Spectroscopy Flashcards
what is infared spectroscopy (IR)?
measures vibrations of bonds to identify functional groups
what is ultraviolet spectroscopy (UV)?
measures electronic transitions to provide information on the electronic bonding in the sample
what is mass spectrometry (MS)?
breaks the molecule into fragments and measures the MW of fragemnts to give information to the structure and functional groups present
what is nuclear magnetic resonance (NMR)?
uses elctromagnetic fields to measure spinning of nuclei to identify chemical environments leading to dentification of functional groups and structure of the sample
what are electromagentic radiations?
travel at the speed of light (c) but at different frequencies and wavelengths
what is frequency (v)?
number of complete wave cycles that pass a fixed point per second, Hz (cycles per second)
what is wavelength (upside down y)?
distance between two peaks
what are photons?
massless packs of energy
E=hv
(h= Planktons constant)
what are the characteristics of the infrared region?
Molecular vibrations can be detected in the infrared region.
The covalent bonds between atoms act like springs. If the bond is stretched /
compressed and then released, the atoms vibrate.
Molecular vibrations can be detected in the infrared region.
The covalent bonds between atoms act like springs. If the bond is stretched /
compressed and then released, the atoms vibrate.
what are IR-Active vibrations?
the bond must have a dipole moment
the vibration must change the dipole moment of the molecule
what are IR-Inactive vibrations?
symmetrical/ has no dipole moment
what are the carbon-carbon bond stretching?
c-c 1200
c=c 1660
c=-c <2200
what are the carbon-hydrogen bond stretching?
c-c-h 2800-3000
c=c-h 3000-3100
c=-c-h 3300
what are the O-H stretching frequencies?
Alcohol (e.g. CH3OH): 3300 cm-1 , Broad
Acid (e.g. CH3COOH): 3000 cm-1 , Broad
what are the N-H stretching frequencies?
Amine: 3300 cm-1 , Broad
1 Spike = N-H of secondary amine (R2NH)
2 Spikes = N-H of primary amine (RNH2)
what are the C-O stretching frequencies?
Alcohols and ethers: 1000 –1200 cm-1
what are the c=o stretching frequencies?
- Intense absorption at 1700 cm-1
- Small overtone peaks at 3400 cm-1
- Aldehyde: 1725 cm-1
- Ketone: 1710 cm-1
what is the stretching frequency of C-H of an aldehyde?
2700 and 2800
what are the C-N stretching frequencies?
c-n 1200
c=n 1660
c=-n >2200
what are the regions of the IR spectrum?
4000-2500= z-H single bond stretches
2500-2000= triple bond stretches
2000-1500=double bond stretches
1500-1000=single bond stretches
1000-400=bending vibrations
what is used for UV/vs spectrophotometry?
- An instrument for irradiating a sample with photons of UV or vis light of particular wavelength/s
- Liquid sample dispensed into a cuvette (solvent not abs. above 200nm) UV – quartz; vis – plastic, polystyrene
- Must be run against a sample blank, (same solution but no drug)
- Measure either:
i. absorption - light not passing through sample
ii. transmission – light passing through sample
what are molecular orbitals?
the region in which there is the highest probability of finding an electron of a molecule
what are HOMOs and LUMOs?
- Highest Occupied Molecular Orbital (HOMO): Highest
energy MO with any electrons in it - Lowest Unoccupied Molecular Orbital (LUMO): The
next highest energy orbital. It will be empty and so is
the lowest energy orbital in which to place an
electron or excite one into.
what happens in the ultraviolet absorption spectroscopy?
- Wavelengths of UV light absorbed by a molecule are
determined by the electronic energy differences
between the HOMO and LUMO. - Absorption is only possible if the molecule contains
two molecular levels separated by energy (Transition
from bonding to anti-bonding orbitals, ᵰ to ᵰ , ᵰ to
σ - The molecule will now be in an “excited state”
- Energy will dissipate rapidly and will return to the
“ground state”
what is lambda max?
wavelength showing the greatest abosorbance
what is the formula for beers law?
A=Ecl
A=absorbance
E= molar absorption coefficient
c=molar conc
l=optical path length
what are chromophores?
structural features of a molecule that absorb specific wavelengths
what are auxochromes?
auxiliary groups attached to a chromophore, unable to absorb, but cause shifts
what are the characteristics of mass spectrometry?
- Provides Molecular Weight
- Provides valuable information about the molecular formula
- Does not use light (different from spectrometry)
- Sample is destroyed
- High-energy electrons fragment the molecules
- Mass of the fragments is measured
- Information used to “reconstruct” the molecule
what are the steps to recieve the mass spectrometry?
- Negatively charged accelerator plate with narrow slit
to allow ions to pass through - Charged particle passes through magnetic field
(bends path) – Curvature of the bend depends on the
m/z ratio (mass / charge) - Slit followed by detector – at any given magnetic
field, only ions of one particular mass are bent exactly
the right amount to pass through the detector. The
detector signal is proportional to the number of ions
striking it
what is the theory of NMR spectroscopy?
- Nuclei of Elemental Isotopes:
- Have a characteristic spin (l)
- Some are integral (1,2,3), some are
fractional (1/2, 3/2, 5/2), some have no spin,
l = 0 (12C, 16O) - The most useful in organic chemistry are
1H, 13C, 19F and 31P, all l = ½
- Spinning Nuclei:
- Create a magnetic moment (μ)
- Can spin in all directions until an external
field is introduced
what is the resonance theory of NMR spectroscopy?
- When a nucleus is subjected to the right combination
of magnetic field and electromagnetic radiation the
spin can flip back and forth - The nucleus is in resonance and this is the energy
detected by NMR
what is shielding?
- Magnetic forcefield exerted by electrons: electrons are circulating and generating a small induced magnetic field that opposes the external field (B0)
- Decrease in the magnetic field felt by the nucleus: this results in a stronger field being needed for resonance to occur
- Electronegative atoms such as oxygen pull electrons away from neighbouring atoms, deshielding them
- Shielded atoms need higher field energy to cause resonance
what are the different chemical shifts variations?
electronegativity (deshielding effect)
distance from electron-withdrawing group
addition of more electron withdrawing groups
other groups with deshelding effects
what is the chemical shift effect of electronegativity?
With increasing electronegativity, there is more deshielding of neighbouring
atoms as the electrons are drawn towards the more electronegative atom.
what is the chemical shift effect of distance from electron-withdrawing group?
The further away an atom is from the electronegative group, the less deshielding
effect is felt by the protons of that atom.
what is the chemical shift effect of addition of more electron withdrawing groups?
If more than one EWG is present, the deshielding
effects are nearly (but not quite) additive.
what is the chemical shift effect of other groups with deshielding effects?
Vinyl (δ5-6) and Aromatic Protons (δ6-9): Electron circulation causes deshielding so lower fields
observed.
Aldehydes (δ9-10): Carbonyl group has deshielding effect
OH and NH: These depend on the concentrations widely
- Concentrated solutions, deshielding caused by H-bonding, NH (δ3.5) and OH (δ4.5)
- Diluted solutions, no H-Bonding, NH and OH (δ2.0)
- Broad peaks are observed due to H-bonding and proton exchange
COOH (δ10+): C=O and O present so even greater deshielding
what does the number of NMR signals present represent?
the number of proton environments
what does chemically equivalent mean?
protons in identical chemical environments
what does the area under the peak represent?
the total number of protons in that environment (ratio)
what is spin-spin splitting?
Protons from different environments can influence the magnetic field of each other if they are significantly close enough to one another. This can result in the splitting of their peaks as seen in the NMR spectrum –Chemically equivalent protons do not split each other
what is the N+1 rule?
If the signal is split by N neighbouring equivalent protons, it will be split into N+1 peaks
how many bonds need to be present for spin-spin splitting to happen?
no more than 3
what are coupling constants?
the distance between the peaks of multipkets measured in Hz
what is homotopic?
if any H are replaced by deuterium (D) nothing changes as the other two groups are equivalents
what is enantiotopic?
if any of the H are replaced
by D the molecule becomes chiral (so they
are called prochiral)
what is diastereotopic?
If either of the prochiral H are replaced by D we create a second chiral centre and the two resulting molecules will be diastereomers (1x S,R,1x R,R).
how to work out the degree of unsaturation?
(2+2a-b+c-e)/2
for a compound formula of CaHbNcOdXe
what is degree of unsaturation also known as?
double bond equivalents
when is a ch3 group further down the proton nmr?
when its closer to an O
what is contained within a fourier transform infared spectrometer?
detector, sample, moving mirror, beam splitter, laser callibration beam, IR sorse, He-Ne laser and fixed mirror
wher do you measure a wavelength?
from crest to crest of a signal
what are the components of plack’s constant?
e= energy of the packet
h= plancks constant (6.626x 10^-34 J.s)
v= frequency (cycles per second)
what is the numerical value of planks constant?
6.626x10^-34 J per second
what is the stretching frequency of C-H?
3000
what is the stretching frequency of c-c?
1200
what is the stretching frequency of c-c?
1200
what is the stretching frequency of c=c?
1660
what is the stretching frequency of c=-c?
2200
what is the stretching frequency of c-n?
1200
what is the stretching frequency of c=n?
1650
what is the stretching frequency of c=-n?
2200
what is the stretching frequency of c-o?
1100
what is the stretching frequency of c=o?
1700
what are the different vibrational moes in infared?
symmetric stretching
antisymetric stretching
bending (scissoring)
what is symmetric stretching?
atoms go away from the central atom
what is antisymmetric stretching?
one atom goes towards the central atom and the other goes away
what is bending (scissoring)?
outer atoms go towards each other with no regard for the central atom
how many vibrational nodes do non-linear molecules have?
3n-6 vibrational modes
(ie, 6 atomes= 12 modes)
what is the name when there are combinations of fundamental vibrations?
complex vibrations
whuat are the rules for IR-active vibrations?
the bond must have a dipole moment
the vibration must change the dipole moment of the molecule
what are the rules for an IR-inactive vibrations?
symmetrical
has no dipole
what are the OH stretching frequencies?
alcohol= 3300 (broad)
acid= 3000 (broad)
what is the c-o stretching frequencies?
alcohol and ethers= 1000-1200
what are the N-H stretching frequencies?
amine= 3300 (broad)
1 spike= secondary amine
2 spikes = primary amine
what can UV/Vis spectrophotometry meaure?
absorption (light not passing through sample)
transmission (light passing through the sample)
what are wavelengths of molecule determined by?
electronic energy differences between HOMO and LUMO
when is absorption only possible?
if the molecule contains two molecular levels seperated by energy
this is then turned into its excited state, then will return to its ground state rapidly
what are chromophores?
structural features of a molecule that absorb specific wavelengthss
what are auxochromes?
auxillary groups attached to a chromophore, unable to absorb, but cause shifts
what are the components of the mass spectrometry machine?
- negatively charged accelerator plate with narrow slit to allow ions to pass through
- charged particles passes through magnetic field- curvature of the bend depends on the m/z ratio
- slit followed by detector- at any given magnetic field, only ions of one particular mass are bent exactly at the right amount to pass through the detector.
what is the rule of 13?
calculation used to determine the molecular formula based on mass
starts with the assumption of only having carbon and hydrogen atoms
then heteroatoms are expored until exact combinations are identified
what are the characteristics of nuclei of elemental isotopes?
have a characteristic spin
some are integral, some are fractional, some have no spin
what are the characteristics of a spinning nuclei?
create a magnetic moment
can spin in all directions until an external field is introduced
why is there a difference where absorption happens?
shielding,
less sheilded protons absorb at lower field
what is the effect of electronegativity in chemical shifts?
with increasing electronegativity, there is more deshielding of neighbouring atoms as the electrons are drawn towards the more electronegative atom
what is the effect of distance from electron withdrawing group on chemical shift?
the further away an atom is from the electronegative group, the less deshielding effect is felt by the protons of that atom
what are the charecteristics of the carbon-13 NMR?
- Determines the magnetic environments of the carbon atoms instead of protons
- 12C (99%) has no magnetic spin
- 13C (1%) does have a magnetic spin – but sensitivity is decreased by x100
- The resonance frequency of 13C is only one fourth of that for 1H NMR (gyromagnetic ratio)
- Hundred of spectra were taken and then averaged (old technique)
- Now we have Fourier-Transform
what is the fourier transform NMR spectroscopy?
- Mathematical technique used to compute a spectrum from the FID
- A pulse is used to introduce non-equilibrium nuclear spin magnetization
- The relaxation of spins to their resonance is time dependent and measured to create
the spectrum
what is the splitting for RCH3?
0-35
what is the splitting for R2CH2?
15-40
what is the splitting for R3CH?
25-50
what is the splitting for R4C?
30-40
what is the splitting for RC=-CR?
65-90
what is the splitting for R2C=CR2?
100-150
what is the splitting for benzene?
110-175
what is the splitting for RCH2Br?
20-40
what is the splitting for RCH2Cl?
25-50
what is the splitting for RCH2NH2?
35-50
what is the splitting for RCH2OH?
50-65
what is the splitting for RCH2OR?
50-65
what is the splitting for R(C=O)OR?
160-185
what is the splitting for R(CO)R?
190-220
