Solvent effects on the infra-red spectra of anilines—VI

speQtro&imica Acta,Vol.25A,pp.1727to 1741.Pergamon Preaf~ 1969.Printed inNorthern

Ireland

Solvent effects on the infra-red spectra of aniline!+VI* Ortho-substituted anilines L. K. DYALL University of Newcastle, Shortland, N.S.W., 2308, Australia (Received13 January

1969)

Abstra&-Measurements of the N-H stretching frequencies are reported for ten orthosubstituted primary aromatic amines dissolved in carbon tetraohloridecontaining varied mole fractions of hydrogen bond acceptors. Evidenoe is obtained for the amines assumingnon-planar con6gurations in many of their hydrogen-bonded aggregates. There are Fermi resonance and solvent polarization effects on the YNH values in some instanoes. The variations in YNH produced by solvent ohange are not useful in diagnosis of intramoleoular hydrogen bonds, but this diagnosis can be made by analysis of the sterio and hydrogenbonding effects which an ortho substituent can produce. When an intramolecular bond is rNH,r,) increased, in comparison present, YNH.,,, is raised, and the value of (rNH,,,,with the correspondingpara-substituted aniline. INTRODUCTION THE presence or absence of intramolecular

hydrogen

bonds in o&o-substituted

aromatic amines has been the subject of much controversy to this question has involved frequencies

[2, 31.

[l].

The expectation

was that the solvent

with any intramolecular bond of an intramolecular hydrogen

being able to form hydrogen

that

stretching

but would have to

for the other amino hydrogen. bond would then be detectable

spectral change when it was replaced by an intermolecular basic solvent. We have since demonstrated

N-H

would hydrogen-bond

freely to the amino hydrogen remote from the ortho substituent, compete presence

Our own approach

measuring the solvent shifts of the

the assumption

bonds to solvent

bond to a sufficiently

of both amino

molecules

The as a

hydrogens

is not justified.

Many

anilines form only 1: 1 aggregates with solvents even when there are no ortho substituents to interfere with this association [4]. Moreover, our detailed analysis of interactions

of secondary

amines

with

solvents

[6] indicates

that

hydrogen

bond energies are only one of several factors controlling the association of solute and solvent. solvent effects substituents.

The

present

paper

on the infrared

therefore

spectra

re-examines

of primary

the whole

aromatic

amines

question

of

with ortho

EXPERIMENTAL Amines were synthesized and purified by standard literature methods. Chloro-2-t-butylaniline was kindly provided by Professor A. N. Hambly. * Part V: L. K. DYALL,b’pectrochim. [l] A. N. [2] L. K. [3]L. K. [4] L. K. [5] L. K.

Acta 25A, 1423 (1969).

HAMBLY, Rev. Pure Appl. Chem. Awrtralia 11,212 (1961). DYALL, AuetraliaraJ. Chem. 13,230 (1900). DYALL,Spectrochint.Acta 17, 291 (1961). DYALL, Spectrochim. Acta 25A, 1423 (1969). DYALL and J. E. KEMP, Spectrochim. Acta 22, 467 (1966). 1727

5-

L. K. DYALL

1728

[4].

Solvents were purified, and spectral measurements The spectral data are presented in Tables l-6.

made, by our usual methods

DISCUSSION 1. Frequency

shifts produced by hydrogen bonding with solvents

Useful guide-lines for the interpretation of our data are provided by our work on primary aromatic amines with no ortho substituents [4]. We found that the formation of one intermolecular hydrogen bond caused a sizeable decrease of the symmetric N-H stretching frequency ( YNH,) but only a small decrease of the antisymmetrical stretching frequency (YNH,,). Hydrogen bonding to a second molecule of solvent very little further change in YNH, but a very large decrease in YNH,,. The change in amine: solvent stoichiometry in the hydrogen-bonded aggregate from 1: 1 to 1: 2 is thus easily recognized. The frequency shifts caused by association of the ortho-substituted amines with a hydrogen bond acceptor have been measured over a range of mole fractions of acceptor in carbon tetrachloride solution (Tables l-5). Only 2-trifluoromethylaniline, with high mole fractions of diethyl ether (Table 2) and acetone (Table 3) gave evidence for 1:2 association with the acceptor. In all other instances, the solvated species observed in equilibrium with free amine at low mole fraction of acceptor persisted through to high mole fractions of acceptor, and is therefore assumed to be the 1: 1 aggregate. In agreement with this conclusion, LAURANSAN Table 1. N-H

stretching frequencies of anilines in acetonitrile-carbon solvents* (all solutions 0.02 molar in amine)

XYUIN

0.000

0.0266

0.0662

0.111

0.169

0.328

0.643

1.000

E - I/2& + 1 na - l/2$ + 1

0.226 0.216

0.262 0.215

0.292 0.214

0.332 0.212

0.360 0.211

0.404 0.206

0.444 0.196

0.480 0.176

vNH.6 vNHs

3513 3423

3612 3421 -3399

3611 3421 3398

3610 3421 3397

3609 3422 -3401

3607 3419 3397

3501

3494

3396

3394

vNHm ( vNH,

3490 3396

3489 3394

3488 3393

3486 3388

3482 3384

3478 3378

3473 3376

3468 3371

vNH,,

3606

3504

vNHs

3376

3376

3504 3484 3376

3503 3483 3377

3376

3504 3484 3375

3482 3376

3483 3376

vN&

3520

3519 3494 3396 3374

3519 3493 3396 3373

3519 3491 3396 3370

3490

3399

3618 3496 3398 3376

3491

vNH,

3619 3497 3399 3377

3369

3368

vNH.8

3621

3618 3492 3396 3378

3617 3493 3393 -3376

3618 3491 -3399 3383

3492

3396

3619 3492 3396 -3374

3490

VNHB

3619 3492 3394 ~3376

3382

3381

2-Trifluoromethylaniline

2-Bromomiline

2.Methoxycerbonylsniline (

2-Nitroaniline

(

6-Methyl.2~nitroaniline I

J.

tetrachloride

(3494) t

+ Dieleatrio constants, E, and refrmtive indices, n, are taken from P. VON R. SCHLEYERand A. ALLEBHAND, Am. Chsm. Sot. 86, 371 (1963). t The band was flat-topped but could not be resolved into its components.

1729

Solvent effects on the i&e-red spectra of milines-VI

Table 2. N-H

stretching frequenciesof anilines in diethyl ethewxrbon tetmchloride solvents* (all solutions 0.02 molar in amine)

o*ooo

0113

0.188

0.479

1.000

0.226 0.216

0.247 o-211

0.259

0.208

0.298 0.197

0.343 0.177

3503 3408

3502 3409 3366

3503 3408 3369

3504 3409 3369

3505

vNH.

vN&,

3513

3510

3509

vNH.

3423

3424 3374

3424 3375

3508 ~3467 3422 3375

3408 3469 3422 3378

vN&,

3490

(3487) t

vNH,

3396

339s 3358

3491 3476 3395 3359

-3490 347s 339s 3358

vNH..

3506

3505 3468 3373 3359

3506 3468 3374 3359

3504 3467 3373 3360

3504 3468 ~3380 3360

3501 3448 3340

3502 3449 3340

3501 3448 3335

-3503 3450 3337

3519 3480 3400 3341

3520 3480 3400 3339

3519 3480 3399 3342

3519 3477 3400 3332

3520 3477 3308 3332

3518 3477 3399 3334

3521 3480 3396 3367

3520 3479 3395 3365

3518 3479 339s 3365

3521 3446 3362 3335

3521 3446 3360 3335

~3516 3447 ~3366 333s

3511 3462 3386 3328

3510 3463 3385 3329

3509 3463 3387 3329

mro E - l/2.? + 1 n* - 1/2n* + 1 2.t-Butyl-S-ohloro-aniline

2.Trifluoromethylaniline

2.Methoxycarbonylmiline

vNH..

(

I I L

3376

vNHs

vNH.w

2.Acetylmiline ( vNH,

2.Nitroaniline

3502 3344

vNH.w

3520

vNH,

3399

1

4Xhloro-2.nitroaniline

vNH,,

3521

vNH, L

3400

6.Methyl-2.nitromilinine

vNH,s

3521

*NH,

339s

vNHa,

3522

vNH,

3365

i

2.Nitro.l-naphthylemine i

LNitro-2.naphthylaemine I

NH.,

3514

NH,

3387

* Dieleotrio oomtmts, E, and refractive indices, ta, are taken from P. VON J. Am. Ohem. Soo. 86, 371 (1963). f Asymmetryof the band suggests an overlapping of two absorptions.

3371

3472 3359

3480 3343

3477 3334 3520 3477 3394 3366

3447 3337 3512 3463 3332

R. SOHLEYERand A. ALLERIUND,

L. K. DYALL

1730

Table 3. N-H stretching frequencies of anilines in acetone-carbon tetrrtohloride and 1,8-cineoleearbon tetrachloride solvents (all solutions 0.15 molar in amine) Acetone Xscetoneor Xctieo1e

vNJ&m

2-t-Butyl-S-chloroaniline

vNH,

2-Trifluoromethylaniline

0.000

0.126

3603 3408

3606 3409 3389

3604 3408 3386

3509

3606 ~3484 ~3421 3391

vNH,,

3513

*NH,

3423

i

2-Bromoaniline

2.Methoxycarbonylaniline

2-Acetylaniline

*NH,,

3490

vNH,

3396

vN&

3606

vNH.

3376

vN&,

3502

vNH,

3344

I I

2-Nitroaniline

vNH,,

3620

vNH,

3399

(

4-Chloro-2-nitroaniline

vNH..

3621

vNH,

3400

I

6-Methyl-2-nitroaniline

vNH,,

3521

vNH.

3396

vNH,,

3622

vNH,

3365

[

2-Nitro-I-naphthylamme

I

I-Nitio-2-naphthylemine

vNHa,

3514

vNHs

3387

3421 3396

0.294

Cineole 1.000 3499 3387

0.114

1.000

3502 3410 3355

3503 3355

3507

3507

3424 3363

3422 3363

3486 3388

3486 3473 3395 3342

3481 3396 3376

3476

3463

3372

3367

3603 3481 3373

3506 3477 3372

3476 3369

3505 3462 3375 3345

3504 3469 3374 3346

3499 3466 3342

3600 3463 3342

3460 3344

3501 3438 3340 -3319

3497 3436

3519 3487 3396 3366

3517 3487 3397 3364

3518 3485 3398 3362

3520 3485 3398 3359

3519 3486 3394 3374

3519 3485 3396 3378

3520 3466 *

3620 3466 *

3458

3349

3348

3346

3608 3472 *

3506 3468 *

3464

3350

3346

3346

3485 3359

3481 3363

3485 3374

3519 3474 3400 3321 3519 3471 3401 3314 3519 3471 3397 3358 3421 3435 3363 3323 3512 3456 3387 3311

3470 3344

3325

3474 3319

3471 3311 3517 3471 3396 3356

3432 3324

3453 3307

* Distortion of the speotrsl traces near 3370 cm-l by solvent absorptions has probably obscured a solute band.

Solvent effects on the i&a-red Table 4. N-H

1731

spectra of aniline-VI

stretching frequencies of aniline5 in dimethylsulphoxide-carbon tetrachloride solvents*

XDMSO

2-Trifluoromethylaniline

vNJ%,

l

vNH,

2-Bromoaniline

2-Methoxyoarbonylaniline

6-Methyl-2-nitroaniline

i

0.386

1 .ooo

0.000

0.054

3513 3423

3504 3421 3348

3501 3421 3347

3498

3497

3348

3345

3352

3489 3466 3395 3330

3465 3394 3326

3462 -3397 3324

3460

3450

3322

3322

3503 3457 3373 3335

3501 3456 3372 3331

3503 3454 3374 3331

-3500 3453 3374 3326

3519 3460 3396 3332

3515 3461 3396 3329

~3508 3459 3397 3325

-3505 3458 -3398 3321

vNH.8

3490

vNH,

3396

vNHa,

3506

vNH,

3376

vNH,.

3521

vNH,

3395

-

0.102

0,203

-3483

3448 3321

3453 3312

* Because of the opacity of DMSO, the sample cell length was reduced as xnMso increased. Amine concentration was oorrespondingly increased from 0.02 to 0.2 molar.

et ab. [6] have demonstrated 1: 1 association of several o&o-substituted anilines with pyridine in carbon tetrachloride solution by means of equilibrium constant measurements. The failure to observe 1: 2 association is not surprising since the second solvent molecule would have to approach that amino hydrogen which is located near the o&o substituent and would encounter severe non-bonded repulsions. In several instances both YNH, and YNH, changed at high mole fractions of acceptor, but we attribute this behaviour to a change in molecular geometry of the amine and not to a change in amine : solvent ratio in the hydrogen-bonded aggregate. We have followed our previous practice [4] of comparing the frequency shifts ( AY) produced at O-1 mole fraction of the various acceptors in carbon tetrachloride solution. However, those solvents using r-bonds as the acceptor site (chlorobenzene, p-xylene) and triethylamine (whose lone pair site is sterically hindered) failed to produce adequate concentrations of the 1: 1 aggregate at low mole fractions. These non-polar solvents were therefore used neat (see Table 6). Simple 1: 1 amine-8olvent association. 2-Trifluoromethylaniline and 2-t-butyl5-chloroaniline both exhibit the “classical” behaviour [4] of an amine using only one of its two amino hydrogens to bond with the solvent (see Fig. 1). The Bellamy plots of the frequency shifts (measured at O-1 mole fraction of acceptor) are linear and pass through the origin (see Table 7). The only exceptional behaviour is shown by 2-t-butyl-5-chloroaniline with dimethylsulphoxide as acceptor (see Table 7). Here the shift in YNH, is very large and probably arises from rotation of the amino group out of the aromatic ring plane (see below). [6] J. LAURANSAN,P. PINEAU and J. LASCOMBE,J.

Chim. Phye. 62, 635 (1966).

1732

L. K.

Table 5. N-H

DYALL

stretching frequencies of aniline8 in pyridine-carbon solvents*

o*ooo

Xpyridine

vNH,,

2-t-Butyl-6.chloromiline

2.Trifluoromethylaniline

vNH,

vNH,

3613 3423

vNH,,

3490

vNH,

3396

vN&.

1

2.Bromoaniline

3603 3408

1

2-Methoxycarbonylanline

vNHm

3606

vNH,

3376

vNHm

3602

vNH,

3344

vNH,s

3620

vNH,

3399

vNH,s

3621

vNH,

3400

*NH,,

3621

vNH,

3396

vNH,,

3622

vNH,

3366

vNH,,

3614

vNH,

3387

(

2.Aoetyhmiline l

2.Nitroaniline I

4.Chloro-2 nitroaniline I

6.Methyl,2-nitroaniline I

2.Nitro-1.naphthylamine I

tetrachloride

0.0964

0.273

0.623

3601 3407 3336

3600 3404 3341

3601 3344

3348

3606 3422 3346

3499 3419 3344

3498 3418 3347

3490

3486 3469 3396 3319

3462 3393 3320

3460

3462

3319

3319

3404 3461 3376 3329

3601 3449 3376 3327

3498 3460 3373 3327

3490 3447 3376 3326

3500 3422 3341 3308

3496 3421 3340 3308

3494 3423 3341 3303

3486 3420 3337 3302

3618 3468 3400 3298

3617 3466 3397 3296

-3614 3465 3397 3294

3292

3618 3464 3399 3290

3618 3463 3398 3291

-3616 3463 3397 3292

3291

3617 3464 3396 3316

3616 3453 3397 3316

3613 3463 3394 3314

-3609 3462 3393 3311

3520 3416 3363 3289

3619 3409 3363 3302

3409

3403

3300

3293

3510 3447 3383 3283

3508 3440 3383 3287

3437

3432

3288

3286

1.000 -3490

3362

3463

3460

* Because of the opacity of pyridine, the sample cell length was reduced &Bxpvridine was inoreased. Amine concentratmn was correspondingly inore&sedfrom O-02 to 0.2 molar.

Solvent effects on the i&a-red Table 6. N-H

1733

spectra of anilines-VI

stretching frequencies of auilines iu pure solvents (all solutions O-02 molar in amine)

N-Methyl&line

Carbon tetrachloride

chlorobenzene

B0IlZ9ne

p_Xylene

3442 3503 3408

343s 3503 3404

3428 3600 3399

3422 3600 3393

uNH,

3613 3423

3508 341s

3608 3412

3507 3408

YNH,,

3490

3483

3481

3480

vNH,

3396

3389

3387

338b

vNHa,

3606

3496

3494

3491

vNH,

3376

3377

3377

337b

vNH,,

3502

3491

3487

3481

*NH,

3344

3347

3347

3344

wNH

vNJ&,

2-t-Butyl-6.chloroa~line

i

vNH,

vNH,s

(

2.Trifluoromethyleniline

l (

2.Bromoaniline

2.Methoxycarbonylaniline

t-Acetylaniline

t

4.Chloro-2.nitroaniline

&Methyl-2.nitroaniline

3488 3467 3394 3305 3506 3438 3376 3329 3604 3404 3341 3302

3620

3610

3608

3602

vNH,

3399

3394

3391

3387

vNHa, i UN%,

3521 3400

3608 3394

3504 3390

3500 3386

3466 3283

YNH,B

3621

3614

3511

3508

vNH,

3396

3394

3394

3393

3621 3454 3396 3338

vNHa,

3522

3610

3504

3498

vNH,

3365

3363

3364

3363

vN&,

3614 3387

3500 3380

3496 3379

3490 3376

2-Nitro-I-naphthylamine

i I-Nitro-2-naphthylamine

3293 3499 3406 3326 3603 3423 3342

vNH,a

l l

2-Nitroaniline

Triethylamine

vNH,

3518 3467 3397 3284

-3619 3406 3364 3300 3436 3281

Behauiour of amines with intra?nolecular hydrogen bonds. 2Acetylaniline and 2-methoxycarbonylaniline are included in the present study because there is general agreement on the presence of a moderately strong intramolecular hydrogen bond in each [l, 71. These amines should, in the presence of a suitable hydrogen bond acceptor, be able to use their second amino hydrogen to form an intermolecular hydrogen bond, whereupon vNH,, will decrease markedly but YNH, will not. Both these amines exhibit this predicted behaviour for hydrogen bond acceptors which are not too strong (see Tables 6 and 7 and Fig. 2). The “strength” or “basicity” of an acceptor is measured here by the AY value it produces with N-methylaniline. The behaviour shown by these two intramolecularly bonded amines is not a useful criterion for identifying such hydrogen bonding. Whereas 2-nitro-lnaphthylamine (Fig. 4) and 6-methyl-2-nitroaniline do both behave in analogous [7] A. N.

HAMBLY

and

B. V.

O’GRADY,

Azlstralian J. Chem. 10,459

(1963).

L. K. DYALL

1734

0

AvNH

of

N-methylaniline,

cm-’

Fig. 1. Comparison of frequency shifts produced by various acceptors with 2trifluoromethylanilineand N-methylaniliue. l = AYNH,,; q = AhvNH,. Code to acceptors: 1, chlorobenzene; 2, benzene; 3, p-xylene; 4, acetonitrile; 5, acetone; 6, diethyl ether; 7, ciueole; 8, dimethylsulphoxide; 9, pyridine; 10, triethylamine.

fashion for the m-type acceptors (Table 6), the other 2nitroanilines do not (see Fig. 3). Some factor additional to hydrogen bonding must determine the frequency shifts. Evidence for molecdar distortiolz. There will be strong repulsion between an ortE+osubstituent and the N-H bond of the aromatic amine [4]. Relief of strain could be achieved by suitable twisting of both the amino group and the 0~~ substituent from the ring plane, but involves loss of delocalization energy. Such loss could amount to several kilocalories per mole for amines such as the 2-nitroanilines whose substituents are mutually conjugated. The aniline must therefore assume a moderately distorted shape in which steric strain and delocalization energies are balanced. Any intramolecular hydrogen bond which is present will also enter into the energy balance. Involvement of the amino hydrogen remote from the o&o substituent in an intermolecular hydrogen bond represents an energy gain, and the amine can now bettor accept the loss of delocalization energy which ensues from loss of substituent coplanarity. That such rotation of substituents does occur has been demonstrated by RAE [8] by means of NMR measurements on nitroanilines in various solvents. [S] I. D. RAE, Australian,J. Chem 20, 1173 (1967).

1735

Solvent effects on the infra-red spectra of anilines-VI

Table 7. Frequency shifts (Av cm-l) for N-H stretching in aromatic amines measured at O-1 mole fraction of acceptor in carbon tetrachloride solution. Av = 0 for carbon tetrachloridesolutions. Av taken as positive for frequency shifts to lower values Acetone

Diethylether

Cineole

Dimethylsulphoxide

34

63

74

94

116

20

-3 19

1 42

1 63

47 63

2 73

0 fl 4

f4

3 26

4 28

3 49

6 60

12 76

8 78

0 6

It2 f3

4

9 20

17

8

38

64

26 70

21 77

-10

0

f2 h.2

23 -1

26 3

38 17

44 31

60 45

66 41

23 -21

f2 +3

31 -1

36 2

64 4

64 26

73 29

80 36

30 -

*2

AvNH,, 1AvNH, AvNH,, ( AvNH,

26 25

33 33

40 68

46 78

49 95

62 101

31 32

36 38

44 68

60 86

61 100

67 110

29 17

36 21

41 28

60 37

60 66

67 79

&4 It4 f3 zt6 fl

2-Nitro- lnephthylemine

AvNH,, IAvNH. AvNH,, i AvNH.

26 2 29 6 20

62 11

66 16

76 30

87 42

105 60

106 76

11 34 -4

zt3: *a f4

1-Nitro-2nephthylamine

AvNH,, AvNH,

38

42 37

62 69

68 76

70 98

67 104

Acetonitrile

Acceptor species

21

AvNH AvNH., AvNH, AvNHas 1AvNHs AvNH., 1AvNH, AvNH., I AvNH.

N-Methylaniline 2-t-Butyl-Schlorcaniline

I

2.Trifluoromethylaniline 2-Bromoaniline 2.Methoxycmbonylaniline

-1

AvNH,,

2-Acetylmiline

1AvNH.

2-Nitroaniline 4-Chloro-2nitroani1ine &Methyl-2nitroani1ine

29

14t

Pyridine

Av,,tr,,,,*

31 +4 6 13 or AvNH. VB. the N-methylaniline AVNH w,~B. The

* This figure is the extrapolation of the plot of AvNH.. extrapolation is from the plot for the most basic acceptors. Data from Table 6 haa been used. t This frequency wea obtained by extrapolation of the values measured at higher mole fractions (see Table 2). The AvNH,,. band obtained at O-1 mole fraction was asymmetric but not resolvable. $ The points for dimethylsulphoxide and pyridine are omitted for extrapolation purposes.

IO

;

60E 0 n I

40-

zi

6 0

I

I

I

40

1

I

so

Au NH of N-methylonlline,

1

I

120

I

0

cm“

Fig. 2. Comparison of frequency shifts produced by various 2-methoxycarbonylaniline and N-methylaniline. l = AvNH,,; See Fig. 1 for number code to acceptors.

acceptors with 0 = AvNH,.

L. K.

1736

T E 0

I z

DYALL

120-

80-

0

40

AVNH

80 of N-methylamlme,

120

I

cm-’

Fig. 3. Comparison of frequency shifts produced by various acceptors with Z-nitroaniline and N-methylaniline. 0 = AYNH,,; 0 = AvNH,. See Fig. 1 for number code to acceptors.

AVNH of

N-methylaniline,

cm-’

Fig. 4. Comparison of frequency shifts produced by various 2-nitro-1-naphthylamine and N-methylaniline. 0 = AvNH,,; See Fig. 1 for number code to acceptors.

acceptors with 0 = AYNH,.

Solvent effectson the infra-red

spectraof anilines-VI

1737

These molecular distortions will have various effects on the N-H stretching frequencies. Relief of steric compression of the N-H bond till lower both of its stretching frequencies (see below), as also will loss of conjugation with the aromatic ring and with an ortlio substituent of the -M type [I]. There will also be changes in these frequencies if any intramolecular hydrogen bond is affected by the rotations of substituents from the aromatic ring plane. Superimposed upon these three effects on the frequencies will be the hydrogen bond shift produced by the acceptor molecule in the solvent. The Bellamy plot of the net shift in either YNH,, or YNH, against vNH of Nmethylaniline for various acceptors could be expected to show a complete lack of correlation, because the frequency shifts for N-methylaniline reflect only intermolecufar hydrogen bonding. Nevertheless, the plots for YNH, are quite informative. Only for 2-trifluoromethyl-, 2-t-butyl&ohloro-, and 2-bromoaniline does the Bellamy plot extrapolate back through all the experimental points to the experimental carbon tetraehloride reference value. The first two amines presumably do not rotate the amino group out of the ring plane when forming intermolecular hydragen bonds, because the compressed N-H bond occupies a space flanked by two “branches” of the ortho substituent and rotation will not easily achieve relief of steric congestion. The large value of AvNH,, for 2-t-butyl-Lchloroaniline with dimethylsulphoxide as acceptor (see Table 7) does, however, suggest that rotation occurs in this one instance. In 2-bromoaniline the rotation of the amino group relieves only a small steric compression and involves only a small loss of conjugation energy, so that the hydrogen bond shift dominates the total frequency shift, The situation is very different for the anilines whose -l% type substituents conjugate strongly with the amino group and also exert considerable steric pressure upon it. There is also an internal hydrogen bond energy involved with these amines. The weaker acceptors (z--types and sometimes the poorer of the lone pair types) produce a progressive twisting of the two snbstitue~ts as the intermolecular bond grows stronger, so that YNR,, falls steadily until the steric congestion is essentially relieved. At this point, the YNH,, values level off, and the further decreases are the small ones normally observed for 1: I aggregates of amine with solvent [4]. At this stage, the intramolecular bond must be broken since the the AvNHS values would otherwise be the larger ones characteristic of an amine forming two hydrogen bonds. The onset of small AvNH,, values does indeed correspond to the onset of appreciable AvNH, values which indicate disruption of the internal hydrogen bond (see Pig. 3). The point in the series of increasingly effective i~te~olec~~ hydrogen bond acceptors at which this disruption occurs is no indication of the strength of the internal. bond since other energy factors are also involved. The Bellamy plots for AvNH, values produced by the stronger acceptors extrapolate back to values of YNH,,~some 30 cm-l lower than the experimental values for carbon tetra~hlo~id~ solutions (see Figs. 2-4 and Table 7). These values are appropriate if the PNH,, values used in the extrapolation no longer include the exaltations of frequency produced by conjugation and steric compression. 8

1738

L. K. DY~LL

The extrapolated value would not include the intramolecular hydrogen bond shift, though this should be small for YNH,,. 2-Nitro-l-naphthylamine and (to a lesser degree) 6-methyl-2-nitroaniline behave differently from the other 2-nitroanilines. Their increments in the AyNH, values for the more basio aeeeptors are large, and level off only for the very strongest acceptors (see Fig. 4). The high AvNH, values sugge& that the intramuie~u~ar bond is surviving along with the external hydrogen bond, but in these 2,6-disubstituted anilinea, the interplay of steric factors is too complex to be unravelled by our infra-red measurements. The PNH, values are not informative. The Bellamy plots do, in some instances, extrapolate back to the vicinity of the experimental values measured on carbon tetrachloride solutions (see Table 7)? which indicates the hydrogen bond shifts to be much larger than those from conjugation and steric effects. For all the amines studied here, the YNH, band is involved in Fermi resonanee with the &NH, overtone in the more basic solvents (see below) and the Bellamy pIots show corresponding curvature. The combination of Fermi resonance and intramolecular hydrogen bonding effects on YNH, of 2-acetylaniline and 2-methoxycarbonylaniline (see Fig. 2) produces chaos in their Bellamy plots.

The effects of bulk solvent properties on N- H stretching frequencies were examined to ensure that any such frequency shifts could be distinguished from changes in the state of amine-solvent aggregation. The data for binary solve&s (Tables 2-5) indicated that variat,ion of the mole fraction of acceptor in carbon tetrachloride solution had very little effect on vNH,, and YNH, of the “free” amine. The YNH values of the solvent-bonded amine are slightly sensitive to variation of solvent composition, but virtually none of our data Iend themselves to precise correlation of these frequency shifts with bulk solvent properties. There are uncertainties in measuring peak maxima when solute-solvent interaction has enhanced solvent absorption (as happens with acetone, dimethylsulphoxide, and pyridine); the vNH, band is often displaced by Fermi resonance; and for some amines (e.g. 2-t~~uoromethylanil~e and 2-bromoaniline in Tables 1 and 3) the YNH,, bands of “free” and “bonded” amine are coalesced. Only the data for acetonitrile-bonded 2-bromoaniline and 2-nitroaniline (Table 1) are suitable for correlation. For each of these amines, both YNH,, and YNH, vary linearly with the solvent polarization term E - 112.5+ 1. We have observed the same dependence of frequency on solvent polarization for the ~~~~-substituted anilines [a]_ The correlations fox 2-bromoaniline curve at low mole fractions of acetonitrile because the bands from the solvent-bonded amine then coalesce with those of the “free” amine.

The ortho-substituted anilines, like the papa-substituted anes [4], display a Fermi resonance interaction of the (TNH, overtone with vNH, in the most basic

1739

Solvent effects on the infra-red spectra of anilines-VI

solvents. This interaction was first proposed by FARMER and THOMSON [9] and is confirmed by our data in Table 8. Solvent effects on the 1600 cm-l region of the spectra of o&o-substituted anilines are not confined to one band arising from the 6NH, vibration. We agree with the conclusion drawn by HAMBLY and O’GRADY [7, lo] from the infra-red spectra of deuterated anilines that there is vibrational coupling of 8NH, with aromatic ring vibrations. Nevertheless, the results given in Table 8 do show that the strong band between 1615 and 1635 cm-l (carbon tetrachloride solution) has more 6NH, character than any other band. This band shifts to higher frequencies in the more basic solvents, and its overtone should do likewise. However, as the YNH, and 26NH, frequencies approach each other more closely, Fermi Table 8. Fermi resonance interaction of NH, deformation with symmetric N-H stretching in &lines. Frequencies in cm- 1. Figures in parentheses are Ed values in units of cm8 mole-l CCI, N-H

stmtahing (as) stretching (8) Overtones

N-H

1600 cm-’ region

N-H

stretching (as) N-H stretching (a) Overtones 1600 cm-’ region

N-H N-H

stmtohing (es) stretching (a) Overtones

1600 cm-’ region

CH,CN

(CH,)W

CP,N t

K-W@

2-Trifluoromethylsniline 3508 (63) 3494 (69) 3469 (22)* 3412 (110) 3394 (140) 3378 (124)* 3232 (6.6) 3262 (21) 3266 (35)

3419 (94) 3362 (112) 3236 (161)

3490 (60) 3362 (97) 3221 (117)

3503 (70) 3342 (so)* 3201 (80)

1630 (341)

1639 (216)

1640 (207)

1660 (138)

1646 (136)

1616 (131) 1601 (67) 1689 (128)

1614 (141) 1600 (77) 1687 (133)

1614 (160) 1601 (70) 1686 (133)

1616 (177) 1601 (72) 1687 (146)

1613 (220) 1600 (74) 1681 (162)

1616 (199)

1637 (148) 1631 (160) 1617 (199) 1601 (78) 1686 (164)

3490 (47) 3396 (67) 3189 (2.3) 3179 (2.0) 1616 (408)

3481 (46) 3387 (76) 3220 (1.1) 3190 (3.2) 1616 (394)

2-Bromosniline 3468 (64) 3472 3371 (106) 3369 3232 (6.6) 3239 3210 1623 (274) 1623

3462 (61) 3319 (90) 3231 (-) 3196 (96) 1627 (184)

1672 (231

1589 (28) 1672 fl9)

1694 (51) 1567 1241

3460 (31) 3322 (83) 3239 (66) 3210 (66) 1636 (161) 1629 (186) 1693 (116)

3464 (78) 3297 (124)

3463 (99) 3292 (116)

3467 (126). 3284 (69)*

3173 (140)

3166 (94)

3168 (SO)

1627 (626)

1626 (698)

1627 (897)

3513 3423 3231 3216 1631

(46) (84) (3.6) (2.7) (344)

3620 (106) 3399 (160) 3246 (0.3) 3196 (0.9) 3163 (2.6) 1624 (690) 1684 (129) 1577 (214)

N-H N-H

stmtahing (as) stretching (8) Overtones

3614 (124) 3387 (107) 3189 (2.1)

1600 cm-’ region

1635 (722) 1611 (160) 1680 (106) 1666 (136)

C&L,

3506 (121) 3391 (198) 3190 (1.6) 3166 (3.6) 1624 (668) 1686 (123) 1676 (235)

GH,)zO

1607 (61) 1669 (18)

2-Nitroeniline 3490 (123) 3480 3368 (169) 3343 3223 (3.6) 3222 3184 (7.2) 3186 1626 (713) 1698 (72) 1574 (306)

(64) (109) (11.2) (12.7) (276)

(146) (147) (0.6) (17.1)

1627 (764) 1699 (59) 1677 (297)

1-Nitro-2-naphthylamine 3496 (114) 3472 (87) 3463 (92) 3379 (167) 3365 (163) 3332 (142) 3192 (1.7) 3239 (7.6) 3248 (29.6) 3203 (11.9) 1636 (626) 1636 (753) 1636 (816) 1610 (124) 1611 (137) 1613 (146) 1682 (119) 1696 (99) 1666 (170) 1666 (219) 1667 (218)

1571 (293)

3173 (71) 1630 (190) 1618 (170) 1695 (83) 1571 174)

1676 (286)

3423 (89) 3292 (149) 3168 (191)

3432 (86) 3286 (141) 3168 (160)

3436 (46) 3281 (79)

1633 (718) 1610 (96)

1633 (691) 1610 (113)

1634 (663) 1616 (138)

1562 (278)

* There is also another band arising from amine not involved in hydrogen bonding with solvent. t The region below 1610 on+ is not transparent in this solvent.

[9] V.C. FARMER andR.H. THOMSON,S~XL%OC&L Actu 16,559 (1900). A. N. HAMBLY and B. V. O'GRADY, Awrtrdian J. Chem. 17,860 (1964).

[lo]

3467 (48) 3305 (68)

1666 (137)

L. EI;. DYALL

1740

resonance occurs. The weak overtone band then ~amatically intensifies, and shifts down to lower frequencies. At the same time, the progressive decrease in YNH, along the series of increasingly effective hydrogen bond acceptors is halted (see Figs. l-3).

We have previously pointed out that an ortho substituent in an aniline can affect the N-H stretching frequency through inductive and mesomeric interactions, sterie pressures, direct field efZ’ects through space, and hydrogen bonding [5]. It is the difficulty of separating these effects one from another which has led to much of the controversy over intramolecular hydrogen bonding in orthosubstituted anilines. The steric pressure on the N-H bond in secondary aromatic amines raises the stretching frequency [5], and would be expected to raise both YNH,, and vNH, in primary amines. Superimposed upon these frequency increases are the other effects of the substituent, but nevertheless several entries in Table 9 reveal steric compression quite clearly. Further examples can be found among data for anilines published by KRUEGER [ 111. Whereas the electricat effect (j-M and +I) of oaks-methyl should lower both N-H stretching frequencies (as happens with p-toluidine), o-toluidine shows slight increases with respect to aniline. A large ortho alkyl group (as in S-t-butyl5chIoroaniline) produces quite large frequency increases in both YNH,, and Table

9. N-E

stretching frequenoies of anilines (carbon tetr~~loride and vNH, (cm-“)

vNH,, Optho substituent in anilines

H

vNH,,

and

vNH,

(cm-l)

vNH,, - vNH,

3479 3395

84

3479 3395

84

H

2-CH,

3482

86

4.CH,

( 3470 3390

80

2-CF,

I 3396 3513 I 3423

90

3XF,

( 3492 3404

88

3603

9s

3x1

i 3490 3402

88

i 3408 3490 i 3396

94

4.Br

I 3486 3389

86

4-NO,

3509 3414

95

3600 3409 3602 3409

91

2-(CH~)~C-5-Cl 2.Br Z-NO, Z-NO,-I-Cl z-NO,-6.CH, 2-NO,-6X1 2GOOCH, 2.COCH,

[ll]

vNH,B - vNH,

Pam or meta substituent in aniline

solutions)

P. J. KRUEGER,

3520 ( 3399 3521 13400

121

3521 i 3395 3512 13391

126

3606 i 3376 3502 3344

130

4.COOCH,

158

I.COCH,

Cum. J. C&m.

121

121

40, 2300 (1962).

93

Solvent effects on the infra-red spectra of anilines-VI

1741

YNH, (Table 9). Alkyl groups can be assumed to exert only small direct field effects, and negligible hydrogen bond effects, on the N-H stretching frequencies. The o&o-trifluoromethyl substituent also produces frequency increases consistent with strong steric compression, though part of the shift would be caused by the strong inductive effect of the substituent operating at short range. The potential hydrogen-bonding substituents (nitro, acetyl, methoxycarbonyl) will all exert strong steric pressures on the N-H bond from the ortho position, so that on this basis alone the vNH frequencies will be higher than they are for the The hydrogen bond shifts will be in the opposite corresponding Ipara isomers. direction, and, to judge by the behaviour of anilines forming one intermolecular hydrogen bond [4, 121, will be very small for vNH,, and appreciable for vNH,. The presence of one intramolecular hydrogen bond can therefore be diagnosed by the combination of a high vNH,, value, and large (vNH,, - vNH,) difference, for the ortho-substituted aniline in comparison with its para isomer. The second of these criteria has already been used by other workers [ 1, 10, 131. Our two criteria are met by the two generally accepted examples of intramolecular hydrogen bonding provided by 2-acetylaniline and 2-methoxycarbonylaniline. On this basis, there can be no doubt that all the 2-nitroanilines listed in Table 9 are internally hydrogen-bonded. Likewise, there is intramolecular 2-nitro-l-naphthylamine, and hydrogen bonding in 1-nitro-2-naphthylamine, 3-nitro-2-naphthylamine [la]. We cannot, however, accept the examples of %nitro-1-naphthylamine [ 141 and the o&ho-aminophenylsulphones [lo] as being proven. In these amines both vNH,, and vNH, are lowered in comparison with the appropriate model compound. These frequency shifts can be explained, at least in part, by rotation of the substituents from the aromatic ring plane to relieve the otherwise very considerable steric strain. Our criteria for internal hydrogen bonding would not be reliable for very weak bonds (such as might exist in 2-bromoaniline) because there is some uncertainty about possible influences of direct field effects. Primary amines using both amino hydrogens to form bonds to basic solvents show lowering of both N-H stretching frequencies [4, 121. Those amines, such as 2,6_dinitroaniline, which form bilateral hydrogen bonds likewise show low values of both vNH,, and vNH, [I, 151. The actual frequencies are, however, little guide to the strength of these hydrogen bonds since there is no measure of the steric compression shift, and there will certainly be some degree of twisting of the substituents from the aromatic ring plane. Acknowledgement-The author is indebted to Professor A. N. HAMBLY for helpful discussions and the gift of samples. [12] [13] [14] [I51

J. J. A. L.

LAURANSAN, P. PINEAU and M.-L. JOSIEN, Ann. C/&n. 9,213(1964). H. RICHARDS and S. WALKER, Trans. Faraday Sot. 57, 418 (1961). BRYSON and R. L. WERNER, Australian J. Chem. 19,456 (1960). K. DYALL and A. N. IIAMEILY, Australian J. Chem. 11,513 (1958).