Raman spectroscopic study of the conformational change of 12-crown-4 due to cation capture

Journal of Molecukw Structure, 162 (1987) 157-167 Elsevier Science Publishers B.V., Amsterdam -Printed

RAMAN SPECTROSCOPIC CHANGE OF 12-CROti-4

in The Netherlands

STUDY OF THE CONFORMATIONAL DUE TO CATION CAPTURE

KUNIO FUKLJSHIMA and YOSHIKO TAMAKI Department of Chemistry, Faculty of Science, Shizuoka University, 836 Oya, Shizuoka (Japan) (Received 23 April 1987)

ABSTRACT Raman spectra of the Li’, Na+, K’, Nq, Mg2’, Ca**, ST*+, Ba’+, Pb*+ complexes of 12crown-4 and also 12crown-4 in various states are observed. The spectra of 12crown-4 change remarkably by complex formation with cations. Normal vibration calculations of various conformations of 12crown-4 are carried out. On the basis of the observed spectra and the results of the calculations, the conformation of 12-crown-4 in the Li’, Na+, K+, NH:, Mg*+ complexes is found to have approximate Did symmetry, while that in the Ca2+, Sr’+, Ba*+ complexes is found to have approximate C,, symmetry. INTRODUCTION

Many reports regarding Raman spectra of crowns capturing cations in their solid state have been published, whereas few studies [l-3] by Raman spectroscopy of ion capture by crowns in solutions have been carried out. On the other hand, there has been no report of a Raman spectroscopic study of 12crown-4. In the present study, Raman spectroscopic investigations of ion capture in solutions and normal vibration calculations were carried out for 12-crown-4, whose smaller cavity is expected to cause various structures of crown-cation complexes, and the molecular strain is expected to affect the conformation of 12-crown-4 in the complexes. EXPERIMENTAL

The sample of 12-crown-4 used was a commercial product of Aldrich Chemicals Company. Raman spectra were recorded on a Model R-800T Raman spectrometer (Japan Spectroscopic Co.) with excitation by the 514.5 nm line (300 mW) of an NEC argon ion laser (Model GLG 3300). The depolarization ratio was measured with a system consisting of a half-wave plate, a lens and a polarizer. Spectra of liquids at room temperature were measured using 0.3 ml Raman cells. Spectra of solid 12-crown-4 were measured using an Oxford-type cryostat and liquid nitrogen. Measurements are shown in Figs. 1 and 2 and Tables l-4. 0022-2860/87/$03.50

0 1987 Elsevier Science Publishers B.V.

158 NORMAL

VIBRATION

CALCULATION

Normal vibration calculation of 12-crown-4, ( -CHz-CHz-O-)4 , was carried out for five models with C4”, CZv, Dzd, and C, symmetry. The internal rotation angles around each bond (starting from the 0 - CH2-CH2 - 0 bond and ending with the CH, - O-CH, - CH2 bond) along the molecular chain are as follows: CJv (0”, -144.7910”, 144.7910”)& Cl” (54.1571”, -139.7910”, 139.7910”, -54.1571”, 149.7910”, -149.7910”)2 ; Dzd -144.7910”, 144.7910”) -54.2528”) 144.7910”) -144.7910”),; (54.2528”) 144.7910”) -54.2528”) 144.7910”, -144.7910”, C, (I) (O”, -144.7910”, 54.2528”) -144.7910”, 144.7910”, -144.7910”, 144.7910”); c, (2) (-54.2528“) 144.7910”) -144.7910”, O”, 144.7910” ,-144.7910”, 54.2528”) -144.7910”, 144.7910”, 0”, -144.7910”, 144.7910”). The following molecular parameters were used: CH = 1.09 a, CC = 1.54 a, CO = 1.43 8,) LHCH = LHCO = LOCO = tetrahedral angle, LCOC = 112” (111.7093” for the CZv model). The modified Urey-Bradley force field [ 41 was used. 2V=

.ZKkiArii

+ EHi~jr~ir~jA~&j

+ EFijAqi

+ X(3/81'2)KAU",j;

where K denotes bond stretching force constants, H angle bending force constants, F repulsive force constants between non-bonded geminal atoms, K intramolecular tension force constants, and Y internal rotation force constants. The calculation was carried out in the Computation Center of Shizuoka University, using the programs BGLZ and LSMB of the Computation Center of the University of Tokyo. The calculated frequencies of the D2d model for the force constants transferred from related molecules (set I of Table 5) [ 51 are shown in parentheses in Table 6, and those for the force constants obtained by refinement by the method of least squares [6] (set II of Table 5) are also shown in Table 6. The calculated frequencies of other models for the set II force constants are shown in Fig. 3. RESULTS

Spectral

AND DISCUSSION

change associated

with change of concentration

of cation

Dependence of Raman spectra on concentration of Na+ was examined for NaBr-12-crown-4-methanol systems, in which degree of capture of cation is higher than that of other systems (log K = 1.41-3.32) [7] . As an example of the dependence, the most remarkable change observed in the region 700 to 1000 cm-’ is shown in Fig. 1. With increasing concentration of Na”, the two bands at 814 and 824 cm-’ (Fig. l(A)) become weaker and one at 845 cm-’ (Fig. l(A)) becomes stronger, and finally, four bands persist at 799, 824, 861 and 911 cm-’ (Fig. l(F)). The NaBr-12-crown-4-methanol system corresponding to Fig. l(F) is used as the base of comparison of the following discussion.

159

IA)

1

I

(B)

(C)

\

I

900

800

700

1

900

I

900

800

800

v/cmml

(El

L

--i&-z-

(F)

.li -iir-z-

Fig. 1. Salt concentration dependence of Raman spectra of 12-crown-4; crown to salt, (A) pure liquid, (B) 16:1, (C) 8:1, (D) 4:1, (E) 2:1, (F) 1:l.

Spectral

difference

due to sort of monovalent

mol ratio of

cations

Table 1 affords the following information. The bands in the spectrum of the 12-crown-4-methanol system, 255, 263,323, 525 cm-’ do not appear in that of the NaBr-12-crown-4-methanol system, and, in the latter, new bands, whose frequencies are different from corresponding ones in the former, appear at 309, 607, 861, and 911 cm-l. These are characteristics of the Na* complex. Corresponding to these, bands having almost the same frequencies appear together with bands of 12-crown-4 for the Li+ complex, showing co-existence of 12-crown-4 and the complex similar to the Na’ complex. In contrast to the case of the Li+ complex, the bands of the K’ and NH: complexes (853 cm-‘, 854 cm-‘) have lower frequency values than those of the Na’ complex (861 cm-‘), showing a slight structural difference between the complexes.

160 TABLE 1 Raman spectra of 12-crown-4 cm“ )”

in crown-monovalent

cation-methanol

systems (< 1000

crown (0.033) methanol (0.967)

crown (0.033) LiCl(O.034) methanol (0.933)

crown (0.032) NaBr (0.052) methanol (0.916)

crown (0.035) KC1 (0.006) methanol (0.959)

crown (0.042) NH,Cl(O.O24) methanol (0.934)

lJ

I

P

”

z

p

Y

Y

p

V

255 263 296

sh 16 sh

? 0 ?

252 262 302 312

sh 19 sh 16

? 0 ? 0

309

53

0.14

323 358

8 55

0 0

355

62

0.11

354

28

504 525 579

7 sh 24

504

21

0.65

500

579

29

0.26

DP ? 0

800

95

P

814 845

100 31

0.23 0.20

900

28

0.75

792 805

sh 89

? 0.18

818 848 865 902

100 40 48 32

0.23 0.22 P 0.61

I

p

I

I

p

262 306

19 40

0 0.29

262 306

13 34

0.15

350

55

0.17

16

0.75

500

25

DP

607 799

18 23

0.27 0.75

580 604 790 805

29 27 sh 86

P I’ ? 0.32

824

1

820 853

100 100

0.23 0.21

350 362 500 530 585 603 794 800 809 822 854

48 sh 21 19 23 28 sh sh 64 87 100

P ? 0.75 0.43 0.35 0.26 ? ? 0.27 0.27 0.13

861 911

100 15

905

44

0.55

905

33

0.75

? 0.12 0.75

P P

aFigures in parentheses represent mole fraction. v = Frequency in cm-’ ; Z = relative intensity; p = depolarization ratio; sh = shoulder; P = p value not determined, but polarized; DP = p value not determined, but depolarized.

Spectral difference due to the type of diualent cation Table 2 and Table 1 affords the following information. Frequencies of the bands of the Ca2+ complex are close to those of corresponding bands of the Na’ complex except for two bands (372 cm-‘, 920 cm-‘), which are higher than corresponding bands of the Na+ complex (354 cm-‘, 911 cm-‘). Frequencies of the Sr2+ and Ba2+ complexes are almost the same as those of the Ca” complex. Although the spectrum of the 12crown4-MgCl, * 6H2Omethanol system is almost the same as that of the 12-crown-4-methanol system, existence of the band at 317 cm-’ suggests formation of the Mg2+ complex. The Pb2+ complex has a different spectrum from those of other complexes, and the spectrum is similar to those of the K+, NH: complexes.

Effect of anion In Table 3, the differing spectra of Na’ complexes with different anions are shown. Although there is no appreciable spectral difference for NaCl-12-

0 0

DP ? 0

8 55

7 sh 24

95 100 31

28

323 358

504 525 579

800 814 845

900

905

806 820 848

500 530 582

355

317

257

v

17 16 29

55

14

12

30

85 100 45

I

DP

P 0.27 0.21

DP DP 0.27

0

0

?

P

crown (0.032) MgCl, .6H,O (0.033) methanol (0.935)

in crowndivalent

14

100

867 920

1

824

19 14

610 793

32

23 19

I

372 508

315

v

DP

0.08

?

0.27 0.75

0.11 0.75

0.16

P

crown (0.036) CaCl, (0.041) methanol (0.923)

cation-methanol

15 24 sh 44 51 sh 100 sh 21

580 607 794 807 820 851 863 906 914

47 sh 38 22

Z

354 367 504

311

v

902

803 815 850

576 598

497

0.28 0.23 ? P 0.26 ? 0.11 ? DP

355

?

305

263

259

”

26 20

21

68

57

12 12

0.18

? 0.24 0.27

0.21 0.31

0.75

0.11

P

0 ?

P

= depolarization

79

6 100

sh

I

crown (0.032) Pb(NO,), (0.011) methanol (0.957)

0.12 0.75

0.15

P

crown (0.037) SrCI, - 6H,O (0.014) methanol (0.949)

systems (< 1000 cm-l)a

aFigures in parentheses represent mole fraction. v = Frequency in cm-’ ., I = relative intensity;p P = p value not determined, but polarized; DP = p value not determined but depolarized.

0.75

P 0.23 0.20

?

0 ?

sh 16 sh

255 263 296

P

I

”

crown (0.033) methanol (0.967)

Raman spectra of 12crown-4

TABLE 2

52

DP

0.16

?

? 0.30 0.61

? 0.13 0.75

0.19

ratio; sh = shoulder;

22

100

860 912

sh

sh 23 30

sh 37 17

I

819

582 604 795

346 360 502

306

”

P

(0.023) methanol (0.942)

crown (0.035) BaCl, .2H,O

162 TABLE 3 Effect of anion on Raman spectra of 12-crown-4-cation crown (0.081) NaCl(O.034) methanol (0.885)

crown (0.032) NaBr (0.052) methanol (0.916) ”

I

complex (< 1000 cm-l)a

P

”

I

crown (0.026) Na,CO, (0.020) methanol (0.954) P

v

I

P

305 350 496 577 601 796 804 816 856 907

41 35 17 1 22 38 sh 42 100 24

0.10 0.14 0.75 P 0.22 DP ? 0.32 0.13 DP

309 354 500

53 28 16

0.14 0.15 0.75

303 349 495

54 29 18

0.15 0.13 0.75

607 799

18 23

0.27 DP

601 796

18 27

0.25 C.69

824 861 911

1 100 15

? 0.12 DP

828 858 907

1 100 16

? 0.13 DP

aFigures in parentheses represent mole fraction. o = Frequency in cm-’ ; I = relative intensity; p = depolarization ratio; P = p not determined, but polarized; DP = p not determined, but depolarized.

crown-4-methanol and NaBr-12-crown-4-methanol systems, the Na,C0312-crown-4-methanol system has a band (816 cm-‘) of lower frequency and greater intensity and also new bands at 577 and 804 cm-‘, which are not bands of CO’,-. This indicates the effect of ion pairing especially for the divalent anion (CO:-).

Conformation of 12crown-4

in the Na’ complex

Information from solvent effect In frequency, some bands of pure liquid 12-crown-4 in Table 4 correspond to the bands of the Na’ complex in Table 1. Therefore, it is expected that pure liquid 12-crown-4 is an equilibrium mixture of a conformer similar to 12-crown-4 in the Na’ complex and another or others. Spectra of 12-crown4 in various solvents with various dielectric constants are shown in Fig. 2. With decreasing dielectric constant, the band of the pure liquid at 850 cm-l is increased in intensity, while that at 822 cm-l is increased in intensity with increasing dielectric constant. Two bands of the pure liquid at 794 and 808 persist in spectra of solutions of solvents having smaller dielectric cm-’ constants, and that at 904 cm-’ persists in spectra of solutions of solvents having greater dielectric constants. These facts indicate that the bands at 794, 808 and 850 cm-l can be assigned to a less polar conformer and that at 822 cm-l to a more polar conformer. The band at 904 cm-’ is considered to be due to both conformers. As the band of the pure liquid at 850 cm-’ clearly corresponds to that of the Na’ complex at 861 cm-‘, the 12-crown-4

163 TABLE 4 Raman spectra of 12-crown-4

(<2000

Pure solid

Pure liquid V

cm-‘)a

I

!J

P

277 323 358 435 504 530 550 579 794 808 822 850 875 904 965 985 1030 1037

23 67 5 14 9 9 21 35 100 79 26 6 34 3 6 sh 24

0.23 0.12 0.12 ? 0.56 ? ? 0.17 0.18 0.17 0.29 0.34 ? 0.61 ? ? ? 0.56

1075 1109 1124 1154

15 24 28 19

? 0.41 0.31 0.52

1261 1297

22 45

0.60 0.57

1465

50

0.63

19

I 277

14

358

24

504

19

567

14

803 823

43 30

903

27

1032 1045 1087 1102 1127 1150 1250 1265 1298 1440 1450 1464

15 15 11 10 100 8 10 10 28 16 17 19

a~ = Frequency in cm-’ ; I = relative intensity; p = depolarization ratio; sh = shoulder.

molecule complex.

is considered

to

assume

a less polar

conformation

in the Na’

Information from normal vibration calculation As a possible model for the less polar conformer, which is taken for the Na’ complex, the D,, model of 12-crown-4 was taken into account, and normal vibration calculation of the model was carried out. Seventy-eight normal vibrations of the model are classified as 11 Al, 9 AZ, 10 B1, 10 Bz and 19 E vibrations. The calculated frequencies for general values of force constants (set I of Table 5) are shown in parentheses in Table 6 except for the CH stretching vibrations. These frequencies correspond well to the observed frequencies of the Na’ complex. It is especially notable that cal-

(B)

1

900

800

700

900

900

800

I

800

v/cm-1

(Fl

, 900

I

800

-

900

800

J,L 9ou

800

Fig. 2. Solvent effect on Raman spectra of 12crown-4: (A) 12crown-4 (pure liquid); (B) benzene solution (x = 0.033); (C) chloroform solution (x = 0.118); (D) methanol solution (x = 0.033); (E) acetonitrile solution (x = 0.072); (F) aqueous solution (x = 0.030). x represents mole fraction of 12crown-4.

culated frequencies of Al species correspond to polarized bands, while those of E species to depolarized bands. Satisfactory convergence to set II force constants in Table 5 by the method of least squares [6] also shows the validity of the correspondence. The presence of a band at 60’7 cm-’ corresponding to AZ vibration, which is Raman inactive for the Dzd model, might be explained as the breaking-down of the selection rule as the Na’ complex itself does not have DZa symmetry because of the larger ionic diameter of the Na’ ion (1.90 a) relative to the cavity size of 1%crown-4 (1.2 a). These results suggest that the conformation of 12crown-4 in the Na’ complex is similar to the DZd model. Conformation

of 12_crown-4

in other complexes

Comparison of spectra suggests: (i) in the Li’ complex, 12-Crown-4 has a structure similar to that of the Na’ complex. Absence of a band near 607

165 TABLE 5 Force constants of 12crown-4 9

K(C--C) WC-O) K( C-H) H( H-C-H) F( H-C-H) H( C-C-H) F(C-C-H) H(C-C-0) F(C-C-0) H( O-C-H) F( O-C-H) q C-O-C) F( C-o-c) WC% -CH,) WCH,--O)

K(--CH,--_)

(in mdyn A’)

Set I

Set II

2.800 2.800 4.200 0.350 0.100 0.150 0.510 0.280 0.600 0.280 0.670 0.525 0.300 0.270 0.109 0.030

2.892 2.779 3.811 0.410 0.043 0.124 0.519 0.313 0.567 0.254 0.703 0.493 0.231 0.309 0.110 0.046

mdyn A mdyn A mdyn A

mdyn A mdyn A mdyn A

cm-’ suggests Dzd symmetry for the complex because of the ionic diameter of Li’ (1.2 a) fitted to the cavity of 12-crown-4; (ii) in the K’ and NH,’ complexes, conformation of 12-crown-4 is almost the same as that of the Na+ complex; (iii) in the Ca 2+, Sr2+, and Ba2+ complexes, as shown in Fig. 3, calculated frequencies of the CzV model are close to those of the DZd model except for the following frequency differences of several vibrations: D2d model, 931 cm-’ (B,), 375 cm-’ (B,), 267 cm-’ (E), 142 cm-’ (A,), CzV model, 952 cm-’ (A,), 396 cm-’ (A,), 282 cm-’ (B,), 253 cm-’ (B, ). The observed frequency difference between the Na’ complex (354 cm-‘) and the CaZ+ complex (372 cm-l) corresponds to the difference of calculated frequencies, 375 and 396 cm-l ; (iv) in the Mg2+ complex, 12-Crown-4 is considered to take a conformation similar to that in the pure liquid of 12crown-4; (v) the Pb2+ complex is considered to take a conformation similar to that of the K’, NH: complexes. Molecular

conformation

in pure liquid and solid of 12-crown-4

Measurement of the solvent effect shows that a less polar form and at least one more conformer (more polar form) exist in pure liquid. If we assume that the more polar form has a structure similar to the CJV model, observed spectra of the pure liquid can be explained well, based on the calculated frequencies in Fig. 3. It is noteworthy that the bands at 822 and 904 cm-’ assigned to a more polar form correspond well to the calculated frequencies of the CJV model. Also in the spectra of the solid, bands are observed at 803, 823, and 903 cm-l, showing the molecular form in the solid has a structure similar to the CgV model.

166 TABLE

6

Calculated frequencies (in cm-‘) of the Dzd model cies of the 1 %crown+Na+ complex (< 2000 cm-‘)a “ohs

Symmetry

with the observed

frequen-

species -4,

A,

compared

B1

E

B,

69(68) 98(98) 142(141) 277(?) 309(P) 354(P) 360(?)* 420(?)* 500(DP) 607(P) 799(DP) 861(P) 911(DP) 940(?)* lolo(?)* 1031(DP)** 1050(DP)** 1068(P)** 1096(DP)** 1119(P) 1240(DP) 1273(DP) 1294(DP) 1310(?) 1356(P)

138(135) 233(225) 267(267)

306(299) 364(366)

343( 348) 375( 364) 494( 508)

404(411) 514(511)

592(585) 810(821) 821(833) 853(869)

859(866)

913(922) 931(942) 1005(1010) 1045(1042)

1034( 1033)

1027( 1028) 1045(1054)

1061( 1073) 1103(1092) 1112(1107) 1251(1274)

1122(1123) 1237( 1263) 1271(1287) 1280(1305)

1291(1307) 1298(1299) 1337( 1330)

1349(1347)

1369( 1360) 1457(1428)

1289( 1306) 1313(1313) 1363(1356) 1459( 1432)

1464(?) 1462(1439) a* = Very weak band;

147 2( 1455)

1466( 1444)

** = crown-Na,CO,-H,O

1470(1451)

system.

CONCLUSIONS

The Li+ complex, in which Li’ (ionic diameter 1.2 A) fits the cavity size on the whole. Even in (1.2 A) of 12crown-4, takes the DZd conformation the Na’ complex, 12-crown-4 assumes a conformation similar to the DZd model, although the larger Na’ ion (ionic diameter 1.90 A) is not considered to be trapped in the center of the cavity. Also for larger cations, K+ (2.66 A), NH: (2.84 a), Pb2+ (2.40 II) and Mg2+ (1.30 A), 12-crown-4 in the complexes is in a conformation similar to that of the Na’ complex, but differing slightly, as the observed frequencies are somewhat different. On the con-

167 500

1000 I

I

I

I

I

I

I

I

I

I

I

I,I,I

observed fresuencies (pure liauid) calculated frequencies

: I 1 ;

) ; ‘f i

11 not observed

> I

!

I

Cpv model

I

calculated freauencies D2d model

I I

4 ,

II

I I I

calculated freauencies Cs model (1) calculated frequencies Cs model (2)

I

’

I

’

I v/cm-1

Fig. 3. Calculated frequencies of models of 12_crown-4.

trary, divalent cations, Ca2+ (1.98 A), Sr” (2.26 A), and Ba2+ (2.70 A), form their crown complexes, in which the conformation of 12-crown-4 is similar to that of the Na+ complex but is more deformed, similar to the CzV model. REFERENCES 1 R. K. Khanna and D. D. Stranz, Spectrochim. Acta, Part A, 36 (1980) 387. 2 L. J. Hilliard, M. R. Rice and H. S. Gold, Spectrochim. Acta, Part A, 38 (1982) 611. 3 H. Takeuchi, T. Arai and I. Harada, J. Mol. Struct., 146 (1986) 197. 4 T. Shimanouchi, J. Chem. Phys., 17 (1949) 245. 5 T. Shimanouchi, Pure Appl. Chem., 7 (1963) 131. 6 D. E. Mann, T. Shimanouchi, J. H. Meal and L. Fano, J. Chem. Phys., 27 (1957) 43. 7 R. M. Izatt, J. S. Bradshaw, S. A. Nielsen, J. D. Lamb and J. J. Christensen, Chem. Rev., 85 (1985) 271.