CHAPTER 3

Infrared Spectrometry

3.1 Introduction

Infrared (IR) radiation refers broadly to that part of the electromagnetic spectrum between the visible and mi- crowave regions. Of greatest practical use to the organic chemist is the limited portion between 4000 and 400 cm I . There has been some interest in the near- IR (14,290-4000 cm-') and the far-IR regions, 700- 200 cm-I.

From the brief theoretical discussion that follows, it is clear that even a very simple molecule can give an extremely complex spectrum. The organic chemist takes advantage of this complexity when matching the spec- trum of an unknown compound against that of an au- thentic sample. A peak-by-peak correlation is excellent evidence for identity. Any two compounds, except en- antiomers, are unlikely to give exactly the same IR spec- trum.

Although the IR spectrum is characteristic of the entire molecule, it is true that certain groups of atoms give rise to bands at or near the same frequency regard- less of the structure of the rest of the molecule. It is the persistence of these characteristic bands that permits the chemist to obtain useful structural information by simple inspection and reference to generalized charts of characteristic group frequencies. We shall rely heavily on these characteristic group frequencies.

Since we are not solely dependent on 1R spectra for identification, a detailed analysis of the spectrum will not be required. Following our general plan, we shall present only sufficient theory to accomplish our pur- pose: utilization of IR spectra in conjunction with other spectral data in order to determine molecular structure.

The importance of IR spectrometry as a tool of the practicing organic chemist is readily apparent from the number of books devoted wholly or in part to discus- sions of applications of IR spectrometry (see the refer- ences at the end of this chapter). There are many com- pilations of spectra as well as indexes to spcctral

collections and to the literature. Among the more com- monly used compilations are those published by Sadtler (1972) and by Aldrich (1985).

3.2 Theory

Infrared radiation of frequencies less than about 100 cm-I is absorbed and converted by an organic mol- ecule into energy of molecular rotation. This absorption is quantized; thus a molecular rotation spectrum consists of discrete lines.

Infrared radiation in the range from about 10,000- 100 cm is absorbed and converted by an organic mol- ecule into energy of molecular vibration. This absorp- tion is also quantized, but vibrational spectra appear as bands rather than as lines because a single vibrational energy change is accompanied by a number of rota- tional energy changes. It is with these vibrational-ro- tational bands, particularly those occurring between 4000 and 400 cm-I, that we shall be concerned. The fre- quency or wavelength of absorption depends on the rel- ative masses of the atoms, the force constants of the bonds, and the geometry of the atoms.

Band positions in IR spectra are presented here as wavenumbers ( T ) whose unit is the reciprocal centi- meter (cm-I); this unit is proportional to the energy of vibration and modern instruments are linear in recip- rocal centimeters. Wavelength (A) was used in the older literature in units of micrometers (pm = m; earlier called microns). Wavenumbers are reciprocally related to wavelength.

Note that wavenumbers are sometimes called "fre- quencies." However, this is incorrect since wavenum- bers (i in units of cm-I) are equal to 1 X 104/h in units of pm, whereas frequencies ( v in Hz) are equal to c/A in cm, c being the speed of light (3 X 101%cm/s), The

108 Chapter 3 Infrared Spectrometry

WAVENUMBERS (cm- ' ) FIGURE 3.34. Ethyl p-toluenesulfonate. A. Asymmetric S(=O), stretch, 1355.5 cm--I. B. Symmetric S(=O), stretch, 1177 cm-l. C. Various strong S-0-C stretching, 1000-769 cm-l.

fonates show negligible differences; electron-donating groups in the para position of arenesulfonates cause higher frequency absorption.

Sulfonic acids are listed in narrow ranges above; these apply only to anhydrous forms. Such acids hydrate readily to give bands that are probably a result of the formation of hydronium sulfonate salts, in the 1230- 1120 cm-I range.

3.6.27 Organic Halogen Compounds

The strong absorption of halogenated hydrocarbons arises from the stretching vibrations of the carbon- halogen bond.

Aliphatic C-Cl absorption is observed in the broad region between 850 and 550 cm-I. When several chlorine atoms are attached to one carbon atom, the band is usually more intense and at the high-frequency end of the assigned limits. Carbon tetrachloride (see Appendix B, No. 10) shows an intense band at 797 cm-l. The first overtones of the intense fundamental bands are frequently observed. Spectra of typical chlo- rinated hydrocarbons are shown in Appendix B: Nos. 10-13. Brominated compounds absorb in the 690- 515 cm-I region, iodo compounds in the 600-500 cm-I region. A strong CH, wagging band is observed for the CH,X (X = C1, Br, and I) group in the 1300- 1150 cm-I region.

Fluorine-containing compounds absorb strongly over a wide range between 1400 and 1000 cm-I because of C-F stretching modes. A monofluoroalkane shows a strong band in the 1100- 1000 cm-I region. As the number of fluorine atoms in an aliphatic molecule in- creases, the band pattern becomes more complex, with multiple strong bands appearing over the broad region

of C-3 absorption (see Fluorolube spectrum, Appen- dix C). The CF, and CF, groups absorb strongly in the 1350-1 120-cm-I region. The spectrum of Fluorolube@, Appendix B, No. 14, illustrates many of the preceding absorption characteristics.

Chlorobenzenes absorb in the 1096- 1089 cm-I region. The position within this region depends on the substitution pattern. Aryl fluorides absorb in the 1250-1100 cm-I region of the spectrum. A monofluo- rinated benzene ring displays a strong, narrow absorp- tion band near 1230 cm-I.

3.6.28 Silicon Compounds

3.6.28.1 Si -H Vibrations Vibrations for the Si -H bond include the Si-H stretch (- 2200 cm-l) and the Si-H bend (800-950 cm-I). The Si-H stretching fre- quencies are increased by the attachment of an electro- negative group to the silicon.

3.6.28.2 SiO-H and Si-0 Vibrations The O H stretching vibrations of the SiOH group absorb in the same region as the alcohols, 3700-3200 cm-I, and strong Si-0 bands are at 830-1110 cm-l. As in alco- hols, the absorption characteristics depend on the de- gree of hydrogen bonding.

The spectrum of silicone lubricant, Appendix B (No. 27), illustrates some of the preceding absorptions.

3.6.28.3 Silicon - Halogen Stretching Vibrations Ab- sorption caused by Si-F stretch is in the 800-1000 re- gion.

Bands resulting from Si- C1 stretching occur at fre- quencies below 666 cm l .

Appendix B 135

NO. 27 NO. 28

CHAPTER 4

Proton Magnetic Resonance Spectrometry

4.1 Introduction Atomic Atomic I Mass Number Example (I)

Nuclear magnetic resonance (NMR) spectrometry is ba- sicallv another form of absor~tion snectrometrv. akin to Half-integer Odd Odd or iH(i), liO(?), l$N($)

2 I I d Z

IR or UV spectrometry. Under appropriate conditions even Integer Even Odd ?H(1), l$N(l), lgB(3)

in a mugneticfield, a sample can absorb electromagnetic Zero Even Even radiation in the radio frequency (rf) region at frequen-

'iC(O), '$O(O), ?%S(O)

cies governed by the characteristics of the sampler Ab- sorption is a function of certain nuclei in the molecule. A plot of the frequencies of the absorption peaks versus peak intensities constitutes an NMR spectrum. This chapter covers proton magnetic resonance (lH NMR) spectrometry.

With some mastery of basic theory, interpretation of NMR spectra merely by inspection is usually feasible in greater detail than is the case for IR or mass spectra. The present account will suffice for the immediate lim- ited objective: identification of organic compounds in conjunction with other spectrometric information. Ref- erences are given at the end of this chapter.

We begin by describing some magnetic properties of nuclei. All nuclei carry a charge. In some nuclei this charge "spins" on the nuclear axis, and this circulation of nuclear charge generates a magnetic dipole along the axis (Fig. 4.1). The angular momentum of the spinning charge can be described in terms of quantum spin num- bers I; these numbers have values of 0, i, 1, $, and so on ( I = 0 denotes no spin). The intrinsic magnitude of

Nuclei with a spin number I of 1 or higher have a non- spherical charge distribution. This asymmetry is de- scribed by an electrical quadrupole moment which, as we shall see later, affects the relaxation time and, con- sequently, the linewidth of the signal and the coupling with neighboring nuclei. In quantum mechanical terms, the spin number I determines the number of orienta- tions a nucleus may assume in an external uniform mag- netic field in accordance with the formulas 21 + 1. We are concerned with the proton whose spin number I is 3.

Thus in Figure 4.2, these are two energy levels and a slight excess of proton population in the lower energy state (N, > Np) in accordance with the Boltzmann dis- tribution. The states are labeled a and /3 or 1 and - 1; AE is given by

the generated dipole is expressed in terms of nuclear where h is Planck's constant, which simply states that magnetic moment, p. AE is proportional to B, (as shown in Fig. 4.2) since h,

Relevant properties, including the spin number I, of y, and , are constants. B, represents the magnetic field several nuclei are given in Appendix H. The spin num- strength.* ber I can be determined from the atomic mass and the atomic number as shown in the next column.

Spectra of several nuclei can be readily obtained * The designations B (magnetic induction or flux density) and H (mag-

(e.g., i ~ , :H, I ~ C , I ~ N , I;F, ~ p ) since they have spin num- netic intensity) are often used interchangeably for magnetic field strength in NMR spectrometry. The SI term tesla (T), the unit of

hers I and a uniform 'pherical charge distribution measurement for B, supercedes the term gauss (G); 1 T = 104 G. The (Fig, 4.1)- Of these, far the most widely used in NMR frequency term hertz (Hz) supercedes cycles per second (cps). MHz . .

spectrometry are 'H (this chapter) and I3C (Chapter 5). is megahertz (lo0 HZ).

4.2 Continuous-Wave (CW) NMR Spectrometry 145

FIGURE 4.1. Spinning charge on proton generates magnetic dipole.

Once two energy levels for the proton have been established, it is possible to introduce energy in the form of radiofrequency radiation (v,) to effect a transition between these energy levels in a stationary magnetic field of given strength B,,. The fundamental NMR equa- tion correlating the applied radiofrequency v, with the magnetic field strength is

since

The introduced radiofrequency vl is given in mega- hertz (MHz). A frequency of 100 MHz is needed at a magnetic field strength B,, of 2.35 tesla (T) for the pro- ton (or any other desired combination of vl and B, at the same ratio. See Appendix H). At this ratio, the sys- tem is in resonance; energy is absorbed by the proton, raising it to the higher energy state, and a spectrum re- sults. Hence the name nuclear magnetic resonance spec- trometry. The constant y is called the magnetogyric

Spin = + 1, $ 2

FIGURE 4.2. Two proton energy levels, from quantum mechanics, in a magnetic field of magnitude 23,. N is population. The direction of the magnetic field ( T T T ) is

I up, parallel to the ordinate, and B,, increases to the right.

ratio, a fundamental nuclear constant; it is the propor- tionality constant between the magnetic moment p and the spin number I.

The radiofrequency v, can be introduced either by con- tinuous-wave (CW) scanning or by a radiofrequency pulse.

4.2 Continuous- Wave (CW) NMR Spectrometry

The problem is how to apply radiofrequency (rf) elec- tromagnetic energy to protons aligned in a stationary magnetic field and how to measure the energy thus ab- sorbed as the protons are raised to the higher spin state. This can best be explained in classical mechanical terms, wherein we visualize the proton as spinning in an ex- ternal magnetic field. The magnetic axis of the proton precesses about the z axis of the stationary magnetic field B, in the same manner in which an off-perpendic- ular spinning top precesses under the influence of grav- ity (Fig. 4.3).

An assemblage of equivalent protons precessing in random phase around the z axis (i.e., in the direction of the stationary magnetic field B,) has a net macroscopic magnetization M, along the z axis, but none in the xy plane (Fig. 4.4).

When an applied rf (v,) is equal to the precessional frequency of the equivalent protons (Larmor frequency u, in MHz), the state of nuclear magnetic resonance is

Precessional

,---+---, Nuclear magnetic

B 0

FIGURE 4.3. Classical representation of a proton precessing in a magnetic field of magnitude B,, in analogy with a precessing spinning top.

CHAPTER 8

B. Solved Problems

Compound 8.1

We start by gathering information in order to establish a molecular formula. We assume that the weak peak at m/z 144 is the molecular ion peak. It is so small that the intensities of its isotope peaks cannot be accurately measured. Since m/z 144 is an even number, there are 0, 2, 4 . . . N atoms present. To begin, we tentatively assume that are no N, S, or halogen atoms present; this posture, of course, is quite shaky.

From left to right, the proton integrator in the 'H NMR spectrum reads: 2, 2, 2, 3, 3-calibrated against the presumed methyl singlet at 6 2.17. From high to low frequency the 13C and DEPT spectra read: C, C, CH,, CH,, CH,, CH,, CH,. Thus, there are 12 protons and 7 carbon atoms in the molecular formula. Note that the DEPT CH subspectrum is omitted since there are no CH groups. The most likely molecular formula under unit mass 144 is C,H120, (Chapter 2, Appendix A). The index of hydrogen deficiency is 2, and this should be immediately explored.

The IR spectrum shows a strong, broad C=O peak at about -1725 cm-I, which accounts for one unsatu- rated site and for one 0 atom. The 'T spectrum shows a ketone C=O group at -6 208, and an ester C=O group at -6 172.5; the latter assignment is reinforced by typical ethyl ester peaks in the IR spectrum at -1160 cm-' and -1030 cm-I. The broad peak at -1725 cm-I must represent both C=O groups. The three 0 atoms in the molecular formula are ac- counted for.

With this information in hand, interpretation of the 'H spectrum is straightforward. The methyl singlet men- tioned above must be attached to the ketone C=O group to give us one end of the molecule, CH,- C --,

I I 0

which also accounts for the base peak in the mass spectrum at m/z 43. The three-proton triplet and the strongly deshielded, two-proton quartet account for the - C -0-CH,-CH, moiety at the other end of

II

Filling in between the two ends of the molecule re- quires little imagination. All that remain in the 'H spec- trum are two two-proton triplets-surely two adjacent CH, groups. Hence:

CH3- C-CH2-CH2- C -0-CH2-CH, II 0

II 0

Ethyl levulinate, Ethyl 4-oxopentanoate

Let us return for a moment to the mass spectrum: Note that the loss of 15 units (loss of CH,) to give a moderate peak of m/z 129 provides confirmation that the weak peak at m/z 144 is indeed the molecular ion peak. Loss of 45 units to give the strong peak at m/z 99 provides further confirmation.

Assignment of the shifts of the CH, groups adjacent to the C=O groups is ambiguous. Assignment can be achieved by obtaining an HMBC spectrum (Chapter 6) which would show correlation (long-range coupling) be- tween the groups adjacent to the ketone C=O group (Chapter 6).

For further discussion, consider and reject the fol- lowing isomers of ethyl levulinate:

0 the molecule. Confirmation is provided by the strong peak at m/z 99 (characteristic loss of 0-CH,-CH,). The NMR spin systems are A,, A,X,, and A,X,.