chapter 12.7-12.9

- Spectroscopy Techniques in Organic Chemistry: Mass spectrometry (molecular size/formula), IR spectroscopy (functional groups), and NMR spectroscopy (carbon-hydrogen framework) are general methods for structure determination.
- UV Spectroscopy: A fourth technique, specifically for conjugated compounds, offers more specialized information and is less commonly used than the others.
 - Purpose: Determines conjugated $\pi$ electron systems.
 - UV Region: Relevant range for organic chemists is $2 \times 10^{-7}$ m to $4 \times 10^{-7}$ m (200 to 400 nm).
- Mechanism of UV Absorption: When an organic molecule absorbs UV radiation, an electron is promoted from a lower-energy orbital to a higher-energy orbital.
 - $\pi \rightarrow \pi$ Excitation: In conjugated molecules (like 1,3-butadiene), a $"\pi"$ electron moves from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).
 - Example: For 1,3-butadiene, a $"\pi \rightarrow \pi*"$ electronic transition occurs at 217 nm.
- Recording UV Spectra: A sample is irradiated with continuously changing UV wavelengths.
 - Absorption: Energy is absorbed when the wavelength matches the energy needed for electron excitation.
 - Display: Spectra plot wavelength vs. absorbance (A), with peaks indicating absorption (unlike IR spectra, which show valleys).
 - Absorbance (A): Defined as $A = \log(I0/I)$ , where $I0$ is incident light intensity and $I$ is transmitted light intensity.
- Molar Absorptivity ( $\epsilon$ ): Quantifies the amount of UV light absorbed.
 - Definition: $\epsilon = A / (c \times l)$ , where A = Absorbance, c = Concentration (mol/L), and l = Sample pathlength (cm).
 - Characteristic Constant: $"\epsilon"$ is a physical constant specific to a substance's $"\pi"$ electron system (e.g., 10,000 to 25,000 for conjugated dienes).
 - Applications: Can be rearranged to $c = A / (\epsilon \times l)$ to measure sample concentration (e.g., $"\beta"$ -carotene example).
- Characteristics of UV Spectra: Usually simple, often showing a single, broad peak.
 - $\lambda_{max}$ : The wavelength at the top of the absorption peak, used to identify its position.
- Structural Information from UV Spectra: The wavelength needed for a $"\pi \rightarrow \pi*"$ transition depends on the energy gap between HOMO and LUMO, which is influenced by the extent of conjugation.
 - Effect of Conjugation: Increased conjugation leads to a decreased energy difference between HOMO and LUMO, resulting in absorption at longer wavelengths (lower energy).
 - Examples: 1,3-butadiene ( $\lambda{max}$ = 217 nm), 1,3,5-Hexatriene ( $\lambda{max}$ = 258 nm), 1,3,5,7-Octatetraene ( $\lambda_{max}$ = 290 nm).
 - Other Systems: Conjugated enones and aromatic rings also have characteristic UV absorptions useful for structure determination.
Ultraviolet (UV) spectroscopy is a fourth technique, but it is applicable only to conjugated compounds.
UV spectroscopy is less commonly used compared to the other three methods because it provides more specialized information.

Mass spectrometry	Molecular size and formula
IR spectroscopy	Functional groups present
NMR spectroscopy	Carbon–hydrogen framework
UV spectroscopy	Conjugated π electron systems

The ultraviolet region spans from the short-wavelength end of the visible region ( $4 \times 10^{-7}$ m) to the long-wavelength end of the X-ray region ( $10^{-8}$ m).
For organic chemists, the most relevant UV range is narrower, from $2 \times 10^{-7}$ m to $4 \times 10^{-7}$ m.
Absorptions in this region are typically measured in nanometers (nm), with $1 \text{ nm} = 10^{-9}$ m.
Therefore, the ultraviolet range of particular interest is from $200$ to $400$ nm.

(Figure 14.11).

The electromagnetic spectrum shows regions (left to right) as follows: X-rays, vacuum ultraviolet, ultraviolet, visible, near-infrared, and infrared. Lambda values of each are mentioned. Energy increases from right to left.

When an organic molecule is irradiated with electromagnetic energy, the radiation is either passed through or absorbed.

With IR irradiation, absorbed energy increases molecular vibrations.
With UV radiation, absorbed energy promotes an electron from a lower-energy orbital to a higher-energy orbital in a conjugated molecule.

In conjugated molecules like 1,3-butadiene:

It has four $\pi$ molecular orbitals: two lower-energy bonding MOs (occupied in the ground state) and two higher-energy antibonding MOs (unoccupied).
On irradiation with UV light ( $h\nu$ ), a $\pi$ electron is promoted from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).
This is called a $\pi \rightarrow \pi$ excitation, as the electron moves from a bonding $\pi$ molecular orbital to an antibonding $\pi$ molecular orbital.
For 1,3-butadiene, UV light of 217 nm is required for this $\pi \rightarrow \pi*$ electronic transition.

Energy diagram shows four p atomic orbitals with single electrons. Ground-state electronic configuration shows two HOMO orbitals each with two electrons. In excited state, one electron shifts to LUMO orbital.

Figure 14.12 Ultraviolet excitation of 1,3-butadiene results in the promotion of an electron from ψ₂, the highest occupied molecular orbital (HOMO), to ψ₃*, the lowest unoccupied molecular orbital (LUMO).

An ultraviolet spectrum is recorded by irradiating a sample with UV light of continuously changing wavelength. When the wavelength corresponds to the energy level required to excite an electron to a higher level, energy is absorbed. This absorption is detected and displayed on a chart that plots wavelength versus absorbance (A), defined as

A=logI0I

where I₀ is the intensity of the incident light and I is the intensity of the light transmitted through the sample.

Note that UV spectra differ from IR spectra in how they are presented. For historical reasons, IR spectra are usually displayed so that the baseline corresponding to zero absorption runs across the top of the chart and a valley indicates an absorption, whereas UV spectra are displayed with the baseline at the bottom of the chart so that a peak indicates an absorption (Figure 14.13).

The ultraviolet spectrum of 1,3-butadiene shows a peak at (217, 0.85) where lambda max equals 217 nanometers. The x-axis and y-axis represent wavelength and absorbance, respectively.

Figure 14.13 The ultraviolet spectrum of 1,3-butadiene, λ_max = 217 nm.

The amount of UV light absorbed is expressed as the sample’s molar absorptivity (ϵ), defined by the equation

ε=Ac×l

where

A=Absorbancec=Concentration in mol/Ll=Sample pathlength in cm

Molar absorptivity is a physical constant, characteristic of the particular substance being observed and thus characteristic of the particular π electron system in the molecule. Typical values for conjugated dienes are in the range ε = 10,000 to 25,000. The units for molar absorptivity, L/(mol · cm), are usually dropped.

A particularly important use of this equation comes from rearranging it to the form c = A/(ε · l), which lets us measure the concentration of a sample in solution when A, ε, and l are known. As an example, β-carotene, the pigment responsible for the orange color of carrots, has ε = 138,000 L/(mol · cm). If a sample of β-carotene is placed in a cell with a pathlength of 1.0 cm and the UV absorbance reads 0.37, then the concentration of β-carotene in the sample is

c=Aεl=0.37(1.38×105Lmol⋅cm)(1.00 cm) =2.7×10−6mol/L

Unlike IR and NMR spectra, which show many absorptions for a given molecule, UV spectra are usually quite simple—often only a single peak. The peak is usually broad, and we identify its position by noting the wavelength at the top of the peak—λ_max, read as “lambda max.”

Spectroscopy Techniques in Organic Chemistry: Mass spectrometry (molecular size/formula), IR spectroscopy (functional groups), and NMR spectroscopy (carbon-hydrogen framework) are general methods for structure determination.
UV Spectroscopy: A fourth technique, specifically for conjugated compounds, offers more specialized information and is less commonly used than the others.
Purpose: Determines conjugated $\pi$ electron systems.
UV Region: Relevant range for organic chemists is $2 \times 10^{-7}$ m to $4 \times 10^{-7}$ m (200 to 400 nm).
Mechanism of UV Absorption: When an organic molecule absorbs UV radiation, an electron is promoted from a lower-energy orbital to a higher-energy orbital.
$\pi \rightarrow \pi$ Excitation: In conjugated molecules (like 1,3-butadiene), a $\pi$ electron moves from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).
Example: For 1,3-butadiene, a $\pi \rightarrow \pi*$ electronic transition occurs at 217 nm.
Recording UV Spectra: A sample is irradiated with continuously changing UV wavelengths.
Absorption: Energy is absorbed when the wavelength matches the energy needed for electron excitation.
Display: Spectra plot wavelength vs. absorbance (A), with peaks indicating absorption (unlike IR spectra, which show valleys).
Absorbance (A): Defined as $A = \log(I0/I)$ , where $I0$ is incident light intensity and $I$ is transmitted light intensity.
Molar Absorptivity ( $\epsilon$ ): Quantifies the amount of UV light absorbed.
Definition: $\epsilon = A / (c \times l)$ , where A = Absorbance, c = Concentration (mol/L), and l = Sample pathlength (cm).
Characteristic Constant: $\epsilon$ is a physical constant specific to a substance's $\pi$ electron system (e.g., 10,000 to 25,000 for conjugated dienes).
Applications: Can be rearranged to $c = A / (\epsilon \times l)$ to measure sample concentration (e.g., $\beta$ -carotene example).
Characteristics of UV Spectra: Usually simple, often showing a single, broad peak.
$\lambda_{max}$ : The wavelength at the top of the absorption peak, used to identify its position.
Structural Information from UV Spectra: The wavelength needed for a $\pi \rightarrow \pi*$ transition depends on the energy gap between HOMO and LUMO, which is influenced by the extent of conjugation.
Effect of Conjugation: Increased conjugation leads to a decreased energy difference between HOMO and LUMO, resulting in absorption at longer wavelengths (lower energy).
Examples: 1,3-butadiene ( $\lambda{max}$ = 217 nm), 1,3,5-Hexatriene ( $\lambda{max}$ = 258 nm), 1,3,5,7-Octatetraene ( $\lambda_{max}$ = 290 nm).
Other Systems: Conjugated enones and aromatic rings also have characteristic UV absorptions useful for structure determination.

in Table 14.2.

Table 14.2 Ultraviolet Absorptions of Some Conjugated Molecules

Name	Structure	λ_max (nm)
2-Methyl-1,3-butadiene		220
1,3-Cyclohexadiene		256
1,3,5-Hexatriene	H₂C=CH―CH=CH–CH=CH₂	258
1,3,5,7-Octatetraene	H₂C=CH–CH=CH–CH=CH–CH=CH₂	290
3-Buten-2-one		219
Benzene		203

Some organic compounds are colored (e.g., $\beta$ -carotene) due to their chemical structures and how they interact with light.
The visible region of the electromagnetic spectrum is from approximately 400 to 800 nm.
Colored compounds possess elaborate systems of conjugation, which cause their ultraviolet (UV) absorptions to extend into the visible region.
For example, $\beta$ -carotene, an orange pigment, has 11 conjugated double bonds, and its maximum absorption ( $\lambda_{max}$ ) occurs at 455 nm, which falls within the visible spectrum.

(Figure 14.14).

The ultraviolet spectrum of beta-carotene shows a curve with a peak (lambda max) at 455 nanometers. The x-axis and y-axis represent wavelength and absorbance, respectively.

Figure 14.14 Ultraviolet spectrum of β-carotene, a conjugated molecule with 11 double bonds. The absorption occurs in the visible region.

"White" light comprises all wavelengths in the visible region.
When white light interacts with $\beta$ -carotene, wavelengths between 400 and 500 nm (blue) are absorbed.
The wavelengths that are transmitted (all colors except blue) are perceived by our eyes, resulting in a yellow-orange color for $\beta$ -carotene.
Conjugation is vital for both the colors of organic molecules and the light-sensitive molecules in our visual system.
Dietary $\beta$ -carotene is a key substance for vision; it is converted to vitamin A in the liver.
Vitamin A is then oxidized to 11-trans-retinal, which subsequently isomerizes via a C11–C12 double bond geometry change to form 11-cis-retinal, essential for vision.

Beta-carotene converts to vitamin A, which further converts to 11-cis-retinal. The carbon atoms in 11-cis-retinal are numbered from left to right.

Two types of light-sensitive receptor cells in the retina:
- Rod cells: Approximately 3 million, responsible for seeing dim light and shades of gray.
- Cone cells: Approximately 100 million, responsible for seeing bright light and colors.
Vision in Rod Cells:
- 11-cis-retinal is converted into rhodopsin (a combination of protein opsin and 11-cis-retinal).
- When light strikes rod cells, isomerization of the C11–C12 double bond occurs, producing trans-rhodopsin (metarhodopsin II).
- This cis–trans isomerization is very fast in the presence of light ( $2 \times 10^{-13}$ seconds) compared to its absence (approx. 1100 years).
- Isomerization of rhodopsin changes its molecular geometry, causing a nerve impulse to be sent to the brain, which is perceived as vision.

In a reaction, Rhodopsin in the presence of light forms Metarhodopsin 2. The cis and trans C C double bond at positions 5 and 6 in reactant and product are labeled, respectively.

Metarhodopsin II is then recycled back into rhodopsin by a multistep sequence involving cleavage to all-trans-retinal and cis–trans isomerization back to 11-cis-retinal.