IR Flashcards
What effect does the force constant (k) have on IR frequency?
As the k value increases, the vibrational frequency increases as well.
Stronger bonds have larger k values
What effect does the reduced mass (mew) have on IR frequency
As mew increases, the frequency DECREASES.
Bonds between heavier atoms have larger mew values.
How does the frequency of C-H bending compare to C-H stretching.
Generally, C-H stretching has a higher frequency (3000) than C-H bending (1340)
How does hybridization affect k
Stronger bonds result in stronger k. So, sp hybridized > sp2> sp3
3300 > 3100 > 2900
What affect does resonance have on k?
Resonance LOWERS k which lowers the absorption frequency
What is the inductive effect and how does it affect vibrational frequency?
The inductive effect is essentially where electrons WANT to move (high concentration to low) and it INCREASES frequency
What is the resonance effect and how does it affect vibrational frequency?
The resonance effect essentially creates a formal charge on independent ions in the molecule which LOWERS frequency.
What is the IHD (Index of Hydrogen Deficiency)?
It is essentially how many Hydrogen atoms we need to remove to make it a saturated compound.
What effect does ring size have on frequency?
As ring size gets smaller, the frequency increases.
Can IR distinguish trans and cis alkenes?
Yes. Trans alkenes will have a signature double peak instead of a cis single peak in the 1000 region (where alkene double bonds are found)
How does conjugation affect C=O bonds?
Conjugation lowers the frequency
