Table 1.

Experimental wavenumbers $\tilde{\nu }$ and relative intensities $I_e$ (normalised to ${\nu }_5+{\nu }_6+{\nu }_7+{\nu }_8+{\nu }_9+{\nu }_{14}=100$) of methyl methanoate modes ${\nu }_n(\Gamma )$ compared to intensities calculated on different levels in the double-harmonic approximation: B = B3LYP, P = PBE0, d = def2-QZVPP, dD = def2-QZVPPD, a = aug-cc-pVQZ, in bold, if the harmonic value disagrees with (anharmonic) experiment in its error bars by more than 20%. For vibrations without a sharp Q-branch, the average of the wavenumbers of the rotational maxima is given. Description of vibrations: $\nu$ stretching, $\delta$ bending, $\tau$ torsion, $\omega$ wagging, ip/op in plane/out of plane, (a)s (a)symmetric. ^a An asymmetric double peak is observed.

Mode	$\tilde{\mathit{\nu }}$/cm${}^{-1}$		${\mathit{I}}_{\mathit{e}}$	B/d	B/dD	B/a	P/d	P/a	Γ	Mode Description
${\nu }_{18}$	132 ± 4		1.9 ± 1.2	6.7	5.7	5.1	7.3	5.6	$a^{″}$	$\tau$(CH${}_3$)
2${\nu }_{18}$	234 ± 4		1.9 ± 1.2						$a^{\prime }$
${\nu }_{17}$	312; 315 a		27.3 ± 2.9	43.7	42.1	42.5	44.6	43.7	$a^{\prime }$	$\delta$($\mathrm{C}-\mathrm{O}-\mathrm{C}$)
${\nu }_{16}$	332 ± 4								$a^{″}$	$\tau$($\mathrm{C}-\mathrm{O}-\mathrm{C}=\mathrm{O}$)
2${\nu }_{16}$	660		0.2 ± 0.6						$a^{\prime }$
${\nu }_{15}$	769		5.1 ± 0.9	8.6	6.3	6.3	8.4	6.3	$a^{\prime }$	$\delta$($\mathrm{O}=\mathrm{C}-\mathrm{O}$) −$\delta$($\mathrm{C}-\mathrm{O}-\mathrm{C}$)
${\nu }_{14}$	928		47.2 ± 2.3	46.4	47.9	47.9	43.2	44.9	$a^{\prime }$	$\nu$($\mathrm{C}-\mathrm{O}$) +$\nu$($\mathrm{O}-\mathrm{C}$)
${\nu }_{13}$	1024 ± 4		2.9 ± 0.8	3.9	3.8	3.8	4.0	3.9	$a^{″}$	${\delta }_{\mathrm{op}}$($\mathrm{C}-\mathrm{H}$)
${\nu }_{12}$	1167		4.3 ± 1	8.4	6.4	6.4	7.0	4.7	$a^{″}$	${\omega }_{\mathrm{op}}$(CH${}_3$)
${\nu }_{11}$	1167								$a^{\prime }$	$\nu$(C–O) −$\nu$($\mathrm{O}-\mathrm{C}$) −${\omega }_{\mathrm{ip}}$(CH${}_3$)
${\nu }_{10}$	1210		3 ± 0.8	3.9	3.8	3.9	5.1	5.7	$a^{\prime }$	$\nu$($\mathrm{C}-\mathrm{O}$) −$\nu$($\mathrm{O}-\mathrm{C}$) +${\omega }_{\mathrm{ip}}$(CH${}_3$)
${\nu }_9$	1370		10.2 ± 1.1	12.2	9.6	9.6	12.7	10.4	$a^{\prime }$	${\delta }_{\mathrm{ip}}$($\mathrm{C}-\mathrm{H}$)
${\nu }_8$	1438		17.1 ± 1.5	23.1	18.5	18.5	25.2	20.5	$a^{\prime }$	umbrella (CH${}_3$)
${\nu }_7$	1452 ± 4								$a^{″}$	${\delta }_{\mathrm{as}}$(CH${}_3$)
${\nu }_6$	1465								$a^{\prime }$	${\delta }_{\mathrm{s}}$(CH${}_3$)
—	1585		1.3 ± 0.8
${\nu }_5$	1755		25.5 ± 1.5	18.4	24.0	24.0	18.9	24.2	$a^{\prime }$	$\nu$(C=O)