Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 1

Exam 2: CHEM/BCMB 4190/6190/8189 (128 points) Thursday, 5 October, 2017 1). For monosubstituted benzene molecules of the type shown at the right, the chemical shifts at the ortho, meta, and para positions can be calculated by the following simple empirical relationship: δ1H = 7.27 + δcorr, where δcorr is an empirical correction factor from a table, and is specific for the ortho (δortho), meta (δmeta), or para (δpara) position. For instance, when R = -OCOCH3, δ1Hpara = 7.27 + (δpara) = 7.27 + (-0.13) = 7.14 ppm. For R = -COR and R = -NH2, the entries for δortho, δmeta, and δpara are missing from the table below. The numerical values for these entries are, from smallest to largest, -0.75, -0.65, -0.25, 0.14, 0.21, and 0.62. Your job is to put these numbers in their correct places in the table. In order to receive credit, you will have to: a). draw appropriate resonance structures to justify your table entries, and b). discuss why certain positions are more/less shielded relative to others. (14 pts)

δ corr R δortho δmeta δpara

-COR -NH2

-OCOCH3 -0.25 0.03 -0.13

R

Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 2

2). Please explain what “shielding” means with respect to NMR. Please use complete sentences and avoid using symbols, equations and drawings. Your explanation should address the relationship between shielding and resonance frequency. (6 points) 3). For a sample of CHCl3, you apply a very long (seconds), low power pulse at the 1H frequency of the single 1H nucleus in the molecule. Immediately after this pulse, you apply a normal, high power 90 degree pulse at the same frequency and then record the FID. Sketch what you would expect the FID to look like. Also, sketch the spectrum you would expect to observe following Fourier transformation of the FID. You must thoroughly explain your answers for credit. (6 points). 4). Circle the word in each pair of parentheses that makes the following statement true: As the value for the shielding constant, σ, increases, this indicates (decreased / increased) shielding of a nucleus, a/an (decrease / increase) in the effective magnetic field experienced by the nucleus, a/an (downfield / upfield) shift in the chemical shift, and a (lower / higher) resonance frequency for the nucleus. (4 points) 5). Why is the term σpara necessary to describe the shielding constant, σ? (4 points)

Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 3

6). The 1H NMR spectra (right) show the signals from two hydrogens on adjacent carbon atoms that are coupled (3J) to one another. From top to bottom, the spectra clearly demonstrate the spectral changes that occur when the difference between the Larmor frequencies of the two hydrogens decreases. The spin system designations (AX, AB, A2) are shown. A space is provided for the A2 spectrum but the spectrum is not shown. a. What are ‘AX’ and ‘AB’ spin systems? (4 points) b. Draw the expected A2 spectrum in the space provided above. Provide an explanation/justification for your answer. Your explanation (and drawing) should include the expected intensity of the signals/peaks in the A2 system relative to the intensities of the peaks in the AX system. (6 points). c. What word or term in NMR spectroscopy is also used to describe nuclei that comprise A2 systems? Please explain. (4 points)

AX

AB

A2

AX Δν/J 20

14

7

1.5

0

CH CH

Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 4

7). A molecule and its 1D, 1H spectrum (at 250 MHz) are shown below (assuming the “R” groups have no hydrogens): a). In the 1D, 1H NMR spectrum of this molecule, how many signals are observed? Please explain your answer. (4 points)

b). What is/are the multiplet structure(s) (i.e. quintet?, doublet of septets?, etc.)? Please explain your answer. (4 points)

c). Explain the intensities of the individual multiplet components (individual peaks) for each signal in the spectrum. You should justify your answer with a calculation or calculations. (4 points)

R H-C-H Rʹ-C-Rʹʹ H-C-H R

Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 5

8). Estimate the magnitudes of the three bond 1H, 1H coupling constant (3JHH) between the two hydrogens labeled “A” and “B” in each of the compounds below. Justify your values with an explanation. (6 points).

9). The four signals that appear in the 1H NMR spectrum of crotonic acid are shown below (next page), and are designated a, b, c, and d. The frequencies for each of the peaks in each multiplet (from TMS, in Hz) are shown above the peaks. You will need to assign each signal to the hydrogen or hydrogens that give rise to it. You must explain your reasoning or otherwise justify your assignments. Your justification must include a complete evaluation of the coupling constants. As part of your explanation you must:

-label the molecular structure of crotonic acid (below) with a, b, c, and d next to the proton(s) that correspond(s) to the signals in the NMR spectrum (4 points) -determine the magnitudes of the coupling constants and report them properly (i.e. 2Jcd, or whatever) (4 points) -explain the relative magnitudes of the chemical shifts for the signals of the two trans ethylenic protons (4 points) -use the values of the coupling constants in your assignment justification, or otherwise explain how they are consistent with your asssignments (4 points) -determine the magnetic field (in MHz) of the magnet that was used to record the spectrum (4 points)

You will need to supply written justification or rationale for your answers in order to receive credit! (you can write/continue your answers on the following page).

Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 6

Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 7

10). The chemical shifts of the hydrogen nuclei in benzene are 7.28 ppm. Estimate the chemical shifts for the equivalent hydrogens of the methyl groups in the dimethylpyrene compound shown (right). To receive credit you will have to provide a good justification for your estimate. (4 points)

11). The chemical shifts (ppm) for the indicated (circled) methyl groups in pinane and α- and β-pinene are shown below. Provide an explanation for the variation of these methyl (circled) shifts between pinane, α-pinene, and β-pinene. (6 points)

Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 8

12). For ethyl phenylacetate, the regions from ~ 0-90 ppm of the 1H-coupled (top) and 1H-decoupled (bottom) 13C NMR spectrum are shown (right). The pulse sequence used to record the decoupled spectrum on natural isotopic abundance phenylacetate is shown below.

Sketch the simple 1H NMR spectrum of the -O-CH2-CH3 group of ethyl phenylacetate. Then sketch the 1H NMR spectrum that you would observe if you recorded the 1H spectrum of natural isotopic abundance phenylacetate with 13C broadband decoupling (pulse sequence shown at right). You will have to explain the appearance of each signal in the spectra, and the differences observed when using the decoupling sequence. (4 points)

Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 9

13). Shown are regions (7.0-8.0 ppm) of two 1H spectra (below, center) of a palladium phosphine compound (below, left). The large singlet at ~7.23 ppm in both spectra is from the hydrogen of CHCl3 (a contaminant). For each spectrum, the pulse sequence used to acquire the spectrum is shown (below, right) to the right of the spectrum. a). Explain in detail the signal with a chemical shift of ~7.2 ppm. Explain which hydrogen(s) in the spectrum give(s) rise to it. Explain your reasoning. Explain the splitting/multiplicity of this signal in each spectrum. (5 points)

b). Explain in detail the signal or signals centered at a chemical shift of ~7.88 ppm. Explain which hydrogen(s) in the spectrum give(s) rise to this signal (or these signals). Explain your reasoning. Explain the splitting/multiplicity of this signal/signals in each spectrum. (5 points)

Ph = phenyl

90° x 1H

31P

90° x 1H

Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 10

14). Shown (right) are two 13C spectra of the compound shown. One was acquired with broadband 1H-decoupling, and the other with gated 1H-decoupling. The signal at 78 ppm is due to solvent. a. Which spectrum was acquired with broadband 1H-decoupling and which was acquired with gated 1H-decoupling. You must explain your reasoning for credit. (4 points)

b. Given the signal-to-noise of each spectrum, what can you conclude about the differences between acquisition parameters for these two spectra? You must explain your reasoning for credit. (4 points)

Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 11

15). Shown (below) is the structure of the compound sucrose octa-acetate. The positions of the glucose moiety are labeled 1-6, and those of the fructose moiety are labeled 1ʹ-6ʹ. Below the structure is the 1H NMR spectrum of sucrose octa-acetate (acquired at 250 MHz), with many of the signal labeled (A-O). It is known that the signals A, B, E, F, and G are from the hydrogens attached to carbons of the glucose ring. The apparent multiplicities of these signals are: A-doublet, B-triplet, E-triplet, F-doublet of doublets, G-complex multiplet. Shown (below, right) are selective decoupling difference spectra of sucrose octa-acetate with dark arrows indicating the frequency of the signal decoupled (‘a’-no decoupling or reference spectrum, ‘b’-decoupling signal A, ‘c’-decoupling signal B, ‘d’-decoupling signal G). In selective decoupling difference experiments, the decoupled spectrum is subtracted from the control spectrum (the control spectrum here is ‘a’), and the decoupling power is very low leading to near complete broadening of signals from coupled nuclei rather than collapse of signals from coupled nuclei to simpler multiplicities. The signal in ‘d’ labeled with an asterisk (*) is an artifact. Assign the signals A, B, E, F, and G to the appropriate hydrogens (1-5). No credit will be given for answers that are not adequately justified. There is space on the next page to continue your answers if needed. (10 points)

*

Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 12

Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 13

THIS PAGE IS BLANK. FEEL FREE TO USE IT AS “SCRATCH” IF YOU LIKE.

Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 14

You may find some of the information in this table useful:

Name _________________________

Exam 2, 2017: CHEM/BCMB 4190/6190/8189 15

You may find some of the following information or equations useful: kB = 1.381 x 10-23 J/K h = 6.626 x 10-34 Js

Avagadro’s number = 6.02214179 x 1023 mol-1 π/2 radians = 90°

γ1H = 26.7519 x 107 rad/T/s, I = 1/2 γ13C = 6.7283 x 107 rad/T/s, I = 1/2 γ10B = 2.8747 x 107 rad/T/s, I = 3 γ15N = -2.7126 x 107 rad/T/s, I = 1/2 γ11B = 8.5847 x 107 rad/T/s, I = 3/2 γ17O = -3.6280 x 107 rad/T/s, I = 5/2

𝐵! =∆! !!!!!!

!!= !∆!

!!

∆𝜈 = 𝜈!" − 𝜈! =!!!

!(∆!)= !!!

!(!!!!!)

𝑆/𝑁 ∝ 𝑁𝑆!/! (signal-to-noise improves with (number of scans)1/2) m = (-I, -I+1, …, I-1, I)

νL = |γ/(2π)| B0 = ω0/(2π)

𝑀! = 𝑀!𝑒!!/T! ∗

tzero=T1ln(2) 1+ γA/γX 1-γA/γX η = γa / (2γx) I = (1 + η) I0 multiplicity=2nI + 1 I ∝ 1/r6 Θ=2πJτ

€

= h /(2π )

€

M0 =Nγ 22B0I(I +1)

3kBT

€

ε ∝dM/dt = γM0B = Nγ 32B02I(I +1)

3kBT

€

E = µZB0 = −mγB0

€

ΔE = µZB0 = γB0

€

Δδ =Δν

observe frequency×106

€

Mz = M0(1− e−t / T1 )

€

Mz = M0(1− 2e− t / T1 )

€

Δυ1/ 2 =1

πT2 *

€

1T2 *

=γΔB02

+1T2

B0 (Tesla, T)

Resonance frequencies (MHz)

1H 13C

9.4 400 100.6 11.74 500 125.7 14.09 600 150.9 18.79 800 201.2

Θ = γB1τp

SW=1/(2DW)=Nyquist frequency (νNQ)/2 AQ=DW*TD DR=2SW/TD=1/AQ (TD≡NP)

π/2 radians = 90°