An Integrated Laboratory Project in NMR Spectroscopy Reggie L. Hudson' and Bradford D. Pendley2 Eckerd College, St. Petersburg, FL 33733 Although Fourier Transform (FT) methods have revolutionized the field of nuclear maenetic resonance soectroscopy ( 1 1 , most predominantly undergmduute collegesand universities du not yet have on-campus mulrinuclear VI'-NMR insrrumentntion. A sur\.ey of the chemistry programs in the Dirertor) of L'nderk-raduafe Rcsrorrh can s e n e as a henrhmark since the departments listed are among the most active in undergraduate research (2). Of the 151 departments listed, only about 20% have an FT-NMR ~ p e c t r o m e t e rWhile .~ this figure probablv has risen since 1985 for the departments in t h e ~ i r ~ c t of o rUndergraduate ~ Research, a much lower percentage would be expected if departments not as active in undergraduate research were included. Even including laree " universities might not substantially alter the percent of chemistry programs where undergraduates have ready access to FT-NMR equipment. The chemistrv. nroeram a t Eckerd Colleee, a four-vear . .. undergraduatr institutwn, is part uf that \,as[ majurity which h a w onlv continuous-wme (C\V) proton NMR spectrometers on campus. Although our research studentscan have multinuclear samples run a t either a regional NMR center or in chemistry-departments of largeruniversities with FT-NMR capability, these options are relatively inconvenient and exninsive for reeularlv scheduled laboratorv courses. Moreover, while we possess NMR computer simulation . oroerams. we have found that for teachine nurooses the . hands-on approach remains irreplaceable for reinforcing lecture material (ex.. - . NMR theorv and instrumentation), for learning lahoratory technique, and for student interest and motivation. This paper describes an advanced NMR project that can he done with a 60-MHz continuous-wave proton spectrometer. Standard synthetic and NMR spectroscopic procedures are employed in the project, and references are given for each of the three parts. The main purposes of the project are to give students experience in second-order NMR analysis, the simplication of spectra by raising spectrometer frequency, and the effect of non-hydrogen nuclei on proton resonances. Related eoals are the reinforcement of svnthetic methods, library search techniques, and NMR top& from earlier courses. The project has evolved through six years of use in the senior year of our integrated laboratory program. Students who have done the project have completed a year of physical chemistry, a semester of instrumental analysis, and have been enrolled concurrently in our advanced organic and inorganic courses. They have had some prior lecture exposure to NMR instrumentation, both CW and FT, and second-order effects. They also have had lahoratory experience in NMR operation and, mainly in organic courses, in spectral analysis. ~~
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Second-Order Analyses A lahoratory problem and its "programmed solution" involving an AB spectrum has been described by Kuhlmann and Braun (3).We have used their quantum mechanical AB analysis of the four-line proton NMR spectrum of 2,3-dihromothiophene and have extended it in several ways. Instead of just analyzing a literature spectrum for chemical shifts, a coupling constant, and line intensities, it is more valuable for
students actually to run and measure the spectrum of 2,3dibromothiophene supnlied hv the instructor. In this wav .. students ran see that the sprctrum is not R result ot'a p0011\ tuned instrument and that it is not in accord wtth firct-order expectations. A second extension, entirely appropriate for an integrated laboratory course, is for students to use literature procedures (4) and carry out a multistep synthesis of the sample compound. Recognition and understanding of the AB quartet pattern is fundamental for the advancement to more complex spin svstems (5-7). For this reason we have occasionallv extended the original laborarur? problem of Kuhlmitn~iand Hraun 131 even further u,ith the f d o a m r two sets uf cumpounds: 111 1,4-dichlorohenzene, 4-bromo'hlorobenzene, I-chloro-4-iodohenzene, and (2) p-toluenesulfooic acid, p-toluenesulfonyl chloride. With the exception of the first compound, all of these show "AB-like" quartets in their proton NMR spectra and can be used by students to examine the variation of chemical shift and spin-spin coupling constant with substituent. Care must be exercised in interpreting these spectra since technically they are AA'BB' patterns rather than AB (7,8). Flrsl-Order Analysis of 2-Bromo-4-Fluoroanisole After learning that a relatively complex quantum mechanical analysis may he needed for even a simple spectrum, students are assigned the task of synthesizing 2-hromo-4fluoroanisole (9)and analyzing its NMR spectrum. They are encouraged to use the library to develop a synthetic procedure starting with 4-fluoroanisole and to make full use of methods (refractive index. IR. GC. hoiliue- .point) from prior courses tu idenriiy their pruduc~. is n,mThe 60-MHz spectrum of ?.bro~no-.l-tl~~
