Infrared vibrations of small molecules and oxo-anions

Fundamental vibrations of H₂O, CO₂, SO₂, NH₃ and CH₄ (gas) and of CO₃²⁻, NO₃⁻ and SO₄²⁻: wavenumber, symmetry, degeneracy, and infrared and Raman activity.


$\ce{H2O}$	ν1	a1	symmetric stretch	3657	2.73	1	weak	active
$\ce{H2O}$	ν2	a1	bend	1595	6.27	1	strong	active
$\ce{H2O}$	ν3	b2	asymmetric stretch	3756	2.66	1	strong	active
$\ce{CO2}$	ν1	σg+	symmetric stretch	1333	7.50	1	absent	active
$\ce{CO2}$	ν2	πu	bend	667	14.99	2	strong	inactive
$\ce{CO2}$	ν3	σu+	asymmetric stretch	2349	4.26	1	very strong	inactive
$\ce{SO2}$	ν1	a1	symmetric stretch	1151	8.69	1	medium	active
$\ce{SO2}$	ν2	a1	bend	518	19.31	1	medium	active
$\ce{SO2}$	ν3	b2	asymmetric stretch	1362	7.34	1	very strong	active
$\ce{NH3}$	ν1	a1	symmetric stretch	3337	3.00	1	weak	active
$\ce{NH3}$	ν2	a1	bend	950	10.53	1	very strong	active
$\ce{NH3}$	ν3	e	asymmetric stretch	3444	2.90	2	weak	active
$\ce{NH3}$	ν4	e	bend	1627	6.15	2	medium	active
$\ce{CH4}$	ν1	a1	symmetric stretch	2917	3.43	1	absent	active
$\ce{CH4}$	ν2	e	bend	1534	6.52	2	absent	active
$\ce{CH4}$	ν3	f2	asymmetric stretch	3019	3.31	3	strong	active
$\ce{CH4}$	ν4	f2	bend	1306	7.66	3	strong	active
$\ce{CO3^2-}$	ν1	a1'	symmetric stretch	1063	9.41	1	absent	active
$\ce{CO3^2-}$	ν2	a2''	out-of-plane bend	879	11.38	1	medium	inactive
$\ce{CO3^2-}$	ν3	e'	asymmetric stretch	1415	7.07	2	very strong	active
$\ce{CO3^2-}$	ν4	e'	bend	680	14.71	2	weak	active
$\ce{NO3-}$	ν1	a1'	symmetric stretch	1050	9.52	1	absent	active
$\ce{NO3-}$	ν2	a2''	out-of-plane bend	831	12.03	1	medium	inactive
$\ce{NO3-}$	ν3	e'	asymmetric stretch	1390	7.19	2	very strong	active
$\ce{NO3-}$	ν4	e'	bend	720	13.89	2	weak	active
$\ce{SO4^2-}$	ν1	a1	symmetric stretch	983	10.17	1	absent	active
$\ce{SO4^2-}$	ν2	e	bend	450	22.22	2	absent	active
$\ce{SO4^2-}$	ν3	f2	asymmetric stretch	1105	9.05	3	very strong	active
$\ce{SO4^2-}$	ν4	f2	bend	611	16.37	3	strong	active

Gas-phase values are fundamentals (CO₂ ν₁ is the mean of the 1285/1388 cm⁻¹ Fermi dyad; NH₃ ν₂ the centre of the 932/968 cm⁻¹ inversion doublet); the ions are typical solid and solution values that shift by tens of cm⁻¹ with the cation and lattice. Infrared intensities are coarse classes.

Sources

T. Shimanouchi, Tables of Molecular Vibrational Frequencies, Consolidated Volume I, NSRDS-NBS 39 (1972), via the NIST Chemistry WebBook.
K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part A, 6th ed. (2009), ch. 2.
G. Herzberg, Molecular Spectra and Molecular Structure II: Infrared and Raman Spectra of Polyatomic Molecules (1945).

Used in these lessons

Infrared spectroscopy: molecular vibrations