Your Name: Question 1. Standard Fragmentations in Mass Spectrometry. (20 points)

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1 Exam #4, ovember 14-16, MU, hemistry 8160, FS07, Dr. Glaser Your ame: Question 1. Standard Fragmentations in Mass Spectrometry. (20 points) For (b) (d), draw the complete structure of the substrate (all atoms, lone pairs, etc), draw the complete structure of the EI-generated radical cation, and draw curved arrow mechanisms for the fragmentations. (a) Predict the intensities of the relative intensities of the peaks for M (:= 100%), M+1, M+2, M+3, and M+4 of hypothetical 1,10-dibromodecapentayne 10 Br 2. Assume that the 13 abundance is 1.1% and that the two isotopes of Br have the same abundance. Show your work and try to be organized. For future reference: This source uses (79)Br : (81)Br = 100 : & (12) : (13) = 100 : M M M M M ormed to M := 100%: M M M M M M+1 = [ *10 *100%] = 11% M+2 = [ *10*9 *100%] + [1 1 *2 *100%] = 1.09% + 200% = 201.1% M+3 = [ *10*9*8 *100%] + [1 1 *2 * 0.011*10 *100%] = 0.10% + 22% = 22.1% M+4 = [ *10*9*8*7 *100%] + [1 1 *2 * *10*9 *100%] + [1 2 *1*1 *100%] = 0.01% + 2.2% + 100% = 102.2% (b) etro-diels-alder Fragmentation. Example: 4-methylcyclohexene. Ionize -1-

2 Exam #4, ovember 14-16, MU, hemistry 8160, FS07, Dr. Glaser Your ame: (c) McLafferty earrangement. Example: 2-exanone Ionize (d) McLafferty+1 earrangement. Example: Butylacetate. Explain the formation of [ 3 () 2 ] Ionize

3 Exam #4, ovember 14-16, MU, hemistry 8160, FS07, Dr. Glaser Your ame: Question 2. Spectra of a MALDI MS Matrix ompound 2,6-Dihydroxyacetophenone. (20 points) (a) Find the MS spectrum of 2,6-dihydroxyacetophenone in the SDBS database ( ), print it, and attach it to this page (staple). What ionization method was used to generate this SDBS spectrum? List the four major peaks and outline a detailed fragmentation mechanism consistent with the MS spectrum. EI: DIET, 75 ev, Source Temp.: 140 Sample Temp.: 120. Four major peaks: m/z rel. Int Ionize 3-3 m/z = 152 m/z = bserved 3 m/z = 43 m/z = m/z =

4 Exam #4, ovember 14-16, MU, hemistry 8160, FS07, Dr. Glaser Your ame: (b) Find the MALDI MS spectrum of 2,6-dihydroxyacetophenone in the text by Pretsch et al. What kind of laser was employed for the generation of this spectrum (provide the approximate wave length of the laser) and in what way does the laser excite the matrix material? (see: apid ommun. Mass Spectrom. 1996, 10, ): Entry 5 in reference. Pulsed nitrogen laser used; = 337 nm; UV/Vis excitation with 337 = Assign the numbered peaks (m/z 23, 39, 153, 175, 191 and 365) and suggest a fragmentation mechanism consistent with the MALDI MS data: 23: a + 39: K = M + 153: [M+] + 175: [M+a] + 191: [M+K] + 365: Quite uncertain, no response required. -4-

5 Exam #4, ovember 14-16, MU, hemistry 8160, FS07, Dr. Glaser Your ame: Question 3. ESI Spectrum of a Tripeptide. (20 points) ead the article omparing the gas-phase fragmentation reactions of protonated and radical cations of the tripeptides GX*1 (Intl. J. Mass Spectrom. 2004, 234, ) and pay special attention to the discussion of the [GX + 2] 2+ ion, a comparatively simple example of a multiply-charged small peptide. Using complete structural drawings, discuss the major initial fragmentations of this dication. ef., p. 107, left column says: of sequence ions from [GX + 2] 2+ may be understood by a slight modification to Scheme 1. Unlike [GX + ]+ which have only one proton (for which both the y1 and b2 fragments compete), [GX + 2] 2+ have two protons, allowing both y1 and b2 fragments to be protonated and thus, in the fragmentation of [GX + 2] 2+, the y1 and its complementary b2 and/or a2 ions are observed with almost equal abundances. The mobile proton also induces the formation of y2 or [y2 + ] 2+ ions via cleavage of amide bond A. For the singly charged y2 ions, the complementary b1 ion is not observed as it is not stable [49] and readily loses to form the a1 ion, which is not detected due to the low mass cut-off of the ion trap. ne possibility is outlined below. ote the additional proton shown in ED. The cleavage now is ETELYTI. This leaves the small piece positively charged. The additional proton migrates from the carbonyl- to the anionic to make the 2 group. Partial credit for any reasonable and consistent suggestion A 2 B m = 115 ( = ) 2 m = m = 232 ( = ) yclize m =

6 Exam #4, ovember 14-16, MU, hemistry 8160, FS07, Dr. Glaser Your ame: Question 4. UV/Vis Spectroscopy of Indicator Dyes. (20 points) (a) Phenolphthalein, 3,3-bis(p-hydroxyphenyl)phthalide, is used an acid-base indicator. The color in acidic media is colorless and absorption occurs in the region(s) UV-region,_< 400_ nm. The color in alkaline media is fuchsia-pink_/ magenta and absorption occurs in the region(s) green, 550 nm. The change of color occurs at the p value of ca. 8. Show the structures involved in the p-dependent equilibrium and state how the structures explain the colors. Quite a lot of information, actually, can be found at Wikipedia: / / deprotonated phenolphthalein Explain the shift in the absorption with p value: eutral compound is sp 3 at central and there is no conjugation between different phenyl rings. UV absorptions of monosubstituted arenes, that is, like benzoate and phenol. Anionic compound is sp 2 at central and conjugation between different phenyl rings is possible (i.e. draw differentb resonance forms). The game has changed in a fundamental way. -6-

7 Exam #4, ovember 14-16, MU, hemistry 8160, FS07, Dr. Glaser Your ame: (b) APM is a fluorescent probe designed for studying protein conformational change (PAS 2005, 102, ) and Table 1 shows the spectral properties of the ethanethiol adduct of APM. Draw the structure of the ethanethiol adduct of APM. See next box. The adduct results by addition of EtS across the double bond of maleic anhydride. The data in Table 1 show that the fluorescence probe features a batho-chromic shift. Using resonance forms, discuss the electronic structures of the ground state and of the electronic excited state. SEt SEt Me 2 Me 2 A B Electronic Ground State: Lots of A and some B. Electronic Excited Singlet State: Some A and lots of B. -7-

8 Exam #4, ovember 14-16, MU, hemistry 8160, FS07, Dr. Glaser Your ame: Question 5. onjugation Effects on UV/Vis Absorptions. (20 points) ompare & contrast the chromophores of molecules A. A 2 Me What type(s) of excitation(s) would you expect for each of the compounds? At approximately what wave length(s) would you expect these transitions? B Me Me What kind(s) of intensity (just provide information about the expected magnitude) would you expect for each of these excitations? A:, -Unsaturated arbonyl (Pretsch et al. p. 388) [1] Acyclic parent system with X = : 193 nm [2] Two additional conjugated double bonds: 2 x +30 nm [3] Amino group if it were in -position (position 2): +95 nm [4] Amino group in position 6: --no increment known-- Estimate based on [1] [3]: 348 nm Estimate for [4]: Probably less than 95 nm. The conjugation length increases, but there also is more opportunity for gauche conformations. Final estimate for A: nm. B:, -Unsaturated arbonyl (Pretsch et al. p. 388) [1] Acyclic parent system with X = : 193 nm [2] Two additional conjugated double bonds: 2 x +30 nm [3] Amino group if it were in -position (position 2): +95 nm [4] Amino group in position 6: --no increment known-- [5] Two double bonds are exocyclic: 2 x +10 nm [6] Alkyls contribute +10 (a); +18 (g); +18 (pos. 5): +46 nm Estimate based on [1] - [3], [5] & [6]: 414 nm Estimate for [4]: Probably more than 95 nm. The conjugation length increases and conformations are rigid. Final estimate for A: nm. : T an, -Unsaturated arbonyl (o increment system available.) aphthalene: around 300 nm Anthracene vs. aphthalene: ca. +80 nm arboxylate attached to Benzene: ca. +23 nm Isoquinoline vs. aphthalene: ca. +20 nm Estimate: 423 nm Final estimate considering DA-nature: nm. -8-