Assigning the 1H NMR Signals of Aromatic Ring 1H atoms

Assigning The 1h Nmr Signals Of Aromatic Ring 1h Atoms-Free PDF

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Possible Signal Appearance, Ha dd Jortho Jmeta, Hb td Jortho Jmeta or ddd Jortho Jortho Jmeta. Hc td Jortho Jmeta or ddd Jortho Jortho Jmeta, Hd dd Jortho Jmeta. The appearance in the spectrum of two triplets of doublets indicates that Jab is almost equal to Jbc. and Jcd From this information alone the signals can be identified as Ha or Hd and Hb or Hc as shown. 2 Changes in Shielding by Carbon Atom Charge, The magnetic field experienced by each 1H atom is influenced by the electron density at the carbon. atom to which it is attached The charge can be estimated in several ways two of which are. provided below ,a Conjugation Depicted by Resonance Structures. Since benzene has an 1H NMR chemical shift of about 7 3 ppm for its H atoms substituted benzenes. will have chemical shifts slightly upfield or downfield of 7 3 ppm For substituents that are. conjugated to the aromatic system resonance structures are a convenient way to estimate. whether a particular position will be relatively shielded or deshielded by the substituent The amino. group NH2 in o isopropylaniline is an electron donating group through conjugation of its p rich. lone pair to the aromatic system , The prediction of extra electron density negative charge at the carbon atoms ortho and para to the.
amino substituent indicate that the chemical shift of Ha and Hc will be more upfield as the H atoms. are more shielded The limitations of this method are significant The resonance structures indicate. that the ortho and para positions will be more shielded but does not indicate which one will be. most shielded This method only works for substituents that are conjugated by resonance and. offers no way to predict the charge distribution caused by non conjugated substituents like the. isopropyl group Even for the amino group which is donating there is no straightforward way to. think about the electron withdrawing nature of the nitrogen via its bond to the ring carbon atom . b Natural Bond Orbital NBO calculations to estimate the Natural Population Analysis NPA . All of the and donating and withdrawing effects of benzene substituents can be estimated via an. NBO calculation in WebMO Gaussian 09 As with all computational approaches this requires an. optimized molecule at a reasonable level of theory and basis set The calculation presented below. was completed at the B3LYP 6 31G d level , deshielded H atoms shielded H atoms. downfield signals upfield signals, Hb and Hd Ha and Hc. 0 22 0 22, While neither the coupling pattern nor carbon atom charge estimation are sufficient to assign all of. the signals the combination of the first two methods is sufficient to unambiguously assign all four of. the signals in the aromatic region ,3 Model compound. An often over looked and under utilized method of assigning the signals in the 1H NMR region is by. comparison to known molecules Let us assume that the 1H NMR spectrum of o isopropylaniline is. not available in any database It is very similar to at least three common compounds benzene . aniline and cumene that might provide estimates of the chemical shift of its H atoms The Spectral. Database for Organic Compounds or SDBS http sdbs riodb aist go jp sdbs cgi bin cre index cgi . provides the 1H NMR spectra for each of these molecules . With these known experimental chemical shifts the impact of each substituent NH2 and iPr can. be quantified by a comparison of the model compound to benzene and provide a closer. approximation of the chemical shifts in o isopropylaniline The hydrogen atom labeled Ha below in. o isopropylaniline should be shifted in a manner similar to the ortho H atoms in aniline 0 675 ppm . and the meta H atoms in cumene 0 109 ppm relative to benzene Combining these shift effects. predicts a chemical shift for the atom labeled Ha in o isopropylaniline shown below . Hd Hb Hc Ha, 7 01 ppm 6 96 ppm 6 68 ppm 6 59 ppm, As can be seen in the comparison between the model compound predictions in blue and the actual.
spectrum there is an error of about 0 12 ppm in the worst predicted signal location The relative. ordering of the signals from high to low chemical shift is correct and in a simple system such as this . the assignment can be made in an unambiguous manner The limitations of this method include the. availability of spectral data for model compounds in the same solvent as the investigated molecule . While the model compound limitation may be overcome by increasing the amount of data available. in the database this method will always be limited in its ability to account for subtle structural. differences in the ring upon substitution of the studied compound caused by multiple substitutions . 4 Curphy Morrison Additivity Constants, This method is a more sophisticated extension of the model compound approach detailed above. where many molecules with a particular functional group have been studied and parameters for the. effect of that substituent on each H atom on the aromatic ring have been determined The. constants will more accurately predict chemical shifts when fewer of them are required and when. the studied compound is similar to those that were used to develop the parameters A very nice list. of the constants is available from Professor Hans Reich via the following links . Curphy Morrison Additivity Constants for Proton NMR . http www chem wisc edu areas reich nmr notes 9 hmr 5 curphy morrison pdf. Curphy Morrison Additivity Constants for Proton NMR vinyl and aryl . http www chem wisc edu areas reich nmr notes 9 hmr 6 vinyl aryl shifts pdf. Each benzene ring H atom is given a standard shift value of 7 36 ppm and adjusted by up to 5 terms. for all of the non H atom substituents on the benzene ring Unfortunately for estimating the shifts. for o isopropylaniline there is no isopropyl substituent listed Since many alkyl groups have similar . small shift effects this will not likely have a large impact on the shift calculation with methyl and. tbutyl groups available Since the chemical shift effect of an iPr group is likely somewhere between. a Me and a tBu group a rough average can be applied with a weight of 1 3 Me and 2 3 tBu An. example calculation for Ha is provided below along with the estimates for Ha Hd . Ha 7 36 standard shift, 0 71 ortho to NH2, 0 09 meta to iPr. 6 56 Estimated shift, Not surprisingly the estimate from the Curphy Morrison Constants is nearly equal to that of the. estimation from model compounds Both of these methods can produce reliable results . 5 NMR calculation using Gaussian 09 WebMO, A computational approach can provide reasonable 1H NMR shift predictions provided that the level. of theory and basis set provide a high quality estimate of the molecular geometry and the magnetic. field of the molecule A simple approach will be employed below where the lowest energy. conformation is optimized and an NMR calculation is performed on that species As always the. nature of the structure as a minimum energy on the potential energy surface should be confirmed. by a vibrational frequency calculation The B3LYP 6 31G d 1H NMR chemical shifts listed below are. the unscaled Isotropic Absolute NMR Shifts referenced to TMS . 6 26 ppm, 6 89 ppm 6 94 ppm, 6 58 ppm, Hd Hb Hc Ha.
6 94 ppm 6 89 ppm 6 58 ppm 6 26 ppm, It is highly unlikely that this simple one conformer approach with a relatively unsophisticated B3LYP. calculation will yield quantitatively accurate predictions This method however is likely to be sufficient. to produce the correct ordering of the signals from high to low chemical shift as shown above A more. well conceived computational approach might determine the chemical shift for each H atom in the. molecule in each conformation and weight the shifts according to the relative energies of each species . With all of this information available the assignment of the 1H signals has become trivial . Assigning the 1H NMR Signals of Aromatic Ring 1H atoms Assigning 1H NMR signals of 1H atoms on an aromatic ring based upon their chemical shift and coupling can be accomplished in a number of different ways which will be detailed below These methods which range from very simple to somewhat sophisticated are complimentary to one another For an example the aromatic region of the 1H NMR of o

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