Homework 2 - Answers - Homework Problem Set 2 Iverson...

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Unformatted text preview: Homework Problem Set 2 Iverson CH310N Due Friday, February 1 NAME (Print): _____________________________ SIGNATURE: _____________________________ Chemistry 310N Dr. Brent Iverson 2nd Homework January 23, 2008 Please print the first three letters of your last name in the three boxes Homework Problem Set 2 Iverson CH310N Due Friday, February 1 Score: _________ Homework Problem Set 2 Iverson CH310N Due Friday, February 1 (1 pt. each) Circle all the statements relating to the theory of NMR that are true. A magnetic field will cause charges to move. A magnetic field will not cause charges to move. Moving charge creates a magnetic field. Moving charge does not create a magnetic field. Electrons, neutrons, and protons are all charged. Neutrons and protons are both charged. Only electrons are charged. Both electrons and protons are charged. Atomic nuclei with an odd atomic mass or an odd atomic number have a quantum mechanical property called spin. The best way to the think about nuclear spin is as if nuclear postive charge (from the protons) were circulating within the nucleus, thereby creating a small magnetic field that can interact with a strong laboratory magnetic field. 1 H and 1 3C nuclei have a spin quantum number of 2. H and 1 3C nuclei have a spin quantum number of 1. H and 1 3C nuclei have a spin quantum number of 1/2. 1 1 The two spins states of a 1H nucleus in a strong magnetic field are +1/2 and -1/2. The two spins states of a 1H nucleus in a strong magnetic field are +1 and -1. The two spins states of a 1H nucleus in a strong magnetic field are +2 and -2. Homework Problem Set 2 Iverson CH310N Due Friday, February 1 (1 pt. each) Suppose you place a sample of a molecule in a very strong laboratory magnetic field. Circle the following statements that are true: A magnetic field will cause the electron density to circulate in the molecule. The circulating electron density will create its own magnetic field that is in the same direction as the laboratory magnetic field. The circulating electron density will create its own magnetic field that opposes the laboratory magnetic field. A 1H nucleus with spin +1/2 will be higher in energy than an equivalent nucleus with a spin of -1/2 A 1H nucleus with spin +1/2 will be lower in energy than an equivalent nucleus with a spin of -1/2 The difference in energy between +1/2 and -1/2 spin states for a given 1H nucleus is directly proportional to the strength of the external magnetic field. If electromagnetic energy of the precise energy diffence between the +1/2 and -1/2 spins states is used to irradiate the sample, the energy is absorbed as the spin "flips" from the +1/2 state to the -1/2 state. If electromagnetic energy of the precise energy diffence between the +1/2 and -1/2 spins states is used to irradiate the sample, the energy is absorbed as the spin "flips" from the -1/2 state to the +1/2 state. "Resonance" in NMR refers to the phenomenon of absorption of energy when a nuclear spin "flips". The magnetic field "felt" by any given nucleus in a molecule is actually the sum of (i) the external magnetic field plus (ii) the magnetic field of the electrons around the nucleus plus (iii) the magnetic fields caused by the different spin states of adjacent nuclei. A 1H nucleus surrounded by greater electron density feels a weaker net magnetic field (external 1 magnetic field plus the induced magnetic field from the circulating electron density) compared to a H nucleus surrounded by less electron density. A 1H nucleus surrounded by greater electron density feels a stronger net magnetic field (external 1 magnetic field plus the induced magnetic field from the circulating electron density) compared to a H nucleus surrounded by less electron density. A 1H nucleus surrounded by greater electron density is considered to be more shielded and comes into resonance (absorbs electromgnetic radiation) at a lower frequency (smaller ppm). A 1H nucleus surrounded by greater electron density is considered to be more shielded and comes into resonance (absorbs electromgnetic radiation) at a higher frequency (larger ppm). Homework Problem Set 2 Iverson CH310N Due Friday, February 1 (1 pt. each) Here are some more. Circle all of the true statements. Increasing shielding is observed for 1H atoms attached to C atoms hybrizied in the order sp1, sp2, sp3 Pi bonds are deshielding to nearby 1 H nuclei. TMS is used as a standard in 1H NMR and is always assigned the value of O ppm The frequency of electromagnetic radiation that is absorbed by "flipping" 1H atoms is plotted on the X axis of an NMR spectrum with no correction for magnetic field strength. The frequency of electromagnetic radiation that is absorbed by "flipping" 1H atoms is plotted on the X axis of an NMR spectrum with a correction for magnetic field strength, resulting in the units of parts per million (ppm). The frequency of electromagnetic radiation that is absorbed by "flipping" 1H atoms is plotted on the X axis of an NMR spectrum with a correction for magnetic field strength, resulting in the units of parts per megabyte (ppm). If electromagnetic energy of the precise energy diffence between the +1/2 and -1/2 spins states is used to irradiate the sample, the energy is absorbed as the spin "flips" from the +1/2 state to the -1/2 state. If electromagnetic energy of the precise energy diffence between the +1/2 and -1/2 spins states is used to irradiate the sample, the energy is absorbed as the spin "flips" from the -1/2 state to the +1/2 state. The magnetic field produced by a nucleus in a 1/2 spin state is different than that of a nucleus in a +1/2 spin state. Spin-spin splitting in molecules is caused by the small magnetic fields changes caused by adjacent (no more than three bonds away) nuclei being in the +1/2 state or -1/2 state. Spin-spin splitting in molecules is caused by some 1H nuclei flipping from the +1/2 state to the -1/2 state, while other 1 H nuclei in the sample are flipping from the -1/2 state to the +1/2 state. The "N+1" rule means that a given signal will be split into N+1 peaks if it is adjacent to N equivalent H atoms in a molecule. The "N+1" rule means that if you play in the Rose Bowl N+1 times, you will win N National Championships. Seems true to me....... Homework Problem Set 2 Iverson CH310N Due Friday, February 1 (1 pt. each) For this problem you need to ignore spin-spin splitting, in that we consider one signal to be derived from one set of equivalent H atoms, regardless of splitting pattern. For each of the next three molecules, on the lines provided state how many sets of equivalent sets of H atoms are present, next state how many different signals will appear in the 1H NMR spectrum of each, and finally, the relative ratios of the integrations of the different signals (i.e. 1:2 or 1:2:6, etc.). Number of sets of equivlaent H atoms H A H B C H A HC C C C H A HB HA C HD HD H B C HD B:C:D:A H B B:A Number of different signals in the NMR spectrum Ratio of signal integrations B H BH B H C C H A A H C A H C 2 2 4:6 (or 2:3) A H AH AH H 4 4 1:2:3:6 BH A H A C H AH C HB C H C H C HD C C HF HE D:E:F:B:C:A 6 6 1:1:1:2:2:3 (1 pt. each) For the following molecule, in the boxes provided, state the number of peaks you expect in each 1H NMR signal due to spin-spin splitting. 4 1 CH2 H3 C O CH3 3 Homework Problem Set 2 Iverson CH310N Due Friday, February 1 (not graded) These two questions will not be graded, but turn them in anyway. They will help you organize your thoughts regarding NMR and MRI. Something linke this might show up on the exam, and that will be graded! In no more than four sentences, explain what happens in a 1 H NMR experiment. The 1H NMR experiment is carried out by placing a sample in a strong magnetic field, then the sample is irradiated with electromagnetic radiation. When the energy of the electromagnetic radiation corresponds exactly to the difference in energy between the +1/2 and -1/2 nuclear spin states, the nuclei absorb the energy as the nuclear spins are excited (i.e. "flip" when they come into "resonance" with the electromagnetic radiation) from the lower energy spin state (+1/2) to the higher energy spin state (-1/2). The NMR spectrum is a plot of absorbance vs. frequency scaled for magnetic field strength, wherein chemical shift (distance from the TMS signal at 0 ppm) is correlated with the functional groups present, the connectivity of atoms in molecules can be determined by splitting, and the relative ratio of different equivalent H atoms in the molecule is proportional to the relative integrations of the different signals. Suppose a relative of yours is having an MRI. In no more than four sentences, explain to them what is happening when they have the MRI scan. The popular medical diagnostic technique of magnetic resonance imaging (MRI) is based on the same principles as NMR, namely the flipping (i.e. resonance) of nuclear spins of protons by radio frequency irradiation when a patient is placed in a strong magnetic field. Magnetic field gradients are used to gain imaging information, and rotation of the gradient around the center of the object gives imaging in an entire plane (i.e. slice inside patient). In an MRI image, you are looking at individual slices that when stacked make up the three-dimensional image of relative amounts of protons, especially the protons from water and fat, in the different tissues. Homework Problem Set 2 Iverson CH310N Due Friday, February 1 On the following pages there are six NMR spectra. The relative integrations are given above each signal. Assign each spectra to the appropriate structure out of the following possibilities. Each NMR spectrum has a letter on it. Write the appropriate letter underneath the molecules in the space provided. Notice that not all of the molecules below will have letters underneath them, as there are only six spectra but ten molecules. You didn't want us to make it too easy, did you? We recommend you print the problems as the spectra are easier to read when printed. CH2 H3 C CH2 CH2 CH3 H3 C C H C CH2 CH3 CH2 CH2 CH3 H3 C F CH CH3 CH3 O C H3 C CH2 E CH3 CH CH3 H3 C H3 C O CH2 CH CH2 CH2 CH3 CH3 C CH H3 C D C O CH2 H3 C CH2 C CH CH3 H3 C A CH3 O C O CH2 CH3 CH3 H3 C B CH2 CH3 H3 C HO CH2 C H CH2 CH3 Homework Problem Set 2 Iverson CH310N Due Friday, February 1 O H H 1 H O H 2 H 3 A H H H Homework Problem Set 2 Iverson CH310N Due Friday, February 1 O H 1 2 H 3 H H B H H H H Homework Problem Set 2 Iverson CH310N Due Friday, February 1 H H 3 3 H 5 H 5 H 6 H 2 H 2 H H 2 4 H 6 H 6 O H 1 C Homework Problem Set 2 Iverson CH310N Due Friday, February 1 H 1 H 1 H 1 H H 1 2 H 1 H 1 D H 1 H 1 H 1 Homework Problem Set 2 Iverson CH310N Due Friday, February 1 H 4 H 4 H 4 O H 3 H 4H 4 H 2 H 4 H 1 H 2 E H 1 H 1 Homework Problem Set 2 Iverson CH310N Due Friday, February 1 H 2 H 2 H 4 H H 1 1 H 4 H H 1 H 3 4 H 4 H H 4 4 F ...
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This homework help was uploaded on 04/21/2008 for the course CH 310n taught by Professor Iverson during the Spring '08 term at University of Texas at Austin.

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