Spectroscopy Chem honors

Spectroscopy Is the interaction between matter and electromagnetic radiation (light) Spectroscopy is commonly used in chemistry to detect, identify, and quantify information about different atoms and molecules We’ve already seen a couple experiments that dealt with spectroscopic studies Photoelectron effect – Albert Einstein Explanation of the atomic structure – Niels Bohr

Analysis Quantitative Analysis – determination of the absolute or relative abundance of one, several, or all particular substance(s) present in a sample Qualitative Analysis – seeks to find the elemental composition of compounds

Spectroscopy Infrared (IR) Spectroscopy – molecules absorb infrared radiation at specific frequencies that are characteristic of their structure [qualitative] Ultraviolet-Visible (UV-vis) Spectroscopy – molecules/atoms/ions absorb ultraviolet-visible radiation at specific frequencies and undergo electronic transitions (transition from the ground state to an excited state) [qualitative / quantitative]

Infrared Spectroscopy Infrared Spectroscopy allow chemists the ability to determine the identity of a given molecule Infrared radiation is absorbed by a given molecule’s stretching and bending frequencies in most covalent bonds The energy absorbed serves to increase the amplitude of the vibrational motions of the bonds in the molecules (We perceive this vibration as heat) Only bonds that have a dipole moment (difference in electronegativity) that changes as a function of time are capable of absorbing infrared radiation

Molecular Vibrations

IR Spectrum

Absorption Frequency of Major Functional Groups

UV-vis Spectroscopy Absorption Spectroscopy in the UV-visible region (200nm – 800nm) Molecules will undergo electric transitions Measures transition between the ground state to the excited state Can be used to collect qualitative and quantitative data

UV-vis Spectroscopy

Electronic States Important Jumps: From π – π* From non-bonding to π*

Determination of Analytes Transition Metal – can be colored because d- electrons within the metal can be excited from one electronic state to another. The color of the metal is strongly affected by the type of ligand bound to the central atom. Organic Molecules – especially those with a high amount of conjugation. Methyl Orange Beta-Carotene

Transition Metal Salt Solutions

Complementary Colors Colors opposite of one another are considered complimentary: Red – Cyan Yellow – Blue Green – Magenta When a specific wavelength of color is absorbed by a molecule from white light, what is seen is the complementary color The wavelength absorbed is known as your λmax

UV-vis Spectrum of beta-carotene Beta-Carotene has a λmax between 400-500nm Absorbs at about 470nm = Around blue and cyan Complementary color is between yellow and red Therefore the color you see is orange!!!!

Absorbance vs. Concentration Absorbance is directly proportional to concentration The more concentrated a substances is, the more molecules are present to absorb light in the UV-Visible region Due to this relationship, we can quantitatively determine the concentration of solution based off of its absorbance values.

Beer’s Law Beer’s Law describes the relationship between concentration and absorbance: A = εbc A = absorbance (value collected from the spectrometer) c = concentration of solution ε = molar absorptivity (probability of the electronic transition) b = path length (the distance light travels through the cuvette – commonly 1.0 cm)

Beer’s Law Experiment Prepare a set of solutions with a known concentration Determine the λmax of the solution Place each solution into the spectrophotometer Collect data (notice the linear relationship between data points) Graph the data and apply a best fit line The slope of the line is equal to the molar absorptivity After collecting the molar absorptivity for your sample set, you can determine the concentration of an unknown solution by collecting its absorbance values from the spectrophotometer