UV/Vis Spectroscopy Overview

In UV-VIS spectroscopy of organic compounds, the most common electronic transitions are n → σ* and n → π* transitions. These transitions occur when electrons in non-bonding (n) orbitals or π orbitals are excited to higher-energy σ* orbitals.

The energy of absorbed light in UV-VIS spectroscopy corresponds directly to the energy difference between the initial and excited states of the electrons within the sample molecule. In other words, the higher the energy of the absorbed light, the larger the energy gap between the electron's ground state and excited state.

When all visible light is absorbed, a substance appears black. If light between 400 and 800 nm is not absorbed, the substance appears colorless. The intensity of the color (how dark the solution appears) is directly related to the concentration of the absorbing species.

<p>A calibration curve is used to establish a relationship between the absorbance of a substance and its concentration. This allows for the determination of the unknown concentration of an analyte by measuring its absorbance and referencing it to the corresponding concentration on the calibration curve.</p> Signup and view all the answers

<p>UV-VIS spectroscopy is highly valued in these fields due to its high accuracy, speed, and affordability. Its sensitivity allows for analyzing complex mixtures, determining concentrations of specific compounds, and characterizing the molecular structures of substances of interest.</p> Signup and view all the answers

<p>The typical UV-VIS spectrum presents a graph plotting the light absorption intensity (absorbance or transmittance) as a function of wavelength. The peaks on this spectrum correspond to specific wavelengths at which the sample absorbs light. These peaks provide information about the identity and relative concentrations of substances in a sample.</p> Signup and view all the answers

<p>Different substances absorb light at specific wavelengths. By varying the wavelength of light used, UV-Vis spectroscopy can identify and quantify different components in a sample based on their unique absorption patterns.</p> Signup and view all the answers

<p>The spectrophotometer scans a range of wavelengths from the UV to the visible region, measuring the absorbance at each wavelength. The resulting data is plotted as absorbance (A) versus wavelength (λ), generating a UV-Vis or electronic absorbance spectrum.</p> Signup and view all the answers

<p>The three main types of atomic orbitals involved in UV-VIS spectroscopy of organic molecules are σ (sigma), π (pi), and n (non-bonding). σ orbitals are associated with single bonds, π orbitals are associated with double or triple bonds (like those found in alkenes or alkynes), and n orbitals are associated with lone pairs of electrons on atoms like oxygen or nitrogen.</p> Signup and view all the answers

<p>A spectrophotometer utilizes a beam splitter to separate the light into two beams. One beam passes through the sample, while the other serves as a reference. A detector compares the intensities of the two beams, and the absorbance is calculated based on the difference in intensity.</p> Signup and view all the answers

<p>The color a compound appears is determined by the wavelengths of light it transmits. When a compound absorbs certain wavelengths, the complementary colors are transmitted, making the compound appear that color. For example, if a compound absorbs red wavelengths, it will appear green because green is the complementary color of red.</p> Signup and view all the answers

<p>Aqueous solutions of Cu(II) ions, like [Cu(H2O)6]2+, absorb wavelengths in the red-orange-yellow portion of the visible spectrum. This means that the complementary colors, green, blue and violet, are transmitted. Since the human eye is more sensitive to blue, the solution appears blue.</p> Signup and view all the answers

<p>Organic molecules typically undergo π→π* and n→π* transitions, involving the excitation of electrons from bonding and non-bonding orbitals to antibonding orbitals. Transition metal ions, however, undergo d-d transitions, involving the excitation of electrons within partially filled d-orbitals.</p> Signup and view all the answers

<p>Conjugation generally leads to a shift of the absorption maximum (λmax) to longer wavelengths (red shift). This is because the delocalized pi electrons in conjugated systems have lower energy and require less energy for excitation.</p> Signup and view all the answers

<p>Inorganic anions usually have broad UV absorption bands due to the excitation of non-bonding electrons. These electrons are less tightly bound and require less energy for excitation, resulting in broad absorption bands in the UV region.</p> Signup and view all the answers

<p>The color of a complex is complementary to the wavelengths of light it absorbs. This means that the color we perceive is the color of the light that is <em>not</em> absorbed. If a compound doesn't absorb visible light, it appears colorless because it transmits all wavelengths of visible light.</p> Signup and view all the answers

<p>A calibration curve in UV-Vis spectroscopy is used to determine the concentration of an unknown sample by relating its absorbance to known concentrations. To create a calibration curve, a series of solutions with known concentrations (standards) are prepared. The absorbance of each standard is measured at the maximum wavelength of absorption (λmax) and plotted against its concentration. The resulting graph, if linear, represents the calibration curve.</p> Signup and view all the answers

<p>Absorbance (A) refers to the amount of light absorbed by a solution, while transmittance (T) refers to the amount of light passing through it. Mathematically, absorbance is the negative logarithm of transmittance: A = -log10 T.</p> Signup and view all the answers

<p>The Beer-Lambert Law states that the absorbance of a solution is directly proportional to the concentration of the analyte and the path length of the light beam through the solution. This relationship can be represented by the equation: A = εbc, where A is absorbance, ε is the molar absorptivity, b is the path length, and c is the concentration. For the Beer-Lambert Law to hold true, the solution must be homogeneous, the light must be monochromatic (single wavelength), and there should be no interactions between the molecules of the analyte.</p> Signup and view all the answers

<p>Measuring absorbance at λmax ensures the most sensitive and accurate measurement. At this wavelength, the analyte absorbs the maximum amount of light, leading to a larger absorbance signal and better discrimination between different concentrations. This is crucial for both standard solutions used to create the calibration curve and unknown samples for analysis.</p> Signup and view all the answers

When certain wavelengths of light are absorbed, creating an observed color.

The colors seen when specific wavelengths of light are absorbed.

A part of a molecule responsible for its color, absorbing specific wavelengths.

Compounds that absorb visible light due to electronic transitions in d-orbitals.

The maximum wavelength at which a substance absorbs light.

A device that divides light into two equally intense beams.

An instrument to measure absorbance by scanning wavelengths of light.

Relating absorbance readings to concentration of a substance.

A plot showing absorbance versus wavelength from UV to visible light.

The relationship between light absorption and color perceived.

A technique measuring the absorption of UV and visible light by a sample.

The movement of electrons from lower to higher energy states when light is absorbed.

A graph showing light absorption intensity against wavelength.

σ, π, and n orbitals involved in electronic transitions.

Higher energy states where electrons transition after absorption.

The ability of a method to detect small differences in analyte concentration.

A group that enhances the color of a compound, enabling color measurement.

A graph plotting absorbance against concentration for analyte measurement.

Describes the linear relationship between absorbance and concentration.

The fraction of light that passes through a solution, expressed as a percentage.