What are the main methods of instrumental methods of structure determination, and what structural clues does each provide?
What factor are all instrumental methods of structure determination based upon?
How a molecule responds to the absorption of energy
What is the source of energy that a molecule absorbs during structure determination?
- In NMR, IR, and UV-Vis spectroscopy; electromagnetic radiation
- In Mass Spec, it's a stream of charged particles such as electrons
What dual properties does electromagnetic radiation possess?
#NAME?
What is the energy of a photon, E, directly proportional to?
- Its frequency (v) v= s^-1 Units: Hertz
- Formula: E = hv
- Planck's constant, h = 6.63 x 10^-34 J�s
Electromagnetic radiation travels at the speed of light, c = 3x10^8 m/s, which is equal to?
- Its frequency ,v, multiplied by its wavelength, ?
- Formula: c = v�?
The range of photon energies is called?
The electromagnetic spectrum; characterized by ? of 400 nm (violet) to 800 nm (red)
If a molecule is exposed to electromagnetic radiation and absorbs a photon, how much is a molecule's energy increased by?
An amount equal to the energy of the absorbed photon
What determines whether electromagnetic radiation is absorbed by a molecule?
- The energy of the photon must = the energy diff. between 2 states
- Known as resonance
What is the formula for the different energy states of a molecule before and after absorption?
The change in energy; ?E = E2 - E1 = hv
What is the difference between increasing the kinetic energy of a molecule versus increasing the energies for electronic, vibrational, and nuclear spin states?
#NAME?
Excitation of a molecule from a lower state (E1) to a higher state (E2) requires the addition of energy equal to?
?E; therefore only this frequency is absorbed, all other frequencies are transmitted
The relationship between frequency and absorption is plotted as?
A spectrum consisting of peaks at characteristic frequencies
UV light lies beyond the visible spectrum w/ wavelengths in the?
200-400 nm range
To correct for concentration and path length effects, absorbance (A) is converted to?
Molar absorptivity "?"; pronounced epsilon
What is the formula for molar absorptivity?
? = A/c � l
c = concentration of moles per liter
l = path length in cm
Absorption of UV radiation excites an electron from the HOMO to the LUMO. What orbitals are these respectively?
In alkenes and polyenes, the HOMO is the highest energy bonding ? orbital, and the LUMO is the lowest energy antibonding ? orbital. Known as a ???* transition.
The HOMO-LUMO energy gap and the max ? for the ???* transition varies with?
- The substituents on the double bonds of a compound
- Adding methyl subs to a double bond causes a 5 nm increase in ?
- Extending conjugation increases ? by 36 nm for each additional double bond in UV-Vis
What is required for a compound to be colored?
It must possess some absorption in the visible range
What is the second type of absorption that is important in UV-Vis examination of organic compounds?
- The n??* transition of the carbonyl (C=O) group
- One of the electrons in a lone pair orbital of (O), given the value (n), is excited to a ?* orbital of the C=O group
What does the n??* transition give rise to?
Weak absorption peaks (max ? < 100) in the 270-300 nm range
What is the structural unit associated with an electronic transition in UV-Vis spectroscopy called?
A chromophore
How does mass spectrometry differ from the other instrumental methods of structural determination?
It does not depend on the absorption of electromagnetic radiation but rather examines ions produced from a molecule in the gas phase
Describe the high-energy electron bombardment method of mass spectrometry.
- An electron of an energy of 10 eV collides w/ an organic molecule
- The energy transferred dislodges one of the molecule's electrons and formed into a cation radical
Describe the electron impact ionization of molecule AB
Molecule A:B bombarded w/ high energy e ? ionized by electron into cation radical AB+ w/ odd # of electrons + 2e
The molecular ion has the same mass (less the negligible mass of a single electron) as the molecule from which it is formed
In an electron impact mass spectrometer, a high energy beam of electrons is used to displace an electron from the organic molecule to form a radical cation known as the molecular ion. If the molecular ion is too unstable then it can fragment to give other smaller ions.
Describe the electron-impact mass spectrometer process.
1. Sample bombarded w/ 70 eV electrons creating ions
2. The positively charged ions (the molecular ion as well as fragment ions) directed into analyzer tube surrounded by a magnet
3. Magnet deflects ions from original trajectory into a circular path
4. Radius of circular path depends on mass-to-charge ratio (m/z)
5. Ions of small m/z deflected more than those of larger m/z
6. Ions of a particular m/z can be gathered by varying either the magnetic field strength or the degree to which the ions are accelerated on entering the analyzer
7. Only positive ions detected
Scanning all m/z values gives the distribution of positive ions called?
A mass spectrum, characteristic of a particular compound
The most intense peak of a mass spectrum is called?
- The base peak; assigned a relative intensity of 100
- Ion abundances proportional to peak intensities
-- Reported as intensities relative to the base peak
The base peak of benzene is 78, but has a small peak one mass unit higher than M+ in the mass spectrum of benzene. What is the origin of this peak?
The origin of this peak and all peaks in the mass spectrum of benzene is due to the fact that a single mass spectrum of benzene is actually a superposition of the spectra of the 3 isotopically different benzenes.
B/ all organic compounds contain (C) and most contain (H)...
Similar isotopic clusters will appear in the mass spectra of all organic compounds
Apparent in (Br) and (Cl)
Why do many fragmentations in mass spectrometry proceed? What does the molecular ion base peak look like?
To form a stable carbocation; extremely small compared to base peak
What is the nitrogen rule?
- A molecule w/ an odd number of (N) has an odd molecular weight
- A molecule w/ only C, H, and O or with an even number of (N) has an even molecular weight
How could one tell between different compounds that have the same molecular weight, but different molecular formulas?
Using a high-resolution mass spectrometer that calculates the exact masses of m/z for molecular ions
Any time a ring or a double bond is present in an organic molecule...
Its molecular formula has two fewer (H) atoms than that of an alkane w/ the same # of (C)
The relationship between molecular formulas, multiple bonds, and rings is referred to as:
- The index of hydrogen deficiency for hydrocarbons aka elements of unsaturation, or the sum of the double bonds and rings
- A saturated, straight-chain alkane will have the molecular formula CnHn+2, n = # of (C) in the molecule
Example: If we put a double bond or a ring structure into a hydrocarbon, the additional (C)-(C) bond means that there must be 2 fewer (H) in the molecule. Therefore, the IHD value = 1
Index of Hydrogen Deficiency Concepts
(O) atoms have no effect of the index of hydrogen deficiency
If a compound has an IHD of 1, the compound has one ring or one double bond and can't have a triple bond
If a compound has an IHD of 2, it can have 2 rings, 2 double bonds, one ring and one double bond, or one triple bond
How does one distinguish between rings and double bonds for a compound w/ an unknown molecular structure?
- By using catalytic hydrogenation
- Each double bond consumes 1 mole of (H), rings are unaffected
- e.g. a molecule w/ a (H) deficiency of 5 that takes up 3 moles of (H) must have 2 rings in its structure
A yellow light has a wavelength of 589 nm, what is its frequency, v?
Solve by rearranging the equation c=v x ?
Therefore, v= c/?= 3 x 10^8/589 x 10^-9
Conjugation in alkenes creates?
Longer wavelengths due to more absorption
What is the major ion fragment expected from the MS of ethyl benzene?
- In alkyl benzenes like ethyl benzene, the dominate fragment will be at the benzylic bond due to resonance stabilization
- Fragmentation at the benzylic bond creates the tropylium ion (C7H7+, m/z = 91)
- The base peak in the mass spectrum of alkylbenzenes confirms C-C bond cleavage at the benzylic carbon
Which is more energetic: 235 nm or 325 nm light? Explain.
- The shortest ? has the higher frequency & would be more energetic
- The light at 235 nm would be the more energetic
1-Pentene and 1,4-pentadiene cannot be differentiated on the basis of their ?max values. They can, however, be identified on the basis of their molar absorptivities (?max). Explain.
Both 1-pentene and 1,4-pentadiene have isolated, localized double bonds. But b/ 1,4-pentadiene contains two double bonds and 1-pentene only contains one, the molar absorptivity for 1,4-pentadiene would be double the value for 1-pentene.
For 3-methyloctane, which end of the molecule is more likely to undergo fragmentation? Explain.
#NAME?
1-Pentanol has its base peak at m/z 31, whereas that for 2-pentanol is at m/z 45. Briefly explain.
The m/z peak for 1-pentanol is CH2OH+, whereas the peak at m/z 45 for 2-pentanol is CH3CHOH+.
Is toluene a good solvent for UV Spectroscopy? Explain.
Toluene contains conjugated double bonds and will absorb in the UV range (200-400nm). Therefore, it would not be a good UV spec solvent
True or false, ultraviolet light has longer wavelengths than visible light
FALSE
Mass spectrometry results in the separation of fragments based upon?
Mass to chage (m/z) ratio.
What does nuclear magnetic resonance spectroscopy depend upon?
The absorption of energy when the nucleus of an atom is excited from its lowest energy spin state to the next higher one
What isotopes give the best NMR spectra for structural information?
(1H) and (13C); (1H) is known as proton nuclear magnetic spectrum (1H NMR) and give info on the environments of the (H) in a molecule; a carbon-13 nuclear magnetic spectrum (13C NMR) does the same for (C)
A proton has two spin states of (+1/2) and (-1/2) with no diff. in energy bet. the two nuclear spin states, and the same likelihood to be (+) or (-). Absorption of electromagnetic radiation can only occur when?
When the two spin states have different energies
What is one way to make the two nuclear spin states of a proton (+1/2) or (-1/2) have different energies?
Placing the sample is a magnetic field
The net absorption of electromagnetic radiation to produce a detectable signal in NMR requires what?
That the lower energy parallel spin state be more highly populated than the higher energy antiparallel spin state.
Explain the use of a magnetic field to vary the energies of the nuclear spin states in a proton, thereby allowing electromagnetic radiation to be absorbed.
A spinning proton behaves like a tiny bar magnet and has a magnetic moment associated with it. In the presence of an external magnetic field (Bo), the lower energy quantum numbers align with (Bo) and the higher energies do not. Like North and South poles on a magnet
What is the difference in energy (delta E) between the two nuclear spin states directly proportional to?
The strength of the applied magnetic field (Bo), which is measured in Tesla (T)
Describe the relationship between the frequency of electromagnetic radiation (s^-1 or Hz), the energy difference between nuclear spin states (kJ/mol or kcal/mol), and magnetic field (T)
The frequency of electromagnetic radiation (s^-1 or Hz) is proportional to the energy difference between nuclear spin states (kJ/mol or kcal/mol) which is itself proportional to the strength of the magnetic field (T)
What are the frequencies for (1H) and (13C) @ 4.7T?
1H = 200 MHz
13C = 50.4 MHz
What are the essential features of an NMR spectrometer?
A powerful magnet to align the nuclear spins
A radiofrequency (rf) transmitter as a source of energy to excite a nucleus from its lowest energy state to the next higher one
A way to monitor absorption of rf radiation and display the spectrum
Getting structures from spectra:
General Advice: be it from real spectra of an experimental sample, or in the context of a question.
Use the H-NMR at the last stage since most useful for "assembling" the structure
Get a molecular formula at the earliest possible opportunity and then calculate the index of hydrogen deficiency (IHD).
- MS
determine the molecular weight
identify the presence of isotopes patterns for Cl or Br
- UV
is the system conjugated ?
- IR
identify the functional groups that are present
- 13C-NMR
how many types of carbon ?
what types of carbon ?
- H-NMR
how many types of hydrogen ?
how many of each type ?
what types of hydrogen ?
how are they connected ?
Having completed an analysis of the available spectra,
list the pieces that have been identifed
Check the pieces, the MW, and/or molecular formula and refine the pieces to fit
assemble the pieces paying particular attention to H-NMR chemical shifts and coupling patterns
Once you think you have the answer, check it with the H-NMR very carefully... it is probably the most critical test to pass
How are NMR spectra acquired?
By using pulsed Fourier-transform nuclear magnetic resonance (FT-NMR) spectrometers
True or false? The molecular ion peak is always the most intense peak on the spectrum.
False, the base peak is always the most intense peak on the spectrum
What is the molecular ion?
The radical cation obtained by the molecular loss of an electron
What are fragment ions?
#NAME?
Vibrational frequencies are sensitive to isotopic replacement. The O-H stretching frequency is near 3600 cm^-1, but that of O-D is about 2630 cm^-1. Which are closer in energy, two adjacent O-H or two adjacent O-D vibrational states?
Absorption frequencies given in wave numbers (cm-1) are directly proportional to the energy difference between two adjacent vibrational states. Thus, the energy difference is smaller for O-D stretching (2630 cm^-1) than for O-H stretching (3600 cm^-1).
Ethylene has no peak in its IR spectrum for C=C stretching. Why?
#NAME?
What is the equation for Hydrogen Deficiency Index (HDI)?
# Carbons - � # Hydrogens - � # Halogens + � # Nitrogens + 1
Arrange the following list of compounds in order of increasing intensity of the C=C stretching absorption, assuming identical molar concentrations in the sample cell.
A. CH3CH=CHCH2CH3
B. CH2=CHCH2CH3
C. CH2=CCl2
- Compound B the least intense b/ C1 of the double bond is bonded to two hydrogen atoms
- Compound A is 2nd b/ both (C) double bond atoms are secondary
- Compound C is the most intense b/ of the presence of the electronegative chlorine atoms creating a dipole; tertiary sub
An IR spectrum exhibits a broad peak in the range of 3500 - 3000 cm^-1 and a strong peak at 1710 cm^-1. Which of the following compounds best fit this data? Provide evidence that supports your selection.
C6H5CH2CH2OH
C6H5CH2CO2H
C6H5COCH3
- The broad peak in the range of 3500 - 3000 cm^-1 and a strong peak at 1710 cm^-1 define the characteristic peaks for a carboxylic acid
- The 1st compound is a ROH & wouldn't have a peak at 1710 cm^-1
- The 3rd compound is a ketone and would not have the broad peak near 3500-3000 cm^-1
Explain what features would be present in an IR spectrum of the following compounds that would distinguish one compound from the others?
2- Pentanone
Pentanal
1-Pentanol
- 1st compound is a ketone w/ a peak near 1715 cm^-1 from the C=O
- The 2nd compound is an aldehyde and has a peak at the C=O near 1725 cm^-1, & two weak peaks associated with the aldehyde proton.
- One near 2750 cm^-1, the other around 2850 cm^-1
- The CH stretch in alkyl chains does not usually extend this far to the right.
- The 3rd compound is an alcohol and its spectrum will have the very broad absorption near 3600-3300 cm^-1 and evidence of the carbon/oxygen single bond near 1300-1000 cm^-1.
How would one use IR spectroscopy to distinguish between a primary, a secondary, and a tertiary amine?
All 3 will have different absorptions associated w/ the amine functional group, in the range 3500-3300 cm^-1
- Primary amines have two bands typically 30 cm^-1 apart
- Secondary amines have one weak peak in this region
- Tertiary amines will not have an NH stretch
How could one distinguish between a carboxylic acid and an ester on an IR peak?
- For a carboxylic acid, the O-H group will appear as a broad absorption near 3300 to 2500 cm^-1
- The carbonyl should appear around 1710 cm^-1
- The ester will yield a carbonyl absorption near 1735 cm^-1 and a medium absorption near 1300-1000 cm^-1 from the C-O single bond stretch
How would one distinguish between an aldehyde and a ketone on an IR peak?
- An aldehyde will have 2 weak absorptions near 2850 & 2750 cm^-1
- The carbonyl will appear around 1725 cm^-1
- The carbonyl peak of the ketone will appear around 1715 cm^-1
What are the uses of IR spectroscopy?
ID the presence or absence of functional groups
Determine if a reaction is completed
The IR region that is of most use in spectroscopy falls between around
400 - 4000 cm^-1
Alongside the wavenumber of the absorption (the x- axis on the IR spectrum), the amount of IR radiation absorbed (the y-axis on the IR spectrum) can also be characteristic of the functional group. What is the formula for the vibrational frequency?
- A bond connecting 2 atoms acts rather like a spring connecting 2 masses. When it is stretched or compressed, there is a restoring force that acts to pull the atoms back.
- The vibrational frequency of a bond is given by the formula opposite where k is the bond force constant and m is the effective mass of the atoms
- Bonds which are strong act like springs which are stiff: k is large and so is the vibrational frequency and wavenumber
- When the mass of the atoms is small, the vibrational frequency and wavenumber tends to be high
What are some important concepts of IR vibrational frequency and vibrational wavenumber?
#NAME?
IR absorption of an alkane
C-H
2850 - 2960
Medium Intensity; Sharp
IR absorption of an alkene
0
IR absorption of an alkyne
?C-H
3300
Strong
C?C
2100 - 2260
Medium
Bending vibrations of an alkyl halide
C-Cl
600 - 800
Strong
C-Br
500 - 600
Strong
IR absorption of an alcohol
O-H
3400 - 3650
Strong; Often broad
C-O
1050 - 1150
Strong
IR absorption of an arene
C-H
3030
Weak
C=C
1660 - 2000
Weak
C=C
1450 - 1600
Weak
IR absorption of an amine
N-H
3300 - 3500
Medium; Often broad
C-N
1030 - 1230
Medium
The wavenumber of absorption associated with bond stretching is dependent on what 2 factors?
#NAME?
What are some trends on the IR peak spectrum?
Bonds to Hydrogen (X-H) 2700 - 4000 cm^-1
Triple Bonds 2100 - 2300 cm^-1
Double Bond 1600 - 1850 cm^-1
Single Bonds 400 - 1500 cm^-1
What effect does conjugation have on the IR absorption of the carbonyl group?
Increase in conjugation of a compound = a decrease in wavenumber
Why are some signals stronger in an IR spectrum vs. others?
- B/ they are more efficient at absorbing IR radiation
- Efficiency of a bond depends on the strength of the bond's dipole
- An increased dipole moment = a increase in strength of the signal, and therefore a decrease in signal transmittance
IR absorption of aldehydes and ketones
C=O
1710 - 1750 cm^-1
Strong
C-H stretch of the CH=O group in aldehydes
2700-2900 cm^-1
Medium
Stretching Vibrations of Single Bonds
O-H in alcohols 3200-3600
O-H in carboxylic acids 2500-3600
R2NH 3350-3500
sp C-H 3310-3320
sp^2 C-H 3000-3100
sp^3 C-H 2850-2950
In IR, the greater the masses of attached atoms...
The lower the IR frequency at which the bond will absorb
Why does an IR peak appear broad?
#NAME?
How can a broad hydroxyl peak signal be made sharper?
- By using a very dilute solution of the sample, or acquiring the IR spectra in the gas phase, hydrogen bonding is prevented through lack of molecular contact; concept of Free vs. Hydrogen Bonded -OH groups
- Even in concentrated solution, larger compounds may sterically hinder hydrogen bonding, preventing exchange. In these situations the broad O-H peak is replaced by a sharp signal around 3600 cm-1
What are some great rules of thumbs for the absorption of C-H peaks?
Since most organic compounds have C-H bonds, a useful rule is that absorption in the 2850 to 3000 cm-1 is due to sp3 C-H stretching; whereas, absorption above 3000 cm-1 is from sp2 C-H stretching or sp C-H stretching if it is near 3300 cm-1.
What stretching vibrations will arenes aka aromatic hydrocarbons have?
- 3030 w/ varying peaks - may be several bands from the C-H bonds
- 1600 & 1500 - from C=C bonds in the ring; will give 2 bands, 3 if conjugated
What bending vibrations will arenes aka aromatic hydrocarbons have?
- 690-900 - str to med, C-H bending and ring puckering/bending instead of the aromatic hydrocarbon being coplanar
What stretching vibrations will aldehydes have?
2690-2840(2 bands) med intensity C-H (aldehyde C-H)
1720-1740 if saturated aldehyde
1675 if ?, ?-unsaturation
What bending vibrations will aldehydes have?
1350-1360 str ?-CH3 bending
1400-1450 str ?-CH2 bending
What stretching vibrations will ketones have?
Remember to take into account the various C-H bonds
1710-1720 str C=O (saturated ketone)
1690 str aryl ketone
1745 str cyclopentanone
1780 str cyclobutanone
What bending vibrations will ketones have?
1400-1450 str ?-CH2 bending
1100 med C-C-C bending
What stretching vibrations will carboxylic acids have?
2500-3300 (acids) overlap C-H str O-H (very broad)
1705-1720 (acids) str C=O (H-bonded)
1210-1320 (acids) str O-C (sometimes 2-peaks)
What bending vibrations will carboxylic acids have?
1395-1440 med C-O-H
What stretching vibrations will alkenes have?
3020-3100 med =C-H & =CH2 (usually sharp)
1630-1680 var C=C (symmetry reduces intensity)
1900-2000 str C=C asymmetric stretch
What bending vibrations will alkenes have?
880-995 str =C-H & =CH2
780-850 med (out-of-plane bending)
675-730 med cis-RCH=CHR
What stretching vibrations will alcohols and phenols have?
3580-3650 var O-H (free), usually sharp
3200-3550 str O-H (H-bonded), usually broad
970-1250 str C-O
What bending vibrations will alcohols and phenols have?
1330-1430 med O-H bending (in-plane)
650-770 var-wk O-H bend (out-of-plane)
What stretching vibrations will amines have?
3400-3500 (dil. soln.) wk N-H (1�-amines), 2 bands
3300-3400 (dil. soln.) wk N-H (2�-amines)
1000-1250 med C-N
What bending vibrations will amines have?
1550-1650 med-str NH2 scissoring (1�-amines)
660-900 var NH2 & N-H wagging (shifts on H-bonding)
What stretching vibrations will nitriles have?
2240-2260 Sharp Med
What stretching vibrations will acyl halides have?
1785-1815 str C=O
Conjugation lowers the C=O frequencies here, as with aldehydes & ketones.
What other vibrations will acyl halides have?
- An overtone near 3600 is often observed in concentrated samples.
- A second overtone related absorption is often seen on the low frequency side of the strong C=O absorption
What stretching vibrations will anhydrides have?
Acyclic (2 bands) 1750 & 1820 cm-1
6-membered ring 1750 &1820
5-membered ring 1785 & 1865
Conjugation lowers the C=O frequencies reported here, as with aldehydes & ketones
What other vibrations will anhydrides have?
The two stretching bands are separated by 60 � 30 cm-1, and for acyclic anhydrides the higher frequency (asymmetric stretching) band is stronger than the lower frequency (symmetric) absorption.
Cyclic anhydrides also display two carbonyl stretching absorptions, but the lower frequency band is the strongest.
One or two -CO-O-CO- stretching bands are observed in the 1000 to 1300 cm-1 region.
What stretching vibrations will esters and cyclic esters/lactones have?
Esters 1740 cm � 10 cm-1 C=O
6-membered lactone 1740 cm � 10
5-membered lactone 1765 cm� 5
4-membered lactone 1840 cm � 5
Conjugation lowers C=O frequencies, as with aldehydes & ketones
Strong CO-O stretching absorptions (one ot two) are found from 1150 to 1250 cm-1
What bending vibrations will esters and cyclic esters/lactones have?
This spectrum is typical of saturated acyclic esters. The C=O frequency is higher than that of simple ketones. A C-O stretching band is seen near 1200.
What stretching vibrations will amides and cyclic amides/lactams have?
1� & 2�-amides 1510 to 1700 cm-1 (2 bands) from C=O
3�-amides 1650� 15 (one band)
6-membered lactams 1670 � 10 (one band
5-membered lactams 1700 � 15
4-membered lactams 1745 � 15
The effect of conjugation is much less than for aldehydes & ketones.
The higher frequency absorption (1665� 30) is called the Amide I band. The lower frequency Amide II band (1620� 30 in 1� amides & 1530� 30 in 2� amides) is largely due to N-H bending trans to the carbonyl oxygen. In concentrated samples this absorption is often obscured by the stronger amide I absorption. Hydrogen bonded association shifts some of these absorptions, as well as the prominent N-H stretching absorptions.
N-H stretch: 3170 to 3500 cm-1. Two bands for 1�-amides, one for 2�-amides.
What other vibrations will amides have?
Primary amides show two strong N-H absorptions (one asymmetric & one symmetric). Strong C=O absorption occurs at lower frequencies than in ketones, due to the extensive p-pi conjugation in amides. There are usually two strong bands in this region, the higher frequency absorption (amide I band) is essentially C=O stretching, the lower frequency (amide II band) is chiefly N-H bending. All these absorptions may display H-bonding association shifts. A C-N stretching band is also seen.
The effect of conjugation is much less than for aldehydes & ketones.
What stretching vibrations will ethers have?
The obvious way to know a molecule is an ether is to see a C-O peak, but no C=O or O-H, since the absence of a C=O or O-H stretch confirms it is not an ester, acid, or alcohol.
The C-O stretch is found between 1000 and 1300.
Aliphalic ethers give one strong asymmetric stretch around 1120, and a very weak symmetric stretch around 850.
Aryl alkyl ethers give two bands around 1250 and 1040, symmetric and asymmetric respectively.
An increase in wavelength correlates to what in wavenumber and frequency?
A decrease in wavenumber and frequency
Why do ketones have a weaker absorption vs. aldehydes?
Since alkyl substituents stabilize the carbocation character of the ionic contributer, ketone carbonyls have slightly lower stretching frequencies, 1715 � 7 cm^-1 vs. aldehydes 1730 � 7 cm^-1
An increase in hydrogen bonding of a solvent will do what to the stretching frequency of aldehydes and ketones?
Hydrogen bonding solvents compared to pure liquid or CCl4 solution spectra will lower these frequencies by 15 to 20 cm^-1
The dipole moment of a structure is increased with?
As noted in the diagram, the dipole moment of this function is increased on stretching (single bond character is greater), and this results in a strong absorption
What 3 factors influence the carbonyl stretching frequency?
1. Conjugation with a double bond or benzene ring
2. Incorporation of the carbonyl group in a small ring (5, 4 or 3-membered)
3. Changing an alkyl substituent of a ketone for an electron releasing or withdrawing group.
How does conjugation with a double bond or benzene ring lower the stretching frequency?
Conjugation extends the dipolar character of the carbonyl group to the double bond (or aromatic ring) so that the beta-carbon atom shares the positive character of the carbonyl carbon. As illustrated by the following resonance equation, this not only explains conjugate addition reactions of nucleophiles, but also suggests that the carbonyl double bond has slightly more single bond character than does an unconjugated function. The bond energy (and force constant) of the conjugated C=O group is correspondingly reduced, and this results in a lower stretching frequency
How does incorporation of the carbonyl group in a small ring (5, 4 or 3-membered), raise the stretching frequency?
Under ideal conditions the carbon atom of a carbonyl group is essentially sp2 hybridized, which implies that the bond angles will be 120� and the C-O sigma bond has 33% s-character. If this group is incorporated in a small ring, the C-CO-C bond angle is reduced to 108� (5-membered ring), 90� (4-membered ring) or 60� (3-membered ring). When this happens, the C-C bonds of the ring assume greater p-character and the C-O sigma bond has correspondingly greater s-character. The double bond of the carbonyl group is therefore shorter and stronger, and exhibits a larger stretching frequency
How does changing an alkyl substituent of a ketone for an electron releasing or withdrawing group change the stretching frequency?
Electron donating substituents on the carbonyl group stabilize the ionic resonance contributor, and increase the single bond character of the C=O bond. The stretching frequency is therefore decreased, as noted in the right hand example below. Electron withdrawing groups have an opposite influence, and increase the stretching frequency of the carbonyl group. Trichloroacetaldehyde (left below) provides a good example.
Primary Amine IR Stretching Vibration Peaks
The N-H stretching absorption is less sensitive to hydrogen bonding than are O-H absorptions. In the gas phase and in dilute CCl4 solution free N-H absorption is observed in the 3400 to 3500 cm-1 region. Primary aliphatic amines display two well-defined peaks due to asymmetric (higher frequency) and symmetric N-H stretching, separated by 80 to 100 cm-1. In aromatic amines these absorptions are usually 40 to 70 cm-1 higher in frequency. A smaller absorption near 3200 cm-1 (shaded orange in the spectra) is considered to be the result of interaction between an overtone of the 1600 cm-1 band with the symmetric N-H stretching band.
C-N stretching absorptions are found at 1200 to 1350 cm-1 for aromatic amines, and at 1000 to 1250 cm-1 for aliphatic amines.
Primary Amine IR Bending Vibration Peaks
Strong in-plane NH2 scissoring absorptions at 1550 to 1650 cm-1, and out-of-plane wagging at 650 to 900 cm-1 (usually broad) are characteristic of 1�-amines.
Secondary Amine IR Stretching Vibration Peaks
Secondary amines exhibit only one absorption near 3420 cm-1. Hydrogen bonding in concentrated liquids shifts these absorptions to lower frequencies by about 100 cm-1. Again, this absorption appears at slightly higher frequency when the nitrogen atom is bonded to an aromatic ring.
The C-N absorptions are found in the same range, 1200 to 1350 cm-1(aromatic) and 1000 to 1250 cm-1 (aliphatic) as for 1�-amines.
Secondary Amine IR Bending Vibration Peaks
A weak N-H bending absorption is sometimes visible at 1500 to 1600 cm-1. A broad wagging absorption at 650 to 900 cm-1 may be discerned in liquid film samples
Tertiary Amine IR Stretching Vibration Peaks
No N-H absorptions. The C-N absorptions are found in the same range, 1200 to 1350 cm-1 (aromatic) and 1000 to 1250 cm-1 (aliphatic) as for 1�-amines.
Tertiary Amine IR Bending Vibration Peaks
Aside from the C-N stretch, these compounds have spectra characteristic of their alkyl and aryl substituents.
spectroscopy
identify new unknown compound
spectroscopy
measures energy diff. between possible states of molecular system by determining frequencies of electromagnetic radiation (light) absorbed by the molecules
spectroscopy possible states
quantized energy levels asso. w/ diff. types of molecular motion like:
molecular rotation
vibration of bonds
nuclear spin transition
electron absorption
spectroscopyq
diff types measure diff. molecular prop => identify presence of specific func groups
spectroscopy advantages
small sample need and can be reused (except mass spec)
Ir spec
measures molecular vibrations, seen as bond stretching, bending, or combo of diff. vib modes.
abs. of IR light
wavelengths of 3000 to 30,000 nm
ir spec
represented on graph we use wavenumber (analog of freq) => 3,500 to 300 cm^-1
Ir spec
light of freq/wavenumbers are absorbed => molecules enter excited vib state
Bond stretching (sym or asym, ir spec)
involves largest change in energy => observed in highest freq. of 4000 to 1500 cm^-1
bending vibrations
observed in lower freq region of 1500 to 400 cm^-1
four types of vibrations
sym and asym bend
sym/asym stretch
Ir spec
can be vibrations that combo bending, stretching and rotating
ir spec
for absorption to be recorded => vibration must result in change in bond dipole moment (sym bonds = silent)
ir spec
to get -> pass IR light (4000 to 400 cm^-1) through sample=> record abs pattern
ir spec
plotted percent transmittance verus. freq.
percent transmittance
equal abs - 1 (max abs at bottom valleys of spectrum)
alkanes abs freq
2800-3000, (C-H)
1200 (C-C)
alkenes
3080-3140 (=C-H)
1645 (C=C)
alkynes
2200 (C?C)
3300 (?C-H)
aromatic
2900-3100 (C-H)
1,475-1625 (C-C)
alcohols
3100-3500 (O-H) (broad)
ethers
1,050-1150 (C=O)
aldehydes
2700-2900 (O)C-H
1725-1750 (C=O)
ketones
1700-1750 (C=O)
acids
1700-1750 (C-O)
2900-3300 O-H (broad)
amines
3100-3500 (N-H) (sharp)
infrared spec
used to ID func. groups
most important are alcohols and carbonyls
IRC
info from freq between 1400 and 4000 cm^-1 (anything lower doesn't matter)
NMR
based on fact that nuclei have magnetic moments that are oriented at random
NMR
when nuclei placed in magnetic field => this tend to align either w/ or against direction of applied force => nuclei then irradiated w/ radio freq. pulses that match energy gap between two states => excite some lower-energy nuclei into ?-state => abs of this radiation leads to excitation at diff. frequencies, depending on atom's magnetic environment
? -state
lower energy, nuclei whose mag. moments are aligned w/ filed
?-state
those whose mag are aligned against field; higher energy
NMR
nuclear magnetic moments of each atom are affected by nearby atoms that also possess magnetic moments, hence a compound may contain many nuclei that resonate at diff. frequencies, producing a complex spectrum
NMR
plot of freq. vs. absorption of energy during resonance
NMR
standarized method of plotting is chemical shift
chemical shift
parts per million of spec freq
chemical shift
increases towards the left (downfield)
TMS = 0 ppm (reference peak)
has magnetic momenet
nuclei w/ odd mass or odd atomic numbers, or both, will have this when placed in magnetic field
H NMR
each distinct set of nuclei gives rise to a separate peak
H NMR
if multiple identical nuclei in relatively identical locations => magnetically equivalent => all resonate at same frequency
H NMR
greater number of protons => taller peak
deshielding (H NMR)
atoms pull electron density away from surrounding atoms => less can shield itself from apparent magnetic field => downfield
shield (H NMR)
EDG helps to do this => further upfield
coupling (H NMR)
when two protons are in close proximity (two C's away, not magnetically identical => this occurs
doublet, triplets, and multiplets
number of peaks that follow n + 1 rule
n = number of protons
coupling constant, J
magnitude of this splitting, measured in Hertz
splitting of peaks
represents number of adjacent hydrogens
Proton NMR
1. number of protons and relative chem environments
2. shows how many adjacent protons by splitting patterns
3. certain func groups
RCH3
0.9
RCH2
1.25
R3CH
1.5
2CH5CH
4.6-6.0
2C; CH
2.0-3.0
Ar2H
6.0-8.5
2CHX
2.0-4.5
2CHOH/2CHOR
3.4-4.0
RCHO
9.0-10.0
RCHCO2
2.0-2.5
2CHCOOH/2CHCOOR
2.0-2.6
2CHOH2CH2OH
1.0-5.5
ArOH
44534
2COOH
10.5-12.0
2NH2
1.0-5.0
NMR Spec
1. # of nonequivalent nuclei, determined from number of peaks
2. mag. environment of nucleus, determined by chemical shift
3. relative numbers of nuclei, determined by integrating peaks areas in H NMR
4. number of neighboring nuclei, determining by splitting pattern
13C NMR
signal occurs 0 210 chem shift downfield from carbon peak of TMS
13C NMR
1. large sample size need (x50 mg)
2. coupling generally not observed
13C NMR
however, couplng is observed between C atoms and protons that are directly attached
(spin decoupling) 13C NMR
ability to record a spectrum w/o coupling of adjacent protons
(spin decoupling) 13C NMR
produces a spectrum of singlets. each corresponding to a separate, magnetically equivalent carbon atom
13NMR
number of 13C resonances will gove us carbon count in molecule
UV spec
study compounds w/ DBs and/or hetero atoms w/ lone pairs that create conjugated systems
UV spec
obtained by passing UV light through sample (usually dissolved in inert, nonabsorbing solvent) => absorbance plotted against wavelength
UV spec
absorbance cause by electronic transitions between orbitals
UV spec
info we get is wavelength of max absorbance => tells extent of conjugation w/in conjugated systems, as well as other structural and compositional info
UV spec
more conjugated system -> lower the energy of transitions
mass spec: basic theory
destructive tech since destroys compound and can't reuse
mass spec: basic theory
high-speed beam of electrons to ionize the sample (eject electron) => particle acc. to put charged particles in flight => a magnetic field to deflect acc. cationic frag => detector records number of particles of each mass that exits deflector area
molecular radical-cation (M+)
initially formed ion in mass spec, resulting from a single electron being removed from a molecule of sample
molecular radical-cation (M+)
unstable species => decompses rapidly into cationic frag and rad. frag.
mass spec: basic theory
since many molecules in sample, this is composed of many lines, each corresponding to speciifc mass/charge ratio (m/z)
mass spec: basic theory
mass/charge - horizontal
relative abundance of various cationic fragments - vertical axis
characteristics of mass spec = tallest peak (highest intensity)
belongs to most common ion called the base peak and is assigned relative abundance value of 100 percent
characteristics of mass spec = tallest peak (highest intensity) = highest m/z ratio (peak furtherest to right)
is generally molecular ion peak or M+(parent ion peak)
molecular ion peak or M+(parent ion peak)
since one electron missing:
1. charge value is usually 1 and the m/z ratio usually can be read as mass of fragment itself
mass spec application
frag pattern compounds => ID or distinguish certain compounds/clues to compound's structure by way of molecular mass
mass spec application
shows weight of molecule w/ a diff frag missing
carbonyls
sharp peak at 1,700 cm^-1
hydroxides
broad peak at 3300 cm^-1
amines
sharper peaks at 3300 and 3400 cm^-1 for primary amines; secondary amines have one peak.
mass spec.
m/z(really just mass) vs. intensity