13. Structure Determination
13. Structure Determination
The analysis of the outcome of a reaction requires that we know the full structure of the products as well as the reactants In the 1gth and early 20th centuries, structures were determined by synthesis and chemical degradation that related compounds to each other Physical methods now permit structures to be determined directly. We will examine mass spectrometry (Ms) infrared(IR) spectroscopy nuclear magnetic resonance spectroscopy (NMR) ultraviolet-visible spectroscopy (VIs)
• The analysis of the outcome of a reaction requires that we know the full structure of the products as well as the reactants • In the 19th and early 20th centuries, structures were determined by synthesis and chemical degradation that related compounds to each other • Physical methods now permit structures to be determined directly. We will examine: – mass spectrometry (MS) – infrared (IR) spectroscopy – nuclear magnetic resonance spectroscopy (NMR) – ultraviolet-visible spectroscopy (VIS)
13. 1 Infrared Spectroscopy and the Electromagnetic Spectrum Energy Frequency(v)in Hz 10 18 16 10 14 12 100 y rays X rays Ultraviolet Infrared Microwaves Radio waves 10-10 10-6 Wavelength ()in m Wavelength ()in m 380nm 500nm 600nm 700nm780nm 38×10-m 7.8×10-7m Thomson· Brooks Cole
13.1 Infrared Spectroscopy and the Electromagnetic Spectrum
Absorption Spectra Organic compound exposed to electromagnetic radiation, can absorb energy of only certain wavelengths(unit of energy) Transmits, energy of other wavelengths Changing wavelengths to determine which are absorbed and which are transmitted produces an absorption spectrum Energy absorbed is distributed internally in a distinct and reproducible way(See Figure 12-1
Absorption Spectra • Organic compound exposed to electromagnetic radiation, can absorb energy of only certain wavelengths (unit of energy) – Transmits, energy of other wavelengths. • Changing wavelengths to determine which are absorbed and which are transmitted produces an absorption spectrum • Energy absorbed is distributed internally in a distinct and reproducible way (See Figure 12-11)
Wavelength (um) 2.5 4 789101214162024 100 8c 40 40003500300026002200200018001600140012001000 800 Wavenumber(cm-1) 92004 Thomson /Brooks Cole
13.2 Infrared Spectroscopy of Organic molecules iR region lower energy than visible light(below red-produces heating as with a heat lamp) 2.5×106mto25×105 m region used by organic chemists for structural analysis iR energy in a spectrum is usually measured as wavenumber(cm- ), the inverse of wavelength and proportional to frequency pecific IR absorbed by organic molecule related o Its structure
13.2 Infrared Spectroscopy of Organic Molecules • IR region lower energy than visible light (below red – produces heating as with a heat lamp) • 2.5 ´ 10-6 m to 2.5 ´ 10-5 m region used by organic chemists for structural analysis • IR energy in a spectrum is usually measured as wavenumber (cm-1), the inverse of wavelength and proportional to frequency • Specific IR absorbed by organic molecule related to its structure
Ultraviolet Visible Near Infrared Far infrared Microwaves infrared 10-5 10 10 10 (cm) A=2.5×10-4cm A=25×10-3cm =2.5m =25m =4000cm =400cm-1 Energy 92004 Thomson/ Brooks Cole
Infrared Energy Modes IR energy absorption corresponds to specific modes, corresponding to combinations of atomic movements, such as bending and stretching of bonds between groups of atoms called"normal modes Energy is characteristic of the atoms in the group and their bonding Corresponds to vibrations and rotations Symmetric Antisymmetric In-plane Out-of-plane stretching Thomson· Brook
Infrared Energy Modes • IR energy absorption corresponds to specific modes, corresponding to combinations of atomic movements, such as bending and stretching of bonds between groups of atoms called “normal modes” • Energy is characteristic of the atoms in the group and their bonding • Corresponds to vibrations and rotations
Interpreting Infrared Spectra Most functional groups absorb at about the same energy and intensity independent of the molecule they are in Characteristic higher energy ir absorptions can be used to confirm the existence of the presence of a functional group in a molecule IR pectrum has lower energy region characteristic of molecule as a whole fingerprint region) See samples in Figure 12-13
Interpreting Infrared Spectra • Most functional groups absorb at about the same energy and intensity independent of the molecule they are in • Characteristic higher energy IR absorptions can be used to confirm the existence of the presence of a functional group in a molecule • IR spectrum has lower energy region characteristic of molecule as a whole (“fingerprint” region) • See samples in Figure 12-13
Regions of the Infrared Spectrum 40002500cm1NH,C-·20001500cm- double H, O-H(stretching) bonds(stretching) 3300-3600N-H.O-H C=01680-1750 30C-H C=C1640-1680cm 2500-2000 cm-l CoC and Below 1500 cm-l C°N( stretching) Ingerprint region Wavelength (um) 2.5 678910 121416 N一H C=0 C≡N C=N Fingerprint regi C≡C 40003500300026002200200018001600140012001000 Wavenumber (cm-l) cHone· Broo cale
Regions of the Infrared Spectrum • 4000-2500 cm-1 N-H, CH, O-H (stretching) – 3300-3600 N-H, O-H – 3000 C-H • 2500-2000 cm-1 CºC and C º N (stretching) • 2000-1500 cm-1 double bonds (stretching) – C=O 1680-1750 – C=C 1640-1680 cm-1 • Below 1500 cm-1 “fingerprint” region
