Perdeuterated methane

Names
Preferred IUPAC name (2H4)methane
Other names Methane-d4 Perdeuteriomethane Tetradeuteriomethane Carbon tetradeuteride[1]
Identifiers
CAS Number	558-20-3
3D model (JSmol)	Interactive image
EC Number	209-191-7
PubChem CID	123070
InChI InChI=1S/CH4/h1H4/i1D4 Key: VNWKTOKETHGBQD-JQYAHLJZSA-N
SMILES [2H]C([2H])([2H])[2H]
Properties
Chemical formula	CD4
Molar mass	20.067 g·mol-1[2]
Appearance	Colorless gas
Odor	Odorless
Melting point	−183[3] °C (−297.4 °F; 90.1 K)
Boiling point	−161[3] °C (−258 °F; 112 K)
Hazards
GHS labelling:
Pictograms	[2]
Signal word	Danger[2]
Hazard statements	H220, H280[2]
Precautionary statements	P203, P210, P222, P280, P377, P381, P403, P410+P403[2]
NFPA 704 (fire diamond)	0 4 0 SA
Related compounds
Related alkanes	Perdeuterated ethane Perdeuterated propane Perdeuterated butane
Related compounds	Methane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Perdeuterated methane (CD₄) is an isotopologue of methane (CH₄).

It is a colorless, odorless gas at standard conditions, chemically very similar to methane but exhibiting significant differences due to the kinetic isotope effect and vibrational isotope effect arising from the doubled atomic mass of deuterium compared to protium.

It has a higher molar mass than methane, which influences several macroscopic properties. The following table compares its key physical constants and methane:

Due to the mass difference, it shows small but measurable deviations in vapor pressure, heat capacity, and phase transition temperatures compared to methane.

The most pronounced differences between it and methane appear in vibrational spectroscopy because of the lower vibrational frequencies of C–D bonds.

it retains tetrahedral (T_d) symmetry. Its fundamental vibrational modes are shifted to significantly lower wavenumbers:

• The C–D stretching modes (ν₁ and ν₃) appear around 2100–2260 cm⁻¹, compared to 2917–3019 cm⁻¹ in methane.
• The deformation modes (ν₂ and ν₄) appear around 990–1100 cm⁻¹, compared to 1306–1534 cm⁻¹ in methane.

These large red shifts make it easily distinguishable in infrared (IR) and Raman spectroscopy. The ν₃ and ν₄ modes are infrared-active, while ν₁ and ν₂ are Raman-active. it has no permanent dipole moment and thus shows only very weak pure rotational transitions in the microwave region.

Chemically, it behaves similarly to methane but reacts more slowly in processes involving C–H/D bond breaking due to the primary kinetic isotope effect (KIE). The C–D bond has lower zero-point energy, resulting in higher activation energy for bond cleavage.

Typical k_H/k_D ratios range from 2 to >7 depending on the reaction and temperature. This effect is particularly important in:

• Atmospheric oxidation by OH radicals or Cl atoms
• Catalytic partial oxidation
• Surface dissociation and chemisorption studies

it is frequently used as an isotopic tracer and in mechanistic studies to probe reaction pathways.

• Solubility: Similar to methane; poorly soluble in water but soluble in organic solvents.
• Phase behavior: it exhibits similar solid phases (I, II, III) as methane, but with slightly shifted transition temperatures due to differences in vibrational entropy.

• Methane