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Data from single-labeled partially deuterated AB support this analysis and render data determination more reliable. 5: Proton Nuclear Magnetic Resonance Spectroscopy (NMR), { "5.01:_Chapter_Objectives_and_Preview_of_Nuclear_Magnetic_Resonance_Spectroscopy" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "5.02:_Theory_of_NMR" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "5.03:_Instrumentation" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "5.04:_Types_of_Protons" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "5.05:_Chemical_Shift" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "5.06:_Integration_of_Proton_Spectra" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "5.07:_Spin-Spin_Splitting_in_Proton_NMR__Spectra" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "5.08:_More_Complex_Spin-Spin_Splitting_Patterns" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "5.09:_Uses_of_(1H)_NMR_Spectroscopy" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "5.10:_Interpreting_(1H)_NMR_Spectra" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "5.11:_(1H)_NMR_problems" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "5.S:_Summary" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()" }, { "00:_Front_Matter" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "01:_Introduction_to_Organic_Spectroscopy" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "02:_Mass_Spectrometry" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "03:_Conjugated_Compounds_and_Ultraviolet_Spectroscopy" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "04:_Infrared_Spectroscopy" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "05:_Proton_Nuclear_Magnetic_Resonance_Spectroscopy_(NMR)" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "06:_Carbon-13_NMR_Spectroscopy" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "07:_Two-Dimensional_NMR_Spectroscopy" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "08:_Structure_Elucidation_Problems" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()", "zz:_Back_Matter" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass230_0.b__1]()" }, [ "article:topic", "author@LaurenReutenauer" ], https://chem.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fchem.libretexts.org%2FBookshelves%2FOrganic_Chemistry%2FIntroduction_to_Organic_Spectroscopy%2F05%253A_Proton_Nuclear_Magnetic_Resonance_Spectroscopy_(NMR)%2F5.10%253A_Interpreting_(1H)_NMR_Spectra, \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\), understand how chemical shift, integration, spin-spin splitting all come together to solve. Webuse the 1 H NMR spectrum of a simple organic compound to determine the number of equivalent sets of protons present. 9.11: Nuclear Magnetic Resonance Spectroscopy Such overlaps lead to deformation of the molecule also being the reason, why the cis-AB is less stable than the trans-AB. Careers. For example, maybe you used p-methyl benzaldehyde. Suppose the NMR recorded in 400 MHz machine, therefore . These two example represent the two main ways of showing spatial orientation in cycloalkanes. Is it possible to go to trial while pleading guilty to some or all charges? The different level of conversion efficiency is related to the inhomogeneity of the RF field, which is by a factor of two smaller for the probe of the 16.4T NMR spectrometer as compared to that of the 9.4-T spectrometer. To double check this, look to see if there are any protons in the aromatic proton region (6.5 ppm - 8 ppm). Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. Thus are in trans-orientation (see point (6) above). The chemical shift is quite far downfield at 3.77 ppm. Tetramethylsilane [TMS;(CH 3) 4 Si] is generally used for standard to determine chemical shift of compounds: TMS =0ppm. 3.4. NMR Since free rotation about the double bond is not possible, these are completely distinct molecules. For coupling the two hydrogens must have different WebThe purpose of this lab is to demonstrate the acid-catalyzed dehydration of cis-and trans- 2-methylcyclohexanol to form a mixture of 1-metyl and 3-methylcyclohexene. [1] [2] The time-profile was calculated for a two-spin system with an effective chemical shift difference of 2.1Hz (which equals to difference (JH2N7 -JH2N8) and a J-coupling equal to 16Hz (JNN). If the two substituents are on opposite sides of the double b Difference Between Cis and Trans Isomers Importantly, in our simulations we varied both the size and the sign of all spinspin interactions. NMR Hence, we can demonstrate the presence of singlet LLSs of the 15N, 15N spin pairs and measure their lifetimes in both isomers of AB. A ring is a DU = 1. Cis or Trans? How to differentiate by NMR? - YouTube Very few peaks show up that far downfield and using the chemical shift table, it indicates that it is an aldehydic proton. 1.5 times larger for APSOC as compared to SLIC (for both cis-AB and trans-AB, at B The chemical shift is quite far downfield at 4.06 ppm (a methylene group attached to another carbon would be at about 1.2ppm). Legal. 2) Both the OH and the methyl group have wedge bonds. Show transcribed image text. WebCis and Trans Isomerism of Cycloalkanes. A community for chemists and those who love chemistry. In addition to SLIC, we also performed APSOC experiments and determined the optimal 1max and rf values. I'm looking through the spectroscopy section of my textbook and it gives the typical splitting constant of J_trans as 11-18 Hz and J_cis as 6-15 Hz, and I'm kinda curious to see if there's a way to figure out which isomer it is if the coupling constant was, say, 13 Hz or so. Just like with 1 H NMR, chemical shift equivalence applies to 13 C NMR. The .gov means its official. Di-substituted cycloalkanes are one class of molecules that exhibit stereoisomerism. Chegg Our experiments also show a remarkably strong effect of the molecular structure on the LLS lifetime. For maintenance of the singlet state, one needs to provide experimental conditions such that this state is an eigen-state of the time-independent Hamiltonian of the spin pair. To populate LLSs in strongly coupled spin pairs, special NMR techniques are needed for performing magnetization-to-singlet (M2S) conversion, while the reverse S2M conversion is required for LLS observation. Kiryutin AS, Pravdivtsev AN, Yurkovskaya AV, Vieth H-M, Ivanov KL. Here 1max was 18Hz, the offset frequency, rf, from the center of the 15N multiplet was set to 9.5 Hz, B0 =9.4 T. The signal intensity is given in percent of the thermal signal of trans-AB. Here we present details of optimization only for trans-AB. 12 (12) 2017 Santalova & Denisenko Figure 1: The signals of olefinic protons in the 1-NMR spectra (700 MHz) of cis-/trans-isomeric fatty acid methyl esters before and after decoupling of allylic protons (the multiplets of the allylic protons are placed in curly brackets): 9-cis-hexadecenoate in CDCl3; b 9-trans cis Versus trans-Azobenzene: Precise Determination of NMR Example \(\PageIndex{1}\) cis Versus trans-Azobenzene: Precise Determination of NMR The final example brings in a more complex splitting pattern. b) Example "b" shows the cycloalkane ring roughly perpendicular to the plane of the page. Finally, all of these isomers may exist as a mixture of two (or more) conformational isomers, as shown in the table. In specially designed molecules, lifetimes of LLSs of 13C spins in liquids can be as long as 1h [7]. Bethesda, MD 20894, Web Policies The magnitude of the coupling constant between hydrogens of adjacent 1. The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Each double bond is 1 DU and the ring is 1 DU, so 3(1) + 1 = 4. Let's sort out what we've got. In both molecules, magnetic non-equivalence is provided by the small difference in nitrogenproton coupling: each nitrogen atom has different couplings to the protons of the nearby phenyl ring as compared to the protons of the remote phenyl ring. 3 JPH (trans) is frequently larger than 3 JPH (cis), but 2 JPH (gem) can be larger than 3 JPH (trans). 500l/h keeping it by 1C warmer than outside). Look up the chemical shift of vinylie proton signals of cis-and 6. Analysis of the proton spectra was accompanied by simulations of the nitrogen spectra, see Fig. What distinguishes top researchers from mediocre ones? NMR Spectroscopy and Proton Equivalence WebDetermine if each of the following alkenes has an NMR Spectroscopy Practice Problems. 1 Answer. Therefore, all the protons are accounted for and there is not integration of 3, so this molecule has NO methyl groups. Browse other questions tagged, Start here for a quick overview of the site, Detailed answers to any questions you might have, Discuss the workings and policies of this site. trans be identified as in the following example. The achieved conversion efficiencies given by the A2 value are 11 and 9.5% for trans-AB and cis-AB, respectively. Unknown molecule1H NMR spectrum: The proton ratio is 1:1:1:1:2. 1H; 1H-29Si HMBC; 29Si; 2D NMR; NMR; double-decker silsesquioxanes; cis/trans isomers. Web2.6: NMR - Introduction. Need to specify cyclic alkenes as cis or trans. 5.10: Interpreting Proton NMR Spectra is shared under a not declared license and was authored, remixed, and/or curated by LaurenReutenauer. 13) UV-vis: valance electron transitions (Ch. This page is devoted to explaining solved 1H NMR problems to help guide you through different scenarios. Separation and Identification of alpha Phase settings used for SOS filtering by SLIC, Pulse sequences of the SLIC (a) and APSOC (b) methods. After that, we synthesized 15N,15N-AB using 15N-nitrobenzene. Finally, we measured the T NMR spectra were recorded for the following samples: (1) 0.15M of 15N,15N-AB in CD3CN; the sample volume was 600l; (2) for measuring the singlet-state lifetime, TLLS, 0.5M of 15N,15N-AB in CD3CN (i.e., almost saturated solution) in a smaller volume of 300l was used to minimize effects of convection [25]. J-coupling constants of AB given in Hz as determined from 1H and 15N spectral analysis of AB, Parameters for trans-AB are given in the first lines of the table cells; parameters for cis-AB are given in brackets in the second line of the table cells. 1 with the NMR data reported earlier by one of us for AB with a single 15N label, see Supplementary Materials. In this section, a new class of isomers, stereoisomers, will be introduced. Solid State Nucl Magn Reson. WebChemistry. In turn, such conditions are fulfilled when the system is brought to a sufficiently low external magnetic field [3] or when a strong spin-locking RF field is applied [2]. Draw you attention to negative results: what peaks might be there if the spectrum matched another possible structure, but is missing. This implies that they are both on the same side of the testosterone ring making them cis. However, that is not a rule. To summarize, homotopic and enantiotopic protons are chemically equivalent and give one signal. Name the following cycloalkanes: Both trans-1,4-dimethylcyclohexane and cis-1,3-dimethylcyclohexane have essentially the same energy, since neither one of them has any strain at all. First, if the molecular formula is known, then start by calculating the degree of unsaturation(DU), which will tell how many double bond, triple bond, or rings are present in the molecule. This section and the following one illustrate some idiosyncrasies that happen when you try to compare the two systems. By applying these methods, we probed LLSs and measured their lifetimes. However, the cis/trans designations are not used if both groups are on the same carbon. Scan this QR code to download the app now. Cis isomers are the isomers that have two similar atoms lying on the same side of the double bond in a molecule. 2. J-couplings are also known as scalar couplings. 8600 Rockville Pike The peak at 2.40 ppm is a quartet that integrates to 2 protons. NMR analysis of streoisomer The piece from this information is. Kiryutin AS, Zimmermann H, Yurkovskaya AV, Vieth H-M, Ivanov KL. In addition, using the techniques outlined here, one can probe the impact of molecular structure on LLS lifetimes. WebQuestion: Question 2 (0.4 points) How can you tell the difference between a cis and a trans alkene using 1h NMR? The knowledge of the NMR parameters allows us to apply M2S/S2M techniques and optimize their performance. Comparison of the TLLS value in the cis and trans-forms allows us to reveal the influence of molecular symmetry on the LLS lifetime. In addition, if you look at the orientations of 2,4-dihydroxy-groups in the same chair conformation, you'd see they are in 1,3-ax,eq-orientation as well. We also used SOS filtering to suppress any background signals. If possible, you should be looking at the same functional group for each of the molecules (SM, E and Z). In both methods, the RF pulses are applied to the 15N spins. In the cis-isomer, the two groups are present on the same side of the double bond whereas, in the trans- isomer, the two groups are present on the opposite sides of the double bond. Chemical Shift Equivalence How Many Unique Signals? Specifically, at B0 =16.4 T the LLS lifetimes are 98s and 13s for trans-AB and cis-AB, respectively. Experts are tested by Chegg as specialists in their subject area. Distinction between cis and trans isomers RF fields are applied to the 15N spins of AB. NMR: Structural Assignment 4. WebYes, stereo isomers are differentiate by NMR spectroscopy. Webcis and trans 1,2-addition product1,4-addition product For a conjugated diene: 0 C 71% 29% 40 C 15% 85% The distribution of products is dependent upon (NMR) - gives a H and C map of the molecule (Ch. The results of optimization of the SLIC experiment are presented in Figs. First, 15N-nitrobenzene was synthesized. Cobalt(II) complexes are the most suitable for detection of this type of isomerism. ! For determining the NMR p Optimization of the SLIC parameters was done by systematic variation of 1 and rf. So I think that should be addressed before explaining the example. Solved How can one figure out the cis and trans stereoisomer 2.8: NMR: Structural Assignment. 15N,15N-AB is a symmetric molecule, which can exist in the cis and trans- forms, see Fig. Do Federal courts have the authority to dismiss charges brought in a Georgia Court? Who are the experts? Martin Karplus was the first to study the variation of coupling constant. Key Term. Being the furthest downfield, the methylene group needs to beattached to the electron-withdrawing group that will be the most deshielding for the protons (Chemical Shift information). WebSynthesis and characterization of trans-1,4-butadiene/isoprene copolymers: Determination of monomer reactivity ratios and temperature dependence. Stack Exchange network consists of 183 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. (1 Mark) Explain how H NMR can be used to calculate the cis/trans ratio in cases where a product mixture is formed. The chemical shift is quite far downfield at 1.98 ppm (a methyl group attached to another carbon would be at about 0.9 ppm). Draw your attention to pieces of data that most strongly support your expected structure. WebWhen alkynel hydrogen atoms are not symmetrically substituted on a double bonded carbon, the hydrogens of a cis and trans isomer will yield a different shift on the NMR spectrum.Because the coupling constant is smaller in a cis isomer than in a trans isomer, the NMR spectrums of the two isomers are different conveying the hydrogens in a cis government site. Later sections will describe how to name these more complex molecules (5.5: Sequence Rules for Specifying Configuration). 3.4. WebChapter 1: NMR Coupling Constants 2 Figure 1-2: The NMR spectrum of 1,1-dichloroethane, collected in a 60 MHz instrument. WebBackground to C-13 NMR; Determine Structure with Combined Spectra; High Resolution Proton NMR Spectra This page describes how you interpret simple high resolution nuclear magnetic resonance (NMR) spectra. The image below shows the two geometric isomers, called cis-2-butene and trans-2-butene. )%2F04%253A_Organic_Compounds-_Cycloalkanes_and_their_Stereochemistry%2F4.02%253A_Cis-Trans_Isomerism_in_Cycloalkanes, \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\), 4.3: Stability of Cycloalkanes - Ring Strain, 5.5: Sequence Rules for Specifying Configuration), draw structural formulas that distinguish between. National Library of Medicine When such a matching is achieved, perturbation terms can induce singlettriplet mixing, required for generating the LLS. However, the entire molecule cannot be designated as either a cis or trans isomer. Hence, the optimization performed here allows us to generate and study LLS in both forms of AB. Following image shows the chair conformation of cyclohexane (in the box): All $\color{red}{\text{red}}$ $\ce{C-H}$ bonds are in axial orientation (ax) and $\color{black}{\text{black}}$ $\ce{C-H}$ bonds are in equatorial orientation (eq). There is a total of 3 protons in the aromatic region, which means that there are three hydrogens coming of the aromatic ring and three groups coming off the aromatic ring. First, the ratio of protons equals the number of protons in our expected product. Consequently, the optimized rf value does not provide maximal conversion efficiency for molecules in different states of protons. However, when there is inhomogeneity of both the B0 and RF fields over the sample using APSOC is preferable, as it is less sensitive to small variations of 1 and . 5.4: NOESY Spectra - Chemistry LibreTexts HHS Vulnerability Disclosure, Help SLIC and APSOC experiments were done for both trans-AB and cis-AB. 2-butanone produces 3 signals in a 1H spectrum, while 2-butanol produces 6 (including the OH proton). a) In example "a" the cycloalkane is shown as being flat and in the plane of the page. The integration tells us the peak corresponds to a methylene group. All samples were degassed using several freezepumpthaw cycles and sealed by flame. Deoxybuzonamine Isomers from the Millipede. Generally, the cis and trans isomerism is found in both organic and inorganic compounds. cis Feng Y, Theis T, Wu TL, Claytor K, Warren WS. Explain 5. J-Coupling A spectrum produced by a 13 C NMR experiment may not always display a 1:1 ratio of signals to individual carbons atoms. Cis and Trans A slightly easier method to determine whether a given compound is meso is to take advantage of a simple principle: meso compounds have an internal mirror plane. We are able to achieve excellent agreement between experiments and simulations. Using the chemical formula and 1H NMR, determine the structure of the unknown molecule. 1-relaxation and affects the resulting signal. Determination of cis-1,4 and trans-1,4 Contents of Polyisoprenes Introduction. WebThe coupling constant, J (usually in frequency units, Hz) is a measure of the interaction between a pair of protons. Beware of overlaps! The use of carbon-13 NMR spectroscopy to differentiate between In other words, the equatorial conformer is more stable by 1.70 kcal/mol. ; (The chain is numbered starting from the carbon atom with the hydroxyl group.) The ratio of C : H in a saturated, acyclic hydrocarbon is n : 2n+2. How to identify cis and trans forms of cyclohexane If you concentrate only on axial positions, you'd see 1,2-ax,ax-bonds are always trans (opposite direction). 2, one can clearly see that the shape of NMR multiplets depends on the sign of protonnitrogen couplings, JNH: variation of sign of the couplings [26] allowed us to achieve perfect agreement with the experimental spectra. The same is true for the meta-protons. The splitting pattern of a singlet indicates that there are no neighbors. and Z Notation For Alkenes In SLIC, singlettriplet conversion occurs in a coherent fashion; hence, the rf-dependence of the signal is expected to contain oscillations. If the two substituents are on the same side of the double bond, the configuration of the bond iscis. So, based on this, bromine gets priority one, the oxygen gets priority two, the methyl carbon is the third and the hydrogen is the lowest priority-four: Step 2: Such a comparison is presented in Fig. As in the SLIC case, when performing optimization we introduced an RF field to the 15N-spins between the RF1 and RF2 fields. Using the chemical formula and the 1H NMR spectrum, determine the structure of the unknown molecule. It is quite far downfield, so it must be near an electron-withdrawing group. [Note: this risks a tsk-tsk with accompanying finger-wag from IUPAC , but it nevertheless Question: How can one figure out the cis and trans stereoisomer ratio through H-NMR? think back to the experiments from Chem 235. Proton magnetic resonance spectra of solutions of six-coordinated M(N-R-pyridinaldimine)(PF 6) 2 complexes (M = Fe(II), Co(II), Ni(II) and Zn(II) show the presence of both cis and trans isomers in equilibrium. The Rh(en) 3+ 3 ion and the trans-Rh(en ) 2 XC1 n+ ions (where X = Cl, OH 2 and NH 3) display only one proton-decoupled C-13 signal.