H3 Chemistry: Use the delta scale and TMS reference

Your success criteria

• Use the delta scale and TMS reference
• Use named chemical evidence.
• Transfer the governing reason to an unfamiliar case.

Anchor shifts to TMS

On a 400 MHz instrument a signal 400 Hz from TMS has δ=1.00 ppm.

The same environment at 600 MHz is 600 Hz from TMS but remains δ=1.00 ppm.

Explore this H3 topic and lesson sequence.

Define the reference and scale

Tetramethylsilane, Si(CH3)4, defines δ=0 ppm and gives one sharp line from twelve equivalent protons.

Use δ=(νsample−νTMS)/ν0×10^6, with both frequencies in the same units.

Why ppm transfers

Absolute Hz separation grows with operating frequency, while division by ν0 removes that field dependence.

TMS is chemically unreactive in typical samples, volatile, and normally appears away from organic proton signals.

Calculate a shift

A 500 MHz spectrum places a signal 750 Hz from TMS: 750/(500×10^6)×10^6=1.50 ppm.

The MHz-to-Hz conversion must be made before cancellation.

• Find δ for 750 Hz at 500 MHz.

Convert ppm to hertz

At 300 MHz, δ=2.00 ppm corresponds to 600 Hz from TMS.

Multiply 2.00×300; ppm and MHz combine numerically to Hz.

• Find the offset for δ=2.00 at 300 MHz.

Start the diagnostic and follow its feedback

Compare instruments

A δ=4.20 signal is 1680 Hz from TMS at 400 MHz and 2520 Hz at 600 MHz.

Its chemical shift is unchanged although its absolute separation changes.

• Calculate both offsets.

Repair an absolute-scale claim

TMS is assigned zero by convention; it does not have zero resonance frequency.

Every nucleus resonates near the instrument frequency, while δ reports only a relative displacement.

• Repair: ‘TMS absorbs at 0 Hz.’

Apply the portable scale

Complete twelve reference calculations, then a 500 MHz assessment and a different 600 MHz retest.

Next: explain inductive deshielding at /learning/h3-nmr-inductive-shielding-lesson.html.

• Annotate νsample, νTMS, ν0 and δ on the fixed table.