Ch 12 - IR & MS

Chemical Ionization

Softer ionization method used when electron bombardment causes excessive fragmentation, helping to preserve the molecular ion peak

Infrared Spectroscopy

A technique used to identify molecular structures by analyzing the absorption of infrared light, revealing functional groups and chemical bonds within a sample, measures absorption of energy ad specific wavelengths corresponding to molecular vibrations

Normal Modes

Specific vibrational patterns of molecules corresponding to combos of atomic movements such as bending & stretching of bonds

Wavenumber

Unit used to measure IR energy, measured in cm-1, inverse of wavelength, proportional to frequency

Conjugation Effect

Lowering of C=O stretching frequency to 1680 cm-1 when carbonyl group is conjugated w/ double bond or aromatic ring

FT-IR Spectrometer

type of infrared spectrometer that uses an interferometer for better sensitivity, faster scanning, improved accuracy through laser calibration

Fingerprint region

The region in an IR spectrum, typically between 400-1500 cm-1, that contains unique absorption patterns characteristic of specific compounds.

Electron Impact Ionization

A method used in mass spectrometry where high-energy electrons collide with a sample, resulting in ionization and fragmentation, forming cation radical

O-H Strecth

A characteristic absorption feature in an infrared spectrum, typically observed in the range of 3200-3600 cm-1 for alcohols, 2500-3500 cm-1 for carboxylic acids due to H bonding

Fragmentation Reactions

the process in mass spectrometry where ions break into smaller pieces, generating daughter ions. This provides structural information about the original molecule, happens after radical cation forms

Carbonyl Stretch

strong, sharp IR absorption between 1670-1780 cm-1, C=O bonds in aldehydes, ketones, esters, carboxylic acids

Base Peak

The most intense peak in a mass spectrum, representing the most stable ion generated during ionization and fragmentation processes, 100% relative intensity

Isotope Peaks

peaks in a mass spectrum that result from the presence of isotopes of the same element, indicating different masses due to variations in neutron count, at M+1 & M+2 positions, heavier isotopes

• C-13

• Cl-37

• Br-81

Nitrogen Rule

Compounds with 0/even numbers of N have even m/z values, odd numbers have odd m/z values

McLafferty Rearrangement

Fragmentation mechanism in carbonyl compounds where a C-H bond 3 atoms away leads to internal proton transfer to C=O group

High Resolution Mass Spectrometry

Mass spec able of separating ions differing in mass as little as 0.0001 amu, distinction between compounds w/ same nominal mass

Mass Spectrometry

An analytical technique used to measure the mass-to-charge ratio of ions, allowing for the identification and quantification of compounds.

Magnetic Deflection

The process of bending the path of charged particles in a magnetic field, often used in mass spectrometry to separate ions based on their mass-to-charge ratio.

Alpha Cleavage

A fragmentation process in mass spectrometry where a bond is cleaved adjacent to a functional group, resulting in the formation of smaller charged fragments, makes stable carbocations

Molecular Ion

Peak in mass spectrum corresponding to unfragmented radical cation, gives molecular weight of compound

T or F: IR, NMR, & UV-VIS are destructive processes

False, only mass spectrometry/MS is

Does MS involve absorption of EM radiation?

No, MS doesn’t involve absorption of EM radiation

Can proteins be sequenced by MS?

Yes, proteins can be sequenced by MS

What is a mass spectrometer designed to do?

1. convert neutral atoms/mol into beam of (+) ions

2. separate ions on basis of their mass to charge (m/z) ratio

3. measure relative abundance of each ion

Which ions are detected in MS?

only (+) charged ions

Why can’t neutral fragments be detected by MS?

Can’t be deflected by magnetic field, are lost througn magnetic deflection

What is the peak that corresponds to unfragmented radical cation?

parent peak

How is molecular weight determined?

From mass of molecular ion

Resolution

measure of how well MS separates ions of diff mass

Low Resolution

Instruments able of separating only ions that differ in nominal mass, by 1+ amu

High resolution

instruments able to separating ions differing in mass as little as 0.0001 amu

How is favorability seen in MS?

Where fragmentation occurs to form new cations, mode that gives the most stable cation is favored

Which ions have the longest lifetimes?

Most stable C+, detected as most abundant ions/strongest peaks in MS