Solvent effects on the infrared spectra of anilines-V

Spectrochimica Acts,Vol.26A,pp1423to 1485. Pergamon Prees1969. PrintedinNorthernIreland

Solvent effects on the infrared spectra of anilinee-V* Anilines with no ortho substituents L. K. DYALL University of Newcastle, Shortland, N.S.W., 2308, Australia (Received

30 December

1968)

Abstract-N-H stretching frequencies of seven primary aromatic amines in carbon tetrachloride solution have been measured in the presence of varying mole fractions of hydrogen bond acoeptor. The first hydrogen bond between amine and solvent causes a much larger shift

in the symmetric N-H stretching frequency than in the antisymmetric, while the second hydrogen bond has a large effect on the antisymmetric frequency and almost none on the symmetric. The frequency shifts show excellent correlation with the Hammett constants of the aniline ring substituents. Fermi resonance and bulk solvent effects on the N-H stretching frequencies are identified in some instances. INTRODUCTION WE previously reported solvent effects on the infrared spectra of some anilines [l] and interpreted the (averaged) shifts in the antisymmetrical (vNH,,) and symmetrical (YNH,) N-H stretching frequencies in relation to intramolecular hydrogen-bonding. It was assumed that anilines normally bonded intermolecularly to two solvent molecules unless prevented from so doing by an on% substituent. However, equilibrium data is now available to demonstrate that the amine-solvent aggregate may have either 1: 1 or 1: 2 stoichiometry even in the absence of ortho substituents [2-61, and our use of averaged frequency shifts would have obscured important information on these states of association. Moreover, frequency shifts will not be observed unless an adequate equilibrium concentration of solvent-bonded amine is present, and our work with secondary aromatic amines [7,8] indicates that the energy of an intramolecular hydrogen bond is only one of several energy quantities determining the position of of the solvent effects on N-H stretching this equilibrium. A total re-examination frequencies of primary amines is needed. This paper reports solvent effects on the spectra of anilines whose interactions with solvents are free of the complications introduced by substitution o&ho to the amino group. The results obtained here will be used to snalyze the more complicated behaviour of o&o-substituted anilines in a later paper. * Part N:

L. K. DYALL, Au.&aZian J. Chem. 20, 93 (1967).

L. K. DYALL, Spectrochim. Acta 17, 291 (1961). H. LADY and K. B. WBETSEL, J. Whys. Chem. 69, 1596 (1965). H. LADY and K. B. WEETSEL, J. Phyrr. Chem. 71, 1421 (1967). LAURANSAN, P. PINEAU end M.-L. JOSIEN, Ann. Chim. 9, 213 (1964). LAURANSAN, P. PINEAU and J. LASCOMBE, J. Chim. Phye. 62, 635 (1966). B. BHO~IK and S. BASU, Tram. Furuday Sot. &&48 (1962). [7] L. K. DYALL and J. E. KEMP, Spctrochim. Acta 22, 467 (1966). [8] L. K. DYALL, Australian J. Chem. 20, 93 (1967). [l]

[2] J. [3] J. [4] J. [5] J. [6] B.

1423

1424

L. K. DYALL EXPERIMENTAL

Materials Liquid amines were purified by distillation and then stored in the dark over molecular sieves (Union Carbide Type 4A). Solid amines were recrystallized to constant m.p. and dried in ‘uacuo over P,O,. Solvents were freshly purified by our usual methods [7]. Spectral

measurement8

The spectra in the N-H stretching region were recorded with a Unicam SP-700 grating spectrophotometer as previously described [7]. Overlapping absorptions were resolved, whenever possible, by graphical reflection about the apparent maximum of the stronger peak. The spectra in the 1700-1550 cm-l range were measured with an Hitachi EPI-G2 spectrometer using polystyrene calibration. Solutions of the amines in pure solvents were prepared w/v, while mixed solvents were prepared w/w. Sample cells were stoppered to prevent loss of volatile solvents. Deliberate temperature variations ( &5’ from ambient) has no measurable effect on the frequencies, and spectral samples were therefore not thermostatted. Sample concentrations were in the range O-01-0.02 molar (suitable for 1 mm cells) whenever this solubility could be achieved. There was no evidence of amine self-association in those solvents whose transparency allowed concentration variation between O-01 and 0.1 molar; the N-H stretching frequencies, extinction coefficients, and band widths remained constant over this concentration range. With the less transparent solvents (acetone, oineole, dimethylsulphoxide, and pyridine), amine concentrations of 0.1 molar were used in 0.1 mm oells, and these spectra were unchanged by increase of concentration to 0.2 molar. DISCUSSION

Solute-solvent

equilibria

A primary amine may use either or both of its amino hydrogens to form hydrogen bonds to solvent molecules possessing a suitable lone electron pair or r-bond acceptor site. Each of these amine-solvent aggregates will have its own pair of N-H stretching frequencies (vNH,, and vNH,), and molecules of “free” amine might be present to contribute yet another pair of bands to this region of the spectrum. All these frequencies must be correctly assigned to corresponding aggregates before comparisons of solvent effects on N-H stretching frequencies can be made. A convenient method of establishing the stoichiometry of amine-solvent hydrogen bond interactions has been demonstrated by JOSIEN et al. [4] for 4-bromoaniline. It consists of measuring the N-H stretching frequencies of the amine in a series of solutions of the amine and hydrogen bond acceptor in an inert solvent. At low mole fractions of the acceptor, the amine-solvent aggregates would have 1: 0 and 1: 1 stoichiometry, with the 1: 2 aggregates appearing at still higher mole fractions. The intensity changes in the various bands enable assignment of the vNH bands to specific aggregates. Morevoer, we find that frequency shifts produced by hydrogen bonding can be distinguished from those produced by bulk solvent effects. Our data, obtained by the mole fraction variation technique for a number of aromatic amines, are presented in Tables 1 (diethyl ether), 2 (acetonitrile), 3 (acetone), 4 (DMSO) and 5 (pyridine). Whenever comparison has been possible, we find that our stoiohiometries agree with those obtained by other workers using equilibrium constant measurements or force constant calculations.

1425

Solvent effects on the infmred spectra of milims-V Table 1. N-H

stretching frequenciesof anilines in diethyl ether-carbon tetrachloride solvents*

E-

n’ N-methylsniline

Xetber

0-0000

0.113

0*188

0.479

0.709

1.000

1/2.9+ 1 l/2?%’+ 1

0.226 0.216

0.247 0.211

0.269 0.208

0.298 0.197

0.320 0.188

0.343 0.177

3442

3442 3389 1:l

3442 3388 1:l

3441 3389 1:l

3443 3388 1:l

3476 ~3448 3397 3366 l:o 1:l 1:2

3468 ~3436

3467 -3440

3366 1:l 1:2

3366 1:l 1:2

3369 1:l 1:2

3468 3390 3367 l:o 1:l

3468 3389 3366 1:o 1:l

3466

3461

3468

3364 1:l

3364 1:l

3366 I:1

3474t 3461

3466 3366 1:2

vNH Amine : solvent aggregate

Anilinc

vNH..

3479

3476

vNH.

3396

Amine :solvent

1: 0

3393 3367 l:o 1:l

aggregate 4-Methylaniline

vNH.. vNH. Amine

:solvent

3472 3389 1: 0

aggregate I-Bmmoaniline

4-Methorycarbonyl~IliliIl~

3480

34787

vNH.

3399

3399 3364 l:o 1:l

3398 3364 l:o 1:l

3364 1:l 1:2

3364 1:l 1:2

3490 3460 3410 3364 l:o I:1 1:2

3488 3461 3410 3364 l:o 1:l 1:2

3488 3461

3489 3463 3410 3362 l:o I:1 1:2

3486 3460

3483 3449

3460

3362 1:l

3362 1:l

3364 1:2

3484 -3444 3406 3368 l:o 1:l I:2

3481 -3437

3480 3447

3448

3369 1:l

3360 1:l

3362 1:2

1:2

1:2

3490 3429

3488 3432

3488 3433

3366 1:l 1:2

3366 1:l 1:2

3368 1:l 1:2

l:o

vNH,.

3600

3492

vNH.

3409

3410 3364 l:o 1:l

1: 0

~ww*t~ vNH,.

3602

3491

vNH.

3409

Amine : solvent

1: 0

3410 3363 l:o

vNH..

3497

3484

vNH.

3407

3406 3369 l:o

aggregate 3.Nitroaniline

Amine : solvent

1:l

1: 0

aggregate 4-Nitroaniline

3466t

3486

Amine : solvent

4.Acetylaniline

3389 1:l

vNH.s

Amine : solvent aggregate

1:l

vNH..

3609

3490

3491

vNH,

3414

3416 3366 l:o 1:l

3417 3366 l:o 1:l

Amine :solvent aggregate

l:o

1:2

3363 1:l 1:2

-

3462 3364 1:2

1:2

All solutions 0.02 molar in amine. * Dielectric constants, E, and refractive indices, n, are taken from P. KIN R. SCELEYEE, and A. ALLEBHAND, J. Am.Chem.Soc.85, 371 (1963). t Asymmetry of the bend suggests an overlapping of two absorptiona.

L. K.

1426 Table 2. N-H

stretching frequencies of anilines in acetonitrile-carbon tetrachloride solvents* 0~0000

0.0266

0.0662

0.111

0.169

0.323

0.643

1.000

E - l/2& + 1 na- 1/e?&*+ 1

0.226 0.216

0.262 0.216

0.292 0.214

0.332 0.212

0.360 0.211

0.403 0.207

0.444 0.196

0.480 0.176

vNH

3442

3441

34397

343ot 3416

Amine:solvent aggregate

3413

3411

1:0

1:O

1:0 1:l

I:0 1:l

1:l

1:l

1:l

vNH.s

vNH.

3472 3389 1:0

3470 3387 1:0 1:l

3467 3383 1:0 1:l

3467 3380 l:o 1:l

3466 3380

3461 3378

3461 3374

3467 3373

1:l

1:l

1:l

1:l

vNHas

3486

3484

3483

vNH,

3399

3396

3390

3479 3384 l:o 1:l

3476 3381

3471 3379

3467 3378

1:l

1:l

3483

3482

3476

3384

3381

3379

1:l

1:l

1:l

3470

3471

3377

3376

1:2

1:2

&eCN

N-Methylaniline

4-Methylaniline

Amine:solvent aggregate I-Bromoaniline

4-Methoxycerbonylaniline

Amine:solvent aggregate

I:0

vNHa,

3600 3409

vNH, Amine : solvent lxggregete

I-Nitroeniline

DYAU

l:o

I:0 1:l

1:0 1:l

3481 3386 1:o 1:l

3497 3407 3392 I:0 1:l

3496 3406 3391 l:o 1:l

3493 3406 3388 l:o I:1

3489 3408 3386 l:o I:1

3600

3496 ~3468

3493 ~3466

3383 1:l 1:2

3381 1:l 1:2

vNHa,

3609

3604

vNH,

3414

3416 3389 l:o 1:l

Amine :solvent aggregate

1: 0

34197 l:o 1:l

3386 1:l

1:l

3486t

3380 1:l 1:2

All solutions 0.02 molar in amine. * Dielectric constants, E, and refractive indices, n, are taken from P. VON R. SCALEYEEend A. ALLERIXAND .7. Am.Chem.Soc. 85,371 (1963). t Aeymmetry of the band indicates an overlapping of two absorptions.

Table 3. N-H

stretching frequencies of anilines in acetone-carbon tetrachloride solvents*

XaFetDne N-Methylaniline

Aniline

4.Methylsniline

vNH

0.000 3442

Amine : solvent aggregate

l:o

vNH,s vNH.

3479 3396

Amine : solvent aggregate

l:o

vNH,,

3472

vNH,

3389

0.126 3433 3408 l:o 1:l

0.294

0.646

1.000

3406

3406

3399

1:l

1:l

1:l

3473

3469

3463

3461

3378

3376

3373

3373

1:l

1:l

I:1

1:l

3466

3466

3469t 3466

4-Bromoauilme

Amine : solvent aggregrtte

l:o

vNHas vNHs

3486 3399

Amine : solvent aggregate

l:o

3376 1:l

3373 1:l

3369 1:l I:2

3370

3480 -3401 3377 l:o I:1

3473

3466

3468

3374

3373

3372

1:l

1:l

1:l

1:2

1427

Solvent effects on the infrared spectra of anilines-V Table 3 (co&.) @OOO

XBcetone 4-MethoxycarbonylWliline

vNH,s

3491

vNH,

3409

-3406 3379 l:o 1:l

l:o

Amine:solvent aggregate 4-Acetylaniline

YNH,,

3602

vNHs

3409

Amine : solvent

l:o

vNH.8

3497

vNH,

3407

Amine :solvent aggregate C-Nitroaniline

vNH,s

3609

vNH.

3414

Amine : solvent aggregate

l:o

l*OOO

3468t

3461

3376 1:l I:2

3373

3484 3469

3467t

3376 1:l 1:2

3373 1:l 1:2

3484 ~3466

l:o

0.646

3484

3492 3461 3409 3379 l:o 1:l

aggregate 3-Nitroaniline

0294

0.126

3600

1:2

-

3463

3477t

-3479 3461

3372 1:2 3463

3377 1:l 1:2

3376 1:l 1:2

3373 1:l 1:2

3371

3496 3462

-3489 3468

3469

3467

3376 1:l 1:2

3372 1:l 1:2

3368

3368

1:2

1:2

1:2

* Opacity of acetone neoeaaitated reduction of sample cell length aa xacetone increased from 0 to 1. Amine ooncentrations were correspondingly increased from 0.02 to 0.16 molar. t Asymmetryof the band indicates an overlapping of two absorptions.

Table 4. N-H stretching and overtone N-H bending frequenciesof anilines in dimethylsulphoxide-carbon tetrechloride solvents* XDWO

N-Methylaniline

4-Methylaniline

vNH

0.0119

0.064

0.103

3440 3362 l:o 1:l

3440 3348 l:o 1:l

3441 3346 l:o 1:l 3447

vNH.,

3472

3466

3461

3461

vNH.

3389

3387 3346 3219

-3383 3340 3221

~3386 3337 3224

3228 3203 l:o

0.203

3442 3363 1:o 1:l

l:o

Amine : solvent aggregate

l:o 1:l

l:o 1:l

l:o 1:l

0.386 3337

l+OO 3328

1:l

1:l

3413t

3390

3338 3228

3336 3229

3336 3230

1:2 1:l

1:2

1:l

vNH.s.

3486

3476

3466

3464 3431

3402

3390

vNH,

3399

3399 3341 3236 3218 l:o 1:l

3400 3338 3234 3219 l:o 1:l

3399 3338 3228

3338 3229

3336 3227

3336 3224

l:o 1:l

1:2

1:2

1:2

3486 3403

3486 3392

3480 3388

3388

3376

3333 3214

3333 3216

3336 3211

3333 3208

3334 3196

1:l 1:2

1:l 1:2

1:l 1:2

1:2

1:2

overtones Amine:solvent aggregate 4-Nitroaniline

3442

Amine : solvent aggregate

overtones

4-Bromoaniline

0.000

3226 3204 l:o

vNH.,

3610

3488

vNH,

3414

3414 3336 3242 3217 l:o 1:l

OVtX+XI~ Amine : solvent aggregate

3229 3209 l:o

Because of opacity of DM80, the sample oell length was reduced aa xDarso increased. Amine oonoentration WBBoor~~pomlingly increased from 0.02 to 0.2 molar. t Asymmetry of the band indioates an overlapping of two nbaorptions. l

L. K.

1428 Table 5. N-H

stretching and overtone N-H bending frequencies of anilinea in pyridine-carbon tetrachloride solvents* Xovridine

N-Methylaniline

Aniline

vNH

0.000

3442

Amine : solvent aggregate

l:o

VNHBB

3479

*NH. OV0rtOKlS3 Amine : solvent aggregcste I-Methylaniline

3207 3198 l:o

vNHas vNH,

3472 3389

Overtones

3228 3203 l:o

Amine: solvent aggregate

3468 3392 3334 3211

3600 3409 3211 1:o

vN&s

3602

vNH.

3409

0.623

1.000

3324

3320

3316

I:1

1:l

1:l

3461 3394 3334 3214

3464

3449

3336 3217

3339 3216

1:l

1:l

1:l

I:1

3466 3387 3333 3211

3464 3384 3334 3211

3454 3387 3333 3214

3448

3428

3336 3217

3336 3216

l:o 1:l

l:o 1:l

l:o 1:l

1:l

1:l 1:21

3466

3449

3336 3213

3337 3211

1:2?

I:21

3466

3467

3337 3210

3328 3202

1:l

1:l

3399 3332 3208

vN&, vN&

0.273

l:o 1:l

3399

Overtones Amine :solvent aggregate

3471 3460 3397 3331 3209

3393 3333 3210

1:o 1:l

l:o 1:l 1:2?

l:o I:1 1:2?

3488 3408 3329 3209 l:o 1:l

3481 3407 3331 3209 l:o 1:l

3477 3401 3333 3208 l:o 1:l

3498 3486 3409 3330 3210 I:0 1:l

~3600 3481 3408 3329 3206 l:o I:1

3478 N3401 3332 3205 l:o 1:l

3469

3460

3337 3207

3338 3200

1:l

1:l

3464 3461

3444

3464t

Overtonea Amine : solvent aggregate

3209 l:o

vN&s

3497

3480

3476

3469

vNH.

3407

3406 3326 3205 l:o 1:l

3404 3328 3207 I:0 1:l

-3399 3333 3206 l:o 1:l

~3603 3484

3482

3481

3407 3328 3196

-3403 3333 3191

Overtones Amine : solvent aggregate 4-Nitroaniline

3480 3470 3395 3336 3211

vNH,

ag@;regate

3-Nitroaniline

3440 3327 l:o 1:l

3477

3225 3204 l:o

0.0954

3442 3327 l:o 1:l

3486

Amine : solvent

I-Aoetylaniline

0.0483

vN&s

Overtones

C-Methoxycarbonylaniline

DYALL

3211 l:o

vN&s

3609

vNHs

3414

Overtones Amine : solvent Stoichiometry

3229 3209 l:o

3414 3327 3207 3195 l:o I:1

1:o I:1

l:o 1:l

3333 3208 I:1 1:2P

3337 3201

3464

3460

3337 3191

3336 3186

1:2P

1:21

1:2?

* Aniline oonoentrationa were 0.02 molar forXpyridine O-273 and below, but were increased to 0.16 molar when higher mole fractions of pyridine were used. t Asymmetry of the bond indicates an overlapping of two ebsorptions.

Solvent effects on

the infrared spectra of aniline-v

1429

Spectral changes from hydrogen bonding The most clear-cut data on frequency shifts were obtained with diethyl ethercarbon tetrachloride solvents (Table 1). The shifts are usually large enought to permit clean resolution of the various bands, and the N-methybniline results indicate that bulk solvent effects are negligible. 4-Methylaniline is a, clear case of 1: 1 association with solvent. The bonded vNH, band appears at 3367 cm-l and persists through increased mole fractions of ether, while the “free” vNH, band at 3389 cm-l diminishes in intensity and then disappears under the wing of the other bend. The vNH,, band does not exhibit this clear-cut behaviour : it has shifted only 14 cm-l on going from xether= 0 to x = 1 and at no stage can separate bands for “free” and hydrogen-bonded amine be seen. The expected two bands are probably coalesced since the vNH, band of 3- and 4-substituted anilines is somewhat broad and weak. Examples of 1: 2 association of amine with ether are provided by 4-acetyl-, 4-methoxycarbonyl-, 3-nitro-, and 4-nitroaniline. At low xether,the appearance of a second vNH, band and a small shift in vNH, indicate 1: 1 association. A second vNH, band appears at higher mole fractions and becomes stronger as xetherincreases. This second vNH,, band is some 40 cm-l lower than that of the 1: 1 aggregate, while vNH, actually incremes by several wavenumbers when the 1: 2 aggregate replaces the 1: 1 aggregate. The overall pattern of frequency shifts from formation of 1: 1 and 1: 2 aggregates agrees with that observed for 4-bromoaniline [4], aminotriethylsilane [9] and methylamine [lo]. Our results with acetonitrile-carbon tetrachloride solvents (Table 2) are similar, except that the frequency shifts are smaller and some of the expected bands cannot therefore be resolved. These results are also complicated by bulk solvent effects on the N-H stretching frequencies (see below), but these two difficulties could be recognized with the aid of the simpler example of N-methylaniline. Acetone-carbon tetrachloride solvents give similar results (Table 3). The solvents involving DMSO and pyridine produced very broad bands with extensive overlap, and a Fermi resonance interaction (see below) maintained the position of the bonded vNH, band almost constant despite the occurrence of general solvent effects on the other bands. Pyridine-carbon tetrachloride solvents (Table 5) provide clear examples (aniline and 4-acetyl- and 4nitroaniline) of separate vNH, bands from the free amine and the 1: 1 amine-solvent aggregate. The apparent vNH, bands of 1: 2 aggregates in this mixed solvent are dubious since pyridine involved in hydrogen bonding shows enhanced absorptions near 3450 cm-l. Fermi resonance interactions It is known for 4-bromoaniline that the overtone of the 1620 cm-’ band (6NH.J sometimes falls sufficiently close to vNH, to give rise to Fermi resonance [4, 111. We now report fundamental and overtone NH, deformation frequencies in a series of solvents (Table 6) in order to identify Fermi disturbances of vNH, for a more representative selection of anilines. M.-TH. FOREL and J. VALADE, Cow@. Rend. 257,387O (1963). H. WOLFF md J. EINTS, Ber. &_msenges. P&8. Chem. 70, 728 (1966). M.-L. JOSIEN, J. China. P&a. 61, 245 (1964).

[9] A. MARCHAND,

[lo] [ll]

1430

L. K.

DYALL

Table 6. Fermi resonance interaction of NH, deformation with symmetric N-H stretching in anilines* CCI,

‘4%

CH,CN

(%H,),O

(CH,),SO t

C,H,N

(C,H,)sN

Aniline N-H

stretching

3479 (24)

3471 (35)

3463 (58)

3465 (66)

3404 (106)

3449 (47)

3460 (33)

(88) N-H stretching

3396 (31)

3386 (68)

3377 (99)

3369 (115)

3338 (116)

3339 (82)

3331 (67)

3207 (3.6) 3198 (2.6) 1618 (300)

3209 (6.8) 3230 (2.4) 1618 (266)

3246 (12.2) 3229 (12.1) 1627 (166)

3242 (19)

3229 (105)

3216 (84)

3202 (61)

1628 (166)

1638 (89)

1629 (100)

1603 (151) 1692 (39)

1602 (176) 1692 (32)

1604 (289) 1588 (26)

1606 (309) 1691 (19)

1603 (476) 1683 (16)

1601 (316)

1632 (108) 1618 (104) 1604 (326)

stretching

3472 (19)

3461 (27)

3467 (46)

3468 (37)

3390 (96)

3428 (64)

3465 (31)

Nz

stretching (8) Overtones

3389 (26)

3380 (47)

3373 (80)

3366 (91)

3336 (161)

3336 (93)

3327 (63)

3206 (3.8) 3227 (1.8) 1622 (178)

3237 (7.2)

3239 (11.6)

3230 (163)

3216 (90)

3198 (82)

1600 cm-’ region

3203 (2.6) 3228 (1.0) 1620 (177)

1627 (143)

1640 (101) 1628 (167)

1640 (92)

1609 (68)

1607 (46)

1637 (96) 1628 (109) 1618 (130)

1639 (110) 1629 (122) 1621 (126)

(8)

Overtonea 1600 cm-’ region

4-Methylaniline N-H

1686 (18)

1686 (18)

1618 (-) 1699 (26) 1686 (18)

1618 (120) 1697 (22) 1686 (17)

1680 (11)

1683 (38)

4.Bromoeniline N-H

stretohing

3486 (27)

3476 (33)

3467 (66)

3466 (46)

3390 (102)

3449 (42)

3461 (30)

N-%

stretching

3399 (49)

3388 (70)

3378 (117)

3366 (136)

3336 (131)

3337 (97)

3326 (73)

3204 (3.8) 3225 (1.6) 1617 (261)

3206 (6.6) 3223 (3.2) 1619 (247)

3249 (10.3) 3228 (8.6) 1631 (146)

3261 (32.9) 3227 (16.9) 1631 (133)

3224 (149)

3211 (116)

3189 (98)

1640 (100)

1634 (99)

1694 (94)

1696 (126)

1597 (216)

1698 (232)

1696 (328)

1638 (128) 1619 (70) 1598 (267)

(fJ) Overtones 1600 am-‘region

I-Nitroeniline N-H

stretching

(m) N-H stretching

3609 (44)

3496 (74)

3471 (79)

3433 (76)

3376

3460 (23)

3476 (22)

3414 (132)

3399 (240)

3376 (260)

3368 (227)

3334 (213)

3337 (124)

3330 (89)

3229 3209 1621 1600

3236 3212 1621 1600

3257 3237 1636 1601

3268 3246 1641 1603

3196 (332)

3181 (199)

3193 (109)

1661 (167) 1600 (866)

1642 (160) 1601 (670)

(insoluble)

(8)

overtones 1600 cm-‘region

(1.9) (10.8) (777) (688)

(4.6) (13.8) (666) (738)

(60) (46) (360) (734)

(-) (136) (218) (1025)

* Fmquenc~es are in cm-‘. Figures m parentheses are apparent extinction coefficients in units of cm* mole-‘. t Extinction coefficients measured for this solvent in the 3500-3300 cm-’ range are over-estimates since the N-H stretohing bands am superimposed on strong background absorption.

The 6NH, band near 1620 cm-l moves to higher frequencies in more basic solvents as expected. The C-C stretching bands of the aromatic ring, near 1600 cm-l, show large intensity changes, and small frequency changes, which imply some degree of The weak overtone of 6NH, shifts to higher coupling with this NH, deformation. frequencies in parallel with the fundamental until, in sufficiently basic solvents, Fermi resonance with the YNH, vibration occurs. The overtone then borrows considerable intensity (so that it swamps out other weak bands in its vicinity) and shifts to lower frequencies as the degree of Fermi resonance increases. These shifts, and the

Solvent effects on the infrared spectra of aniline-V

1431

corresponding disturbance of vNH,, are shown in Fig. 1. The YNH values of Nmethylaniline are included to provide a set of undisturbed vNH shifts. C,,H,,O in Fig. 1 is 1,8-cineole. Recognition of the Fermi resonance interaction is important in assigning this part of the spectrum. BRYSON and WERNER [12], when calculating N-H force constants for anilines in pyridine solution, selected the two strongest bands in the 3200-3500 cm-l region to be vNH, and vNH,. In many instances, they overlooked the weak vNH, band and chose vNH, and 26NHz instead.

.

+

0

(CH,),SO

+o

0

C d-h,0

+o

.

o+

.

(C,H,),

0 0+

.

CH3CN 0

0.

Cl&

.

cc&

00

.

+

0

+

0

-I

I

3200

3300

Frequency,

? LOO

cm-’

Fig. 1. Fermi resonanceinteraction of YNH. of aniline with overtone NH, defor mation. 0--YNH~ of aniline. *Overtone SNH, of aniline. +-NH of N-methylaniline.

Relative solvent shifts from hydrogen bonding The solvent-induced shifts in vNH, and vNH, will be produced by a combination of bulk solvent effects and specific hydrogen bond interaction. Data in Tables 2-5 show that many of the hydrogen bond acceptors we have used do exert large polarization effects on vNH frequencies of the hydrogen-bonded aggregates. Ideally, these bulk solvent effects could be eliminated by extrapolation of the data to zero concentration of the acceptor species to obtain the hydrogen-bond shift. In practice, however, only low concentrations of some of our amines could be achieved at very low mole fractions of acceptor, and the equilibrium conbentration of hydrogen-bonded [12] A. BRYSON and R.L.WERNER,AU&UI~~~

J.Chem.

l&456

(1960).

L. K.

1433

DYALL

Table 7. N-H stretching frequency shifts of anilines at mole fraction 0.1 of hydrogen bond acceptor in carbon tetrachloride solutions (reference frequencies measured on carbon tetrachloride solutions) Aoetonitrile

Acetone

Diethyl ether

Cineole

DMSO

Pyndme

Relative shift*

21:

34

53

74

94

115

1.00

tHammett u or u-

N-Methylaniline

vNH

4-Methylaniline

vNHa, vNH,

5 9

6 13

4 22

6 35

21 62

8 55

0.46

Aniline

*NH,, vNH.

6 11

I3 17

3 28

7 40

16 6.5

11 61

0.53

0.000

4-Bromoaniline

vNH,, vNH,

4 14

6 22

6 35

9 49

21 61

14 68

0.64

0.232

4-Methoxyoarbonylaniline

vNH,s vNH,

7 21

9 30

8 46

13 61

20 73

19 78

0.83

0.636 (0

4-Aoetylaniline

vNHa, vNH,

8 22

10 30

11 46

16 63

20 72

21 80

0.87

0.874 (a-)

vNH,. vNH,

11 22

13 30

13 48

16 63

23 71

22 79

0.88

*NH,, vNH,

13 31

14 39

19 68

20 73

24 81

27 86

1.14

3-Nitroaniline

4-Nitroaniline

-0.170

0.710

1.27 (0

* Expressed as the slope of the frequency shlfts for the amine plotted on the ordinate scale against those of N-Methylaniline. t Hemmett u for the substituent in the aniline. For the methoxyosrbonyl, acetyl, and 4-nitro substituents, Hammett u- ia used. $ Overlapping bands prevented this shift from being directly measured. The 21 cm-’ shift 1~88 obtained by extrapolation from higher mole fractions (see data in Table 2).

aggregate was then too low to yield well-defined spectra. Rather than use dubious extrapolations, we have chosen to compare solvent shifts at O-1 mole fraction of the acceptor in carbon tetrachloride solution. At this low level of acceptor, bulk solvent effects are small, and are effectively cancelled when the shifts in vNH, and YNH, of the aniline are plotted against the vNH shift of N-methylaniline. The choice of O-1 mole fraction yields suitable concentrations of the 1: 1 amine-acceptor aggregate for 0.02 molar amine with all the acceptors used. In no instance was there any evidence of 1: 2 aggregates being present. For all the amines used here, the frequency shifts decreased in the order pyridine > DMSO > cineole > diethyl ether > acetone > acetonitrile (see Table 7). The plot of vNH, vs. vNH of N-methylaniline is linear for each of the seven primary amines, although the Fermi resonance of vNH, with 26NH, produced deviations for most amines with DMSO or pyridine as acceptors (see Fig. 2). The relative frequency shifts of YNH, to those of N-methylaniline should be determined by the electron deficiency at the amino hydrogens. so that the largest shifts will be produced by the electron-attracting substituents in the aniline ring [13, 141. The slopes of our relative frequency shift plots have a correlation coefficient ? = O-993 with Hammett e (see Table 7). Hammett (T- is used for 4-substituents of [13] [la]

E. A. CUTMORE and H. E. HALLAM, Trunrr.B’czruduySot. 58,40 (1962). P. J. KRUEGER and H. W. THOMPSON, Proc. Roy. Sot. A243, 143 (1957).

Solvent effects on the infrared spectra of anilines-V

1433

the -M type to allow for the enhanced resonance interaction with the +M amino group. The frequency shifts of each amine can also be plotted against those of aniline, and again the correlation of the slopes with Hammett ~7(or CY-)is very good (?: = 0.985). The YNH, frequencies of the 1: 1 aggregates show only a rough linear correlation with YNH of N-methylaniline (see Fig. 2) or with YNH, of aniline. The scatter will arise, at least in part, from this vNH, band being coalesced with that of the free

N-melhylonilin Au,

cm-’

Fig. 2. Relative frequency shifts of 4-acetylaniline and N-methylaniline measured at 0.1 mole fraction of acceptor in carbon tetrachloride solution. 0-AwNHas of 4-acetylaniline, O-AVNH# of 4-acetylaniline.

amine at mole fraction 0.1 of some acceptors. The vNH, bands of the 1: 2 aggregates are better defined and their frequencies do correlate with YNH of N-methylaniline. The frequency shifts for those 1: 2 aggregates which could be formed were measured with the acceptor used as pure solvent (Table 8). The bulk solvent effects are probably not cancelled out in the plots of relative frequency shifts, but nevertheless these plots are linear and yield slopes which correlate quite well with the Hammett constants of the aniline substituents. The 1: 2 aggregates with pyridine show shifts far too small to fit on these plots, which confirms our belief that the broad bands near 3450 cm-l assigned to these aggregates (Table 5) are actually enhanced solvent absorption. Bulk solvent eflects The solvent shift of an infrared stretching frequency is believed to include contributions from the solvent polarization, E - l/2& + 1, and solvent polarizability, n2 - 1/2n2 + 1, where E is the dielectric constant and n the refractive index of the solvent [15]. Of the various amine-solvent species we have observed, only the 1: 1 [16] G. L. CALDOW

and H. W. THOMPSON,

Proc.

Roy. Hoc. k?&&

1 (1960).

L. K.

1434

DYALL

Table 8. Antisymmetrical N-H stretching frequency shifts (Av) of 1:2 aggregates of anilines with hydrogen bond acceptors. Carbon tetrachloride solutions used as frequency reference, pure acceptor species used as solvent Frequency shift (cm-l)

N-Methylaniline 4-Methylaniline Aniline C-Bromoaniline C-Methoxycarbonylaniline 4-Acetylaniline 3-Nitroaniline 4-Nitroaniline

Acetonitrile

Acetone

Diethyl ether

31 -

43 17 -

53 39 29

-

39 39 34 52

48 52 49 76

38

(AY)

Dimethylsulphoxide

Cineole 74

Relative shift * 1.00 -

47 52

114 82 75 95

0.65 -0.70

75 81 77 106

122 122 115 134

1.02 1.10 1.00 1.30

-

Hammett u or 6 -0~170 0.000 0.232 0.636 (6) 0.874 (6) 0.710 1.27 (6)

* Expressed as the slope of the frequency shift for the amine plotted on the ordinate scale against those of N-methylaniline. 30

TE ” ; c 5 6 ii Es

1

/ 20-

/’

/’ 1’ 0 .d 1’

IO-

,A”

/.

0’

,,/~//’ 0

“/’

Fig. 3. Dependence of N-H stretching frequenoies of 1:l aggregates on solvent 4-methoxycarbonylaniline in acetonitrile-carbon tetrachloride polarization: solvents. Data from Table 3. O---V~~~; O-+NHe.

exist throughout a wide enough range of solvent composition to allow correlation of frequency shifts with solvent properties. Even then, there are difficulties with overlapping absorptiona, Fermi resonance, and enhancement of solvent Only the data from carbon absorptions from the solute-solvent interactions. tetrachloride-diethyl ether and carbon tetrachloride-acetonitrile solvents (Tables 1 and 2) are usable. The results for 4-methoxycarbonylaniline in aoetonitrile-carbon tetrachloride solvents show a linear dependence of both vMH,, and YNH, on the solvent polarization term (Table 2 and Fig. 3). The curvature of the ~JNH, plot at low mole fractions of acetonitrile can be attributed to problems of band resolution. The other anilines aggregaks

Solvent effects on the infrmcd spectra of anilines-V

1436

substantiate this correlation of frequency with solvent polarization, but the correlations are restricted either by overlapping of bands at low mole fractions of acetonitrile, or by incursion of 1: 2 aggregates at high mole fractions. The frequencies of the 1: 1 aggregates in carbon tetrachloride-diethyl ether solvents (Table 1) show no variation with solvent composition, even though the variation in the solvent polarization term is about half that of the carbon-tetrachlorideacetonitrile system. The polarization shift is probably being offset by a polarizability shift in the opposite direction. The vNH, frequencies measured in solvents containing DMSO or pyridine are almost unaffected by changes in solvent composition. The expected shifts to lower frequency are offset by an increased degree of Fermi resonance with 26NH,. The position of this overtone therefore is sensitive to changes in solvent composition (see Tables 4 and 5).