CHEMICAL SPECTROSCOPY

by

R. E.~DD Lecturer in Chemistry,

King's College,

University of Durham,

Newcastle-upon-Tyne (England)

ELSEVIER PUBLISHING COMPANY AMSTERDAM - NEW YORK

1962

•

CONTENTS

Introduction . . . . . .

CHAPTER l. EXPERIMENTAL METHODS OF SPECTROSCOPY

Wavelength and Frequency . Wave number, 6

Energy and Frequency . . . General Experimental Arrangement Sources ............ .

Diseharge tubes, 13 - Ares and sparks, 15 - Ineandeseent sourees, 15 -Continuum diseharge sourees, 17 - Monochromatic high frequency sources, 18

Spectrographs and Monochromators Prisms and prism materials . Gratings Filters ...... . Detectors ..... .

Photographie detection, 31 - Photocells, 32 - Thermal deteetors, 34 -Mierowave deteetors, 35

Presentation . . . . . Photographs, 36 - Direct recording, 37 - Comparative recording, 38 -Standard presentation, 39

Resolution. . . . . . . . . . . . . . . . . . . . . . Wavelength or Frequency Calibration ........ .

Optical spectrometers, 42 - Microwave and radio frequencies, 45 Intensity Measurement . . . . . . . . . . . . : . . .

Beer's law, 47 - Additivity of optical density, 49 - Integrated intensity, 50 - Raman scattering intensity, 51 - Microwave absorption intensity, 52

Sample Treatment . . . . . . . . . . . . . . . . . . . . . Absorption eells, 53 - Measurement and adjustment of path length, 54 -Solvent eompensation, 55 - Other factors, 55 - Solvents, 56 - Solids, 58 -Temperature variation, 58 - Quantities required, 59 - Mierowave absorp­tion, 59

Raman Spectra Polarization, 62

Fluorescence . References . . .

l

3

4

7 8

12

19 23 27 29 31

39 42

47

53

60

viii CONTENTS

CHAPTER 2. CHARACTERIZATION OF STATES: ELECTRONIC STATES

Quantum Mechanical Basis .............. . Fundamentalpostulates, 72 - Characteristic states, 73 - Degeneracy, 74 -Uncertainty principle, 74

General Interpretation of Spectra . . . . . . . . . . . . . . Separation of motion in molecules, 76 - Band spectra, 78 - Fluorescence and Raman spectra, 79 - Effect of magnetic and electric fields, 80 - Popu­lation of levels, 81

Spectral Activity: Selection Rules . . . . . . . Electronic states of Hydrogen and Alkali Metals

Hydrogen atom, 84 - Alkali metals, 88

Angular Momentum . . . . . . . . . . . . . . . . . . . . Space quantization, 92 - Relativistic treatment, 93 - Multiplet spectra, 96 - Electron spin and the Vector model, 96 - Space quantization of J , loo - Zeeman effect, 101

Spin Resonance . . . Paramagnetic resonance, 103 - Nuclear spin resonance, 104

Helium .... . ........ . ...... . Singlet and triplet levels, 108 - Symmetry of observed states, l 10 - Energy and electron correlation, l l l

67

71

75

92

103

107

Pauli's Principle . . . . . . . . . . . . . . . . . . . . . . II2 The periodic table, u3 - Deducing configuration from spectral terms, u6

Electronic ~tates in Molecules . . . . . . . . . . . . . . . . rr8 Molecular orbitals, u8 - The hydrogen molecule-ion, H 2+, 122 - Hydrogen molecule, 122 - Multiplicity of molecular states, 125

References. . . . . . . . . . . . . . . . . . . . . . . . . 127

CHAPTER 3. CHARACTERIZATION OF STATES: NUCLEAR MOTION 129

Vibrational States of Diatomic Molecules . . . . . . . . 129 Harmonie diatomic oscillator, 131 - Vibrational spectrum, 133 - Raman spectrum, 136 - Anharmonic oscillator, 137 - Vibration in electronic transitions, 139 - Analysis of a band system, 141 - Sequences, 144 - Poten-tial energy curves, 147 - Force constants in diatomic molecules, 150

Dissociation Energy . . . . . . . . . . . . . . . . . . . . 151'

Vibrational States of Polyatomic Molecules. . . . . . . . . . 157· Normal coordinates, 159 - Quantum mechanical description, 163 - Sym-metry of the mechanical model, 164 - Symmetry of vibrational states, 168 -Selection rules for infra-red spectra, l 70 - Selection rules for Raman spectra, r72 - Centrosymmetric molecules, 173 - Polarization of Raman lines, 174 -Use of character tables, 175 - Determination of molecular symmetry, 177 -Limitations, 181 - Assignment of frequencies, 182 - Force constants, 184

Rotational States . . . . . . . . . . . . . . . . . . . . . 185 Linear and diatomic molecules, 186 - Spherical top molecules, 193 - Sym­metrie top molecules, 193 - Asymmetrie top molecules, 196 - Stark effect, 197 - Centrifugal stretching, 200 - Interaction between vibration and

CONTENTS

rotation, 200 - Rovibrational transitions, 201 - Unresolved bands, 206 -Molecular dimensions from rotational structures, 207 - Rotation in elec­tronic states, 210

ix

Nuclear spin. . . . . . . . . . . . . . . . . . . . . . . . 2n Symmetry properties including nuclear spin, 212 - Interaction with rota­tion, 213

References. . . . . . . . . . . . . . . . . . . . . . . . . 216

CHAPTER 4. lNTERACTIONS AND EMPIRICAL CORRELATIONS

Chromophores, 218

Ultraviolet and Visible Absorption Spectra . . . . . . Single bonds and saturated molecules, 219 - Non·bonding electrons, 221 -N -+ R transitions, 221 - Double bonds, 222 - Conjugation and isolation, 223 - Aromatic hydrocarbons, 226 - Conjugated ketones--quinone, 229 -Nitro and nitroso compounds, 230 - Azo compounds, 231 - Dyes, 231 -Use and specificity of electronic spectra, 233

Infra-red Correlations. . . . . . . . . . . . . . . . . . . . Characteristic group frequencies, 238 - Use of group frequency correlations, 240 - Specificity of infra-red spectra, 243

Raman Correla tions . . . . . . . . . . . Aqueous solutions, 245 - Non-aqueous solutions, 247

Microwave Correlations . . . . . . . . . . . . . . . . . Nuclear Magnetic Resonance . . . . . . . . ..... .

Magnetic dipole interaction, 249 - N.m.r . in liquids and gases, 252 - Chem­ical shift, 252 - Correlations, 254 - Electron-coupled nuclear spin inter­actions, 256 - N .m.r. fingerprints, 261

Intramolecular Perturbations . . . . . . . . . . . . . . . . Electronic spectra, 262 - Vibrational spectra, 263

Complexes and Ionic Association . . . . . . . . . . . . . . Crystal field theory, 267 - Selection rules, 272 - Spectrochemical series, 273 - Ligand field theory, 273 - Charge-transfer spectra, 274 - Charge­transfer spectra of ions in solution, 275 - Ionic association-complex ions, 276 - lonic association-ion pairs, 278 - Anions, 279

Intermolecular Perturbations . . . . . . . . . . . . . . . . Change of state, 280 - Change of symmetry, 281 - Solvent effects in elec­tronic spectra, 282 - Solvent effects in vibration spectra, 283 - Hydrogen bonding, 284 - Solvent effects in nuclear magnetic resonance, 285 - Chem­ical exchange, 285

References . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER 5. lNTENSITY • • • • • • • . • • . • • . • • • • • •

Concentration Dependence . . . . . . . . . . . . . . . . . Emission spectrophotometry, 290 - Atomic absorption spectroscopy, 292 -Absorption spectrophotometry, 292 - Beer's law, 293 - Analysis of mix­tures, 294

218

219

244

262

266

280

290

290

X CONTENTS

Chemical Equilibria Determination of acid-base ionization constants, 296 - Quantitative Raman studies, 300 - Ionic association, 302 - Stoicheiometry of complex ions, 304 - Stability constants, 306

295

Kinetic Applications . . . . . . . . . . . . . . . . . . . . 308 Flash photolysis, 310

Population of Levels . . . . . . . . . . . . . . . . . . . . 313

Relaxation and resonance absorption, 314 - Rotational states, 316 - Vibra-tional and electronic absorption spectra, 318

Franck-Condon principle . . . . . . . . . . . . . . . . . . 318 Absorption, 319 - Emission, 320 - Dissociation processes: photochemistry, 321

Absolute Intensity . . . . . . . . . . . . . . . . . . . . . 323 Selection rules, 324 - Absolute intensity in absorption, 325 - Empirical correlations, 327

References. . 328

SUBJECT INDEX 331

Table I.

Table 2. Table 3.

Table 4· Table 5·

Table 6. Table 7. Table 8.

Table 9. Table 10. Table l I.

Table 12.

Table 13. Table 14. Table 15.

Table 16.

Table 17.

Table 18.

Table 19. Table 20. Table 21.

LIST OF T ABLES

Page

Correction to vacuurn of wavelength or wavenurnber. . . . . . 5 Wavelength, energy and frequency . . . . . . . . . . . . . . 8 Transparency of various optical rnaterials, and suitability for prisrns,

lenses and windows . . . . . . . . . . . . . . . . . . . . 23 Recornmended prism materials . . . . . . . . . . . . . . . . 26 Wavelengths (in air) and corresponding wavenumber (in vacuurn)

of mercury lines useful for calibration of spectrometers 44 Molar decadic extinction coefficien t . . . . . . . . . . . . . . 48 Verification of Ritz' combination principle. . . . . . . . . . . 85 Spin quanturn number and magnetic moment of sorne cornrnon

particles . . . . . . . . . . . . . . . . . . . . . . . . . l 06 Operation of Pauli's principle when n = I, 2 and 3 . . . . . . l 14 Operation of Pauli's principle in relativistic (n, k, m) atomic states II5 Part of Deslandres Table for the fourth positive group of CO, with

AG and LlG1 values . . . . . . . . . . . . . . . . . . . . 142 Vibrations parameters frorn emission spectra of sorne diatomic

molecules. . . . . . . . . . . . . . • . . . . . . . . . . 145 Character table for the point group C1v. • • • • • • • . • • • 167 Spectral activity of various configurations of XYZ, XY2 , XY3 178 Rotational constants for sorne diatomic and linear molecules from

pure rotational transitions in the Raman effect and microwave absorption . . . . . . . . . . . . . . . . . . . . . . 192

Rotational constants for some symmetric top molecules by the Rarnan and rnicrowave methods . . . . . . . . . . . . . . 195

Rotational constants for some asymmetric top molecules (rnicro-wave absorption) . . . . . . . . . . . . . . . . . . . . . 197

Comparison of values of rotational constants and dirnensions ob-tained by various spectroscopic rnethods . . . 209

Some representative group stretching frequencies. . . . . . . . 239 Chemical shifts for protons in various positions . . . . . . . . 255 N urnerical values of e - 4 E / kT for various LI E and various temperatures 3 l 3