ACSI HL Spectroscopic Technique 2023 (Student)

IBDP Chemistry HL/ Spectroscopic Identification of Organic Compounds Page 1 Anglo−Chinese School (Independent) Year 6 (2023) IBDP Chemistry HL (IBDP syllabus Topic 11) 1 1.3 Spectroscopic Identification of Organic Compounds − E ssential Ide a: Analytical techniques can be used to determine the structure of a compound, analyse the composition of a substance or determine the purity of a compound. Spectroscopic techniques are used in the structural identification of organic and inorganic compounds. (IBDP syllabus Topic 21) 2 1.1 Spectroscopic Identification of Organic Compounds − E ssential Idea: Although spectroscopic characterization techniques form the backbone of structural identification of compounds, typically no one technique results in a full structural identification of a molecule. TOPIC 11 MEASUREMENT AND DATA PROCESSING (Part 2)

IBDP Chemistry HL/ Spectroscopic Identification of Organic Compounds Page 2 11.3 Spectroscopic Identification of Organic Compounds Nature of science: Improvements in instrumentation —mass spectrometry, proton nuclear magnetic resonance and infrared spectroscopy have made identification and structural determination of compounds routine. (1.8) Models are developed to explain certain phenomena that may not be observable—for example, spectra are based on the bond vibration model. (1.10) Understandings: • The degree of unsaturation or index of hydrogen deficiency (IHD) can be used to determine from a molecular formula the number of rings or multiple bonds in a molecule. • Mass spectrometry (MS), proton nuclear magnetic resonance spectroscopy ( 1H NMR) and infrared spectroscopy (IR) are techniques that can be used to help identify compounds and to determine their structure. Applications and skills: • Deduction of the oxidation states of an atom in an ion or a compound. Determination of the IHD from a molecular formula. • Deduction of information about the structural features of a compound from percentage composition data, MS, 1H NMR or IR. Guidance: • The electromagnetic spectrum (EMS) is given in the data booklet in section 3. The regions employed for each technique should be understood. • The operating principles are not required for any of these methods. • The data booklet contains characteristic ranges for IR absorptions (section 26), 1H NMR data (section 27) and specific MS fragments (section 28). For 1H NMR, only the ability to deduce the number of different hydrogen (proton) environments and the relative numbers of hydrogen atoms in each environment is required. Integration traces should be covered but splitting patterns are not required.

IBDP Chemistry HL/ Spectroscopic Identification of Organic Compounds Page 3 21.1 Spectroscopic Identification of Organic Compounds Nature of science: Improvements in modern instrumentation— advances in spectroscopic techniques (IR, 1H NMR and MS) have resulted in detailed knowledge of the structure of compounds. (1.8) Understandings: • Structural identification of compounds involves several different an alytical techniques including IR, 1H NMR and MS. • In a high resolution 1H NMR spectrum, single peaks present in low resolution can split into further clusters of peaks. • The structural technique of single crystal X –ray crystallography can be used to identify the bond lengths and bond angles of crystalline compounds. Applications and skills: • Explanation of the use of tetramethylsilane (TMS) as the reference standard. • Deduction of the structure of a compound given information from a range of analytical characterization techniques (X–ray crystallography, IR, 1H NMR and MS). Guidance: • Students should be able to interpret the following from 1H NMR spectra: number of peaks, area under each peak, chemical shift and splitting patterns. Treatment of spin-spin coupling constants will not be assessed but students should be familiar with singlets, doublets, triplets and quartets. • High resolution 1H NMR should be covered. • The precise details of single crystal X -ray crystallography need not be known in detail, but students should be aware of the existence of this structural technique in the wider context of structural identification of both inorganic and organic compounds. • The operating principles are not required for any of these methods.

IBDP Chemistry HL/ Spectroscopic Identification of Organic Compounds Page 4 11.3 Spectroscopic Identification of Organic Compounds Nature of science “Improvements in modern instrumentation – advances in spectroscopic techniques (IR, 1H NMR and MS) have resulted in detailed knowledge of the structure of compounds. (1.8).” Scientists are constantly innovating to improve not just their research methodologies, but also the instruments that assist them in their research so as to obtain more precise and accurate data. An example is the infrared spectrometer. Since its invention in 1957, for more than 20 years, each dispersive infrared spectrometer had to scan the reference and sample at every wavelength of the infrared spectrum. The process was tedious and very time– consuming. In the 1980’s, Fourier–transformation infrared spectrometers (FTIR) started to appear in many research laboratories. These spectrometers are built with Fourier transformation functions in their software to allow all wavelengths to be used simultaneously and the process is completed in a fraction of the original time. Improvements in instrumentation also allow scientists to observe phenomenon that are not possible in the past. One example is the use of the NMR as body scanners on July 3, 1977. It was modified to provide a non–intrusive scan of the inside of the human body in 3– dimensions. Today, the technique, called magnetic resonance imaging (MRI), is commonly found in many hospitals. “Models are developed to explain certain phenomena that may not be observable – for example, spectra are based on the bond vibration model. (1.10)” The key objective of acquiring scientific knowledge and understanding is to make accurate predictions. The scientists will make repeated observations on a phenomenon to increase precision, design a model to explain the phenomenon, and apply it in a new situation to test the model. Models provides prediction to the outcome, and in so doing, scientists do make a lot of assumptions as models are supposed to be simple. In many cases, the models yield a set of close approximations to the actual phenomenon. One example is the ideal gas model, which firstly, re quires us to imagine or visualize since molecules cannot be seen with our naked eyes and secondly, ideal gases do not exist. Thus, the model collapses when the molecules become too large and heavy. And so, scientists have developed more sophisticated models to consider other factors like the space occupied by the gas particles and the intermolecular attractive forces to provide more accurate predictions. In this chapter, there will be many models which require visualization, such as how IR interacts with t he atoms and causes vibrations in the form of bond stretching and bending, fragmentation of molecules and how they accelerate between charged plates and deflect in a magnetic field in MS, and how an external magnetic field affects the spin states of protons in NMR and their relaxation to the ground state emits radio frequencies that can be detected. Finally, Bragg’s law will be introduced in X -ray crystallography to determine the distance between the planes of atoms or molecules and hence, the distance betw een the atoms or molecules in a solid crystal.

IBDP Chemistry HL/ Spectroscopic Identification of Organic Compounds Page 5 11.3.1 Difference between Spectroscopy and Spectrometry The two terms, spectroscopy and spectrometry, are often used synonymously by many students and authors. However, there is a subtle difference be