Introduction to spectroscopy 1-infrared spectroscopy Spectroscopy At this point in the course, we have learned many organic reactions that can be used to synthesize organic molecules. How do you determine whether the product that you have obtained in an organic reaction is the product that you had expected?Also, how would you determine the structure of unknown organic compounds that are isolated from natural sources? Definition Spectroscopy:
It is an analytical technique concerned withstudying of the interaction of electromagnetic radiation and matter in orderto determine the structure. Types of Spectroscopy Infrared spectroscopy (IR)
measures the bond vibration frequencies in a molecule and is used to determine the functional group Mass spectrometry (MS) fragments the molecule and measures the masses Nuclear magnetic resonance spectroscopy (NMR) detects signals from hydrogen atoms and can be used to distinguish isomers electromagnetic relationships:
c Wavelength ():Crest-to-crest distance between waves Frequency (v):Number of cycles per second number of cycles/sec or s-1 or Hertz (Hz) electromagnetic relationships:
Electromagnetic radiation displays the properties of both particles and waves The particle component is called a photon The energy (E) component of a photon is proportional to the frequency. Where h is Plancks constant and n, is the frequency in Hertz (cycles per second) E = hn The speed of light, c, is constant, the frequency, n, is inversely proportional to how long the oscillation is, or wavelength: c ___ n = c = 3 x 1010 cm/s l hc ___ E = hn = l Electromagnetic (EM) Spectrum
Frequency, n in Hz ~1019 ~1017 ~1015 ~1013 ~1010 ~105 Wavelength, l ~.0001 nm ~0.01 nm 10 nm 1000 nm 0.01 cm 100 m Energy (kcal/mol) > 300 300-30 300-30 ~10-4 ~10-6 g-rays X-rays UV IR Microwave Radio nuclear excitation (PET) core electron excitation (X-ray cryst.) electronic excitation (p to p*) molecular vibration molecular rotation Nuclear Magnetic ResonanceNMR (MRI) Visible High frequency (u) Short wavelength (l)
= High energy Low frequency (u) Long wavelength (l) = Low energy The Spectrum and Molecular Effects
Absorption Spectra Organic compound exposed to electromagnetic radiation, can absorb energy of certain wavelengths. Changing wavelengths to determine which are absorbed and which are transmitted produces an absorption spectrum Energy absorbed is shown as dips in spectrum Infrared Absorption of Ethyl Alcohol CH3CH2OH Infrared Spectroscopy Infrared radiation 1. what type of bonds are present
IR is used to tell: 1. what type of bonds are present 2. some structural information Infrared Spectroscopy
Infrared spectroscopy is the measurement of the wavelength and intensity of the absorption of mid-infrared light by a sample. Mid-infrared is energetic enough to excite molecular vibrations to higher energy levels. The wavelength of infrared absorption bands is characteristic of specific types of chemical bonds, and infrared spectroscopy finds its greatest utility for identification of organic and organometallic molecules. The high selectivity of the method makes the estimation of an analyte in a complex matrix possible. The IR Spectroscopic Process
As a covalent bond oscillates due to the oscillation of the dipole of the molecule a varying electromagnetic field is produced The greater the dipole moment change through the vibration, the more intense the EM field that is generated The IR Spectroscopic Process
When a wave of infrared light encounters this oscillating EM field generated by the oscillating dipole of the same frequency, the two waves couple, and IR light is absorbed The coupled wave now vibrates with twice the amplitude IR beam from spectrometer coupled wave EM oscillating wave from bond vibration Dipole Changes During Vibrations Absorption of infrared radiation is thus confined largely to molecular species that have small energy differences between various vibrational and rotational states. In order to absorb infrared radiation, a molecule must undergo a net change in dipole moment as a consequence of its vibrational or rotational motion. The dipole moments is determined by the magnitude of the charge difference and the distance between the two occurs during the vibration or rotation of homonuclear species such as O2, N2, or Cl2; consequently centers of Types of Molecular Vibrations:
charge. No net change in dipole moment, such compounds cannot absorb in the infrared A polar bond is usually IR-active A nonpolar bond in a symmetrical molecule will absorb weakly or not at all Types of Molecular Vibrations: Vibrations fall into the basic categories of stretching and bending. A stretching vibration involves a continuous change in the interatomic distance along the axis of the bond between two atoms. Bending vibrations are characterized by a change in the angle between two bonds and are of four types: scissoring, rocking, wagging, and twisting. Different Types of Vibration
Symmetric Stretch Asymmetric Stretch Scissoring Rocking Wagging Twisting Stretch Vibration or oscillation along the line of the bond
Bend Vibration or oscillation not along the line of the bond H C H C symmetric asymmetric H C H C H C H C twist wag rock scissor out of plane in plane What is the Infrared? Just below red in the visible region
Wavelengths usually m More common units are wavenumbers, or cm-1, the reciprocal of the wavelength in centimeters ( cm-1) Wavenumbers are proportional to frequency and energy Energy required to distort covalent bonds (stretching, compression, wagging, etc.) Light that is absorbed shows up as a peak in absorbance or a dip in transmittance. Absorption of Infrared light in the wavelength range of 2
Absorption of Infrared light in the wavelength range of 2.5x10-6 m to 25x 10-6 m (2.5x10-4 cm to 25x 10-4 cm)=(2.5 m to 25 m) causes bond vibrations Specific wavelengths (or frequencies) can be correlated with specific functional groups Wave Number => (cm-1) => proportional to energy n Remember Energy is inversely proportional to wavelength but proportional to wavenumber n Number of possible modes (1) If Linear molecule: 3N 5
Vibrational Modes Number of possible modes (1) If Linear molecule: 3N 5 Example: Carbon dioxide is a linear molecule and thushas 3 x 3 5 = 4 modes (2) If Nonlinear molecule: 3N 6 Example : Triatomic molecule such as water, sulfur dioxide, or nitrogen dioxide have 3 x 3 6 = 3 vibrational modes Infrared Instruments An infrared spectrophotometer is;
an instrument that passes infrared light through an organic molecule and produces a spectrum that contains a plot of the amount of light transmitted on the vertical axis against the wavelength of infrared radiation on the horizontal axis. *In infrared spectra the absorption peaks point downward because the vertical axis is the percentage transmittance of the radiation through the sample. Absorption of radiation lowers the percentage transmittance value. Since all bonds in an organic molecule interact with infrared radiation, IR spectra provide a considerable amount of structural data. How does it work? A molecule is exposed to IR.
It starts to undergo distortions based on the types of bonds present. The spectrum represents the presence of chemical groups (Functional groups). The IR Spectrum Each stretching and bending vibration occurs with a characteristic frequency as the atoms and charges involved are different for different bonds The y-axis on an IR spectrum is in units of % transmittance In regions where the EM field of an osc. bond interacts with IR light of the same n transmittance is low (light is absorbed) In regions where no osc. bond is interacting with IR light, transmittance nears 100% The x-axis of the IR spectrum is in units of wavenumbers, n, which is the number of waves per centimeter in units of cm-1(Remember E = hn or E = hc/l) How are these interpreted?
The spectrum is divided into two distinct sections. 4000cm-1 to 1300cm-1 is the functional group region. 1300cm-1 400cm-1 is the fingerprint region. Bands in these areas can be used as evidence of certain functional groups. Fingerprint region tends to have small bands, that are difficult to individually resolve. Summary of IR Absorptions
Bonds to H Triple bonds Double bonds Single Bonds CC 2200 cm-1 (weak or absent if internal)
The IR Spectrum The detection of different bonds 1-Lighter atoms will allow the oscillation to be faster higher energy. This is especially true of bonds to hydrogen C-H, N-H and O-H 2-Stronger bonds will have higher energy oscillations Triple bonds > double bonds > single bonds in energy C-C cm-1 C=C cm-1 CC cm-1(weak or absent if internal) 3-As opposed to chromatography or other spectroscopic methods, the area of a IR band (or peak) is not directly proportional to concentration of the functional group producing the peak 4-The intensity of an IR band is affected by two primary factors: a.Whether the vibration is one of stretching or bending b. Electronegativity difference of the atoms involved in the bond c. For both effects, the greater the change in dipole moment in a given vibration or bend, the larger the peak. d. The greater the difference in electronegativity between the atoms involved in bonding, the larger the dipole moment, Hence polar bonds such as O-H and C=O give more intense absorptions than non-polar C-H, C-C and C=C bonds. e. Typically, stretching will change dipole moment more than bending Infrared Group Analysis
Since most types of bonds in covalent molecules have roughly the same energy, i.e., C=C and C=O bonds, C-H and N-H bonds they show up in similar regions of the IR spectrum Remember all organic functional groups are made of multiple bonds and therefore show up as multiple IR bands (peaks) Infrared Group Analysis How to Analyze an IR Spectrum;Look for whats there and whats not there
C-H absorption The wavenumber will tell you sp3(C-C), sp2(C=C), sp (CC) and perhaps aldehyde. Carbonyl (C=O) absorption Its presence means the compound is an aldehyde, ketone, carboxylic acid, ester, amide, anhydride or acyl halide. Its absence means the compound cannot be any of the carbonyl-containing compounds. O-H or N-H absorption This indicates either an alcohol, N-H containing amine or amide, or carboxylic acid. CC and CN absorptions Be careful: internal triple bonds often do not show up in IR spectra. C=C absorption Can indicate whether compound is alkene or aromatic. EXAMPLE 2-aminopentane Amines - Primary Shows the N-H stretch for NH2 as a doublet between cm-1 (symmetric and anti-symmetric modes) EXAMPLE pyrrolidine Amines Secondary N-H band for R2N-H occurs at cm-1 as a single sharp peak weaker than O-H Tertiary amines (R3N) have no N-H bond and will not have a band in this region Examples of Infrared Spectra O-H Stretch of a Carboxylic Acid
This O-H absorbs broadly, cm-1, due to strong hydrogen bonding Carbonyl Stretching Exact absorption characteristic of type of carbonyl compound The C=O bond of simple ketones, aldehydes, and carboxylic acids absorb around 1710 cm-1 Usually, its the strongest IR signal Carboxylic acids will have O-H also Aldehydes have two C-H signals around 2700 and 2800 cm-1 Practice Q: Identify the chemical species represented in this IR spectra? Types of peaks V-shape peak Broad U-shape peak
-NH bond for 2o amine (R2NH) Broad U-shape peak -OH bond W-shape peak -NH bond for 1o amine (RNH2) Sharp & strong peak C=O at1710 cm-1 Strengths and Limitations
IR alone cannot determine a structure Some signals may be ambiguous The functional group is usually indicated The absence of a signal is definite proof that the functional group is absent Correspondence with a known samples IR spectrum confirms the identity of the compound Web References