Chemical Shift and TMS

• Shielded protons absorb signals on the right side of the spectrum.
• Deshielded protons absorb signals on the left side of the spectrum.

Chemical Shift

• Differences in resonance frequency are very small, making them hard to measure.
• For example, the difference between proton in chloromethane and fluoromethane is 72 Hz at 1.41 Tesla.
• A reference compound is used to measure resonance frequency of each proton relative to it.
• Tetramethylsilane TMS ((CH3)4Si) serves as standard because its protons are more shielded.
• TMS protons are moe shielded than most organic compounds due to electronegativity.
• Compounds don't have better-shielded hydrogens than TMS.
• Proton resonance is measured by how far (in Hz) compounds shift from TMS resonance, depending on field strength.

Tesla and Frequency

• At 1.4 (14100 Gauss) Tesla, protons resonate at 60 MHz, and at 2.3 (23500 Gauss) Tesla, at 100 MHz.
• The ratio of frequency and strength is the same: [100/60 = 23500/14100 = 5/3]
• The shift from TMS in a 100 MHz field is 5/3 larger than in a 60 MHz field for a given proton.

Chemical Shift (δ)

• Comparing data from instruments with different field strengths presents a challenge.
• This is solved by using a parameter independent of field strength.
• Chemical shift (δ) is the shift in Hz divided by the instrument's frequency (in MHz).
• δ = (shift in Hz) / (spectrometer frequency in MHz)
• δ represents the amount a proton resonance shifts from TMS, measured in parts per million (ppm) of the spectrometer's operating frequency.

Delta Values

• Delta (δ) values remain consistent across instruments operating at 60 MHz, 100 MHz, or 220 MHz,
• For CH3Br, the shift is 162 Hz at 60 MHz and 270 Hz at 100 MHz, with δ = 2.7 in both cases [162/60 = 270/100 = 2.7 ppm]
• Chemical shift is reported in ppm, with TMS set at δ = 0.00.
• In the tau (τ) scale, TMS has a value of 10.00, where τ = 10 - δ.

Factors Affecting Chemical Shift

• Electronegativity of nearby atoms
• Hybridization of adjacent atoms
• Diamagnetic effects from adjacent pi bonds

Electronegativity and Chemical Shift

• CH3F: Electronegativity = 4.0, Chemical Shift (δ) = 4.26
• CH3OH: Electronegativity = 3.5, Chemical Shift (δ) = 3.47
• CH3Cl: Electronegativity = 3.1, Chemical Shift (δ) = 3.05
• CH3Br: Electronegativity = 2.8, Chemical Shift (δ) = 2.68
• CH3I: Electronegativity = 2.5, Chemical Shift (δ) = 2.16
• (CH3)4C: Electronegativity = 2.1, Chemical Shift (δ) = 0.86
• (CH3)4Si: Electronegativity = 1.8, Chemical Shift (δ) = 0.00

Hybridization and Chemical Shift

• Alkyl (RCH3, R2CH2, R3CH): Chemical Shift (δ) = 0.8 - 1.7
• Allylic (R2C=C(R)CHR2): Chemical Shift (δ) = 1.6 - 2.6
• Acetylenic (RC≡CH): Chemical Shift (δ) = 2.0 - 3.0
• Vinylic (R2C=CHR, R2C=CH2): Chemical Shift (δ) = 4.6 - 5.7
• Aldehydic (RCHO): Chemical Shift (δ) = 9.5 - 10.1

Diamagnetic Effects

• A carbon-carbon triple bond shields an acetylenic hydrogen, shifting its signal to a lower frequency (to the right), smaller δ value.
• A carbon-carbon double bond deshields vinylic hydrogens, shifting their signal to a higher frequency (to the left), larger δ value.
• Alkyl groups (RCH3) has a chemical shift(δ) of 0.8-1.0
• Acetylenic groups (RC≡CH) have a chemical shift (δ) of 2.0 - 3.0
• Vinylic groups (R2C=CH2) have a chemical shift (δ) of 4.6 - 5.7

Chemical Shift Values for Benzene

• In a magnetic field, benzene's six π electrons circulate, creating a ring current.
• The induced magnetic field reinforces the applied magnetic field near the protons.
• The protons experience a stronger magnetic field, requiring a higher frequency for resonance, thus are deshielded and absorb downfield.
• In a magnetic field, loosely held π electrons of the double bond create a magnetic field that reinforces the applied field in the vicinity of the protons.
• The protons now feel a stronger magnetic field, requiring a higher frequency for resonance, and the absorption is downfield.

Carbon-Carbon Triple Bond

• π electrons circulate, but the induced magnetic field opposes the applied magnetic field.
• The proton feels a weaker magnetic field, so a lower frequency is needed for resonance.
• The nucleus is shielded, and the absorption is upfield.

Reinforcing Fields

• Aromatic compounds have a higher reinforcing field, resulting in a higher shift compared to a single π system in a double bond.
• The direction of the field, in contrast to the position of the proton in an sp (triple bond) hybridized molecule, opposes field, resulting in a shielding effect..

NMR Spectrophotometer Components

• Magnet
• Sample and sample holder
• Radiofrequency generator
• Detector
• Recorder
• In the NMR spectrophotometer, the sample is dissolved in a solvent with no interfering protons (e.g., CDCl3), internal standard TMS is added.
• The sample in a cylindrical glass tube is placed between the faces of the magnet's pole pieces.
• A radiofrequency generator irradiates the sample with a short pulse of radiation, causing resonance.
• A detectors measure the energy released when nuclei return to their low energy state to record the spectrum.
• Modern NMR spectrometers use superconducting magnets with coils cooled in liquid helium for resistance-free electricity.

Spectrometer Components Details

• Magnet: provides a stable and homogeneous field, typically 15 inches, capable of producing 23,500 gauss for 100MHz.
• Sample and sample holder: uses a 1–30 mg sample in dilute solution (2–10%).
• The sample holder is a glass tube, about 5 mm in diameter and 15-20 cm in length.
• Radiofrequency oscillator; installed perpendicular to the magnetic field, transmits radio waves, of mixed frequencies (60-300MHz).
• A sweep generator supplies DC current to a secondary magnet.
• RF detector/receiver: Installed perpendicular to both magnetic and oscillator coil, tuned to the same frequency as transmitter.
• When precession frequency matches RF, the nuclei induces (emf) in the detector coil, sending an amplified signal to the recorder.
• Recorder: displays a spectrum: plot showing resonance signal strength/magnetic field strength on its Y/X axis..
• The strength of the resonance signal is proportional to the number of resonating nuclei at that field strength.

Instrument Operation

• A specifically made sample long cylindrical glass tube is placed in the NMR instrument.
• The sample is dissolved in a proton-free solvent (CDCl3 or CCl4,) with a bit of TMS internal reference added.
• The sample is positioned between magnetic poles, a RF generator-attached coil is included (e.g., 60 MHz)
• This coil provides Energy to change spin orientation of proton orientation.
• When resonance happens, detector coil senses radiofrequency signal, .

Instrument processes

• With increasing magnetic field strength, the precessional frequency of all protons increases, with resonance occurring at 60 MHz.
• When magnetic field increases, the recorder pen moves from left to right.
• Thus deshielded protons resonates faster (on the left (downfield), while shielded protons chart's right side (upfield).
• Because highly shielded protons precess more slowly than less shielded protons the field is increased.
• This induces them to precess at 60 MHz, leading shielded protons to the right and deshielded to the left.
• TMS has a peak value of δ = 0 ppm as the most shielded proton.

Pulsed Fourier Transform (FT) Instrument

• Advanced NMR technique uses frequency pulses to resonate all precessing nuclei at the same time.
• Emit radio frequencies as a result of their relaxation, which the computer design soft ware helps in recording.
• This software calculates the shift from pulse frequency. For example, for an acetone proton (single type).

Solvents for Preparing sample

• The solvent used for dissolving a sample must have:
  • No protons
  • Low boiling point
  • Non-polar nature
  • Inexpensive
• Deuterated chloroform is typically preferred as a solvent.
• Deuterium oxide, Dimethyl sulfoxide (DMSO), Carbon tetrachloride, Carbon disulfide, Trifluoromethane, Carboxylic acid can be used,