Spectroscopy Part 1: Organic Structure Determination PDF

Spectroscopy Part 1 Organic Structure Determination How do you determine the molecular structure of a suspicious white powder? The historical method was to perform many tedious chemical reactions, but nobody does that anymore! Instead, instrumental me...

Spectroscopy Part 1 Organic Structure Determination How do you determine the molecular structure of a suspicious white powder? The historical method was to perform many tedious chemical reactions, but nobody does that anymore! Instead, instrumental methods are used, the techniques of analytical chemistry. There are many useful analytical chemistry methods, but we will look at only two of these: 1. Infrared (IR) Spectroscopy: Yields information about organic functional groups in a molecule. 2. Nuclear magnetic resonance (NMR) Spectroscopy: A very important method that yields structure. 1 IR Spectroscopy Spectroscopy describes how electromagnetic radiation interacts with molecules. Molecules at room temperature have energy, and some of this energy is stored in bond vibrations. 1.1 An IR Spectrum An infrared absorption (IR) spectrum tells us the frequencies of bond vibrations in a molecule. There are usually many different bond vibrations, many different peaks in an infrared spectrum, but don't worry, we don't need to know much about most of these observed peaks (see later). many vibrations IR absorbance O C CH3 C-H stretching vibrations H WEAK C=O stretching vibration STRONG ~5 x 1013 s-1 ν increasing vibration frequency wavenumber, cm-1 With What Frequencies do Bonds Vibrate? IR bond vibrations are often localized on bonds or groups of atoms in the functional groups. Different bonds vibrate with different frequencies, measured in units of wavenumber, cm-1. Stronger bonds have higher vibration frequencies. bond BDE (kcal/mol) ν (cm-1) C C ~85 1200 INCREASING INCREASING bond strength C C ~145 1600 frequency C C ~200 2100 Bonds involving lighter (smaller) atoms have higher vibration frequencies Spectroscopy : page 1  bond BDE (kcal/mol) ν (cm-1) DECREASING C C ~85 1200 INCREASING atom mass C D ~100 2100 frequency C H ~100 3000 Approximate Bond Vibration Regions in the IR Spectrum stronger bonds stronger bonds single bond region bonds to H triple double (fingerprint) the lightest atom bond bond vibrations are "mixed" region region O-H C-H C N C C region region C C C O 3500 3000 2000 1750 1500 1000 increasing vibration frequency cm-1 decreasing vibration frequency How Strong Are the Absorption Peaks (How Big Are the Peaks in the Spectra)? Bond vibrations have larger peaks in an IR spectrum if the bond is polar. The more polar the bond, the larger the peak in an IR spectrum. Examples +H O + large dipole moments, O + C C N STRONG (large) absorptions R H H R C C R C C R C C H C symmetrical R H zero dipole small dipole very small dipole moment, but there are moment, no moments, WEAK many, so observed! absorption! absorptions 1.2 Real Absorption Bands Vibrations of C–H bonds around 3000 cm−1, C–H bonds H atoms are light, bonds to H atoms tend to be high frequency (large n), ca. 2700 to 3500 cm−1. Stronger C–H bonds will vibrate with higher frequencies, weaker C–H bonds will vibrate with lower frequencies. We expect that stronger C–H bonds will have higher frequency absorption in IR spectroscopy, they do: Spectroscopy : page 2  sp H sp3 H C C H C C C H sp2 H H increasing bond strength increasing expected vibrational frequency C(sp3)–H bonds are weaker than C(sp2)–H and C(sp)H bonds and vibrate at frequencies lower than 3000 cm−1. The dipole moments for C–H bonds are very small; however, there are usually many C–H bond vibrations, and the many small peaks “add up” so that the peaks due to these vibrations can still be observed in the spectrum. sp3 H C 3000 3000 cm-1 70 H very sharp, close to 1600 cm-1, very 60 characteristic (but often weak). 50 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) The peak is often weak because of a small dipole moment, but is usually sharp and close to 1600 cm−1. If there is a benzene ring, there should almost always also be C(sp2)–H bond vibrations, and these will be observed at >3000 cm−1. 1.3 Reading an IR Spectrum The following chart will be provided so that you don’t have to memorize frequencies for different bond vibrations. stronger bonds stronger bonds single bonds to -H region triple bond region double bond region bond fingerprint O-H N-H C-H C NC C C O C C region small range of peak frequencies C O H C N usually larger range of peak frequencies C strong H broad peak N H C O 1660–1600 O C H C N O H C 3300-3250 H 2200 2820–2710 C 1680 2 peaks O C CH 3100–3000 C OR N H C H 2200 1730 broad with spikes O ~3300 1600 2980–2850 C O H 1710 O broad ~3300-3400 O C C O H NR2 broad ~3000 1650 4000 3500 3000 2500 2000 1700 1500 wavenumber, cm-1 IR peaks will always appear over a range of frequencies, this is one of the challenges in reading IR spectra, sometimes the frequencies will not exactly match with any chart or correlation table. Spectroscopy : page 10 Example Spectrum: You must distinguish “real” peaks from impurity or other artifact peaks, such as contaminating water, which is often seen as a weak peak around 3300 cm−1 (any “real” peaks in this region would be strong). Note that a benzene ring adjacent to a C=O bond represents a common example of a more generic conjugated C=C adjacent to C=O. -1 fingerprint region (ignore < 1500 cm ) H2O ignore ignore H sp3 C H H O H C C sp2 CH3 O C conjugated 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) Example Problem: Provided is an IR spectrum of the hydroxyketone structure provided. Assign all important vibrational peaks above 1500 cm−1. NOT aldehyde, no 2730 + 2820 peaks O OH H O H C H centered H H O ignored ~3300 C sp3 therefore C not acid H all