The Spectroscopic Process
- In UV spectroscopy, the sample is irradiated with the broad spectrum of the UV radiation
- If a particular electronic transition matches the energy of a certain band of UV, it will be absorbed
- The remaining UV light passes through the sample and is observed
- From this residual radiation a spectrum is obtained with “gaps” at these discrete energies – this is called an absorption spectrum.
- A. Observed electronic transitions
- 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 (s, p), there is a corresponding anti-bonding orbital of symmetrically higher energy (s*, p*)
- The lowest energy occupied orbitals are typically the s; likewise, the corresponding anti-bonding s* orbital is of the highest energy
- p-orbitals are of somewhat higher energy, and their complementary anti-bonding orbital somewhat lower in energy than s*.
- Unshared pairs lie at the energy of the original atomic orbital, most often this energy is higher than p or s (since no bond is formed, there is no benefit in energy)
- Here is a graphical representation.
8. From the molecular orbital diagram, there are several possible electronic transitions that can occur, each of a different relative energy.
9. 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:
Solvents used for obtaining UV Spectra :
Common solvents and cutoffs:
95% ethanol 205
The UV 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 lmax
Practical application of UV spectroscopy
- UV was the first organic spectral method, however, it is rarely used as a primary method for structure determination
- It is most useful in combination with NMR and IR data to elucidate unique electronic features that may be ambiguous in those methods
- It can be used to assay (via lmax and molar absorptivity) the proper irradiation wavelengths for photochemical experiments, or the design of UV resistant paints and coatings
- The most ubiquitous use of UV is as a detection device for HPLC; since UV is utilized for solution phase samples vs. a reference solvent this is easily incorporated into LC design
i. Remember the electrons present in organic molecules are involved in covalent bonds or lone pairs of electrons on atoms such as O or N
ii. 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
iii. A functional group capable of having characteristic electronic transitions is called a chromophore (color loving)
iv. Structural or electronic changes in the chromophore can be quantified and used to predict shifts in the observed electronic transitions
- Alkanes – only posses s-bonds and no lone pairs of electrons, so only the high energy s à s* transition is observed in the far UV
This transition is destructive to the molecule, causing cleavage of the s-bond
- Alcohols, ethers, amines and sulfur compounds – in the cases of simple, aliphatic examples of these compounds the n à s* is the most often observed transition; like the alkane s à s* it is most often at shorter l than 200 nm
See how this transition occurs from the HOMO to the LUMO
- Alkenes and Alkynes – in the case of isolated examples of these compounds the p à p* is observed at 175 and 170 nm, respectively
Even though this transition is of lower energy than s à s*, it is still in the far UV – however, the transition energy is sensitive to substitution
- Carbonyls – unsaturated systems incorporating N or O can undergo n à p* transitions (~285 nm) in addition to p à p*
- Despite the fact this transition is forbidden by the selection rules (e = 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 p à p* transition in the vacuum UV (188 nm, e = 900); sensitive to substitution effects
Carbonyls – n à p* transitions (~285 nm); p à p* (188 nm)
From our brief study of these general chromophores, only the weak n à p* transition occurs in the routinely observed UV . The attachment of substituent groups (other than H) can shift the energy of the transition.
Substituents that increase the intensity and often wavelength of an absorption are called auxochromes. Common auxochromes include alkyl, hydroxyl, alkoxy and amino groups and the halogens
Substituents may have any of four effects on a chromophore
i. Bathochromic shift (red shift) – a shift to longer l; lower energy
ii. Hypsochromic shift (blue shift) – shift to shorter l; higher energy
iii. Hyperchromic effect – an increase in intensity
iv. Hypochromic effect – a decrease in intensity
Some common structures and their Absorbance maximum are given below :
How to determine structures using UV spectra data :
We will find that the effect of substituent groups can be reliably quantified from empirical observation of known conjugated structures and applied to new systems
This quantification is referred to as the Woodward-Fieser Rules which we will apply to three specific chromophores:
i. Conjugated dienes
ii. Conjugated dienones
iii. Aromatic systems
For acyclic butadiene, two conformers are possible – s-cis and s-trans
The s-cis conformer is at an overall higher potential energy than the s-trans; therefore the HOMO electrons of the conjugated system have less of a jump to the LUMO – lower energy, longer wavelength.
Two possible p à p* transitions can occur for butadiene Y2 à Y3* and Y2 à Y4*
The Y2 à Y4* transition is not typically observed:
i. The energy of this transition places it outside the region typically observed – 175 nm.
ii. For the more favorable s-trans conformation, this transition is forbidden
The Y2 à Y3* transition is observed as an intense absorption
The Y2 à Y3* transition is observed as an intense absorption (e = 20,000+) based at 217 nm within the observed region of the UV
While this band is insensitive to solvent (as would be expected) it is subject to the bathochromic and hyperchromic effects of alkyl substituents as well as further conjugation.
Woodward and the Fiesers performed extensive studies of terpene and steroidal alkenes and noted similar substituents and structural features would predictably lead to an empirical prediction of the wavelength for the lowest energy p à p* electronic transition
This work was distilled by Scott in 1964 into an extensive treatise on the Woodward-Fieser rules in combination with comprehensive tables and examples – (A.I. Scott, Interpretation of the Ultraviolet Spectra of Natural Products, Pergamon, NY, 1964)
A more modern interpretation was compiled by Rao in 1975 – (C.N.R. Rao, Ultraviolet and Visible Spectroscopy, 3rd Ed., Butterworths, London, 1975).
Woodward-Fieser Rules – Dienes
The rules begin with a base value for lmax of the chromophore being observed:
acyclic butadiene = 217 nm
The incremental contribution of substituents is added to this base value from the group tables:
|Each exo-cyclic C=C||+5|
Isoprene – acyclic butadiene = 217 nm
one alkyl subs. + 5 nm
Experimental value 220 nm
– acyclic butadiene = 217 nm
one exocyclic C=C + 5 nm
2 alkyl subs. +10 nm
Experimental value 237 nm
There are two major types of cyclic dienes, with two different base values
Heteroannular (transoid): Homoannular (cisoid):
e = 5,000 – 15,000 e = 12,000-28,000
base lmax = 214 base lmax = 253
The increment table is the same as for acyclic butadienes with a couple additions:
|Where both types of diene are present, the one with the longer l becomes the base|
In the pre-NMR era of organic spectral determination, the power of the method for discerning isomers is readily apparent
Consider abietic vs. levopimaric acid: