IR, NMR, and MS CHEM 315 Lab 8

Molecular Structure and Spectra The most powerful and efficient methods currently in use to characterize the structure of organic molecules involve absorption and emission spectroscopic and spectrometric techniques

Molecular Structure and Spectra Absorption Spectroscopy involves the detection and study of electromagnetic radiation absorbed by an organic molecule. The absorbed radiation can be from almost any part of the electromagnetic spectrum, but the most important areas for Organic Chemistry are the infrared (IR), Ultraviolet/visible (UV/VIS), and radio frequency (RF) regions

Molecular Structure and Spectra In Emission Spectroscopy, electromagnetic radiation emitted by the molecule is detected and interpreted to provide structural information

The Electromagnetic Spectrum

Test Questions Past tests have all included questions related to spectroscopic characterization concepts – Given spectrum, choose the correct molecule – Given molecule, choose the correct spectrum

IR Absorption Spectroscopy The data obtained from IR is usually used to identify functional groups associated with the compound being studied Radiation from the middle of the infrared spectrum is used

IR Absorption Spectroscopy Energies of photons within this region correspond to energy differences between vibrational states in covalently bonded molecules. Also called Vibrational Spectroscopy

IR Absorption Spectroscopy Vibrational states are quantized, so molecules will only absorb specific energies. When IR radiation is absorbed, molecules will vibrate at a faster rate as if they were connected by springs

Vibrational modes There are two types of vibrational modes: – Stretches: bond length changes – Bends: bond angles change

Vibrational modes There are two types of stretches, symmetric and assymmetric There are four types of bends, rocking, scissoring, wagging, and twisting

IR Spectroscopy Not all possible vibrational modes absorb IR Only those that change the dipole of the molecule during movement are IR active. The symmetrical stretch of O=C=O is an example of IR-inactive vibrations

IR spectrum Plotted as percent transmittance (%T) vs Frequency. Frequency is typically given in reciprocal wavelengths, or wave numbers (cm -1 ) The typical wave number range is cm -1 The highest energy vibrations are on the left, near 4000 cm -1

Example

Intensity of the bands Three principal factors: – Concentration of compound – Bond Polarity – Level of asymmetry

Absorption Frequencies Frequencies of specific IR bands are affected by any factor that affects vibrational energy Examples: – Stretching > Bending – Shorter, stronger bonds require higher energy – Triple bond> double bond> single bond

Absorption Frequencies Another important factor involves inductive effects. Hydrogen Bonding must also be considered

Important Bands

Fingerprint Region Most bending frequencies are found which overlaps significantly with single bond stretching and combination vibrational modes. This region known as Fingerprint region.

Examples The following are example spectra of simple compounds containing many of the common functional groups. Make sure to note the common bands of each.

UV/VIS Absoption Spectroscopy Applications of UV/VIS are quite specialized. Primarily used for degree of unsaturation and conjugation in organic compounds

Saturated/Unsaturated Molecules The terms saturated and unsaturated refer to the hydrogen content of organic compounds. – Saturated compounds contain the maximum possible number of hydrogen atoms – Unsaturated compounds contain fewer hydrogens because of multiple bonds, rings, and other functional groups

Degree of Unsaturation (DU) DU can be calculated using the following: DU = 2C N – (H + X) 2 Where: C= # carbons, N=# nitrogens, H=# hydrogens, X=# Halogens

Conjugated double bonds Conjugations refers to a specific arrangement of double bonds in a molecule A compound is conjugated when its double bonds alternate with single bonds. Ex.

Range of Radiation The wavelength (λ) in UV/VIS is commonly given in nanometers (nm) Range most important for molecular structure is relatively narrow, falling roughly between 200nm and 500nm – Range starts in near UV and overlaps into the violet/blue region.

What happens Energy of UV/VIS much higher than IR Causes a change in electron configuration – Promotes electrons from bonding (ψ) to antibonding (ψ * ) molecular orbitals The energy absorbed during this transition (called π→π * transition) is measured and interpreted for structural information

Spectra

Estimated Degree of Conjugation In general, the longer the conjugated double bond system, the lower energy required thus the longer wavelength absorbed. Most λmax absorption peaks are found between 200 and 400 nm.

Special Cases When conjugation system is really long, for example β-carotene with 11 conjugated double bonds and a λmax=455 nm, the absorption may fall in the visible part of the spectrum and appear colored to the human eye

General Rules For every conjugated double bond, the absorption wavelength is increased by about 30-40nm. Additional isolated double bonds do not increase the wavelength Alkyl groups add about 5nm to absorption

Mass Spectrometry Bombard molecule with high energy electron beam, dislodges an electron creating a radical cation. Destructive technique because the radical cation immediately starts to break apart, called fragmentation. Energy is not measured, only radical cation and pieces

Mass Spectrometry Most important molecular characterization clues: – Estimate of molecules mass – Molecules fragmentation pattern Pattern is usually unique and can sometimes be compared to database.

The Spectrum X axis is a mass to charge ratio – Charge typically +1, so basically just mass Y axis is relative intensity – Most abundant, called base peak, is assigned 100% – Others assigned relative to base peak

How it works – Original radical cation referred to as Molecular Ion – Molecular Ion immediately fragments – Some fragments are cations or radical cations – Species sorted by mass using strong magnetic field which deflects charged species paths – Neutral species not detected

Three important features Base peak – Most abundant, most stable fragment Molecular Ion Peak – Usually first significant peak from right, highest m/z ratio. – Represents molecular weight of compound – M+1 peak, next to MI peak, represents isotopes

Three important features Fragmentation pattern – Provides clues to types of functional groups present M-15= methyl M-18= water M-29= ethyl M-35= Cl M-43=propyl

Example

Proton NMR The most important spectroscopic tool currently available for the characterization of organic compounds is NMR. Several kinds of NMR are commonly used but most tests questions focus on proton ( 1 H) NMR

Proton NMR Uses Radio Frequency (RF) radiation Absorptions not caused by energy changes of electrons, instead caused by radiation absorbed by nuclei Not all nuclei can absorb, but H and C can which is why it is so useful

How it work – Spins of protons (hydrogen nuclei) are randomly orientated, but when exposed to strong external magnetic field, align either parallel or anti-parallel to external field – When a parallel proton absorbs exactly the right energy, supplied by RF radiation, it flips to anti-parallel, and absorption is recorded

The spectrum Y axis represents relative intensity of absorption X axis is a ppm scale (δ) Peak positions, called chemical shift, are plotted from high frequency (downfield) on left to low frequency (upfield) on right

The spectrum

Four Features of NMR 1)Number of Absoption signals (peaks) - will get 1 signal for each set of Equivalent Hydrogens Examples:

Four Features of NMR 2) Chemical Shift – This is the position of the peak and depends on the electron environment surrounding the nuclei -Next slide shows the relative chemical shift ranges for protons associated with the functional groups most commonly seen on tests.

Four Features of NMR 3) Integral – The area under each peak will correspond to the amount of energy absorbed, thus corresponds to the number of nuclei absorbing – By comparing the ratios of the areas, we obtain the ratios of nuclei those peaks represent. – Integrals can be shown multiple ways

Four Features of NMR 4) Signal Splitting – The type of peak will depend on the number of adjacent hydrogens – n + 1 rule: the type of peak will be the number of adjacent hydrogens, n, plus 1 1= singlet (s)4=quartet (q) 2= doublet (d)5= pentets (p) 3= triplet (t) 6+= multiplet (m)

Common Splitting Patterns