Infrared And Raman Spectroscopy (Dean's Analitical Chemistry Handbook).pdf

Source: DEAN’S ANALYTICAL CHEMISTRY HANDBOOK

SECTION 7

INFRARED AND RAMAN

SPECTROSCOPY

Table 7.1 Wave Number–Wavelength Conversion Table

7.3

7.1 THE NEAR-INFRARED REGION

7.4

7.1.1 Correlation of Near-Infrared Spectra with Molecular Structure

7.4

Figure 7.1 Near-Infrared Spectra-Structure Correlations and Average Molar

Absorptivity Data

7.5

Table 7.2 Absorption Frequencies in the Near Infrared

7.7

7.1.2 Solvents for the Near-Infrared Region

7.8

Figure 7.2 Solvents for Near-Infrared Spectrophotometry

7.9

7.2 THE MID-INFRARED REGION

7.9

7.2.1 Infrared-Transmitting Materials

7.9

Table 7.3 Infrared-Transmitting Materials

7.10

7.2.2 Radiation Sources

7.10

7.2.3 Infrared Spectrometers

7.11

7.2.4 Detectors

7.12

7.2.5 Preparation of Samples

7.13

Figure 7.3 Infrared Transmission Characteristics of Selected Solvents

7.14

7.2.6 Internal Reflectance

7.17

7.3 CORRELATION OF INFRARED SPECTRA WITH MOLECULAR STRUCTURE

IN THE MID-INFRARED REGION

7.18

7.3.1 C — H Frequencies

7.18

Figure 7.4 Colthup Chart of Structure–Spectra Correlations in the

Mid-Infrared Region 7.19

Table 7.4 Absorption Frequencies of Alkanes 7.21

7.3.2 Alkenes 7.22

7.3.3 Alkynes 7.22

7.3.4 Alcohols and Phenols 7.22

7.3.5 Amines 7.22

Table 7.5 Absorption Frequencies of Alkenes >C == C< 7.23

7.3.6 Carbonyl Group 7.24

7.3.7 Ethers 7.24

Table 7.6 Absorption Frequencies of Triple Bonds 7.25

7.3.8 Other Functional Groups 7.25

Table 7.7 Absorption Frequencies of Alcohols and Phenols 7.26

7.3.9 Compound Identification 7.26

Table 7.8 Absorption Frequencies of Primary, Secondary, and Tertiary Amines 7.27

Table 7.9 Absorption Frequencies of Carbonyl Bands 7.29

Table 7.10 Absorption Frequencies of Ethers and Peroxides 7.32

Table 7.11 Absorption Frequencies of Sulfur Compounds 7.33

Table 7.12 Absorption Frequencies of Aromatic and Heteroaromatic Bands 7.34

Table 7.13 Absorption Frequencies of the Nitro Group 7.38

Table 7.14 Absorption Frequencies of Double Bonds Containing Nitrogen Atoms 7.38

Table 7.15 Absorption Frequencies of Cumulated Double Bonds

7.40

Table 7.16 Absorption Frequencies of Boron Compounds

7.41

Table 7.17 Absorption Frequencies of Phosphorus Compounds

7.42

Table 7.18 Absorption Frequencies of Silicon Compounds

7.43

Table 7.19 Absorption Frequencies of Halogen Compounds

7.44

7.1

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INFRARED AND RAMAN SPECTROSCOPY

7.2

SECTION SEVEN

7.4 QUANTITATIVE ANALYSIS

7.45

Figure 7.5 Baseline Method for Calculation of the Transmittance Ratio

7.46

7.5 THE FAR-INFRARED REGION

7.46

7.5.1 Sources, Optical Materials, and Detectors

7.46

Figure 7.6 Transmission Regions of Selected Optical Materials, and Useful

Ranges of Sources and Detectors in the Far-Infrared Region

7.47

Figure 7.7 Solvents for the Far-Infrared Region

7.48

7.5.2 Solvents and Sampling Techniques

7.48

7.5.3 Spectra–Structure Correlations

7.49

7.6 RAMAN SPECTROSCOPY

7.49

Figure 7.8 Spectra–Structure Correlation Chart in the Far-Infrared Region for

Alkanes, Alkenes, Cycloalkanes, and Aromatic Hydrocarbons

7.50

Figure 7.9 Spectra–Structure Correlation Chart for the Far-Infrared for Heterocyclic

and Organometallic Compounds and Aliphatic Derivatives

7.52

Figure 7.10 Spectra–Structure Correlation Chart in Far-Infrared for Inorganic Ions

7.54

7.6.1 Principles

7.54

7.6.2 Instrumentation for Dispersive Raman Scattering

7.55

7.6.3 Instrumentation for Fourier-Transform Raman Spectroscopy

7.56

7.6.4 Sample Handling

7.56

Figure 7.11 Obscuration Ranges of the Most Useful Solvents for

Raman Spectrometry

7.57

7.6.5 Diagnostic Structural Analysis

7.57

Table 7.20 Raman Frequencies of Alkanes

7.58

Table 7.21 Raman Frequencies of Alkenes

7.59

Table 7.22 Raman Frequencies of Triple Bonds

7.60

Table 7.23 Raman Frequencies of Cumulated Double Bonds

7.61

7.6.6 Quantitative Analysis

7.61

Table 7.24 Raman Frequencies of Alcohols and Phenols

7.62

Table 7.25 Raman Frequencies of Amines and Amides

7.62

Table 7.26 Raman Frequencies of Carbonyl Bands

7.63

Table 7.27 Raman Frequencies of Other Double Bonds

7.65

Table 7.28 Raman Frequencies of Nitro Compounds

7.65

Table 7.29 Raman Frequencies of Aromatic Compounds

7.66

Table 7.30 Raman Frequencies of Sulfur Compounds

7.70

Table 7.31 Raman Frequencies of Ethers

7.72

Table 7.32 Raman Frequencies of Halogen Compounds

7.72

Bibliography

7.73

The infrared region of the electromagnetic spectrum includes radiation at wavelengths between 0.7

and 500 m m or, in wave numbers, between 14 000 and 20 cm –1 . The relationship between wave num-

ber and wavelength scales in the infrared region is given in Table 7.1. Molecules have specific fre-

quencies that are directly associated with their rotational and vibrational motions. Infrared absorp-

tions result from changes in the vibrational and rotational state of a molecular bond. Coupling with

electromagnetic radiation occurs if the vibrating molecule produces an oscillating dipole moment

that can interact with the electric field of the radiation. Homonuclear diatomic molecules such as

hydrogen, oxygen, or nitrogen, which have a zero dipole moment for any bond length, fail to inter-

act. These changes are subtly affected by interaction with neighboring atoms or groups, as are res-

onating structures, hydrogen bonds, and ring strain. This imposes a stamp of individuality on each

molecule’s infrared absorption spectrum as portions of the incident radiation are absorbed at specific

wavelengths. The multiplicity of vibrations occurring simultaneously produces a highly complex

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INFRARED AND RAMAN SPECTROSCOPY

7.3

TABLE 7.1 Wave Number–Wavelength Conversion Table

(Wave number is reciprocal of wavelength. The wave number (in cm –1 ) = 10 000/wavelength)(in µ m). For exam-

ple, 15.4

m is equal to 649 cm –1 .)

Wavelength

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

( m

Wave number, cm –1

1.0

10 000

9091

8333

7692

7143

6667

6250

5882

5556

5263

2.0

5000

4762

4545

4348

4167

4000

3846

3704

3571

3448

3.0

3333

3226

3125

3030

2941

2857

2778

2703

2632

2564

4.0

2500

2439

2381

2326

2273

2222

2174

2128

2083

2041

5.0

2000

1961

1923

1887

1852

1818

1786

1754

1724

1695

6.0

1667

1639

1613

1587

1563

1538

1515

1493

1471

1449

7.0

1429

1408

1389

1370

1351

1333

1316

1299

1282

1266

8.0

1250

1235

1220

1205

1190

1176

1163

1149

1136

1124

9.0

1111

1099

1087

1075

1064

1053

1042

1031

1020

1010

10.0

1000

990

980

971

962

952

943

935

926

917

11.0

909

901

893

885

877

870

862

855

847

840

12.0

833

826

820

813

806

800

794

787

781

775

13.0

769

763

758

752

746

741

735

730

725

719

14.0

714

709

704

699

694

690

685

680

676

671

15.0

667

662

658

654

649

645

641

637

633

629

16.0

625

621

617

613

610

606

602

599

595

592

17.0

588

585

581

578

575

571

568

565

562

559

18.0

556

552

549

546

543

541

538

535

532

529

19.0

526

524

521

518

515

513

510

508

505

503

20.0

500

498

495

493

490

488

485

483

481

478

21.0

476

474

472

469

467

465

463

461

459

457

22.0

455

452

450

448

446

444

442

441

439

437

23.0

435

433

431

429

427

426

424

422

420

418

24.0

417

415

413

412

410

408

407

405

403

402

25.0

400

398

397

395

394

392

391

389

388

386

26.0

385

383

382

380

379

377

376

375

373

372

27.0

370

369

368

366

365

364

362

361

360

358

28.0

357

356

355

353

352

351

350

348

347

346

29.0

345

344

342

341

340

339

338

337

336

334

30.0

333

332

331

330

329

328

327

326

325

324

31.0

323

322

321

319

318

317

316

315

314

313

32.0

313

312

311

310

309

308

307

306

305

304

33.0

303

302

301

300

299

298

297

296

295

34.0

294

293

292

291

290

289

288

287

35.0

286

285

284

283

282

281

280

279

36.0

278

277

276

275

274

273

272

271

37.0

270

269

268

267

266

265

264

38.0

263

262

261

260

259

258

257

39.0

256

255

254

253

252

251

40.0

250

Source: J. A. Dean, ed., Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, New York, 1992.

absorption spectrum that is uniquely characteristic of the functional groups that make up the mole-

cule and of the overall configuration of the molecule as well. It is therefore possible to identify sub-

stances from their infrared absorption spectrum. 1

1 N. B. Colthup, L. H. Daly, and S. E. Wiberley, Introduction to Infrared and Raman Spectroscopy , 3d. ed., Academic,

New York, 1990.

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INFRARED AND RAMAN SPECTROSCOPY

7.4

SECTION SEVEN

For qualitative analysis, one of the best features of an infrared spectrum is that the absorption or

the lack of absorption in specific frequency regions can be correlated with specific stretching and

bending motions and, in some cases, with the relationship of these groups to the rest of the mole-

cule. Thus, when interpreting the spectrum, it is possible to state that certain functional groups are

present in the material and certain others are absent. The relationship of infrared spectra to molecular

structure will be treated in Sec. 7.3.

7.1 THE NEAR-INFRARED REGION

The near-infrared region (NIR), which meets the visible region at about 12 500 cm –1

(800 nm) and

N−−H, and O−−H stretching frequencies, which are adequate for studying many organic compounds.

The instruments used in NIR have fused quartz optics with either a quartz prism or a grating mono-

chromator and photoconductor detectors. NIR uses a tungsten source that covers the range of 700 to

about 2500 nm (4000 to 14 000 cm –1 ). The techniques of near-infrared are closer to those of ultravio-

let and visible spectrophotometry than to infrared in that long-path-length cells (0.1 to 10 cm) and

dilute solutions are generally used. Quartz, glass, or Corex cells may be used up to 2.4 m m. Special

grades of silica are readily available for use up to 3 m m. Because of the sharpness of the absorption

bands in the near-infrared region, it is desirable to use high resolution for quantitative work, that is,

spectral slit widths of the order of a few wave numbers.

m), contains primarily overtones and combination bands of C

−−

7.1.1 Correlation of Near-Infrared Spectra with Molecular Structure

The absorptivity of near-infrared bands is from 10 to 1000 times less than that of mid-infrared bands.

Thicker sample layers (0.5 to 10 mm) must be used to compensate. On the other hand, minor impu-

rities in a sample are less troublesome.

Most analytical applications in the near-infrared region have been concerned with organic com-

pounds and generally with quantitative functional-group analysis. This is because near-infrared spec-

tra are mainly indicative of the hydrogen vibrations of the molecule, that is, C −− H, S −− H, O −− H,

N−− H, etc., and few vibrations are dependent on the carbon or inorganic skeleton of the molecule.

The data on the regions in which various functional groups absorb and the average intensities of the

absorption bands have been collected in Fig. 7.1 and Table 7.2. Most of the data in Fig. 7.1 were

obtained in carbon tetrachloride solution. 2

Significant spectral features are as follows:

1. O −−

H stretching vibration near 7140 cm –1 (1.40 m m).

2. N −−

H stretching vibration near 6667 cm –1 (1.50 m m).

3. C −−

H stretching and deformation vibrations of alkyl groups at 4548 cm –1

(2.20 m m) and 3850

cm –1 (2.60 m m).

4. Absorption bands due to water at 2.76, 1.90, and 1.40 m m (3623, 5263, and 7143 cm –1 ).

5. Aromatic amines: (a) Primary amines have bands near 1.97 and 1.49 m m (5076 and 6711 cm –1 ).

(b) Secondary amines show only the band at 1.49 m m. (c) Tertiary amines exhibit no appreciable

absorption at either wavelength.

Rather than measure a property or a concentration singly, many more signals—a full spectrum—

are taken, and a complex mathematical model (chemometrics) is used to calculate the parame-

ters. Calibration involves taking spectra of many samples from various positions throughout the

2 R. F. Goddu and D. A. Delker, Anal. Chem. 32 :140 (1960).

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extends to about 4000 cm –1 (2.50

INFRARED AND RAMAN SPECTROSCOPY

7.5

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