Structure Determination: MS, IR, NMR (A review) Dr. Sheppard CHEM 2412 Fall 2014 McMurry (8th ed.) sections 12.1-3, 12.5-8, 13.1-5, 13.7-11, 13.13

Spectroscopy Analytical techniques Help determine structure Destroy little or no sample Light absorbed by the sample is measured as wavelength varies Types: Mass spectrometry (MS) Fragments the molecule and measures the masses Infrared (IR) spectroscopy Measures the bond vibration frequencies in a molecule and is used to determine the functional group Nuclear magnetic resonance (NMR) spectroscopy Number, type and connectivity of atoms in a molecule Ultraviolet (UV) spectroscopy Uses electron transitions to determine bonding patterns (conjugated p systems)

Mass Spectrometry

Mass Spectrometry Used with Gas Chromatography Mixture of compounds separated by gas chromatography, then identified by mass spectrometry Determines MW and provides information about structure A beam of high-energy electrons breaks molecules into ions (fragments) M → M•+ + e- M•+ → A+ + X A+ → B+ + Y etc. Ions are separated and detected; mass determined

The Mass Spectrum Plot relative abundance vs. mass-to-charge ratio Base peak = strongest (most abundant/stable ion) Molecular ion/parent peak (M+) = mass of compound 14 14 15

Mass Spectrum of Hexane 14 14 14 15

Isotopes Present in their usual abundance 81Br Isotopes Present in their usual abundance Hydrocarbons contain 1.1% 13C, so there will be a small M+1 peak If S is present, M+2 will be 4% of M+ If Cl is present, M+2 is one-third of M+ If Br is present, M+2 is equal to M+ If I is present, peak at 127; large gap

Infrared Spectroscopy

IR Spectroscopy Units are wavenumbers (4000-400 cm-1) Measures molecular vibrations No two molecules will give exactly the same IR spectrum (except enantiomers)

IR Spectrum Simple stretching: 1500-4000 cm-1 Baseline Absorbance/Peak Simple stretching: 1500-4000 cm-1 Complex vibrations: 400-1500 cm-1 The “fingerprint region” Interpretation: Looking for presence/absence of functional groups Correlation tables Polar bonds is usually the most IR-active

IR Correlation Table (McMurry 8th ed.)

Hexane , Hexene and Hexyne Spectra

Alcohol and Amine Spectra

Carbonyl Spectra

NMR

NMR Most powerful technique for structure determination Number and type of atoms in a molecule Connectivity of atoms Used to study a wide variety of nuclei: 1H, 13C, 15N, 19F, 31P, etc. Radio-frequency radiation used to transition between energy states (nuclear spin) Spinning nucleus acts as a bar magnet Aligns with or against external field Absorption of light causes spin flip “Resonance” Measured by spectrometer

Nuclei in a Molecule Depending on their chemical environment, atoms in a molecule are shielded by different amounts Chemically equivalent nuclei Interchanged through bond rotation or element of symmetry Have same absorption Chemically different nuclei have different absorption

Chemical Shifts (d scale, in ppm)

TMS = Reference Compound NMR Spectra TMS = Reference Compound

13C-NMR Signal = one sharp line for each different type of carbon The number of different signals indicates the number of different kinds of carbon The chemical shift indicates the functional group Used to support 1H-NMR analysis

1H-NMR More info than 13C-NMR The number of signals How many different kinds of protons are present The location (chemical shift) of the signals Is the proton shielded or deshielded The intensity (integration) of the signal The number of protons of that type Signal splitting (multiplicity) The number of protons on adjacent atoms

1H-NMR Number of Signals One signal for each type of H in a molecule 2 4 3

1H-NMR Chemical Shifts More shielded = upfield (to the right) Less shielded = downfield (to the left)

1H-NMR Integration The signal intensity (area under signal) is proportional to the number of protons giving rise to that signal Shown by integration line Height of vertical line ≈ area under peak ≈ # H’s in set Measure height with ruler or look at graph paper Ratio of height = ratio of hydrogens

1H-NMR Spin-Spin Splitting Signals can be split into multiple peaks The (n+1) rule: A signal is split by n neighboring protons, into (n + 1) peaks

1H-NMR Spin-Spin Splitting Coupling constants (J) Distance between the peaks of a split signal Measured in Hz (usually 0-18) Gives info on type of H

Stereochemical Nonequivalence Usually, two protons on the same carbon are equivalent and do not split each other If the replacement of each of the protons of a -CH2 group with an imaginary “Z” gives stereoisomers, then the protons are non-equivalent and will split each other Results in more complex splitting patterns Examples:

Solving NMR Problems Given: Goal: Determine structure 1H-NMR Molecular formula (typically) IR (sometimes) 13C-NMR (sometimes) Goal: Determine structure If the molecular formula is known: Determine the number of elements of unsaturation Sum of number of rings + p bonds Index of Hydrogen Deficiency IHD = C – ½(H + X) + ½N + 1 Example: C5H3N2O2Cl IHD = 5 – ½(3 + 1) + ½(2) + 1 = 5