Conformational isomerism of chlorophenylthioureas and intramolecular NH … Cl hydrogen bonds: IR spectra of NH stretching vibrations

Journal of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 350 (1995) 205-214

Conformational isomerism of chlorophenylthioureas and intramolecular N - H . . - C 1 hydrogen bonds: IR spectra of N - H stretching vibrations Yoshiyuki Mido*, Masayuki Sakoda, Ken-ichiro Fujiwara Department 0["Chemistry Faculty ~?fScience, Kobe University, Nada-ku, Kobe 657, Japan

Received 12 October 1994

Abstract

The u N - H regions of the IR spectra of thioureas with chlorophenyl (C1Ph) groups and those with halophenyl groups were measured in dilute C C I 4 solution. The observed uN H bands were classified into eight groups according to the wavenumbers and the two substituent groups. The suggested conformational states and the formation of intramolecular N - H . - . CI hydrogen bonds in these compounds were discussed in comparison with those of the urea analogs. It was found that these thiourea derivatives are more stable in the cis form than the urea analogs and that thioureas with o-C1Ph groups form fewer intramolecular N H--. CI hydrogen bonds than do the urea analogs.

I. Introduction

In a series of studies on the vibrational spectra and molecular conformations of substituted ureas and thioureas, we have characterized the three conformations existing in solution of these compounds, shown in Fig. 1, by analyzing the N - H stretching region of their I R spectra [1-10]. In most cases, the most stable form for these compounds is the trans conformation. M o n o and disubstituted ureas ( M R U and R U R ' ) and even simple trisubstituted ureas ( D R U R ' with R = primary alkyl group) take the trans or t r a n s - t r a n s molecular form [1 4,7-9,11-15]. M o n o - and disubstituted thioureas ( M R T U and R T U R ' ) , however, are in equilibrium between the trans and cis forms and between the t r a n s - t r a n s * Corresponding author.

and trans-cis forms, respectively [5,9,10,15-20]. Simple trisubstituted thioureas ( D R T U R ' ) take only the trans form [6], although compounds D R T U P h with one phenyl group are in equilibrium between the trans and cis forms [7,8,21 23]. Trisubstituted ureas and thioureas with two phenyl groups ( R P h U P h and R P h T U P h ) take only the cis form [7,8]. Disubstituted thioureas with t-butyl group(s) (RTUtB and s-DtBTU) [5], and trisubstituted ureas with R = i-propyl or cyclohexyl (DiPUR' and D c H U R ' ) are in equilibrium between the out and trans forms [3]. However, D i P T U R ' , DcHTUR', DiPTUPh, D c H T U P h , DiPUPh and D c H U P h take only the out form [6-8,14]. The favorable conformations and their populations in solution are governed primarily by steric hindrance between the C = X group and the substituent(s) [3,4,6,21], attraction between the

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206

Y. Mido et al./Journal of Molecular Structure 350 (1995) 205-214

R ~c/N~R

~c-/N"H

II

X trans

II

X cis

~c/N'~R

II

X out

X=O,S

Fig. 1. Three conformations of substituted ureas and thioureas.

substituents [7,8,12] and between an N - H hydrogen atom and the substituent [16,21,22], and interactions between the solute and the solvent molecules [7,12,20,21]. We have been very interested in the fact that a few u H - H bands of N, N'-disubstituted ureas and thioureas with ortho-chlorophenyl (o-C1Ph) groups appear at considerably lower wavenumbers in solution than those of the m-C1Ph and p-C1Ph analogs. A previous IR study suggested that molecules of N, N'-disubstituted ureas (C1PhUC1Ph) are in the trans-trans conformation and that C1PhUC1Ph with o-CIPh group(s) forms intramolecular N H...C1 hydrogen bonds to give N - H bands at lower wavenumbers [9]. In the present study we have examined the u N - H bands of thioureas with chlorophenyl group(s) (C1PhTUCIPh and DRTUC1Ph) in solution and clarified their conformational states in comparison with those of the urea analogs. We have further studied the u N - H bands of thioureas with halophenyl groups (s-DXPhTU and M P h T U X P h , X = F , C1, Br in the o- and ppositions) in solution to confirm the u N - H bands associated with intramolecular N H...C1 hydrogen bonds.

2. Experimental All compounds examined were prepared in our laboratory by the reaction of pertinent phenyl isothiocyanate (phenyl isocyanate for urea derivatives) and amine or aniline. In many cases reagents dissolved in benzene were used. The crude products were purified by repeated recrystallization from suitable solvents. All of the compounds are white crystalline powders. Solutions of 2.0 x 10 -3 m o l d m 3 or saturated concentrations, prepared using spectral grade CC14, were

examined at room temperature. The IR spectra of the solutions contained in 2 cm cells were recorded on Jasco DS-701G IR and Perkin-Elmer 1600 FT-IR spectrometers with 4 cm -~ resolution. The observed uN H wavenumbers are believed to be accurate within =t=2 cm ~ unless the bands are broad or shoulders.

3. Results and discussion 3.1. N,N I-bis ( chlorophenyl) thioureas

(CIPhTUCIPh) In the IR spectra of N,N'-bis(chlorophenyl)thioureas (C1PhTUC1Ph), two to four u N - H bands were observed in dilute solutions in CC14. It is noted that the number of the u N - H bands is almost double that for the urea analogs (C1PhUC1Ph) [9]. For example, s-DoC1PhTU (= oC1PhTUoC1Ph) shows three u N - H bands while the urea analog (s-DoC1PhU) shows only one band, and oCIPhTUpC1Ph shows four u N - H bands while oC1PhUpC1Ph shows two bands. Therefore, C1PhTUC1Ph molecules are expected to be in the trans-cis conformation or in equilibrium between several conformers in solution. Some typical u N - H spectra are shown in Fig. 2, and the observed u N - H wavenumbers and relative band intensities are summarized in Table 1, together with those for N-chlorophenyl-N'phenylthioureas (C1PhTUPh) [9]. Taking account of the wavenumbers and the substituents, namely m-C1Ph, p-ClPh and Ph (class I) and o-C1Ph, 2,6-, 2,5- and 2,4-C12Ph (class II), we can classify the observed bands into five groups; band A: 3411 3402 cm 1, class I; band B: 3401-3398 cm i, class II; band C: 3382-3372 cm 1, classes I and II; band D: 3372-3361 cm -1, class I; band E: 3350-3341 cm 1, class II. s-DoC1PhTU exhibits bands B, D and E, at 3401, 3369 and 3349 cm -1, respectively. The spectrum of s-DoC1PhTU ((a) in Fig. 2) is almost the same as the spectra of 2,5- (f) and 2,4-C12PhTUoCIPh (g), but is slightly different in band intensity only from that of 2,6-ClzPhTUoC1Ph (e). These bands can be assigned to the characteristic N - H bonds on the basis of the previous relations on urea analogs

207

Y. Mido et al./Journal of Molecular Structure 350 (1995) 205-214 1

I

I

I

I

I

I

I

t

I

I

I

f

D E

i

E

B

A

(a) I

I

I

(b) I

I

D

I

(e) I

(f)

B

I

I

B

(g)

I

I

f

7 %,

E

(d)

(c) I

3500

3400

I

3500

I

3400

I 3,(h) I

I

3300 3500

1400

Wavenumber

A I

3500

D

I

3400

I

(J)

A

(i) I

3500

I

I

3400

3300

(cm-')

Fig. 2. IR spectra in the uN-H regions of chlorophenylthioureas in dilute solution in CCl4: (a) s-DoCIPhTU, (b) oCIPhTUpCIPh, (c) mCIPhTUpCIPh, (d) oCIPhTUPh, (e) 2,6- (f) 2,5- and (g) 2,4-CI2PhTUoCIPh, (h) 2,6-, (i) 2,5- and (j) 2,4-C12PhTUmCIPh

(C1PhUC1Ph) [9]. Band B arises from the trans N H bond adjacent to the o-CIPh group, while band A arises from the trans N H bond adjacent to the m-C1Ph or p-C1Ph group. Bands D and E, being observed in almost all of the spectra of C1PhTUC1Ph with one or two C1Ph groups belonging to class II, may be associated with the intramolecular hydrogen-bonded N - H bonds in the trans and cis forms (to be denoted by

trans(bonded) and cis(bonded) forms), respectively. The coincidence in the wavenumbers of bands B, D and E for s-DoC1PhTU with those for 2,6-, 2,5- and 2,4-CIzPhTUoC1Ph indicates that the effect of the chlorine atom in the 2 position of 2,6-, 2,5- and 2,4-C12Ph groups upon the u N - H wavenumbers is similar to that of the chlorine atom in the o-C1Ph group. Therefore, the molecules of these compounds in solution are

208

Y. Mido et al./Journal of Molecular Structure 350 (1995) 205-214

Table 1 Observed N - H stretching wavenumbers (cm -1) and relative intensities for N, N'-disubstituted thioureas with chlorophenyl groups (CIPhTUC1Ph) in dilute CCI solution o-Clorophenyl (o-ClPh) A o-C1Ph m-C1Ph

3407

p-CIPh

3407

B

A

C

D

E

A

C

D

E

A

C

D

E

3401

3371

3350

3407

3380

3370

3348

3407

3380

3370

3348

3409

3378

3363

3344

1

½b

~3

1

L2

sh

!3

1

21

sh

1

½ sh

½ sh

3407

3380

3408

3378

3380

~1

3381 ~1

3401 1

2,4-C12Ph

Phenyl (Ph)

E

1

2,5-Cl2Ph

C

p-Chlorophenyl (p-C1Ph)

D

1

2,6-C12Ph

m-Clorophenyl (m-CIPh)

3370

sh

3370 sh

3372 2

3398

3368

1

~ sh

3399 1

3369 ~sh

3348

]

3348 l

3350 1

3348 ~

3350 ~

3408

3J

3380

1

1

1

1

1

1

3408

3380

3408

3380

3409

3378

1

1

1

1

1

1

3407 l

3404 1

3405 1

3375

3409

3373

3411

1

1

1

1

3382 ½ sh 3380 ½sh

3365 sh? 3365 ½sh

3344 21 3348 ½

3402 1 3405 1

3383 3363 ½sh sh? 3 3 8 1 3365 sh? ½sh

3343 21 3348 ½

3408 1 3408 1

21

3372 1

3378 sh? 3376 ~i

3362 3341 ½sh 23 3 3 6 1 3344 1 ½sh

b, broad; sh, shoulder.

suggested to be in equilibrium between the transtrans(bonded) and trans-cis(bonded) conformers. oC1PhTUpC1Ph gives bands A, C, D and E, and the spectrum (Fig. 2(b)) is almost the same as that of oC1PhTUmC1Ph or oCIPhTUPh (d) in Fig. 2. In the spectra of 2,5- (Fig. 2(i)), 2,4-CI2PhTUmC1Ph (Fig. 2(j)), 2,5- and 2,4-C12PhTUpCIPh, the wavenumbers of these u N - H bands are coincident with those for oC1PhTUpC1Ph, but the intensities are different. Band C can be ascribed to the u N - H band for the N - H bond in the cis conformation. The molecules of these compounds are expected to be in a complicated equilibrium of several conformers. The spectra of 2,6-ClzPhTUmCIPh (Fig. 2(h)), 2,6-ClzPhTUpC1Ph and 2,6-CIzPhTUPh are quite different from the spectrum of oC1PhTUpC1Ph, and a discussion of this difference will be made later. mC1PhTUpC1Ph, which has two C1Ph groups both belonging to class I, exhibits bands A and C at about 3408 and 3380 cm -1 with similar band intensities, and the spectrum (c) in Fig. 2 is similar to the spectra of s-DmC1PhTU, s-DpCIPhTU, mC1PhTUPh and pCIPhTUPh. The molecules of these compounds are in the trans-cis conformation in solution. This assignment is reasonable, since the molecules of RTUPh in the same conformation exhibit two bands at similar wavenumbers;

for instance, the isopropyl analog (iPTUPh) exhibits bands at 3417 and 3388 cm -1 [9,19]. 2,6-CI2PhTUmCIPh shows two bands at about 3410 and 3373 cm -1 with similar intensities. The spectrum (h) in Fig. 2 is similar to those of 2,6CI2PhTUpCIPh and 2,6-C12PhTUPh, but appreciably different from the spectra of 2,5-C12Ph (i) and 2,4-C12Ph analogs (j), as referred to above. The 3410 cm -l band clearly corresponds to band A, but the assignment of the band at about 3373 cm -1 to band C or band D is subtle, since the wavenumber (3373cm -l) is between the regions appearing for band C (3382-3372cm -j) and band D (3372-3361 cm-1). As will be mentioned later, this band may be preferentially assigned to band C belonging to class II (band C2). Thus, these 2,6-ClzPh molecules in solution are suggested to exist in the trans-cis conformation without intramolecular hydrogen bonding. It is strange, however, that the chlorine atoms of the 2,6-C12Ph group form no intramolecular N - H . . . C 1 hydrogen bonds. A possible, interpretation is as follows. Since the chlorine atoms are symmetrically located in the 2,6-C12Ph group bound to the N - H nitrogen atom, either of the two is sterically hindered by the C=S group in the trans case and by the other nitrogen atom in the cis case [21]. This situation is illustrated in Fig. 3.

209

Y, Mido et al./Journal of Molecular Structure 350 (1995) 205-214

simpler than those of N,N'-disubstituted thioureas, the band assignments are expected to be more straightforward than those of disubstituted thioureas. In order to confirm the above assignment for the five bands A - E , the u N - H spectra of N, N-dialkyl-N~-chlorophenylthioureas (DRTUC1Ph) and N-alkyl-N-phenyl-N'-chlorophenylthioureas (RPhTUC1Ph) have been examined in dilute solution. The observed uN H bands are classified, as summarized in Table 2, by taking account of the wavenumbers and the substituent adjacent to the N - H bond, namely m-C1Ph and p-CIPh (class I) or o-C1Ph (class II). The classification is as follows: band O1" 3449-3442 cm l (class I); band O 2 : 3 4 2 4 - 3 4 1 9 cm I (class II); band A: 3433-3422 cm -1 (class I); band B: 3412 3409 cm 1 (class II); band C1:3389-3382 cm 1 (class I); band C 2 : 3 3 8 2 - 3 3 7 4 cm i (class II); band D: 3370--3369 cm -l (class II); band E: 3349-3348 c m i (class II). The bands corresponding to bands A - E of C1PhTUC1Ph were all observed in the spectra of DRTUCIPh and

CI c, /N~.

/N..

/l~

C

H

Fig. 3. Steric hindrance of the 2,6-dichlorophenyl group in 2,6CI2PhTUpC1Ph.

Moreover, the two chlorine atoms tend to be equally attracted by the N - H hydrogen atom. Accordingly, the planes of the dichlorophenyl group and the thiourea skeleton cannot be coplanar, and the chlorine atoms, being far from the N - H hydrogen atom, cannot form any intramolecular hydrogen bonds.

3.2. DRTUClPh and RPhTUClPh Since trisubstituted thioureas have only one N - H bond and their u N - H spectra should be

Table 2 Observed N - H stretching wavenumbers (cm -l) and relative intensities for N, N-dialkyI-N'-chlorophenylthioureas (DRTUCIPh) in dilute CC14 solution DR

o-Chlorophenyl (o-ClPh) 02

Dimethyl (DM) Diethyl (DE) Dipropyl (DP) Di-i-propyl (D/P) Dibutyl (DB) Di-i-butyl (D/B) Dicyclohexyl (DcH) Methyl phenyl (MPh) Ethyl phenyl (EPh) Butyl phenyl (BPh)

m-Chlorophenyl (m-CIPh)

p-Chlorophenyl (p-CIPh)

A

CI

3382

3422

1

1

B

C2

3409 1 3412 1 3411 1

D

E

O~

3374 l10 3377 sh

3428 l

A

CI

3388

3427

3389

1

1

3383 ½

3430 1 3431 1

3382 !3 3384

3431 1 3433 1

3384

3424 1

:~1

3449 1

3411 I

3409 1

3377 ~osh 3377 l sh

3428 1 3429 1

3419 1

b, broad; sh, shoulder.

O1

3382 ½ 3385 3

3442

I

3386 I

3443

1

1

3370

3349

3386

1

1

1

l

3369

3349

3385

3389

l

3389

1

l

l

3369

3348

3386

3387

1

1

1

1

210

Y. Mido et al./Journal of Molecular Structure 350 (1995) 205-214

RPhTUCIPh. Band C of C1PhTUC1Ph should be further divided into two characteristic bands (bands C1 (class I) and C2 (class II)) by the two substituent groups. Bands O1 and 02, which appear only in the spectra of DllaTUCIPh and DcHTUC1Ph, originate from the out N - H bond [4]. The u N - H wavenumbers of these trisubstituted thioureas are higher, by about 20 cm 1 for band A and by about 10 cm -l for all other bands, than those of disubstituted thioureas CIPhTUC1Ph. The difference in the u N - H wavenumbers is due to the difference in the number of substituents, and these values are comparable to the value of about 15 cm-1 between tri- and disubstituted alkly thioureas [6]. It should be noted that compounds DRTUC1Ph with an o-C1Ph group exhibit neither band E nor band D, forming no intramolecular hydrogen bonds, though CIPhTUCIPh and RPhTUC1Ph with o-CIPh groups all exhibit hydrogen-bonded u N - H bands. F r o m the observed wavenumbers and the band intensities, it is suggested that in solution, compounds D R T U C I P h with R = primary alkyl group are in equilibrium between the trans and cis forms. D i P T U C I P h and DcHTUC1Ph take the out form. C o m p o u n d s RPhTUoC1Ph are in equilibrium between the trans(bonded) and cis(bonded) forms, whereas RPhTUmC1Ph and RPhTUpC1Ph take only the cis form.

3.3. s - D X P h T U and M P h T U X P h

The I R spectra of N,N'-bis(halophenyl)thioureas and N-alkyl-N-phenyl-Nr-halophenylthioureas ( s - D X P h T U and M P h T U X P h ) with X = F, C1 and Br in the ortho and para positions have been examined in solution. The observed u N - H wavenumbers for these derivatives are summarized in Table 3, together with those for X P h T U P h [9] for reference. s - D o X P h T U give three u N - H bands. As the mass of the halogen atom increases, two lower wavenumber bands (the band at 3381 cm -1 of s - D o F P h T U is considered to be doubly degenerate) shift to the low wavenumber side, and the other one remains unshifted at about 3405 cm -1. However, s - D p X P h T U give two u N - H bands at about 3410 and 3380 cm 1, independent of the halogen atom. C o m p o u n d s M P h T U o X P h exhibit two u N - H bands (the band at 3387 cm -1 of M P h T U o F P h is doubly degenerate), both of which shift to lower wavenumbers similarly to the two shifting bands of s-DoXPhTU, while only one band of M P h T U p X P h remains unshifted at 3389 cm -l. oC1PhTUPh and oBrPhTUPh exhibit four u N - H bands, two of which are halogen-shifted and the other two are unshifted (the band at 3386 cm i of o F P h T U P h is triply degenerate). The u N - H band data for X P h T U P h are almost the same as those for s - D X P h T U

Table 3 Observed N-H stretching wavenumbers(cm- l) and relative intensities for N, N '-bis(halophenyl)thioureas(s-DXPhTU) and N-methylN-phenyl-N'-halophenylthioureas (MPhTUXPh) in dilute CC14solution XPh

s-DXPhTU

o-FPh

3405

3381

1

1

o-CIPh o-BrPh p-FPh

XPhTUPha

3387

3409

3386

1

1

3401

3371

3350

3372

3350

3410

3379

3363

3346

1

½b

23

1

1

1

!2

sh

±2

3389

3410 1 3413

3381 ~2 3382

1

1

3389

3408

3377

1

1

3387

3412

3381

3398

3365

1

sh

3411

3381

1

1

p-C1Ph

3407

3380

1

1

p-BrPh

3407

3379

1

1

a

MPhTUXPh

b, broad; sh, shoulder. Ref. [9].

3330 ~3

3365 ~3

3334 1

I

1

3359 sh

3330 2

Y. Mido et al./Journal o,1Molecular Structure 350 ( 1995 ) 205 214

except for the presence of band C at about 3380 c m - l for oXPhTUPh. The fact that, as the mass of the halogen a t o m increases, the oXPh analogs give two halogenshifting bands, while the p X P h analogs give one unshifting band, is conclusive evidence that the shifting bands originate from the intramolecular N - H . . . C I hydrogen bonded N - H bonds. Therefore, bands D and E at about 3371 and 3350 cm 1 for s-DoC1PhTU and MPhTUoC1Ph arise from the intramolecular hydrogen bonded N - H bonds in the trans and cis conformations, respectively. On the contrary, the unshifting bands at about 3405 cm 1 for s - D o X P h T U correspond to the unshifting bands at about 3410 cm 1 for sD p X P h T U , both originating from the trans conformation without intramolecular hydrogen bonding. The latter bands correspond to band A and the former bands to band B for C1PhTUCIPh. The bands at about 3380 cm 1 of s - D p X P h T U correspond to the bands at about 3389 cm -~ of M P h T U p X P h , both of which are associated with band C arising from the cis conformation without intramolecular hydrogen bonding. The wavenumber difference of 9 cm 1 between the cis bands for M P h T U p C I P h (trisubstituted) and sD p X P h T U (disubstituted thioureas) is small compared with the difference of about 15 cm 1 between the trans bands of tri- and disubstituted alkyl thioureas [6]. It should be noted, however,

21 l

that there is no wavenumber difference between the trans(bonded) bands for M P h T U o X P h and sD o X P h T U or between the cis(bonded) bands. F r o m the above band assignment and the band intensity data, it is shown that s - D o X P h T U and o X P h T U P h are in equilibrium between the t r a n s trans(bonded) and the trans-cis(bonded) forms, and M P h T U o X P h is in equilibrium between the trans(bonded) and cis(bonded) forms, whereas sD p X P h T U and p X P h T U P h take the trans cis form and M P h T U p X P h takes the cis form. 3.4. Urea analogs, s - D X P h U and M P h U X P h

The I R spectra of N,N'-bis(halophenyl)ureas and N-alkyl-N-phenyl-N'-halophenylureas (sD X P h U and M P h U X P h ) have been examined in solution and compared with those of s - D X P h T U and M P h T U X P h . The observed uN H wavenumbers for these compounds are summarized in Table 4, together with those for X P h U P h [9] for reference, s-DoXPhU and M P h U o X P h both exhibit one halogen-shifting uN H band. The wavenumbers for the hydrogen bonded bands of s-DoXPhU, which are appreciably different from those for M P h U o X P h (trisubstituted urea), coincide with those of the trans(bonded) bands fbr oXPhUPh. On the contrary, s-DpXPhU, as well as p X P h U H , give two unshifting uN H bands at about 3442 and 3430 cm i, while compounds

Table 4 Observed N H stretching wavenumbers (cm-I) and relative intensities for N, N'-bis(halophenyl)ureas (s-DXPhU) and N-methyI-Nphenyl-N '-halophenylureas (MPhUXPh) in dilute CC14 solution XPh

s-DXPhU

MPhUXPh

XPhUPha

o-FPh

3443

3437

3443 1

o-C1Ph

3419

3407

3430 1

3415

3432

1

1

o-BrPh

3407

3392

3430

3444

3430

3436

3443

3428

p-CIPh

3442

3428 1 3429 1

3434

3441 23 3441 2

3427 1 3426 1

" Ref. [9].

3440

3433

4

l

p-FPh

p-BrPh

3

3403

1

4

3401 3387 ,3 4

Y. Mido et al./Journal of Molecular Structure 350 (1995) 205-214

212

Table 5 Comparison of the conformational states of thiourea derivatives (RTUR') in solution with those of the corresponding urea derivatives (RUR') Compound a

RTUR'

RUR'

Equilibrium between the trans-trans(bonded) and trans-cis(bonded) forms

Trans(bonded)-trans(bonded) form

R

R'

oClPh 2,6-Cl2Ph 2,5-Cl2Ph 2,4-C12Ph

oClPh

oClPh 2,5-C12Ph 2,4-C12Ph oC1Ph 2,5-C12Ph 2,4-C12Ph

pClPh mC1Ph

IIl

mC1Ph pC1Ph

mC1Ph pC1Ph Ph

Trans-cis form

Trans trans form

IV

2,6-C12Ph

mCIPh pC1Ph Ph

Trans-cis form

Trans trans(bonded) form

DR

oCIPh

I

II

(a)

(b)

V

(a)

Trans-trans(bonded) form Complicated equilibrium between the trans, cis, trans(bonded) and cis(bonded) forms

Ph

Equilibrium between the trans, trans(bonded) and cis(bonded) forms

Trans(bonded) form Equilibrium between the trans and cis forms

VI

(b)

DR

mC1Ph pClPh oCIPh mC1Ph pC1Ph

(c)

DiP DcH

(a)

RPh

oXPh

(b)

RPh

pXPh

Trans form Out form

Out form

Equilibrium between the trans(bonded) and cis(bonded) forms Cis form

Cis(bonded) form Cis form

a Compounds 1-1V are disubstitutions. Compounds I have two CIPh groups of class I, compounds II have two CIPh groups of classes I and II, and compounds III have two C1Ph groups of class II. Compounds IV are exceptions. Compounds V-VI are trisubstitutions; D, di; R, primary alkyl; lP, isopropyl; cH, cyclohexyl; oXPh, ortho-halophenyl; pXPh, para-halophenyl group.

M P h U p X P h exhibit one unshifting band at about 3435 cm -1 . It is clear that the u N - H wavenumbers for the substituted ureas are generally higher than those of the thiourea analogs in the same conformation [5]. The wavenumber difference between the bands for the disubstituted ureas and thioureas studied here is estimated to be 20-30 cm -1 for the trans conformation and 42-52 cm 1 for the trans(bonded) or cis(bonded) conformation. The wavenumber difference between the bands for M P h U o X P h and M P h T U o X P h is estimated to be 27-35 cm -1 , if the shifting band of M P h U o X P h originates from the

trans(bonded) form. The difference in the wavenumber differences 42-52 cm I and 27-35 cm -1, is due to the dissimilar u N - H wavenumber for s-DoXPhU and the corresponding M P h U o X P h and the similar wavenumber for the thiourea analogs s - D o X P h T U and M P h T U o X P h . F r o m the above band assignment and the band intensity data, it is concluded that s - D o X P h U and M P h U o X P h molecules take the trans(bonded)trans(bonded) form and the trans(bonded) form in solution, respectively, and oXPhUPh molecules are in equilibrium between the t r a n s trans(bonded) and trans-cis(bonded) forms.

Y. Mido et al./Journal of Molecular Structure 350 (1995) 205-214

However, the pXPh analogs of di- and tri-substituted ureas take the trans-trans and the trans form, respectively. 3.5. Conformational states in solution

From the characteristic u N - H bands and their relative intensities, we have determined the conformational states of chlorophenylthiourea molecules in solution. For example, from the observation that s-DmC1PhTU and DMTUmC1Ph exhibit bands A and C with similar intensities, s-DmCIPhTU molecules have been established to exist only in the trans-cis conformation, and DMTUmC1Ph molecules are in equilibrium between the trans and cis conformers with similar populations. The conformational states thus determined are summarized in Table 5. At first sight it is seen that all the thiourea compounds except V(c) and VI(b) involve the cis conformation, whereas only a few urea compounds involve the cis conformation. The exceptional thiourea compounds V(c) and VI(b) are in the same conformational state as the urea analogs. All the urea compounds with the o-C1Ph group(s) apart from compound V(c) involve hydrogen bonded conformation(s), while the thiourea compounds with the o-C1Ph group(s) do not always involve the hydrogen bonded conformation(s). It is suggested, therefore, that the thiourea compounds tend to take the cis conformation more than the urea analogs do, and that the thiourea compounds with the o-C1Ph groups are less likely to form intramolecular N-H..-C1 hydrogen bonds than the urea analogs. These features can be explained in terms of steric hindrance and hydrogen bonding ability. The steric hindrance between the C=X group and the substituents in the thiourea derivatives is larger than that in the urea analogs, and since the electronegativity of a thiourea sulfur is smaller than that of a urea oxygen, the hydrogen bonding ability of the former derivatives is lower than that of the latter.

4. Conclusion

In this work we have studied the conformational

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states of substituted thioureas (C1PhTUCIPh and DRTUC1Ph) in dilute solution by examining the u N - H bands and comparing them with those of the corresponding ureas. Eight types of observed u N - H bands are characterized by the wavenumbers and the two substituent groups. The lowest and second lowest wavenumber bands observed for the thiourea derivatives with o-ClPh group(s) are associated with intramolecular hydrogen bonded N - H bonds in the cis and trans conformations, respectively. The assignment has been made from the observation that the two u N - H bands of s-DoXPhTU and MPhTUoXPh shift appreciably to the low wavenumber side with changing X from F and Cl to Br, whereas the corresponding band(s) for their pXPh analogs remain unshifted. From the conformational states determined, it is suggested that the thiourea derivatives are more likely to adopt the cis conformation than the urea analogs, and the thiourea compounds with o-C1Ph group(s) tend to form less intramolecular N - H . . . C I hydrogen bonds than the urea analogs.

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