 1235 s 1163 vs 1109 vw 1102 vw 1048 m 1028 vw 1007 vw

2964 s p 2941 w sh p 2868 vw p 2845 vw p 1440 vw sh dp 1433 w dp 1413 vw sh dp

Assignment

2861 vw 2842 vw 1439 w 1428 w

1274 m p 1259 vw 1236 w p 1163 m p 1109 vw p

1273 m 1254 vw

1048 w dp

1048 m

1167 m 1108 vw

928 vw 907 w 822 m 741 w 706 s 658 m 575 s

908 vw p 823 vw p 707 vs p 661 w p 576 vs p 510 m p 368 w p 233 s p 186 m p 105 m p

702 vs 577 s

232 m 189 m 106 w

n 5X n5 n 14 X n 14 n 6 , n 6X n 15 X n 15 n 7 , n X7

CH 2 symmetric stretch combinationroverton combinationroverton CH 2 deformation a CH 2 deformation a CH 2 deformation CH 2 wag CH 2 wag CH 2 twist CH 2 twist a CH 2 wag a CH 2 twist a CH 2 twist C–C stretch

n 16 X n 16 X n 17 n 17 n8 n X8 n9 n X9 X n 10 X n 10 , n 11 n 11 X n 18 , n 18

CH 2 rock CH 2 rock a CH 2 rock a CH 2 rock C–Cl stretch C–Cl stretch C–I stretch C–I stretch ClCC bend ClCCrICC bend ICC bend ClCCI torsion

n iX , refers to gauche conformer. p s Polarised band, dp s depolarised band. Qualitative intensity descriptions: vs s very strong, s s strong, m s medium, w s weak, vw s very weak, sh s shoulder of larger band. a , The iodine atom is attached to this carbon atom.

148

X. Zheng, D.L. Phillipsr Vibrational Spectroscopy 16 (1998) 145–155

ab initio harmonic force field and the normal-mode vibrational frequencies for each conformer. The optimized molecular geometry of the anti conformer in terms of internal coordinates is displayed in Fig. 1.

3. Results and discussion

of varying polarity w10–13,16x. These studies w10– 13,16x found that the solvent effects on the vibrational frequency and intensity changes of the anti and gauche peaks could be quantitatively described by the following two equations: D nrn s C Ž ´ y 1 . r Ž 2 ´ q 1 . log Ž 2 A arA g . s X q Y Ž ´ y 1 . r Ž 2 ´ q 1 .

It is already well known that all of the vibrational fundamental frequencies are active in the infrared andror Raman spectra for both the anti Ž Cs symmetry. and gauche Ž C1 symmetry. conformers of 1-X– 2-Y dihaloethanes w2–16x. Each isomer of the molecules in these types of dihaloethanes should have 18 fundamental modes of vibrations. The anti conformation has the irreducible representation 11AX q 7AY . It is possible that 36 distinct Ž18 for each isomer. fundamental bands can be observed but in practice this number of bands is usually not observed because the corresponding vibrational modes in the two isomers of the molecule may have virtually the same vibrational frequencies which will result in infrared andror Raman bands that are common to both conformers. Fig. 2 shows the FT-IR and FT-Raman spectra of liquid 1-chloro-2-iodoethane and an FT-Raman spectrum of 77 K solid 1-chloro-2iodoethane. Comparison of the liquid FT-Raman and FT-IR spectra in Fig. 2 shows that some of the features are complementary to one another in intensity with some of the peaks common to both spectra. Table 2 lists the vibrational frequencies and qualitative intensities for the vibrational bands observed in our FT-IR and FT-Raman spectra shown in Fig. 2. Assignment of vibrational bands unique to the anti conformer is not obvious because all vibrational modes of 1-chloro-2-iodoethane should be infrared and Raman active which makes it ambiguous which vibrational bands belong to which conformer. However, assignments of vibrational bands unique to the gauche conformer can be accomplished by observing intensity changes in solvents of different polarity since the energy difference of the different rotational isomers changes. The more polar isomer Žgauche in our case. vibrational peaks will have more intensity relative to anti isomer vibrational peaks in more polar solvents. Several studies on rotational isomerism have quantitatively observed the shifts in vibrational frequencies and intensities with solvents

Ž 1. Ž 2.

where C, X, and Y are constants, ´ is the dielectric constant of the solvent, D n is the frequency difference between the vapor and solution phases, A g and A a are the absorbances of a band unique to gauche and anti isomers, respectively. The observation of vibrational intensity changes ŽEq. Ž2.. was found to be substantially more reliable and accurate than the use of vibrational frequency shifts ŽEq. Ž1.. in determining which peaks are mainly due to gauche or anti conformers. This rule has proven to be sufficiently reliable to make vibrational assignments on several closely related dihaloethanes w10–12x. Solvent features made it very difficult to integrate many of the

Fig. 3. FT-Raman spectra of 1-chloro-2-iodoethane in CH 3 OH, CH 3 COCH 3 , C 4 H 8 O, CS 2 , and cyclohexane ŽC 6 H 12 . solvents. The larger gauche and anti Raman bands are labeled with a G or A, respectively.

X. Zheng, D.L. Phillipsr Vibrational Spectroscopy 16 (1998) 145–155

infrared bands of 1-chloro-2-iodoethane accurately and we focused on the FT-Raman spectra for many vibrational bands since this was much less of a problem in the Raman spectra. FT-Raman spectra of 1-chloro-2-iodoethane were obtained in solvents with varying polarity and these are displayed in Fig. 3. The spectra shown in Fig. 3 have several peaks that noticeably increase in intensity as the solvent polarity increases and they appear to be mainly due to features associated with the gauche conformation of the molecule. Table 3 lists the relative intensities of the major Raman peaks in the spectra of Fig. 3 of 1-chloro-2-iodoethane in different solvents Žcyclohexane, CS 2 , tetrahydrofuran, acetone, and methanol.. The dielectric constants of the solvents and neat liquid 1-chloro-2-iodoethane are also given in Table 3. Fig. 4 compares a fit of Eq. Ž2. to the intensity changes of the anti and gauche C–I stretch experimental peaks as the polarity of the solvent is varied. We obtain good agreement between Eq. Ž2. and the gauche and anti peaks experimental vibrational intensities as the solvent polarity changes. This allows us to assign many vibrational peaks in the Raman and IR spectra to predominantly anti and gauche conformations. It is also useful to obtain a low temperature solid state vibrational spectrum of 1-chloro-2-iodoethane. Fig. 2 shows a low temperature Ž77 K. solid state Raman spectrum of 1-chloro-2-iodoethane and compares it with a room temperature Raman spectrum. Only anti Raman peaks appear in the low temperature Ž77 K. spectrum and this indicates that the anti conformation is more stable than the gauche conformation for neat 1-chloro-2-iodoethane in the condensed phase. This is in contrast to the 1-fluorohaloethanes where the gauche form is more stable and only gauche Raman peaks appear in the low temperature Ž77 K. solid state spectrum. For all the 1,2-dihaloethanes, the anti conformation is more stable in the gas phase due to the steric interactions of the two halogen atoms in the molecule. Upon solvation, the solvent–solute interactions tend to stabilize the gauche conformer more than the anti conformer Žparticularly in more polar solvents.. For the 1-fluoro-2-haloethanes, the solvent-solute interactions appear greater than the steric interactions and the gauche conformation is more stable than the anti conformation in the condensed phase w11,12,14x. However, our

149

results for 1-chloro-2-iodoethane found that the anti isomer was the more stable conformation in the condensed phase. This is most likely due to the significantly greater steric interactions between the chlorine and iodine atoms in 1-chloro-2-iodoethane than the two halogen atoms in the 1-fluoro-haloethane molecules. Ab initio calculations have proven very helpful in making vibrational assignments of several dihaloethanes w14,15x. Durig et al. w14x have recently revised the previous vibrational assignments of 1-fluoro-2-haloethanes based on a combination of ab initio calculations Žusing 3-21G) basis sets. and experimental Raman and IR spectra. The ab initio calculated vibrational frequencies can provide the energetic order of frequencies by which to assign the spectra w14,15,18x. The ab initio optimized molecular geometries for the anti and gauche conformations given in Table 1 show that the rŽ C1 –C2 . internal coordinate is about the same in both conformations which suggests that the vibrational frequency associated with the C–C stretch will be almost the same. The large amount of repulsive interaction between the chlorine and iodine atoms is stronger in the gauche conformation than in the anti conformation and this leads to the /ŽIC 1C 2 . and /ŽClC 2 C 1 . angles of the gauche conformation to be greater than those of the anti conformation. This implies that the CCI and CCCl bending vibrational frequencies of the gauche conformer will be higher than those of the anti conformer. Table 4 compares our ab initio calculated vibrational frequencies with our experimental vibrational frequencies. In order to more easily compare the ab initio and the experimental vibrational frequencies, we have scaled the ab initio vibrational frequencies. The scaled ab initio vibrational frequencies and the scaling factors are shown in Table 4. We have also carried out a normal coordinate calculation using an adapted version of the Snyder and Schachtschneider FG program Ždescribed in detail in Ref. w19x. and the optimized ab initio geometries and valence force field. The force field was adjusted slightly to better fit the experimental frequencies. Table 5 shows a comparison of the calculated and experimental frequencies as well as the approximate potential energy distribution for diagonal force constants that make significant contributions to each of the calculated vibrational frequencies. The potential

150

Table 3 Solvent polarity and temperature effects on relative Raman intensities of gauche and anti forms Frequency Žcmy1 . a

23 46.8 2.8 16.4 53.7 100

Relative intensity C 6 H 12 17.7 20.2 43.7 3.0 14.6 56.5 100

Relative intensity C4 H8O 21.0 47.3 4.6 29.9 58.0 100

Relative intensity CH 3 OH 26 22.8 49.5 4.2 32 55.7 100

Relative intensity CH 3 COCH 3 37.3 21 48.9 7.1 38.8 54.9 100

3.2

1.9

4.8

4.0

3.6

6.7 16.2 2.2

13.3 2.8

4.8 17.5 4.1

4.9 16.9 3.7

7.5 20 4.3

19.1

21.2

20.8

20.4

23

Solidified by liquid nitrogen smaller unchanged smaller disappeared disappeared unchanged disappeared unchanged vanishing disappeared more visible shift to 1052 smaller disappeared more visible unchanged disappeared unchanged larger

a: bs bend, s ssymmetric stretch, as sasymmetric stretch, r s rock, w s wag, tw s twist. a , The iodine atom is attached to this carbon atom. G sGauche; A s Anti. Solvent dielectric constants, ´ , are 2.023 for cyclohexane Žor C 6 H 12 ., 2.641 for CS 2 , 7.58 for tetrahydrofuran Žor C 4 H 8 O., 20.7 for acetone Žor CH 3 COCH 3 ., and 33.62 for methanol Žor CH 3 OH.. The dielectric constant, ´ , is 10.06 for neat liquid 1-chloro-2-iodoethane.

X. Zheng, D.L. Phillipsr Vibrational Spectroscopy 16 (1998) 145–155

105 torsion ŽG and A. 186 b ŽICC. ŽA. 233 b ŽICC. ŽG.rb ŽClCC. ŽA. 368 b ŽClCC. ŽG. 510 s ŽC–I. ŽG. 576 s ŽC–I. ŽA. 661 s ŽC–Cl. ŽG. 707 s ŽC–Cl. ŽA. 823 r Ž a CH 2 . ŽG. 908 r ŽCH 2 . ŽG. 928 r ŽCH 2 . ŽA. 1047 s ŽC–C. ŽG and A. 1162 w Ž a CH 2 . ŽG and A. 1236 tw ŽCH 2 . ŽG. 1254 tw ŽCH 2 . ŽA. 1274 w ŽCH 2 . ŽA. 1413 b Ž a CH 2 . ŽG. 1433 b Ž a CH 2 . ŽA. 1439 b ŽCH 2 . ŽG and A.

Relative intensity CS 2

X. Zheng, D.L. Phillipsr Vibrational Spectroscopy 16 (1998) 145–155

Fig. 4. Comparison of gauche and anti C–I stretch Raman band intensities wusing RT ln Ž2) A a r A g . from Eq. Ž2.x as a function of solvent polarity Žusing the Onsager function of Ž ´ y1.rŽ2 ´ q1. from Eq. Ž2... The solid circles are the experimental data and the line is the best fit to the experimental data using Eq. Ž2..

energy distributions and the ab initio scaled calculated vibrational frequencies were used to make tentative assignments of the experimental vibrational frequencies.

151

One anticipates that the C–X stretch vibrational mode of the anti isomer will occur at a higher frequency that of the gauche conformer because this frequency in isomers of substituted ethanes has been found to increase as the electronegativity of the group which is anti to C–X increases w12x. We assign the 510 cmy1 and the 661 cmy1 peaks to the gauche C–I stretch ŽG-n C – I . and C–Cl stretch ŽGn C – Cl . vibrational modes. The 576 cmy1 and 707 cmy1 peaks are assigned to the anti C–I stretch ŽA-n C – I . and C–Cl stretch ŽA-n C – Cl . vibrational modes based on the potential energy distributions, the ab initio calculations, the results of our low temperature Raman spectra and the results of our FT-Raman solvent polarity experiments. The CCX bending vibration of the gauche conformer should occur at a higher frequency than that of the anti conformer since the gauche conformer has a stronger interaction between the two halogen atoms which leads to a larger /ŽXCC. in the gauche isomer than in the anti isomer. For the lower frequency region ; 100 to 400 cmy1 the ab initio

Table 4 Observed and calculated frequencies Žcmy1 . for the anti and gauche conformers of 1-chloro-2-iodoethane

ni X

A

Y

A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Fundamentals a CH 2 sym. str. CH 2 sym. str. CH 2 deform. a CH 2 deform. CH 2 wag a CH 2 wag CC stretch CCl stretch CI stretch CCCl bend CCI bend a CH 2 asym. str. CH 2 asym. str. CH 2 twist a CH 2 twist CH 2 rock a CH 2 rock torsion

Anti

Label

Ab initio

Scaled

Observed

3302 3293 1648 1637 1455 1354 1068 746 641 245 189 3377 3354 1437 12618 1031 821 111

2973 2965 1442 1432 1473 1185 961 671 577 214 165 3040 3019 1258 1103 902 718

2964 2964 1439 1433 1274 1162 1047 707 576 233 186 3033 3007 1254 1108 928 741 105

A-n 1 A-n 2 A-n 3 A-n4 A-n 5 A-n 6 A-n 7 A-n 8 A-n 9 A-n 10 A-n 11 A-n 12 A-n 13 A-n 14 A-n 15 A-n 16 A-n 17 A-n 18

Gauche

Label

Ab initio

Scaled

Observed

3278 3265 1642 1623 1471 1322 1069 683 562 402 243 3350 3335 1427 1270 998 916 93

2951 2939 1437 1421 1288 1157 962 615 506 352 212 3016 3002 1249 1112 873 802

2964 2941 1439 1413 1293 1162 1047 661 510 368 233 2973 2973 1236 1102 908 823 105

G-n 1 G-n 2 G-n 3 G-n4 G-n 5 G-n 6 G-n 7 G-n 8 G-n 9 G-n 10 G-n 11 G-n 12 G-n 13 G-n 14 G-n 15 G-n 16 G-n 17 G-n 18

Scaling factors for ab initio calculated vibrational frequencies: 0.9 for stretch vibrational modes; 0.875 for bend vibrational modes; and 1.0 for torsion vibrational modes. Sym.s symmetric, asym.s asymmetric, deform.s deformation and str.s stretch. a , The iodine atom is attached to this carbon atom. G s Gauche; A s Anti.

X. Zheng, D.L. Phillipsr Vibrational Spectroscopy 16 (1998) 145–155

152

Table 5 Observed and calculated frequencies Žcmy1 . of 1-chloro-2-iodoethane and their approximate potential energy distribution

ni

Fundamentals

Anti

Label

Approximate potential energy distribution

2964 2964 1439 1433 1274 1162 1047 707 576 233 186 3030 3007 1254 1108 928 741

A-n 1 A-n 2 A-n 3 A-n4 A-n 5 A-n 6 A-n 7 A-n 8 A-n 9 A-n 10 A-n 11 A-n 12 A-n 13 A-n 14 A-n 15 A-n 16 A-n 17

103

105

A-n 18

C–H Ž98%. C–H Ž98.7%. HCH Ž74.8%., CCH Ž11.5%., ClCH Ž11.4%. HCH Ž72.2%., ICH Ž18.1%. CCH Ž50.2%., ClCH Ž41.8%. CCH Ž59.8%., ICH Ž47.6%., CCrCCH Žy11.7%., ICHrCCH Žy11.5%. C–C Ž98.0%., CCrCCH Žy21.7%. C–Cl Ž51.8%., ClCC Ž21.4%., ICC Ž16.5%., C–I Ž11.0%. C–I Ž64.0%., C–Cl Ž30.2%. ClCC Ž53.3%., ICC Ž20.0%., ICCrClCC Ž11.9%. ICC Ž49.5%., C–I Ž16.8%., C–Cl Ž14.3%. C–H Ž100.1%. C–H Ž101.1%. CCH Ž67.8%., ICH Ž8.4%., ClCH Ž12.4%., CCHrCCH Ž14.5%. CCH Ž32.0%., ClCH Ž41.6%., ICH Ž38.3%. CCH Ž11.4%., ClCH Ž36.5%., ICH Ž50.3%. CCH Ž87.3%., ICH Ž5.7%., ClCH Ž10%., xCC x Ž9.1%., CCHrCCH Žy18.6 %. xCC x Ž85.5%.

2965 2946 1437 1417 1285 1169 1047 661 509 372 239 2974 2969 1236 1101 905 822 77

2964 2941 1439 1413 1293 1162 1047 661 510 368 233 2973 2973 1236 1102 908 823 105

G-n 1 G-n 2 G-n 3 G-n4 G-n 5 G-n 6 G-n 7 G-n 8 G-n 9 G-n 10 G-n 11 G-n 12 G-n 13 G-n 14 G-n 15 G-n 16 G-n 17 G-n 18

C–H Ž102.5%. C–H Ž97.9%. HCH Ž72.3%., ICH Ž11.2%., CCH Ž9.4%. HCH Ž77.2%., ClCH Ž14.2%., ICH Ž6.8%. CCH Ž55.2%., ClCH Ž42.1%. ClCH Ž52.6%., CCH Ž40.4%. C–C Ž102%. C–Cl Ž60.0%., CCH Ž19.2%. C–I Ž63.1%., ICC Ž18.3%., C–Cl Ž7.9%. ClCC Ž32.8%., C–I Ž26.3%., C–Cl Ž10.2%. xCC x Ž48.2%., ICC Ž26.1%., ClCC Ž18.9%. C–H Ž97.3%. C–H Ž101%. CCH Ž61.0%., ICH Ž45.7%., ClCH Ž8.9%. ICH Ž63%., CCH Ž57%. CCH Ž47.0%., ICH Ž24.0%., ClCH Ž7.0%., xCC x Ž12%. CCH Ž30.9%., ClCH Ž29.9%., ICC Ž12.4%., ICH Ž7.8%. xCC x Ž30.2%., ICC Ž38.0%.

Calculated

Observed

2768 2962 1440 1431 1271 1155 1047 711 574 231 179 3029 3001 1248 1111 937 741

torsion

B. Gauche conformer X A 1 a CH 2 sym. str. 2 CH 2 sym. str. 3 CH 2 deform. 4 a CH 2 deform. 5 CH 2 wag 6 a CH 2 wag 7 CC stretch 8 CCl stretch 9 CI stretch 10 CCCl bend 11 CCI bend Y A 12 a CH 2 asym. str. 13 CH 2 asym. str. 14 CH 2 twist 15 a CH 2 twist 16 CH 2 rock 17 a CH 2 rock 18 torsion

A. Anti conformer Y A 1 a CH 2 sym. str. 2 CH 2 sym. str. 3 CH 2 deform. 4 a CH 2 deform. 5 CH 2 wag 6 a CH 2 wag 7 CC stretch 8 CCl stretch 9 CI stretch 10 CCCl bend 11 CCI bend Y A 12 a CH 2 asym. str. 13 CH 2 asym. str. 14 CH 2 twist 15 a CH 2 twist 16 CH 2 rock 17 a CH 2 rock 18

Sym.s symmetric, asym.s asymmetric, deform.s deformation, str.s stretch. a , The iodine atom is attached to this carbon atom. G s Gauche; A s Anti.

calculations agree well with the experimentally observed vibrational frequencies Žsee Table 4.. We assign the broad peak ; 105 cmy1 to the torsional vibrational modes of both the anti and gauche conformers. Using the energy decreasing order of G-

bCCCl ) A-bCCCl ) G-bCCI ) A-bCCI from the ab initio calculations and the spectral features given in Figs. 2 and 3, we assign the 189 cmy1 peak as A-bCCI , the 233 cmy1 peak as A-bCCCl and G-bCCI , and the 368 cmy1 peak as G-bCCCl . The observations

X. Zheng, D.L. Phillipsr Vibrational Spectroscopy 16 (1998) 145–155

that the 368 cmy1 peak completely disappeared in the low temperature 77 K Raman spectrum and increased in intensity as the solvent polarity increases strongly suggests that this vibrational mode is predominantly from the gauche conformer. The observations that the 189 cmy1 peak intensity relative to the 707 cmy1 peak intensity is nearly unchanged in the 77 K Raman spectrum and the room temperature Raman spectrum as well as in different room temperature solvents strongly suggests that the 189 cmy1 peak is predominantly from the anti conformer. We have assigned the 233 cmy1 band to both A-bCCCl and G-bCCI based on the following two observations: the ab initio calculated frequencies for these two modes are very close to one another Ž245 cmy1 and 243 cmy1, respectively. and the 233 cmy1 band lost some intensity in the 77 K Raman spectrum relative to the room temperature Raman spectrum. The approximate potential energy distributions in Table 5 also supports our assignment of the 233 cmy1 band of chloro-2-iodoethane to both the A-bCCCl and G-bCCI vibrational modes. The assignment of the CH 2 bending modes follow the general order of decreasing frequency for the CH 2 deformation) CH 2 wag ) CH 2 twist ) CH 2 rock w11,14x. For CH 2 bending vibrations, the vibrational mode associated with a CH 2 group attached to the iodine atom will occur at a lower frequency than the mode associated with a CH 2 group attached to the chlorine atom because of the effect of the reduced masses on these vibrational modes. For CH 2 stretching vibrations Žwhich occur at higher frequencies than the CH 2 bending vibrations., the vibration arising from the CH 2 group to which the iodine is bonded occurs at a higher frequency than the vibration due to the CH 2 group to which the chlorine is bonded w11,14x. Our ab initio calculations also show these trends in the CH 2 vibrational frequencies Žsee Table 4.. Our assignments of the higher frequency modes are based on the energy order of the ab initio

153

calculations, the approximate potential energy distribution of Table 5, and the observed trends in closely related dihaloethane molecules. Assignments made in the higher frequency region are tentative since several of these features are not well-resolved and some weak bands are common to both isomers though the ab initio calculation give a useful frequency order for these vibrations. A check on the tentative assignments we have made for the vibrational modes of 1-chloro-2iodoethane was performed by using the Mizushima Sum Rule w2x and the Bernstein and Pullin empirical sum rule w3x. The results of these calculations are given in Table 6 and it was found that the Mizushima sum rule had a 1.4% difference between the gauche and anti conformers while the Bernstein and Pullin empirical sum rule had only a 0.05% difference between the anti and gauche conformers. The Mizushima sum rule w2x places more weight on contributions from higher frequency vibrational modes while the Berstein and Pullin empirical sum rule w3x reflects average contributions from all vibrational modes. The results of these two rules given in Table 6 indicates the assignments made to the lower frequency modes Ž- 750 cmy1 . is much more certain than the assignments made for the higher frequency vibrational modes. The low frequency bands Ž- 750 cmy1 . are mostly well-resolved and generally unique to each isomer and this makes assignments of these vibrational bands easier and more accurate. We have carried out temperature dependent FTRaman studies of neat 1-chloro-2-iodoethane and in cyclohexane solution in order to determine the relative stabilities of the anti and gauche conformers. We measured the intensities of the anti conformer 576 cmy1 C–I stretch peak and the gauche 510 cmy1 C–I stretch peak relative to one another as a function of temperature. Using this data we constructed a van’t Hoff plot Žshown in Fig. 5. and determined

Table 6 Sum rule for liquid ClCH 2 CH 2 I Mizushima sum rule

Bernstein and Pullinn sum rule

Gauche

Anti

Ýn i2 = 10y7

Ýn i2 = 10y7

4.844

4.915

Gauche % difference 1.4

Anti y4

Ýn i = 10 2.4151

Ýn i = 10y4 2.4161

% difference 0.05

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X. Zheng, D.L. Phillipsr Vibrational Spectroscopy 16 (1998) 145–155

Fig. 5. The van’t Hoff plots of the intensities of the anti conformer 576 cmy1 C–I stretch peak and the gauche 510 cmy1 C–I stretch peak relative to one another as a function of temperature for neat 1-chloro-2-iodoethane ŽA. and 1-chloro-2-iodoethane in cyclohexane solution ŽB.. Each plot shows the best fit linear regression as a solid line. We determined D H s 0.12"0.01 kcalrmole for neat liquid 1-chloro-2-iodoethane and D H s 0.40"0.03 kcalrmole for 1-chloro-2-iodoethane in cyclohexane solution from the van’t Hoff plots.

best fit linear regression. The slope of the van’t Hoff plot is related to the change of enthalpy and the plots in Fig. 5 gave D H s 0.12 " 0.01 kcalrmole for neat liquid 1-chloro-2-iodoethane and D H s 0.40 " 0.03 kcalrmole for 1-chloro-2-iodoethane in cyclohexane solution. The anti conformation was found to be more stable relative to the gauche conformation when in cyclohexane solution than in the neat 1-chloro-2iodoethane. This is likely due to the greater solvent–solute stabilization in the neat liquid than in cyclohexane solution for the gauche conformation. We are very confident of the vibrational assignments for most of the low frequency vibrational modes Ž- 750 cmy1 .. However, the higher frequency modes are tentative in nature since we cannot unambiguously assign several of these bands since they are not well-resolved and some weak bands are

common to each isomer. We did not have an FT-IR spectrometer with a Mylar beam splitter to look at vibrational peaks below 450 cmy1 and we also did not have a sample device for taking low temperature crystal 1-chloro-2-iodoethane infrared spectra. This information would also be helpful to confirm our present vibrational assignments. In particular, our assignment of the 105 cmy1 Raman band to the torsional vibration of both anti and gauche conformers is somewhat speculative since the FT-Raman spectrometer’s notch filter starts to cut off near 100 cmy1 . It is possible the torsion vibrations are at different vibrational frequencies as indicated by our ab initio calculations. It would be helpful for a far infrared spectrum to be obtained to definitively locate the gauche and anti torsional vibrations of 1chloro-2-iodoethane. Further work that includes vibrational spectra of several deuterated 1-chloro-2iodoethanes like ICD 2 CH 2 Cl and ICH 2 CD 2 Cl could also be done in order to unambiguously assign the higher frequency vibrations Žlike the different CH 2 vibrational modes. of 1-chloro-2-iodoethane.

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