Spectroscopy and Structure

Chapter 13

Spectroscopy and Structure

Determination of structure: spectroscopic methods

• Near the beguiling of our study , we outlined the general steps an organic chemist takes when he is confronted with an unknown compound and sets out to find the answer to the question:  We have seen, in more detail, some of the ways in which he carries out the various steps: determination of molecular weight and molecular formula; detection of the presence or absence of certain functional groups; degradation to simpler compounds; conversion into derivatives; synthesis by an unambiguous route.

• By now, we are familiar with some of the features of the organic chemical landscape; so long as v^e do not wander too far from home, we can find our way about without becoming lost. We are ready to learn a little about how to interpret the kind of information these modern instruments give, so that they can help us to see more clearly the new things we shall meet, and to recognize them more readily when we encounter them again. The instruments most directly concerned with our primary interest, molecular structure, are the spectrometers measurers o spectra. Of the various spectra, we shall ^ual^^ and nucleajjna&w^^^ since they are thc_workhorsesjpf the organic chemical laboratory today ; of these, we shall spend most of our time with nmr.

• In all this, \\e must constantly keep in mind that \\hat ask learn at this stage must be great simplified. There are many exceptions to the general i/actions \\e shall learn: there are main pitfalls into \\hitch \\e can stumble Our ability to apply spectroscopic methods to the determination of organic structure is limited by our understanding of organic chemistry as a \vole and in this \\e are, of course, onl> beginners. But so long as \se are aware of the dangers of a little learning and are \\ailing to make mistakes and profit from them, it is \\Ortonville for us to become beginners in this area of organic chemistry, too.

The mass spectrum

• In the mass spectrometer, molecules are bombarded with a beam of energetic electrons. The molecules are ionized and broken up into many fragments, some of \\high are positive ions. Each kind of ion has a particular ratio of mass to charge, or /;/ e Value. For most ions, the charge is I, so that m/e is simply the mass of the ion. Thus, for neopentane:

• The set of ions is analyzed in such a way that a signal is obtained for each value of m/e that is represented; the intensity of each signal reflects the relative abundance of the ion producing the signal. The largest peak is called the base peak: its intensity is taken as 100, and the intensities of the other peaks arc expressed relative to it. A plot or even a list showing the relative intensities of signals at the various m/e values is called a mass spectrum and is highly characteristic of a particular compound.

• Jurats. Spectrac can, be used in two general ways: to prevailed identity of two confounds. And (b) toJveJpLestjahlisJillie .suitors of a^jieyv^^motmot ^ "Two compounds are shown to be identical by the fact that they have identical physical properties: melting point, boiling point, density, refractive index, etc. The greater he number_ofj^ysical.prqperties measured, the stronger the evidence. 

• The compound. Sometimes the M+ peak is the base peak, and is easily recognized; often, though, it is not the base peak it may even be very small and considerable work is required to locate it. Once identified, it gives the most accurate molecular weight obtainable.

• M + e~ > M* + 2eMoiecular ion (Parent ion) m/e SB mol. 

• We might at first think that the M+ peak would be the peak of highest m/e value. This is not so, however. Most elements occur naturally as several isotopes; generally, the lightest one greatly predominates, and the heavier ones occur to lesser extent. The relative abundances of several heavy isotopes.

• The molecular weight that one usually measures and works with is the sum of the average atomic weights of the elements and reflects the presence of these heavy isotopes. This is. not true, however, of the molecular weight obtained from the mass spectrum; here, the M+ peak is due to molecules containing only the commonest isotope of each element.

• Consider benzene, for example. The M f peak, m/e 78, is due only to ions of formula C6H6 +. There is a peak at m/e 79, the M + 1 peak, which is due to C5 13CH6 + and C6 H5 D + . There is an M + 2 peak at m/e 80, due to C4 13 C2 H6 % C5 13CH5 D% and C6H4D2 +. Now, because of the low natural abundance of most heavy isotopes, these isotopic peaks are generally much less intense than the M+ peak; just how much less intense depends upon which elements they are due to. In the case of benzene, the M + 1 and M + 2 peaks are, respectively, 6.58% and 0.18% as intense as the M* peak. (Table 13.1 shows us, however, that a monochord compound would have an M -f 2 peaks about one-third as intense as the M * peak, and a monoreme compound would have M and M -f 2 peaks of about equal intensity.)

• It is these isotopic peaks that make it possible for us to determine the molecular formula of the compound. Knowing the relative natural abundances of isotopes, one can calculate for any molecular formula the relative intensity to be expected for each isotopic peak: M + 1, M + 2, etc. The results of such calculations are available in tables. Consider, for example, a compound for which 'M* is 44.

The electromagnetic spectrum

• shorter the wavelength^ the higher the frequency. " When a beam of electromagnetic radiation is passed through a substance, the radiation can be either absorbed or transmitted, depending upon its frequency and the structure of the molecules it encounters. Electrogenic adagial ea. ^energy, and hejice ,when a molecule absorbs radiationfu gams energy.

• The energyjoined byjjiejiplecule jn this way may bring about increased vibration or rotation of the atoms, or may raise electron^ to higher energy levels. The particular 'Frequency of radiation that a given molecule can absorb~de"pends upon the changes in vibrations or rotations or electronic slates that are permitted to a molecule of that structure. The spectrum of a compound is a plot that shows how much electromagnetic radiation is absorbed (or transmitted) at each frequency. It can be highly characteristic of the compound's structure.

The infrared spectrum

• Interpretation of an infrared spectrurn is not a simple matter. Bands may be obscured by the overlapping of other bands. Overtones (harmonics) may appear at just twice the frequency of the fundamental band. The absorption band of a particular group may be shifted by various structural features conjugation, electron withdrawal by a neighboring substituent, angle strain or van der Waals strain, hydrogen bonding and be mistaken for a band of an entirely different group. (On the other hand, recognized for what they are, such shifts reveal the structural features that cause them.) .
• In our work we shall have modest aims: to learn to recognize a few of the more striking absorption bands, and to gain a little practice in correlating infrared data with other kinds of information. We must realize that we shall be taking from an infrared spectrum only a tiny fraction of the information that is there, and which can be gotten from it by an experienced person with a broad understanding of organic structure.

JThe ultraviolet spectrum

• Light of wavelength between about 4000 A and 7500 A (400-750 mpO is visible. Just beyond the red end of the visible spectrum (A greater than 750 m^) lies the infrared region which we have just discussed. Just beyond the violet end of the visible spectrum (A less than 400 mijiu) lies the ultraviolet region.

• In a transition to a higher electronic level, a molecule can go from any of a number of sub-levels corresponding to various vibrational and rotational states to any of a number of sub-levels; as a result, ultraviolet absorption bands are broad. Where an infrared spectrum shows many sharp peaks, a typical ultraviolet spectrum shows only a few broad humps. One can conveniently describe such a spectrum in terms of the position of the top of the hump (hmAX) and the intensity of that absorption (emA1t , the extinction coefficient).

• When we speak of a molecule as being raised to a higher electronic level, we mean that an electron has been changed from one orbital to another orbital of higher energy. This electron can be of any of the kinds we have encountered: a a electron, a TT electron, or an n electron (a non-bonding electron that is, one of an unshared pair). A a electron is held tightly, and a good deal of energy is required to excite it: energy corresponding to i,Hraviolet light of short wavelength, in a region "far" ultraviolet outside the range of the usual spectrometer. It is chiefly excitations of the comparatively loosely held n and TT electrons that appear in the (near) ultraviolet spectrum, and, of these, only jumps to the lower more stable excited stastes.

• The electronic transitions of most concern to the organic chemist are: (a) // -> TT*, in which the electron of an unshared pair goes to an unstable (and bonding) IT orbital, as, for example,

• In contrast to the infrared spectrum, the ultraviolet spectrum is not used primarily to show the presence of individual functional groups, but rather to show relationships between functional groups, chiefly conjugation: conjugation between two or more carbon-carbon double (or triple) bonds; between carboncarbon and carbon-oxygen double bonds; between double bonds and an aromatic ring; and even the presence of an aromatic ring itself. It can, in addition, reveal the number and location of substituents attached to the carbons of the conjugated system. 

The nuclear magnetic resonance (Namr) spectrum

• Like electrons, the nuclei of certain atoms are considered to spin. The spinning of these charged particles the circulation of charge generates a magnetic moment along the axis of spin, so that these nuclei act like tiny bar magnets. One such nucleus and the one we shall be mostly concerned withies the proton, the nucleus of ordinary hydrogen, 1 H

• Now, if a proton is placed in an external magnetic field, its magnetic moment, according to quantum mechanics, can be aligned in either of two ways: with or against the external field. Alignment with the field is the more stable, and energy must be absorbed to "flip" the tiny proton magnet over to the less stable alignment, against the field.

• Just how much energy is needed to flip the proton over depends, as we might expect, on the strength of the extcn.il field: the stronger the field, the greater the tendency to remain lined up with it, and the higher the frequency (Remember: A = hv) of the radiation needed to do the job. 

• In a field of 14,092 gauss, for example, the energy required corresponds to electromagnetic radiation of frequency 60 MHz (60 megahertz or 60 million cycles per second): radiation in the radiofrequency range, and of much lower energy (lower frequency, longer wavelength) than even infrared.

• Now, if the situation were as simple as we have so far described it, all the protons in an organic molecule would absorb at exactly the same field strength. and the spectrum would consist of a single signal that would tell us little about the structure of the molecule. But the frequency at which a proton absorbs depends on the magnetic field which that proton feels, and this effective field strength is not exactly the same as the applied field strength. The effective field strength at each proton depends on the environment of that proton on, among other things, the electron density at the proton, and the presence of other, nearby protons. Each protomer, more precisely, each set of equivalent protons will have a slightly different environment from every other set of protons, and hence will require a slightly different applied field strength to produce the same effective field strength: the particular field strength at which absorption takes place.

• Now, if the situation were as simple as we have so far described it, all the protons in an organic molecule would absorb at exactly the same field strength. and the spectrum would consist of a single signal that would tell us little about the structure of the molecule. But the frequency at which a proton absorbs depends on the magnetic field which that proton feels, and this effective field strength is not exactly the same as the applied field strength. The effective field strength at each proton depends on the environment of that proton on, among other things, the electron density at the proton, and the presence of other, nearby protons. Each protomer, more precisely, each set of equivalent protons will have a slightly different environment from every other set of protons, and hence will require a slightly different applied field strength to produce the same effective field strength: the particular field strength at which absorption takes place.

• The result is a spectrum showing many absorption peaks, whose relative positions, reflecting as they do differences in environment of protons, can give almost unbelievably detailed information about molecular structure.

• In the following sections, we shajMookjrt various aspects of the Namr spectrum.

Nmr. Number of signals. Equivalent and non-equivalent protons

• In a given molecule, protons with the same environment absorb at the same (applied) field strength; protons with different environments absorb at different (applied) field strengths. A set of protons with the same environment are said to be equivalent; the number of signals in the nmr spectrum tells us, therefore, how many sets of equivalent protons how many "kinds" of protons a molecule contains.

• For our purposes here, equivalent protons are simply chemically equivalent protons, and we have already had considerable practice in judging what these are. Looking at each of the following structural formulas, for example, we readily pick out as equivalent the protons designated with the same letter:

• Realizing that, to be chemically equivalent, protons must also be stereochemically equivalent, we find we can readily analyze the following formulas, too:

• signals, and it takes only a little work with models or stereochemical formulas to see that this should indeed be so.

• The environments of the two protons on C-l are not the same (and no amount of rotation about single bonds will make them so); the protons are not equivalent and will absorb at different field strengths.

• We can tell from a formula which protons are in different environments and hence should give different signals. We cannot always tell -particularly \\ith stereochemically different protons just how different these environments are; they may not be different enough for the signals to be noticeably separated, and we may Seehawer signals than we predict.

• Now, just how did we arrive at the conclusions of the last few paragraphs? Most of us perhaps without realizing it judge the equivalence of protons by following the approach of isomer number (Sec. 4.2). This is certain!) the easiest \vay to do it. We imagine each proton in turn to be replaced by some other atom Z. If replacement of either of t\\o protons by Z would yield the same product or enantiomeric products, then the t\vo protons are chemically equivalent. We ignore the existence of conformational isomers.

Nmr, positions of signals. Chemical shift.

• Just as the number of signals in an nmr spectrum tells us how many kinds of protons a molecule contains, so the positions of the signals help to tell us what kinds of protons they are: aromatic, aliphatic, primary, secondary, tertiary; benzylic, vinylic, acetylenic; adjacent to halogen or to other atoms or groups. These different kinds of protons have different electronic environments, and it is the electronic environment that determines just where in the spectrum a proton absorbs.

• When a molecule is placed in a magnetic field as it is when one determines a nmr spectrum its electrons are caused to circulate and, in circulating, they generate secondary magnetic fields: induced magnetic fields.

• Circulation of electrons about the proton itself generates a field aligned in such a way that at the proton it opposes the applied field. The field felt by the proton is thus diminished, and the proton is said to be shielded. Circulation of electrons specifically, n electrons about nearby nuclei generates a field that can either oppose or reinforce the applied field at the proton, depending on the proton's location. If the induced field opposes the applied field, the proton is shielded, as before. If the induced field reinforces the applied field, then the field felt by the proton is augmented, and the proton is said to be DeShields

• The reference point from which chemical shifts are measured is, for practical reasons, not the signal from a naked proton, but the signal from an actual compound: usually tetramethylsilane, (CH3 ) 4Si. Because of the low electronegativity of silicon, the shielding of the protons in the silane is greater than in most other organic molecules; as a result, most nmr signals appear in the same direction from the tetramethylsilane signal: downfield.

• The most commonly used scale is the B (delta) scale. The position of the tetramethylsilane signal is taken as 0.0 ppm. Most chemical shifts have S values between and 10 (minus 10, actually). A small 8 value represents a small downfield shift, and a large 8 value represents a large downfield shift.

• One commonly encounters another scale: the r (tan) scale, on which the tetranethylsilane signal is taken as 10.0 ppm. Most r values lie between and 10. The two scales are related by the expression r = 10 8.

• An nmr signal from a particular proton appears at a different field strength han the signal from tetramethylsilane. This difference the chemical shift is measured not in gauss, as we might expect, but in the equivalent frequency units Remember: v = y// /27r), and it is divided by the frequency of the spectrometer ised. Thus, for a spectrometer operating at 60 MHz, that is, at 60 x 10 6 Hz:

• . observed shift (Hz) x 10* 60 x 10 (HZ)

• Furthermore, it has been found that a proton with a particular environment shows much the same chemical shift, whatever the molecule it happens to be part of. Take, for example, our familiar classes of hydrogens: primary, secondary, and tertiary. In the absence of other nearby substituents, absorption occurs at about these values:

• RCH3 8 0.9 R2CH2 5 1.3 R3CH 8 1

• All these protons, in turn, differ widely from aromatic protons which, because of the powerful deshielding due to the circulation of the * electrons (see Fig. 13.4, p. 419), absorb far downfield:

• Ar-H 8

• Attachment of chlorine to the carbon bearing the portion causes a downfield shift. If the chlorine is attached to the carbon once removed. from the carbon bearing the proton, there is again a downward shift, but this time much weaker.

Nmr. Peak area and proton counting

• Let us look again at the nmr spectra (Fig. 13.5, p. 422) of toluene, />-xylene, and mesitylene, and this time focus our attention, not on the positions of the signals, but on their relative intensities, as indicated by the sizes of the absorption peaks.

• Judging roughly from the peak heights, we see that the (high-field) peak for side-chain protons is smaller than the (low-field) peak for aromatic protons in the case of toluene, somewhat larger in the case of p-xylene, and considerably larger in the case of mesitylene.

• It is not surprising that this is so. The absorption of every quantum of energy is due to exactly the same thing: the flipping over of a proton in the same effective magnetic field. The more protons flipping, the more the energy absorbed, and the greater is the area under the absorption peak.

• Areas under nmr signals are measured by an electronic integrator and are usually given on the spectrum chart in the form of a stepped curve; heights of steps are proportional to peak areas. Nmr chart paper is cross-hatched, and we can conveniently estimate step heights by simply counting squares. We arrive at a set of numbers that are in the same ratio as the numbers of different kinds of protons. 

Nmr. Splitting of signals. Spin-spin coupling

• A nmr spectrum, we have said, shows a signal for each kind of proton in a molecule; the few spectra we have examined so far bears this out. If we look much further, however, we soon find that most spectra are or appear to be much more complicated than this. Figure 13.8 (p. 427), for example, shows the nmr spectra for three compounds,

• CH2 Br-CHBr2 CH3-CHBr2 CH3-CH2 Br

• The answer is that we are observing the splitting _of mrir^signals causedJ>y^ spin -spin coupling. The signal we expect from each set of equivalent protons is *Sppearing, not as a single peak, but as a group of peaks. Splitting reflects the environment of the absorbing protons: not with respect to electrons, but with respect to other, nearby protons. It is as though we were permitted to sit on a proton and look about in all directions: we can see and count the protons attached to the carbon atoms next to our own carbon atom and, sometimes, even see protons still farther away.

• Let us take the case of adjacent carbon atoms carrying, respectively, a pair of secondary protons and a tertiary proton, and consider first the absorption by one of the secondary protons:

•  CH-CHj

• The magnetic field that a secondary proton feels at a particular instant is slightly increased or slightly decreased by the spin of the neighboring tertiary proton: increased if the tertiary proton happens at that instant to be aligned uv7/i the applied.

•  In the doublet is exactly the same as the separation of peaks in the triplet. (Spin-spin coupling is a reciprocal affair, and the effect of the secondary protons on the tertiary proton must be identical with the effect of the tertiary proton on the secondary protons.) Even if they were to appear in a complicated spectrum of many absorption peaks, the identical peak separations would tell us that this doublet and triplet were related: that the (two) protons giving the doublet and the (one) proton giving the triplet are coupled, and hence are attached to adjacent carbon atoms.

• If we turn once more to Fig. 13.8 (p. 427), we no longer find these spectra so confusing. We now see not just five or six or seven peaks, but instead a double and a triplet, or a double and a quartet, or a triplet and a quartet.

 Nmr. Coupling constants

• Nmr. Coupling constants ^tance betweenjgeaks in mujjirjet js__a measurejrf the effectiveness of ^and is called the-coupling constant /. Coulam. Tanke chemical shift).is not a matter of induced magnetic fields. The value of the coupling constant as measured, in Hz remains the same, whatever the applied magnetic field (that is, whatever the radiofrequency used). In this respect, of course, spin-spin splitting differs from chemical shift, and, when necessary, the two can be distinguished on this basis:. the spectrum is run at a second, different radiofrequency; when measured in Hz, peak separations due to splitting remain constant, whereas peak separations due to chemical shifts change. (When divided by the radiofrequency and thus converted into ppm, the numerical value of the chemical shift would, of course, remain constant.).

• For example, in any substituted ethylene or in any pair of geometric isomers J is always larger between trans protons than between cis protons; furthermore, the size of J varies in a regular way with the electronegativity of substituents. so that one can often assign configuration without having both isomers in hand.

• A coupling constant is designated as -I- or - to permit certain theoretical correlations; for many compounds this sign has been determined. We shall be concerned only with the absolute size of /, as reflected in the distance between peaks.

• Although we shall not work very much with the values of coupling constants, we should realize that, to an experienced person, they can often be the most important feature of an nmr spectrum: the feature that gives exactly 'the kind of information about molecular structure that is being looked for.

Nmr. Complicated spectra. Deuterium labeling

Most nmr spectra that the organic chemist is likely to encounter are considerably more complicated than the ones given in this book. How are these analyzed ? First of all, many spectra showing a large number tff peaks can be completely analyzed by the same general methods we shall use here. It just takes practice. Then again, in many cases complete analysis is not necessary for the job at hand. Evidence of other kinds may already have limited the number of possible structures, and all that is required of the nmr spectrum is that it let us choose among these. Sometimes all that we need to know is how many kinds of protons there are or, perhaps, how many kinds and how many of each kind.

• omy one structural feature is still in doubt for example, does the molecule contain two methyl groups or one ethyl group? and the answer is given in a set of peaks standing clear from the general confusion.

• Because a deuteron has a much smaller magnetic moment than a proton, it absorbs at a much higher field and so gives no signal in the proton nmr spectrum. Furthermore, its coupling with a proton is weak and it ordinarily broadens, but does not split, a proton's signal; even this effect can be eliminated by double irradiation.

• One can use deuterium labeling to find out which signal is produced by which proton or protons: one observes the disappearance of a particular signal when a proton in a known location is replaced by deuterium. One can use deuterium labeling to simplify a complicated spectrum so that a certain set of signals can be seen more clearly: see, for example, Fig. 13.18, p. 438.

Equivalence of protons: a closer look

We have seen that equivalence or non-equivalence of protons is fundamental to the nmr spectrum, since it affects both the number of signals and their splitting. Let us look more closely at equivalence, and see how it is affected by the rate at which certain molecular changes occur:

• Each of these molecular changes can change the environment both electronic and protonic of a given proton, and hence can affect both its chemical shift and its coupling with other protons. The basic question that arises is whether or not the nmr spectrometer sees the proton in each environment or in an average of all of them. The answer is, in short, that it can often see the proton in either way, depending upon the temperature, and in this ability lies much of the usefulness of nmr spectroscopy.

• In comparing it with other spectrometers, Professor John D. Roberts of the California Institute of Technology has likened the nmr spectrometer to a camera with a relatively long shutter time that is, to a "slow" camera. Such a camera photographs the spokes of a wheel in different ways depending upon the speed with which the wheel spins: as sharp, individual spokes if spinning is slow; as blurred spokes if spinning is faster; and as a single circular smear if spinning is faster yet. In the same way, if the molecular change is relatively fast, the nmr spectrometer sees a proton in its average environment a smeared-out picture; if the molecular process is slow, the spectrometer sees the proton in each of its environments.

• In this section we shall examine the effects of rotations about single bonds on the nmr spectrum, and in later sections the effects of the other molecular changes. Let us return to ethyl chloride , and focus our attention on the methyl protons. If, at any instant, we could look at an individual molecule, we would almost certainly see it in conformation I. One of the methyl protons is ami to the chlorine and two protons are gauche; quite clearly, the anti-proton is in a different environment from the others, and for the moment is not equivalent to them.

• Yet, we have seen, the three methyl protons of ethyl chloride give a single nmr signal (a triplet, because of the adjacent methylene group), and hence must be magnetically equivalent. How can this be? The answer is, of course, that rotation about the single bond is compared with the nmr ** shutter speed 1 * a fast process; the nmr "camera" takes a smeared-out picture of the three protons.

•  Each proton is seen in an average environment, which is exactly the same as the average environment of each of the other two: one-third ami, and two-thirds gauche.

The electron spin resonance (esr) spectrum

• The electron spin resonance (esr) spectrum Let us consider a free radical placed in a magnetic field and subjected to electromagnetic radiation; and let us focus our attention, not on the nuclei, but on the odd, unpaired electron. This electron spins and thus generates a magnetic moment, which can be lined up with or against the external magnetic field. Energy is required to change the spin state of the electron, from alignment with the field to the less stable alignment, against the field. This energy is provided by absorption of radiation of the proper frequency. An absorption spectrum is produced.

• The esr spectrum is thus analogous to the nmr spectrum. An electron has, however, a much larger magnetic moment than the nucleus of a proton, and more energy is required to reverse the spin. In a field of 3200 gauss, for example, where nmr absorption would occur at about 14 MHz, esr absorption occurs at a much higher frequency: 9000 MHz, in the microwave.

• Like nmr signals, esr signals show splitting, and from exactly the same cause, coupling with the spins of certain nearby nuclei: for example, protons near carbon atoms that carry or help to carry the odd electron. For this reason, esr spectroscopy can be used not only to detect the presence of free radicals and to measure their concentration, but also to give evidence about their structure: what free radicals they are, and how the odd electron is spread over the molecule. 

Spectroscopic analysis of hydrocarbons. Infrared spectra

• In this first encounter with infrared spectra, we shall see absorption bands due to vibrations of carbon -hydrogen and carbon-carbon bonds: bands that will constantly reappear in all the spectra we meet, since along with their various functional groups, compounds of all kinds contain carbon and hydrogen. We must expect to find these spectra complicated and, at first, confusing. Our aim is to learn to pick out of the confusion those bands that are most characteristic of certain structural features.

• Bands due to carbon-carbon stretching may appear at about 1500 and 1600 cm' 1 for aromatic bonds, 'at 1650 cm" 1 for double bonds (shifted to about 1600 cm' 1 by conjugation), and at 2100 cm" 1 for triple bonds. These bands, however, are often unreliable. (They may disappear entirely for fairly symmetrically substituted alkynes and alkenes, because the vibrations do not cause the change in dipole moment that is essential for infrared absorption.) More generally useful bands are due to the various carbon-hydrogen vibrations.

• Absorption due to carbon-hydrogen stretching, which occurs at the high-frequency end of the spectrum, is characteristic of the hybridization of the carbon holding the hydrogen: at 2800-3000 cm * for tetrahedral carbon; at 3000 3100 cm" 1 for trigonal carbon (alkenes and aromatic rings): and at 3300 cm' 1 for digonal carbon (alkynes).

• Absorption due to various kinds of carbon- hydrogen bending, which occurs at lower frequencies, can alo be characteristic of structure. Methyl and methylone groups absorb at about 1430 1470cm ! ; for methyl, there is another band, quite characteristic, at 1375 cm" '. The isopropyl " split" is characteristic: a doublet, with equal intensity of the two peaks, at 1370 and 1385 cm" l (confirmed by a band at 1170 cm l ). tert-Buiy\ gives an unsymmetrical doublet: 1370 cm' 1 (strong) and 1395 cm" 1 (moderate).

• Now, what do we look for in the infrared spectrum of a hydrocarbon. To begin with, we can rather readily tell whether the compound is aromatic or purely aliphatic. The spectra in Fig. 13.2 (p. 411) show the contrast that is typical: aliphatic absorption is strongest at higher frequency and is essentially missing below 900 cm ~ l ; aromatic absorption is strong at lower frequencies (C H out-ofplane bending) between 650 and 900 cm' 1 . In addition, an aromatic ring will show C H stretching at 3000- 3 100 cm ~ * ; often, there is carbon-carbon stretching at 1 500 and 1 600 cm - ! and C H in-plane bending in the 1000-1 100 cm - l region.

Spectroscopic analysis of hydrocarbons. Nmr

The application of nmr spectroscopy to hydrocarbons needs no special discussion beyond that already given in Sees. 13.6-13.11. For hydrocarbons as for other kinds of compounds, \\c shall find that where the infrared spectrum helps to tell us what kind of compound we are dealing with, the nmr spectrum will help to tell us what compound.