Lecture 2 Intro to Spectroscopy NMR Spectroscopy How it works Chemical Shift in 1H NMR Equivalent Non equivalent Hydrogens Organic Chemistry From Yesterday to Today Late 1700 s Atomic Theory 1800 s Organic Structural Theory Combustion Analysis Functional Group Tests 1900 s Synthesis and Analysis Today Automated Synthesis Spectroscopic Analysis Spectral Analysis Each type of spectral analysis has its value in determining confirming the structure of a compound Spectroscopy allows us to see the molecule NMR Nuclear Magnetic Resonance Spectroscopy Different types of nuclei in a molecule 1H 13C 1H NMR Aids in the determination of bond connectivity within a molecule the pieces of a molecule IR Infrared Spectroscopy Confirms the presence of functional groups within a molecule MS Mass Spectrometry Determines the mass of a compound Also aids in the determination of pieces of the molecule Types of Analysis NMR Nuclear Magnetic Resonance Spectroscopy Uses radio waves electromagnetic radiation Interacts with sample s nuclei in the presence of a magnet Effect nuclei flip and relax known as resonance IR Infrared Spectroscopy IR radiation Interacts with molecule as a whole Effect bond vibrations within molecule MS Mass Spectrometry No radiation used Interacts with and destroys molecule fragments molecule Effect creates ions and neutral fragments of molecule 1 H NMR Spectrum of Ethanol CH3CH2OH ppm Transmittance IR Spectrum of Hexanol Wavenumber cm 1 Mass Spectrum of Phenetole 94 Intensity OCH2CH3 MW 122 122 77 m z mass to charge ratio Nuclear Magnetic Resonance Use To assist in the elucidation of a molecule s structure Information Gained Different chemical environments of nuclei being analyzed 1H nuclei chemical shift The number of nuclei with different chemical environments number of signals in spectrum Determine the number of protons that are adjacent to one another splitting patterns The numbers of protons with the same chemical environment integration Determine how many protons are bonded to the same carbon integration Determine which protons are adjacent to one another coupling constants How does NMR work Basic Idea In the presence of an applied magnetic field B o the NMR instrument 1 Irridate the sample with radiofrequency radiation 2 Nuclei resonance excite magnetic transitions 3 Measure the energy absorbed released by nuclei 4 Obtain a spectrum How does NMR work Facts that allow NMR to work 1 2 3 Nuclei have a spin like electrons Nuclei that have odd mass or odd atomic number have a quantized spin angular momentum and a magnetic moment The allowed spin states a nucleus can adopt is quantized and is determined by its nuclear spin quantum number I H and 13C nuclei have I 1 2 Thus there are two allowed spin states 1 2 and 1 2 1 1 H NMR Spectroscopy 1 The nuclei can either align with 1 2 or oppose 1 2 the applied magnetic field Bo from the NMR instrument When the nuclei absorb the radiofrequency pulse a specific energy is absorbed since the spin states are quantized the spin flips resonance When the pulse is over the spin relaxes back to its original state The spin releases the energy that it had originally absorbed this is the energy that is measured H nuclei have magnetic spin I 1 2 This happens to each 1H nuclei in the sample but not every 1H nuclei are the same How does NMR work Getting a Spectrum Pulse sample with radiofrequency radiation spin flip resonance After pulse the excited nuclei lose their excitation energy and return to their original state relax As the nuclei relax they emit electromagnetic radiation results in free induction decay FID FID contains all emitted frequencies QuickTime and a TIFF LZW decompressor are needed to see this picture Fourier transform FT is performed on the FID FT extracts the individual frequencies on the different nuclei results in a spectrum How does NMR work Higher energy state magnetic field opposes applied field Nuclei are charged and if they have spin they are magnetic Energy of transition energy of radiowaves Lower energy state magnetic field aligned with applied field Applied Magnetic Field Bo An NMR Diagram On the Inside RF transmitter N RF Receiver S Note modern NMRs use superconducting magnets to attain very strong magnetic fields Chemical Shifts Not all proton nuclei resonate at the same frequency Proton nuclei are surrounded by electrons in slightly different chemical environments nuclei are shielded by valance electrons that surround them As a result the nuclei are shielded from B o to an extent that depends on the electron density around it A shielded nucleus will feel a diminished B o and will absorb radiofrequency radiation at a lower frequency have a lower ppm value A deshielded nucleus will feel a stronger B o and will absorb radiofrequency radiation at a higher frequency have a higher ppm value Different nuclei will be shielded differently and as a result will have different resonance frequency different ppm values different chemical shifts Chemical Shifts Protons near an electronegative group will be deshielded feel a stronger Bo have a higher ppm value Electronegative groups OH OR Cl F Br N Other deshielding groups C C phenyl C O N H O H X CH O CH COCH CHO CO2H 12 0 11 0 10 0 9 0 8 0 7 0 C CH C CH ArH CH3 CH CH2 6 0 5 0 4 0 Chemical Shift ppm 3 0 2 0 1 0 0 0 Shielding Deshielding Effect 1 a b H NMR Spectrum of Ethanol c a CH3CH2OH TMS Three signals three different types of H s b c downfield ppm upfield Chemical Shifts TMS Tetramethylsilane Me4Si is the internal reference used TMS s chemical shift is set at zero since most peaks appear more downfield from it The Delta Scale An arbitrary scale 1 1 part per million ppm of the spectrometer operating frequency For example if using an 80 MHz instrument to run a 1H NMR spectrum 1 would be 1 ppm of 80 000 000 Hz or 80 MHz Since the radiofrequency absorption of a nuclei depends on the magnetic field strength chemical shift in Hz would vary from instrument to instrument Thus report the nuclei absorption in relative terms as opposed to absolute terms Hz This way the chemical shifts will be the same for nuclei of a sample despite what instrument you use leads to correlation charts Equivalent Non Equivalent Hydrogens As seen in the 1H NMR spectrum of ethanol the number of signals equals the number of different types of protons in a compound General rules Protons attached to the same sp3 carbon are equivalent homotopic if there are no chiral centers in the molecule if there are could be