Introduction to Spectroscopy Spectroscopy involves an interaction between
Introduction to Spectroscopy • Spectroscopy involves an interaction between matter and light (electromagnetic radiation) • Light can be thought of as waves of energy or packets (particles) of energy called photons • Properties of light waves include wavelength and frequency
Electromagnetic Radiation Spectrum • There are many wavelengths of light that can not be observed with your eyes
Spectroscopy used in Chemistry • When light interacts with molecules, the effect depends on the wavelength of light used • This chapter focuses on IR spectroscopy
15. 2 IR Spectroscopy • An IR spectrophotometer irradiates a sample with all frequencies of IR light • Detector determine the degrees of absorption at all frequencies to show the presence of certain bonds (functional groups) Media used to determine the IR spectra: • Samples are deposited neat on a salt (Na. Cl) plate. • Or, sample dissolved in a solvent or embedded in a KBr pellet
Bond vibration patterns • Molecular bonds can vibrate by stretching or by bending in a number of ways • This chapter will focus mostly on stretching frequencies
Energy level from Bond Vibration • The energy necessary to cause vibration depends on the type of bond
IR absorption from Bond Vibration • For each bond, vibrational energy levels are separated by gaps (quantized) • If a photon (light) strikes the molecule with the exact amount of energy needed, a molecular vibration will occur • Energy is eventually released from the molecule generally as heat • Infrared (IR) Light generally causes molecular vibration • HOW IR light is absorbed molecular structure
IR Spectroscopy • A signal on the IR spectrum has three important characteristics: wavenumber, intensity, and shape
IR Frequency: Wavenumber •
Classification of IR in Structure Analysis • The region above 1500 cm-1 : diagnostic region. DIAGNOSTIC REGION FINGERPRINT REGION • The region below 1500 cm-1 : the fingerprint region.
IR of –OH group: Ether vs. Alcohol CH 3 OCH 3 CH 2 OH
IR of C=O group: Alcohol vs. Carboxylic acid Ethanol (-OH) acetic acid (>C=O & -OH)
Wavenumber: Bond and Mass • The wavenumber for a stretching vibration depends on the bond strength and the mass of the atoms bonded together • Heavier atoms causes lower frequency IR
IR Wavenumber: Bond and Mass Stronger bond causes higher frequency stretch vibration (higher wavenumber) Heavier atoms causes lower frequency stretch vibration
IR wavenumber: Hybridization • Same C-H bond, different hybridization: • C-H bond (carbon being sp hybridized) is strongest.
IR pattern for C-H of dif. hybridization • The region ≈3000 cm-1 : sp 3, sp 2, sp
IR for hydrocarbon w/o unique C-H • List the absorption around wavenumbers (in cm-1 ) of 3000 cm-1 for the molecules below: A. 3300 B. 3100 C. 2900 I II III
Resonance affects wavenumber • Consider a carbonyl that has two resonance contributors • C=O ~1720 cm-1, C-O 1120 cm-1 • If there were more contributors with C-O single bond character than C=O double bond character, the observed wavenumber is in between C-O and C=O bonds.
Examples of Resonance Effect
15. 4 IR Signal Strength • The strength of IR signals can vary
Molecular Symmetry vs. Signal Strength • Note the general strength of the C=O stretching signal vs. the C=C stretching signal • Typical C=C bond has absorption at 1500 -2000 cm-1 • A symmetrical molecule with a completely nonpolar C=C bond: 2, 3 -dimethyl-2 -butene, no IR signal in the 1500 -2000 cm-1 region
#Bonds affects IR Signal Strength • Stronger signals are also observed when there are multiple bonds of the same type vibrating • Although C-H bonds are not very polar, they often give very strong signals, due to the large number of C-H bonds • Because sample concentration can affect signal strength, the Intoxilyzer 5000 can be used to determine blood alcohol levels be analyzing the strength of C-H bond stretching in blood samples
15. 5 IR Signal Shape • Some IR signals are broad, while others are very narrow • O-H stretching signals are often quite broad
Hydrogen bonding affects Shape • When possible, O-H bonds form H-bonds that weaken the O-H bond strength • The H-bonds are transient, so the sample will contain molecules with varying O-H bond strengths • Varying O-H bond strength leads to broad band signals. • If no H-bonding: The O-H stretch signal will be narrow if a dilute solution of an alcohol is prepared in a solvent incapable of H-bonding
Example of signal for O-H bond • In a sample with an intermediate concentration, both narrow and broad signals are observed. • Explain the cm-1 readings for the two O-H stretching peaks
IR Signal Shape for Carboxylic Acid • Consider how broad the O-H stretch is for a carboxylic acid and how its wavenumber is around 3000 cm-1 rather than 3400 cm-1 for a typical O-H stretch
Carboxylic Acid forms Dimers • H-bonding is often more pronounced in carboxylic acids, because they can forms H-bonding dimers (two molecules bonded through H-bonding)
Practice: Predicting IR Signal Shape • For the molecule below, predict all of the stretching signals in the diagnostic region: • X-H: O-H, -C-H, =C-H • X=Y: C=C, C=O (conjugated with C=C)
Signal Shape for Amine (N-H) • Primary and secondary amines exhibit N-H stretching signals. • Tertiary amines has no N-H bond. • Because N-H bonds are capable of H-bonding, their stretching signals are often broadened • N-H bonds are less polar than O-H bond, thus weak signals.
IR Signal for Amines
IR Signal Shape for –NH 2 • The appearance of two N-H signals for the primary amine is NOT simply the result of each N-H bond giving a different signal • Instead, the two N-H bonds vibrate together in two different ways
Symmetry of Vibration • A single molecule can only vibrate symmetrically or asymmetrically at any given moment, so why do we see both signals at the same time? • Similarly, CH 2 and CH 3 groups can also vibrate as a group giving rise to multiple signals • Practice with conceptual checkpoint 15. 10
Effect of solvation on IR • Explain how an experiment involving isotopic labeling might be used to explore the type of fragmentation that occurs in the MS analysis of organic compounds.
15. 6 Analyzing an IR Spectrum • Table 15. 2: key signals for functional groups • Often, the molecular structure can be identified from an IR spectra 1. Focus on the diagnostic region (above 1500 cm-1) a) b) c) d) 1600 -1850 cm-1 – check for double bonds (C=O or C=C) 2100 -2300 cm-1 – check for triple bonds (C N or C C) 2700 -4000 cm-1 – check for X-H bonds (C-H, O-H, N-H) Analyze wavenumber, intensity, and shape for each signal
Analyzing an IR Spectrum 2. Focus on the 2700 -4000 cm-1 (X-H) region • Practice with Skill. Builder 15. 1
15. 7 Using IR to Distinguish Between Molecules • As we have learned in previous chapters, organic chemists often carry out reactions to convert one functional group into another • IR spectroscopy can often be used to determine the success of such reactions • For the reaction below, how might IR spectroscopy be used to analyze the reaction? • Practice with Skill. Builder 15. 2
Practice: Using IR to Distinguish Between Molecules • For the reactions below, identify the key functional groups, and describe how IR data could be used to verify the formation of product • Is IR analysis qualitative or quantitative?
Additional Practice Problems • Explain how an experiment involving isotopic labeling might be used to explore the type of fragmentation that occurs in the MS analysis of organic compounds.
- Slides: 38