IB NOTES: Modern Analytical Chemistry

Definitions: Qualitative Analysis: The detection of the __________________ but not the __________ of a substance in a mixture. Quantitative Analysis: The measurement of the ________________ of a particular substance in a mixture.

Principles of Spectroscopy a. Spectroscopy is based on the ____________________. Electromagnetic radiation is a form of energy using waves and has two main characteristics.  _______________ b. Energy of the radiation can be calculated by Planck’s equation. E=hf  What does h stand for?

Example #1: Calculate the energy of a photon of visible light with a frequency of 3.0 x s -1. Express your answer in kJ mol -1

Example #2: A beam of radiation has energy of 4.00 x J per photon. Calculate the frequency of the radiation.

C. How do each of the following types of radiation affect molecules? i. Radio Waves II. Microwaves iii. Infrared Radiation iv. Visible and Ultraviolet light v. X-rays vi. Gamma ray

Absorption vs. Emission Spectra When radiation is passed through a sample of atoms/molecules some radiation is absorbed and used to excite the atoms from a lower energy state to a higher energy state. The spectrometer produces an absorption spectrum by analyzing the transmitted radiation. Draw an example of an absorption spectrum: An emission spectrum is produced when radiation from an excited sample is emitted as the atoms/molecules return to their “ground” state. Draw an example of an emission spectrum:

 Think of a chemical bond as a spring. It naturally vibrates and bends with its own rhythm that depends upon the unique bond. The mass of the atoms also affects the frequency. Light atoms vibrate at a high frequency whereas heavy atoms have lower frequencies. Bonds are vibrating all the time, however if we give it just the right amount we can cause that bond to vibrate at a fast rate. 

 IR can cause a bond to ________ or __________. However, a bond will only interact with electromagnetic IR if it is ______________.

 Both samples allow ____________________ to pass through. How much of the ___________________ passes through is called _____________________. The machine compares the values and is able to measure the intensities of the two beams. The machine can then identify the types of ______________ contained in the sample.

 See Page in Data Booklet with Table of Bonds and Wavenumber/cm -1. Each bond is given a range of wavenumbers since absorption can be affected by neighboring bonds. Only functional groups can be easily ID’d using IR spectra.  Example #3: A molecule absorbs IR at a wavenumber of 1720 cm -1. Which functional group could account for this absorption?  I. aldehydes II. EstersIII. Ethers A. I only B. I and II C. I, II and III D. None of the above

 Each IR spectra also has what is called a ____________________________. This is caused by bending vibrations. It allows you to differentiate between very similar compounds (ie. 1-propanol and 2- propanol).

▪ Make sure to review the basic steps and function of a mass spectrometer ▪ Determining the Molecular Mass of a Compound.  How does a Mass Spec tell us the Molar Mass? 

 Example: An unknown compound has the following mass composition:  C: 40.0%H: 6.7%O: 53.3%  The largest mass recorded on the mass spectrum of the compound corresponds to a relative molecular mass of 60. Calculate the empirical formula and determine the molecular formula of the compound.   The IR spectrum shows an absorption band at 1700 cm -1 and a very broad band between cm- 1. Deduce its molecular structure. 

 As the chemical sample passes through the mass spec it breaks apart into fragments. These fragments give us information about the initial compound. There are certain “common” fragments that will often break off (i.e. –H, -CH 3, -CH 2 CH 3, -OH). Be able to determine the mass of these fragments as well as the mass of the remaining molecule.

 It is important to remember your carbocations! When you are looking at possible products remember this:  Order of stability of carbocations primary < secondary < tertiary

Nuclear Magnetic Resonance (NMR) Spectroscopy I.The Principles of NMR a.Any nuclei with an odd number of protons (ex: 1 H, 13 C, 19 F 31 P etc) spin like bar magnets. When placed in an magnetic field some atoms align with the field and those with higher energy align against it. Therefore you end with ____________ nuclear energy levels. For our purposes we will only be focused on ­­­­­­­­_______________.

 In an NMR it is important to find the exact amount of energy needed to “flip” the nuclei and spin in the opposite direction. This is called __________________. (What equation could we use to find the energy?) It corresponds to the radio wave section of the electromagnetic spectrum.

The Chemical Shift  Different numbers of electrons effectively _________________ the nucleus, impacting the amount of energy it takes to flip the nuclei. (Remember we are only interested in the H atoms – but these H can be bonded to any other atom).  The standard chemical or ____________________ used is _____________________ or TMS. Its structure is :  The position of any recorded NMR signal relative to the TMS peak is called the _______________________________. Refer to Table 18 of the IB Data Booklet.

Interpreting NMR Spectra  Examine the following NMR Spectra and determine what H formed each peak.  What do the numbers next to the peaks mean?

Splitting Patterns This only occurs with high resolution NMR. Interpreting a high resolution spectrum The n+1 rule  The amount of splitting tells you about the number of hydrogens attached to the carbon atom or atoms next door to the one you are currently interested in. The number of sub-peaks in a cluster is one more than the number of hydrogens attached to the next door carbon(s).  So - on the assumption that there is only one carbon atom with hydrogens next door to the carbon we're interested  Singletnext door to carbon with no hydrogens attached  Doubletnext door to a CH group  Tripletnext door to a CH 2 group  Quartetnext door to a CH 3 group

Magnetic Resonance Imaging (MRI)  MRI is used to study the chemical _______________ which makes up 70% of the human body. The patient is placed in a strong magnetic field and bombarded with radio wave pulses. A computer decodes the signals and creates an image. 