1 H NMR BASICS

1 H NMR BASICS

Hydrogen NMR There are three isotopes of hydrogen used in NMR spectroscopy:  1Hydrogen, 2Deuterium and 3Tritium. Each isotope resonates at a  very different frequency for example  if 1H resonates at 400 MHz then 2H resonates at 61.402 MHz.  Only one isotope is observed at a time  because the spectrometer transmits and receives over a very limited frequency range.  The chemical shift ranges for all three nuclei are virtually identical  and can be used for preliminary analysis but there the similarity ends. 3Tritium is not commonly measured by NMR because it is radioactive. Each type of signal has a characteristic chemical shift range (fig. 1)  that can be used for initial assignment. Fig. 1. Chemical shift ranges of protons  according to their chemical environment Choose the structure that most closely represents the hydrogen in question.  R = alkyl or H, Ar = aryl. 1Hydrogen (Proton) NMR The 1D 1H (Proton) NMR experiment is the most common NMR experiment. The proton (1Hydrogen nucleus) is the most sensitive nucleus apart from tritium) and usually yields sharp signals. Even though its chemical  shift range is narrow, its sharp signals make proton NMR very useful. A typical analysis of a 1H NMR spectrum may proceed as follows: The number of protons of each type in the spectrum of a pure sample can be obtained directly from the integrals of each multiplet. This is only true if the multiplets are well separated and do not overlap the solvent or residual water signals and provided that the molecule is not undergoing slow conformational exchange. A routine NMR spectrum yields integrals with an accuracy of +/-10%.  Accuracies of +/-1% can be achieved by increasing the relaxation delay  to five times the longitudinal relaxation time (T1) of the signals of interest. Where multiplets overlap, the total integral of the spectral region may be used. From the table of the proton chemical shifts one obtains information about  each type of proton and can carry out a preliminary assignment. Consider ethanol as an example (Fig. 2). Using chemical shifts, the peak at 1.2 ppm is in the expected range for CH3 (methyl) and at 3.7 and 3.9 ppm are compatible with CH2 (methylene). The chemical shift of OH is very dependent on solvent and other experimental conditions so cannot be assigned by chemical shift alone. Using integration, we expect CH3 to have an integral of three, CH2 to have an integral of two and OH to have an integral of one. This is the case and so the assignment is complete. Fig. 2. 1H NMR spectrum of ethanol in CDCl3 For other molecules this is not sufficient and the multiplet structure is needed to complete the assignment. The multiplets (fig. 3) arise from spin-spin couplings that are transmitted through chemical bonds and yield information  about the immediate molecular environment.  In the case of CH3 and OH, they are split by the two neighboring protons of the CH2  to yield a triplet pattern called AX2. (Click here to see a list of common  homonuclear splitting patterns and a description of heteronuclear coupling.) The CH2 is split by the single OH proton and the three CH3 protons to form the AX3Y pattern.  Fig. 3. Multiplet structures from the 1H NMR spectrum of ethanol in CDCl3   Having determined the multiplicity, one may measure the coupling constants.  These are measured in Hz (not ppm), as they are field independent. If you find that two (and only two) multiplets contain the same coupling constant then  you know that they arise from nearby protons. The coupling constant gives an indication of the distance between the protons. In general 10 to 18 Hz means two bond or three bond trans to a C=C double bond. Coupling constants between 1 to 10 Hz  indicate three-bonds or more bonds if delocalized. Less than 1 Hz usually means four or more bonds. In addition to homonuclear couplings, the multiplets may be split by other nuclei such as 19Fluorine or 31Phosphorus. (If suchheteronuclear couplings are undesirable they may be decoupled. The best pulse sequence in such a case is that for inverse gated decoupling.) Properties of 1H Property Value Spin ½ Natural abundance 99.9845% Chemical shift range 13 ppm, from -1 to 12 Frequency ratio (Ξ) 100.000000% Reference compound TMS < 1% in CDCl3 = 0 ppm Linewidth of reference 0.08 Hz T1 of reference 14 s Receptivity rel. to 1H at natural abundance 1.000 Receptivity rel. to 1H when enriched 1.000 Receptivity rel. to 13C at natural abundance 5870 Receptivity rel. to 13C when enriched 5871 2Deuterium NMR 2Deuterium (heavy hydrogen) NMR is usually used for field frequency lock. At natural abundance it has very low sensitivity but when enriched it is of  medium sensitivity. Deuterium usually yields broad signals whose line width typically varies between a few hertz and a few kilohertz.  The spectrum has the same narrow chemical shift range as for   1H but its low resolution and lower sensitivity make it a poor alternative. Deuterium-deuterium couplings are about 40 times smaller that proton-proton couplings and are therefore not observed.  However, in partially deuterated molecules small proton-deuterium couplings can be obseved. The main use of deuterium spectra is for determining the effectiveness of chemical deuteration (fig. 4). Fig. 4. 1H and 2H NMR spectra of  Amphetamine sulfate–d3 showing successful specific deuteration of the methyl Properties of 2H Property Value Spin 1 Natural abundance 0.0155% Chemical shift range 13 ppm, from -1 to 12 Frequency ratio (Ξ) 15.350609% Reference compound TMS-d12 neat = 0 ppm Linewidth of reference 1.7 Hz T1 of reference 1 s Receptivity rel. to 1H at natural abundance 1.50 × 10-6 Receptivity rel. to 1H when enriched 9.65 × 10-3 Receptivity rel. to 13C at natural abundance 8.78 × 10-3 Receptivity rel. to 13C when enriched 56.7 Linewidth parameter 0.41 fm4 3Tritium NMR 3T is the only nucleus more sensitive than proton (1H). Being a spin-½ isotope of hydrogen, spectra of fully tritiated compounds  look similar to 1H with effectively the same chemical shifts  but with slightly higher sensitivity, dispersion and coupling constants. However,   3T is very radioactive so most NMR studies are carried out with partially and specifically labeled compounds. Properties of 3H Property Value Spin ½ Natural abundance 0.0000000000000003% Chemical shift range 13 ppm, from -1 to 12 Frequency ratio (Ξ) 106.663974% Reference compound TMS-t1 <1% in CDCl3 = 0 ppm Linewidth of reference ~0.1 Hz T1 of reference ~20 s Receptivity rel. to 1H at natural abundance 4 × 10-18 Receptivity rel. to 1H when enriched 1.21 Receptivity rel. to 13C at natural abundance 2 × 10-14 Receptivity rel. to 13C when enriched 7103

Carbon-13 NMR

Carbon-13 NMR (¹³C NMR or sometimes simply referred to as carbon NMR) is the application of nuclear magnetic resonance (NMR) spectroscopy to carbon. It is analogous to proton NMR (1H NMR) and allows the identification of carbon atoms in an organic moleculejust as proton NMR identifies hydrogen atoms. As such ¹³C NMR is an important tool in chemical structure elucidation in organic chemistry. ¹³C NMR detects only the 13C isotope of carbon, whose natural abundance is only 1.1%, because the main carbon isotope,12C, is not detectable by NMR since it has zero net spin.

Implementation

¹³C NMR has a number of complications that are not encountered in proton NMR. ¹³C NMR is much less sensitive to carbon than ¹H NMR is to hydrogen since the major isotope of carbon, the ¹²C isotope, has a spin quantum number of zero and so is not magnetically active and therefore not detectable by NMR. Only the much less common ¹³C isotope, present naturally at 1.1% natural abundance, is magnetically active with a spin quantum number of 1/2 (like ¹H) and therefore detectable by NMR. Therefore, only the few ¹³C nuclei present resonate in the magnetic field, although this can be overcome by isotopic enrichment of e.g. protein samples. In addition, thegyromagnetic ratio (6.728284 10⁷ rad T⁻¹ s⁻¹) is only 1/4 that of ¹H, further reducing the sensitivity. The overall receptivity of ¹³C is about 4 orders of magnitude lower than ¹H.

^[1]

Another potential complication results from the presence of large one bond J-coupling constants between carbon and hydrogen (typically from 100 to 250 Hz). In order to suppress these couplings, which would otherwise complicate the spectra and further reduce sensitivity, carbon NMR spectra are proton decoupled to remove the signal splitting. Couplings between carbons can be ignored due to the low natural abundance of ¹³C. Hence in contrast to typical proton NMR spectra which show multiplets for each proton position, carbon NMR spectra show a single peak for each chemically non-equivalent carbon atom.

In further contrast to ¹H NMR, the intensities of the signals are not normally proportional to the number of equivalent ¹³C atoms and are instead strongly dependent on the number of surrounding spins (typically ¹H). Spectra can be made more quantitative if necessary by allowing sufficient time for the nuclei to relax between repeat scans.

High field magnets with internal bores capable of accepting larger sample tubes (typically 10 mm in diameter for ¹³C NMR versus 5 mm for ¹H NMR), the use of relaxation reagents,^[2] for example Cr(acac)₃ (chromium (III) acetylacetonate, CAS number 21679-31-2), and appropriate pulse sequences have reduced the time needed to acquire quantitative spectra and have made quantitative carbon-13 NMR a commonly used technique in many industrial labs. Applications range from quantification of drug purity to determination of the composition of high molecular weight synthetic polymers.

¹³C chemical shifts follow the same principles as those of ¹H, although the typical range of chemical shifts is much larger than for ¹H (by a factor of about 20). The chemical shift reference standard for ¹³C is the carbons in tetramethylsilane (TMS), whose chemical shift is considered to be 0.0 ppm.

Typical chemical shifts in ¹³C-NMR

DEPT spectra

DEPT spectra of propyl benzoate

DEPT stands for Distortionless Enhancement by Polarization Transfer. It is a very useful method for determining the presence of primary, secondary andtertiary carbon atoms. The DEPT experiment differentiates between CH, CH₂and CH₃ groups by variation of the selection angle parameter (the tip angle of the final ¹H pulse):

• 135° angle gives all CH and CH₃ in a phase opposite to CH₂
• 90° angle gives only CH groups, the others being suppressed
• 45° angle gives all carbons with attached protons (regardless of number) in phase

Signals from quaternary carbons and other carbons with no attached protons are always absent (due to the lack of attached protons).

The polarization transfer from ¹H to ¹³C has the secondary advantage of increasing the sensitivity over the normal ¹³C spectrum (which has a modest enhancement from the NOE (Nuclear Overhauser Effect) due to the ¹H decoupling).

APT spectra 

Another useful way of determining how many protons a carbon in a molecule is bonded to is to use an Attached Proton Test, which distinguishes between carbon atoms with even or odd number of attached hydrogens. A proper spin-echo sequence is able to distinguish between S, I₂S and I₁S, I₃S spin systems: the first will appear as positive peaks in the spectrum, while the latter as negative peaks (pointing downwards), while retaining relative simplicity in the spectrum since it is still broadband proton decoupled.

Even though this technique does not distinguish fully between CH_n groups, it is so easy and reliable that it is frequently employed as a first attempt to assign peaks in the spectrum and elucidate the structure.^[3]

1. ^ R. M. Silverstein, G. C. Bassler and T. C. Morrill (1991). Spectrometric Identification of Organic Compounds. Wiley.
2. ^ Caytan, Elsa; Remaud, Gerald S.; Tenailleau, Eve; Akoka, Serge, GS; Tenailleau, E; Akoka, S (2007). "Precise and accurate quantitative 13C NMR with reduced experimental time". Talanta 71 (3): 1016–1021. doi:10.1016/j.talanta.2006.05.075. PMID 19071407
3. ^ Keeler, James (2010). Understanding NMR Spectroscopy (2nd ed.). John Wiley & Sons. p. 457. ISBN 978-0-470-74608-0.

• Carbon NMR spectra, where there are three spectra of ethyl phthalate, ethyl ester of orthophthalic acid: completely coupled, completely decoupled and off-resonance decoupled (in this order).

• For an extended tabulation of ¹³C shifts and coupling constants.

Raman Imaging Breaks The Nanometer Barrier

Raman Imaging Breaks The Nanometer Barrier

Spectroscopy: Chemical analysis technique uses double-resonance approach to zoom in on single molecules.

 

An experimental TERS image (top left) of a single porphyrin molecule (right) and its theoretical simulation (bottom left).

Credit: Guoyan Wang & Yan Liang

http://cen.acs.org/articles/91/i23/Raman-Imaging-Breaks-Nanometer-Barrier.html

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High-Throughput qNMR

It is well known that NMR is a very convenient technique for quantification, provided the amount of the material is within the limits of detection in NMR. 

When it comes to the actual calculations, this is a very straightforward process that does not require any fancy mathematics. Typically you will select the most convenient signal(s) or multiplet(s) in your spectrum and calculate the integral which can then be mapped to the corresponding concentration units by using a scaling factor that was previously calculated using some internal or external references.

 

read all at

http://nmr-analysis.blogspot.in/2012/12/high-throughput-qnmr.html

Infrared Spectroscopy,Characterisation of Organic Compounds

 This discussion is on infrared (IR) spectroscopy which tells about the functional groups in the molecule.

William de Wiveleslie Abney

Almost any compound having covalent bonds absorbs various frequencies of electromagnetic radiation in the infrared region of the electromagnetic spectrum. This region lies at wavelength longer than those associated with visible light, which range from approximately 400 to 800 nm, but lies at wavelengths shorter than those associated with microwaves, which are longer than 1 mm. For chemical purposes, we are interested in the vibrational portion of the infrared region. It includes radiations with wavelengths between 2.5 μm and 25 μm.


In the early use of IR spectroscopy in 1882, William Abney managed to identify 52 benzene derivatives from the IR spectroscopy. IR spectroscopy is used to see the the functional group in the molecule as the bonds vibrates at certain wavenumber and it only vibrate at only certain allowable frequencies. 

The covalent bonds in a molecule can be described in similar way with a 2 balls (atoms) that connected with a spring (the bond). The bond distance continually changes, but an equilibrium or average bond distance can be defined. Whenever the spring is stretched or compressed beyond this equilibrium distance, the potential energy of system increases. As the spring moves in a harmonic oscillation, the energy is proportional to the frequency of vibration which is determined by the force constant (K) of the spring and the masses of two bonded atoms. The natural frequency of vibration of a bond is given by the equation

which is derived from Hooke's Law for vibrating springs. The reduced mass, μ, of the system is given by

From those equations, two things should be noticeable immediately. One is that stronger bonds have a larger force constant K and vibrate at higher frequencies than weaker bonds. Therefore, triple bonds will vibrate at higher frequencies (higher wavenumber) than double bonds or single bonds.

The second is that bonds between atoms of higher masses vibrate at lower frequencies than bonds between lighter atoms as shown below.

Besides that, when a bond vibrates, not all modes of vibration are allowed. When vibration modes do not provides no dipole moment change, so it is not allowed; Only vibration modes give dipole moment change that is allowed. The result of unallowed vibration modes is there is no peak or signal in the spectra.

1-octyne (left) and 4-octyne (right)

Alkenes

Alkenes show many more peaks than alkanes. The principal peaks of diagnostic value are the C-H stretching peaks for the sp² carbon at value greater than 3000 cm^-1, along with C-H peaks for the sp³ carbon atoms. Also the C=C stretching peak near 1650 cm^-1, with higher intensity of cis- than trans- configuration.

cis-pentene (above) and trans-pentene (below)

O-H and N-H stretching

The signals for O-H and N-H stretching occur around 3300 cm^-1, but they look different. O-H stretching broad bands centering between 3400 and 3300 cm^-1. In solution, it will also be possible to observe a free O-H stretching band at about 3600 cm^-1 (sharp and weaker) to the left of the hydrogen bonded O-H peak.

1-butanol

Meanwhile, primary amines, R-NH₂, show two N-H stretching bands in the range 3500-3300 cm^-1, whereas secondary amines, R₂N-H, show only one band in that region. Tertiary amines will not show an N-H stretch. Because of these features, it is easy to differentiate among primary, secondary, and tertiary amines by inspection of the N-H stretch region.

butylamine (above), dibutylamine (middle), and tributylamine (below)

Carbonyl stretching

The carbonyl group is present in aldehydes, ketones, acids, esters, amides, acid chlorides, and anhydrides. This group absorbs strongly in the range from 1850 to 1650 cm^-1 because of its large change in dipole moment. In figure below provides the normal bas value for the C=O stretching vibrations of the various functional groups. The C=O frequency of a ketone, which is approximately in the middle of the range, is usually considered the reference point for comparison of these values. In this section we will focus on aldehydes, ketones, acids, and esters.

Normal base value for the C=O stretching vibrations for carbonyl groups

Aldehydes show a very strong band for the carbony group that appear in the range of 1740-1725 cm^-1. A very important doublet can be observed in the C-H stretch regio for the aldehyde C-H near 2850 and 2750 cm^-1. The presence of this doublet allows aldehydes to be distinguished from other carbonyl-containing compounds. In the other sides, ketones show a very strong band for C=O group that appears in the range of 1720-1708 cm^-1.

Nonanal (left) and 2-nonanone (right)

Carboxylic acids show a very strong band for the C=O group that appears in the range of 1730-1700 cm^-1 and the O-H stretch appears in the spectrum as a very broad band extending from 3400 - 2400 cm^-1. This broad band centers on about 3000 cm^-1 and partially obsecures the C-H stretching bands. If the very broad O-H stretch band is seen, along with a C=O peak, it almost certainly indicates the compound is a carboxylic acid.

Nonanoic acid

Besides, there are variations of carbonyl compounds that can shift the C=O stretch frequency. The first one, there is conjugation of C=O with C=C, it lowers the stretching frequency to around 1680 cm^-1.

4-methyl pent-3-en-2-one (mesityl oxide)

The carbonyl amide absorbs at an even lower frequency, 1640-1680 cm^-1, but the carbonyl ester absorbs at higher frequency, 1730 - 1740 cm^-1.

Butyl propanoate (left) and N-butyl propanamide (right)

Besides that, carbonyl groups in small rings (5 carbons or less) absorb at an even higher frequency.

The C=O stretching vibrations for cyclic ketones

Carbon-Nitrogen Stretching

The C-N stretching absorbs around 1200 cm^-1, and as the bond stronger the C=N stretch absorbs around 1660 cm^-1 and is much stronger than the C=C absorption in the same region. The nitriles group absorb strongly just above 2200 cm^-1. The alkyne C=Csignal is much weaker and is just below 2200 cm^-1.

Octanenitrile

To summarise this section, we will see the approximation range of those signals in IR spectroscopy.

IR spectroscopy correlation chart