electronic spectroscopy of molecules. regions of electromagnetic spectrum radio-waves region...

Electronic Spectroscopy of molecules

Regions of Electromagnetic Spectrum

Radio-wavesRegion

MicrowavesRegion

Infra-redRegion

VisibleRegion

Ultra-violetRegion

X-ray Region

-ray Region

Frequency (HZ)

106 - 1010 1010 - 1012 1012 - 1014 1014 - 1015 1015 - 1016 1016 - 1018 1018- 1020

Wavelength 10m – 1 cm 1 cm – 100µm 100µm – 1µm 700 – 400 nm 400-10 nm 10nm – 100pm

100pm – 1 pm

NMR, ESR RotationalSpectroscopy

Vibrational spectroscopy

ElectronicSpectroscopy

ElectronicSpec.

Energy 0.001 – 10 J/mole

Order of some 100 J/mole

Some 104

J/moleSome 100kJ/mole

Some 100skJ/mole

107- 109

J/mole109- 1011

J/mole

Frequency ()

Wavelength ()

where h = Planck’s constant = 6.624 x 10-34 Joules sec = frequency of electromagnetic radiation in cycle per sec

= c/

where c = velocity of light; = wavelength of electromagnetic radiation

Therefore, E = hc/

But 1/ = = wave number in cm-1

Thus, E = hc

E = h

Energy of lightFrequency of light

Electromagnetic Radiation

Higher frequency () -- Higher Energy -- Lower wavelength

UVX-rays IR-rays RadioMicrowave

Visible

UV Spectroscopy

Longer Wavelength, Lower Energy

Also known as electronic spectroscopy

Involves transition of electrons within a molecule or ion from a lower to higher electronic energy level or vice versa by absorption or emission of radiation falling in the uv-visible range,

Visible range is 400-800 nm Near uv is 200-400 nm Far uv is 150-200 nms

UltraViolet Spectroscopy

Visible rangeUltraViolet range

Longer Wavelength, Lower Energy

150 nm 200 nm

Flame Test for Cations

lithium sodium potassium copper

A flame test is an analytic procedure used in chemistry to detect the presence of certain elements, primarily metal ions, based on each element's characteristic emission spectrum. The color of flames in general also depends on temperature.

Flame Test

1. Electron absorbs energy from the flame goes to a higher energy state.

2. Electron goes back down to lower energy state and releases the energy it absorbed as light.

LightPhoton

Emission of Energy(2 Possibilities)

Continuous Energy Loss Quantized Energy Loss

or

Emission of Energy

Continuous Energy Loss

• Any and all energy values possible on way down

• Implies electron can be anywhere about nucleus of atom

Continuous emission spectra

Quantized Energy Loss

• Only certain, restricted, quantized energy values possible on way down

• Implies an electron is restricted to quantized energy levels

Line spectra

Emission Spectrum

Line Emission Spectrum (Quantized Energy Loss)

Continuous Emission Spectrum

Atomic Spectra of Hydrogen Atom

http://hyperphysics.phy-astr.gsu.edu/hbase/hyde.html

http://hyperphysics.phy-astr.gsu.edu/hbase/hyde.html

Atomic Spectra of Hydrogen Atom by Bohr´s Theory:

n = 1

n = 2

n = 3

n = 4

n = 5

n = 6 n = 7 n = 8

Line Emission Spectrum of Hydrogen Atoms

H2 Emission Spectrum

Line Spectra vs. Continuous Emission Spectra

The fact that the emission spectra of H2 gas and other molecules is a line rather than continuous emission spectra tells us that electrons are in quantized energy levels rather than anywhere about nucleus of atom.

Regions of Electromagnetic Spectrum

Pure rotational spectra: Permanent electric dipole moment – Fine StructureIR or Vibrational Spectra: Change of dipole during motionElectronic Spectra: Changes in electron distribution in a molecule are always accompanied by a dipole change.

ALL MOLECULES DO GIVE AN ELECTRONIC SPECTRUM AND SHOW VIBRATIONAL AND ROTATIONAL STRUCTURE IN THEIR SPECTRA FROM WHICH ROTATIONAL CONSTANTS AND BOND VIBRATION FREQUENCIES MAY BE DERIVED.

The Born-Oppenheimer Approximation:

Different Types of Molecular Energy in Electronic Spectra

A change in the total energy of a molecule may then by written,

Origin of Electronic Spectra

In the ground state electrons are paired

If transition of electron from ground state to excited state takes place in such a way that spins of electrons are paired, it is known as excited singlet state.If electrons have parallel spins, it is known as excited triplet state.


Excitation of uv light results in excitation of electron from singlet ground state to singlet excited state

Transition from singlet ground state to excited triplet state is forbidden due to symmetry consideration

UV Spectrum of Isoprene

Types of Electrons in Molecules

The lowest energy transition (and most often obs. by UV) is typically that of an electron in the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO).

For any bond (pair of electrons) in a molecule, the molecular orbitals are a mixture of the two contributing atomic orbitals; for every bonding orbital “created” from this mixing (, ), there is a corresponding anti-bonding orbital of symmetrically higher energy (*, *)

The lowest energy occupied orbitals are typically the likewise, the corresponding anti-bonding orbital is of the highest energy

-orbitals are of somewhat higher energy, and their complementary anti-bonding orbital somewhat lower in energy than *.

Unshared pairs lie at the energy of the original atomic orbital, most often this energy is higher than or (since no bond is formed, there is no benefit in energy)

Possible Electronic Transitions

Observed electronic transitions: graphical representation

Energy

nAtomic orbitalAtomic orbital

Molecular orbitals

Occupied levels

Unoccupied levels

The difference in energy between molecular bonding, non-bonding and anti-bonding orbitals ranges from 125-650 kJ/mole

This energy corresponds to EM radiation in the ultraviolet (UV) region, 100-350 nm, and visible (VIS) regions 350-700 nm of the spectrum

A AMO

- Single bonds are usually too high in excitation energy for most instruments (185 nm) -- vacuum UV.

Types of electron transitions:

i) , , n electrons

UV Spectroscopy

Sigma () – single bond electron

MO’s Derived From the 2p Orbitals

y

y

Pi () – double bond electron

Low energy bonding orbital (π)High energy anti-bonding orbital (π*)

Non-bonding electrons (n): don’t take part in any bonds -- neutral energy level.

Example: Formaldehyde

UV Spectroscopy

Observed electronic transitions:

From the molecular orbital diagram, there are several possible electronic transitions that can occur, each of a different relative energy:

Energy

n

n

n

alkanes

carbonyls

unsaturated cmpds.

O, N, S, halogens

carbonyls

UV Spectroscopy

Possible Electronic Transitions

30

Observed electronic transitions:• Although the UV spectrum extends below 100 nm (high energy),

oxygen in the atmosphere is not transparent below 200 nm

• Special equipment to study vacuum or far UV is required

• Routine organic UV spectra are typically collected from 200-700 nm

• This limits the transitions that can be observed:

n

n

alkanes

carbonyls

unsaturated cmpds.

O, N, S, halogens

carbonyls

150 nm

170 nm

180 nm √ - if conjugated!

190 nm

300 nm √

UV Spectroscopy

UV Spectrum of Isoprene

Selection Rules

Not all transitions that are possible are observed

For an electron to transition, certain quantum mechanical constraints apply – these are called “selection rules”

For example, an electron cannot change its spin quantum number during a transition – these are “forbidden”

Other examples include:• the number of electrons that can be excited at one time• symmetry properties of the molecule• symmetry of the electronic states

To further complicate matters, “forbidden” transitions are sometimes observed (albeit at low intensity) due to other factors.....

UV Spectroscopy

Instrumentation – Sample Handling

In general, UV spectra are recorded solution-phase

Cells can be made of plastic, glass or quartz

Only quartz is transparent in the full 200-700 nm range; plastic and glass are only suitable for visible spectra

Concentration is empirically determined

A typical sample cell (commonly called a cuvet):

UV Spectroscopy

Instrumentation – Sample Handling

Solvents must be transparent in the region to be observed; the wavelength where a solvent is no longer transparent is referred to as the cutoff

Since spectra are only obtained up to 200 nm, solvents typically only need to lack conjugated systems or carbonyls

Common solvents and cutoffs:

acetonitrile 190chloroform 240cyclohexane 195 1,4-dioxane 21595% ethanol 205n-hexane 201methanol 205isooctane 195water 190

UV Spectroscopy

35

The Spectrum

The x-axis of the spectrum is in wavelength; 200-350 nm for UV, 200-700 for UV-VIS determinations

Due to the lack of any fine structure, spectra are rarely shown in their raw form, rather, the peak maxima are simply reported as a numerical list of “lamba max” values or max

max = 206 nm 252

317376

O

NH2

O

UV Spectroscopy

Wavelength (nm)

Ab

s

When a beam of monochromatic radiation is passed through a solution of an absorbing medium, the rate of decrease of intensity of radiation with thickness of the absorbing medium is directly proportional to the intensity of incident radiation as well as the concentration of the solution........

l = width of the cuvette

A = lc = log I0/I

Where A is absorbance is the molar absorbtivity with units of L mol-1 cm-1 l is the path length of the sample (typically in cm). c is the concentration of the compound in solution, expressed in mol L-1

Beer-Lambert Law:

= intensity of the incident light

= intensity of the transmitted lightIncident light Transmitted light

A = log (Original intensity/ Intensity)

% T = log (Intensity/ Original intensity) x 100

The Spectrum

The y-axis of the spectrum is in absorbance, A

From the spectrometers point of view, absorbance is the inverse of transmittance:

A = log10 (I0/I) or log10 (I/I0)

From an experimental point of view, three other considerations must be made:

a longer path length (l ) through the sample will cause more UV light to be absorbed – linear effect

the greater the concentration (c) of the sample, the more UV light will be absorbed – linear effect

some electronic transitions are more effective at the absorption of photon than others – molar absorptivity, this may vary by orders of magnitude…

UV Spectroscopy

A = lc = log I0/I

The Spectrum

These effects are combined into the Beer-Lambert Law: A = c l

for most UV spectrometers, l would remain constant (standard cells are typically 1 cm in path length)

concentration is typically varied depending on the strength of absorption observed or expected – typically dilute – sub .001 M

molar absorptivities vary by orders of magnitude:• values of 104-106 are termed high intensity absorptions• values of 103-104 are termed low intensity absorptions• values of 0 to 103 are the absorptions of forbidden transitions

A is unitless, so the units for are cm-1 · M-1 and are rarely expressed

Since path length and concentration effects can be easily factored out, absorbance simply becomes proportional to , and the y-axis is expressed as directly or as the logarithm of

UV Spectroscopy

Observed electronic transitions:

From the molecular orbital diagram, there are several possible electronic transitions that can occur, each of a different relative energy:

Energy

n

n

n

alkanes

carbonyls

unsaturated cmpds.

O, N, S, halogens

carbonyls

UV Spectroscopy: Electronic transitions

n *, * requires unsaturated functional groups most commonly used, energy good range for UV/Vis

~ 200 - 700 nm n * : ~ 10-100 * : ~ 1000 –

10,000

The valence electrons are the only ones whose energies permit them to be excited by near UV/visible radiation.

(bonding)

(bonding)

n (non-bonding)

(anti-bonding)(anti-bonding)

Four types of transitions

*

n*

n*

*

* transition in vacuum UV ( ~ 150 nm)n* saturated compounds with non- bonding electrons

~ 150-250 nm ~ 100-3000 ( not strong)

Definition

Remember the electrons present in organic molecules are involved in covalent bonds or lone pairs of electrons on hetero-atoms such as O or N

Since similar functional groups will have electrons capable of discrete classes of transitions, the characteristic energy of these energies is more representative of the functional group than the electrons themselves.

A functional group capable of having characteristic electronic transitions is called a chromophore (color loving). A Chromophore is a covalently unsaturated group responsible for electronic absorption e.g C=C, C=0, and NO2 etc.

Structural or electronic changes in the chromophore can be quantified and used to predict shifts in the observed electronic transitions.

UV Spectroscopy: Chromophores

Alkanes (CH4, C2H6 etc.) – only posses -bonds and no lone pairs of electrons, so only the high energy * transition is observed in the far UV (or vacuum UV), ~ 150 nm

C C

C C

UV Spectroscopy: Organic Chromophores

Alcohols, ethers, amines and sulfur compounds – in these compounds

the n * is the most often observed transition at shorter value (< 200 nm);

like the alkane * transition also possible.

Note how this transition occurs from the HOMO to the LUMO

CN

CN

nN sp3C N

C N

C N

C N

anitbonding orbital


n* transition lower in energy than σ*

max max

H2O 167 1480

CH3OH 184 150

CH3Cl 173 200

CH3I 258 365

(CH3)2S 229 140

(CH3)2O 184 2520

CH3NH2 215 600

(CH3)3N 227 900


Alcohols, ethers, amines and sulfur compounds

Explain why max and the corresponding max is different.

n* transition - - between 150 and 250 nm.

n* t

ransitio

n

Alkenes and Alkynes – in the case of isolated examples of these compounds the * is observed at 175 and 170 nm, respectively

Even though this transition is of lower energy than *, it is still in the far UV – however, the transition energy is sensitive to substitution


~ 170 - 190 nm

C C

Example: ethylene absorbs at max = 165 nm = 10,000 (intense band)

= hv =hc/

hv


Alkenes

Carbonyls – unsaturated systems incorporating N or O can undergo n * transitions (~285 nm) in addition to *

Despite the fact this transition is forbidden by the selection rules ( = 15), it is the most often observed and studied transition for carbonyls

This transition is also sensitive to substituents on the carbonyl

Similar to alkenes and alkynes, non-substituted carbonyls undergo the * transition in the vacuum UV (188 nm, = 900); sensitive to substitution effects


n * C O

Carbonyls – n * transitions (~285 nm); * (188 nm)

n

σ σ* transitions omitted for clarity

O

O

C O


C O

C O

n

The n* transition is at even longer wavelengths (low energy transition) but is not as strong as * transitions. It is said to be “forbidden.”

Example: Acetone: nmax = 188 nm ; = 1860 (intense

band)nmax = 279 nm ; = 15

hv

n


n

n* and * Transitions

Most UV/vis spectra involve these transitions.

* are generally more intense than n*

max max type

C6H13CH=CH2 177 13000 *

C5H11CC–CH3 178 10000 *

O

CH3CCH3 186 1000 n*

O

CH3COH 204 41 n*

CH3NO2 280 22 n*

CH3N=NCH3 339 5 n*


Chromophore Example Solvent max (nm) max Type of transition

Alkene n-Heptane 177 13,000 *

Alkyne

C5H11CC-CH3

n-Heptane 178

196

225

10,000

2,000

160

*

_

_

Carbonyl n-Hexane

n-Hexane

186

280

180

293

1,000

16

Large

12

n*

n*

n*

n*

Carboxyl Ethanol 204 41 n*

Amido Water 214 60 n*

Azo Ethanol 339 5 n*

Nitro CH3NO2Isooctane 280 22 n*

Nitroso C4H9NO Ethyl ether 300

665

100

20

_

n*

Nitrate C2H5ONO2Dioxane 270 12 n*

C6H13HC CH2

CH3CCH3

O

CH3CH

O

CH3COH

O

CH3CNH2

O

H3CN NCH3

Absorption Characteristics of Some Common Chromophores

Substituent Effects

The attachment of substituent groups (other than H) can shift the energy of the transition

Auxoxhromes:

Substituents that increase the intensity and often wavelength of an absorption are called auxochromes. An auxochrome represents a saturated group, which when attached to a chromophore changes both the intensity as well as the wavelength of the absorption maximum.

Common auxochromes include alkyl (such as -CH3, Et etc), hydroxyl (-OH), alkoxy (-OR) and amino groups (-NR2) and the halogens (such as X = Cl, I etc.)


In General – Substituents may have any of four effects on a chromophore

i. Bathochromic shift (red shift) – a shift to longer ; lower energy

ii. Hypsochromic shift (blue shift) – shift to shorter ; higher energy

iii. Hyperchromic effect – an increase in intensity

iv. Hypochromic effect – a decrease in intensity

200 nm 700 nm

H

yp

oc

hro

mic

Hypsochromic

Hy

pe

rch

rom

ic

Bathochromic

UV Spectroscopy: Substituent Effects

Conjugation – most efficient means of bringing about a bathochromic and hyperchromic shift of an unsaturated chromophore:

H2CCH2

-carotene

O

O

max nm 175 15,000

217 21,000

258 35,000

n * 280 27 * 213 7,100

465 125,000

n * 280 12 * 189 900

UV Spectroscopy: Substituent Effects

electronic spectroscopy of molecules. regions of electromagnetic spectrum radio-waves region...

Documents